22Solid Lubricants and

Self-Lubricating Films*

22.1 IntroductionGeneral Characteristics of Solid Lubricants • New Products, Practices, and Approaches in Solid Lubrication

22.2 Classification of Solid LubricantsLamellar Solid Lubricants

22.3 Lubrication Mechanisms of Layered Solids22.4 High-Temperature Solid Lubricants

Lubricious Oxides, Fluorides, and Sulfates • Composites • New Approaches to Solid Lubrication at High Temperatures

22.5 Self-Lubricating CompositesTraditional Materials • New Self-lubricating Composite Coatings and Structures

22.6 Soft Metals22.7 Polymers 22.8 Summary and Future Directions

22.1 Introduction

In most tribological applications, liquid or grease lubricants are used to combat friction and wear; butwhen service conditions become very severe (i.e., very high or low temperatures, vacuum, radiation,extreme contact pressure, etc.), solid lubricants may be the only choice for controlling friction and wear.Some of the key advantages of solid lubricants in tribological applications over liquid and grease lubricantsare summarized in Table 22.1. A combination of solid and liquid lubrication is also feasible and mayhave a beneficial synergistic effect on the friction and wear performance of sliding surfaces. Solid lubri-cants can be dispersed in water, oils, and greases to achieve improved friction and wear properties underconditions of extreme pressures and/or temperatures (Barnett, 1977; Broman et al., 1978; Kimura et al.,1999; Erdemir, 1995).

When present at a sliding interface, solid lubricants function the same way as their liquid counterparts.Specifically, they shear easily to provide low friction and to prevent wear damage between the slidingsurfaces. Several inorganic materials (e.g., molybdenum disulfide, graphite, hexagonal boron nitride,

*Work supported by U.S. Department of Energy, Office of Transportation Technology, under Contract W-31-109-Eng-38.

The submitted manuscript has been created by the University of Chicago as Operator of Argonne NationalLaboratory (“Argonne”) under Contract No. W-31-109-ENG-38 with the U.S. Department of Energy. The U.S.Government retains for itself, and others acting on its behalf, a paid-up, nonexclusive, irrevocable worldwide licensein said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and displaypublicly, by or on behalf of the Government.

Ali ErdemirArgonne National Laboratory

boric acid) can provide excellent lubrication (Sutor, 1991; Klauss, 1972; Lancaster, 1984; Sliney, 1982;McMurtrey, 1985; Lansdown, 1999). Most of these solids owe their lubricity to a lamellar or layeredcrystal structure. A few others (e.g., soft metals, polytetrafluoroethylene, polyimide, certain oxides andrare-earth fluorides, diamond and diamond-like carbons, fullerenes) can also provide lubricationalthough they do not have a layered crystal structure. In fact, diamond-like carbon films are amorphous,but provide some of the lowest friction coefficients of all the solid materials (Erdemir et al., 2000). Becausea special chapter (see Chapter 24) in this Handbook is devoted to the friction and wear behavior ofdiamond and diamond-like carbon, they will not be covered here.

The solid lubricants with a layered crystal structure are graphite, hexagonal boron nitride, boric acid,and the transition-metal dichalcogenides MX2 (where M is molybdenum, tungsten, or niobium, and Xis sulfur, selenium, or tellurium). Figure 22.1 shows the layered crystal structures of these solids. Certainmonochalcogenides (e.g., GaSe and GaS) have lattice structures similar to those of dichalcogenides; hence,they can also provide low friction when present at a sliding interface (Erdemir, 1994).

Major shortcomings of solid lubricants include:

1. Except for soft metals, most solid lubricants are poor thermal conductors and, hence, cannot carryaway heat from sliding interfaces.

2. Depending on test environment and contact conditions, their friction coefficients may be high orfluctuate significantly.

