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Considerations in vacuum tribology (adhesion, friction, wear, and solid lubrication in vacuum)
Tribology International 32 (1999) 605–616 www.elsevier.com/locate/triboint
Foreword
Considerations in vacuum tribology (adhesion, friction, wear, and solid lubrication in vacuum) Kazuhisa Miyoshi
*
National Aeronautics and Space Administration, Glenn Research Center, Cleveland, OH 44135, USA
Abstract The success of many tribological products used or tribological systems operated in vacuum depends on adequate control of adhesion between two or more materials. Adhesion (e.g., in adhesive bonding) is the mechanical force or strength required to separate the surfaces in contact. This foreword is concerned with adhesion, which has greatly contributed, and should continue to contribute, to tribological problems, such as high friction, solid-state welding, scuffing or scoring, high wear, and a short lubricant life, in vacuum. Published by Elsevier Science Ltd. Keywords: Adhesion; Solid lubrication; Vacuum tribology
1. Analogy between adhesion and friction in vacuum When smooth, atomically clean solid surfaces are brought into contact under a normal load, the atoms must be in contact at some points. Thus, interatomic forces will come into play [1–4] and cause some adhesion at these points. In a vacuum environment atomically clean solids will exhibit strong adhesive bonds and high coefficients of friction when brought into contact. In practical cases adhesion develops in the film formation processes of joining, bonding, and coating. Beneficially, it is a crucial factor in the structural performance of engineering materials, including monolithics, composites, and coatings, used in engines, power trains, gearboxes, and bearings. The joining of solid to solid, fiber to matrix, and coating to substrate is determined by adhesion. Destructively, adhesion occurs in solid-state contacts, causing high friction and heavy surface damage, particularly in vacuum environments. Adhesion, a manifestation of mechanical strength over an appreciable area, has many causes, including chemical bonding, deformation, and the fracture processes involved in interface failure. Adhesion undoubtedly depends on the environment, the surface cleanliness, the
* Guest Editor. Tel.: +1-216-433-6078; fax: +1-216-433-5544. E-mail address: [email protected] (K. Miyoshi). 0301-679X/99/$ - see front matter. Published by Elsevier Science Ltd. PII: S 0 3 0 1 - 6 7 9 X ( 9 9 ) 0 0 0 9 3 - 6
area of real contact, the properties of the solids and the interface, and the modes of junction rupture. In particular, the vacuum environment and the surface cleanliness strongly influence the adhesion behavior of solids. A number of bulk and surface properties of solids have been shown to affect the nature and magnitude of the adhesive bond forces that develop for solids. Surface properties include electronic surface states, ionic species present at the surface, the chemistry, and the surface energy of the contacting materials. Bulk properties include elasticity, plasticity, fracture toughness, cohesive bonding energy, defects, and the crystallography of the solids. An analogy between adhesion and friction in vacuum can exist. For example, Fig. 1 shows two comparisons of adhesion and friction results obtained in vacuum: adhesion (pull-off force) versus coefficient of friction and atomically clean surface versus as-received, contaminated surface. The pull-off force, which reflects interfacial adhesion, was measured both for metal–ceramic couples cleaned by argon sputtering and for asreceived, contaminated metal–ceramic couples under a load (0.2 to 2 mN) in ultrahigh vacuum (10⫺8 Pa) at room temperature. The coefficient of friction was also measured for the cleaned metal–ceramic couples and the contaminated metal–ceramic couples in sliding contact under a load (0.05 to 0.5 N) in ultrahigh vacuum (10⫺8 Pa) at a sliding velocity of 3 mm/min. With the clean metal–ceramic couples adhesion and friction strongly
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Fig. 2. Pull-off force (adhesion) as function of temperature for SiC {0001} flat surfaces in contact with sintered polycrystalline SiC pins in ultrahigh vacuum.
Fig. 1. Pull-off force (adhesion) as function of Young’s modulus (a) and coefficient of friction as function of shear modulus (b) of metals in contact with polycrystalline manganese–zinc ferrite in ultrahigh vacuum.
depended on the Young’s and shear moduli for the metals. Adhesion and friction decreased as the Young’s and shear moduli of the metals increased. With the asreceived, contaminated metal–ceramic couples adhesion and friction were less than with the clean metal–ceramic couples and were independent of the Young’s and shear moduli for the metals. Even at high temperatures the analogy between adhesion and friction in vacuum has been observed, as shown in Figs. 2 and 3. These figures present the pulloff forces and the coefficients of static and dynamic friction for as-received ceramic flat surfaces in contact with clean ceramic pins as a function of temperature. Comparing Figs. 2 and 3 shows that the static and dynamic
friction characteristics were the same as those for adhesion. The adhesion and friction remained low at temperatures up to 300°C. This low adhesion and friction can be associated with the presence of contaminants on the as-received flat surfaces. Adhesion and friction increased rapidly between 300 and 400°C. Although adhesion and friction decreased slightly at 600°C, they remained relatively high between 400 and 700°C. The high adhesion and friction between 400 and 700°C can be attributed to the absence of adsorbed contaminants, such as carbon and water vapor. The somewhat low values of adhesion and friction at 600°C are probably due to the α-SiO2 to β-SiO2 transition at about 583°C for a small amount of silicon dioxide contaminants [5]. Above 800°C the adhesion and friction decreased rapidly, resulting from the graphitization of the silicon carbide surfaces with heating. Note that the chemical information on the ceramic surfaces was obtained from X-ray photoelectron spectroscopy and Auger electron spectroscopy surface analyses.
2. Relation of surface energy and real area of contact to friction All the clean metal–ceramic couples, including the metal–diamond couples, exhibited a correlation between the surface and bulk properties of the metal and the
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Fig. 3. Coefficients of static (a) and dynamic (b) friction as functions of temperature for SiC {0001} flat surfaces in sliding contact with sintered polycrystalline SiC pins in ultrahigh vacuum.
