Engineering Tribology Episode 2 Part 9 ppsx

SOLID LUBRICATION AND SURFACE TREATMENTS 425 various test machines were also increased by the addition of suspended molybdenum disulphide [49,50]. Although in most cases 1% concentration by weight of molybdenum disulphide in oil is sufficient, improvements were still obtained at higher concentrations reaching 5% [49]. 0 50 100 150 10 100 1000 10 4 10 5 10 6 Number of cycles to disruption Slip amplitude [µm] Vacuum evaporation Sputtering 100V bias 200V bias Ion Plating FIGURE 9.15 Comparison of the durability of a gold lubricant film produced by different coating techniques under fretting conditions [46]. However, an increase in wear when molybdenum disulphide is added to oil has also been reported [51]. Under moderate conditions of sliding speed and load where molybdenum disulphide is not expected to improve lubrication, abrasive impurities in the solid lubricant can cause rapid wear [51]. Silica in particular accentuates wear when in concentrations above 0.01%, and pyrites (iron sulphide) are also destructive [51]. The quality, i.e. cleanliness, of the solid lubricant added to oil is therefore critical. Although solid lubricant additives are suitable for extremes of loads and speeds, they are not suitable for reducing wear under moderate conditions. Molybdenum disulphide suspensions provide a limited reduction in friction and wear when added to an oil containing sulphur based additives or zinc dialkyldithiophosphate. On the other hand, the presence of detergents or dispersants in the oil, such as calcium sulphonate, inhibits the lubricating action of molybdenum disulphide [48,50]. The mechanism of lubrication by molybdenum disulphide dispersed in oil has unfortunately received very little attention. It is widely believed, however, that molybdenum disulphide provides a complimentary role to surfactants. Where there is a worn surface devoid of surfactant, it is hypothesized that molybdenum disulphide particles adhere to form a lubricating film. A conceptual model of solid lubrication by molybdenum disulphide which occurs only when there are no surfactants to block adhesion by lamellae of solid lubricant to the worn surface is illustrated schematically in Figure 9.16. It has been found that molybdenum disulphide lubricates by film formation on a worn surface at high temperatures where all surfactants, both natural and artificial, are unlikely to adsorb on worn surfaces [52]. However, evidence which confirms that molybdenum disulphide is only effective beyond the desorption temperature of the specific surfactants is absent from the published literature. Solid lubricants are also used to improve the frictional characteristics of polymers [33]. In general they do offer some improvement but the effectiveness of solid lubricants added to polymers depends on the type of polymer used. The greatest improvements in polymer friction and wear characteristics are achieved with polymers of moderate lubricity such as nylon and polyimide [53]. For example, the addition of graphite to nylon results in a TEAM LRN 426 ENGINEERING TRIBOLOGY reduction of the coefficient of friction from 0.25 to 0.18 and a small reduction in wear [53]. On the other hand, it has also been shown that molybdenum disulphide when added to nylon oxidizes during wear and does not develop an effective transfer film [54]. Under these conditions, the friction performance of nylon/molybdenum disulphide blend was found to be inferior to plain nylon [54]. Adhesion blocked by adsorbed films Adsorbed film of surfactants MoS 2 present below desorption temperature MoS 2 present above desorption temperature Adhesion of lamella Inter-lamellar sliding Desorbed surfactants Inter-lamellar sliding inhibited FIGURE 9.16 Conceptual model of the mechanism of lubrication by molybdenum disulphide suspended in oil. In polyimides the addition of the same amount of graphite reduced the coefficient of friction to less than half of pure polyimide and significantly reduced wear. Although molybdenum disulphide showed the same reduction of coefficient of friction as graphite/polyimide blend its reduction in wear rate was inferior to that of graphite/polyimide blend [53]. Improvements achieved by adding molybdenum disulphide and graphite to polytetrafluoroethylene (PTFE) are very limited [55,56]. The coefficients of friction for PTFE filled with graphite and molybdenum disulphide are very similar to that of unfilled PTFE and slightly lower than those obtained with most other fillers [55]. Interest in graphite has recently been extended by the incorporation of carbon fibres into polymers. Carbon fibres offer a unique combination of mechanical reinforcement and lubricity [57]. It has been shown that a carefully formulated polyimide/carbon fibre composite can sustain high contact loads and maintain a friction coefficient close to 0.2 at temperatures reaching 300°C with very low wear rates [58,59]. 