Application of biocompatible thin organic coatings to improve tribology of ti6al4v alloy

.. .APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY OF TI6AL4V ALLOY BHARAT PANJWANI (B.Tech, IIT Kanpur, India) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING... and its alloys In this thesis, application of thin organic coatings to improve tribology of titanium and its alloys has been explored with emphasis on biomedical applications Ti6Al4V alloy, a... tribological limitations of Ti6Al4V alloy In this study, following approaches have been used:  Use of PFPE to improve the tribological properties of Ti6Al4V alloy  Use of PFPE overcoat to improve the tribological

APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY OF TI6AL4V ALLOY BHARAT PANJWANI NATIONAL UNIVERSITY OF SINGAPORE 2011 APPLICATION OF BIOCOMPATIBLE THIN ORGANIC COATINGS TO IMPROVE TRIBOLOGY OF TI6AL4V ALLOY BHARAT PANJWANI (B.Tech, IIT Kanpur, India) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Preamble Preamble This thesis is submitted for the degree of Master of Engineering in the Department of Mechanical Engineering, National University of Singapore under the supervision of Associate Professor Sujeet Kumar Sinha. No part of this thesis has been submitted for any degree or diploma at any other University or Institution. As far as the author is aware, all work in this thesis is original unless reference is made to other work. Parts of this thesis have been published and under review for publication as listed below: Journal 1. B. Panjwani, N. Satyanarayana and S. K. Sinha. "Tribological characterization of a biocompatible thin film of UHMWPE on Ti6Al4V and the effects of PFPE as top lubricating layer", Journal of the Mechanical Behavior of Biomedical Materials 4 (2011) 953-960. (a part of Chapter 4) 2. B. Panjwani and S. K. Sinha. “Evaluation of tribological properties of PFPE over-coated 3-glycidoxypropyltrimethoxy silane self-assembled monolayer on Ti6Al4V surface”, manuscript in preparation. (a part of Chapter 5) Conference 1. B. Panjwani, N. Satyanarayana and S. K. Sinha, “Improving the tribology of Ti6Al4V through a biocompatible thin UHMWPE coating”, ICMAT 2011 - International Conference on Materials for Advanced Technologies, Singapore from 26 Jun to 1 July, 2011. i Acknowledgements Acknowledgements This is the great opportunity to acknowledge and express my thanks to people for their support and encouragement in my postgraduate studies. First of all, I would like to express my earnest gratitude and sincere thanks to my supervisor Associate Professor Sujeet Kumar Sinha for providing me this priceless opportunity to pursue my postgraduate studies. I am pleased to thank my graduate advisor Assoc. Prof. Sujeet Kumar Sinha for his invaluable guidance, supervision, encouragement, support and offering this great opportunity to work with him. I would like to express my special thanks to Dr. Nalam Satyanarayana for his consistent help and support offered during my research work. I would also like to thank Dr. R. Arvind Singh, Dr. Mohammed Abdul Samad and Dr. Myo Minn for their support and valuable discussions. I would like to say thanks to all my colleagues, Ehsan, Jonathan, Keldron, Nam Beng, Prabakaran, Robin, Sandar, Sekar, Srinivas, Yaping, Yemei, for stimulating research environment of mutual support and help in the team. I would also like to thank all my friends, Amit, Archit, Chandra, Luv, Meisam, Sashi, Srinivasa, Tapesh for their friendship and support. I am grateful to the lab staff, Mr. Thomas Tan Bah Chee, Mr. Abdul Khalim Bin Abdul, Mr. Ng Hong Wei, Mr. Maung Aye Thein, Mr. Juraimi Bin Madon, Mr. Suhaimi Bin Daud, for their continuous support and assistance. Many thanks to Mr. Juraimi Bin Madon for his technical expertise and support offered ii Acknowledgements in the fabrication of fixtures. I would also like to express my sincere thanks to the ME dept office staff, Ms. Teo Lay Tin, Sharen and Ms. Thong Siew Fah, for their support. Finally, I want to express my gratitude and sincere thanks to my family for their support, love and encouragement. iii Table of Contents TABLE OF CONTENTS Page Number Preamble i Acknowledgements ii Table of Contents iv Summary ix List of Tables xii List of Figures xiii List of Notations xvii Chapter 1 Introduction 1 1.1 Importance of tribology 1 1.2 Brief history of tribology 2 1.3 Tribological applications 3 1.3.1 Industrial tribology 3 1.3.2 MEMS/NEMS tribology 3 1.3.3 Biomedical tribology 4 1.4 Importance of titanium and titanium alloys 4 1.4.1 Industrial applications 5 1.4.2 Consumer durables 6 1.4.3 Medical applications 7 1.4.4 MEMS applications 7 1.5 Titanium and titanium alloys tribology 8 iv Table of Contents 1.6 Objectives of the thesis 9 1.7 Methodology in the present thesis 10 1.8 Structure of the thesis 11 CHAPTER 2 Literature Review 12 2.1 Surface engineering and tribology 12 2.2 Existing tribology solutions for titanium alloys 13 2.2.1 Surface treatments 13 2. 2.1.1 Thermally sprayed coatings 13 2. 2.1.2 Electroplating and electroless plating systems 14 2. 2.1.3 Physical vapor-deposited coatings 14 2. 2.1.4 Surface modifications 15 2.2.2 Thermo-chemical processes 16 2.2.2.1 Nitriding 16 2.2.2.2 Oxidizing 17 2.2.3 Energy beam surface alloying 18 2.2.3.1 Laser gas nitriding 18 2.2.3.2 Electron beam alloying 18 2.2.4 Duplex treatments 2.3 Ti6Al4V alloy surface treatments for biomedical applications 2.3.1 Plasma nitriding 2.3.2 Bio-ceramic coatings 19 19 19 20 v Table of Contents 2.4 Thin film coatings in tribology 2.4.1 Polymer coatings in tribology 2.4.1.1 UHMWPE polymer coating tribology 2.4.2 Self-assembled monolayers coatings 20 21 22 23 2.4.2.1 Applications of SAMs coatings on titanium 26 2.4.2.2 Applications of SAMs coatings in MEMS tribology 27 2.5 Friction and wear mechanisms in polymer tribology 28 2.6 Solution-based coating methods for polymers 32 2.7 Use of PFPE as a top layer 33 2.8 Pretreatment methods 34 2.9 Biocompatibility testing 37 2.9.1 In-vitro testing 38 2.9.2 In-vivo animal testing 39 2.9.3 Clinical testing 39 CHAPTER 3 Materials and Experimental Procedures 40 3.1 Materials 40 3.2 Coatings preparation procedure 42 3.3 Polymer coating thickness measurement method 45 3.4 Contact angle measurement 46 3.5 Optical microscope 47 3.6 FE-SEM surface morphology observation 47 3.7 AFM surface topography measurement 48 vi Table of Contents 3.8 FTIR-ATR analysis 49 3.9 XPS analysis 50 3.10 Cytotoxicity assessment 51 3.11 Tribological characterization 52 CHAPTER 4 Tribological Characterizations of Thin UHMWPE Film and PFPE Overcoat 54 4.1 Physical characterizations 55 4.1.1 Coating thickness measurement 55 4.1.2 Water contact angle results 55 4.1.3 SEM surface morphology 57 4.1.4 AFM surface morphology 58 4.2 Chemical characterizations 59 4.2.1 FTIR analysis results 59 4.2.2 XPS analysis results 59 4.3 Tribological characterization of UHMWPE coating 60 4.4 Investigation of underlying wear mechanism 63 4.5 Effect of PFPE overcoat on UHMWPE coating 65 4.6 Explanation of wear resistance increase by PFPE overcoat 66 4.7 Biocompatibility assessments 67 4.7.1 Cytotoxicity test results 67 4.8 Potential applications of coatings 68 vii Table of Contents CHAPTER 5 Tribological Evaluations of Molecularly Thin GPTMS SAMs Coating with PFPE Top Layer 70 5.1 Physical characteristics of the coatings 71 5.1.1 Water contact angle results 71 5.1.2 AFM morphology results 73 5.2 Chemical characteristics of UHMWPE coating 75 5.2.1 XPS analysis results 75 5.3 Tribological characterizations 76 5.4 Optical microscopy of wear track and counterface surface 80 5.5 Biocompatibility test 84 5.5.1 Cytotoxicity test results 85 5.6 Potential applications of GPTMS/PFPE coating 85 CHAPTER 6 Conclusions 87 CHAPTER 7 Future Recommendations 90 References 92 Appendix A Cytotoxicity Test Procedures 107 viii Summary Summary Titanium and its alloys have been extensively used in many biomedical and industrial applications due to their high specific strength with acceptable elastic modulus, corrosion resistance and biocompatibility. However, high coefficient of friction and low wear resistance of titanium and its alloys limit their usage in some applications. To improve the tribological properties of titanium and its alloys, various surface modifications, coatings and treatments have been explored. In spite of these developments, there is still a need to further investigate effective solutions to improve tribological properties of titanium and its alloys. In this thesis, application of thin organic coatings to improve tribology of titanium and its alloys has been explored with emphasis on biomedical applications. Ti6Al4V alloy, a commonly used titanium alloy, has been chosen as substrate material in the studies of this thesis. In the first study, ultra-high molecular weight polyethylene (UHMWPE) polymer thin film (thickness of 19.6±2.0 µm) was coated onto substrate using dipcoating method. measurement, Physical Field characterizations emission-scanning (contact electron angle, microscopy thickness (FE-SEM) morphology and atomic force microscopy (AFM) imaging), biocompatibility test (cytotoxicity) and chemical characterizations (Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS)) were carried out for the obtained UHMWPE coating. Tribological characterization of this coating was carried out using 4 mm diameter ix Summary Si 3 N 4 ball counterface in a ball-on-disk tribometer for different normal loads (0.5, 1.0, 2.0 and 4.0 N) and rotational speeds (200 and 400 rpm). This coating exhibited low friction coefficient (0.15) and high wear life (> 96,000 cycles) for the tested conditions. Perfluoropolyether (PFPE) overcoat on UHMWPE coating further increased the wear resistance of coating as tested at even higher rotational speed (1000 rpm). UHMWPE coatings (with and without PFPE overcoat) meet the requirements of cytotoxicity test using the ISO 10993-5 elution method. Due to their low surface energy, wear resistance and noncytotoxic nature, the thin coatings of UHMWPE and UHMWPE/PFPE can find various applications in biomedical implants and devices. Despite having suitable properties for biomedical applications, higher thickness of UHMWPE and UHMWPE/PFPE coatings may prevent their usage in micro-electro-mechanical systems (MEMS) biomedical applications. In the second study of this thesis, 3-glycidoxypropyltrimethoxy silane (GPTMS) selfassembled monolayers (SAMs) with PFPE overcoat has been deposited onto substrate. For comparison, PFPE coating has also been formed onto same substrate. Ti6Al4V alloy specimens with PFPE overcoat and GPTMS/PFPE composite coating showed low coefficient of friction and high wear durability as tested at 0.2 N normal load and rotational speed of 200 rpm. The wear durability of the obtained GPTMS/PFPE coating is much higher than that for only PFPE coating. Obtained coatings were also characterized by contact angle measurement, AFM imaging and XPS analysis. Formed PFPE and GPTMS/PFPE coatings are x Summary biocompatible in nature. Due to the combination of hydrophobicity, low friction coefficient, high wear resistance and noncytotoxicity, these coatings can find usage in biomedical applications where low coating thickness may be crucial. Molecular thickness (< 4 nm) of these coatings is particularly advantageous for their applications in biomedical MEMS devices. xi List of Tables List of Tables Page Number Table 4.1 Measured water contact angle values for different specimens. 56 Table 4.2 Summary of specimens. 61 Table 5.1 Measured water contact angle values for different specimens. 72 Table 5.2 Measured surface roughness for different Specimens in AFM imaging. 74 Table 5.3 Coefficient of friction for specimens tested in the study. 77 tribological tests on Ti6Al4V/UHMWPE xii List of Figures List of Figures Page Number Figure 1.1 Research methodology followed in the research studies. 10 Figure 2.1 Schematic of typical SAM molecule structure and attachment with substrate. 25 Figure 2.2 General classification of the wear of polymers [Briscoe and Sinha 2002]. 30 Figure 2.3 Schematic representations of wear mechanisms (N: normal load; V: sliding velocity). (a) Adhesive wear. (b) Abrasive wear. 31 Figure 2.4 A schematic diagram of contact angle measurement. 36 Figure 3.1 Experimental apparatus. (a) Dip-coating machine. (b) Clean air furnace. 44 Figure 3.2 Optima contact angle measurement set-up. 46 Figure 3.3 Experimental instruments. (a) Optical microscope set-up. (b) Ball-on-disk tribometer. (c) Ball-on-disk tribometer stage. 47 Figure 3.4 Ball-on-disk tribometer schematic (R: Track radius; r: Ball radius; F: Normal load; ω: Rotational speed of the disk). 53 Figure 4.1 Step-height measurement method. 55 Figure 4.2 Measured water contact angle values for different specimens. (a) Ti6Al4V. (b) Ti6Al4V/O 2 plasma treated. (c) Ti6Al4V/O 2 plasma treated/UHMWPE. (d) Ti6Al4V/O 2 plasma treated/UHMWPE/PFPE. 57 Figure 4.3 Surface morphology of Ti6Al4V/UHMWPE surface using FESEM. (a) At lower magnification, 100x. (b) At higher magnification, 500x. 57 xiii List of Figures Page Number Figure 4.4 AFM morphology of surfaces. (a) Polished Ti6Al4V alloy surface (scan area: 40µm×40µm, vertical scale: 1µm). (b) Ti6Al4V/UHMWPE surface (scan area: 40µm×40µm, vertical scale: 5µm). 58 Figure 4.5 FTIR-ATR spectrum of the UHMWPE coating on Ti6Al4V substrate. 59 Figure 4.6 XPS spectra of specimens. (a) UHMWPE coating on Ti6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V. 60 Figure 4.7 Variation of friction coefficient as a function of the sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si 3 N 4 ball as the counterface (track radius: 3 mm, normal load: 0.5 N, spindle speed: 200 rpm). 62 Figure 4.8 Variation of friction coefficient vs. sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si 3 N 4 ball as the counterface (track radius: 2 mm, normal load: 4 N, spindle speed: 400 rpm). 62 Figure 4.9 Wear track morphology. (a) FESEM morphology of wear track for bare Ti6Al4V alloy for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after 1,000 cycles, magnification 60X. (b) FESEM morphology of wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 sliding cycles, magnification 80X. (c) AFM surface morphology inside the wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 cycles (scan area: 40µm×40µm, vertical scale: 500 nm). 64 xiv List of Figures Page Number Figure 4.10 Wear track and counterface analysis. (a) EDX analysis of wear track for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after completion of 175,000 cycles. (b) EDX analysis of Ti6Al4V surface without polymer coating. (c) Optical image of Si 3 N 4 ball for high normal load tribology test after completion of 175,000 cycles, 100X. (d) Optical image of Si 3 N 4 ball after cleaning with acetone for high normal load sliding tribology test after completion of 175,000 sliding cycles, 100X. 65 Figure 4.11 Effect of PFPE overcoat on wear life (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm). 66 Figure 5.1 Water contact angle measurement from representative samples. (a) Ti6Al4V. (b) Ti6Al4V (after O 2 plasma treatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heat treated). 72 Figure 5.2 Water contact angle measurement from representative samples. (a) Ti6Al4V/GPTMS. (b) Ti6Al4V/GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE (heat treated). 72 Figure 5.3 AFM imaging (scan area: 5µm×5µm, vertical scale: 100 nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) Ti6Al4V/PFPE (heat treated). (d) Ti6Al4V/GPTMS. (e) Ti6Al4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE (heat treated). 74 Figure 5.4 Wide scan XPS spectra of Ti6Al4V/GPTMS specimens. 76 Figure 5.5 Comparison of C1s peaks for bare Ti6Al4V/GPTMS and Ti6Al4V in C1s scan. 76 Figure 5.6 Wear durability (number of sliding cycles before failure) of tested specimens in the study. 77 Figure 5.7 Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE (heat treated) and (c) bare Ti6Al4V) using Si 3 N 4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm). 78 xv List of Figures Page Number Figure 5.8 Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b) Ti6AL4V/GPTMS/PFPE (heat treated) and (c) Ti6Al4V/GPTMS) using Si 3 N 4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm). 79 Figure 5.9 Optical micrographs of Ti6Al4V specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. 80 Figure 5.10 Optical micrographs of Ti6Al4V/GPTMS specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. 81 Figure 5.11 Optical micrographs of Ti6Al4V/PFPE specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. 81 Figure 5.12 Optical micrographs of Ti6Al4V/PFPE (heat treated) specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Figure 5.13 Optical micrographs of Ti6Al4V/GPTMS/PFPE specimen’s wear track and counterface surface after 100,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. 82 82 xvi List of Notations List of Notations ADLC: Amorphous diamond-like carbon AFM: Atomic force microscopy ASTM: American society for testing and materials CVD: Chemical vapor deposition DLC: Diamond-like carbon EDX: Energy-dispersive x-ray spectroscopy Epoxy SAM: 3-Glycidoxypropyltrimethoxysilane FE-SEM: Field emission- scanning electron spectroscopy FTIR-ATR: Fourier transform infrared spectroscopy-attenuated total reflectance GPTMS: Glycidoxypropyltrimethoxysilane HSS: High speed steel ISO: International standards organization L-B: Langmuir-Blodgett method MEM: Minimum essential medium MEMS: Micro-electro-mechanical systems MPa: Mega Pascal NEMS: Nano-electro-mechanical systems PDMS: Polydimethylsiloxane PE: Polyethylene PEEK: Poly ether ether ketone PFPE: Perfluoropolyether PI: Polyimide xvii List of Notations PMMA: Polymethylmethacrylate PS: Polystyrene PTFE: Polytetrafluoroethylene PVD: Physical vapor deposition RMS: Root mean square roughness SAMs: Self-assembled monolayers SEM/EDS: Scanning electron microscope equipped with x-ray energy dispersion spectroscopy Si 3 N 4 : Silicon nitride TO: Thermal oxidation UHMWPE: Ultra-high-molecular-weight polyethylene XPS: X-ray photoelectron spectroscopy xviii Chapter 1: Introduction Chapter 1 Introduction 1.1 Importance of tribology Tribology is defined as the discipline to study the science and technology of interacting surfaces in relative motion and of associated subjects and practices [Jost 1966]. The word “Tribology” was originated from the Greek word “Tribos” which means rubbing [Dowson 1979]. Tribology investigates the principles and related practices of friction, lubrication and wear phenomena to understand the interaction of contact surfaces in a given environment. Friction is defined as the resistance between interacting surfaces under relative motion. Wear can be described as the material removal phenomenon due to interaction of surfaces in relative motion. Friction and wear are often unavoidable phenomena in sliding and rolling surface contacts. Lubrication is the method employed to reduce friction and wear of contact surfaces in relative motion by interposing a material called lubricant. Lubricant can be of any material state such as solid, liquid and gas or a combination of them. Friction and wear play important roles in many places in natural phenomena as well as man-made devices such as automobile, manufacturing etc. Friction and wear are often undesirable factors in many applications and adversely affect the performance and efficiency of systems thus scientists and 1 Chapter 1: Introduction engineers strive to come up with means to minimize friction and wear to increase life and durability of such systems. Tribology has grown into an important discipline for studying friction, lubrication and wear principles in order to improve the efficiency of mechanical systems. 1.2 Brief history of tribology Although full appreciation of significance of tribology as an independent discipline has been recognized only recently, human civilization had realized the importance of friction and wear phenomena since ages. Wheel is the most important mechanical invention of human civilizations and has been important milestone in the journey to come up with solutions to address friction and wear phenomenon in transportation. Known oldest wheel, discovered in Mesopotamia, dates back to 3500 BC although archaeologists believe that it was invented around 8,000 BC. In 1880 BC, the Egyptians used sledges to transport large statues and made use of water to lubricate sledges. Leonardo Da Vinci (1452-1519) is known as the first person to study friction systematically as indicated by the sketches discovered several hundred years later. Amonton (1699) stated through experiments that friction force is directly proportional to the applied normal load and is independent of the apparent area of contact. These two laws are known as Amonton’s laws of friction. Charles Augustine Coulomb (1785) discovered the third law that kinetic friction force is independent of sliding velocity. These three 2 Chapter 1: Introduction laws of friction were discovered on the basis of experimental observations and were related to dry friction. 