adhesion, friction, metal wear, and metal transfer to the ceramic [6]. It is possible that the metal’s bulk properties depend on the magnitude of its surface properties. It is interesting to consider then the role that the metal’s basic surface and bulk properties, as found in the literature
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(such as its surface energy and its ductility), play in adhesion and friction. The surface energy per unit area g of a metal is directly related to the interfacial bond strength per unit area at the metal–ceramic interface. The values of g suggested by Tyson [7] and Miedema [8] for various metals at room temperature increased with an increase in the Young’s and shear moduli. This behavior was opposite to that for the coefficient of friction, which decreased with an increase in g or in the Young’s and shear moduli. Obviously, g alone does not explain the friction trend and characteristics. Certainly, if g is low, the interfacial bond strength per unit area is weak, but that weakness does not result in a mechanically weak interface in the real area of contact between the metal and ceramic surfaces. What is presumably missing here is the ductility of the metals. A metal’s ductility influences the real area of contact and accordingly the adhesion and friction at the metal– ceramic interface. Ceramics such as silicon carbide, unlike metals, are not considered to be ductile; these materials behave in a ductile manner only when subjected to high compressive stresses. Because of the marked difference in the ductility of ceramics and metals, solid-state contact between the two materials can result in considerable plastic deformation of the softer metal. The real area of contact then for such a couple was calculated from the experimentally measured Vickers microhardness of the metal. The real area of contact A was simply determined from the ratio of normal load to hardness. The calculated value of A depended strongly on the shear modulus of the metal, decreasing as the shear modulus of the metal increased. The real area of contact obviously behaved in the same way as the coefficient of friction [9]. Thus, ductility is indeed important in determining friction characteristics. Therefore, the role of the metal’s total surface energy in the real area of contact gA, which is the product of the surface energy per unit area g and the real area of contact A, was considered and assessed. The value of gA also decreased as the shear modulus of the metal increased [9]. Fig. 4 clearly shows that the coefficient of friction for metal–ceramic couples increased as the product gA increased. In other words, the coefficient of friction m can be expressed as a function of gA m⫽f(gA) To reduce friction and to provide lubrication, therefore, the product of the real area of contact A and the surface energy g must be minimized in vacuum. 3. Importance of materials in couples to friction in vacuum Fig. 5 presents examples of coefficients of friction for clean metal–metal couples, clean metal–nonmetal
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Fig. 6. Coefficients of friction for diamond film in contact with diamond pin and for c-BN film in contact with diamond pin in ultrahigh vacuum. Fig. 4. Coefficients of friction for various metals in sliding contact with SiC {0001} surfaces in ultrahigh vacuum as function of total surface energy of metal in real area of contact.
couples, and clean nonmetal–nonmetal couples measured in ultrahigh vacuum. The coefficient of friction for clean interacting surfaces in ultrahigh vacuum strongly depends on the materials coupled. The judicious selection of counterpart materials can reduce the coefficient of friction in ultrahigh vacuum. Another example (Fig. 6) shows the coefficients of friction in ultrahigh vacuum for an as-deposited cubic boron nitride (c-BN) film in sliding contact with a chemical-vapor-deposited (CVD) diamond pin and for a CVD diamond film in contact with a natural diamond pin as a function of the number of passes. The materials combination of c-BN and diamond provided low coefficients of friction, whereas the diamond–diamond couple had considerably higher coefficients of friction as a result of dangling bonds in ultrahigh vacuum.
Fig. 5.
4. Adhesion effect on sliding wear in vacuum Inspection of all the metal and ceramic surfaces after sliding contact in vacuum revealed that the metal deformation was principally plastic and that the cohesive bonds in the metal fractured [10]. All the metals that were examined in the previous section failed by shearing or tearing and transferred to the ceramic during sliding. Because the interfacial bond between the metal and the ceramic is generally stronger than the cohesive bond within the metal, separation generally took place in the metal when the junction was sheared. Pieces of the metal were torn out and transferred to the ceramic surface. For example, when an atomically clean silicon carbide (SiC) surface was brought into contact with a clean aluminum surface, the interfacial adhesive bonds that formed in the area of real contact were so strong that shearing or tearing occurred locally in the aluminum. Consequently, aluminum wear debris particles were transferred to the SiC surface during sliding, as verified by a scanning
Coefficients of friction for clean solid in sliding contact with itself and with other materials in ultrahigh vacuum.
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electron micrograph and an aluminum Kα X-ray map (Fig. 7). The morphology of metal transfer to ceramic revealed that metals with a low shear modulus exhibited much more wear and transfer than those with a higher shear
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modulus. Further, the more chemically active the metal, the greater was the metal wear and transfer to the ceramic. Table 1 summarizes the type of metal transfer to single-crystal SiC that was observed after multipass sliding. Generally, the metals listed at the bottom of the table had a higher shear modulus and less chemical affinity for silicon and carbon. Therefore, those metals exhibited less wear and transferred less metal to the SiC. Sometimes the strong adhesion and high friction between a metal and a ceramic can locally damage the ceramic surface if that surface contains imperfections, such as microcracks or voids [9]. For example, Fig. 8 shows scanning electron micrographs of the wear tracks generated by 10 passes of rhodium and titanium pins on the SiC {0001} surface along the 具101¯ 0典 direction. The cracks observed in the wear tracks propagated primarily along cleavage planes of the {101¯ 0} orientation. Fig. 8(a) reveals a hexagonal light area, which is the beginning of a wear track, and a large crack. Cracks were generated primarily along the {101¯ 0} planes, propagated, and then intersected during loading and sliding of the rhodium pin over the SiC surface. It is anticipated from Fig. 8(a) that subsurface cleavage cracking of the {0001} planes, which are parallel to the sliding surface, also occurs. Fig. 8(b) reveals a hexagonal pit surrounded by a copious amount of thin titanium film. The hexagonal fracturing is caused primarily by cleavage cracking along the {101¯ 0} planes and subsurface cleavage cracking along the {0001} planes. The smooth surface at the bottom of the hexagonal pit is due to cleavage of the {0001} planes. Fig. 9 illustrates the SiC wear debris produced by 10 passes of aluminum pins sliding on an SiC surface. The scanning electron micrographs reveal evidence of multiangular SiC wear debris particles with transferred aluminum wear debris on the SiC wear track. These multiangular wear debris particles had crystallographically oriented sharp edges and were nearly hexagonal, rhombic, parallelogramic, or square. These shapes may be related to surface and subsurface cleavage of {101¯ 0}, {112¯ 0}, and {0001} planes. Similar hexagonal pits and multiangular wear debris with crystallographically oriented sharp edges were also observed with single-crystal SiC in contact with itself. Fig. 10 clearly reveals the gross hexagonal pits on the wear scar of the SiC pin and a nearly fully hexagonal and flat wear particle. The wear debris had transferred to the flat SiC specimen. Thus, crystallographically oriented cracking and fracturing of the SiC flat resulted from sliding both a metal pin and a ceramic pin.
5. Surface roughness effect on friction and wear Fig. 7. Aluminum transferred to SiC {0001} surface before and after single-pass sliding in ultrahigh vacuum. (a) Initial contact area. (b) Aluminum Kα X-ray map (1.5×104 counts). (c) Aluminum wear debris.
