Friction, Lubrication, and Wear Technology (1997) Part 14 ppsx

Adhesive Wear Adhesive wear occurs when two surfaces slide against each other with intended motion, producing fragments from one surface that adhere to the other (Ref 6). It arises from the strong adhesive forces set up when two materials come into intimate contact. It generally occurs when lubrication is inadequate and results in metal transfer, usually called galling. Soft bearing coatings allow the embedding of abrasive particles and also permit deformation for alignment of bearing surfaces. Adequate lubrication is required. Coatings of this type are generally low in cost, because they wear in preference to the mating surface (Fig. 6a). Fig. 6 Cutaway views of thermal spray coated bearings showing areas that exhibit adhesive wear. (a) Soft bearing material. (b) Hard bearing material The following performance factors apply to soft bearing coatings: • Good lubrication must be provided or the wear rate will be excessive • The coating must be soft enough to trap the many abrasive particles that will be carried by the lubricant • These coatings generally have poor abrasive wear resistance • The inherent nature of therma l spray coatings enhances their usefulness as bearing 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 Coating process (a) Applications Adhesive and abrasive wear Soft bearing coatings: Aluminum bronze OFW, EAW, OFP, PA, HVOF Tobin bronze OFW, EAW Babbitt OFW, EAW, OFP Tin OFW, EAW, OFP Babbitt bearings, hydraulic press sleeves, thrust bearing shoes, piston guides, compressor crosshead slippers Hard bearing coatings: 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 Bumper crankshafts for punch press, sugar cane grinding roll journals, antigalling sleeves, rudder bearings, impeller shafts, pinion gear journals, piston rings (internal combustion); fuel pump rotors Aluminum oxide PA Chromium oxide PA Tungsten carbide PA, HVOF Chromium 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 (oil industry); concrete mixer screw conveyors; grinding hammers (tobacco industry); core 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 PA, HVOF Servomotor shafts, lathe and grinder dead centers, cam followers, rocker arms, piston rings (internal combustion), cylinder liners Aluminum bronze OFW, EAW, PA, HVOF Cu-Ni-In PA, HVOF Fretting: Small-amplitude oscillatory displacement applications at low temperature (<540 °C, or 1000 °F) Cu-Ni PA, HVOF Aircraft flat tracks (air-frame component); expansion joints and midspan supports (jet engine components) Co-Cr-Ni-W PA, HVOF Fretting: Small-amplitude oscillatory displacement applications at high temperature (>540 °C, or 1000 °F) Chromium carbide PA, HVOF Compressor air seals, compressor stators, fan duct segments and stiffeners (all jet engine components) Chromium carbide PA, HVOF Tungsten carbide PA, HVOF WC/Ni-Cr-B-SiC (fused) OFP, HVOF WC/Ni-Cr-B-SiC (unfused) OFP, HVOF Erosion Chromium oxide PA Exhaust fans, hydroelectric valves, cyclone dust collectors, dump valve plugs and seats, exhaust valve seats Cavitation Ni-Cr-B-SiC-Al- Mo PA Wear rings (hydraulic turbines), water turbine buckets, water turbine nozzles, diesel engine cylinder liners, pumps Ni-Al/Ni-Cr-B- SiC PA Type 316 stainless steel PA Ni-Cr-B-SiC (fused) OFP, HVOF Ni-Cr-B-SiC (unfused) HVOF Aluminum bronze PA, HVOF Cu-Ni PA, HVOF (a) OFW, oxyfuel wire spray; EAW, electric arc wire spray; OFP, oxyfuel powder spray; PA, plasma arc spray; HVOF, high-velocity oxyfuel powder spray Hard bearing coatings are highly resistant to adhesive wear. They are used where embeddability and self-alignment are not important and where lubrication is marginal (Fig. 6b). The following performance factors apply to hard bearing coatings: • Lubrication should be good, but is not as important as for soft bearing coatings because the high wear resistance of these materials allows them to withstand momentary unlubricated service • Applications that require hard bearing coatings are usually characterized by high load and low speed • Surfaces should generally be of equal hardness • Although like coatings can be used for sliding against each other, unlike combi nations are frequently used for example, a coating running against a wrought metal. This reduces seizing and scuffing • Wear rate generally increases with temperature Applications for hard bearing 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 materials for this application should generally be