Table 3 PROPERTIES REQUIRED BY SOLID LUBRICANTS To Provide Low-Friction and Wear Thin films Self-lubricating materials Good film-forming ability (powders) Ability to form transfer films Ductility Low-moderate elastic modulus Good adhesion to substrate Adequate strength for required load capacity Film continuity Low-Shear Strength General High thermal/oxidative/hydrolytic stabilities High softening/melting points Chemically inert High-thermal conducltvity/diffusivity Corrosion protection of substrate Appropriate electrical conductivity No abrasive impurities Low toxicity/environmental compatibility Low-thermal expansion electrical contacts, it is being increasingly supplanted by MoS 2 for three reasons. First is the wide variability in graphites from different sources; MoS 2 quality is more rigidly con- trolled by specifications. Second, the low friction of MoS 2 does not depend on adsorbed vapors and is, in general, lower in vacuum than in air. Finally, the load-carrying capacity of MoS 2 is generally superior. MoS 2 has a lamellar structure, but with interlamellar bonding being between adjacent layers of S atoms. The bonding is relatively weak, via Van der Waals forces only, and MoS 2 is therefore an intrinsic solid lubricant. Adsorbed vapors usually increase friction but the effects are comparatively small. The thermal stability of MoS 2 in nonoxidizing environments is of the order of 1100°C, but in air oxidation begins to become significant at around 350 to 400°C. The normal air-oxidation product is MoO 3 , once believed to be abrasive but now known to be virtually innocuous. 10 A major concern with MoS 2 is the presence of abrasive impurities. 11 The reasons for concern are twofold. First and foremost, chemical analyses provide no information about the form of the impurity; abrasion by hard particles, such as SiO 2 , depends greatly on their shape and size. Second, other factors in addition to impurities can play a role in abrasiveness, e.g., crystallite modifications or anisotropy in hardness. Other Lamellar Solids The only dichalcogenides, other than MoS 2 , with real promise appear to be sulfides and selenides of Mo, Ta, W, and Nb. Since these are synthesized directly from the elements, the compositions are not always stoichiometric and the crystal structure not wholly hexagonal. Some compounds are nevertheless superior to MoS 2 in two main areas. TaS 2 , TaSe 2 , and WS 2 have greater oxidation stability while TaS 2 , TaSe 2 , and NbSe 2 have much greater electrical conductivity. 12 Experimental determinations of frictional properties and endurance of surface films are somewhat conflicting, but no synthetic dichalcogenides appear to be consistently superior to MoS 2 . Together with uncertainties about composition and high expense, this has precluded their widespread use. WS 2 and NbSe 2 have found limited application in self-lubricating composites. 272 CRC Handbook of Lubrication Copyright © 1983 CRC Press LLC The most recent synthetic solid lubricant to receive serious attention is poly (carbon monofluoride), usually referred to as graphite fluoride, (CF x ) n . 13 Prepared by direct elemental synthesis, differing reaction temperatures lead to variation in x from about 0.25 to 1.1. Friction is largely independent of composition for x >0.6, but film endurance increases monotonically until x reaches about 1.0. 14 Thermal stability depends markedly on compo- sition and ranges from 200 to over 500°C. 15 In comparison to graphite, graphite fluoride is distinctly superior in a number of respects. For burnished films of powder, the load-carrying capacity of (CF x ) n is greater, 16 wear life is longer, 13 and effective lubrication occurs in both vacuum 17 and inert gases. 18 Comparisons with MoS 2 , however, are less favorable; (CF x ) n is variously reported as being superior 14 or not, 15 depending on the lest method and on the formulation of the lubricant film. As a very general summary, (CF x ) n appears to offer little over MoS 2 in most applications. A number of other lamellar solids with crystal structures of the CdI 2 or CdCl 2 type also form coherent surface films from powder and exhibit low friction. However, their thermal, oxidative, and hydrolytic stabilities are generally much inferior to those of MoS 2 . BN, similar in crystal structure to graphite, seems to be largely ineffective as a high-temperature lubricant due to its inability to form surface films. Metal Salts Numerous other inorganic salts with low shear strength and film-forming ability have shown promise as solid lubricants. The main interest is in their high-temperature potential, and PbO and CaF 2 are particularly important. PbO provides effective thin film lubrication from room temperature to about 350°C, and again from 500°C upwards. Between these temperatures, however, it oxidizes in air to Pb 3 O 4 , which has poor lubricating properties. Attempts to bridge this gap have been made by addition of SiO 2 to form a silicate phase containing excess PbO which is then protected against oxidation. 