To verify and understand the surface roughness effect on friction and wear in vacuum, we conducted friction
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Table 1 Metals transferred to SiC {0001} surfaces after 10 sliding passes in ultrahigh vacuum Metal
Al Zr Ti Ni Co Fe Cr Rh W Re a b
Form (size) of metal transferred
Small particlea
Piled-up particlesb
Multilayer agglomeration
Large lump particleb
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
No No No No No No No Yes Yes Yes
Extent of metal transfer
Shear modulus, GPa
Most
27 34 39 75 76 81 117 147 150 180
Most
Submicrometer. Several micrometers.
experiments with diamond films having different surface roughnesses in sliding contact with a natural, hemispherical diamond pin in three environments: humid air, dry nitrogen, and ultrahigh vacuum [11]. Diamond films with a root-mean-square surface roughness ranging from 15 to 160 nm were produced by microwave-plasmaassisted chemical vapor deposition. 5.1. Friction In humid air and in dry nitrogen abrasion occurred and dominated the friction and wear behavior. The natural diamond pin tended to dig into the surface of diamond films during sliding and produce a wear track (groove). When interactions between the diamond pin surface and the initially sharp tips of asperities on the diamond film were strong, the friction was high. Surface roughness of diamond films can have an appreciable influence on the initial friction of these films: the greater the initial surface roughness, the higher the initial coefficient of friction (Fig. 11(a)). The results are consistent with the works of other researchers [12,13]. As sliding continued and the pin passed repeatedly over the same track, the coefficient of friction was appreciably affected by the wear on the diamond films (i.e., a blunting of the tips of the asperities). When repeated sliding produced a smooth groove with blunted asperities on the surface of the diamond film, the coefficient of friction was low, and the initial surface roughness effect became negligible. This showed that the equilibrium coefficient of friction was independent of the initial surface roughness of the diamond film (Fig. 11(b)). In vacuum, as in humid air and in dry nitrogen, the natural diamond pin dug into the surface of diamond films during sliding and produced a wear track (groove). However, the coefficient of friction increased with an
increase in the number of passes (see Fig. 6), just the opposite of that in humid air and in dry nitrogen. Further, the initial surface roughness of the diamond film had no effect on friction (Fig. 12). These results lead us to ask, which is more important for rough diamond surfaces in vacuum: abrasion or adhesion? The answer is adhesion because the initial coefficients of friction were independent of the initial surface roughness of the diamond films. When sliding continued, the wear dulled the tips of the diamond grains and enlarged the contact area in the wear track, thereby increasing friction. The increase in equilibrium friction that resulted from cleaning off the contaminant surface film by sliding and from increasing the contact area was greater than the corresponding decrease in abrasive friction that resulted from blunting the tips of surface asperities. This relationship is brought out clearly in Fig. 12; here the equilibrium coefficients of friction (1.5 to 1.8) are greater than the initial coefficients of friction (1.1 to 1.3) regardless of the initial surface roughness of the diamond films. In vacuum, therefore, the friction arises primarily from adhesion between the sliding surfaces of the diamond pin and the diamond films. 5.2. Wear The wear rate (wear coefficient) of the diamond films depended on their initial surface roughness whether in humid air, in dry nitrogen, or in vacuum (Fig. 13). It generally increased with increasing initial surface roughness. The wear rate in vacuum varied by a factor of 4 (Fig. 13(b)), whereas the wear rate in humid air or in dry nitrogen varied by a factor of 6 to 10 (Fig. 13(a)). Further, the wear rates of the diamond films in vacuum were considerably higher than those of diamond films in
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Fig. 8. Scanning electron micrographs of wear tracks on single-crystal SiC {0001} surface after 10 passes of rhodium and titanium pins in ultrahigh vacuum. Sliding direction, 具101¯ 0典 (a) Rhodium pin; hexagonal cracking. (b) Titanium pin; hexagonal pit.
humid air or in dry nitrogen. Obviously, under these vacuum conditions adhesion between the sliding surfaces of the diamond pin and the diamond film played an important role in the wear process and provided the high wear rates.
6. Solid film lubrication and possible risk of solidstate welding in vacuum 6.1. Solid film lubrication A solid lubricant is defined as “any material used as a thin film or a powder on a surface to provide protection from damage during relative movement and to reduce friction and wear.” Solid lubrication is achieved by using self-lubricating solids or by imposing a solid material having low shear strength and high wear resistance
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Fig. 9. Scanning electron micrographs of wear tracks and multiangular SiC wear debris on flat single-crystal SiC {0001} surface after 10 passes of aluminum pin in ultrahigh vacuum. Sliding direction, 具101¯ 0典.
between the interacting surfaces in relative motion. The imposed solid material may be a coating, a loose powder, or a dispersion in oils and greases. In the field of vacuum tribology dry solid lubricants are used when liquid lubricants do not meet the advanced requirements of modern technology. They are less expensive than oil and grease lubrication systems for many applications. Solid lubricants also reduce weight, simplify lubrication, and improve materials and processes. Fig. 14 and Tables 2 and 3 [14,15] list applications needed to meet the critical operating conditions for which liquid lubricants are ineffective or undesirable. Changes in critical environmental conditions, such as pressure, temperature, and radiation, affect lubricant efficiency. Note that in the cost-conscious automotive industry solid lubricants are replacing oils and greases in many applications and are helping to make highly efficient automobiles possible. Oils or greases cannot be used in many applications because of the difficulty in applying them, sealing prob-
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Fig. 10. Scanning electron micrographs of wear debris on singlecrystal SiC {0001} pin surface after 10 passes of SiC pin in ultrahigh vacuum. (a) Fracture pit. (b) Wear debris. Sliding direction, 具101¯ 0典.
lems, weight, or other factors, such as environmental conditions. Solid lubricants may be preferred to liquid or gas films for several reasons. In high-vacuum environments, in space-vacuum environments, in food-processing machines, or in semiconductor manufacturing equipment a liquid lubricant would evaporate and contaminate the product, such as optical and electronic equipment or food. At high temperatures liquid lubricants decompose or oxidize; suitable dry solid lubricants can extend the operating temperatures of sliding systems beyond 250 or 300°C while maintaining relatively low coefficients of friction. At cryogenic temperatures liquid lubricants are highly viscous and are not effective. Under radiation or corrosive environments liquid lubricants decompose or will be contaminated. Further, in the weight-conscious aerospace industry dry solid lubricants lead to substantial weight savings relative to the use of liquid lubricants. The elimination (or limited use) of liquid lubricants and their replacement
Fig. 11. Initial (a) and equilibrium (b) coefficients of friction as function of initial surface roughness of diamond films in humid air and in dry nitrogen.
Fig. 12. Initial (mI) and equilibrium (mF) coefficients of friction as function of initial surface roughness of diamond in ultrahigh vacuum.
by solid lubricants reduce aircraft or spacecraft weight and therefore have a dramatic impact on mission extent and craft maneuverability. Under high vacuums, high temperatures, cryogenic temperatures, radiation, space
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Table 2 Application of solid lubricants: areas where fluid lubricants are undesirable Requirement
Applications
Avoid contaminating product or environment
Food-processing machines Optical equipment Space telescopes Metal-working equipment Surface-mount equipment Tape recorders Microscopes and cameras Textile equipment Paper-processing machines Business machines Automobiles Medical and dental equipment Spectroscopes Maintain servicing or lubrication Aircraft in inaccessible or unlikely areas Space vehicles Satellites Aerospace mechanisms Nuclear reactors Resist abrasion in dirt-laden Aircraft environments Space vehicles (rovers) Automobiles Agricultural and mining equipment Off-road vehicles and equipment Construction equipment Textile equipment Provide prolonged storage or Aircraft equipment stationary service Railway equipment Missile components Nuclear reactors Telescope mounts Heavy plants, buildings, and bridges Furnaces
Fig. 13. Wear rates of diamond as function of initial surface roughness. (a) In humid air and in dry nitrogen. (b) In ultrahigh vacuum.