TABLE 22.1 Comparison of Solid and Liquid Lubricants in Tribological Applications

Application Environment and/or Condition Solid Lubricants Liquid and Grease Lubricants

Vacuum Some solids (i.e., transition-metal dichalcogenides) lubricate extremely well in high vacuum, have very low vapor pressure

Most liquids evaporate, but perfluoropolyalkylethers (PFPE) and polyalfaolefins (PAO) have good durability

Pressure Can endure extreme pressures May not support extreme pressures without additives

Temperature Relatively insensitive; can function at very low and high temperatures; low heat generation due to shear

May solidify at low temperatures and decompose or oxidize at high temperatures; heat generation varies with viscosity

Electrical conductivity Some provide excellent electrical conductivity Mostly insulatingRadiation Relatively insensitive to nuclear radiation May degrade or decompose over timeWear Provide excellent wear performance or durability at

slow speeds and under fretting conditions; lifetime is determined by lubricant film thickness and wear rate

Provide marginal performance and durability at slow speeds and under fretting conditions; need additives for boundary lubrication

Friction Extremely low friction coefficients are feasible Depends on viscosity, boundary films, and temperature

Thermal conductivity and heat dissipation capability

Excellent for metallic lubricants; poor for most inorganic or layered solids

Good

Storage Can be stored for very long times (dichalcogenides are sensitive to humidity and oxygen)

May evaporate, drain, creep, or migrate during storage

Hygiene Better industrial hygiene due to little or no hazardous emissions; since they are in solid state, there is no danger of spillage that can contaminate environment

May release hazardous emissions; liquid lubricants may spill or drip and contaminate environment; fire hazard with certain oils and greases

Compatibility with tribological surfaces

Compatible with hard-to-lubricate surfaces (i.e., Al, Ti, stainless steels, and ceramics)

Not suitable for use on non-ferrous or ceramic surfaces

Resistance to aqueous and chemically aggressive environments

Relatively insensitive to aqueous environments, chemical solvents, fuels, certain acids and bases

May be affected or altered by acidic and other aqueous environments

3. They have finite wear lives and their replenishment is more difficult than that of liquid lubricants.4. Oxidation and aging-related degradation may occur over time and present some problems with

transition-metal dichalcogenides.5. Upon exposure to high temperatures or oxidative environments, they may undergo irreversible

structural-chemistry changes that in turn lead to loss of lubricity and the generation of someabrasive, nonlubricious by-products.

FIGURE 22.1 Schematic illustration of layered crystal structures of (a) graphite, (b) hexagonal boron nitride, (c)molybdenum disulfide (representing transition metal dichalcogenides), and (d) boric acid.

Oxygen

Hydrogen

Boron

3.18 A°

2.96 A°

3.35 A°

Molybdenum

Sulfur

Carbon

(b)(a)

(d)(c)

22.1.1 General Characteristics of Solid Lubricants

Well-known solid lubricants (graphite, HBN, and transition-metal dichalcogenides) owe their lubricityto a unique layered structure. As illustrated in Figure 22.1, the crystal structures of these solids are suchthat while the atoms lying on the same layer are closely packed and strongly bonded to each other, thelayers themselves are relatively far apart, and the forces that bond them (e.g., van der Waals) are weak.When present between sliding surfaces, these layers can align themselves parallel to the direction ofrelative motion and slide over one another with relative ease, thus providing low friction. In addition,strong interatomic bonding and packing in each layer is thought to help reduce wear damage. While thismechanism is largely responsible for low friction and is essential for long wear life, a favorable crystalstructure in itself is not sufficient for effective lubrication. The presence or absence of certain chemicaladsorbates is also needed for providing easy shear in most solids. For example, moisture or some other

In contrast, MoS2 and other transition-metal dichalchogenides work best in vacuum or dry runningconditions, but degrade rather quickly in moist and oxidizing environments (Winer, 1967; Farr, 1975;Kanakia and Peterson, 1987). The friction coefficients of self-lubricating metal dichalcogenides aretypically in the range of 0.002 to 0.05 in vacuum or dry and inert atmospheres, but increase rapidly to0.2 in humid air. It is generally agreed that no solid can provide very low friction and wear, regardlessof test environment and/or conditions.

Soft metallic lubricants have crystal structures with multiple slip planes and do not work-hardenappreciably during sliding contact. Dislocations and point defects generated during shear deformationare rapidly nullified by the frictional heat produced during sliding contact. Most high-temperature solidlubricants rely on thermal softening and/or limited chemical reaction with sliding surfaces that makethem shear with relative ease; whereas self-lubricating polymers consist of long molecular chains withhigh chemical inertness and/or very low surface energy, making them non-stick or largely insensitive tochemical bonding.