19 With this mixture, lu- brication is possible between 250 and 700°C, but below 250°C friction becomes high, and film endurance low. CaF 2 and eutectic mixtures of CaF 2 /BaF 2 also provide effective lubri- cation in the range 250 to 1000°C; high friction (f>0.3) below 150°C can be partially alleviated by the addition of Ag. 20 A series of metal oxides, tungstates, and molybdates also show promise as high-temperature lubricants, with reasonably low-friction coefficients at 700°C, e.g., f ~ _ 0.2 (MoO 3 , K 2 MoO 4 ) and f ~ _ 0.3 (Co 2 O 3 , NiMoO 4 ). None, however, are effective at room temperature. Synthetic mixed metal sulfides, e.g., AsSbS 4 ,Ce 2 (MoS 4 ) 3 , are claimed to increase the load-carrying capacity of greases more than comparable additions of MoS 2 . 21 Their per- formance as solid films, however, is inferior to that of MoS 2 . Reaction Films The ability of oxide and other reaction films on metals to prevent intermetallic contact and reduce wear, and sometimes friction, is well known. Coefficients of friction of oxide films are not particularly low (0.4 to 0.8), but during continuous sliding at high temperature, increased rates of oxidation can combine with substrate softening and plastic flow to generate a complex, oxide-rich, surface layer which may greatly reduce wear. 22 Deliberate introduction of readily oxidizable alloying elements, e.g., Si or Fe, into Ni-alloys enhances the production of such layers. Soft Metal Films Several low shear strength metals can be deposited as lubricating films on harder substrates by conventional electroplating or by newer techniques of vacuum deposition — evaporation, sputtering, ion-plating. Most metals of interest — In, Pb, Sn, Ag, Au, Cu, Zn, T1, Ba, and Bi have low-solid solubility in Fe. Thin metal-film lubrication is most relevant to high Volume II 273 Copyright © 1983 CRC Press LLC Most elements into most metals via ion-bombardment temperatures or to applications where sliding is limited, e.g., rolling element bearings. Ag- Pd films have been used at temperatures up to 1000°C, and Pb films have been very successful for long-term rolling bearing lubrication in space mechanisms. 23 Au is also of interest in the latter application, but test results have proved extremely variable. Vacuum sputtering and ion-plating permit close control of film composition and thickness and can provide outstanding adhesion to the substrate. Optimum film thickness for maximum wear life is generally very similar to that required to give minimum coefficient of friction, 0.1 to 1 µm. Diffusion Coatings An alternative to deposition of a surface film for reducing friction and wear of metals is the thermal diffusion of foreign atoms into a surface. Some commonly available treatments of this type, fisted in Table 4, have different objectives: to increase wear resistance by increasing surface hardness (C,N in steels), to produce a low-shear strength surface to inhibit scuffing or seizure (S in steels), or to provide either of the above in conjunction with increased corrosion-resistance (Sn-Cu in steels). Analogous to diffusion treatments, although not involving high bulk temperatures, is the recently developed “ion-implantation” in which surfaces are bombarded with ions of the element of interest accelerated to high energies. The surface usually increases in hardness and also develops a compressive stress which improves fatigue resistance. Although depth of penetration is small, ~ _ 100 nm or less, beneficial effects on wear appear to persist long after removal of material to this depth. 