1.3 Tribological applications Tribology as a discipline has grown tremendously in sync with the scientific and technical developments in the world. Being a multidisciplinary discipline, tribology keeps reinventing itself with developments in science and technology. Today, tribology has found place in every aspects of everyday life. Due to the growth of knowledge and interest in tribology for different applications, this discipline has been further divided into different areas. 1.3.1 Industrial tribology One of the important factors affecting the performance of the machines is the nature of interacting surfaces. Thus friction and wear become important considerations in the functioning of machines. Industrial tribology has found important place in production, manufacturing, fabrication, aviation, aerospace and marine sectors. 1.3.2 MEMS/NEMS tribology With the development of MEMS/NEMS applications, it has been observed that friction and wear at small length scales become limitations for efficiency and durability of devices. At small length scales, surface forces become predominant compared to inertial forces. Thus MEMS/NEMS devices require specialized 3 Chapter 1: Introduction solutions to address tribological limitations to reduce friction and increase wear durability. 1.3.3 Biomedical tribology With the development of biomedical engineering, researchers are investigating the application of tribological principles for the improvement of functioning of medical implants and patients’ comfort. With continued research by tribologists, this area has grown tremendously and has been able to make useful contributions to biomedical engineering. Tribology has found importance in improving implants life and in reducing patient trauma in biomedical applications. 1.4 Importance of titanium and titanium alloys British mineralogist and chemist, William Gregor, discovered titanium metal in 1791. A Berlin chemist, Martin Klaporth, independently isolated titanium oxide in 1795. He named it titanium after Greek mythological name “Titans”. The most popular titanium alloy Ti6Al4V was developed in the late 1940s in the United States [Leyens and Peters 2003]. Titanium and its alloys are widely used in biomedical, aerospace, aviation, marine, chemical industry, sports and leisure applications due to its high specific strength and excellent corrosion resistance. The ASTM defines a number of alloy standards from Grade 1 to 38 (ASTM B861 - 10 Standard Specifications for Titanium and Titanium Alloy 4 Chapter 1: Introduction Seamless Pipe). Ti6Al4V is the most commonly used titanium alloy for biomedical and industrial applications. Its chemical composition consists of 6% aluminum, 4% vanadium, 0.25% (maximum) iron, 0.2% (maximum) oxygen and remaining of titanium. It is significantly stronger than pure titanium while stiffness and thermal properties are same as that of pure titanium (although thermal conductivity is about 60% lower in Grade 5 Ti compared to that of pure Ti). Grade 5 is heat treatable and is an excellent combination of strength, corrosion resistance, weldability and fabricability. Grade 5 can be used upto 400 degrees Celsius temperature [Leyens and Peters 2003]. Ti6Al4V is most widely used among titanium alloys [Donachie 2000] and is considered as the "workhorse" of the titanium industry. 1.4.1 Industrial applications Titanium and its alloys are used in industrial applications due to its high specific strength, fatigue strength and creep resistance at high temperature [Leyens and Peters 2003]. The much higher payoff for weight reduction in aircraft and spacecraft is the driving factor for the usage of titanium and titanium alloys. In jet engine, titanium is the second most common material after Ni-based super-alloys. It is widely used in airframe and gas turbine engine due to the weight saving considerations. 5 Chapter 1: Introduction Highly stressed components of helicopters such as rotor mast and head are made from titanium alloys. In space applications, titanium alloys are used extensively due to small payload requirement of space vehicles. Titanium is reactive metal but is extremely corrosion resistant due to its stable oxide layer at surfaces. Due to its corrosion resistant behavior, titanium alloys are popular in chemical, process and power generation industries. Heat exchangers, condensers, containers, apparatus and steam turbine blades are made from titanium alloys. It is also used in photochemical refineries and flue gas desulphurization plants. Titanium alloys show excellent corrosion resistance in seawater and sour hydrocarbons, thus they are widely used in marine and offshore applications. Titanium alloys are used in automobile industry to improve performance at reduced weight although their use has been limited to racing and high performance sports car due to higher cost. 1.4.2 Consumer durables In sports and leisure, titanium alloys are used in making golf clubs, tennis racquets, baseball bats, pool cues, high speed cycling, scuba diving equipment, expedition and trekking equipment [Leyens and Peters 2003]. Titanium alloys have also found usage in architecture due to its excellent immunity to environmental corrosion and a low coefficient of thermal expansion. In jewellery and fashion industry, titanium alloys are gaining popularity due to its lightweight, corrosion resistant, hypo-allergic nature and possibility of 6 Chapter 1: Introduction creating a large range of surface finishes by utilizing anodizing and heat treatment. Besides titanium alloys are finding place in musical instruments, optical instruments, information technology and security applications due to its versatile properties. 1.4.3 Medical applications Excellent compatibility with the human body makes titanium a key material for biomedical implant materials. It is resistant to corrosion from body fluids. Their excellent fatigue property, high specific strength and low modulus of elasticity make it a preferred material for orthopedic devices. Bone fracture plates, screws, nails and plates for cranial surgery are made from titanium alloys [Leyens and Peters 2003]. Shape memory property of Nitinol (a titanium alloy) makes it suitable for some specialized applications such as stent. Titanium has widespread usage in dental implants due to its biocompatibility and low thermal conductivity. 1.4.4 MEMS applications Titanium is also been proposed as a potential MEMS (Micro-electromechanical systems) material for its physical and mechanical properties. Titanium and titanium alloy MEMS can be preferably used in biomedical applications due to its excellent biocompatibility. As a potential MEMS material for its physical and mechanical properties as well as biocompatibility, titanium alloy can be used in many MEMS 7 Chapter 1: Introduction applications [Aimi et al. 2004]. In MEMS applications, lubrication is required to reduce adhesion, friction and wear to ensure the reliability and durability of devices. The durability and reliability of MEMS/NEMS devices are affected by surface properties such as adhesion, friction, and wear [Bhushan 2003, 2004, 2005]. This requires the application of ultra-thin lubricant films having low friction and low adhesion as well as high wear durability to protect the contact surfaces in MEMS/NEMS devices. 1.5 Titanium and titanium alloys tribology Titanium and titanium alloys have found many applications due to its high strength-to-weight ratio, excellent corrosion resistance and biocompatibility. Unalloyed titanium is as strong as steel but has 45% less weight. Titanium can be alloyed with aluminium, vanadium, molybdenum and iron to produce lightweight strong alloys to produce alloys of importance in biomedical, industrial, marine, automotive and aerospace applications. Application of titanium alloys in many areas is limited by its tribological properties such as high friction coefficient, poor wear durability and low surface hardness. Its poor tribological properties are caused by severe adhesive wear with a strong tendency to seizure, low abrasion resistance and the lack of mechanical stability of the oxide layer [Budinski 1991; Yildiz et al. 2009]. Titanium tribology has found great interest among researchers due to possible application of titanium alloys with improved tribological properties in many potential areas. 8 Chapter 1: Introduction In industrial applications, various surface treatments such as thermochemical processes, energy beam surface alloying and duplex treatments have been proposed by researchers to address the tribological limitations of titanium alloys [Bloyce 1998]. For biomedical applications, plasma nitriding and bio-ceramic coatings are widely investigated solutions to improve the tribological properties of alloys in orthopedic implants [Molinari et al. 1997; Yildiz et al. 2008; Fei et al. 2009]. 1.6 Objectives of the thesis The objective of this thesis is to evaluate some of the potential solutions for surface modifications of titanium and titanium alloys to improve its tribological properties. In the first study, UHMWPE and UHMWPE/PFPE thin film coatings were evaluated to address Ti6Al4V alloy tribological limitations. Experimental characterizations of the physical, chemical and tribological properties of coatings were carried out. This study also investigates the underlying mechanism for excellent UHMWPE thin film tribological properties. In this study, following approaches have been used:  Use of UHMWPE polymer coating to improve the tribological properties of Ti6Al4V alloy.  Use of PFPE as a top layer to further improve the wear resistance of UHMWPE film. 9 Chapter 1: Introduction In the second study, GPTMS self-assembled monolayers (SAMs) coating with PFPE overcoat was evaluated to address tribological limitations of Ti6Al4V alloy. In this study, following approaches have been used:  Use of PFPE to improve the tribological properties of Ti6Al4V alloy.  Use of PFPE overcoat to improve the tribological properties of GPTMS SAMs coated Ti6Al4V alloy. 1.7 Methodology in the present thesis To achieve above objectives, UHMWPE polymer and GPTMS SAMs coatings were deposited onto Ti6Al4V alloy substrate. Oxygen plasma treatment was used to clean and improve adhesion properties of Ti6Al4V alloy substrate with coatings. PFPE top layer was used to enhance the wear durability of coatings. Following process diagram (Fig. 1.1) represents the summary of the steps followed in this thesis for different studies. Figure 1.1: Research methodology followed in the research studies. 10 Chapter 1: Introduction 1.8 Structure of the thesis Present thesis consists of a total of seven chapters. Literature review in the field of titanium and titanium alloys tribology as well as thin film coatings is presented in Chapter 2. Chapter 3 provides the detailed information of materials and experimental characterization techniques (physical, chemical, biological, tribological) used in this thesis. Chapter 4 presents the results and discussions for the tribological evaluation of UHMWPE thin film coating on Ti6Al4V alloy substrate and the effect of PFPE overcoat. Chapter 5 consists of results and discussions for the tribological characterization of PFPE overcoated bare Ti6Al4V alloy and GPTMS SAMs deposited Ti6Al4V alloy. Chapter 6 summarizes the conclusions drawn from studies and Chapter 7 presents the recommendations for future studies from the work presented in this thesis. 11 Chapter 2: Literature Review Chapter 2 Literature Review 2.1 Surface engineering and tribology All solids are bound by surfaces. Surfaces act as an interface between solid and its environment. Many of the properties of the solids are governed by the solid’s interaction with the environment through the surfaces. Surface is the most important part in many engineering applications since most failures such as corrosion, fatigue and wear initiate at surfaces. Even though many solids have desired bulk properties, they lack suitable surface properties. Surface engineering involves modifying surface properties of the components to suit different applications. Surface engineering techniques are used in the automotive, aerospace, missile, power, electronic, biomedical, textile, petroleum, petrochemical, chemical, power, steel, cement, machine tools and construction industries. Tribological properties such friction coefficient and wear are mainly affected by surface properties. For tribological applications, surface engineering consists of surface modifications, surface coatings and treatment techniques. Surface modification also consists of changing the texture of a component. Objective of surface engineering in tribology is to reduce friction coefficient and increase wear durability at contact interfaces in many applications. Many materials possess required bulk properties such as strength and toughness but do not possess suitable surface properties for tribological applications. In these cases, modifying surface properties to reduce coefficient of 12 Chapter 2: Literature Review friction and wear resistance by surface engineering serves the purpose [Bhushan and Gupta 1991]. 2.2 Existing tribology solutions for titanium alloys Higher friction coefficient and low wear durability of titanium and titanium alloys are attributed to the presence of mechanically unstable titanium oxide film and the high surface energy leading to adhesive wear [Budinski 1991; Yildiz et al. 2009]. In titanium and titanium alloys, surface wear occurs by adhesive and abrasive mechanisms as well as by subsurface damage via plastic deformation. The different processes used for the tribological improvements of titanium alloys are surface treatments (thermally sprayed coatings, electroplating and electroless plating systems, physical vapour-deposited coatings and surface modifications), thermo-chemical processes (nitriding and oxidising), energy beam surface alloying (laser gas nitriding and electron beam alloying) and duplex treatments [Bloyce 1998]. 2.2.1 Surface treatments 2.2.1.1 Thermally sprayed coatings In thermal spraying process, solid rod or powder of metal and/or ceramic is partially or fully melted and sprayed onto the substrate on which it re-solidifies. No special pre-treatment is required for titanium and titanium alloys before thermal spraying. Plasma spraying, detonation gun, high-velocity oxy-fuel and vacuum plasma spraying are used to deposit these coatings [Bloyce 1998]. 13 Chapter 2: Literature Review Hard materials such as WC-Co, Mo and Cr-Ni are sprayed onto Ti6Al4V to provide wear resistant coatings. 75 µm of molybdenum is sprayed onto the stems of titanium automotive valves to prevent galling. Molybdenum coating exhibits low coefficient of Friction, lubricant retention ability, hardness and wear resistant properties. In aero engines and other gas turbine applications, sprayed coatings on titanium are used to improve wear durability. Tungsten carbide-cobalt (WC-Co) material is sprayed onto titanium alloy mid-span support faces to provide protection against fretting wear in the low-pressure compressor. Thermal spraying methods are more suitable for localized areas than for complete surface of components. 2.2.1.2 Electroplating and electroless plating systems Since passivating film of TiO 2 acts as a weak interface between the coating and titanium alloy substrates, pretreatments methods such as abrasive blasting and copper strike are required prior to plating. Hard chrome plating, coatings based on electroless nickel and soft metallic coatings including silver and copper, are used on titanium alloy substrates to protect against wear. Oil seal collars, pistons, racing car fly-wheels and bearing housings are plated with hard chrome [Bloyce 1998]. 2.2.1.3 Physical vapour-deposited coatings Metals, alloys, compounds or metastable materials are deposited on different substrates using physical vapour deposition (PVD) methods. TiN coating 14 Chapter 2: Literature Review using PVD method is coated onto titanium alloy parts for racing cars and aerospace components where strength-to-weight ratio is an important consideration. TiN is coated onto pump parts and valve components in oil, chemical and food industries due to its corrosion resistance property [Bloyce 1998]. Diamond-like carbon (DLC), amorphous diamond-like carbon (ADLC), hydrogenated carbon films (a-C:H) [Kustas et al 1993] and MoS 2 [Buchholtz and Kustas 1996] coatings by PVD method are finding applications due to their low coefficients of friction and wear durability. Applications of the most of the developed PVD coatings is limited to low contact stress areas to avoid plastic deformation of the substrate. 2.2.1.4 Surface modifications Ion implantation is one of the widely used surface modification techniques for titanium alloys [Perry 1987]. Commonly implanted species are nitrogen and carbon for Ti6Al4V alloy. Improvement in the wear durability is caused by the increase in surface hardness. An increase in hardness results in resistance to the plastic deformation and thus, oxide layer can support higher stresses. Various titanium prosthetics in biomedical applications are ion-implanted. By anodizing titanium and titanium alloys, layers of TiO 2 less than 100 nm thickness are produced on the surface to increase the wear resistance of the alloys. Anodizing improves the tribological properties of titanium and titanium alloys compared to untreated material but it is not able to support higher loads. 15 Chapter 2: Literature Review Anodizing is frequently used on titanium fasteners and considered as minimum base treatment for titanium and titanium alloys to improve tribological properties [Bloyce 1998]. 2.2.2 Thermo-chemical processes Titanium can be thermo-chemically alloyed with interstitial elements such as boron, carbon, nitrogen and oxygen. Phase equilibrium exists for boron and carbon with titanium element so only a thin compound layer can be produced with boron and carbon which means that solid solution hardening does not exist below the thin compound layer. Thus this condition is similar to surface modification produced by PVD coatings [Bloyce 1998]. Solid-solution-hardened diffusion zones exist below the surface compound layers in the case of nitriding and oxidizing so these methods are more useful considering depth-hardening criteria. 2.2.2.1 Nitriding Nitriding of titanium and titanium alloys has been widely used effectively as a surface treatment for protection against wear [Molinari et al. 1997]. Components treated by nitriding include racing engine components, surgical instruments, racing car components, watchcases, precision mechanical parts and golf club heads. Plasma ion or glow discharge nitriding have long been applied in the surface treatment of titanium-based materials. Treatment gases such as nitrogen, nitrogen-hydrogen mixtures, nitrogen-argon mixtures or cracked ammonia and temperatures in the range 700-900 ºC are used in plasma nitriding. 16 Chapter 2: Literature Review Racing car steering racks, gears and ball valves are treated using plasma nitriding [Bloyce 1998]. 2.2.2.2 Oxidising Tribological properties of titanium and its alloys can be improved by oxidizing. Oxygen in solution in αTi results in the significant strengthening of the material although it affects adversely the properties such as tensile ductility, fracture toughness and fatigue crack growth. Considering balance of the change in the properties, oxidising is used in limited cases. Controlled oxidizing in lithium carbonate salt baths is used for the production of titanium pistons. A treatment based on oxidizing in air has been used to surface treat Ti22V4Al alloy valve spring retainers. Diffusion hardening has been successfully applied in the surface treatment of the biomedical alloy Ti-13Nb-13Zr alloy to improve tribological properties [Mishra et al. 1994]. Ti6Al4V alloy treated by a thermal oxidation (TO) process exhibits low coefficient of friction and low wear rates. Improvement in properties can be attributed to the formation of oxide layer and a hardened diffusion zone. TO is used for surface treatment of auto-engine components. To address the tribological limitations of titanium alloys by oxidising, different solutions such as plasma nitriding, plasma immersion ion implantation, plasma spraying, PVD, CVD and laser surface treatments have been explored [Bloyce 1998]. 