environments, or corrosive environments, solid lubrication may be the only feasible system. Numerous solid lubricants, such as permanently bonded lubricating films, have been developed to reduce friction and wear in applications of this type where liquid lubricants are ineffective or undesirable. The simplest kind of solid lubricating film is formed when a low-friction solid lubricant, such as molybdenum disulfide (MoS2), is suspended in a carrier and applied to the surface like a normal lubricant. The carrier may be a volatile solvent, a grease, or any of several other types of material. After the carrier is squeezed out or evaporates from the surfaces, a layer of MoS2 provides lubrication. Solid lubricants are also bonded to rubbing surfaces with various types of resin, which cure to form strongly adhering coatings with good frictional properties. In some plastic bearings the solid lubricant is sometimes incorporated into the plastic. During operation some of
the solid lubricant may be transferred to form a lubricating coating on the mating surface. In addition to MoS2, tungsten disulfide (WS2), polytetrafluoroethylene (PTFE), polyethylene, and a number of other materials are used to form solid films. Sometimes, combinations of several materials, each contributing specific properties to the film, are used. Because of recent innovations in the physical and chemical vapor deposition processes, solid lubricating materials, such as diamond, diamond-like carbon (DLC), MoS2, WS2, and PTFE films, are grown economically on ceramics, polymers, and metals and used as solid lubricating films. 6.2. Possible risk of solid-state welding in vacuum Solid lubricants designed for vacuum tribology applications must not only display low coefficients of friction (0.01 to 0.1) but also maintain good durability and environmental stability. The ability of a lubricant to allow rubbing surfaces to operate under load without scuffing, scoring, galling, seizing, welding, or any other
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Fig. 14. Ranges of application of solid lubricants in (a) high-vacuum, (b) high-temperature, (c) cryogenic temperature, and (d) radiation environments.
manifestation of material destruction in hostile environments is an important lubricant property. For solid lubricating films to be durable under sliding conditions they must have low wear rates and high interfacial adhesion strength between the films and the substrates. Solid film lubricants have finite wear lives or endurance lives. For example, Fig. 15 presents coefficients of friction for magnetron-sputtered MoSx films (x⬇1.7) on 440C stainless steel substrates in sliding contact with 440C stainless steel balls in ultrahigh vacuum as a function of the number of passes [16]. Films of MoSx were deposited by magnetron radiofrequency sputtering to a
nominal thickness of 110 nm on the 440C stainless steel disks. As shown in Fig. 15, where the plots are extended to the endurance life, the coefficient of friction rapidly rose to a fixed value of 0.15 that was the onset of scuffing or scoring. The onset of scuffing or scoring refers to localized surface damage associated with local solid-state welding between the 440C stainless steel ball and the 440C stainless steel substrate disk. The phenomenon of scuffing or scoring in vacuum tribology and lubricated systems is of great practical importance, since it leads to unacceptably high adhesion, coefficients of friction, and wear and to catastrophic results.
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Table 3 Application of solid lubricants: areas where fluid lubricants are ineffective Environment High vacuum
Applications Room temperature or cryogenic temperatures
Clean room High temperature
High temperatures
Air atmosphere
Molten metals (sodium, zinc, etc.) Cryogenic temperatures
Radiation (gamma rays, fast neutrons, X-rays, beta rays, etc.)
Corrosive gases (chlorine, etc.) High pressures or loads
Fretting corrosion (general)
References [1] Adhesion and cold welding of materials in space environments, ASTM Special Technical Publication No. 431. Philadelphia (PA, USA): American Society for Testing and Materials, 1967. [2] Buckley DH. Surface effects in adhesion, friction, wear and lubrication, vol. 5. Elsevier Book Series, Elsevier Scientific Publishing Co., 1981. [3] Buckley DH. The use of analytical surface tools in the fundamental study of wear. Wear 1978;46(1):19–53. [4] Tabor D. Status and direction of tribology as a science in the 80s—understanding and prediction, new directions in lubrication, materials, wear, and surface interactions. In: Loomis W.R., editor, vol. 1. Park Ridge (NJ, USA): Noyes Publications, 1985, pp. 1– 17 (Also NASA CP-2300).
Vacuum products Space mechanisms Satellites Space telescope mounts Space platforms Space antennae Semiconductor manufacturing equipment X-ray tubes X-ray equipment Furnaces Furnaces Metal-working equipment Compressors Nuclear reactors Molten metal plating equipment Space mechanisms Satellites Space vehicles Space propulsion systems Space telescope mounts Space platforms Space antennae Turbopumps Liquid nitrogen pumps Butane pumps Freon pumps Liquid natural gas pumps Liquid propane pumps Refrigeration plants Nuclear reactors Space mechanisms Satellites Space vehicles Space platforms Space antennae Semiconductor manufacturing equipment Metal-working equipment Bridge supports Plant supports Building supports Aircraft engines Turbines Landing gear Automobiles
[5] Weast RC, editor. CRC handbook of chemistry and physics: a ready reference book of chemical and physical data 68th ed. Boca Raton (FL, USA): CRC Press, 1987. [6] Miyoshi K. Fundamental considerations in adhesion, friction, and wear for ceramic–metal contacts. Wear 1990;141:35–44. [7] Tyson WR. Surface energies of solid metals. Can Metall Q 1975;14(4):307–14. [8] Miedema AR. Surface energies of solid metals. Z Metall 1978;69(5):287–92. [9] Miyoshi K. Adhesion, friction, and wear behavior of clean metal– ceramic couples. In: Proceedings of the International Tribology Conference, Yokohama, 1995:1853–8, vol. III. [10] Miyoshi K, Buckley DH. Friction and wear behavior of singlecrystal silicon carbide in sliding contact with various metals. ASLE Trans 1979;22(3):245–56.
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[11] Miyoshi K et al. Friction and wear of plasma-deposited diamond films. J Appl Phys 1993;74(7):4446–54. [12] Casey M, Wilks J. The friction of diamond sliding on polished cube faces of diamond. J Phys D: Appl Phys 1973;6:1772–81. [13] Hayward IP, Singer IL, Seitzman LW. The tribological behaviour of diamond coatings. In: Proceedings of the Second International Conference on the New Diamond Science and Technology. Pittsburgh (PA, USA): Materials Research Society, 1991:785–9. [14] Kakuda K, editor. Special issue on rolling bearings, ball screws, and rolling guides—fundamentals and applications. Tokyo (Japan): Nippon Seiko Co. Ltd, 1988. [15] Lancaster JK. Solid lubricants. In: Booser ER, editor. CRC handbook of lubrication—theory and practice of tribology. Boca Raton (FL, USA): CRC Press, 1984:269–90. [16] Miyoshi K, et al. A vacuum (10⫺9 Torr) friction apparatus for determining friction and endurance life of MoSx films. STLE Trans 1993;36(3):351–358. (Also NASA Technical Memorandum 104478, 1992).