based on operating temperature and surface finish requirements. The following performance factors must be considered: • The coating must be hard. In particular, surface hardness should exceed the hardnes s of the abrasive grains present • The most common abrasive is silica (sand), with a hardness of approximately 820 HK. (For comparison, tungsten carbide/cobalt composite is 1400 to 1800 HK; Al 2 O 3 is approximately 2100 HK) • Information about the abrasive ho w often it is replenished, whether it is sharp and brittle, how it breaks down is important in selecting the coating and estimating its performance • If the system is closed, debris created by the wear process will also contribute to the wear rate and thus must also be considered • The coating must exhibit oxidation resistance at the service temperature Applications and recommended materials for coatings resistant to abrasive grains at low and high temperatures are listed in Table 1. Fig. 7 Abrasive wear on the flight surfaces of a helical screw used in spiral conveyor applications Surface Fatigue Wear Repeated loading and unloading cause cyclic stress on a surface, eventually resulting in the formation of surface or subsurface cracks. The surface ultimately fractures and large fragments are lost, leaving pits. This phenomenon can occur only in systems where abrasive and adhesive wear are not present for example, in systems with high surface contact loads. An area of surface must be stressed repeatedly, without constant removal of particles, to fail in fatigue. Fretting, erosion, and cavitation are typical examples of this type of wear. Fretting. Some fretting-resistant coatings resist wear caused by repeated sliding, rolling, or impacting over a track (Fig. 8a). The repeated loading and unloading cause cyclic stresses inducing surface or subsurface cracks. Other coatings resist wear caused when contacting surfaces undergo oscillatory displacement of small amplitude (Fig. 8b). This type of wear is difficult to anticipate, because no intended motion is designed into the system. Vibration is a common cause of fretting. The following performance factors apply to coatings for fretting resistance: • The coating must be resistant to ox idation at the service temperature. If an oxide forms, it must be tough and tenacious; a loosely adherent oxide will cause severe abrasive wear • A surface that is free of stress, particularly tensile stress, is desirable. High- shrink coatings tend to have high surface stress and do not perform as well as low-stress coatings • Brittle coatings fail rapidly. Tough coatings tend to perform better • Coatings with hard particles distributed in a soft matrix are generally the most durable Applications and recommended materials for fretting-resistant coatings are listed in Table 1. Fig. 8 Two types of 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 mechanism is closely analogous to abrasion. When the angle of impingement is normal to the surface, material is displaced by plastic flow or is dislodged by brittle failure. The following performance factors apply to erosion-resistant coatings: • If the angle of particle impact is less than 45°, the coating selected should be harder and more abrasion resistant • If the angle of particle impact is greater than 45°, the coating should be softer and tougher • At high service temperatures, coatings should have high hot hardness and oxidation resistance at temperatures and environments ranging from 540 to 815 °C(1000 to 1500 °F) • When the carrier is liquid, the corrosion resistance of the coating must be considered Applications and recommended materials for coatings used to resist particle erosion are listed in Table 1. Fig. 9 Typical erosive wear of a fan blade assembly generated by the high- velocity impingement of a gas or liquid on the blade surface Cavitation is caused by mechanical shock that is induced by bubble collapse in liquid flow (Fig. 10). Materials that resist fretting-type surface fatigue are resistant to cavitation. The most effective coating properties are toughness, high wear resistance, and corrosion resistance. The following performance factors apply to cavitation-resistant coatings: • Relative motion between a liquid and metal surface, including bubble generation and bubble collapse, must exist for cavitation to occur • Liquids will penetrate sprayed coatings unless fused; therefore, all coatings should be sealed • Selection of a coating must be influenced by its