17 Chapter 2: Literature Review 2.2.3 Energy beam surface alloying Energy beam surface alloying method changes the chemical composition of the material in the liquid state during surface melting. Greater depth of hardening at surface is obtained through this method compared to other surface engineering methods. 2.2.3.1 Laser gas nitriding Nitrogen is a widely researched alloying element for titanium and titanium alloys. Laser gas nitriding is carried out using CO 2 laser, a gas jet of blowing nitrogen or nitrogen and argon at the melt pool. Different structures are obtained by controlling the amount of nitrogen in the gas jet [Bloyce 1998]. 2.2.3.2 Electron beam alloying Different alloying elements can be used to increase surface hardness and wear durability by using electron beam alloying method [Bloyce 1998]. This process can produce different microstructures with defined properties by controlling the amount of alloying elements. Silicon and carbon alloyed surfaces exhibit significant improvements in tribological performance. Titanium alloyed with silicon and carbon results in a tough hard layer, whereas titanium alloyed with silicon and nitrogen results in a hard wear-resistant layer. Electron beam alloying method is a line-of-sight method and is not suitable for complex geometries. 18 Chapter 2: Literature Review 2.2.4 Duplex treatments Combination of two processes can be used to obtain the best improvement in tribological properties as observed in different studies. Relatively deep cases in material surfaces can be achieved by combining surface alloying processes and thermo-chemical treatments. Low-friction hard surfaces can be obtained using a combination of thermo-chemical processes and PVD coating processes [Dong et al. 1996]. 2.3 Titanium alloys surface modifications for biomedical applications Plasma nitriding, CVD, PVD, plasma immersion ion implantation, plasma spraying and laser surface treatments are used to improve wear and corrosion properties of titanium and its alloys in medical implants applications [Yildiz et al. 2009]. 2.3.1 Plasma nitriding Thermo-chemical diffusion plasma nitriding process is one of the common methods to produce hard and wear resistant nitrides on the surface of titanium alloys. Compound and diffusion layers formed on the surface as a result of the nitriding process increase the surface hardness, wear and corrosion resistance of the titanium alloy medical implants. 19 Chapter 2: Literature Review 2.3.2 Bio-ceramic coatings Different bio-ceramic coatings made of inorganic material such as TiN, TiAlN and Al 2 O 3 on the surface of titanium alloys exhibit anti-allergic and noncancerous nature as well as good corrosion resistance and excellent tribological properties. Al 2 O 3 coating shows lower friction coefficient (0.4), high strength, wear resistance, chemical inertness and excellent corrosion resistance in hip– prosthesis applications [Yildiz et al. 2009]. TiAlN deposited by PVD techniques is used in many implant applications. TiAlN coating has good mechanical properties, high wear resistance and biocompatibility. 2.4 Thin film coatings in tribology Thin film surface coating of functional material is commonly used in various fields of technology such as optical devices, electrical equipment, tools for cutting, forming etc [Hedenquist et al. 1992; Sproul 1996; Zweibel 2000]. Depending upon the application, pure metals, compounds and ceramics are used as coating materials. Thin surface coating technique has many advantages. Surface properties can be tailored by coating while bulk properties of the materials are retained. This provides capability to provide optimized properties for the desired application. Materials that are difficult to synthesize utilizing other methods can be used as a coating material. Since coating requires usage of a small amount, expensive material can also be used as thin film coatings. Thin film coatings have also become popular in tribology due to their effectiveness and potential applications [Holmberg 1994]. Thin film coating 20 Chapter 2: Literature Review method can be applied to improve tribological performance of materials by applying a thin layer of a low friction coefficient and wear resistant material. Various methods such as gaseous state and solution state as well as molten and semi-molten state processes can be applied to deposit coatings. 2.4.1 Polymer coatings in tribology A polymer can be defined as a molecule composed of many (poly) parts (mer) joined together by chemical covalent bonds. If all monomer segments of polymer are the same, it is called a homopolymer. If the monomer segments of polymer are different, it is called a copolymer. Polyethylene is a polymer formed from monomer ethylene (C 2 H 4 ). Ethylene is a gas having a molecular weight of 28. The chemical formula for polyethylene (PE) can be written as -(C 2 H 4 )-, where n is the degree of polymerization. Polymer thin films, when coated onto various metallic substrates such as steel and aluminium, show excellent tribological properties [NASA report 1980; Fusaro 1987]. Hard coatings such as metal, ceramic, nitride, carbide and oxide are used widely to provide wear resistant layers onto components in different engineering applications. Recently, researchers are showing interest to explore applications of polymer coatings in different applications due to their good wear and corrosion resistance, self lubricating characteristics, low noise emission, cost effectiveness and impact loading performance [Demas and Polycarpou 2008; Zhang et al 2010]. Due to the higher cost and operational difficulties of PVD and CVD coating 21 Chapter 2: Literature Review techniques, polymer coatings can provide a good alternative protecting technology [Klein 1988]. Commonly used polymers for coatings are polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), ultra high molecular weight polyethylene (UHMWPE), epoxy, polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA). PTFE coating is used in tribological applications due to its excellent properties such as low friction coefficient, high chemical resistance and high temperature stability. However, PTFE coating has poor wear and abrasion resistance which lead to failure in the machine parts [Unal et al. 2004]. PEEK is a suitable polymer for coating due to its excellent tribological properties, chemical resistance and high strength. Many researchers [Lu and Friedrich 1995; Friedrich and Schlarb 2008] have investigated friction and wear behavior of PEEK based composites. 2.4.1.1 UHMWPE polymer coating tribology UHMWPE is a linear homopolymer. In UHMWPE, the degree of polymerization can be upto 200,000. UHMWPE can have average molecular weight of upto 6 million g/mol. UHMWPE molecular weight can not be measured by conventional means and is measured using intrinsic viscosity. UHMWPE polymer has outstanding physical and mechanical properties. It is a unique polymer with excellent impact resistance, abrasion resistance, chemical inertness and lubricity. UHMWPE exhibits high wear resistance compared to PMMA, polystyrene (PS), PEEK, and PE. It has been used in many industrial applications. 22 Chapter 2: Literature Review Bulk UHMWPE is commonly used for orthopedic applications due to its excellent wear resistance. Besides usage of bulk UHMWPE, there are different studies to explore the application of UHMWPE coating for different tribological applications. UHMWPE, as a thin film coating, has shown low coefficient of friction and wear resistance in some studies [Satyanarayana et al. 2006; Minn and Sinha 2008; Abdul Samad et al. 2010]. Due to its excellent wear resistance, UHMWPE coating has the potential to provide a solution for tribological limitations of titanium alloys in biomedical and industrial applications. 2.4.2 Self-assembled monolayers coatings Molecularly thin films have received widespread interest due to their ability to tailor the functionality of the constituent molecules. Molecular thin films facilitate the investigation of intermolecular forces, molecule-substrate and molecule-solvent interactions. These interactions affect interfacial properties such as wettability, biocompatibility and corrosion resistance of the surfaces for different types of materials [Mrksich 2000; Yan et al. 2000]. Chemical transformations on molecular films can be investigated in details which can provide new mechanistic insights as well as methods to tailor surface properties. Such transformations allow functionalities such as the tethering of biologically important molecules to surfaces at precisely controlled positions. These functionalities are of the significant importance in different studies of chemical biology and micro-array technology [Petty 2002; Pirrung 2002]. Development of 23 Chapter 2: Literature Review molecular films by the chemisorptions between the substrate and head group of organic molecules provides a method for making ultra-thin organic films having controlled thickness [Azzam et al. 2002]. Ultra-thin organic molecular layers have also found application as the lubricants for MEMS systems [Bhushan et al. 1995 (a); Komvopoulos 1996; Rymuza 1999]. There are primarily two methods to form lubricant layers; Langmuir–Blodgett method and Self-assembly method [Ulman 1991]. Langmuir–Blodgett (L-B) method cannot be applied for three dimensional surfaces and is used mainly for flat surfaces such as magnetic recording media [Ando et al. 1989; Bhushan et al. 1995 (b)] as well as L–B films have only physically bonding with the substrate by vander Waals forces [Koinkar and Bhushan 1996]. Compared to this, self-assembled monolayers (SAMs) have easy preparation methods and possess excellent properties such as low thickness, stable chemical and physical properties as well as good covalent bonding with the substrate. Moreover, the properties of the SAMs can be tailored by changing the type and length of the molecules, terminal group and the degree of cross-linking within the layer. These properties make SAMs more attractive than the L–B films [Ulman 1991]. SAMs are ordered molecular assemblies formed spontaneously by the immersion of a substrate into a solution of the active surfactant due to adsorption of the surfactant with affinity of its head group to substrate [Ulman 1991]. SAMs consist of three building blocks (Fig. 2.1). A head group bonds covalently strongly to a substrate. A tail group constitutes outer surface of the film. A body 24 Chapter 2: Literature Review chain connects head and tail group. For strong binding of SAM molecules to the substrate, head should contain a group that bonds chemically with the substrate. Molecular structure and cross-linking of SAMs also affect the friction and wear properties of coatings. Figure 2.1: Schematic of typical SAM molecule structure and attachment with substrate. SAMs are usually formed by solution-based method (immersing a substrate in a SAMs solution that is reactive to the substrate surface) though vapor based deposition methods are also used [Ulman 1991]. In general, SAM molecules are not aligned perpendicularly to the substrate and have tilt angle with respect to surface. Alkane-thiolates adsorbed on Au exhibit a tilt angle of 30-35o with the surface normal [Ulman 1996]. By having SAMs with different terminal groups, the film surface can be made hydrophilic or hydrophobic. A non-polar methyl (-CH 3 ) or trifluoromethyl (-CF 3 ) terminal group are used for a hydrophobic surface film with low surface energy. To achieve a hydrophilic film, surface terminal groups such as alcohol (OH) or carboxylic acid (-COOH) groups are used. The commonly used surface 25 Chapter 2: Literature Review head groups are thiol (-SH), silane (e.g. trichlorosilane -SiCl 3 ), and carboxyl groups (-COOH). The commonly used substrates are gold, silver, silicon and steel. There are many research studies of various SAMs having interaction with different substrate. The alkyl-silane SAMs on Si [DePalma and Tillman 1989; Ruhe et al. 1993; Srinivasan et al. 1998; Cha and Kim 2001; Ren et al. 2002; Sung et al. 2003] and alkyl-thiol SAMs on Au [Lio et al. 1997; Bhushan and Liu 2001; Liu et al. 2001] have been widely studied. 2.4.2.1 Applications of SAMs coatings on titanium Increasingly, titanium based substrates are gaining importance. SAMs have been widely investigated as a versatile tool for surface modification in various applications such as microelectronics, corrosion resistance and biosensors. They also facilitate studies of wetting, adhesion, friction and related phenomena [Ulman 1991]. For decades, mineral or metallic surface properties have been tuned by SAMs [Van Alsten et al. 1999; Barriet et al. 2003; Noel et al. 2006; Satyanarayana and Sinha 2005; Raman et al. 2006]. Material degradation can be controlled without altering the visual aspect of the material by forming SAMs of alkyl perfluorinated chains which are known for their chemical inertness. The formation of SAMs on metals [Folkers et al. 1995] or metal oxide surfaces [Gao et al. 1997] is widely employed for the fabrication of model surfaces with highly controlled chemical properties. SAMs can be used in the modification of metal oxide surfaces for the investigation of protein adsorption 26 Chapter 2: Literature Review [Shumaker-Parry et al. 2000], for the study of cell behavior [Noel et al. 2006] as well as for the fabrication of tailored sensor surfaces [Nyquist et al. 2000]. Passivation of metal surfaces, adhesion promotion, and interface corrosion protection in metal/lacquer systems are other examples of industrial applications of SAMs [Van Alsten et al. 1999]. Surface modification of titanium and its alloys is of great importance for their practical applications in biomedical implants. A recent interest has emerged for organic functionalization of the native oxide surfaces of tantalum, titanium and related alloys due to their wide use as biocompatible materials in implants [Viornery et al. 2002; Gawalt et al. 2003]. Covalent surface modification of titanium dioxide is of great interest in view of its applications in medical implants, catalysis and polymer fillers. 2.4.2.2 Applications of SAMs coatings in MEMS tribology Self-assembled monolayers (SAMs) can reduce adhesion and stiction as well as control friction and wear at the contact interfaces. Thus a lot of attention has been paid to study them in tribological applications [Choo et al. 2007; Singh and Yoon 2007]. Even though some SAMs exhibit low coefficient of friction, the wear resistance achieved by these monomolecular layers is not sufficient to provide high wear life to the high velocity moving MEMS components. Therefore, researchers have realized the importance of the mobile portion as the top layer which can provide self-repairability due to the migration of mobile molecules into 27 Chapter 2: Literature Review the sliding contact resulting into high wear resistance [Katano et al. 2003]. This concept has been well studied for hard disk lubrication. 2.5 Friction and wear mechanisms in polymer tribology This section of the thesis has a brief review of polymer tribology with emphasis on the associated friction and wear mechanisms. In polymer tribology, friction between two sliding surfaces is primarily affected by two terms, ploughing term and adhesion term [Briscoe and Sinha 2002]. (a) Ploughing term: This term of the friction is the result of plowing of the asperities of the counterface into polymer besides the sub-surface deformation involving plastic flow and fracture depending upon contact conditions. (b) Adhesion term: This term of the friction is caused by the adhesive interaction of very low thickness layer of the polymer in contact with counterface. If hardness of one of the sliding surface is high relative to another surface, plowing term becomes important. Adhesion term depends upon the interfacial shear stress at the contact surfaces in the absence of hard asperities. Shear stress (τ) depends upon the contact pressure P as given in Equation (2.1) [Bowden and Tabor 1986]. τ = τ 0 + αP (2.1) 28 Chapter 2: Literature Review where τ o is defined as the initial shear stress; α is the pressure coefficient; τ is defined as the friction force (F) divided by real contact area (A); P is calculated as the ratio of applied normal load (L) to real contact area (A) [Briscoe et al. 1973]. Thus Equation (2.1) can be rewritten as Equation (2.2) as given below. F/A = τ 0 + α (L/A) (2.2) Initial shear stress (τ 0 ) can be neglected in most conditions (especially high load conditions) [Briscoe and Tabor 1975]. Therefore, Equation (2.2) modifies as Equation (2.3) as follows. F=αL (2.3) Where α = μ (coefficient of friction) is the ratio of friction force to the applied normal load. Due to visco-elastic nature of polymers, their tribological behavior is more complex than that for metals or ceramics. For polymers, friction coefficient depends upon load, contact geometry and loading time [Bowden and Tabor 1986]. General classification of the wear of polymers is shown in Fig. 2.2 [Briscoe and Sinha 2002]. 29 Chapter 2: Literature Review Figure 2.2: General classification of the wear of polymers [Briscoe and Sinha 2002]. In the generic scaling approach, friction is defined as interfacial and bulk types which result into interfacial and cohesive nature of wear. In the second approach, phenomenological methods are used to define wear by the different prevailing mechanisms at the interface. Third approach defines wear using material characteristics of different polymer types (Fig. 2.2). There are different mechanisms that contribute to wear such as adhesion, abrasion, fatigue, erosion and corrosion in sliding conditions (see Fig. 2.3 for schematic representations of adhesive and abrasive wear mechanism). In most polymers, wear occurs primarily by adhesive, abrasive and fatigue mechanisms [Opondo and Bessell 1982]. In adhesive wear, isolated spots on sliding interfaces adhere and afterwards, shear takes place in sliding at some point other than original adhering interface. Adhesive wear is the purest and most important form 30 Chapter 2: Literature Review of wear and it can not be eliminated fully in sliding conditions. This form leads to material transfer from the worn part to the counterface. Load and geometry of contact as well as surface energy of sliding surfaces affect the characteristics of the transfered layer. The nature of formed transfer layer greatly affects friction and wear rate of polymers. In some sliding conditions, polymer has linear molecules and thin transfer layer protects the counterface. This condition generates very less friction and wear rate by resulting sliding between polymer and polymer due to easy shearing in the sliding conditions. Molecular orientation with respect to sliding direction also affects the friction coefficient. If molecular orientation is aligned with sliding direction, friction force reduces [Briscoe and Sinha 2002]. Thick transfer layers in sliding conditions lead to high wear rate. Molecule types and glass transition temperature of polymer also affect nature of formed transfer layer. Figure 2.3: Schematic representations of wear mechanisms (N: normal load; V: sliding velocity). (a) Adhesive wear. (b) Abrasive wear. Abrasive wear is affected by two-body and three-body mechanisms. In two body abrasive wear, polymer surface and counterface are involved. If 31 Chapter 2: Literature Review generated debris or foreign particles are trapped between sliding surfaces, it results into three-body abrasive wear. Abrasive wear depends upon bulk material properties of polymer as investigated by many researchers [Ratner et al. 1964; Lancaster 1969 and 1973]. Equation (2.4) describes the relationship between wear rate and bulk material properties. V L Se (2.4) Where V = Wear Volume;  = Proportionality constant; L = Normal load;  = Sliding velocity;  = Hardness of the polymer; S = Ultimate tensile stress; e = % Elongation to break. Equation (2.4) has been validated experimentally in different studies [Ratner et al. 1964]. Fatigue wear occurs due to the repetitive cyclic loading in sliding conditions and wear particles are generated in the form of delaminated flakes by the phenomenon of the fatigue crack growth. Fatigue wear is generally smaller than adhesive and abrasive wear but can result into severe damage for mechanical components such as bearings. 