Volume 18 - Friction, Lubrication, and Wear Technology Part 22 pdf

There are no clear-cut trends for the effects of lubricants on the coefficients of friction. PTFE lubricants demonstrate the lowest static coefficients of friction at all temperatures when compared to metallic lubricants (intercalated graphite, SbSbS 4 and CaF 2 ). However, graphite powder can offer a lower static coefficient of friction than PTFE (Fig. 10 and 11). Fig. 10 Effect of lubricant and test temperature on the coefficients of friction of carbon fiber/PEEK composites. (a) Static coefficient of friction. (b) Dynamic coefficient of friction Fig. 11 Effect of lubricant and test temperature on the coefficie nts of friction of carbon fiber/PEEK composites. (a) Static coefficient of friction. (b) Dynamic coefficient of friction Coefficients of friction are expected to decrease with increasing thermal stability of the resin (for example, melt point, T g , or continuous-use temperature). This trend is not well demonstrated by 30% carbon-fiber-reinforced PEEK, PEK, or HTX (Fig. 12). Fig. 12 Effect of test temperature on the coefficients of friction of 30% carbon-fiber- reinforced PEEK, PEK, and HTX composites. (a) Static coefficient of friction. (b) Dynamic coefficient of friction The coefficient of friction increased for the PTFE-lubricated, carbon-fiber-reinforced PEEK composite with increasing temperature (Fig. 13). Fig. 13 Plot of coefficient of friction versus test temperature for LCL-4033 The reinforced, lubricated PPS composite demonstrates a clear trend for the dynamic coefficient of friction to increase with temperature. The static coefficient of friction reaches a maximum at approximately 150 °C (300 °F) and then rapidly diminishes with further increases in temperature (Fig. 14). Fig. 14 Plot of coefficient of friction versus test temperature for Lubricomp O-BG Conclusions. The results of the tests conducted on the composites listed in Table 2 can be summarized as follows: • PEEK composites formulated with lower molecular weight resin demons trated lower wear factors than analogs using higher molecular weight resin at elevated temperatures. This is probably due to superior wet-out of the resin and fiber in the low-molecular-weight composites • Intercalated graphite is a very effective (and expensive) high-temperature lubricant for carbon-fiber- reinforced high-molecular-weight PEEK • For service to 205 °C (400 °F), Lubricomp O-BG (PPS) and LCL- 4033 (PEEK) composites are the materials of choice for wear and friction applications • For service at 260 °C (500 °F), lubricated PEEK composites are the materials of choice for wear and friction applications • For service at temperatures in excess of 260 °C (500 °F), composites based on PEK and HTX will be required for wear and friction applications References 1. M.P. Wolverton, J.E. Theberge, and K.L. McCadden, How Plastics Wear at High Temperatures, Mach. Des., Vol 55 (No. 3), p 111 2. K. Friedrick, "Friction and Wear of Composites," Composites, Workshop, Center for Composite Materials, University of Delaware, 16 May 1986 3. J.F. Archard and W. Hirst, Wear of Metals under Unlubricated Conditions, Proc. R. Soc. (London) A, Aug 1956 Thermal Spray Coatings Burton A. Kushner and Edward R. Novinski, Perkin-Elmer Corporation, Metco Division Introduction THE THERMAL SPRAY COATING PROCESS is by far the most versatile modern surfacing method with regard to economics, range of materials, and scope of applications. The thermal spray process permits rapid application of high- performance materials in thicknesses from a few mils to more than 25 mm (1 in.) on parts of a variety of sizes and geometries. Thermal spraying requires minimal base-metal preparation, can be applied in the field, and is a low- temperature (>95 °C, or >200 °F) method compared with techniques such as weld overlay. Typical part configurations include piston rings, journals, conveyors, shifter forks, extrusion dies, transformer cases, ship hulls, ship tanker compartments, and suspension bridges. Thermal spraying reduces wear and corrosion and greatly prolongs part service life by allowing use of a high- performance coating material over a low-cost base metal. Application areas of the process can be categorized as (Ref 1): • Wear resistance • Oxidation resistance • Corrosion resistance • Restoration of dimension • Abradable clearance control • Thermal barriers • Electrical conductivity or resistivity • Biomedical More than 200 coating materials with