resistance to the liquid used in a particular application • Hardness is an important factor, but coatings must also be tough. Brittle coatings fail quickly • Coatings that work harden are especially resistant to the repeated pounding of cavitation Applications and recommended materials for coatings resistant to cavitation are listed in Table 1. Fig. 10 Typical surface fatigue wear produced by cavitation in a pump impeller component References 1. E.R. Novinski, Application of Thermal Spray Processes to Conserve Critical Materials, Workshop on Conservation and Substitution Technology for Critical Materials, Conference Proceedings, Vanderbilt University, 16 June 1981 2. H. Herman, Plasma Sprayed Coatings, Sci. Am., Sept 1988 3. H.S. Ingham and A.P. Shepard, Flame Spray Handbook, Vol III, Perkin-Elmer Corp., Metco Division, 1965 4. G.L. Kutner, Thermal Spray by Design, Adv. Mater. Proc. Met. Prog., Oct 1988, p 67 5. M.L. Thorpe, Thermal Spraying Becomes a Design Tool, Mach. Des., 24 Nov 1983 6. F.N. Longo, Handbook of Coating Recommendations, Perkin-Elmer Corp., Metco Division, 1972 Electroplated Coatings Rolf Weil and Keith Sheppard, Stevens Institute of Technology Introduction IT IS OFTEN NECESSARY to coat a material that is subject to friction and, possibly, wear. The coating can be tailored to the tribological demands of the environment and can provide a wider choice in selecting a base material to meet special requirements, such as strength or low cost. Like other of surfaces used in tribological applications, electroplated coatings have two primary categories. Hard coatings are normally used to resist many forms of wear, such as those that involve abrasive, adhesive, and erosion process. Some degree of toughness in these coatings is often desirable, in order to resist cracking. Soft coatings are sometimes used on bearing surfaces to provide low shear strength. They are typically used at ambient temperatures and low loads. An additional consideration in coating selection is a requirement to provide corrosion protection. Corrosive wear places particularly severe demands on the protective abilities of such a coating, because the thin surface film that is primarily responsible for limiting the corrosion kinetics is continually being worn away. Coatings that are used to control friction and wear can be electrochemically deposited either with or without an externally applied current. Deposition without an external current is called electroless plating. For many wear applications, electrochemical deposition is the most rapid and economical means to apply coatings ranging from 10 to 500 m (0.4 to 20 mils) in thickness (Ref 1). Adhesion to the substrate is a very important requirement of all coatings. Because adhesion primarily depends on the cleanliness of the substrate surface to be coated, proper pretreatment of the substrate is usually necessary. Deposition Fundamentals Electroplating is the coating of an electrically conducting surface by application of an electrical potential in a suitable solution that contains the ions of the metals to be deposited. The electrode to be coated is the cathode. The counter- electrode, the anode, can be of the soluble type, so that it supplies metal ions to the solution. Alternatively, the anode can be insoluble, in which case the ions of the metals to be deposited must be continuously or periodically added to the plating solution to compensate for the depletion. The deposition rate depends primarily on the current density. If all supplied electrons reduce the metal ions, then the deposition rate can be readily calculated from Faraday's law, which states that 96,500 coulombs (1 Faraday) deposit 1 gram (0.035 oz) equivalent weight (atomic weight divided by valence). However, hydrogen evolution or other secondary reactions may use some of the current supplied. Thus, only a fraction of the supplied electrons reduces the metal ions. In this case, the plating efficiency is less than 100%. (The plating efficiency is the ratio of the metal yield to that calculated from Faraday's law, which assumes that there are no other reactions.) The microstructures of electrodeposits, which to a large extent determine their properties, depend on a number of factors. These factors include the microstructure of the surface to be coated, the plating conditions (that is, the current density), the temperature and composition of the plating solution, as well as the degree and type of agitation. The composition