Fig. 15. Coefficients of friction for MoSx films as function of number of passes for various loads: (a) 0.49 N, (b) 1 N, (c) 2 N, (d) 3.6 N.
Foreword
Considerations in vacuum tribology (adhesion, friction, wear, and solid lubrication in vacuum) Kazuhisa Miyoshi
*
National Aeronautics and Space Administration, Glenn Research Center, Cleveland, OH 44135, USA
Abstract The success of many tribological products used or tribological systems operated in vacuum depends on adequate control of adhesion between two or more materials. Adhesion (e.g., in adhesive bonding) is the mechanical force or strength required to separate the surfaces in contact. This foreword is concerned with adhesion, which has greatly contributed, and should continue to contribute, to tribological problems, such as high friction, solid-state welding, scuffing or scoring, high wear, and a short lubricant life, in vacuum. Published by Elsevier Science Ltd. Keywords: Adhesion; Solid lubrication; Vacuum tribology
1. Analogy between adhesion and friction in vacuum When smooth, atomically clean solid surfaces are brought into contact under a normal load, the atoms must be in contact at some points. Thus, interatomic forces will come into play [1–4] and cause some adhesion at these points. In a vacuum environment atomically clean solids will exhibit strong adhesive bonds and high coefficients of friction when brought into contact. In practical cases adhesion develops in the film formation processes of joining, bonding, and coating. Beneficially, it is a crucial factor in the structural performance of engineering materials, including monolithics, composites, and coatings, used in engines, power trains, gearboxes, and bearings. The joining of solid to solid, fiber to matrix, and coating to substrate is determined by adhesion. Destructively, adhesion occurs in solid-state contacts, causing high friction and heavy surface damage, particularly in vacuum environments. Adhesion, a manifestation of mechanical strength over an appreciable area, has many causes, including chemical bonding, deformation, and the fracture processes involved in interface failure. Adhesion undoubtedly depends on the environment, the surface cleanliness, the
* Guest Editor. Tel.: +1-216-433-6078; fax: +1-216-433-5544. E-mail address: [email protected] (K. Miyoshi). 0301-679X/99/$ - see front matter. Published by Elsevier Science Ltd. PII: S 0 3 0 1 - 6 7 9 X ( 9 9 ) 0 0 0 9 3 - 6
area of real contact, the properties of the solids and the interface, and the modes of junction rupture. In particular, the vacuum environment and the surface cleanliness strongly influence the adhesion behavior of solids. A number of bulk and surface properties of solids have been shown to affect the nature and magnitude of the adhesive bond forces that develop for solids. Surface properties include electronic surface states, ionic species present at the surface, the chemistry, and the surface energy of the contacting materials. Bulk properties include elasticity, plasticity, fracture toughness, cohesive bonding energy, defects, and the crystallography of the solids. An analogy between adhesion and friction in vacuum can exist. For example, Fig. 1 shows two comparisons of adhesion and friction results obtained in vacuum: adhesion (pull-off force) versus coefficient of friction and atomically clean surface versus as-received, contaminated surface. The pull-off force, which reflects interfacial adhesion, was measured both for metal–ceramic couples cleaned by argon sputtering and for asreceived, contaminated metal–ceramic couples under a load (0.2 to 2 mN) in ultrahigh vacuum (10⫺8 Pa) at room temperature. The coefficient of friction was also measured for the cleaned metal–ceramic couples and the contaminated metal–ceramic couples in sliding contact under a load (0.05 to 0.5 N) in ultrahigh vacuum (10⫺8 Pa) at a sliding velocity of 3 mm/min. With the clean metal–ceramic couples adhesion and friction strongly
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Fig. 2. Pull-off force (adhesion) as function of temperature for SiC {0001} flat surfaces in contact with sintered polycrystalline SiC pins in ultrahigh vacuum.
Fig. 1. Pull-off force (adhesion) as function of Young’s modulus (a) and coefficient of friction as function of shear modulus (b) of metals in contact with polycrystalline manganese–zinc ferrite in ultrahigh vacuum.
depended on the Young’s and shear moduli for the metals. Adhesion and friction decreased as the Young’s and shear moduli of the metals increased. With the asreceived, contaminated metal–ceramic couples adhesion and friction were less than with the clean metal–ceramic couples and were independent of the Young’s and shear moduli for the metals. Even at high temperatures the analogy between adhesion and friction in vacuum has been observed, as shown in Figs. 2 and 3. These figures present the pulloff forces and the coefficients of static and dynamic friction for as-received ceramic flat surfaces in contact with clean ceramic pins as a function of temperature. Comparing Figs. 2 and 3 shows that the static and dynamic
friction characteristics were the same as those for adhesion. The adhesion and friction remained low at temperatures up to 300°C. This low adhesion and friction can be associated with the presence of contaminants on the as-received flat surfaces. Adhesion and friction increased rapidly between 300 and 400°C. Although adhesion and friction decreased slightly at 600°C, they remained relatively high between 400 and 700°C. The high adhesion and friction between 400 and 700°C can be attributed to the absence of adsorbed contaminants, such as carbon and water vapor. The somewhat low values of adhesion and friction at 600°C are probably due to the α-SiO2 to β-SiO2 transition at about 583°C for a small amount of silicon dioxide contaminants [5]. Above 800°C the adhesion and friction decreased rapidly, resulting from the graphitization of the silicon carbide surfaces with heating. Note that the chemical information on the ceramic surfaces was obtained from X-ray photoelectron spectroscopy and Auger electron spectroscopy surface analyses.
2. Relation of surface energy and real area of contact to friction All the clean metal–ceramic couples, including the metal–diamond couples, exhibited a correlation between the surface and bulk properties of the metal and the
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Fig. 3. Coefficients of static (a) and dynamic (b) friction as functions of temperature for SiC {0001} flat surfaces in sliding contact with sintered polycrystalline SiC pins in ultrahigh vacuum.