2.6 Solution-based coating methods for polymers Many solution-based coating techniques have been developed by making use of the physical interaction between the deposited molecules and the substrate [Advincula et al. 2004]. Following are some of the widely used solution-based coating techniques:  Spin coating 32 Chapter 2: Literature Review  Dip-coating  Printing/droplet evaporation  Doctor blading  Spray coating In the above mentioned deposition techniques, molecules are adsorbed from solution onto substrate and solvent evaporates during the coating process. Layers with desired thickness and homogeneity can be deposited without significant effort by controlling deposition conditions. In this thesis, dip-coating method was used to coat thin films of UHMWPE on Ti6Al4V substrate as well as to overcoat PFPE. In dip-coating process, a substrate is dipped into a liquid coating solution for the specified time duration and is withdrawn afterwards from the solution at a controlled speed. Dipcoating method is an excellent method for producing high-quality and uniform coatings. Being a cost effective method, it can be easily used in laboratory set-up. 2.7 Use of PFPE as a top layer Perfluoropolyether (PFPE) lubricants were developed in the early 1960’s and have been used as lubricants in different applications. PFPE lubricants, a unique class of lubricants and functional fluids, have found usage in different applications because of their versatile nature. PFPE lubricants have following advantageous properties:  Good lubrication properties 33 Chapter 2: Literature Review  Low volatility  Non-flammable  Chemical inertness  Low surface tension  Excellent oxidative and thermal stability  Effective in wide temperature range  Non-cytotoxic and biologically inert  Good radiation resistance PFPE has been applied as a top layer to improve the wear durability of high speed steel (HSS) tools [Fox-Rabinovich et al. 2002]. It was observed that after applying PFPE as a top layer, coefficient of friction decreased and wear resistance of tool improved. Improved properties were attributed to the top layer of PFPE. Use of a mobile hydrocarbon-based lubricant as top layer also improved the wear life of monolayers deposited onto Si based MEMS in a study [Eapen et al. 2005]. PFPE overcoat also resulted in the improved tribological properties of UHMWPE films deposited on Si substrate [Satyanarayana et al. 2006; Minn et al. 2008]. The method of using PFPE as a top layer to improve the tribological properties of polymer and SAMs coatings has also been investigated in this thesis. 2.8 Pretreatment methods Pretreatment methods for surface cleaning and surface energy modification play a very important role in the adhesion of thin films coating onto 34 Chapter 2: Literature Review a substrate. The wetting property of the substrate affects significantly the adhesion between the film and the substrate. High surface energy of the substrate is one of the most important factors that strongly influence the adhesion strength of the coating [O’Brien et al. 2006]. Hydrophobic substrate surface results into low adhesion strength of the applied coating. Surface energy of the surface can be estimated using water contact angle (θ), which is defined by the Young’s Equation [Wu. 1982]. The contact angle is defined as the angle at which a liquid/vapor interface meets a solid surface. The contact angle is a system specific property and is determined by equilibrium of the drop under the action of interfacial energies (Fig. 2.4). Surface energy of the interface can be calculated by water contact angle of the surface. The relation between the contact angle (θ) and interfacial energies is defined as given below (Equation 2.5). γ LV cosθ = γ SV – γ SL (2.5) γ SV = Surface free energy of solid S. γ LV = Surface free energy of liquid L. γ SL = Interfacial free energy between solid and liquid. θ = Contact angle. 35 Chapter 2: Literature Review Figure 2.4: A schematic diagram of contact angle measurement. Depending upon the surface energy, surfaces can be divided into high energy surfaces and low energy surfaces. High energy surface exhibits hydrophilic nature while low energy surface exhibits hydrophobic nature [Wu 1982]. Different physical and chemical treatments can be used for modifying surface energy. Higher surface energy (hydrophilic surface) of the substrate promotes adhesion of the deposited coating. After going through various surface treatments for titanium alloys, oxygen plasma was chosen as pretreatment method for Ti6Al4V alloy substrate for coatings of UHMWPE polymer and SAMs. Oxygen plasma is an environmentalfriendly surface pretreatment technique with cost effectiveness and easier processing method. Plasma cleaning is one of the effective methods for pretreatment of metals to increase surface energy to improve the adhesion between the film and the substrates. Pre-treatment generates functional groups which cause an increase in the hydrophilic nature of the surface. In normal conditions, surfaces exposed to the atmosphere are covered by organic or inorganic contaminants, CO 2 and hydrocarbon. These contaminants 36 Chapter 2: Literature Review prevent good bonding of the coating. Oxygen plasma discharge subjects the surface to very high energy bombarding electrons which break the molecular bonds on the surface. Thus, contaminants covering the surface are removed in the form of CO 2 due to the reaction of carbon with the free oxygen radicals in the plasma. Oxygen radicals generated in the oxygen plasma oxidize the surface resulting in functionalized groups. “Functionalized groups making the surface hydrophilic” is called the oxidative effect. Higher wettability assists in improved adhesion property of the surface [Loh 1999]. Oxygen plasma treatment has been used to create pure and stoichiometric surface oxide layer on Ti. Oxide layers generated by oxygen plasma method exhibit reduced stiction for hydrocarbon contaminations [Aronsson et al. 1997]. Oxygen plasma treatment also hydroxylizes Ti surface [Yoshinari et al. 2006] to facilitate covalent attachment of SAMs on surface. Gas plasma has been used in SAMs deposition studies as pretreatment for metal oxides to attach SAMs [Mahapatro et al. 2006]. Stability of octadecyltriethoxysilane and octadecyltrichlorosilane SAMs deposited on mica was enhanced greatly by the application of gas plasma pretreatment process [Kim et al. 2002, 2008]. 2.9 Biocompatibility testing Biocompatibility refers to the evaluation of the suitability of materials for use in implantable medical devices. Biocompatibility examines the interaction between a medical device and tissues as well as physiological systems of the patient treated with the device. An evaluation of biocompatibility is an important 37 Chapter 2: Literature Review and early part of the overall testing of a device for intended medical application. The biocompatibility of a device depends on different factors. Some of the important factors are the following: • Physical and chemical nature of device materials. • Types of patient tissue that will come in contact with the device. • Duration of device exposure. ISO 10993 “Biological Evaluation of Medical Devices” documents the testing strategies that are acceptable in Europe and Asia. Part 1 of the ISO 10993 standard has the general guidance on selection of tests. Part 2 covers requirements of animal welfare. Parts 3 through 19 are guidelines for specific test procedures and guidelines on testing-related issues. Different biocompatibility tests such as cytotoxicity, hemocompatibility, sensitization, irritation, systemic toxicity, genotoxicity and implantation have been advised in ISO 10993 as per the requirements of device such as nature and duration of contact. Any medical implant needs to go through following categories of tests before getting approval to be used in medical practice. 2.9.1 In-vitro testing In this method, testing is carried out using cells and tissues outside the body in an artificial environment. 38 Chapter 2: Literature Review 2.9.2 In-vivo animal testing Studies are carried out for implant material in an animal model. 2.9.3 Clinical testing Tests are carried out in human subjects to verify the safety and effectiveness of a medical device for intended applications. 39 Chapter 3: Materials and Experimental Procedures Chapter 3 Materials and Experimental Procedures This chapter provides detailed information on the materials used in sample preparation, coating preparation procedures and descriptions of experimental methodologies used in physical, chemical, biological and tribological characterizations of the formed coatings in this thesis. 3.1 Materials Ti6Al4V-ELI Grade 5 (as per ASTM B265) specimens were used as the substrate to form coatings. Size of the Ti6Al4V alloy specimen used for coating deposition was 25mm×25mm (5mm thickness). Ti6Al4V alloy specimens were procured from Titan Engineering, Singapore. In the chapter 4 of this thesis, grinding of titanium alloy samples was done using SiC abrasive papers using 800, 1000 and 1200 grit sizes successively. After grinding, samples were polished using 5 μm alumina powder paste and 1 µm diamond paste subsequently. Afterwards, samples were sonicated using acetone, ethanol and distilled water for the duration of 10 min in each solution and dried with N 2 gas. After grinding, polishing and cleaning, surface roughness of obtained Ti6Al4V alloy specimens was measured as 44 nm. Roughness of the Ti6Al4V alloy specimen was measured using AFM in a scan area of 40μm×40μm. AFM roughness measurements were carried out at three different 40 Chapter 3: Materials and Experimental Procedures locations onto the sample surface and it provided same roughness value within the deviation of 5 nm. UHMWPE polymer powder was procured from Ticona Engineering Polymers, Germany through a local Singapore supplier. UHMWPE polymer powder was of GUR X143 grade. Physical properties of the UHMWPE powder used are as follows:  Melt flow index 190/15 = 1.8±0.5 G/10 min  Density = 0.33±0.03 g/cm3  Average particle size = 20±5 μm Decahydronapthalin (decalin) was used as the solvent to dissolve UHMWPE powder. PFPE (Z-dol 4000) with chemical formula (HOCH 2 CF 2 O(CF 2 CF 2 O) p -(CF 2 O) q -CF 2 CH 2 OH; ratio p/q is 2/3) was procured from Solvay Solexis, Singapore. H-Galden (ZV60) was used as solvent for PFPE and procured from Ausimont INC. Chemical formula of H-Galden is (HCF 2 O-(CF 2 O) p - (CF 2 CF 2 O) q -CF 2 H, ratio p/q is 2/3). Ethanol, acetone, and distilled water were used for cleaning of titanium alloy sample surfaces before any surface treatment and coating. In the chapter 5 of this thesis, grinding of titanium alloy samples was done using SiC abrasive papers using 160, 320, 800 and 1000 grit sizes successively. After grinding, samples were polished using 1 µm and 0.25 µm diamond pastes subsequently. Afterwards, samples were sonicated using acetone, ethanol and distilled water for the duration of 15 min in each solution and dried with N 2 gas. Resulting surface roughness (rms) of Ti6Al4V specimens (after grinding, 41 Chapter 3: Materials and Experimental Procedures polishing and cleaning) was 12 nm (measured by AFM in a scan area of 5μm×5μm). 3-Glycidoxypropyltrimethoxysilane (CH 2 –O–CH–CH 2 –O–(CH2) 3 – Si–(OCH 3 ) 3 ) (purchased from Aldrich) was used for the preparation of SAMs solution. Toluene was used as the solvent to form SAMs solution. A commercial PFPE Zdol 4000 (molecular weight 4000 g/mol, with OH terminal groups at both ends, monodispersed) was used to form an overcoating layer. Hydrofluoropolyether solvent (H-Galden ZV) purchased from Ausimont Inc. was used for the preparation of PFPE solution without further purification. Toluene (99.5% anhydrous), ethanol (99.8%), acetone and distilled water were also used for cleaning the samples. 3.2 Coatings preparation procedure In the chapter 4 of this thesis, oxygen plasma treatment was carried out on dried Ti6Al4V alloy specimens (after grinding, polishing and cleaning) in Harrick Plasma Cleaner. RF power supply of 18 W was used for oxygen plasma treatment. Titanium alloy surfaces were treated with oxygen plasma under vacuum for the time duration of 10 min. Oxygen plasma treatment on titanium alloy surfaces removes contaminations and helps in better adhesion of the UHMWPE coatings. After oxygen plasma treatment, specimens were used for dip-coating in UHMWPE solution. Dip-coating was carried out within 30 min of oxygen plasma treatment to avoid any contamination of the surface after plasma cleaning. 42 Chapter 3: Materials and Experimental Procedures UHMWPE powder dissolution was carried out in decalin first by heating the polymer solution to 80 °C for 20 min followed by heating to 160 °C for 40 min. UHMWPE polymer powder solution in decalin was prepared at 3 wt% concentration. To assist dissolution process, magnetic stirrers were used. After the heating process, solution turns transparent from white which indicates uniform and complete dissolution. Dip-coating process was carried out immediately using dipping and withdrawal speeds of 1.9 mm/s and an intermediate soaking time of 35 sec in solution (See Fig. 3.1 (a) for dip-coating machine). After completion of dipcoating, samples were kept in air for 1 min. Afterwards, samples were kept in clean air furnace at 100 ◦C for a time duration of 20 hrs (See Fig. 3.1 (b) for clean air furnace). Samples were cooled slowly to room temperature after heat treatment. Thus obtained UHMWPE coated samples were kept in desiccator before using it for Ti6Al4V/UHMWPE specimen characterizations. To achieve overcoat of PFPE on UHMWPE, UHMWPE coated samples were dip coated in PFPE solution. PFPE dip coating solution was prepared using 0.5 wt% PFPE in H-Galden solvent. Dip-coating was carried out in PFPE solution for 1 min soaking time and using dipping as well as withdrawal speeds of 2.1 mm/s. For PFPE coating, no heat treatment was carried. These samples were stored in desiccator till further characterizations for Ti6Al4V/UHMWPE/PFPE specimens. 43 Chapter 3: Materials and Experimental Procedures Figure 3.1: Experimental apparatus. (a) Dip-coating machine. (b) Clean air furnace. In the chapter 5 of this thesis, Ti6Al4V alloy specimens (after grinding, polishing and cleaning) were oxygen plasma treated for 10 min. Plasma treated Ti6Al4V alloy specimens were immersed into the epoxy SAM solution (using toluene as the solvent) at a concentration of 1 vol% and left for 18 h. Afterwards, the samples were washed with toluene and ethanol to remove any physisorbed SAM molecules and dried with N 2 gas. Samples were kept in desiccator overnight before any further characterization for GPTMS coating. This procedure is similar to that reported in an earlier study [Luzinov et al. 2000] except that the process was carried out in ambient atmosphere of 25°C and a relative humidity of ~70% in this study. PFPE was dip-coated onto the GPTMS SAMs modified samples. Dipping and withdrawal speeds of 2.1 mm/s were used for the dip-coating process with an intermediate soaking time of 60 sec in solution. After dip coating, heat treatment was carried out for samples at 150 ◦C for approximately 1 hr in clean air furnace. 44 Chapter 3: Materials and Experimental Procedures 3.3 Polymer coating thickness measurement method In the literature, polymer film thickness has been measured by using different methods such as non-contact laser sectioning/SEM [Minn and Sinha 2008], profilometer [Satyanarayana et al. 2006], AFM [Lobo et al. 1999] and focused ion beam/SEM [Abdul Samad et al 2010]. In this study, stylus profilometer in 2D scan mode was used to measure thickness. Contact force of 25.5 mg and scan distance of 500 µm was used for scanning. Step-height method was used to measure the thickness from the scanned profile. Scratches were created in UHMWPE coating using a sharp corner of another bare Ti6Al4V alloy sample to expose Ti6Al4V substrate. This method made scratch only on the polymer coating as the scratching tip and the substrate were of same materials. For measurements of average and standard deviation, data from three samples were used. In each sample, three scratches were made and data from three different locations on each scratch were used to measure thickness. Method used in this study was based on the assumption that bare Ti6Al4V sample corner will not penetrate another Ti6Al4V substrate significantly to affect validity of measurement. To validate this assumption, scratches were created using similar method on bare Ti6Al4V alloy samples. It was observed that the scratch-depth created in this way on bare Ti6Al4V alloy substrate did not exceed 0.7 µm in any of the case. This value has been taken into account in standard deviation measurement of the coating thickness. 45 Chapter 3: Materials and Experimental Procedures 3.4 Contact angle measurement Contact angle measurement is one of the methods to evaluate surface free energy of surfaces. Contact angle measurement is simple, inexpensive and widely used technique for characterizing the wettability of surfaces. Contact angle is sensitive to the conditions such as chemistry and topography of top layer. Water (polar) is the popular liquid to measure the contact angle. Other liquids such as formamide (polar), hexadecane (non-polar) and diiodomethane (non-polar) are also used in contact angle measurements. In this study, static contact angle is used to measure hydrophilic or hydrophobic nature of the different surfaces (See Fig. 3.2 for contact angle measurement set-up). VCA Optima Contact Angle System (AST Products, Inc. USA) was used to measure static contact angle. Distilled water droplets with a volume of 0.5 µl were used for the measurements. To arrive at average contact angle and standard deviation of different surfaces, data were collected for three different samples for five different locations for each sample. Figure 3.2: Optima contact angle measurement set-up. 46 Chapter 3: Materials and Experimental Procedures 3.5 Optical microscope Olympus microscope was used to study counterface and wear track conditions before and after completion of tribological tests (See Fig. 3.3 (a) for optical microscope set-up). This microscope uses monochromatic light source and allows observations at 50, 100, 200 and 500 magnifications. Each silicon nitride ball, used in the tribological tests, was observed under optical microscope before test at various magnifications to ensure cleanliness of the counterface. After completion of each tribological test, wear track and counterface were observed to investigate underlying wear and friction mechanisms. Figure 3.3: Experimental instruments. (a) Optical microscope set-up. (b) Ball-on-disk tribometer. (c) Ball-on-disk tribometer stage. 3.6 FE-SEM surface morphology observation Surface morphology of the Ti6Al4V and UHMWPE coated Ti6Al4V surfaces was observed using Hitachi S-4300 Field Emission Scanning Electron 47 Chapter 3: Materials and Experimental Procedures Microscopy (FESEM). Thin gold coating was carried out on polymer deposited Ti6Al4V surfaces to make the surface conductive. JEOL, JFC-1200 Fine Coater was used to perform gold coating. A current of 10 mA and the coating duration of 30 sec were used to deposit gold film on polymer coated surfaces. 