different characteristics of toughness, coefficient of friction, hardness, and other properties are available. These materials can be grouped as follows: • Pure metals • Metal alloys • Cermets • Ceramics • Carbides • Polymers • Special composite materials All thermal spray processes rely on three basic operational mechanisms: • Heating a coating material in either wire or powder form to a molten or plastic state • Propulsion of particles of the heated material • Impact of the material onto a workpiece whereby the particles rapidly solidify and adhere both to one another and to the substrate to form a dense, functional, protective coating The particles bond to the substrate mechanically and, in some cases, metallurgically. Particle velocity, substrate roughness, particle size, material chemistry, particle temperature, and substrate temperature influence the bond strength of the coating material. The process was originally referred to as flame spraying, metal spraying, flame plating, or metallizing when it was limited to the oxygen-fuel (oxyfuel) wire spray method. Thermal Spray Processes Currently, five different commercially available thermal spray methods are in use: • Oxyfuel wire (OFW) spray • Electric arc wire (EAW) spray • Oxyfuel powder (OFP) spray • Plasma arc (PA) powder spray • High-velocity oxyfuel (HVOF) powder spray Selection of the appropriate thermal spray method is typically determined by: • Desired coating material • Coating performance requirements • Economics • Part size and portability The oxyfuel wire spray process (also called wire flame spraying or the combustion wire process) is the oldest of the thermal spray coating methods and among the lowest in capital investment. The process utilizes an oxygen-fuel gas flame as a heating source and coating material in wire form. Solid rod feed stock has also been used. During operation, the wire is drawn into the flame by drive rolls that are powered by an adjustable air turbine or electric motor (Fig. 1). The tip of the wire is melted as it enters the flame and is atomized into particles by a surrounding jet of compressed air and propelled to the workpiece. Fig. 1 Atomization of wire feedstock from the nozzle of an OFW spray gun Spray rates for this process range from 2.3 to 55 kg/h (5 to 120 lb/h) and are dictated by the melting point of the material and the choice of fuel gas. The wire spray gun is most commonly used as a hand-held device for on-site application, although an electric-motor-driven gun is recommended for fixed-mounted use in high-volume, repetitive production work. The OFW process is widely used for corrosion protection of large outdoor structures, such as bridges and storage tanks, and for restoration of dimension to worn machinery components. It is a good choice for all-purpose spraying. Coatings can be applied rapidly and at low cost, and a wide variety of metal coating materials are available. Typical spray materials include austenitic and martensitic stainless steels, nickel aluminide, nickel chromium alloy, bronze, Monel, babbitt, aluminum, zinc and molybdenum. Electric arc wire spraying also applies coatings of selected metals in wire form. Push-pull motors feed two electrically charged wires through the arc gun to contact tips at the gun head (Fig. 2). An arc is created that metals the wires at temperatures above 5500 °C (10,000 °F). Compressed air atomizes the molten metal and projects it onto a prepared surface. Fig. 2 Spray pattern generated by two electrically charged wires melted at the nozzle of an EAW spray gun The EAW process is excellent for applications that require heavy coating buildup or that have large surfaces to be sprayed. The arc system can produce a spray pattern ranging from 50 to 300 mm (2 to 12 in.) and can spray at high speeds. It has built-in flexibility, allowing coating characteristics, such as hardness or surface texture, to be tailored to specific applications. The EAW method is characterized by strong coating adhesion because of the high particle temperatures produced. Because the process uses only electricity and compressed air, it allows equipment to be moved relatively easily from one installation to another, and eliminates the need to stock oxygen and fuel gas supplies. Materials applied by the EAW method are similar to those used in the OFW process. The oxyfuel powder spray method extends the range of available coatings and subsequent applications to include ceramics, cermets, carbides, and fusible