9.3 WEAR RESISTANT COATINGS AND SURFACE TREATMENTS Wear resistant coatings consist of carefully applied layers of usually hard materials which are intended to give prolonged protection against wear. Abrasive wear, adhesive wear and fretting are often reduced by wear resistant coatings. There are numerous methods of applying hard materials. For example, sputtering and ion-plating are used in a similar manner as in the deposition of solid lubricants to generate thin coatings. Other methods are used to deposit very thick layers of hard material. Applications of wear resistant coatings are found in every industry, and for example, include mining excavator shovels and crushers [60], cutting and forming tools in the manufacturing industries [61], rolling bearings in liquefied natural gas pumps [62], etc. In most of these applications, wear rather than friction is the critical problem. Another benefit of hard-coating technology is that a cheap substrate material can be improved by a coating of an exotic, high-performance material. Most engineering items are made of steel and it is often found that some material other than steel is needed to fulfil the wear and friction requirements. Many wear resistant materials are brittle or expensive and can only be used as a coating, so improved coating technology has extended the control of wear to many previously unprotected engineering components. TEAM LRN SOLID LUBRICATION AND SURFACE TREATMENTS 427 9.3.1 TECHNIQUES OF PRODUCING WEAR RESISTANT COATINGS There are many different methods of applying wear-resistant or hard coatings to a metal substrate currently in use [e.g. 63-65]. New techniques continue to appear as every available technology is adapted to deposit a wear resistant coating more efficiently. The wear resistance of a surface can also be improved by localized heat treatment, i.e. thermal hardening, or by introducing alloying elements, e.g. nitriding or carburizing. Many of these methods have been in use for many years but unfortunately suffer from the disadvantage that the substrate needs to be heated to a high temperature. Carburizing, nitriding and carbonitriding in particular suffer from this problem. Various coating techniques available with their principal merits and demerits are listed in Table 9.1. T ABLE 9.1 Available techniques for modifying the surface to improve its tribological characteristics. Wide range of coating thicknesses, but adhesion to substrate is poor and only certain materials can be coated by this technique Physical and chemical vapour deposition Thin discrete coating; no limitations on materials Ion implantation Thin diffuse coating; mixing with substrate inevitable Thick coatings; coating material must be able to meltLaser glazing and alloying Electroplating Friction surfacing Simple technology but limited to planar surfaces; produces thick metal coating Explosive cladding Rapid coating of large areas possible and bonding to substrate is good. Can give a tougher and thicker coating than many other methods Very thick coatings possible but control of coating purity is difficultThermal spraying Suitable for very thick coatings only; limited to materials stable at high temperatures; coated surfaces may need further preparation Surface welding The thinner coatings are usually suitable for precision components while the thicker coatings are appropriate for large clearance components. Coating Techniques Dependent on Vacuum or Gas at Very Low Pressure Plasma based coating methods are used to generate high quality coatings without any limitation on the coating or substrate material. The basic types of coating processes currently in use are: physical vapour deposition (PVD), chemical vapour deposition (CVD) and ion implantation. These coating technologies are suitable for thin coatings for precision components. The thickness of these coatings usually varies between 0.1 - 10 [µm]. These processes require enclosure in a vacuum or a low pressure gas from which atmospheric oxygen and water have been removed. As mentioned already the use of a vacuum during a coating process has some important advantages over coating in air. The exclusion of contaminants results in strong adhesion between the applied coating and substrate and greatly improves the durability of the coating. · Physical Vapour Deposition This process is used to apply coatings by condensation of vapours in a vacuum. The extremely clean conditions created by vacuum and glow discharge result in near perfect TEAM LRN 428 ENGINEERING TRIBOLOGY adhesion between the atoms of coating material and the atoms of the substrate. Porosity is also suppressed by the absence of dirt inclusions. PVD technology is extremely versatile. Virtually any metal, ceramic, intermetallic or other compounds which do not undergo dissociation can be easily deposited onto substrates of virtually any material, i.e. metals, ceramics, plastics or even paper. Therefore the applications of this technology range from the decorative to