3.7 AFM surface topography measurement Atomic force microscopy (AFM) is a very high resolution microscopy having resolution on the order of the fractions of a nanometer. In AFM imaging, cantilever with a sharp tip at its end is used to scan the specimen surface. Tip interaction with the surface causes tip to deflect due to operating forces between the tip and the sample. Deflection of the tip is measured by a laser reflected from the top surface of the cantilever into photodiodes. AFM operates in primarily two modes. Contact Mode and Tapping mode [Digital Instruments MultiModeTM Instruction Manual 1997]. In contact mode, tip scans the surface and tip deflection is used as feedback signal. During scanning, force between the tip and the surface is kept constant by maintaining a constant tip deflection. In the tapping mode imaging, the tip is oscillated up and down at near its resonance frequency while scanning the sample surface. The reflected laser beam from the tip generates a sinusoidal signal in the photodiode array whose amplitude provides the image of the scanned surface. As the oscillating tip is scanned over the surface, tip interactions with the surface cause the change in the amplitude of oscillating tip. In contact mode, scanning over the surface can cause damage or 48 Chapter 3: Materials and Experimental Procedures modify surface features. Thus tapping mode is preferred over contact mode to measure topography of polymer coatings. Tip geometry also affects the accuracy of measurements. Sharp tip generates accurate representation of film while blunt tip provides erroneous imaging results. Besides tip geometry, the spring constant and the resonance frequency of tip are important considerations for the accuracy of imaging. A small spring constant is effective in measuring small forces and a high resonance frequency is recommended for making tip insensitive to vibrations and mechanical noise. Topographic (roughness) measurement of specimens’ surfaces was carried out using Atomic force microscope (Dimension 3000 AFM, Digital Instruments, USA). Images were scanned in air using a monolithic silicon tip using the tapping mode. In measurements, set point voltage used was 1-2 V and scan rate of 0.5 Hz was used. 3.8 FTIR-ATR analysis FTIR-ATR (Fourier transform infrared spectroscopy-attenuated total reflectance) is an analytical technique to identify organic and inorganic materials by measuring the absorption of infrared radiation by the sample material versus wavelength. Molecular components and structure of the material can be identified by infrared absorption. When an infrared radiation is irradiated over the material, molecules are excited by absorbed IR radiation into a higher vibrational state. The energy difference between excited vibrational state and ground state determines 49 Chapter 3: Materials and Experimental Procedures the wavelength of absorbed light by a particular molecule. Thus absorbed wavelengths by the sample are characteristic of its molecular structure. In FTIR spectroscopy, an interferometer is used to modulate the wavelength generated from a broadband infra-red source. A detector is used to measure the intensity of transmitted or reflected light as a function of its wavelength. Thus detector is able to provide the interferogram (intensity of light as a function of its wavelength). Interferogram is analyzed by computer using Fourier transforms to obtain a single-beam infrared spectrum. The FTIR spectra are plotted as intensity versus wave number (in cm-1). The wavenumber is the reciprocal of the wavelength. The intensity is plotted as the percentage of light transmittance or absorbance vs wavenumber. Bio-Rad FTIR model 400 spectrophotometer was used to collect FTIRATR spectrum in air for UHMWPE film. This spectrum was collected from 32 scans at a resolution of 4 cm−1 at four different locations. The spectra were collected at 5 replicate points. Plasma treated bare Ti6Al4V was used for background scan for ATR-FTIR spectrum. 3.9 XPS analysis X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique to measure chemical state, elemental composition, empirical formula and electronic state of the elements of the material. XPS is operated in ultra high vacuum (UHV) conditions. 50 Chapter 3: Materials and Experimental Procedures In XPS spectra, material is irradiated with x-rays and kinetic energy and numbers of electrons that escape from the top 1 to 10 nm of the material surface are analyzed. X-rays are usually generated either by Mg or Al source. X-rays cause a core electron to be emitted from the sample. The kinetic energy of thus emitted electron is detected using an electron multiplier and is equal to the difference between the energy of the X-ray (1253 eV for Mg, 1486 eV for Al) and the binding energy of the electron. In this study, XPS is used to study the chemical state of the sample material. XPS (Thermo Fisher Scientific Theta Probe) was used to study chemical states of the coatings. A monochromatized Al Kα X-ray source (1486.6 eV photons) was used at a constant dwell time of 100 ms and pass energy of 40 eV. A photoelectron take-off angle of 90o was used to obtain the core level signals. All binding energies (BE) were referenced to the C1s hydrocarbon peak at 284.6 eV. 3.10 Cytotoxicity assessment The cytotoxicity test was carried out by NAMSA (Northwood, OH, USA). This test was done according to the guidelines of “International Organization for Standardization 10993-5: Biological Evaluation of Medical Devices, Part 5: Tests for In Vitro Cytotoxicity” (see Appendix A for details on cytotoxicity testing methods). Before cytotoxicity testing, Ti6Al4V/UHMWPE specimens were sterilized using ethylene oxide and degassed. Fifteen specimens were used for each test for the cytotoxicity testing using ISO elution method-1×MEM extract method. 51 Chapter 3: Materials and Experimental Procedures In a single preparation, test specimens were extracted in single strength minimum essential medium (1×MEM) at a temperature of 37 ◦C for the time duration of 24 hours. The negative control, reagent control and positive control were also prepared as per the requirements mentioned in testing standards. Triplicate monolayers of L-929 mouse fibroblast were dosed with prepared extracts. Incubation was carried out at a temperature of 37 ◦C in the presence of 5% CO 2 for a time duration of 48 hours. Following the incubation, the triplicate monolayers were microscopically (100 magnification) examined for any abnormal cell morphology and cellular degeneration. 3.11 Tribological characterization Tribological tests were carried out in this study using UMT-2 (Universal Micro Tribometer, CETR, USA) (see Fig. 3.3 (b) and (c) for tribometer set-up). It can be operated in both ball-on-disk and ball-on-plate modes. This tribometer can apply normal loads upto 500 gm (5 N) and rotational speeds of upto 5000 rpm (1.05 m/s at a track radius of 2 mm). In this study, tests were carried out in ballon-disk mode under dry conditions. Ball-on-disk mode measures dynamic friction coefficient. Many contact geometries can be simulated using ball-on-disk mode thus this mode was chosen to evaluate tribological properties of coatings in this thesis. Ball-on-disk mode schematic has been shown in Fig. 3.4. A Si 3 N 4 ball of 4 mm diameter was used as the counterface for tribological tests. Si 3 N 4 ball had the surface roughness of 5 nm as provided by the supplier. The ball was thoroughly cleaned using acetone before each test. Si 3 N 4 ball is used as the counterface since 52 Chapter 3: Materials and Experimental Procedures Si 3 N 4 material has much higher hardness when compared to Ti6Al4V alloy and is inert towards organic species. For tribological characterization, the spindle rotational speeds of 200 and 400 rpm were used in different tests. Track diameter of 4 mm was used for all the tests. The spindle rotational speed of 200 rpm gives a sliding speed of 41.9 mm/s at the used track diameter. Applied normal loads from 0.2 N to 4.0 N were used for various tests. At least three repetitions were carried out for every tested load and speed combination. Tribological tests were carried out in a class 100 clean booth environment. Temperature of 25±2 °C and a relative humidity of 55±5 % were maintained in the clean booth for tests. The wear life for the tribological tests was taken as the number of life cycles after which the coefficient of friction exceeded 0.3 or a visible wear appeared on the substrate whichever occurred earlier [Miyoshi 2001]. The FESEM and optical microscope were used to observe the worn track surfaces after appropriate number of sliding cycles to infer wear mechanism. Figure 3.4: Ball-on-disk tribometer schematic (R: Track radius; r: Ball radius; F: Normal load; ω: Rotational speed of the disk). 53 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Chapter 4 Tribological Characterizations of Thin UHMWPE Film and PFPE Overcoat In view of the stated objective of this thesis in Chapter 1, first study of this thesis titled “Tribological characterizations of thin UHMWPE film and PFPE overcoat” investigates the potential of thin UHMWPE polymer coating to address tribological limitations of Ti6Al4V alloy. Rationale behind the selection of UHMWPE polymer coating in this study has been explained in chapter 2. UHMWPE polymer thin film was coated onto Ti6Al4V alloy substrate using dip-coating method. Results of physical characterizations (contact angle, thickness measurement, FE-SEM morphology and AFM imaging), biocompatibility test (cytotoxicity) and chemical characterizations (FTIR-ATR and XPS) of UHMWPE thin film have been reported in this chapter. Use of PFPE overcoat to further improve the wear resistance of UHMWPE coating has been evaluated at higher RPM. It was observed that the coating of PFPE onto a Ti6Al4V substrate without any intermediate layer does not achieve high wear durability. Wear track and counterface conditions have been examined to infer underlying friction and wear mechanism. Potential applications of this coating have been suggested based upon prior literature studies and the results of this study. 54 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat 4.1 Physical characterizations 4.1.1 Coating thickness measurement Thickness of UHMWPE coating obtained in this study was measured using stylus profilometer by step-height method. Fig. 4.1 shows the schematic of thickness measurement in step-height method. Measured average thickness and standard deviation of the characterized UHMWPE coatings are 19.6 µm and 2.0 µm respectively. Figure 4.1: Step-height measurement method. 4.1.2 Water contact angle results For different specimens, static contact angle measurements were carried out using VCA Optima Contact Angle System. After grinding, polishing, cleaning and drying processes, Ti6Al4V alloy surface exhibits water contact angle of 55.1±4.3◦. This measured value matches quite closely with value observed in an earlier study 50.0±3.1◦ [Ponsonnet et al. 2003]. Bare Ti6Al4V (after polishing, cleaning and drying processes), when exposed to O 2 plasma treatment, showed water contact angle of 10.4±1.1◦. This low water contact angle value indicates increased surface free energy of the surface due to plasma treatment. Increased surface energy of bare Ti6Al4V after oxygen plasma treatment affects the adhesion of UHMWPE coating on bare Ti6Al4V substrate positively. Surface energy is one of the significant factors which affect the adhesion of a coating on a 55 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat substrate [Silbernagl et al. 2009]. UHMWPE coating formed in this study exhibited a water contact angle of 135.5±3.3◦ and UHMWPE/PFPE composite layer showed a water contact angle of 128.5±2.9◦. High water contact values of Ti6Al4V/UHMWPE and Ti6Al4V/UHMWPE/PFPE coatings indicate low surface energy of the obtained coatings. Bulk UHMWPE water contact angle values as documented in literature are close to 90◦ [Chen et al. 2003]. Water contact angle is significantly influenced by the surface chemistry and surface conditions. The differences in the water contact angle of obtained UHMWPE coating in this study when compared to that of bulk UHMWPE can arise due to different surface conditions such as roughness [Torrisi et al. 2003]. Table 4.1 summarizes static water contact angle values for different specimens prepared in this study. Fig. 4.2 shows the pictures of water contact angle measurements of representative specimens from different surface modifications. Table 4.1: Measured water contact angle values for different specimens. Specimen Type Ti6Al4V Ti6Al4V/O 2 plasma treated Ti6Al4V/O 2 plasma treated/UHMWPE Ti6Al4V/O 2 plasma treated/UHMWPE/PFPE Bulk UHMWPE [Chen et al. 2003] Water contact angle (degree) 55.1±4.3 10.4±1.1 135.5±3.3 128.5±2.9 90.0 56 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Figure 4.2: Measured water contact angle values for different specimens. (a) Ti6Al4V. (b) Ti6Al4V/O 2 plasma treated. (c) Ti6Al4V/O 2 plasma treated/UHMWPE. (d) Ti6Al4V/O 2 plasma treated/UHMWPE/PFPE. 4.1.3 SEM surface morphology The surface morphology of the UHMWPE film on Ti6Al4V surface, observed under FE-SEM, has been shown in Fig. 4.3. UHMWPE film exhibits fibrous structure as observed in the surface morphology of the film. This coating shows polymer chains protruding to the surface containing valleys between them. Fig. 4.3 (a) and (b) show UHMWPE coating surface morphology at 100 and 500 magnification respectively. Images indicate the presence of uniformly distributed polymer chain density in the UHMWPE coating. Figure 4.3: Surface morphology of Ti6Al4V/UHMWPE surface using FESEM. (a) At lower magnification, 100x. (b) At higher magnification, 500x. 57 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat 4.1.4 AFM surface morphology AFM morphology imaging results are shown in Fig. 4.4. AFM imaging was done for the scan area of 40µm×40µm. This scan area is much smaller than used for SEM imaging. AFM morphology of Ti6Al4V alloy surface after grinding, polishing and cleaning is shown in Fig. 4.4 (a). AFM measurement of Ti6Al4V alloy surface shows an average roughness (Ra) of 44 nm. Fig. 4.4 (b) shows sub-micron features of the UHMWPE polymer coating formed in this study. Fig. 4.4 (b) shows the presence of islands and valleys in the formed UHMWPE coating. The roughness (rms) of the UHMWPE coating measured using scan area of 40µm×40µm is 0.795 µm. The roughness (rms) of the bulk UHMWPE measured in an earlier study was 0.332 µm [Satyanarayana et al. 2006]. Different roughness value of the formed UHMWPE coating compared to that of the bulk UHMWPE can cause differences in the observed water contact angle results of the coating from the bulk polymer even though both have same chemical structure. Figure 4.4: AFM morphology of surfaces. (a) Polished Ti6Al4V alloy surface (scan area: 40µm×40µm, vertical scale: 1µm). (b) Ti6Al4V/UHMWPE surface (scan area: 40µm×40µm, vertical scale: 5µm). 58 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat 4.2 Chemical characterizations 4.2.1 FTIR analysis results Chemical nature of the coating was analyzed using FTIR-ATR analysis spectrum. Spectrum was observed to be identical for all measurements at different locations. Fig. 4.5 shows the FTIR spectrum of UHMWPE coating on Ti6Al4V alloy substrate. FTIR spectrum indicates CH stretching modes at 2866 and 2939 cm−1, a band at 1475 cm−1 corresponding to polyethylene–methylene (CH 2 ) bending and CH 2 rocking mode at 731 cm−1. This spectrum’s characteristics are similar to the observed UHMWPE or PE polymers spectra characteristics in earlier studies [Elliott 1969; Kang and Nho 2001]. This FTIR analysis validated that present coating’s chemical nature is similar to that of bulk UHMWPE. Figure 4.5: FTIR-ATR spectrum of the UHMWPE coating on Ti6Al4V substrate. 4.2.2 XPS analysis results XPS spectra of UHMWPE coating on Ti6Al4V alloy substrate has been shown in Fig. 4.6 (a). Observed XPS spectra of UHMWPE film coating indicates 59 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat high carbon content due to the presence of polyethylene (CH 2 ) group of UHMWPE polymer. Absence of titanium peak in the observed XPS spectra confirms the fact that the Ti6Al4V substrate is completely covered by the UHMWPE polymer film and there are no pin-holes. XPS spectra of UHMWPE/PFPE coating on Ti6Al4V substrate has been shown in Fig. 4.6 (b). The presence of strong F1s peak in XPS spectra confirms the presence of PFPE overcoat on UHMWPE. Figure 4.6: XPS spectra of specimens. (a) UHMWPE coating on Ti6Al4V. (b) UHMWPE/PFPE coating on Ti6Al4V. 4.3 Tribological characterization of UHMWPE coating Bare Ti6Al4V alloy tribological properties were characterized using Si 3 N 4 as counterface in ball-on-disk tribometer and it exhibited a higher friction coefficient of 0.6~0.7 as shown in Fig. 4.7. Higher friction coefficient of bare Ti6Al4V alloy is attributed to high surface energy leading to adhesive wear and the presence of mechanically unstable titanium oxide film [Budinski 1991; Yildiz et al. 2009]. After coating of UHMWPE film onto the Ti6Al4V alloy surface, the friction coefficient was reduced significantly from ~0.6 to ~0.15. Bulk UHMWPE polymer has excellent lubrication properties and its tribological properties have 60 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat been widely investigated and documented in earlier research studies [Wang et al. 1995]. In the initial study, tribological tests were carried out for the normal loads of 0.5, 1.0 and 2.0 N. Five samples were characterized for each testing conditions. Tests were carried out for the duration of 96,000 cycles (8 hours) at 200 rpm. Sliding test beyond 96,000 cycles was not carried out due to long duration of testing. The UHMWPE film did not fail in any of these tests for the tested duration of 96,000 cycles. Table 4.2 shows the summary of the tribological tests carried out on Ti6Al4V/UHMWPE film. Table 4.2: Summary of tribological tests on Ti6Al4V/UHMWPE specimens. Test Condition No. 1 2 3 Test Parameters (Track Diameter, Normal Load, Spindle Speed) 6 mm, 0.5 N, 200 RPM 4 mm, 1.0 N, 200 RPM 4 mm, 2.0 N, 400 RPM Maximum Friction Coefficient during Test Duration Tested cycles 0.15 0.15 0.11 96,000 (not failed) 96,000 (not failed) 96,000 (not failed) Fig. 4.7 shows the variation of friction coefficient vs. number of sliding cycles for the test condition No. 1 in Table 4.2. The test specimens exceeded 96,000 cycles without failure. For comparison, bare Ti6Al4V substrate tribological properties were also evaluated for the same testing parameters using Si 3 N 4 as counterface in the ballon-disk tribometer. Bare Ti6Al4V substrate showed a higher coefficient of friction of 0.6~0.7 as shown in Fig. 4.7. After coating of UHMWPE polymer film onto the Ti6Al4V alloy surface, the coefficient of friction was reduced to ~0.15 as shown in Table 4.2 and Fig. 4.7. 61 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Figure 4.7: Variation of friction coefficient as a function of the sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si 3 N 4 ball as the counterface (track radius: 3 mm, normal load: 0.5 N, spindle speed: 200 rpm). Afterwards, four samples were tested at a high normal load of 4 N (track radius = 2 mm, spindle speed = 400 rpm) till film failure was observed. Fig. 4.8 shows the variation of friction coefficient vs. number of cycles of revolutions for a typical run at this high normal load test condition. Test result shows wear durability of 187,000±8,000 cycles and a maximum coefficient of friction of 0.10 before failure of the film. Figure 4.8: Variation of friction coefficient vs. sliding cycles (for bare Ti6Al4V and Ti6Al4V/UHMWPE) using Si 3 N 4 ball as the counterface (track radius: 2 mm, normal load: 4 N, spindle speed: 400 rpm). 62 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat 4.4 Investigation of underlying wear mechanism To investigate the underlying wear mechanism in the above tribological studies, wear track and counterface conditions were investigated for one of the high load test conditions (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) where test was stopped after 175,000 sliding cycles to investigate counterface and wear track conditions even though the film had not failed. FESEM image of the wear track of Ti6Al4V alloy (Fig. 4.9 (a)) showed that the specimen surface, without UHMWPE coating, is extensively damaged only after 1,000 sliding cycles and there is significant accumulation of wear debris around the wear track. Compared to this, wear track morphology of Ti6Al4V/UHMWPE is smooth even after 175,000 sliding cycles as seen in Fig. 4.9 (b). AFM morphology (Fig. 4.9 (c)) of the surface inside the wear track showed an average roughness (rms) of 35 nm which is very smooth compared to the initial roughness (0.795 µm) of the UHMWPE polymer coating. Protrusions and valleys of the polymer coating are flattened (ironed) in the initial running-in period of the sliding cycles of the tribology test. 63 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Figure 4.9: Wear track morphology. (a) FESEM morphology of wear track for bare Ti6Al4V alloy for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after 1,000 cycles, magnification 60x. (b) FESEM morphology of wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 sliding cycles, magnification 80x. (c) AFM surface morphology inside the wear track for Ti6Al4V/UHMWPE specimen for high load tribology test after the completion of 175,000 cycles (scan area: 40µm×40µm, vertical scale: 500 nm). EDX analysis at different locations inside the wear track confirms the presence of polymer film proving that the coating was not removed even after 175,000 cycles of sliding (Fig. 4.10 (a)). Six locations were investigated in EDX analysis and in all cases EDX analysis indicated high percentage of carbon (> 97%) and no Ti presence (only trace Ti peak was visible because of the detection of the substrate). Compared to this, EDX analysis of bare Ti6Al4V alloy showed high percentage of Ti (as seen in Fig. 4.10 (b)). Au peaks have also been observed in the EDX analysis because of the gold coating deposited on the specimens before FESEM/EDX analysis to make coating conductive as required in EDX analysis. The optical image of Si 3 N 4 ball (Fig. 4.10 (c)) at the end of tested 175,000 sliding cycles on the UHMWPE film showed that there was only a little polymer transfer to the counterface. After cleaning the ball with acetone, no permanent physical damage was observed on the Si 3 N 4 ball surface (Fig. 4.10 (d)), indicating that the UHMWPE coating was able to protect the counterface as well during tribological tests. 