Ambient temperature has a strong influence on the lubricity of solid lubricants. Graphite can providelubrication up to 400°C, while HBN can withstand temperatures up to 1000°C. Most transition metaldichalcogenides tend to oxidize at elevated temperatures, and thus lose their lubricity. MoS2 can providelubrication up to 400°C, while WS2 endures up to 500°C (Sliney, 1982). In general, those with higheroxidation resistance or chemical/structural stability perform the best at elevated temperatures. Oxide-and fluoride-based solid lubricants (e.g., CaF2, BaF2, PbO, and B2O3) (Sliney, 1993), as well as some softmetals (e.g., Ag, Au), function quite well at elevated temperatures (Erdemir et al., 1990c; Erdemir andErck, 1996; Maillat et al., 1993), but all fail to provide low friction at room or lower ambient temperatures.The lubricity of these solids at elevated temperatures is largely controlled by their ability to soften andresist oxidation.

Solid lubricants can be applied to a tribological surface in a variety of forms. The oldest and simplestmethod is to sprinkle, rub, or burnish the fine powders of solid lubricants on surfaces to be lubricated.Fine powders of certain solid lubricants were also used to lubricate sliding bearing surfaces with greatsuccess (Heshmat and Heshmat, 1999; and Higgs et al., 1999). Certain solid lubricants have been blendedin an aerosol carrier and sprayed directly onto the surfaces to be lubricated. Powders of solid lubricantscan be strongly bonded to a surface by appropriate adhesives and epoxy resins to provide longer wearlife (Gresham, 1997). They can also be dispersed or impregnated into a composite structure. Certainsolids (e.g., HBN and boric acid) have been mixed with oils and greases in powder form to achieveimproved lubrication under extreme pressure and temperature conditions (Kimura et al., 1999; Erdemir,1995). However, in most modern applications, thin films of solid lubricants are preferred over powdersor bonded forms. They are typically deposited on surfaces by advanced vacuum deposition processes(e.g., sputtering, ion plating, and ion-beam-assisted deposition) to achieve strong bonding, dense micro-structure, uniform thickness, and long wear life (Spalvins, 1969, 1971, 1980; Erdemir, 1993). Ion-beamdeposition and mixing can also be used to enhance the durability of solid lubricant coatings (Bhattacharya

et al., 1993; Erck et al., 1992). However, the lifetimes of most solid lubricants are still limited because ofthe finite lubricant film thickness. To increase their durability, a self-replenishment or resupply mecha-nism is needed but very difficult.

22.1.2 New Products, Practices, and Approaches in Solid Lubrication

In recent years, several new lubricants and modern lubrication concepts have been introduced to achievebetter lubricity and longer wear life in demanding tribological applications. Some of the traditional solidlubricants were prepared in the forms of metal, ceramic, and polymer-matrix composites and usedsuccessfully in a variety of engineering applications (Rohatgi et al., 1992; Prasad and McConnell, 1991;Gangopadhyay and Jahanmir, 1991; Friedrich, 1995). Carbonaceous films produced by catalytic crackingof carbon-bearing gases were also shown to provide good lubricity at elevated temperatures (Lauer andBunting 1988; Blanchet et al., 1994). Recent developments in PVD and CVD deposition technologieshave led to the synthesis of a new generation of adaptive, self-lubricating coatings with composite ormultilayer architectures (Jayaram et al., 1995; Zabinski et al., 1992, 1995; Voevodin et al., 1999). Theseexotic architectures, based on layers of a self-lubricating dichalcogenide (e.g., MoS2, WS2, etc.) and a softmetallic or hard ceramic layer, were shown to work extremely well under demanding tribological con-ditions. Multifunctional nanocomposite films (consisting mainly of MoS2 and Ti) have also been pro-duced by magnetron sputtering and are quite hard, moisture insensitive, and self-lubricating, thus raisingthe prospect for dry-sliding applications, as well as dry metal-cutting and -forming (Fox et al., 1999).Duplex/multiplex surface treatments and multilayer coatings with self-lubricating capabilities have alsomade their way into the commercial marketplace and have been meeting the ever-increasing performancedemands of more severe applications.