microelectronics, over a significant segment of the engineering, chemical, nuclear and related industries. In recent years, a number of specialized PVD techniques have been developed and extensively used. Each of these techniques has its own advantages and range of preferred applications. Physical vapour deposition consists of three major techniques: evaporation, ion-plating and sputtering. Evaporation is one of the oldest and most commonly used vacuum deposition techniques. This is a relatively simple and cheap process and is used to deposit coatings up to 1 [mm] thick. During the process of evaporation the coating material is vaporized by heating to a temperature of about 1000 - 2000°C in a vacuum typically 10 -6 to 1 [Pa] [64]. The source material can be heated by electrical resistance, eddy currents, electron beam, laser beam or arc discharge. Electric resistance heating usually applies to metallic materials having a low melting point while materials with a high melting point, e.g. refractory materials, need higher power density methods, e.g. electron beam heating. Since the coating material is in the electrically neutral state it is expelled from the surface of the source. The substrate is also pre-heated to a temperature of about 200 - 1600°C [64]. Atoms in the form of vapour travel in straight lines from the coating source towards the substrate where condensation takes place. The collisions between the source material atoms and the ambient gas atoms reduce their kinetic energy. To minimize these collisions the source to substrate distance is adjusted so that it is less than the free path of gas atoms, e.g. about 0.15 - 0.45 [m]. Because of the low kinetic energy of the vapour the coatings produced during the evaporation exhibit low adhesion and therefore are less desirable for tribological applications compared to other vacuum based deposition processes. Furthermore, because the atoms of vapour travel in straight lines to the substrate, this results in a ‘shadowing effect’ for surfaces which do not directly face the coating source and common engineering components such as spheres, gears, moulds and valve bodies are difficult to coat uniformly. The evaporation process is schematically illustrated in Figure 9.17. Vacuum pump Resistance heater Coating Substrate Vapour Coating material (molten) FIGURE 9.17 Schematic diagram of the evaporation process. Ion-plating is a process in which a phenomenon known as ‘glow discharge’ is utilized. If an electric potential is applied between two electrodes immersed in gas at reduced pressure, a stable passage of current is possible. The gas between the electrodes becomes luminescent hence the term ‘glow discharge’. When sufficient voltage is applied the coating material can TEAM LRN SOLID LUBRICATION AND SURFACE TREATMENTS 429 be transferred from the ‘source’ electrode to the ‘target’ electrode which contains the substrate. The process of ion-plating therefore involves thermal evaporation of the coating material in a manner similar to that used in the evaporation process and ionization of the vapour due to the presence of a strong electric field and previously ionized low pressure gas, usually argon. The argon and metal vapour ions are rapidly accelerated towards the substrate surface, impacting it with a considerable energy. Under these conditions, the coating material becomes embedded in the substrate with no clear boundary between film and substrate. Usually prior to ion-plating the substrate is subjected to high-energy inert gas (argon) ion bombardment causing a removal of surface impurities which is beneficial since it results in better adhesion. The actual coating process takes place after the surface of the substrate has been cleaned. However, the inert gas ion bombardment is continued without interruptions. This causes an undesirable effect of decreasing deposition rates since some of the deposited material is removed in the process. Therefore for the coating to form the deposition rate must exceed the sputtering rate. The heating of the substrate by intense gas bombardment may also cause some problems. The most important aspect of ion-plating which distinguishes this process from the others is the modification of the microstructure and composition of the deposit caused by ion bombardment [65]. Ion plating processes can be classified into two general categories: glow discharge (plasma) ion plating conducted in a low vacuum of 0.5 to 10 [Pa] and ion beam ion plating (using an external ionization source) performed in a high vacuum of 10 -5 to 10 -2 [Pa] [64]. The ion-plating process is schematically illustrated in Figure 9.18. High voltage power supply − + Vacuum pump Resistance heater ≈0.1 Pa argon gas Coating Substrate Plasma Coating material (molten) FIGURE 9.18 Schematic diagram of the ion-plating process. Sputtering is based on dislodging and ejecting the atoms from the coating material by bombardment of high-energy ions of heavy inert or reactive