64 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Figure 4.10: Wear track and counterface analysis. (a) EDX analysis of wear track for high normal load tribology test (track radius = 2 mm, normal load = 4 N, spindle speed = 400 rpm) after completion of 175,000 cycles. (b) EDX analysis of Ti6Al4V surface without polymer coating. (c) Optical image of Si 3 N 4 ball for high normal load tribology test after completion of 175,000 cycles, 100x. (d) Optical image of Si 3 N 4 ball after cleaning with acetone for high normal load sliding tribology test after completion of 175,000 sliding cycles, 100x. Since polymer coating presence is observed on the substrate and the counterface surfaces which suggest that after certain number of sliding cycles, contact is between polymer/polymer which can protect the substrate and the counterface surfaces. The phenomenon of sliding between the polymer coating on the substrate and the transferred polymer film on the counterface can result in low friction coefficient and high wear resistance as observed in similar studies if the polymer transfer layer formed on counterface is thin and material transfer is not extensive [Briscoe and Sinha 2002]. 4.5 Effect of PFPE overcoat on UHMWPE coating Tribological tests were carried out at high rotational speed (1000 rpm) for UHMWPE/PFPE coating to evaluate the effect of PFPE coating on UHMWPE film wear durability. For the comparison, UHMWPE coating was also tested for 65 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat similar conditions (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm). Five samples were tested for each surface coating to obtain the wear life. UHMWPE on Ti6Al4V showed a wear life of 28,000±8,000 cycles at these sliding conditions. After PFPE coating, wear life increased to 60,000±14,000 cycles. Thus PFPE top layer improved the wear life of UHMWPE film as evaluated at the high rotational speed testing condition. This comparison has also been plotted in a bar chart shown in Fig. 4.11. Figure 4.11: Effect of PFPE overcoat on wear life (track radius = 2 mm, normal load = 4 N, spindle speed = 1000 rpm). 4.6 Explanation of wear resistance increase by PFPE overcoat The observed improvement in wear resistance by PFPE overcoat is similar to the work done in earlier research studies [Satyanarayana et al. 2006; Abdul Samad et al. 2010]. PFPE exhibits lower surface energy [Makkonen 2004], excellent lubrication properties and thermal stability [Liu and Bhushan 2003]. These properties are beneficial for severe tribological condition for the tested condition of 1000 rpm in this study. 66 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Since UHMWPE polymer surface does not have any reactive chemical groups to react with PFPE molecules, possibility of any chemical interaction between UHMWPE and PFPE can be eliminated. Therefore as hypothesized in earlier studies, PFPE molecules can be entrapped during coating into rough features of UHMEPE film such as valleys and can result into good lubrication properties during sliding testing due to availability of lubricant [Satyanarayana et al. 2006; Abdul Samad et al. 2010]. PFPE spreads well on polymer surfaces due to low surface tension. 4.7 Biocompatibility assessments Different biocompatibility tests have been suggested by ISO 10993 guidelines for biomedical devices based upon mode of contact, application area and duration of contact in different applications. However cytotoxicity test is common test applicable to all implant applications and used as a preliminary test to evaluate the feasibility of using any coating in biomedical implant applications. This test assesses the potential cytotoxic effects of leachable extracted from test specimens’ surfaces. 4.7.1 Cytotoxicity test results In this study, UHMWPE and UHMWPE/PFPE coatings were tested for cytotoxicity on Ti6Al4V alloy substrate using ISO elution method-1×MEM (minimum essential medium) extract method. In cytotoxicity testing carried out in this study, no cytotoxicity effects such as cell lysis were seen in any of the test 67 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat wells during microscopic examination in the ISO elution method-1×MEM extract. No change in the pH value was noticed after duration of 48 hours. Grade should be less than the grade 2 (mild reactivity) for the tested coatings to meet the requirements of the ISO elution method-1×MEM extract. Based upon test analysis results, it was inferred that coatings exhibited cytotoxicity level of grade 0 (reactivity: none) according to test guidelines and thus meet the requirements of the ISO elution method-1×MEM extract. 4.8 Potential applications of coatings As observed in this study, the UHMWPE and UHMWPE/PFPE thin film coatings exhibit hydrophobicity, high wear durability and noncytotoxicity. Hydrophobic coatings surfaces are water repellent and do not need a liquid lubrication medium to reduce friction. Due to the low friction coefficient and selfcleaning nature of surfaces, hydrophobic coatings can find many applications in biomedical area. For example, in coronary guide-wire application, hydrophobic coatings improve tactile feedback thus making it easier to grip by cardiologist during operation. In endoscopic applications, hydrophobic coatings prevent the sticking of organic material on the surface of the device. Low surface energy of hydrophobic coatings has been observed to be beneficial in bio-film inhibition for some in-vivo applications [Roosjen et al. 2006]. In a metallic stent, a hydrophobic and noncytotoxic coating on stent surface can assist in improving the compatibility of metal with body fluids by inhibiting release of potential cytotoxic leachables from metal surface into body environment [Pendyala et al. 2009]. 68 Chapter 4: Tribological characterizations of thin UHMWPE film and PFPE overcoat Similarly, wear resistant property of a biocompatible coating is advantageous in orthopedics and bio-devices applications. In considerations of the abovementioned potential applications, UHMWPE and UHMWPE/PFPE thin film coatings studies can find usage in biomedical devices. 69 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer CHAPTER 5 Tribological Evaluations of Molecularly Thin GPTMS SAMs Coating with PFPE Top Layer As investigated in the Chapter 4, a thin film of UHMWPE coating exhibited low friction coefficient and high wear resistance. Use of PFPE as the top layer further improved the wear resistance of the obtained thin UHMWPE coating. Due to the combination of hydrophobicity, noncytotoxicity and excellent tribological properties, potential applications of these coatings in biomedical instruments were suggested. In spite of having suitable properties for the biomedical applications, higher thickness of UHMWPE (~19.6 µm) may prevent its usage in biomedical MEMS applications. This chapter investigates the feasibility of using molecularly thin GPTMS/PFPE coating (~4 nm) to address the tribological limitations of titanium and its alloys in MEMS and in particular bio-MEMS applications. In this study, GPTMS SAMs with PFPE overcoat has been deposited onto Ti6Al4V alloy substrate. For comparison studies, PFPE has also been coated onto Ti6Al4V alloy substrate. Obtained coatings have been characterized by contact angle measurement, AFM imaging, XPS analysis and tribological tests. Wear track and counterface surfaces have been examined to understand the underlying friction and wear mechanism. 70 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer 5.1 Physical characteristics of the coatings 5.1.1 Water contact angle results After polishing, cleaning and drying processes, bare Ti6Al4V alloy showed a water contact angle value of 73±4◦. This measured value is close to the value of 72◦ observed in an earlier study [Wang et al. 1997]. After 10 min of oxygen plasma treatment of Ti6Al4V alloy, water contact angle was reduced to 6±1◦. This is due to the hydrophilic nature of Ti6Al4V alloy surface after oxygen plasma exposure indicating increased surface energy. Surface energy is one of the factors which significantly influence the adhesion of the coating onto the substrate [Silbernagl et al. 2009]. PFPE coating on Ti6Al4V alloy exhibited a water contact value of 52±3◦. PFPE coating on Ti6Al4V alloy after heat treatment showed increased water contact angle value of 110±2◦. GPTMS coating on Ti6Al4V alloy exhibited a water contact angle value of 60±2◦ which is in a good agreement with the value of 62◦ reported in an earlier study [Elender et al. 1996]. PFPE overcoat onto GPTMS coated Ti6Al4V alloy showed a water contact angle of 90±5◦. After heat treatment, water contact angle value of Ti6Al4V/GPTMS/PFPE was increased to 112±6◦. Table 5.1 summarizes the water contact angle values (average and standard deviation) for different specimens types prepared in this study. Fig. 5.1 and 5.2 show the water contact angle measurement pictures from the representative specimens prepared in this study. 71 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer Table 5.1: Measured water contact angle values for different specimens. Specimen Type Ti6Al4V Ti6Al4V (after O 2 plasma treatment) Ti6Al4V/PFPE Ti6Al4V/PFPE (heat treated) Ti6Al4V/GPTMS Ti6Al4V/GPTMS/PFPE Ti6Al4V/GPTMS/PFPE (heat treated) Water Contact Angle (°) 73±4 6±1 52±3 110±2 60±2 90±5 112±6 Figure 5.1: Water contact angle measurement from representative samples. (a) Ti6Al4V. (b) Ti6Al4V (after O 2 plasma treatment). (c) Ti6Al4V/PFPE. (d) Ti6Al4V/PFPE (heat treated). Figure 5.2: Water contact angle measurement from representative samples. (a) Ti6Al4V/GPTMS. (b) Ti6Al4V/GPTMS/PFPE. (c) Ti6Al4V/GPTMS/PFPE (heat treated). Thus, GPTMS/PFPE coating is hydrophobic in nature indicating low surface energy of the obtained coating. Observed increase in the water contact 72 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer angle value after heat treatment of PFPE coatings is due to the reduction in the available surface hydroxyl groups [Tao and Bhushan 2005]. Low surface energy is one of the desirable properties for MEMS component coatings since high surface energy leads to stiction resulting in the failure of components. Low surface energy of the coatings suggests their potential applications in preventing stiction arising due to the surface and capillary forces [Mastrangelo 1997]. 5.1.2 AFM morphology results Bare Ti6Al4V (after polishing, grinding and cleaning) showed the surface roughness (rms) of 11.8 nm as measured from AFM imaging using the scan area of 5µm×5µm. Scratches created in the polishing process can be observed in AFM imaging (Fig. 5.3 (a)) results. Bare Ti6Al4V with PFPE overcoat exhibited the surface roughness (rms) of 1.7 nm. PFPE coating on Ti6Al4V, after heat treatment, showed the surface roughness (rms) of 1.2 nm. GPTMS coating deposited on Ti6Al4V showed the surface roughness (rms) of 6.8 nm. PFPE overcoat onto GPTMS coating exhibited the surface roughness (rms) of 2.5 nm. After heat treatment, PFPE coating on GPTMS showed the surface roughness (rms) of 5.0 nm. Table 5.2 summarizes the measured surface roughness (RMS and Ra) values of different specimens using 5µm×5µm scan area. Fig 5.3 shows the AFM imaging results from different representative specimen types prepared in this study. 73 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer Table 5.2: Measured surface roughness for different Specimens in AFM imaging. Specimen Type Ti6Al4V Ti6Al4V/PFPE Ti6Al4V/PFPE (heat treated) Ti6Al4V/GPTMS Ti6Al4V/GPTMS/PFPE Ti6Al4V/GPTMS/PFPE (heat treated) RMS (nm) 11.8 1.7 1.2 6.8 2.5 5.0 Ra (nm) 9.6 1.3 0.7 4.9 2.1 3.4 Figure 5.3: AFM imaging (scan area: 5µm×5µm, vertical scale: 100 nm). (a) Ti6Al4V. (b) Ti6Al4V/PFPE. (c) Ti6Al4V/PFPE (heat treated). (d) Ti6Al4V/GPTMS. (e) Ti6Al4V/GPTMS/PFPE. (f) Ti6Al4V/GPTMS/PFPE (heat treated). Thickness of GPTMS coating obtained in an earlier study using similar deposition method has been reported to be the order of (~1 nm) [Luzinov et al. 2000]. Thickness of PFPE coating deposited using similar procedure followed in this study has been reported to be the order of (2~3 nm) in an earlier research study [Eapen et al. 2002]. Since surface roughness of the used Ti6Al4V alloy substrate is quite high (11.8 nm) in comparison with the deposited GPTMS SAMs 74 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer and PFPE coatings in this study, it is difficult to infer data such as thickness and topography of deposited coatings from AFM imaging results. As observed in the AFM imaging results (Table 5.2), GPTMS coating has reduced the surface roughness of the substrate surface. Observed significant change in the surface roughness also suggests the possibility of forming GPTMS as multi-layer rather than monolayer probably due to high humidity ambient conditions [Luzinov et al. 2000]. PFPE coatings onto different specimens also resulted into the reduction of surface roughness value significantly which can be attributed to the possibility of linear and flexible PFPE nano-particles filling up the surface textures features such as valleys. 5.2 Chemical characteristics of UHMWPE coating 5.2.1 XPS analysis Wide scan XPS spectra of Ti6Al4V/GPTMS specimen showed strong C1s, O1s and Ti2p peaks as shown in Fig. 5.4. Due to nanometer thickness (< 1 nm) of GPTMS SAMs coating, substrate is also detected in XPS spectra. Fig. 5.5 shows the comparison of C1s peak of Ti6Al4V/GPTMS and Ti6Al4V specimens. High resolution XPS spectrum of C1s scan for Ti6Al4V/GPTMS specimen showed two strong peaks. Peak at 284.6 eV represents (C–C) bonds and 286.4 eV corresponds to the (C–O) bonds in the epoxy SAM molecules. These two strong peaks are indicative of epoxy SAMs presence as seen in earlier studies [Wong and Krull 2005; Cloarec et al. 2002]. Compared to this, Ti6Al4V showed only one C1s peak. Observed change in the XPS core-level spectra of C1s for 75 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer Ti6Al4V/GPTMS specimen in comparison with the bare Ti6Al4V specimen indicates the deposition of GPTMS SAMs coating. Figure 5.4: Wide scan XPS spectra of Ti6Al4V/GPTMS specimens. Figure 5.5: Comparison of C1s peaks for bare Ti6Al4V/GPTMS and Ti6Al4V in C1s scan. 5.3 Tribological characterizations Bare Ti6Al4V alloy and surface modified Ti6Al4V alloy specimens’ tribological properties were evaluated using Si 3 N 4 as counterface in a ball-on- 76 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer disk tribometer. Table 5.3 tabulates initial friction coefficient (evaluated at the end of 600 sliding cycles) values of specimens investigated in this study. Fig. 5.6 summarizes the wear life (average and standard deviation) of prepared specimens in this study. Bare Ti6Al4V alloy and GPTMS deposited Ti6Al4V specimens showed wear life of less than 100 sliding cycles for tested conditions (not shown in Fig. 5.6). Wear durability data of samples from each specimen type were collected from minimum six different samples using three different tracks on each sample. Table 5.3: Coefficient of friction for specimens tested in the study. Specimen Type Ti6AL4V Ti6AL4V/PFPE Ti6AL4V/PFPE (heat Treated) Ti6AL4V/GPTMS Ti6AL4V/GPTMS/PFPE Ti6AL4V/GPTMS/PFPE(heat Treated) Initial Coefficient of Friction 0.5~0.6 0.12~0.13 0.11~0.12 0.5~0.6 0.11~0.12 0.12~0.13 Figure 5.6: Wear durability (number of sliding cycles before failure) of tested specimens in the study. Fig. 5.7 shows the variation in the coefficient of friction with sliding cycles for representative PFPE coated Ti6Al4V alloy specimens. For comparison, 77 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer result of the tribological characterization of bare Ti6Al4V alloy has also been tabulated for the same testing conditions. Bare Ti6Al4V alloy showed high coefficient of friction (0.5~0.6) as shown in Fig. 5.7 and Table 5.3. High surface energy and mechanical instability of oxide layer result into poor tribological properties of bare Ti6Al4V alloy [Budinski 1991; Yildiz et al. 2009]. PFPE modified Ti6Al4V alloy (with heat treatment as well as without heat treatment) specimens have showed lower coefficient of friction although sample with heat treatment exhibited lower wear durability (see Fig. 5.6 and Fig 5.7). Heat treatment increases the bonded portion of PFPE on the substrate. Reduction in the wear durability can be attributed to the reduction in the mobile portion of PFPE overcoat after heat treatment. Reduction in the wear durability after heat treatment of PFPE coating has been observed in an earlier study onto a different substrate [Miyake et al. 2006]. Figure 5.7: Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/PFPE, (b) Ti6AL4V/PFPE (heat treated) and (c) bare Ti6Al4V) using Si 3 N 4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm). 78 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer Fig. 5.8 shows the variation in the coefficient of friction with sliding cycles from representative PFPE coated Ti6Al4V/GPTMS specimens. Similar evaluation has also been obtained for the tribological test of Ti6Al4V/GPTMS specimen. Epoxy SAMs (GPTMS) modified Ti6Al4V alloy exhibited high coefficient of friction (0.5~0.6) at the tested condition of 20 gm normal load and 200 rpm (see Table 5.3 and Fig. 5.8). Observed tribological properties of epoxy SAMs modified substrate are similar to the observation in an earlier study [Sidorenko et al. 2002]. PFPE overcoat onto epoxy-SAMs modified Ti6Al4V alloy (with and without heat treatment) reduced coefficient of friction and increased the wear durability (see Fig. 5.6 and 5.8). Epoxy-SAMs modified Ti6Al4V alloy specimens with PFPE overcoat showed high wear durability (90,700±13,900 sliding cycles) although heat treatment has resulted into some reduction in the wear durability (74,700±24,000 sliding cycles). Figure 5.8: Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/GPTMS/PFPE, (b) Ti6AL4V/GPTMS/PFPE (heat treated) and (c) Ti6Al4V/GPTMS) using Si 3 N 4 ball as the counterface (track radius: 2 mm, normal load: 0.2 N, spindle speed: 200 rpm). 79 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer PFPE overcoat (with and without heat treatment) on bare Ti6Al4V alloy and GPTMS SAMs modified alloy has reduced the coefficient of friction to 0.11~0.13 for different specimens investigated in this study as observed in Table 5.3. 5.4 Optical microscopy of wear track and counterface surface Fig. 5.9 shows the optical micrographs of the wear track and counterface surface after completion of 5,000 cycles of the sliding test for bare Ti6Al4V specimen. There was extensive wear debris accumulation around the track (Fig 5.9 (a)) and counterface surface of Si 3 N 4 ball showed severe damage (Fig 5.9 (b)). Similar wear track and counterface conditions were observed for GPTMS deposited Ti6Al4V specimens after completion of 5,000 sliding cycles (as seen in Fig. 5.10). Fig. 5.10 (a) and Fig. 5.10 (b) show the accumulation of wear particles near wear track and damaged counterface surface respectively. Figure 5.9: Optical micrographs of Ti6Al4V specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. 