Recently, carbon and WS2 were prepared in the form of hollow nanotubes and demonstrated to providehigh mechanical strength and very low friction coefficients under certain sliding conditions (Tenne, 1992;Falvo, 1999). Nanostructured ZnO films were also shown to be quite lubricious and relatively insensitiveto variations in ambient pressure, environment, and temperature (Zabinski, 1997). A series of adaptivelubrication strategies has also been introduced in recent years and shown to be effective over a widerange of temperatures and pressures (Walck, 1997). Minute oxygen deficiency or sub-stoichiometry inrutile was shown to lead to the formation of low-shear crystallographic planes and hence high lubricity(Gardos, 1988, 1990, 1993). A series of plasma-sprayed composite coatings consisting of silver and alkalinehalides (i.e., CaF2, BaF2) as the self-lubricating entities and CrC and/or Cr2O3 as the wear-resisting entitieswere also shown to provide excellent lubrication over a wide temperature range (DellaCorte, 1998;DellaCorte and Fellenstein, 1997). Furthermore, H3BO3 powders, films forming on B2O3, and B4C coatingswere shown to be quite lubricious and highly effective under extreme sliding conditions (Erdemir, 1991;Erdemir et al., 1990b, 1991c, 1999).

Thiomolybdates and oxythiomolybdates of Cs, Zn (i.e., Cs2MoO2S2, ZnMoO2S2), and a few other alkalimetals were found to be effective in controlling friction and wear at elevated temperatures (King, 1990).These solids possess a lamellar structure like MoS2 but can endure much higher temperatures than MoS2.Furthermore, certain complex oxides and oxide-fluorides (i.e., ZnO/SnO/SrF2, NiO/BaTiO3,MgO/ZnO/CaF2, NiO/SrF2) were shown to be rather lubricious at elevated temperatures (Erdemir et al.,1998). Erdemir (1999) introduced a new crystal-chemical approach to the selection, classification, andmechanistic understanding of lubricious oxides used to combat friction and wear at elevated tempera-tures. Based on this approach, one can predict the shear rheology and hence lubricity of an oxide oroxide mixture at elevated temperatures.

John and Zabinski (1999) investigated the lubrication properties of some sulfate-based (i.e., CaSO4,BaSO4, and SrSO4) coatings at high temperatures as a potential replacement for alkaline halides (i.e.,CaF2, BaF2). They found that these sulfates became highly lubricious at 600°C and were able to providefriction coefficients of ≈0.15 to sliding surfaces. Structural studies revealed the presence of a carbonatecrystal structure, along with a sulfate crystal structure after testing. The carbonate crystal structure

consisted of alternating layers of alkali-earth atoms and carbonate ions. It was proposed that such alayered structure may have been responsible for the low-friction nature of these sulfates at high temper-atures. The new lubricants and lubrication approaches mentioned above are some of the most notabledevelopments in recent years and certainly have the potential to overcome difficult lubrication problemsthat may arise in the future.

In this chapter, solids with self-lubricating capabilities are reviewed first and classified on the basis oftheir crystal structures, chemistry, and operational limits. A summary of the recent understanding of thelubrication mechanisms of both traditional and new solid lubricants is presented next. Then, the presentstate-of-the-art in advanced solid lubrication methods and practices is provided. Particular emphasis isplaced on the synthesis and/or applications of solid lubricant films on tribological surfaces by means ofadvanced surface engineering processes such as ion-beam-assisted deposition, ion-beam mixing, andunbalanced magnetron sputtering. Traditional and new applications for self-lubricating composite solidlubricants are also emphasized. This chapter primarily focuses on developments evolved during the lastdecade because several excellent reviews, book chapters, and books cover the earlier developments (Sutor,1991; Lancaster, 1984; Sliney, 1982, 1993; Singer, 1989, 1992, 1998; McMurtrey, 1985; Klauss, 1972;Miyoshi, 1996). Also, major emphasis is placed on inorganic solid lubricants with layered crystal structuresand those that provide lubrication at high temperatures. Soft metals and polymers are briefly discussedbecause there are several excellent articles providing in-depth information on the properties and appli-cations of these solid lubricants (Sliney, 1986; Dayson, 1971; Sherbiney and Halling, 1977; Wang et al.,1995; Briscoe, 1990; Zhang, 1997; Bahadur and Gong, 1992; Friedrich et al., 1995).