24 274 CRC Handbook of Lubrication Table 4 SOME SURFACE TREATMENTS TO REDUCE FRICTION AND/OR WEAR OF METALS Diffusion Treatments Non-Implantation Copyright © 1983 CRC Press LLC Chemical conversion coatings listed in Table 4 comprise “built-up” films produced by reactions in salt solutions. Thicknesses are typically 2 to 25 µm. The films are porous and are most important in the present context as substrates on which to deposit lubricating solids. Without additional lubrication, solid, or liquid, they are of little value for long-term reduction in friction and wear of metals. Polymers Polymers are used for solid lubrication in three main ways: as thin films, as self-lubricating materials, or as binders for lamellar solids. PTFE is outstanding in this group and, in thin film form, can exhibit lower friction than any other known polymer (~0.03 to 0.1). Its other main advantages are effectiveness over a wide temperature range, – 200 to + 250°C, and general lack of chemical reactivity. The low friction of PTFE is attributed to the smooth molecular profile of the polymer chains which, after orientation in early stages of sliding, can then slip easily over each other. 25 PTFE films are conventionally produced by spraying followed by sintering at temperatures above 325°C. Coating formulations are also available in which PTFE particles are bonded with a synthetic resin curing at a lower temperature. Arecent technique of radio-frequency sputtering can produce very uniform, thin films with excellent adhesion to metals. 26 Since load-carrying capacity and endurance of PTFE films on metals are generally inferior to those of the best MoS 2 coatings, and low thermal conductivity limits the maximum speed, they tend to be used mainly in moderate conditions of sliding or where contamination by MoS 2 might create problems. Anti-stick coatings in food processing equipment and in plastics molding are major areas. The only other polymers widely used as thin-film lubricants are the polyimides. 27 Their maximum useful temperature for long-term use, ~ _ 300°C, exceed that of PTFE but the frictional properties are inferior, f ~ _ 0.13 to 0.3. By far the greatest use of polymers in solid lubrication is in self-lubricating composites as direct replacements for lubricated metals. 28,29 Of the hundreds of polymers commercially available, the few finding widespread use as self-lubricating materials are listed in Table 5. Reinforcing fibers, fillers, and additives commonly incorporated to improve particular prop- Volume II275 Table 5 PLASTICS AND FILLERS FOR SELF-LUBRICATING COMPOSITES Copyright © 1983 CRC Press LLC erties are also given. PTFE almost invariably requires reinforcement when used in bulk as it is extremely susceptible to viscoelastic deformation under load. Reinforcements are also commonly used with some thermosetting resins, e.g., phenolics, to increase toughness. Friction and wear properties of the latter are improved by addition of lamellar solids such as MoS 2 , or PTFE powder or flock. Some additives can also be multifunctional. A good example is graphite which, particularly in fiber form, not only reduces friction and wear but also increases the strength, stiffness and thermal conductivity of polymer composites. FRICTION AND WEAR TESTING Three separate objectives are involved in performance-testing solid lubricants and self- lubricating materials: provision of design data, selection or development of materials, and quality control. Unfortunately, reliable design information is available only from tests either in the intended application or in a very close laboratory simulation. For materials selection, development and quality control, however, a variety of accelerated test procedures can be used, 30 and the most common are illustrated in Figure 1. With tests involving nonconformal geometry (Figures la to e), thin-film solid lubricants are usually applied to the larger, rotating surface because this makes the greatest contribution to the total wear life. With conformal geometries, both surfaces are usually coated. The wear life of thin-film lubricants is obtained by determining the time or sliding distance before the coefficient of friction rises to some arbitrarily fixed value such as 0.2. Amount of wear is seldom measured per se, although an average wear rate can be inferred from the film thickness and time to failure. Load- carrying capacity is frequently found by increasing the applied load in increments until failure occurs, either by increased friction (thin films), by greatly increased wear, or by excessive temperature rise (self-lubricating composites). Relative ratings of different materials may vary significantly between one test and another. One attempt to provide a basis for comparison suggests that the fundamental parameter affecting wear life is the number of cycles of compression/flexure to which each element 276 CRC Handbook of Lubrication FIGURE 1. Wear-testing apparatus for solid lubricants. Initial point contact: (a) four-ball; (b) hemisphere on disc (may be 3 pins). Initial line contact: (c) block on ring (Timken, LFW1); (d) Reciprocating pad on ring; and (e) Falex. Conforming contact: (f) journal bearing (Almen-Wieland); (g) thrust bearing (LFW3); and (h) Press-fit (LFW4). a e fgh b c d Copyright © 1983 CRC Press LLC of the film is subjected. 