80 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer Figure 5.10: Optical micrographs of Ti6Al4V/GPTMS specimen’s wear track and counterface surface after completion of 5,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Fig. 5.11 shows the optical micrograph images of the wear track and counterface surface after completion of 10,000 sliding cycles for Ti6Al4V/PFPE specimen. As observed in Fig. 5.11 (a), wear track shows mild impression of counterface with absence of debris. There is a little material transfer to counterface surface. After cleaning with acetone, counterface surface is smooth with no sign of any permanent damage. Compared to this, wear track surface after completion of 10,000 sliding cycles using Ti6Al4V/PFPE (heat treated) specimen shows the accumulation of debris and severe damage to counterface surface attributed to the reduction in wear resistance after heat treatment (see Fig. 5.12). Figure 5.11: Optical micrographs of Ti6Al4V/PFPE specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. (c) Counterface after cleaning, magnification 100x. 81 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer Figure 5.12: Optical micrographs of Ti6Al4V/PFPE (heat treated) specimen’s wear track and counterface surface after completion of 10,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. Fig. 5.13 shows the wear track and counterface surface after completion of 100,000 sliding cycles using the coating of GPTMS/PFPE onto Ti6Al4V alloy in one of the sliding test where coating had not failed. As seen in Fig 5.13 (a), no wear debris was observed around wear track although mild scratch had formed by the impression of the counterface. There was a little material transfer to the counterface of Si 3 N 4 ball (see Fig 5.13 (b)). After cleaning with acetone, counterface showed smooth surface with no signs of damage as observed in Fig 5.13 (c). Figure 5.13: Optical micrographs of Ti6Al4V/GPTMS/PFPE specimen’s wear track and counterface surface after 100,000 sliding cycles. (a) Wear track, magnification 50x. (b) Counterface, magnification 100x. (b) Counterface after cleaning, magnification 100x. 82 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer As seen in different optical micrograph images, before failure of the film, mild scratching on the wear track without the presence of wear debris and smooth counterface surface accompanied by little material transfer were observed. After failure of the film, wear debris accumulation along the wear track and worn counterface surface were noticed. Different optical microscope images support the wear durability data shown in Fig. 5.6. PFPE coating on bare Ti6Al4V alloy as well as GPTMS SAMs deposited Ti6Al4V alloy has resulted into lowering of friction coefficient and the increase in wear durability. This can be attributed to lower surface energy [Makkonen 2004] as well as flexible and mobile nature of PFPE molecules [Mate 1992] resulting into low resistance in the shearing at the sliding interface. PFPE overcoat on bare Ti6Al4V alloy as well as GPTMS coated Ti6Al4V may consist of three parts as speculated in earlier studies [Satyanarayana and Sinha 2005; Satyanaryana et al. 2007]. PFPE top layer consists of molecules trapped in surface texture, strongly adsorbed portion and the mobile portion. Strongly adsorbed portion results due to strong physical adsorption and hydrogen bonding interaction as well as covalent bonding formed due to the heat treatment. During the initial sliding cycles, mobile as well as bonded portion of PFPE will lubricate the contact region. After squeezing of the mobile portion of PFPE, lubrication will be supported by the bonded portion of PFPE molecules. After complete removal of the mobile and bonded portion of PFPE, high friction and wear will be followed leading to the failure of the coating. 83 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer High wear resistance of Ti6Al4V/GPTMS/PFPE specimens compared with Ti6Al4V/PFPE can be explained due to the possible interaction of PFPE molecules with GPTMS intermediate layer. Chemical interaction of PFPE with epoxy group has been reported in previous research studies [Ellis 1993; Elender et al. 1996]. It has been observed in some studies that dual-lubricant layer combination consisting of initially bonded layer having strong-polar top group with top mobile layer having weaker polar group shows improved tribological properties compared with the use of only any one of the monolayers [Katano et al. 2003; Satyanaryana et al. 2007]. 5.5 Biocompatibility assessment Biocompatibility of GPTMS has been evaluated in earlier research studies. In one study, noncytotoxicity of GPTMS was validated by the assessment of cytocompatibility of hybrids of gelatin–GPTMS and chitosan–GPTMS in an invitro study [Ren et al. 2002]. In another study, biocompatibility of GPTMS SAM was confirmed using both bacterial culture (E.coli DH5α, Gram negative bacteria) and plant tissue culture (wheat seed) [Kumar et al. 2011]. Biocompatibility of PFPE polymer has been examined by researchers and long-term biocompatibility of PFPE has been validated in different studies for biomedical applications [Xie et al. 2006; Sweeney et al. 2008]. In this study, H-Gladen ZV60 was used as the solvent for PFPE. To confirm that H-Galden has not affected the biocompatibility of PFPE, PFPE top 84 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer layer coated onto UHMWPE film was tested for cytotoxicity using ISO elution method-1×MEM extract method. 5.5.1 Cytotoxicity test results In cytotoxicity testing carried out on PFPE over-coated UHMWPE film, no cytotoxicity effects such as cell lysis were seen in any of the test wells during the microscopic examinations of the ISO elution method- 1×MEM extract. No change in the pH value was noticed after the duration of 48 hours. Grade of the test results should be less than the grade 2 (mild reactivity) to meet the requirements of the ISO elution method-1×MEM extract. Based upon test analysis results, it was inferred that coatings exhibited cytotoxicity level of grade 0 (reactivity: none) according to the test guidelines, and thus PFPE top layer coating meets the requirements of the ISO elution method-1×MEM extract. In view of the mentioned previous research studies and carried out cytotoxicity test in this study, noncytotoxic nature of GPTMS and PFPE coatings are indicated. It suggests the noncytotoxic nature of composite coating of GPTMS/PFPE investigated in this study. 5.6 Potential applications of GPTMS/PFPE coating GPTMS/PFPE coating exhibits hydrophobicity, noncytotoxicity and excellent tribological properties (low friction and high wear durability). Hydrophobic coatings exhibit low surface energy. Lower friction and waterrepellent nature of the hydrophobic coatings are useful in different biomedical 85 CHAPTER 5 Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer applications. Hydrophobic coatings on coronary guidewire assist in improving tactile feedback to cardiologist. Low surface energy of biomedical coatings has been found to be useful in bio-film inhibition for few in-vivo applications [Roosjen et al. 2006]. Coatings, having low surface energy and noncytotoxic nature, on metallic stent are beneficial in improving the biocompatibility of metal surface with body fluids by inhibiting the release of cytotoxic extracts from the metal surface into the body environment [Pendyala et al. 2009]. Thus, the obtained composite coating of GPTMS/PFPE can find different applications in biomedical devices. Due to the molecularly low thickness of this coating (< 4 nm), it can be particularly useful in biomedical MEMS applications where thickness of the surface coating is a crucial consideration for its usage. 86 Chapter 6: Conclusions Chapter 6 Conclusions In the first study of this thesis titled: “Tribological characterizations of thin UHMWPE film and PFPE overcoat”, a thin film of UHMWPE (thickness of 19.6±2.0 µm) was coated onto Ti6Al4V alloy substrate. Prior to polymer coating, oxygen plasma pre-treatment of the substrate surface was carried out for cleaning as well as better adhesion of the polymer coating to the substrate. Physical characterizations (contact angle measurement, thickness measurement, SEM morphology and AFM imaging), biocompatibility test (cytotoxicity assessment) and chemical characterizations (FTIR-ATR and XPS) were also carried out for the UHMWPE polymer thin coating. Tribological characterization of the coating was carried out at different load conditions and rotational speeds using a ball-ondisk tribometer against a counterface of 4 mm diameter Si 3 N 4 ball. From the observations and results of this research study, following important conclusions can be drawn:  Obtained UHMWPE polymer coating has similar physical and chemical structure as that of bulk UHMWPE polymer as validated by different physical and chemical characterization of the coating.  Oxygen plasma treatment is an effective surface treatment method for cleaning and good adhesion of polymer coating on Ti6Al4V alloy substrate as indicated by increased surface energy of the substrate after plasma treatment. 87 Chapter 6: Conclusions  Formed UHMWPE polymer coating on Ti6Al4V alloy substrate exhibits excellent tribological properties such as low coefficient of friction (0.15) and high wear durability (> 96,000 cycles) under tested conditions. Good mechanical and tribological properties of bulk UHMWPE polymer also results into useful tribological properties of polymer coating.  UHMWPE and UHMWPE/PFPE coatings are non-cytotoxic in nature as per the test requirements of the ISO elution method-1×MEM extract.  PFPE overcoat on UHMWPE polymer coating enhances wear resistance for the tested high speed loading condition of 1000 rpm and the normal load of 4 N by providing additional thermal stability and lubrication properties.  Due to the combination of low surface energy, wear resistance and noncytotoxicity of the coating, the thin film of UHMWPE (with and without PFPE overcoat) can find usage in various applications of biomedical devices. In the second study of this thesis titled: “Tribological evaluations of molecularly thin GPTMS SAMs coating with PFPE top layer”, the potential of molecularly thin GPTMS/PFPE coating (~4 nm) to address the tribological limitations of titanium and titanium alloy has been evaluated. Physical characterizations (contact angle measurement, AFM imaging), chemical characterization (XPS) and tribological tests were carried out for obtained coatings specimens. 88 Chapter 6: Conclusions From the results and observations of this study, following important conclusions can be drawn:  PFPE overcoat onto Ti6Al4V/GPTMS surface exhibits low surface energy as indicated by high water contact angle of coatings.  PFPE overcoat on bare ti6Al4V alloy after oxygen plasma treatment exhibits improved tribological properties such as low coefficient of friction (0.11 ~ 0.13) and high wear durability compared to bare Ti6Al4V alloy.  PFPE overcoat on GPTMS deposited Ti6Al4V alloy exhibits low friction coefficient (0.11 ~ 0.13) and high wear resistance. Wear resistance of PFPE overcoat on GPTMS deposited Ti6Al4V alloy is significantly higher (90,700±13,900 sliding cycles) than PFPE overcoat without GPTMS deposition (26,700±4,900) which can be attributed to increased bonding of PFPE with substrate due to GPTMS intermediate layer.  PFPE overcoat also reduces surface roughness of the substrate due to filling up of surface texture by nano-particles of PFPE.  Low coefficient of friction, high wear resistance, hydrophobicity and biocompatible nature of the coatings are suitable for their proposed applications in biomedical instruments. Due to molecular layer thickness of the coatings (~4 nm), these coatings are particularly suitable for biomedical MEMS applications. 89 Chapter 7: Future Recommendations CHAPTER 7 Future Recommendations Below are recommendations for future studies based upon the results and understanding generated in this research work:  In this thesis, cytotoxicity tests for the UHMWPE and UHMWPE/PFPE coatings were carried out. Cytotoxicity test is the preliminary test to assess the feasibility of coatings for biomedical applications. To further assess the suitability of obtained coatings in biomedical devices, additional biocompatibility tests such as hemocompatibility, sensitization, irritation studies, systemic toxicity, genotoxicity etc are recommended as per the guidelines of ISO 10993 “Biological Evaluation of Medical Devices” to address specific application requirements.  Besides in-vitro biocompatibility tests, in-vivo animal testing and clinical testing are also recommended for future studies to assess the potential usage of these coatings in biomedical applications.  Optimization studies of process parameters to obtain lower thickness UHMWPE coating without deteriorating tribological properties are recommended to broaden application areas.  Obtained composite coating of GPTMS/PFPE can be tested for different biocompatibility tests such as hemocompatibility, sensitization, irritation, systemic toxicity, genotoxicity etc as per the guidelines of ISO 10993 90 Chapter 7: Future Recommendations “Biological Evaluation of Medical Devices” to satisfy specific application requirements.  In-vivo animal testing and clinical testing are also recommended in future studies for the obtained GPTMS/PFPE coating.  Optimization of PFPE (wt %) solution, dip-coating immersion time, dipping and withdrawal speeds on Ti6Al4V alloy substrate can be carried out to obtain PFPE coatings with improved tribological properties.  Heat treatment temperature as well as time duration optimization studies can be carried out to investigate the potential to obtain coatings with improved wear resistance at higher loads. 91 References References Abdul Samad, M., Satyanarayana, N., Sinha, S.K., 2010. Tribology of UHMWPE film on air-plasma treated tool steel and effect of PFPE overcoat. Surface & Coatings Technology 204, 1330-1338. Advincula, R.C., Brittain, W.J., Caster, K.C., Ruhe, J., 2004. Polymer Brushes: Synthesis, Characterization and Applications. Wiley-VCH Verlag GmbH & Co. Aimi, M.F., Rao, M.P., Macdonald, N.C., Zuruzi, A.S., Bothman, D.P., 2004. High-aspect-ratio bulk micromachining of titanium. Nature Materials 3, 103-105. Ando, E., Goto, Y., Morimoto, K., Ariga, K., Okahata, Y., 1989. Frictional properties of monomolecular layers of silane compounds. Thin Solid Films 180, 287-291. Aronsson, B.O., Lausmaa, J., Kasemo, B., 1997. Glow discharge plasma treatment for surface cleaning and modification of metallic biomaterials. Journal of Biomedical Materials Research 35, 49–73. Azzam, W., Wehner, B.I., Fischer, R.A., Terfort, A., Woll, C., 2002. Bonding and Orientation in Self-Assembled Monolayers of Oligophenyldithiols on Au Substrates. Langmuir 18, 7766-7769. Barriet, D., Lee, T.R., 2003. Fluorinated Self-Assembled Monolayers: Composition, Structure and Interfacial Properties. Current opinion in colloid & interface science 8, 236-242. Bhushan, B., Gupta, B.K., 1991. Handbook of tribology: materials, coatings, and surface treatments. New York: McGraw-Hill. 92 References Bhushan, B., Israelachvili, J.N., Landman, U., 1995 (a). Nanotribology: friction, wear and lubrication at the atomic scale. Nature 374, 607-616. Bhushan, B., Kulkarni, A.V., Koinkar, V.N., Boehm, M., Odoni, L., Martelet, C., Belin, M., 1995 (b). Microtribological characterization of self-assembled and Langmuir-Blodgett monolayers by atomic and friction force microscopy. Langmuir 11, 3189-3198. Bhushan, B., Liu, H., 2001. Nanotribological properties and mechanisms of alkylthiol and biphenyl thiol self-assembled monolayers studied by atomic force microscopy. Physical Review B 63, 2454121-24541211. Bhushan, B., 2003. Adhesion and stiction: mechanisms, measurement techniques and methods for reduction. Journal of Vacuum Science & Technology B 21, 2262–2296. Bhushan, B., 2004. Springer handbook of nanotechnology. Springer, Berlin Heidelberg New York. Bhushan, B., 2005. Nanotribology and nanomechanics: an introduction. Springer, Berlin Heidelberg New York. Bloyce, A., 1998. Surface engineering of titanium alloys for wear protection. Proceedings of the Institution of Mechanical Engineers 212 Part J, 467-476. Bowden, F.P., Tabor, D., 1986. The friction and lubrication of solids (Clarendon Press, Oxford). Briscoe, B.J., Scruton, B., Willis, R.F., 1973. The shear strength of thin lubricant films. Proceedings of the Royal Society of London, Series A 333, 99114. 93 References Briscoe, B.J., Tabor., D., 1975. The effect of pressure on the frictional properties of polymers. Wear 34, 29-38. Briscoe, B.J., Sinha, S.K., 2002. Wear of polymers. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 216, 401-413. Buchholtz, B.W., Kustas, F.M., 1996. Effects of surface pretreatment of Ti-6Al4V on the adhesion and wear lifetimes of sputtered MoS2 coatings. Tribology Transactions 39(2), 330-337. Budinski, K.G., 1991. Tribological properties of titanium alloys. Wear 151, 203217. Cha, K-H., Kim, D-E., 2001. Investigation of the tribological behavior of octadecyltrichlorosilane deposited on silicon. Wear 251, 1169-1176. Chen, J.S., Sun, Z., Guo, P.S., Zhang, Z.B., Zhu, D.Z., Xu, H.J., 2003. Effect of ion implantation on surface energy of ultrahigh molecular weight polyethylene. Journal of Applied Physics 93, 5103-5108. Choo, J.H., Spikes, H.A., Ratoi, M., 2007. Friction reduction in low-load hydrodynamic lubrication with a hydrophobic surface. Tribology International 40, 154-159. Cloarec et al. 2002. Immobilization of homooligonucleotide probe layers onto Si/SiO2 substrates: characterization by electrochemical impedance measurements and radiolabelling. Biosensors & Bioelectronics 17, 405–412. 94 References Demas, N.G., Polycarpou, A.A., 2008. Tribological performance of PTFE-based coatings for air-conditioning compressors. Surface & Coatings Technology 203, 307–316. DePalma, V., Tillman, N., 1989. Friction and wear of self-assembled trichlorosilane monolayer films on silicon. Langmuir 5, 868-872. Donachie, M.J., 2000. Titanium: a technical guide, second edition, ASM International, Materials Park, OH. Dong, H., Bloyce, A., Bell, T., 1996. Duplex surface engineering of Ti-6Al-4V alloy. In Surface Performance of Titanium (Eds J. K. Gregory, H. J. Rack and D. Eylon), (Metallurgical Society of AIME, Warrendale, Pennsylvania). 23-31. Dowson, D., 1979. History of Tribology. Longmans, London. Eapen, K.C., Patton, S.T., Zabinski, J.S., 2002. Lubrication of microelectromechanical systems (MEMS) using bound and mobile phases of Fomblin Zdol®. Tribology Letters 12, 35-41. Eapen, K.C., Patton, S.T., Smallwood, S.A., Phillips, B.S., Zabinski, J.S., 2005. MEMS Lubricants based on bound and mobile phases of hydrocarbon compounds: film deposition and performance evaluation. Journal of Microelectromechanical systems 14, 954-960. Elender, G., Kuhher, M., Sackmann, E., 1996. Functionalisation of Si/SiO2 and glass surfaces with ultrathin dextran films and deposition of lipid bilayers. Biosensors & Bioelectronics 11, 565-577. Elliott, A., 1969. Infra-red Spectra and Structure of Organic Long-chain Polymers, Edward Arnold (Publishers) Ltd. 95 References Ellis, B., 1993. The kinetics of cure and network formations. In Chemistry and Technology of Epoxy Resins. Edited by Bryan Ellis, Blackie Academic & Professional, New York, 72-116. Folkers, J.P., Gorman, Ch.B., Laibinis, P.E., Buchholz, S., Whitesides, G.M., Nuzzo, R.G. 1995. Self-Assembled Monolayers of Long-Chain Hydroxamic Acids on the Native Oxide of Metals. Langmuir 11, 813–824. Fox-Rabinovich, G.S., Kovalev, A.I., Wainstein, D.L., Shuster, L.Sh., Dosbaeva, G.K., 2002. Improvement of Duplex PVD coatings for HSS cutting tools by PFPE (perfluropolyether ‘Z-Dol’). Surface & Coatings technology 160, 99-107. Freidrich, K., Schlarb, A.K., 2008. Tribology of Polymer Nanocomposites: Friction and wear of bulk materials and coatings. Tribology and Interface engineering 55. Fusaro, R.L., 1987. Tribological Properties of Polymer Films and Solid Bodies in a Vacuum Environment: Prepared for the 1987 Annual Meeting of the American Society of Lubrication Engineers. NASA Report. Gao, W., Dickinson, L., Grozinger, Ch., Morin, F.G., Reven, L., 1997. Order−Disorder Transitions in Self-Assembled Monolayers: A 13 C Solid-State NMR Study. Langmuir 13, 115–118. Gawalt, E.S., Avaltroni, M.J., Danahy, M.P., Silverman, B.M., Hanson, E.L., Midwood, K.S., Schwarzbauer, J.E., Schwartz, J., 2003. Bonding Organics to Ti Alloys: Facilitating Human Osteoblast Attachment and Spreading on Surgical Implant Materials. Langmuir 19, 200-204. 