22.2 Classification of Solid Lubricants

Solid lubricants can be categorized into several subclasses. Table 22.2 provides such a classification basedon the chemistry, crystal structure, and lubricity of the most widely used and recently developed solidlubricants. As can be realized from Table 22.2, the range of friction coefficients is rather large for a givensolid lubricant. This is mainly because friction is very sensitive to test environment, condition, and/orconfiguration. Ambient temperature and the type of counterface materials can also make a big differencein the frictional property of a given solid lubricant. The specific form or shape of the solid lubricants(i.e., thin films, powders, bulk, composite, and crystalline or amorphous states) can also play a majorrole. For example, the wide range of friction coefficients for MoS2 (i.e., 0.002 to 0.25) stems from severalfactors affecting its shear rheology and hence frictional properties. These factors include film microstruc-ture and chemistry, test environment, ambient temperature, contact pressure, film thickness, stoichiom-etry, and purity. Deposition and/or lubricant application methods can also play a major role in frictionalperformance of MoS2 films. Due to their porous, columnar structure, MoS2 films deposited by conven-tional sputtering methods tend to exhibit higher friction and shorter wear lives than films produced bymore robust ion-beam-assisted deposition and closed-field unbalanced magnetron sputtering techniques.The MoS2 films deposited by these advanced physical vapor deposition methods can have near-perfectstoichiometry, purity, and basal plane orientation parallel to the substrate surfaces. These highly opti-mized films can, in turn, provide friction coefficients as low as 0.002 in ultrahigh vacuum (Martin et al.,1994; Donnet et al., 1993).

In general, no single lubricant can provide reasonably low and consistent friction coefficients overbroad test conditions, temperatures, and environments. Each lubricant listed in Table 22.2 functionsrather nicely under certain test conditions, but not under all conditions. Researchers have mixed two ormore of these lubricants to broaden the operational range, but in most cases the improvements wereeither transitory or short-lived.

22.2.1 Lamellar Solid Lubricants

Lamellar or layered solid lubricants are the class that is most studied by scientists and widely used byindustry. Among the best-known examples are transition-metal dichalcogenides (e.g., MoS2), graphite,

HBN, and H3BO3. MoS2, graphite, and boric acid are natural minerals, extracted from deposits aroundthe world. Other lamellar solids, such as WS2, fluorinated graphite, and transition-metal diselenides andditellurides, are synthetic and are used at much smaller scales than graphite, HBN, and MoS2. MoS2 andWS2 are well-suited for aerospace and cryogenic applications, while HBN is preferred for lubrication atelevated temperatures. HBN is widely used as a release agent in high-temperature metal-forming oper-ations. Graphite and H3BO3 work extremely well in moist air. The lubricity of graphite persists up to400°C, while H3BO3 begins to decompose at about 170°C. These two solids do not provide lubricationin dry or vacuum environments. Graphite fluoride is produced by the fluorination of graphite. This

TABLE 22.2 Solid Materials with Self-lubricating Capability

Classification Key ExamplesTypical Range of

Friction Coefficienta

Lamellar solids MoS2

WS2

HBNGraphiteGraphite fluorideH3BO3

GaSe, GaS, SnSe

0.002–0.250.01–0.20.150–0.70.07–0.50.05–0.150.02–0.20.15–0.25

Soft metals AgPbAuInSn

0.2–0.350.15–0.20.2–0.30.15–0.250.2

Mixed oxides CuO–Re2O7

CuO–MoO3

PbO–B2O3

CoO–MoO3

Cs2O–MoO3

NiO–MoO3

Cs2O–SiO2

0.3–0.10.35–0.20.2–0.10.47–0.20.180.3–0.20.1

Single oxides B2O3

Re2O7

MoO3

TiO2 (sub-stoichiometric)ZnO

0.15–0.60.20.20.10.1–0.6

Halides and sulfates of alkaline earth metals

CaF2, BaF2, SrF2

CaSO4, BaSO4, SrSO4

0.2–0.40.15–0.2

Carbon-based solids DiamondDiamond-like carbonGlassy carbonHollow carbon nanotubesFullerenesCarbon-carbon and carbon-graphite-based composites

0.02–10.003–0.50.15—0.150.05–0.3

Organic materials/polymers Zinc steariteWaxesSoapsPTFE

0.1–0.20.2–0.40.15–0.250.04–0.15

Bulk or thick-film (>50 µm) composites

Metal-, polymer-, and ceramic-matrix composites consisting of graphite, WS2, MoS2, Ag, CaF2, BaF2, etc.