gases, usually argon. In sputtering the coating material is not evaporated and instead, ionized argon gas is used to dislodge individual atoms of the coating substance. For example, in glow-discharge sputtering a coating material is placed in a vacuum chamber which is evacuated to 10 -5 to 10 -3 [Pa] and then back-filled with a working gas, e.g. argon, to a pressure of 0.5 to 10 [Pa] [64]. The substrate is positioned in front of the target so that it intercepts the flux of dislodged atoms. Therefore the coating material arrives at the substrate with far less energy than in ion-plating so that a distinct boundary between film and substrate is formed. When atoms reach the substrate, a process of very rapid condensation occurs. The condensation process is critical to coating quality and unless optimized by the appropriate selection of coating rate, argon gas pressure and bias voltage, it may result in a porous crystal structure with poor wear resistance. The most characteristic feature of the sputtering process is its universality. Since the coating material is transformed into the vapour phase by mechanical (momentum exchange) rather TEAM LRN 430 ENGINEERING TRIBOLOGY than a chemical or thermal process, virtually any material can be coated. Therefore the main advantage of sputtering is that substances which decompose at elevated temperatures can be sputtered and substrate heating during the coating process is usually negligible. Although ion-plating produces an extremely well bonded film, it is limited to metals and thus compounds such as molybdenum disulphide which dissociate at high temperatures cannot be ion-plated. Sputtering is further subdivided into direct current sputtering, which is only applicable to conductors, and radio-frequency sputtering, which permits coating of non- conducting materials, for example, electrical insulators. In the latter case, a high frequency alternating electric potential is applied to the substrate and to the ‘source’ material. The sputtering process is schematically illustrated in Figure 9.19. + − Vacuum pump ≈1 Pa argon gas Coating Substrate Coating material Bombardment of coating material by gas ions Dislodgement of atoms Plasma RF generaor or DC power supply Vapour Deposition of dislodged atoms FIGURE 9.19 Schematic diagram of the sputtering process. · Chemical Vapour Deposition In this process the coating material, if not already in the vapour state, is formed by volatilization from either a liquid or a solid feed. The vapour is forced to flow by a pressure difference or the action of the carrier gas toward the substrate surface. Frequently reactant gas or other material in vapour phase is added to produce a metallic compound coating. For example, if nitrogen is introduced during titanium evaporation then a titanium nitride coating is produced. The coating is obtained either by thermal decomposition or chemical reaction (with gas or vapour) near the atmospheric pressure. The chemical reactions usually take place in the temperature range between 150 - 2200°C at pressures ranging from 50 [Pa] to atmospheric pressure [64]. Since the vapour will condense on any relatively cool surface that it contacts, all parts of the deposition system must be at least as hot as the vapour source. The reaction portion of the system is generally much hotter than the vapour source but considerably below the melting temperature of the coating. The substrate is usually heated by electric resistance, inductance or infrared heating. During the process the coating material is deposited, atom by atom, on the hot substrate. Although CVD coatings usually exhibit excellent adhesion, the requirements of high substrate temperature limit their applications to substrates which can withstand these high temperatures. The CVD process at low pressure allows the deposition of coatings with superior quality and uniformity over a large substrate area at high deposition rates [64]. The CVD process is schematically illustrated in Figure 9.20. · Physical-Chemical Vapour Deposition This is a hybrid process which utilizes glow discharge to activate the CVD process. It is broadly referred to as ‘plasma enhanced chemical vapour deposition’ (PECVD) or ‘plasma assisted chemical vapour deposition’ (PACVD). In this process the techniques of forming TEAM LRN SOLID LUBRICATION AND SURFACE TREATMENTS 431 solid deposits by initiating chemical reactions in a gas with an electrical discharge are utilized. Many of the phenomena characteristic to conventional high temperature CVD are employed in this process. Similarly the same principles that apply to glow discharge plasma in sputtering apply to CVD. In this process the coating can be applied at significantly lower substrate temperatures, of about 100 - 600°C, because of the ability of high-energy electrons produced by glow discharge, at pressures ranging from 1 to 500 [Pa], to break chemical bonds and thus promote chemical reactions. Virtually any gas or vapour, including