96 References Harder, P., Grunze, M., Dahint, R., Whitesides, G.M., Laibinis, P.E., 1998. Molecular Conformation in Oligo(ethylene glycol)-Terminated Self-Assembled Monolayers on Gold and Silver Surfaces Determines Their Ability To Resist Protein Adsorption. Journal of Physical Chemistry B 102, 426–436. Hedenqvist, P., Olsson, M., Hogmark, S., Bhushan, B., 1992. Tribological studies of various magnetic heads and thin-film rigid disks. Wear 153, 65-78. Holmberg, K., 1994. Coatings tribology: properties, techniques, and applications in surface engineering / Kenneth Holmberg, Allan Matthews. Amsterdam; New York: Elsevier. Jost, H.P., 1966. Lubrication (tribology) education and research, Jost Report, (Department of Education and Science, HMSO, London) 4. Jost, H.P., 1990. Tribology - Origin and future. Wear 136, 1-17. Kang, P.H., Nho, Y.C., 2001. The effect of γ-irradiation on ultra-high molecular weight polyethylene recrystallized under different cooling conditions. Radiation Physics and Chemistry 60, 79–87. Katano, T., Oka, M., Nakazawa, S., Aramaki, T., Kusakawa, K., 2003. Characteristics of dual lubricant layers. IEEE Tranactions on Magnetics 39, 2489– 91. Kim, S., Christenson, H.K., Curry, J.E., 2002. The Effect of Humidity on the Stability of an Octadecyltriethoxysilane Monolayer Self-Assembled on Untreated and Plasma-Treated Mica. Langmuir 18, 2125–2129. 97 References Kim, S., Sohn, H., Boo, J.H., Lee, J., 2008. Significantly improved stability of n-octadecyltrichlorosilane self-assembled monolayer by plasma pretreatment on mica. Thin Solid Films 516, 940– 947. Kingshott, P., Griesser, H.J., 1999. Surfaces that resist bioadhesion. Current Opinion in Solid State and Materials Science 4, 403-412. Klein, L.C., 1988. Sol-Gel Technology for Thin films, fibres, performs, electronics and specilaity shapes, Noyes Publications, Park Ridge, NJ. Koinkar, V.N., Bhushan, B., 1996. Microtribological studies of unlubricated and lubricated surfaces using atomic force/ friction force microscopy, Journal of Vacuum Science and Technology A 14, 2378–2391. Komvopoulos, K., 1996. Surface engineering and microtribology for microelectromechanical systems. Wear 200, 305-327. Kumar, S., Singh, J., Agrawal, V.V., Ahamad, M., Malhotra, B.D., 2011. Biocompatible self-assembled monolayer platform based on (3- glycidoxypropyl)trimethoxysilane for total cholesterol estimation. Analytical Methods, Advance Article. DOI: 10.1039/c1ay05231d. Kustas, F.M., Misra, M.S., Wei, R.P., Wilbur, J., 1993. Diamond-like carbon coatings on Ti-6Al-4V. Tribology Transactions 36(1), 113-119. Lancaster, J.K., 1969. Relationship between the wear of polymers and their mechanical properties, Tribology Convention. Institution of Mechanical Engineers, London, 100-108. Lancaster, J.K., 1973. Basic mechanisms of friction and wear of polymers. Plastics and Polymers 41, 297-306. 98 References Leyens, C., Peters, M., 2003. Titanium and titanium alloys: fundamentals and applications. Weinheim : Wiley-VCH. Li, D.G., Yu, X.J., Dong, Y.H., 2007. The different self-assembled way of n- and t-dodecyl mercaptan on the surface of copper. Applied Surface Science 253, 4182-4187. Lio, A., Charych, D.H., Salmeron, M., 1997. Comparative atomic force microscopy study of the chain length dependence of frictional properties of alkanethiols on gold and alkylsilanes on mica. Journal of Physical Chemistry B 101, 3800-3805. Liu, H., Bhushan, B., Eck, W., Stadler, V., 2001. Investigation of the adhesion, friction, and wear properties of biphenyl thiol self-assembled monolayers by atomic force microscopy. Journal of Vacuum Science and Technology A 19, 1234-1240. Liu, H., Bhushan, B., 2003. Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM. Ultramicroscopy 97, 321–340. Lobo, R.F.M., Pereira-da-Silva, M.A., Raposo, M., Faria, R.M., Oliveira Jr, O.N., 1999. In situ thickness measurements of ultra-thin multilayer polymer films by atomic force microscopy. Nanotechnology 10, 389-393. Loh, J.H., 1999. Plasma surface modification in biomedical applications. Medical Device Technology 10, 24-30. Lu, Z.P., Friedrich, K., 1995. On sliding friction and wear of PEEK and its composites, Wear 181-183, 624-631. 99 References Luo, Y., Ge, S., Jin, Z., Fisher, J., 2009. Effect of surface modification on surface propertiesand tribological behaviours of titanium alloys. Proc. IMechE Part J: J. Engineering Tribology 223. special issue paper, 311-316. Luzinov, I., Julthongpiput, D., Liebmann-Vinson, A., Cregger, T., Foster, M.D., Tsukruk, V.V., 2000. Epoxy-Terminated Self-Assembled Monolayers: Molecular Glues for Polymer Layers. Langmuir 16, 504-516. Mahapatro, A., Johnson, D.M., Patel, D., Feldman, M., Ayon, A., Agrawal, M., 2006. The use of alkanethiol self-assembled monolayers on 316L stainless steel for coronary artery stent nanomedicine applications: an oxidative and in vitro stability study. Nanomedicine 2, 182–190. Makkonen, L., 2004. Friction as a result of surface tension. 27th Annual Meeting of the Adhesion Society, Wilmington, NC, USA, 399–401. Mastrangelo, C.H., 1997. Adhesion-related failure mechanisms in micromechanical devices. Triboogy Letter 3, 223–238. Mate, C.M., 1992. Atomic-force-microscope study of polymer lubricants on silicon surfaces. Physical Review Letters 68, 3323–3326. Meth, S., Sukenik, C.N., 2003. Siloxane-anchored thin films on silicon dioxidemodified stainless steel. Thin Solid Films 425, 49-58. Minn, M., Sinha, S.K., 2008. DLC and UHMWPE as hard/soft composite film on Si for improved tribological performance. Surface & Coatings Technology 202, 3698–3708. 100 References Mishra, A.K., Davidson, J.A., Poggie, R.A., 1994. Diffusion hardening - a new surface hardening process for titanium alloys. In Surface Modification Technologies VII (Eds T. S. Sudarshan, K. Ishizaki, M. Takata and K. Kamata), 453-471 (Institute of Materials, London). Miyake, S., Wang, M., Ninomiya S., 2006. Nanotribological properties of perfluoropolyether-coated magnetic disk evaluated by vertical and lateral vibration wear tests. Surface & Coatings Technology 200, 6137–6154. Miyoshi, K., 2001. Solid lubrication fundamentals and applications, Marcel Dekker, Inc. Modi, M., Sitaraman, S.K., 2004. Interfacial fracture toughness measurement of a Ti/Si interface. Journal of Electronic Packaging 126, 301-307. Molinari, A., Straffelini, G., Tesi, B., Bacci, T., Pradelli, G., 1997. Effects of load and sliding speed on the tribological behaviour of Ti6Al4V plasma nitrided at different temperatures. Wear 203, 447-454. Mrksich, M., 2000. A surface chemistry approach to studying cell adhesion. Chemical Society Reviews 29, 267-273. NASA 1980. Comparison of the tribological properties at 25OC of seven different Polyimide Films bonded to 301 stainless steel: NASA, Report. Noel, S., Lecaude, N., Alamarguy, D., 2006. Grafting of Bifunctional Fluorinated Polyether Molecules on Metallic Surfaces: Application to the Protection of Electrical Contacts. Surface and Interface Analysis 38, 326-329. 101 References Nyquist, R.M., Eberhardt, A.S., Silks III, L.A., Li, Z., Yang, X., Swanson, B.I., 2000. Characterization of Self-Assembled Monolayers for Biosensor Applications. Langmuir 16, 1793–1800. O’Brien, E.P., White, C.C., Vogt, B.D., 2006. Correlating interfacial moisture content and adhesive fracture energy of polymer. Advanced Engineering Materials 8, 114-118. Opondo, M.N., Bassell, T.J., 1982. Wear of rigid PVC in relation to water pump designs. Tribology International 15 (2), 75-83. Pendyala, L., Jabara, R., Robinson, K., Chronos, N., 2009. Passive and active polymer coatings for intracoronary stents: novel devices to promote arterial healing. Journal of Interventional Cardiology 22, 37–48. Perry, A.J., 1987. Ion implantation of titanium alloys for biomaterial and other applications. Surface Engineering 3(2), 154-160. Petty, M.C., 2002. Applications of organised molecular films to electronic and opto-electronic devices. Studies in Interface Science 16, 317-367. Pirrung, M.C., 2002. How to Make a DNA Chip. Angewandte Chemie International Edition 41, 1276-1289. Ponsonnet, L., Reybier, K., Jaffrezic, N., Comte, V., Lagneau, C., Lissac, M., Martelet, C., 2003. Relationship between surface properties (roughness, wettability) of titanium and titanium alloys and cell behaviour. Materials Science and Engineering C 23, 551–560. 102 References Raman, A., Dubey, M., Gouzman, I., Gawalt, E.S., 2006. Formation of SelfAssembled Monolayers of Alkylphosphonic Acid on the Native Oxide Surface of SS316L. Langmuir 22(15), 6469-6472. Ratner, S.N., Farberoua, I.I., Radyukeuich, O.V., Lure, E.G., 1964. Correlation between wear resistance of plastics and other mechanical properties. Soviet Plastics 7, 37-45. Ren, S., Yang, S., Zhao, Y., Zhou, J., Xu, T., Liu, W., 2002. Friction and wear studies of octadecyltrichlorosilane SAM on silicon. Tribology Letters 13, 233239. Roosjen, A., Norde, W., van der Mei, H.C., Busscher, H.J., 2006. The use of positively charged or low surface free energy coatings versus polymer brushes in controlling biofilm formation. Progress in Colloid and Polymer Science 132, 138– 144. Ruhe, J., Novotny, V.J., Kanazawa, K.K., Clarke, T., Street, G.B., 1993. Structure and tribological properties of ultrathin alkylsilane films chemisorbed to solid surfaces. Langmuir 9, 2383-2388. Rymuza, Z., 1999. Control tribological and mechanical properties of MEMS surfaces. Part 1: critical review. Microsystem Technologies 5, 173-180. Satyanarayana, N., Sinha, S.K., 2005. Tribology of PFPE Overcoated SelfAssembled Monolayers Deposited on Si Surface. Journal of Physics D: Applied Physics 38, 3512-3522. 103 References Satyanarayana, N., Sinha, S.K., Ong, B.H., 2006. Tribology of a novel UHMWPE/PFPE dual-film coated onto Si surface. Sensors and Actuators A 128, 98–108. Satyanaryana, N., Gosvami, N., Sinha, S.K., Srinivasan, M.P., 2007. Friction, adhesion and wear durability studies of ultra-thin PFPE overcoated 3Glycidoxypropyltrimethoxy silane SAM on Si surface. Philosophical Magazine 87, 3209-3227. Shumaker-Parry, J.S., Campbell, Ch.T., Stormo, G.D., Silbaq, F.S., Aebersold, R.H., 2000. Proceedings of Medical and Fiber Optic Sensors and Delivery Systems; SPIE: San Jose, CA 3922, 158-166. Sidorenko, A., Ahn, H.S., Kim, D.I., Yang, H., Tsukruk, V.V., 2002. Wear stability of polymer nanocomposite coatings with trilayer architecture. Wear 252, 946-955. Silbernagl, D., Sturm, H., Cappella, B., 2009. Influence of Film−Substrate Adhesion on the Mechanical Properties of Thin Polymer Films. Langmuir 25, 5091–5097. Singh, R.A., Yoon, E.S., 2007. Friction of chemically and topographically modified Si (1 0 0) surfaces. Wear 263, 912-919. Sproul, W.D., 1996. Physical vapour deposition tool coatings, Surface and coatings technology 81, 1-7. Srinivasan, U., Houston, M.R., Howe, R.T., Maboudian, R., 1998. Alkyltrichlorosilanebased self-assembled monolayer films for stiction reduction in silicon micromachines. Journal of Microelectromechanical systems 7, 252-260. 104 References Sung, I-H., Yang, J-C., Kim, D-E., Shin, B-S., 2003. Micro/nano-tribological characteristics of self-assembled monolayer and its application in nano-structure fabrication. Wear 255, 808-818. Sweeney et al. 2008. Synthetic corneal inlays. Clinical and Experimental Optometry 91(1), 56–66. Tambe, N.S., Bhushan, B., 2005. Nanotribological characterization of selfassembled monolayers deposited on silicon and aluminium substrates. Nanotechnology 16, 1549-1558. Tao, Z., Bhushan, B., 2005. Bonding, degradation, and environmental effects on novel perfluoropolyether lubricants. Wear 259, 1352–1361. Torrisi, L., Gentile, C., Visco, A.M., Campo, N., 2003. Wetting modifications of UHMWPE surfaces induced by ion implantation. Radiation Effects and Defects in Solids 158, 731–741. Ulman, A., 1991. An Introduction to Ultra Thin Organic Films: From LangmuirBlodgett to Self-assembly. Academic Press, San Diego, CA. Unal, H., Mimaroglu, A., Kadioglu, U., Ekiz, H., 2004. Sliding friction and wear behaviour of polytetrafluoroethylene and its composites under dry conditions. Material and designs 25, 239-45. Van Alsten, J.G., 1999. Self-Assembled Monolayers on Engineering Metals: Structure, Derivatization, and Utility. Langmuir 15, 7605–7614. Viornery, C., Guenther, H.L., Aronsson, B.O., Pechy, P., Descouts, P., Graetzel, M., 2002. Osteoblast culture on polished titanium disks modified with phosphonic acids. Journal of Biomedical Materials Research Part A 62, 149-155. 105 References Wang, A., Sun, D.C., Stark, C., Dumbleton, J.H., 1995. Wear mechanisms of UHMWPE in total joint replacements. Wear 181-183, 241-249. Wang, R., Hashimoto, K., Fujishima, A., Chikuni, M., Kojima, E., Kitamura, A., Shimohigoshi, M., Watanabe, T., 1997. Light-induced amphiphilic surfaces. Nature (London) 388, 431–432. Wong, A.K.Y., Krull, U.J., 2005. Surface characterization of 3- glycidoxypropyltrimethoxysilane films on silicon-based substrates. Analytical and Bioanalytical Chemistry 383 187–200. Wu, S., 1982. Polymer interface and adhesion, Marcel Dekker, Inc. Xie et al. 2006. Two-Year Preclinical Testing of Perfluoropolyether Polymer as a Corneal Inlay. Investigative Ophthalmology & Visual Science 47, 574-581. Yan, L., Huck, W.T.S., Whitesides, G.M., 2000. Supramolecular Polymers, Ciferri, A., Ed., Marcel Dekker: New York. Yildiz, F., Yetim, A.F., Alsaran, A., Efeoglu, I., 2009. Wear and corrosion behaviour of various surface treated medical grade titanium alloy in bio-simulated environment. Wear 267, 695–701. Yoshinari, M., Hayakawa, T., Matsuzaka, K., Inoue, T., Oda, Y., Shimono, M., Ide, T., Tanaka, T., 2006. Oxygen plasma surface modification enhances immobilization of simvastatin acid. Biomedical research 27(1), 29–36. Zhang, J., Polycarpou, A.A., Economy, J., 2010. An improved tribological polymer-coating system for metal surfaces. Tribology Letters 38, 355–365. Zweibel, K., 2000. Thin film PV manufacturing: materials costs and their optimisation, Solar energy materials and solar cells 63, 375-386. 106 Appendix A: Cytotoxicity Test Procedures Appendix A Cytotoxicity Test Procedures In-vitro tests for cytotoxicity are carried out as per the guidelines of “International Organization for Standardization 10993-5: Biological Evaluation of Medical Devices, Part 5: Tests for In Vitro Cytotoxicity”. Cytotoxicity tests evaluate the response of cells in contact with devices or their extracts. "Part 5: Tests for In Vitro Cytotoxicity" contains details about the procedure of testing using cell culture using direct or indirect contact with device or filter diffusion. Extracts of devices or materials are exposed to a cell culture system such as L929 mouse fibroblast cell line in the test. After exposure, loss of cell viability indicates the presence of cytotoxic extracts. Extracts are samples of potentially cytotoxic substances that can leach out from the device material into the tissue from device during usage. Extraction medium is chosen according to the nature and use of device as well as test method. Extracts are prepared by incubating the device at a recommended surface area to extractant volume ratio. This ratio is depended upon the thickness of testing device. Surface area to extractant volume ratio is 60 cm2 per 20 ml extraction volume if the device thickness is greater than or equal to 0.5 mm and 120 cm2 per 20 ml extraction volume if device thickness is less than or equal to 0.5 mm. In case surface area of device can not be determined, a weight to volume ratio (4 g per 20 ml extraction volume) is used for extraction. Extractable 107 Appendix A: Cytotoxicity Test Procedures substances should be maximized and device should be exposed to extreme conditions without a significant degradation. Other method of cytotoxicity testing by direct contact is the agar diffusion or overlay assay. In this method, test device is placed directly on the mammalian cell layer. Mammalian cell layer is protected from mechanical abrasion by using an intermediate agar layer. Similar to other methods of cytotoxicity assessment, the presence of cytotoxic leachables is indicated by the observed loss of cell viability. The direct contact assay method requires the placement of device or material onto the cell culture medium without the presence of an agar intermediate layer. In a filter diffusion test, test device or material is placed on one side of filter and exposed to cells grown on the opposite side of filter. In general, cytotoxicty test involves a cell culture dish having one-half million to one million cells. Device cytotoxicity is measured by the cell viability after a period of exposure (approximately 24–72 hours). Cytotoxicity can be measured by qualitative and quantitative analysis. To validate test results, positive control materials (such as organo-tin-impregnated polyvinyl chloride material) and negative control materials (such as USP-grade high-density polyethylene RS) are also tested. Following the test period, the cell layers are examined microscopically for any abnormal cell morphology and cellular degeneration. 108 [...]... modifications, coatings and treatments have been explored In spite of these developments, there is still a need to further investigate effective solutions to improve tribological properties of titanium and its alloys In this thesis, application of thin organic coatings to improve tribology of titanium and its alloys has been explored with emphasis on biomedical applications Ti6Al4V alloy, a commonly... tribological limitations of Ti6Al4V alloy In this study, following approaches have been used:  Use of PFPE to improve the tribological properties of Ti6Al4V alloy  Use of PFPE overcoat to improve the tribological properties of GPTMS SAMs coated Ti6Al4V alloy 1.7 Methodology in the present thesis To achieve above objectives, UHMWPE polymer and GPTMS SAMs coatings were deposited onto Ti6Al4V alloy substrate... The objective of this thesis is to evaluate some of the potential solutions for surface modifications of titanium and titanium alloys to improve its tribological properties In the first study, UHMWPE and UHMWPE/PFPE thin film coatings were evaluated to address Ti6Al4V alloy tribological limitations Experimental characterizations of the physical, chemical and tribological properties of coatings were... applications due to its high strength -to- weight ratio, excellent corrosion resistance and biocompatibility Unalloyed titanium is as strong as steel but has 45% less weight Titanium can be alloyed with aluminium, vanadium, molybdenum and iron to produce lightweight strong alloys to produce alloys of importance in biomedical, industrial, marine, automotive and aerospace applications Application of titanium alloys... excellent UHMWPE thin film tribological properties In this study, following approaches have been used:  Use of UHMWPE polymer coating to improve the tribological properties of Ti6Al4V alloy  Use of PFPE as a top layer to further improve the wear resistance of UHMWPE film 9 Chapter 1: Introduction In the second study, GPTMS self-assembled monolayers (SAMs) coating with PFPE overcoat was evaluated to address... and titanium alloys before thermal spraying Plasma spraying, detonation gun, high-velocity oxy-fuel and vacuum plasma spraying are used to deposit these coatings [Bloyce 1998] 13 Chapter 2: Literature Review Hard materials such as WC-Co, Mo and Cr-Ni are sprayed onto Ti6Al4V to provide wear resistant coatings 75 µm of molybdenum is sprayed onto the stems of titanium automotive valves to prevent galling... and Kustas 1996] coatings by PVD method are finding applications due to their low coefficients of friction and wear durability Applications of the most of the developed PVD coatings is limited to low contact stress areas to avoid plastic deformation of the substrate 2.2.1.4 Surface modifications Ion implantation is one of the widely used surface modification techniques for titanium alloys [Perry 1987]... treatment was used to clean and improve adhesion properties of Ti6Al4V alloy substrate with coatings PFPE top layer was used to enhance the wear durability of coatings Following process diagram (Fig 1.1) represents the summary of the steps followed in this thesis for different studies Figure 1.1: Research methodology followed in the research studies 10 Chapter 1: Introduction 1.8 Structure of the thesis... further increased the wear resistance of coating as tested at even higher rotational speed (1000 rpm) UHMWPE coatings (with and without PFPE overcoat) meet the requirements of cytotoxicity test using the ISO 10993-5 elution method Due to their low surface energy, wear resistance and noncytotoxic nature, the thin coatings of UHMWPE and UHMWPE/PFPE can find various applications in biomedical implants and... (e) Ti6Al4V/ GPTMS/PFPE (f) Ti6Al4V/ GPTMS/PFPE (heat treated) 74 Figure 5.4 Wide scan XPS spectra of Ti6Al4V/ GPTMS specimens 76 Figure 5.5 Comparison of C1s peaks for bare Ti6Al4V/ GPTMS and Ti6Al4V in C1s scan 76 Figure 5.6 Wear durability (number of sliding cycles before failure) of tested specimens in the study 77 Figure 5.7 Variation of friction coefficient as a function of the sliding cycles ((a) Ti6AL4V/ PFPE,