adhesion, friction, metal wear, and metal transfer to the ceramic [6]. It is possible that the metal’s bulk properties depend on the magnitude of its surface properties. It is interesting to consider then the role that the metal’s basic surface and bulk properties, as found in the literature
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(such as its surface energy and its ductility), play in adhesion and friction. The surface energy per unit area g of a metal is directly related to the interfacial bond strength per unit area at the metal–ceramic interface. The values of g suggested by Tyson [7] and Miedema [8] for various metals at room temperature increased with an increase in the Young’s and shear moduli. This behavior was opposite to that for the coefficient of friction, which decreased with an increase in g or in the Young’s and shear moduli. Obviously, g alone does not explain the friction trend and characteristics. Certainly, if g is low, the interfacial bond strength per unit area is weak, but that weakness does not result in a mechanically weak interface in the real area of contact between the metal and ceramic surfaces. What is presumably missing here is the ductility of the metals. A metal’s ductility influences the real area of contact and accordingly the adhesion and friction at the metal– ceramic interface. Ceramics such as silicon carbide, unlike metals, are not considered to be ductile; these materials behave in a ductile manner only when subjected to high compressive stresses. Because of the marked difference in the ductility of ceramics and metals, solid-state contact between the two materials can result in considerable plastic deformation of the softer metal. The real area of contact then for such a couple was calculated from the experimentally measured Vickers microhardness of the metal. The real area of contact A was simply determined from the ratio of normal load to hardness. The calculated value of A depended strongly on the shear modulus of the metal, decreasing as the shear modulus of the metal increased. The real area of contact obviously behaved in the same way as the coefficient of friction [9]. Thus, ductility is indeed important in determining friction characteristics. Therefore, the role of the metal’s total surface energy in the real area of contact gA, which is the product of the surface energy per unit area g and the real area of contact A, was considered and assessed. The value of gA also decreased as the shear modulus of the metal increased [9]. Fig. 4 clearly shows that the coefficient of friction for metal–ceramic couples increased as the product gA increased. In other words, the coefficient of friction m can be expressed as a function of gA m⫽f(gA) To reduce friction and to provide lubrication, therefore, the product of the real area of contact A and the surface energy g must be minimized in vacuum. 3. Importance of materials in couples to friction in vacuum Fig. 5 presents examples of coefficients of friction for clean metal–metal couples, clean metal–nonmetal
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Fig. 6. Coefficients of friction for diamond film in contact with diamond pin and for c-BN film in contact with diamond pin in ultrahigh vacuum. Fig. 4. Coefficients of friction for various metals in sliding contact with SiC {0001} surfaces in ultrahigh vacuum as function of total surface energy of metal in real area of contact.
couples, and clean nonmetal–nonmetal couples measured in ultrahigh vacuum. The coefficient of friction for clean interacting surfaces in ultrahigh vacuum strongly depends on the materials coupled. The judicious selection of counterpart materials can reduce the coefficient of friction in ultrahigh vacuum. Another example (Fig. 6) shows the coefficients of friction in ultrahigh vacuum for an as-deposited cubic boron nitride (c-BN) film in sliding contact with a chemical-vapor-deposited (CVD) diamond pin and for a CVD diamond film in contact with a natural diamond pin as a function of the number of passes. The materials combination of c-BN and diamond provided low coefficients of friction, whereas the diamond–diamond couple had considerably higher coefficients of friction as a result of dangling bonds in ultrahigh vacuum.
Fig. 5.
4. Adhesion effect on sliding wear in vacuum Inspection of all the metal and ceramic surfaces after sliding contact in vacuum revealed that the metal deformation was principally plastic and that the cohesive bonds in the metal fractured [10]. All the metals that were examined in the previous section failed by shearing or tearing and transferred to the ceramic during sliding. Because the interfacial bond between the metal and the ceramic is generally stronger than the cohesive bond within the metal, separation generally took place in the metal when the junction was sheared. Pieces of the metal were torn out and transferred to the ceramic surface. For example, when an atomically clean silicon carbide (SiC) surface was brought into contact with a clean aluminum surface, the interfacial adhesive bonds that formed in the area of real contact were so strong that shearing or tearing occurred locally in the aluminum. Consequently, aluminum wear debris particles were transferred to the SiC surface during sliding, as verified by a scanning
Coefficients of friction for clean solid in sliding contact with itself and with other materials in ultrahigh vacuum.
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electron micrograph and an aluminum Kα X-ray map (Fig. 7). The morphology of metal transfer to ceramic revealed that metals with a low shear modulus exhibited much more wear and transfer than those with a higher shear
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modulus. Further, the more chemically active the metal, the greater was the metal wear and transfer to the ceramic. Table 1 summarizes the type of metal transfer to single-crystal SiC that was observed after multipass sliding. Generally, the metals listed at the bottom of the table had a higher shear modulus and less chemical affinity for silicon and carbon. Therefore, those metals exhibited less wear and transferred less metal to the SiC. Sometimes the strong adhesion and high friction between a metal and a ceramic can locally damage the ceramic surface if that surface contains imperfections, such as microcracks or voids [9]. For example, Fig. 8 shows scanning electron micrographs of the wear tracks generated by 10 passes of rhodium and titanium pins on the SiC {0001} surface along the 具101¯ 0典 direction. The cracks observed in the wear tracks propagated primarily along cleavage planes of the {101¯ 0} orientation. Fig. 8(a) reveals a hexagonal light area, which is the beginning of a wear track, and a large crack. Cracks were generated primarily along the {101¯ 0} planes, propagated, and then intersected during loading and sliding of the rhodium pin over the SiC surface. It is anticipated from Fig. 8(a) that subsurface cleavage cracking of the {0001} planes, which are parallel to the sliding surface, also occurs. Fig. 8(b) reveals a hexagonal pit surrounded by a copious amount of thin titanium film. The hexagonal fracturing is caused primarily by cleavage cracking along the {101¯ 0} planes and subsurface cleavage cracking along the {0001} planes. The smooth surface at the bottom of the hexagonal pit is due to cleavage of the {0001} planes. Fig. 9 illustrates the SiC wear debris produced by 10 passes of aluminum pins sliding on an SiC surface. The scanning electron micrographs reveal evidence of multiangular SiC wear debris particles with transferred aluminum wear debris on the SiC wear track. These multiangular wear debris particles had crystallographically oriented sharp edges and were nearly hexagonal, rhombic, parallelogramic, or square. These shapes may be related to surface and subsurface cleavage of {101¯ 0}, {112¯ 0}, and {0001} planes. Similar hexagonal pits and multiangular wear debris with crystallographically oriented sharp edges were also observed with single-crystal SiC in contact with itself. Fig. 10 clearly reveals the gross hexagonal pits on the wear scar of the SiC pin and a nearly fully hexagonal and flat wear particle. The wear debris had transferred to the flat SiC specimen. Thus, crystallographically oriented cracking and fracturing of the SiC flat resulted from sliding both a metal pin and a ceramic pin.
5. Surface roughness effect on friction and wear Fig. 7. Aluminum transferred to SiC {0001} surface before and after single-pass sliding in ultrahigh vacuum. (a) Initial contact area. (b) Aluminum Kα X-ray map (1.5×104 counts). (c) Aluminum wear debris.