hardfacing coatings. Using either gravity flow or pressurized feed, powder is fed into the gun and carried to the gun nozzle (Fig. 3), where it is melted and projected by the gas stream onto a prepared surface. For general-purpose spraying, the gravity-flow system is used. When exacting coating consistency and/or high spray rates are desired, the pressurized feed system is used. Oxyfuel powder guns are the lowest cost thermal spray equipment and are easiest to set up and change coating materials. The OFP method finds widest use in short-run machinery maintenance work and in the production spraying of abradable clearance-control seals for gas turbine engines. Fig. 3 Cross-sectional view of an O FP spray system showing powder feed material being transported by the carrier gas and then melted by the oxy-fuel mixture Plasma arc powder is one of the most sophisticated and versatile thermal spray methods. Temperatures that can be obtained with commercial plasma equipment have been calculated to be greater than 11,000 °C (20,000 °F) and are far above the melting or even the vaporization point of any known material. Decomposition of materials during spraying is minimized because of the high gas velocities produced by the plasma, resulting in extremely short residence time in the thermal environment. The plasma process also provides a controlled atmosphere for melting and transport of the coating material, thus minimizing oxidation, and the high gas velocities produce coatings of high density. The plasma gun operates on the principle of raising the energy state of a gas by passing it through an electric arc. The release of energy in returning the gas to its ground state results in exceedingly high temperatures. A gas such as nitrogen or argon enters a direct-current arc between a tungsten cathode and a copper anode that make up the nozzle (Fig. 4). Both components are cooled by a constant flow of water through internal passages. Here the plasma gas first dissociates (in the case of nitrogen, into two atoms), followed by ionization that releases free electrons. The electrons recombine outside the electric arc, and energy is released as heat and light. In addition, frequent collisions transfer energy from the electrons to the positive ions, accelerating them until the plasma reaches a state of equilibrium. The result is a thermal plasma, in which the energy of the electrons has been turned into enthalpy, or heat content (Ref 2). At this point, powdered coating material suspended in a gas is injected into the plasma and is subsequently melted and propelled at high velocity to the workpiece. In practice, a small amount of a secondary gas, such s hydrogen or helium, is mixed with the primary plasma gas to increase operating voltage and thermal energy. Fig. 4 Schematic of a plasma arc powder spray system showing routing of plasma gas and powder material at the output nozzle The high temperatures and high gas velocities produced by the plasma process result in coatings that are superior in mechanical and metallurgical properties to low-velocity OFW or OFP coatings. The plasma process is particularly efficient for spraying high-quality coatings of ceramic materials, such as zirconium oxide for turbine engine combustors and chromium oxide for printing rolls. The plasma process is also readily field-portable and is thus used for large on-site applications such as power utility plant boiler tubes. Current plasma spray technology permits fully automatic start/stop operation and closed-loop computer control for power level, plasma gas flow, and powder feed rate. System problems can be diagnosed via computer modem to the equipment manufacturer, and system performance can be documented and stored on a digital recorder or data logger. A variation of the plasma spray process is to conduct spraying within a vacuum or low-pressure chamber. Although this significantly adds to cost, the quality of metallic coatings is improved by minimizing oxides within the coating, reducing porosity, and increasing coating adhesion. With any thermal spray powder method, the degree to which a given flame effectively melts and accelerates the powder depends on the type of coating material and the size and shape of the particles. Each particular coating material and gun combination has an optimum particle size. Particles much smaller than ideal will overheat and vaporize; much larger particles will not melt and may fall from the flame or rebound from the target (Ref 2). The