0.05–0.4

Thin-film (<50 µm) composites Electroplated Ni and Cr films consisting of PTFE, graphite, diamond, B4C, etc., particles as lubricants

Nanocomposite or multilayer coatings consisting of MoS2, Ti, DLC, etc.

0.1–0.5

0.05–0.15

a Friction values given in this table represent friction measurements made on each solid lubricant over a wide range oftest conditions, environments, and temperatures. The objective here is to show how friction varies depending on testconditions, as well as from one solid to another.

process increases the spacing between the carbon-carbon layers in graphite from about 0.34 nm to valuesas high as 0.8 nm, resulting in easier shear and hence better lubricity, even in dry environments. Amongthe lamellar solids, MoS2 and WS2 have the best overall load-carrying capacity as thin films on rigidsubstrates.

Most lamellar solids have good wetting capability or chemical affinity for ferrous surfaces. On a roughor porous sliding surface, they fill in the valleys between asperities and/or pores, thus providing asmoother surface finish and better support. When applied properly, these solids can also withstandextreme contact pressures without being squeezed out of the load-bearing surfaces. WS2 is preferred overMoS2 when applications involve relatively higher temperatures. However, WS2 is a synthetic lubricantand thus is expensive. Selenides of W, Nb, and Mo can provide even higher temperature capabilities thantheir sulfide analogs, but they too are expensive and are used on much smaller scales. Certain selenidesand tellurides (e.g., V, Nb) provide excellent electrical conductivity.

22.2.1.1 Transition-Metal Dichalcogenides

Transition-metal dichalcogenides, MX2 (where M is Mo, W, Nb, Ta, etc., and X is sulfur, selenium, ortellurium), are among the lowest-friction materials known in dry and vacuum environments (Winer,1967; Farr, 1975; Kanakia and Peterson, 1987; Singer et al., 1990; Donnet, 1996). They are also well-suitedfor cryogenic applications. MoS2 and WS2 are the best-known examples and the most widely useddichalcogenides. MoS2 is a natural mineral known as molybdenite, whereas WS2 and other dichalco-genides are man-made and therefore expensive. The hardness values of these solids on the Mohs scaleare 1.5 to 2 and their specific gravities lie between 4.7 and 5.5. They are chemically stable and resist attackby most acids, except aqua regia and hot and highly concentrated HCl, H2SO4, and HNO3. At roomtemperature in ultrahigh vacuum, these solid lubricants provide some of the lowest friction coefficients,but moisture in air has a detrimental effect on their lubricity (Peterson, 1953; Fusaro, 1978). Oxidationof MoS2 does not begin until the temperature reaches about 375°C. At approximately 500°C, rapidoxidation begins and MoO3 and SO2 are produced. The thermal and oxidative stability of WS2 is betterthan that of MoS2 (Sliney, 1982).

22.2.1.1.1 Preparation and Uses of DichalcogenidesMoS2 and other dichalcogenides are applied on tribological surfaces as thin, strongly bonded solid filmsproviding very long wear lives and super-low friction coefficients. Depending on application conditions(load, speed, temperature, etc.) and the form (crystalline or amorphous), size, purity, stoichiometry, andfilm thickness, the friction coefficients of MoS2 and other dichalcogenides vary considerably. In moistair, the lifetimes of lubricant films are rather short and typical values for friction coefficients are 0.05 to0.25. Burnished films tend to be short-lived and give higher friction than thin sputtered films (Spalvins,1971; Fusaro, 1978; Peterson, 1953). Bonded and composite forms of MoS2 last much longer, but theirfriction coefficients are generally high (Gresham, 1977).

The advanced physical vapor deposition (PVD) methods used in the deposition of high-quality MoS2

films include magnetron sputtering (Spalvins, 1969, 1971, 1980; Stupp, 1981), ion-beam-assisted depo-sition (IBAD) (Bolster, 1991; Wahl et al., 1995; Seitzman et al., 1995; Dunn et al., 1998), and ion-beammixing (Kobs et al., 1986; Bhattacharya et al., 1993; Rai, 1997). A pulsed laser deposition (PLD) methodcan also be used to deposit high-quality MoS2 and other composite films with excellent tribologicalperformance (Zabinski, 1992; Prasad, 1995). Sputtering has been and is still the most widely used method.Recently, closed-field unbalanced magnetron sputtering of MoS2 has become very popular and is highlyeffective in many tribological applications, including metal-forming and -cutting operations (Fox et al.,1999). Fil2.2 compares the endurance lives of MoS 2 films prepared by various methods.