and the pH of the plating solution in the vicinity of the cathode (which generally differs from that of the bulk) can have a large effect on the structure and properties of the deposit. In the absence of significant surface inhibition, it is possible for the deposit to reproduce the structure of the substrate surface. However, even minute quantities of certain substances can greatly affect the structure. These substances may be intentionally added to the plating solution or be present as impurities from the water, chemicals, or secondary reactions. Foreign substances can alter the grain orientation by being preferentially adsorbed on certain crystal planes. Then, grains with other planes exposed on their surfaces can grow preferentially until they essentially compose the entire deposit, thereby producing a fiber texture, that is, most of the grains have same crystallographic direction normal to the surface. The adsorption of some foreign material can greatly reduce the grain size. These materials impede grain growth, thereby requiring almost continuous renucleation. Some addition agents are present in plating solutions to level the deposit surface, that is, to make it smoother than the substrate. They do so by being preferentially adsorbed on asperities. By blocking growth on the asperities, the recesses receive most of the depositing metal, thereby producing a smoother surface. High internal stresses, particularly of the tensile type, can adversely affect the wear properties of electrodeposits. They can arise from the lattice misfit between the substrate and an epitaxial deposit, from the coalescence of crystallites, or from the diffusion of hydrogen out of the surface layers. These stresses can be large enough to cause cracking, as is the case in some chromium deposits. Most electrodeposits are also characterized by a high dislocation density, which results in strength, hardness, and ductility values that are similar to those of the same metal in the cold-worked condition. Pulse plating has been used to enhance the tribological properties of electrodeposits. Generally, a square-wave current pulse is employed. The current is on for a certain time period and off for another. The ratio of the time to the sum of the on and off times is called the duty cycle. By varying the pulse frequency, the duty cycle, and the current density, the structure and properties of electrodeposits can be beneficially changed. Electroless plating is an autocatalytic reaction in which a reducing agent supplies an internal current. The advantages of electroless plating are that nonmetallic substrates can be coated and the deposit thickness is uniform. The autocatalytic reaction has to be activated on some surfaces, particularly those that are nonmetallic. Thus, the areas to be plated can be controlled by specific activation. The disadvantages are a slow deposition rate and a higher cost. It is also necessary to maintain the composition of the plating solution by continuous or frequent additions of the depleted chemicals and removal of the impurities. Wear-resistant composite can be produced by the codeposition of fine particulate matter. Hard particles, such as diamond, silicon carbide, and aluminum oxide, are kept in suspension by agitation and become occluded in the deposit. Solid lubricants, such as polytetrafluoroethylene (PTFE) particles, have also been codeposited. As much as 30 vol% of the particles can be attained in both electrodes and electroplated coatings (Ref 2). In some applications, only limited areas of a component are subjected to wear. If the deposition of precious metals is involved, then it may be economically desirable to coat these areas selectively. Such limited coverage can be achieved by jet plating, laser-enhanced and laser-jet plating, and by physically masking off areas not to be plated, as in photolithography (Ref 3). In jet plating, a fine jet of the plating solution is directed onto the areas to be coated. Because the current is constrained within the jet, only the areas where it is applied become coated. The jet also provides rapid ion transport to the depositing surface and therefore permits to the deposition rates. In laser-enhanced plating, the heating effect of the laser enhances mass transport locally. It can also influence the deposition kinetics such that metal is not plated outside of the heated area. Laser-jet plating, which is a combination of jet and laser-enhanced plating, can provide