To verify and understand the surface roughness effect on friction and wear in vacuum, we conducted friction
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Table 1 Metals transferred to SiC {0001} surfaces after 10 sliding passes in ultrahigh vacuum Metal
Al Zr Ti Ni Co Fe Cr Rh W Re a b
Form (size) of metal transferred
Small particlea
Piled-up particlesb
Multilayer agglomeration
Large lump particleb
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
No No No No No No No Yes Yes Yes
Extent of metal transfer
Shear modulus, GPa
Most
27 34 39 75 76 81 117 147 150 180
Most
Submicrometer. Several micrometers.
experiments with diamond films having different surface roughnesses in sliding contact with a natural, hemispherical diamond pin in three environments: humid air, dry nitrogen, and ultrahigh vacuum [11]. Diamond films with a root-mean-square surface roughness ranging from 15 to 160 nm were produced by microwave-plasmaassisted chemical vapor deposition. 5.1. Friction In humid air and in dry nitrogen abrasion occurred and dominated the friction and wear behavior. The natural diamond pin tended to dig into the surface of diamond films during sliding and produce a wear track (groove). When interactions between the diamond pin surface and the initially sharp tips of asperities on the diamond film were strong, the friction was high. Surface roughness of diamond films can have an appreciable influence on the initial friction of these films: the greater the initial surface roughness, the higher the initial coefficient of friction (Fig. 11(a)). The results are consistent with the works of other researchers [12,13]. As sliding continued and the pin passed repeatedly over the same track, the coefficient of friction was appreciably affected by the wear on the diamond films (i.e., a blunting of the tips of the asperities). When repeated sliding produced a smooth groove with blunted asperities on the surface of the diamond film, the coefficient of friction was low, and the initial surface roughness effect became negligible. This showed that the equilibrium coefficient of friction was independent of the initial surface roughness of the diamond film (Fig. 11(b)). In vacuum, as in humid air and in dry nitrogen, the natural diamond pin dug into the surface of diamond films during sliding and produced a wear track (groove). However, the coefficient of friction increased with an
increase in the number of passes (see Fig. 6), just the opposite of that in humid air and in dry nitrogen. Further, the initial surface roughness of the diamond film had no effect on friction (Fig. 12). These results lead us to ask, which is more important for rough diamond surfaces in vacuum: abrasion or adhesion? The answer is adhesion because the initial coefficients of friction were independent of the initial surface roughness of the diamond films. When sliding continued, the wear dulled the tips of the diamond grains and enlarged the contact area in the wear track, thereby increasing friction. The increase in equilibrium friction that resulted from cleaning off the contaminant surface film by sliding and from increasing the contact area was greater than the corresponding decrease in abrasive friction that resulted from blunting the tips of surface asperities. This relationship is brought out clearly in Fig. 12; here the equilibrium coefficients of friction (1.5 to 1.8) are greater than the initial coefficients of friction (1.1 to 1.3) regardless of the initial surface roughness of the diamond films. In vacuum, therefore, the friction arises primarily from adhesion between the sliding surfaces of the diamond pin and the diamond films. 5.2. Wear The wear rate (wear coefficient) of the diamond films depended on their initial surface roughness whether in humid air, in dry nitrogen, or in vacuum (Fig. 13). It generally increased with increasing initial surface roughness. The wear rate in vacuum varied by a factor of 4 (Fig. 13(b)), whereas the wear rate in humid air or in dry nitrogen varied by a factor of 6 to 10 (Fig. 13(a)). Further, the wear rates of the diamond films in vacuum were considerably higher than those of diamond films in
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Fig. 8. Scanning electron micrographs of wear tracks on single-crystal SiC {0001} surface after 10 passes of rhodium and titanium pins in ultrahigh vacuum. Sliding direction, 具101¯ 0典 (a) Rhodium pin; hexagonal cracking. (b) Titanium pin; hexagonal pit.
humid air or in dry nitrogen. Obviously, under these vacuum conditions adhesion between the sliding surfaces of the diamond pin and the diamond film played an important role in the wear process and provided the high wear rates.
6. Solid film lubrication and possible risk of solidstate welding in vacuum 6.1. Solid film lubrication A solid lubricant is defined as “any material used as a thin film or a powder on a surface to provide protection from damage during relative movement and to reduce friction and wear.” Solid lubrication is achieved by using self-lubricating solids or by imposing a solid material having low shear strength and high wear resistance
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Fig. 9. Scanning electron micrographs of wear tracks and multiangular SiC wear debris on flat single-crystal SiC {0001} surface after 10 passes of aluminum pin in ultrahigh vacuum. Sliding direction, 具101¯ 0典.
between the interacting surfaces in relative motion. The imposed solid material may be a coating, a loose powder, or a dispersion in oils and greases. In the field of vacuum tribology dry solid lubricants are used when liquid lubricants do not meet the advanced requirements of modern technology. They are less expensive than oil and grease lubrication systems for many applications. Solid lubricants also reduce weight, simplify lubrication, and improve materials and processes. Fig. 14 and Tables 2 and 3 [14,15] list applications needed to meet the critical operating conditions for which liquid lubricants are ineffective or undesirable. Changes in critical environmental conditions, such as pressure, temperature, and radiation, affect lubricant efficiency. Note that in the cost-conscious automotive industry solid lubricants are replacing oils and greases in many applications and are helping to make highly efficient automobiles possible. Oils or greases cannot be used in many applications because of the difficulty in applying them, sealing prob-
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Fig. 10. Scanning electron micrographs of wear debris on singlecrystal SiC {0001} pin surface after 10 passes of SiC pin in ultrahigh vacuum. (a) Fracture pit. (b) Wear debris. Sliding direction, 具101¯ 0典.
lems, weight, or other factors, such as environmental conditions. Solid lubricants may be preferred to liquid or gas films for several reasons. In high-vacuum environments, in space-vacuum environments, in food-processing machines, or in semiconductor manufacturing equipment a liquid lubricant would evaporate and contaminate the product, such as optical and electronic equipment or food. At high temperatures liquid lubricants decompose or oxidize; suitable dry solid lubricants can extend the operating temperatures of sliding systems beyond 250 or 300°C while maintaining relatively low coefficients of friction. At cryogenic temperatures liquid lubricants are highly viscous and are not effective. Under radiation or corrosive environments liquid lubricants decompose or will be contaminated. Further, in the weight-conscious aerospace industry dry solid lubricants lead to substantial weight savings relative to the use of liquid lubricants. The elimination (or limited use) of liquid lubricants and their replacement
Fig. 11. Initial (a) and equilibrium (b) coefficients of friction as function of initial surface roughness of diamond films in humid air and in dry nitrogen.
Fig. 12. Initial (mI) and equilibrium (mF) coefficients of friction as function of initial surface roughness of diamond in ultrahigh vacuum.
by solid lubricants reduce aircraft or spacecraft weight and therefore have a dramatic impact on mission extent and craft maneuverability. Under high vacuums, high temperatures, cryogenic temperatures, radiation, space
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Table 2 Application of solid lubricants: areas where fluid lubricants are undesirable Requirement
Applications
Avoid contaminating product or environment
Food-processing machines Optical equipment Space telescopes Metal-working equipment Surface-mount equipment Tape recorders Microscopes and cameras Textile equipment Paper-processing machines Business machines Automobiles Medical and dental equipment Spectroscopes Maintain servicing or lubrication Aircraft in inaccessible or unlikely areas Space vehicles Satellites Aerospace mechanisms Nuclear reactors Resist abrasion in dirt-laden Aircraft environments Space vehicles (rovers) Automobiles Agricultural and mining equipment Off-road vehicles and equipment Construction equipment Textile equipment Provide prolonged storage or Aircraft equipment stationary service Railway equipment Missile components Nuclear reactors Telescope mounts Heavy plants, buildings, and bridges Furnaces
Fig. 13. Wear rates of diamond as function of initial surface roughness. (a) In humid air and in dry nitrogen. (b) In ultrahigh vacuum.