high velocity oxyfuel powder spray process (also known as the hypervelocity oxy-fuel powder spray process, the oxyfuel detonation [OFD] process, and the D-gun process) represents the state of the art for thermal spray metallic coatings. The HVOF process uses extremely high kinetic energy and controlled thermal energy output to produce very-low-porosity coatings that exhibit high bond strength, fine as-sprayed surface finish, and low residual stresses. The HVOF process with an oxygen-fuel mixture consisting of oxygen and either acetylene, propylene, propane, or hydrogen fuel gas, depending on coating requirements. The fuel gas flows through a siphon system, where it is thoroughly mixed with oxygen (Fig. 5a). In one design, the mixed gases are ejected from the gun nozzle and are ignited. The high- velocity gases produce uniquely characteristic multiple shock diamond patterns, which are visible in the flame (Fig. 5b). Combustion temperatures approach 2750 °C (5000 °F) and form a circular flame configuration. Powder is injected into the flame axially to provide uniform heating, and powder particles are accelerated by the high-velocity gases, which typically approach a speed of 1350 m/s (4500 ft/s). Fig. 5 Schematics showing input and output of an HVOF powder spray device. (a) Key components of an HVOF system. (b) Close-up view of HVOF spray system output The low residual coating stress produced in the HVOF process allows significantly greater thickness capability than the plasma method, while providing lower porosity, lower oxide content, and higher coating adhesion. Coatings produced by the HVOF process also have much better machinability compared with other methods, and coating porosity has closely approached wrought materials, as verified by recent gas permeability testing. HVOF systems are available with closed- loop computer control and robotics capability. Process Parameters Several important factors must be considered when selecting an appropriate thermal spray method: • Surface preparation • Deposition rate • Coating thickness limitations • Bond coat materials • Coating finishing method Surface Preparation. Bonding of thermal spray coatings relies primarily on mechanical interlocking with the substrate material. Good surface preparation cannot be overemphasized; most coating failures can be traced to poor practice at this first step of the operation. Nonporous surfaces should be cleaned to remove organic contamination by vapor degreasing or by washing with hot detergent solutions or steam cleaning. Porous castings may require heat treatment to pyrolize contamination at approximately 200 to 300 °C (400 to 600 °F). Cleaned parts should be uniformly abrasive blasted to achieve a white metal condition and a minimum of 6 m (250 in.) R a (arithmetic average surface roughness) finish using either aluminum oxide or chilled, angular iron steel grit. The grit and air supply must be oil-free so that the cleaned surfaces are not recontaminated. Blasted parts should be handled with clean gloves and protected from shop soil until the coating operation. Spraying should be done within 2 h of blasting. Parts should be reblasted if this time interval is exceeded. Machined undercuts should have at least a 45° taper and must be cleaned and grit blasted as described above. Deposition Rate. The coating deposition rate is limited by the method used and the melting point of the coating material. Other considerations include the deposit efficiency and target efficiency. Deposit efficiency is the quantity of coating material deposited relative to the quantity being sprayed. Target efficiency relates to the area of the spray pattern relative to the size of the part or target. Both factors influence cost. Coating Thickness Limitations. All thermal spray coatings exhibit a degree of internal stress as a result of shrinkage from a molten state to a solid state. These stresses accumulate as the coating thickness increases and result in a shear force at the substrate interface. Ductile coating materials tend to exhibit low stress; the opposite is true for hard coating materials, such as carbides or ceramics. Also, very porous coatings exhibit lower stress than denser coatings. When the internal stress exceeds the adhesion, the coating can delaminate from the substrate or crack. Equipment manufacturers usually provide the practical thickness limit for each spray material. [...]