improved deposit characteristics. The jet then acts as a waveguide for the laser. The principal electrochemically deposited materials used in tribological applications are chromium, nickel, and both precious and soft metals. The characteristics of each type of deposit are described in this article. References 4, 5, 6, 7, 8, 9, 10, 11 provide additional information relevant to electrochemically deposited metals and alloys used in tribological applications. Chromium Hard chromium coatings are widely used because of their low coefficient of friction and good wear properties. They are deposited at higher temperatures and current densities than decorative chromium. The plating solution for hard chromium has a lower ratio of chromic oxide to sulfuric acid (the main constituents of the solution) than that used for decorative coatings. The thickness of hard chromium deposits varies from about 0.1 to 100 m (0.004 to 4 mils), whereas decorative thicknesses usually range from about 0.1 to 0.2 m (4 to 8 in.). Very strict antipollution to regulations govern the discharge of solutions containing hexavalent chromium ions, which are used for hard-chrome plating. Therefore, plating solutions that contain mostly trivalent chromium ions are of interest. These solutions generally contain formic acid or one of its salts. Carbon will deposit in these solutions. The deposits can therefore be heat treated to precipitate a chromium carbide. The hardness of hard chromium varies from about 900 to 1100 on the Knoop and Vickers hardness scales. These values are considerably higher than the hardness of bulk chromium. Deposits from trivalent solutions are softer than those that are plated from hexavalent chromium solutions. However, after heat treating at about 700 °C (1290 °F), a hardness comparable to that of hard chromium can be achieved (Ref 12). Chromium deposits are characterized by high internal tensile stresses that can reach 1000 MPa (145 ksi). These stresses can reduce the fatigue properties of coated components. Hydrogen is also codeposited with chromium and can diffuse into components, causing hydrogen embrittlement. Heat treatments are typically required to relieve the stresses and hydrogen effects, but can reduce the hardness. The coefficients of friction of hard chromium against hard materials are generally the lowest of any electrochemically deposited coatings. The actual values vary considerably, depending on the test method, the mating surfaces of the materials, and the degree of lubrication. Some values of static and sliding coefficients of friction are listed in Table 1. The static coefficient is calculated from the force to initiate movement of one component of a couple against the other. The force to maintain movement enters into the calculation of the sliding coefficient. It is important to note that only coefficients of friction obtained under the same conditions can be compared. The values should not be considered as absolute. Table 1 Coefficients of friction Couple Static coefficient Sliding coefficient Chromium-plated steel versus itself 0.14 0.12 Chromium-plated steel versus steel 0.15 0.13 Steel versus steel 0.30 0.20 Source: Ref 13 The wear rates of hard chromium can vary greatly, depending, again, on the type of test. The rates also vary with the mating material and whether adhesive or abrasive wear predominates. Some dry abrasive wear data from a Taber abrasion test for three chromium deposits labeled CrA, CrB, and CrC are shown in Fig. 1. The Taber test measures, for a certain number of cycles, the weight loss that results from abrasion with resilient, abrasive wheels at a load of 9.8 N (11 kgf). Figure 1 shows that deposit CrC had less wear than deposit CrA. Fig. 1 Effect of number of cycles on mass loss in the Taber abrasion test for uncoated steel substrate (Fe), three chromium deposits (CrA, CrB, CrC), and three electroless nickel deposits: as- plated nickel (EN), heat treated at 400 °C (750 °F) (EN400), and heat treated at 600 °C (1110 °F) (EN600). Source: Ref 1 Figure 2 represents data obtained in a Falex wear test for the same three chromium deposits. In this test, a pin is rotated between two V-shaped blocks. Deposit CrA showed less wear than deposit CrC, illustrating the effect of a lubrication test method on the results. Figure 1 also shows that the chromium coating improved the abrasion resistance of the steel substrate. The hardest chromium deposits do not necessarily exhibit the least wear. The low friction coefficients and good wear properties of chromium have been attributed to a self-healing Cr 2 O 3 film that forms on the surface. In general, hard chromium has a lower wear rate than either electroplated or electroless nickel, which are the two competing materials. This effect is also illustrated in Fig. 1 and 2. [...]