environments, or corrosive environments, solid lubrication may be the only feasible system. Numerous solid lubricants, such as permanently bonded lubricating films, have been developed to reduce friction and wear in applications of this type where liquid lubricants are ineffective or undesirable. The simplest kind of solid lubricating film is formed when a low-friction solid lubricant, such as molybdenum disulfide (MoS2), is suspended in a carrier and applied to the surface like a normal lubricant. The carrier may be a volatile solvent, a grease, or any of several other types of material. After the carrier is squeezed out or evaporates from the surfaces, a layer of MoS2 provides lubrication. Solid lubricants are also bonded to rubbing surfaces with various types of resin, which cure to form strongly adhering coatings with good frictional properties. In some plastic bearings the solid lubricant is sometimes incorporated into the plastic. During operation some of
the solid lubricant may be transferred to form a lubricating coating on the mating surface. In addition to MoS2, tungsten disulfide (WS2), polytetrafluoroethylene (PTFE), polyethylene, and a number of other materials are used to form solid films. Sometimes, combinations of several materials, each contributing specific properties to the film, are used. Because of recent innovations in the physical and chemical vapor deposition processes, solid lubricating materials, such as diamond, diamond-like carbon (DLC), MoS2, WS2, and PTFE films, are grown economically on ceramics, polymers, and metals and used as solid lubricating films. 6.2. Possible risk of solid-state welding in vacuum Solid lubricants designed for vacuum tribology applications must not only display low coefficients of friction (0.01 to 0.1) but also maintain good durability and environmental stability. The ability of a lubricant to allow rubbing surfaces to operate under load without scuffing, scoring, galling, seizing, welding, or any other
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Fig. 14. Ranges of application of solid lubricants in (a) high-vacuum, (b) high-temperature, (c) cryogenic temperature, and (d) radiation environments.
manifestation of material destruction in hostile environments is an important lubricant property. For solid lubricating films to be durable under sliding conditions they must have low wear rates and high interfacial adhesion strength between the films and the substrates. Solid film lubricants have finite wear lives or endurance lives. For example, Fig. 15 presents coefficients of friction for magnetron-sputtered MoSx films (x⬇1.7) on 440C stainless steel substrates in sliding contact with 440C stainless steel balls in ultrahigh vacuum as a function of the number of passes [16]. Films of MoSx were deposited by magnetron radiofrequency sputtering to a
nominal thickness of 110 nm on the 440C stainless steel disks. As shown in Fig. 15, where the plots are extended to the endurance life, the coefficient of friction rapidly rose to a fixed value of 0.15 that was the onset of scuffing or scoring. The onset of scuffing or scoring refers to localized surface damage associated with local solid-state welding between the 440C stainless steel ball and the 440C stainless steel substrate disk. The phenomenon of scuffing or scoring in vacuum tribology and lubricated systems is of great practical importance, since it leads to unacceptably high adhesion, coefficients of friction, and wear and to catastrophic results.
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Table 3 Application of solid lubricants: areas where fluid lubricants are ineffective Environment High vacuum
Applications Room temperature or cryogenic temperatures
Clean room High temperature
High temperatures
Air atmosphere
Molten metals (sodium, zinc, etc.) Cryogenic temperatures
Radiation (gamma rays, fast neutrons, X-rays, beta rays, etc.)
Corrosive gases (chlorine, etc.) High pressures or loads
Fretting corrosion (general)
References [1] Adhesion and cold welding of materials in space environments, ASTM Special Technical Publication No. 431. Philadelphia (PA, USA): American Society for Testing and Materials, 1967. [2] Buckley DH. Surface effects in adhesion, friction, wear and lubrication, vol. 5. Elsevier Book Series, Elsevier Scientific Publishing Co., 1981. [3] Buckley DH. The use of analytical surface tools in the fundamental study of wear. Wear 1978;46(1):19–53. [4] Tabor D. Status and direction of tribology as a science in the 80s—understanding and prediction, new directions in lubrication, materials, wear, and surface interactions. In: Loomis W.R., editor, vol. 1. Park Ridge (NJ, USA): Noyes Publications, 1985, pp. 1– 17 (Also NASA CP-2300).
Vacuum products Space mechanisms Satellites Space telescope mounts Space platforms Space antennae Semiconductor manufacturing equipment X-ray tubes X-ray equipment Furnaces Furnaces Metal-working equipment Compressors Nuclear reactors Molten metal plating equipment Space mechanisms Satellites Space vehicles Space propulsion systems Space telescope mounts Space platforms Space antennae Turbopumps Liquid nitrogen pumps Butane pumps Freon pumps Liquid natural gas pumps Liquid propane pumps Refrigeration plants Nuclear reactors Space mechanisms Satellites Space vehicles Space platforms Space antennae Semiconductor manufacturing equipment Metal-working equipment Bridge supports Plant supports Building supports Aircraft engines Turbines Landing gear Automobiles
[5] Weast RC, editor. CRC handbook of chemistry and physics: a ready reference book of chemical and physical data 68th ed. Boca Raton (FL, USA): CRC Press, 1987. [6] Miyoshi K. Fundamental considerations in adhesion, friction, and wear for ceramic–metal contacts. Wear 1990;141:35–44. [7] Tyson WR. Surface energies of solid metals. Can Metall Q 1975;14(4):307–14. [8] Miedema AR. Surface energies of solid metals. Z Metall 1978;69(5):287–92. [9] Miyoshi K. Adhesion, friction, and wear behavior of clean metal– ceramic couples. In: Proceedings of the International Tribology Conference, Yokohama, 1995:1853–8, vol. III. [10] Miyoshi K, Buckley DH. Friction and wear behavior of singlecrystal silicon carbide in sliding contact with various metals. ASLE Trans 1979;22(3):245–56.
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[11] Miyoshi K et al. Friction and wear of plasma-deposited diamond films. J Appl Phys 1993;74(7):4446–54. [12] Casey M, Wilks J. The friction of diamond sliding on polished cube faces of diamond. J Phys D: Appl Phys 1973;6:1772–81. [13] Hayward IP, Singer IL, Seitzman LW. The tribological behaviour of diamond coatings. In: Proceedings of the Second International Conference on the New Diamond Science and Technology. Pittsburgh (PA, USA): Materials Research Society, 1991:785–9. [14] Kakuda K, editor. Special issue on rolling bearings, ball screws, and rolling guides—fundamentals and applications. Tokyo (Japan): Nippon Seiko Co. Ltd, 1988. [15] Lancaster JK. Solid lubricants. In: Booser ER, editor. CRC handbook of lubrication—theory and practice of tribology. Boca Raton (FL, USA): CRC Press, 1984:269–90. [16] Miyoshi K, et al. A vacuum (10⫺9 Torr) friction apparatus for determining friction and endurance life of MoSx films. STLE Trans 1993;36(3):351–358. (Also NASA Technical Memorandum 104478, 1992).
Fig. 15. Coefficients of friction for MoSx films as function of number of passes for various loads: (a) 0.49 N, (b) 1 N, (c) 2 N, (d) 3.6 N.