... Small-amplitude oscillatory displacement applications at low temperature (540 °C, or 1000 °F) Erosion Cavitation Coating process (a) Molybdenum Mo/Ni-Cr-B-SiC Co-Mo-Cr-Si Aluminum bronze Cu-Ni-In Cu-Ni Co-Cr-Ni-W Chromium carbide Chromium carbide Tungsten carbide WC/Ni-Cr-B-SiC (fused) WC/Ni-Cr-B-SiC... bearing coatings: PA Mo/Ni-Cr-BSi blend OFW, Molybdenum EAW, PA OFW, EAW High-carbon steel OFP, PA Alumina/titania OFP, PA, Tungsten HVOF carbide PA, HVOF Co-Mo-Cr-Si PA Fe-Mo-C Aluminum oxide PA Chromium oxide PA Tungsten carbide PA, HVOF Chromium PA, HVOF carbide Ni-Cr-B-SiC/WC OFP, HVOF (fused) Ni-Cr-B-SiC OFP, HVOF (fused) Ni-Cr-B-SiC HVOF (unfused) Abrasive wear Surface fatigue wear Fretting: Intended... valve plugs and seats, exhaust valve seats Compressor air seals, compressor stators, fan duct segments and stiffeners (all jet engine components) PA, HVOF OFP, HVOF OFP, HVOF PA PA Wear rings (hydraulic turbines), water turbine buckets, water turbine nozzles, diesel engine cylinder liners, pumps Ni-Al/Ni-Cr-BSiC Type 316 stainless steel Ni-Cr-B-SiC (fused) Ni-Cr-B-SiC (unfused) Aluminum bronze Cu-Ni PA... coatings are listed in Table 1 Recommended coatings include nickel-, iron-, cobalt-, and molybdenum-base alloys, ceramics, and tungsten carbides (see Table 1) Abrasive Wear Abrasive wear occurs when hard foreign particles, such as metal debris, metallic oxides, and dust from the environment, are present between rubbing surfaces (Fig 7) These particles abrade material off both surfaces Selection of coating... fretting wear (a) Stem and seat wear caused by the intended cyclic up -and- down motion of an engine valve (b) Wear caused by the unplanned but unavoidable oscillatory motion of a press-fitted shaft on the inner ring of a bearing Erosion is caused when a gas or a liquid that ordinarily carries entrained particles impinges on a surface with velocity (Fig 9) When the angle of impingement is small, the wear- producing... Plating: Fundamentals and Applications, G.O Mallory and J.B Haydu, Ed., American Electroplaters and Surface Finishers Society, 1990, p 26 9-2 88 3 L.T Romankiw and T.A Palumbo, Electrodeposition in the Electronic Industry, Electrodeposition Technology, Theory and Practice, L.T Romankiw and D.R Turner, Ed., Proceedings, Vol 8 7-1 7, The Electrochemical Society, 1987, p 1 3-4 1 4 Metals Handbook, 9th ed., Surface... Budinski, Wear Characteristic of Industrial Plating, Selection and Use of Wear Tests for Coatings, STP 769, ASTM, 1982, p 11 8- 131 17 X Changgeng, D Zonggeng, and Z Lijun, The Properties of Electrodeposited Ni-P-SiC Composite Coatings, Plat Surf Finish., Vol 75 (No 10), 1988, p 54 18 R Weil and K Parker, Chapter 4, Properties of Electroless Nickel Plating, Electroless Plating: Fundamentals and Applications,... Wolowodiuk, and D.R Blessington, Additive-Free Hard Gold Plating for Electronic Applications, Plat Surf Finish., Vol 67 (No 6), 1980, p 50 22 G.L Ide and J.B Vanhumbeeck, Palladium and Palladium Alloy Electroplating for Contact Applications, Electrodeposition Technology, Theory and Practice, L.T Romankiw and D.R Turner, Ed., Proceedings, Vol 8 7-1 7, The Electrochemical Society, 1987, p 17 9-1 90 23 A.D Knight and. .. coatings Pores act as reservoirs for lubricant; with reduced particle junctions, there is less tendency for adhesive wear Applications and recommended materials for soft bearing coatings are listed in Table 1 Table 1 Thermal spray coatings for friction and wear applications Type of wear Coating material Adhesive and abrasive wear Adhesive wear Soft bearing coatings: OFW, Aluminum EAW, OFP, bronze PA,... but a nickel undercoat will There are some applications of electrodeposited Ni-W, Ni-Mo, and Ni-Cr alloys that are hard and wear resistant These alloys also offer good corrosion protection Electroless Nickel The tribological and associated properties of electroless nickel were extensively discussed by Weil and Parker (Ref 18) The reducing agents for electroless nickel are sodium hypophosphite, sodium . carbide PA, HVOF Ni-Cr-B-SiC/WC (fused) OFP, HVOF Ni-Cr-B-SiC (fused) OFP, HVOF Abrasive wear Ni-Cr-B-SiC (unfused) HVOF Slush-pump piston rods, polish rod liners, and sucker rod couplings. mandrels (dry-cell batteries); buffing and polishing fixtures; fuel-rod mandrels Surface fatigue wear Molybdenum OFW, PA Mo/Ni-Cr-B-SiC PA Fretting: Intended motion applications Co-Mo-Cr-Si. Mo/Ni-Cr-B- Si blend PA Molybdenum OFW, EAW, PA High-carbon steel OFW, EAW Alumina/titania OFP, PA Tungsten carbide OFP, PA, HVOF Co-Mo-Cr-Si PA, HVOF Adhesive wear Fe-Mo-C PA