31 Even in very carefully controlled conditions, repeat determinations of wear life can show considerable scatter. With the Timken apparatus, Figure 1e, scatter in wear life determinations can exceed ±100%. With Falex tests, Figure 1e, scatter is usually less than ±50%. Falex tests are commonly incorporated into specification require- ments for thin film lubricants. The four-ball machine, Figure la, is widely used for evaluating solid lubricant additives in oils; the pin/disc and pin/ring arrangements (Figures 1b to d) are used for wear testing self-lubricating composites as well as thin film lubricants; reciprocating line-contact arrange- ments (Figure 1d) show promise for wear testing thin, self-lubricating, bearing-liner ma- terials; 32 the press-fit test (Figure 1h) is used for dry powders and rubbed films and the journal and thrust-bearing configurations (Figures 1f and g) simulate bearing applications for both thin films and self-lubricating composites. OPERATIONALPERFORMANCE Thin Film Lubricants Rubbed Films The simplest way to coat a solid lubricant on a metal surface is by burnishing of dry powder (MoS 2 , graphite, etc.) with a soft tissue. MoS 2 films produced in this way range from 0.1 to 10 µm thick, depending on rubbing time. Film thickness also increases with increasing humidity. 33 Bonding of lamellar solids to the substrate appears to involve three mechanisms: (1) particles can be physically trapped within surface depresssions, (2) crys- tallites may be mechanically embedded into the substrate and act as nuclei around which film growth occurs via intercrystallite cohesion, and (3) the lubricant may interact chemically with the substrate. The importance of the last component is supported by observations that effectiveness of MoS 2 film formation on different metals correlates with the strength of the metal-sulfur bond. 34 Behavior of rubbed MoS 2 films shows some general trends with operational parameters. Friction rises with increasing relative humidity, 35 possibly as a result of increased hydrogen bonding between adsorbed water molecules. Initial reduction in friction with increasing temperature can be attributed to desorption of water vapor, but reduction in wear life as temperatures rise above 200°C is more probably a consequence of increasing oxidation of the MoS 2 . Effects of substrate roughness on wear life are consistent with the idea that mechanical entrapment of particles plays a major role in film formation; if the topography is very smooth, little lubricant is contained within the surface depressions, but if the surface is very rough metal peaks may protrude through the lubricant film. Relation of wear life to substrate hardness involves an uncertain trend. 36,37 The possibility that MoS 2 might induce corrosion of ferrous substrates in humid environ- ments has been the subject of much controversy. Oxidation of MoS 2 is accelerated by moisture, and after prolonged storage of powder in air at room temperature, MoO 3 , adsorbed H 2 O, and H 2 SO 4 can all be present as surface contaminants. For this reason, pH limits of aqueous extracts from MoS 2 powder are required by most specifications, 38 or a direct cor- rosion test. 39 MoS 2 powder is commonly protected against oxidation during storage either by adsorption of long chain organic inhibitors or by enclosure in an inert gas atmosphere. Bonded Coatings To overcome the dependence of burnished film thickness on relative humidity, and to obtain greater film thickness and wear lives, lamellar solids are often incorporated within a synthetic resin binder to produce a “bonded coating”. An enormous number of coating formulations has been developed 40 and some of the more widely used constituents are listed in Table 6. MoS 2 is by far the most common. Relevant specifications are given in Table 7. Volume II 277 Copyright © 1983 CRC Press LLC With the possible exception of polyimides, most binders have intrinsically poor frictional properties and the optimum lubricant to binder ratio