polymers, can be used as source material [64]. For example, during this process a diamond coating can be produced from carbon in methane or in acetylene [88]. Amorphous diamond-like coatings in vacuum can attain a coefficient of friction as low as 0.006 [96]. Although contamination by air and moisture tends to raise this coefficient of friction to about 0.02-0.07, the diamond-like coating still offers useful wear resistance under these conditions [97-99]. The mechanism responsible for such low friction is still not fully understood. The PECVD process is schematically illustrated in Figure 9.21. Resistance heater Substrate Exhaust Coating Inlet of reactant gases FIGURE 9.20 Schematic diagram of the CVD process. RF generator or DC power supply − + Substrate Coating Inlet of reactant gases Exhaust FIGURE 9.21 Schematic diagram of the PECVD process. · Ion Implantation The energy of ions in a plasma can be raised to much higher levels than is achieved either in ion-plating or sputtering. If sufficient electrical potential is applied then the plasma can be converted to a directed beam which is aimed at the material to be coated allowing the controlled introduction of the coating material into the surface of the substrate. This process is known as ion implantation. During the process of ion implantation, ions of elements, e.g. nitrogen, carbon or boron, are propelled with high energy at the specimen surface and penetrate the surface of the substrate. This is done by means of high-energy ion beams containing the coating material in a vacuum typically in the range 10 -3 to 10 -4 [Pa]. A specialized non-equilibrium microstructure results which is very often amorphous as the original crystal structure is destroyed by the implanted ions [66]. The modified near-surface TEAM LRN 432 ENGINEERING TRIBOLOGY layer consists of the remnants of a crystal structure and interstitial implanted atoms. The mass of implanted ions is limited by time, therefore compared to other surfaces, the layers of ion-implanted surfaces are very shallow, about 0.01 to 0.5 [µm]. The thickness limitation of the implanted layer is the major disadvantage of this method. The coatings generated by ion implantation are only useful in lightly loaded contacts. The technique allows for the implantation of metallic and non-metallic coating materials into metals, cermets, ceramics or even polymers. The ion implantation is carried out at low temperatures. Despite the thinness of the modified layer, a long lasting reduction in friction and wear can be obtained, for example, when nitrogen is implanted into steel. The main advantage of the ion implantation process is that the treatment is very clean and the deposited layers very thin, hence the tolerances are maintained and the precision of the component is not distorted. Ion implantation is an expensive process since the cost of the equipment and running costs are high [64]. The ion implantation process is schematically illustrated in Figure 9.22. Ions Current Filament: coating element Non-ionized material retained Ion accelerator Ion separator Electrostatic flow controller Raster on substrate Vacuum pump Magnets Ionization FIGURE 9.22 Schematic diagram of the ion implantation process. More detailed information about surface coating techniques can be found in [45,64,65]. Coating Processes Requiring Localized Sources of Intense Heat A localized intense source of heat, e.g. a flame, can provide a very convenient means of depositing coating material or producing a surface layer of altered microstructure. Coating methods in common use that apply this principle are surface welding, thermal spraying and laser hardening or surface melting. · Surface Welding In this technique the coating is deposited by melting of the coating material onto the substrate by a gas flame, plasma arc or electric arc welding process. A large variety of materials that can be melted and cast can be deposited by this technique. During the welding process a portion of the substrate surface is melted and mixed together with the coating material in the fusion zone resulting in good bonding of the coating to the substrate. Welding is used in a variety of industrial applications requiring relatively thick, wear resistant coatings ranging from about 750 [µm] to a few millimetres [64]. Welding processes can be easily automated and are capable of depositing coatings on both small components of intricate shape and large flat surfaces. TEAM LRN SOLID LUBRICATION AND SURFACE TREATMENTS 433 There is a variety of specialized welding processes, e.g. oxyfuel gas welding (OGW), shielded metal arc welding (SMAW), submerged arc welding (SAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), etc., which are described in detail in [e.g. 64]. A schematic diagram of the typical welding process is shown in Figure 9.23. Completed weld Parent metal Products o f combustion protect weld pool Filler wire FIGURE 9.23 Schematic diagram of the welding