... in wear applications include aluminum piston heads, aircraft engine shafts, components of gas turbines and engine mounts in the aircraft industry, and such automotive parts as differential pinion ball shafts, fuel injectors, ball studs, disk brake pistons, transmission thrust washers, knuckle pins, and hose couplings In mining applications and associated material-handling equipment, where abrasive wear. .. Plating: Fundamentals and Applications, G.O Mallory and J.B Haydu, Ed., American Electroplaters and Surface Finishers Society, 1990, p 269-288 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 87-17, The Electrochemical Society, 1987, p 13-41 4 Metals Handbook, 9th ed., Surface... Cleaning, Finishing and Coating, Vol 5, American Society for Metals, 1982 5 F.A Lowenheim, Ed., Modern Electroplating, 3rd ed., John Wiley and Sons, 1974 6 L Durney, Ed., Electroplating Engineering Handbook, 4th ed., Van Nostrand Reinhold, 1984 7 J.K Dennis and T.E Such, Nickel and Chromium Plating, Butterworth, 1972 8 G.O Mallory and J.B Haydu, Ed., Electroless Plating: Fundamentals and Applications,... Electroplaters and Surface Finishers Society, 1990 9 J.Cl Puippe and F Leaman, Ed., Theory and Practice of Pulse Plating, American Electroplaters and Surface Finishers Society, 1986 10 W.H Safranek, Ed., The Properties of Electrodeposited Metals and Alloys: A Handbook, 2nd ed., American Electroplaters and Surface Finishers Society, 1986 11 Electrodeposited Coatings Database, American Electroplaters and Surface... Connectors and Contact Interface Metallurgy, Electrochemical Technology in Electronics, L.T Romankiw and T Osaka, Ed., Proceedings, Vol 88-23, The Electrochemical Society, 1988, p 161-170 24 M Antler, The Tribology of Contact Finishes: Mechanisms of Friction and Wear, Plat Surf Finish., Vol 75 (No 10) 1988, p 46 25 B.M Luce and D.G Foulke, Silver, Chapter 14, Modern Electroplating, 3rd ed., John Wiley and. .. abrasion test dc, direct current Source: Ref 14 Chromium is widely used for wear resistance in automotive and aircraft components, such as pistons and shock absorbers (Ref 13) Other applications include coatings on drills, taps, dies, extrusion screws, and rolls The wear resistance of gun barrels can also be improved by chrome plating Salvaging of worn parts by chromium electrodeposition is an important... better withstand wear at elevated temperatures and are therefore more widely used under these conditions Coefficients of friction of electroless nickel in the as-deposited condition (EN) and heat treated at 400 °C (750 °F) (EN400) and at 600 °C (1110 °F) (EN600) are listed in Table 2 They are compared to the three chromium alloys depicted in Fig 1 and 2 The counter surfaces were diamond and plain carbon... pulse plating and deposition temperature on the wear rate of chromium deposits are shown in Fig 3 The improve wear resistance at higher pulse frequencies and temperatures corresponds to increases in hardness The wear resistance in some applications can also be improved by the inclusion in the deposit of hard particles or those of a solid lubricant (Ref 15) Fig 3 Effect of pulse frequency and solution... 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 87-17, The Electrochemical Society, 1987, p 179-190 23 A.D Knight and. .. electrodeposition technology, along with the provision of a self-lubricating surface (Ref 3) It is also apparent that the need to maintain submicron spacing between the magnetic layer and the heads in advanced storage systems requires much better control of the electrochemical plating processes than is generally exercised References 1 D.T Gawne and U Ma, Friction and Wear of Chromium and Nickel Coatings, Wear, . D.T. Gawne and U. Ma, Friction and Wear of Chromium and Nickel Coatings, Wear, Vol 129, 1989, p 123 2. N. Feldstein, Composite Electroless Plating, Electroless Plating: Fundamentals and Applications, . 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,. Applications and recommended materials for fretting-resistant coatings are listed in Table 1. Fig. 8 Two types of fretting wear. (a) Stem and seat wear caused by the intended cyclic up -and- down