usually ranges from 1:1 to 4:1. High ratios minimize friction while low ratios maximize wear life. Other additives can also be included in the coating. Sb 2 O 3 generally increases the wear life of MoS 2 coatings when added at a concentration of around 30% by weight, and is believed to function as a sacrificial antioxidant. Inhibitors, such as dibasic lead phosphite, reduce substrate corrosion and other metal sulfides can increase wear life. Graphite additions increase wear life but are falling into disfavor because of possible electrochemical corrosion. Bonded coatings are generally applied from dispersions in a volatile solvent by spraying, brushing, or dipping. Spraying is usually the most consistent, but dipping is widely used because of low cost. Recommended thicknesses range from 5 to 25 µm, but even thicker coatings may be useful in low-stress applications. Surface pretreatment is essential both to remove organic contamination and to provide a suitable topography for mechanical “key- ing”. Optimum roughness depends on the finishing process used: abrasion 0.5 µm Ra, grit- blasting 0.75 µm Ra, grinding 1.0 µm and turning 1.25 µm Ra. An alternative, or additional, pretreatment is phosphating for steels and analogous chemical conversion treatments for other metals. It is more difficult to generalize performance trends for bonded coatings than for rubbed films of lamellar solids because their properties depend on the type of binder and on the test method, in low stress conditions wear life usually increases with film thickness but at high stresses the reverse may occur. 41 Sliding speed usually has little effect on either friction or wear until it becomes so high that frictional heating begins to soften or degrade organic resin binders. The most important variable is temperature. With organic binders, wear life tends to decrease with increasing temperature but with inorganic binders the converse is sometimes observed because of low-temperature brittleness. Probably best all-round per- formance over the widest temperature range is given by formulations incorporating high- temperature resin binders such as polyimides. Binder properties may also affect the way in which wear life depends on relative humidity. Significant reductions in both wear life and load-carrying capacity of solid lubricant films occur in the presence of conventional oils. 42 In some cases the reduction in performance is a consequence of the resin binder being attacked by certain fluids, e.g., acrylics by chlorinated organic solvents. More generally, fluids tend to cause adhesion failures at the substrate interface and also impede reaggregation of lubricant debris produced during wear. Despite these reductions in performance, some MoS 2 -bonded coatings persist sufficiently long in the presence of oils to facilitate running-in, 43 and to reduce tool wear during machining operations. 44 The most promising high-temperature coatings are those incorporating CaF 2 /BaF 2 eutectic. These may be applied by spraying from dispersions, followed by fusing at around 1000°C, or bonded with metal salts such as monoaluminum phosphate. 45 Thicker coatings, 0.1 mm upwards, can be produced by plasma-spraying mixtures of CaF 2 /BaF 2 with metals, oxides, or graphite, followed by machining and a final heat treatment to enrich the lubricant phase in the surface. 46 Applications include seals for gas turbine regenerators and high-temperature air-frame bearings. Thin coatings of mixed fluorides have also been used on retainers of ball bearings for hostile environments. 47 For cryogenic applications, bonded coatings con- taining either MoS 2 or PTFE are generally satisfactory, although some resin binders can become rather brittle. PTFE films tend to lose adhesion to metal substrates on cooling to low temperatures as a result of their high thermal expansion coefficients; this may be offset by low expansion fillers in the coatings, e.g., lithium aluminum silicate. Self-Lubricating Composites The main applications of self-lubricating composites are for dry bearings, gears, seals, sliding electrical contacts, and retainers in rolling element bearings. This section concentrates on the influence of composition and sliding conditions on wear. Volume II 279 Copyright © 1983 CRC Press LLC Polymer Composites Because low thermal conductivity inhibits dissipation of frictional heat, thermoplastics undergo large increases in wear above critical loads and speeds as a consequence of surface melting. Effects on thermosetting resins are less dramatic because oxidative degradation, leading to surface embrittlement, is a function of exposure time as well as temperature. Thermal conductivity of the counterface is also relevant and at high sliding speeds can become more important than the conductivity of the polymer composite itself. Limiting speeds for polymers sliding against themselves are, in general, several hundred times lower than those for polymers sliding against metals. 