process. · Thermal Spraying This is the most versatile process of deposition of coating materials. During this process the coating material is fed to a heating zone where it becomes molten and then is propelled to the pre-heated substrate. Coating material can be supplied in the form of rod, wire or powder (most commonly used). The distance from the spraying gun to the substrate is in the range of 0.15 to 0.3 [m] [64]. The molten particles accelerated towards the substrate are cooled to a semimolten condition. They splatter on the substrate surface and are instantly bonded primarily by mechanical interlocking [64]. Since during the process a substantial amount of heat is transmitted to the substrate it is therefore water cooled. There are a number of techniques used to melt and propel the coating material and the most commonly applied are: flame spraying, plasma spraying, detonation-gun spraying, electric arc spraying and others. Flame Spraying utilizes the flame produced from combustion gases, e.g. oxyacetylene and oxyhydrogen, to melt the coating material. Coating material is fed at a controlled rate into the flame where it melts. The flame temperature is in the range of 3000 to 3500°C. Compressed air is fed through the annulus around the outside of the nozzle and accelerates the molten or semimolten particles onto the substrate. The process is relatively cheap, and is characterized by high deposition rates and efficiency. The flame sprayed coatings, in general, exhibit lower bond strength and higher porosity than the other thermally sprayed coatings. The process is widely used in industry, i.e. for corrosion resistant coatings. A schematic diagram of this process is shown in Figure 9.24. Plasma Spraying is different from the plasma-based coating methods described previously since the coating metal is deposited as molten droplets rather than as individual atoms or ions. The technique utilizes an electric arc to melt the coating material and to propel it as a high-velocity spray onto the substrate. In this process gases passing through the nozzle are ionized by an electric arc producing a high temperature stream of plasma. The coating material is fed to the plasma flame where it melts and is propelled to the substrate. The temperature of the plasma flame is very high, e.g. up to 30,000°C and can melt any coating TEAM LRN 434 ENGINEERING TRIBOLOGY material, e.g. ceramics [89]. The highest temperatures are achieved with a monoatomic carrier gas such as argon and helium. Molecular gases such as hydrogen and nitrogen produce lower plasma temperatures because of their higher heat capacity. Therefore plasma spraying is suitable for the rapid deposition of refractory compounds which are usually hard in order to form thick hard surface coatings. The very high particle velocity in plasma spraying compared to flame spraying results in very good adhesion of the coating to the substrate and a high coating density. The application of an inert gas in plasma spraying gives high purity, oxides free deposits. Although it is possible to plasma spray in open air the oxidation of the heated metal powder is appreciable and the application of inert gas atmosphere is advantageous. The quality of coating is critical to the wear resistance of the coating, i.e. adhesion of the coating to the substrate and cohesion or bonding between powder particles in the coating must be strong. These conditions often remain unfulfilled when the coating material is deposited as partially molten particles or where the shrinkage stress on cooling is allowed to become excessive [67]. Plasma spraying is commonly used in applications requiring wear and corrosion resistant surfaces, i.e. bearings, valve seats, aircraft engines, mining machinery and farm equipment. A schematic diagram of the plasma spraying process is shown in Figure 9.25. Wire feed Fuel gases and oxygen Compressed air Semimolten spray stream Water-cooled substrate FIGURE 9.24 Schematic diagram of the flame spraying process. Tungsten electrodes Plating Powder feed of coating material Plasma flameSpark Water cooling Water cooling Ar, He, H 2 , N 2 High voltage Substrate FIGURE 9.25 Schematic diagram of the plasma spraying process. Detonation-Gun Spraying is similar in some respects to flame spraying. The mixture of a metered amount of coating material in a powder form with a controlled amount of oxygen and acetylene is injected into the chamber where it is ignited. The powder particles are heated TEAM LRN [...]... Tribology Conference, Tokyo, 8-10, July 198 5, Vol 1, Japan Society of Lubrication Engineers, Tokyo, 198 5, pp 20 9 -21 4 92 A.W Batchelor, N.L Loh and M Chandrasekaran, Lubrication of Stellite at Ambient and Elevated Temperatures by Transfer Films from a Graphite Slider, Wear, Vol 198 , 199 6, pp 20 8 -21 5 93 R Ahmed and M Hadfield, Rolling Contact Performance of Plasma Sprayed Coatings, Wear, Vol 22 0, 199 8,... 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