48 Wear rates of polymer composites depend strongly on the surface roughness of metal counterfaces. In early stages of sliding, wear rate varies typically with initial Ra roughness raised to a power of 2 to 4; 49 for this reason smooth counterfaces are always recommended for applications such as dry bearings. During running-in, however, the initial counterface roughness is frequently reduced, either by transfer of the polymer and/or fillers or by polishing/abrasive action of fillers, leading to a reduction in wear rate. Steady-state roughness and steady-state rate of wear depend both on the composite composition and on relative hardness of the fillers and counterface. 50 Relationships between steady-state rate of wear and initial counterface roughness thus become very variable and examples are shown in Figure 2. Although an optimum counterface roughness for minimum wear is sometimes suggested, experimental results are conflicting. For PTFE composites and other polymers incorporating solid lubricants which rely on transfer film formation on the counterface to achieve low wear, wear behavior is strongly influenced by environmental factors. Relative humidity is particularly important and in- creasing humidity can either reduce or increase wear depending on the type of filler; there are no systematic trends. 51 Liquid water, however, increases wear by inhibiting transfer film formation and the aggregation of wear debris. Other fluids, including conventional hydro- carbon lubricants, produce similar effects although to a smaller extent. For polymer com- posites which do not rely on transfer film formation, e.g., nylons and acetals, hydrocarbon lubricants usually reduce wear 52 and are often effective in extremely small amounts. Small pockets of fluid within the bulk structure can provide a continuous source of lubricant. 53 Applications of polymer composites are extremely diverse. For dry bearings, some of the most successful composites are of complex construction, e.g., a layer of sintered bronze of graded porosity on a steel backing and filled with PTFE/Pb, 3 or a fabric liner of interwoven PTFE and glass fibers impregnated with synthetic resin and adhesively bonded to a steel backing. 54 Composites of the latter type are widely used in aerospace applications; a typical modern aircraft may contain several hundred. For transfer lubrication of rolling-element bearings, a particularly successful composite for retainers is PTFE/glass fiber/MoS 2 . 55,56 Metal-Lamellar Solid Composites Awide variety of metal-solid lubricant mixtures have been developed and some examples are listed in Table 8. With those containing lamellar solids, low friction is achieved via transfer. Since transfer film formation is an inefficient process, a high proportion of solid lubricant, 25% or more, is usually needed. Since such composites are mechanically weak, low friction tends to be associated with high wear and vice versa, as shown in Figure 3. For any given materials, however, conditions which reduce friction, such as increased temperature with fluoride or oxide films, usually reduce wear rate also. A great deal of effort has been devoted to material combinations and/or composite fab- rication to obtain both low friction and wear. Incorporation of PTFE in lamellar solid-metal composites appears to facilitate transfer film formation, and carbides in Ta-Mo-MoS 2 improve strength. 57 Fabrication techniques use conventional powder metallurgy, infiltration of porous metals, electrochemical codeposition, plasma spraying, and machining of holes or recesses 280 CRC Handbook of Lubrication Copyright © 1983 CRC Press LLC Volume II 283 Table 9 CLASSIFICATION OF CARBONS AND GRAFITES Copyright © 1983 CRC Press LLC [...]... Administration, Washington, D.C., 19 56 10 Grattan, P A and Lancaster, J K., Abrasion by lamellar solid lubricants Wear, 10 , 453, 19 67 11 Giltrow, J P and Lancaster, J K., The role of impurities in the abrasiveness of MoS2, Wear, 20, 13 7, 19 72 12 Magie, P M., A review of the properties and potentials of the new heavy metal derivative solid lubricants, Lubr Eng., 22, 262, 19 66 13 Fusaro, R L and Sliney, H... Trans., 13 , 56, 19 70 14 Play, D and Godet, M., Study of the Lubricating Properties of (CFx)n, Coll Int CNRS, 233, 4 41, 19 75; NASA Rep TM 7 519 1, National Aeronautics and Space Administration, Washington, D.C., 19 75 15 McConnell, B D., Snyder, C E., and Strang, J R., Analytical evaluation of graphite fluoride and its lubrication performance under heavy loads, paper 76-AM-5C-3, ASLE Trans., 19 76 preprint 16 ... Figure 1 illustrates this point for nitrogen, the principal component of air: the viscosity is 18 × 10 –6Pa·sec up to 40 atm pressure, 20 × 10 –6 at 10 0 bar, and 53 × 10 –6 at 10 00 bar The viscosities of airat several pressures from 1 to 10 0 bar are shown in Figure 2 as a function of absolute temperature This shows that the effect of pressure increases at lower temperatures Viscosities of a number of common... III., 19 71, 326 Copyright © 19 83 CRC Press LLC 2 91- 300 4 /10 /06 12 :47 PM Page 2 91 Volume II 2 91 PROPERTIES OF GASES Donald F Wilcock INTRODUCTION Increasing interest in and application of gas bearings requires knowledge of a number of gas properties which are not as readily available as the properties of common liquid lubricants This is particularly true in process fluid lubrication where gases other... Society of Lubrication Engineers, Park Ridge, III., 9 71, 3 71 73 Cook, C R., Lubricants for high temperature extrusion, Proc ASLE Conf Solid Lubr., SP-3, American Society of Lubrication Engineers, Park Ridge, III., 19 71, 13 74 Messina, J., Rust-inhibited, non-reactive perfluorinated polymer greases, Proc ASLE Conf Solid Lubr., SP-3, American Society of Lubrication Engineers Park Ridge, III., 19 71, 326... 27, 396, 19 71 43 Kawamura, M., Hoshida, K., and Acki, I., Running-in effect of bonded solid film lubricants on conventional oil lubrication, Proc ASLE 2nd Int Conf Solid Lubr., SP-6, American Society of Lubrication Engineers, Park Ridge, III., 19 78, 10 1 44 Harley, D and Wainwright, P., Development of a dry film tool lubricant, Proc ASLE 2nd Int Conf on Solid Lubr., SP-6, American Society of Lubrication. .. effectiveness of oil soluble additions and graphite dispersed in mineral oil, Proc 2nd ASLE Int Conf on Solid Lubr., SP-6, American Society of Lubrication Engineers, Park Ridge, III., 19 78, 51 71 Barlz, W J., Solid lubricant additives — effect of concentration and other additives on anti-wear performance, Wear, 17 , 4 21, 19 71 72 Groszek, A J and Witheredge, R E., Surface properties and lubricating action of graphite... Selection of Materials for Dry Sliding Various attempts have been made to provide general guidelines for selection of materials for specific applications For dry bearings, one approach is to identify major application requirements as listed down the left hand side of Table 11 , and then select the group of Copyright © 19 83 CRC Press LLC 286 CRC Handbook of Lubrication FIGURE 4 Order -of- magnitude wear rates of. .. Lubr Eng., 28, 16 1, 19 72 17 Martin, C., Sailleau, J., and Roussel, M., The ultra-high vacuum behavior of graphite-fluoride filled self-lubricating materials, Wear, 34, 215 , 19 75 18 Fusaro, R L., Effect of Fluorine Content, Atmosphere and Burnishing Technique on the Lubricating Properties of Graphite Fluoride, TN-D-7574, National Aeronautics and Space Administration, Washington, D.C., 19 74 19 Bisson, E... velocity of the particles is an expression of the gas temperature, increasing with temperature When a gas particle hits a solid surface and bounces off, the change in momentum of the particle exerts a force on the surface The sum of the countless surface collisions is the pressure the gas exerts on the surface If one of a pair of parallel surfaces is moving, it will impart an additional component of velocity . of Lubrication Engineers, Park Ridge, III., 19 78, 51. 71. Barlz, W. J., Solid lubricant additives — effect of concentration and other additives on anti-wear per- formance, Wear, 17 , 4 21, 19 71. 72 principal component of air: the viscosity is 18 10 –6 Pa·sec up to 40 atm pressure, 20 10 –6 at 10 0 bar, and 53 10 –6 at 10 00 bar. The viscosities of airat several pressures from 1 to 10 0 bar are. abrasiveness of MoS 2 , Wear, 20, 13 7, 19 72. 12 . Magie, P. M., A review of the properties and potentials of the new heavy metal derivative solid lubricants, Lubr. Eng., 22, 262, 19 66. 13 . Fusaro,