MoS2 films may also contain lattice and volume defects (i.e., large voids or porosities) in their micro-structures (Spalvins, 1980; Lince and Fleischauer, 1987; Hilton and Fleischauer, 1991). Furthermore,some contain significant amounts of oxygen and carbon impurities that may have been present in thedeposition chamber or introduced during film deposition. They may also come from the source or targetmaterial used during deposition. Depending on the level of contaminants, resultant MoS2 films may show

significant differences in their tribological properties (Suzuki, 1998). Crystalline films with porous colum-nar structures tend to wear out rather quickly. Tilting or bending of columnar grains with poor cohesionduring sliding results in fracture of the top portions of each column; the remaining lower part is smearedon the surface and the basal planes of the MoS2 crystal are eventually oriented parallel to the slidingsurfaces (Spalvins, 1980; Hilton, 1991). The rate and degree of reorientation appear to depend on theinitial microstructure. Recent studies have indicated that films with dense morphology, preferred basalorientation, and high purity provided the best overall performance. In fact, the lowest friction coefficients(0.002 to 0.01) were reported on a phase-pure (oxygen-free) and stoichiometric MoS2 film (Donnet,1993; Martin et al., 1993). These super-low friction coefficients are obtained in ultrahigh vacuum andare attributed to a combination of perfect basal orientation of the MoS2 layers and to the absence of anyadsorbed species or contaminants on sliding surfaces.

22.2.1.1.2 Modern PracticesIncreasing demand for higher performance, longer wear life, and better efficiency in advanced mechanicalsystems that depend primarily on solid lubrication for safe operation has intensified interest in new andexotic lubrication practices in recent years. One of the major reasons for this interest was that conventionallubrication practices could no longer meet the performance and durability needs of advanced mechanicalsystems. Most studies have concentrated on MoS2 and have resulted in a better understanding of thefriction and wear mechanisms of this solid lubricant. Such mechanistic understanding is, in turn, usedto develop the new and better lubrication practices that are in wide use by industry today.

With the advanced PVD methods mentioned earlier, MoS2 films can be grown at subzero, room, orelevated temperatures. At lower deposition temperatures or under high-energy ion bombardment, onecan obtain amorphous and sub-stoichiometric films with relatively poor tribological properties. Duringsliding or upon annealing, the crystallinity, and hence the lubricity, of MoS2 may be restored (Zabinskiet al., 1994). Ion-beam mixing of sputtered MoS2 or WS2 films (50 to 70 nm thick) with sapphire, Si3N4,

and ZrO2 substrates can also result in an amorphous microstructure with a sub-stoichiometry of MoS1.8.In these studies, 2-MeV Ag+ ions at 5 × 1015 cm–2 dose were used. During tribological tests in dry N2,

FIGURE 22.2 Endurance lives of MoS2 films produced by various methods. (From Singer, I.L., Bolster, R.N.,Seitzman, L.E., Wahl, K.J., and Mowery, R.L. (1994), Advanced Solid Lubricant Films by Ion-Beam Assisted Despo-sition., Naval Research Laboratory, NRL/MR/6170-94-7633.)

Maximum Thrust Bearing Enduranceof *1 µm* MoS2

Coatings

burnished

dc sputtered

rf sputtered

NRL S-modulated

OSMC pure

OSMC AuPd

NRL Pb-alloyed

0 2 4 6 8 10 12

Revolutions to Failure (Millions)

friction coefficients of 0.03 to 0.04 were measured in both the as-deposited and ion-irradiated films.However, the sliding lives of Ag+ ion-irradiated films were found to increase 10- to 1000-fold over thoseof as-sputtered films on all ceramic surfaces studied. The improvements in wear lives were correlatedwith a significant improvement in film/substrate adhesion (Bhattacharya et al., 1993). Figure 22.3 showsthe friction performance and durability of sputtered and Ag+ ion-beam mixed MoS2 films on sapphiresubstrates. Similar improvements in wear lives of WS2 film were found after ion-beam mixing (Rai et al.,1997).

Recently, researchers have developed novel means to dope MoS2 films with certain metals (e.g., Au,Ni, Ti, Pb, C, etc.) and compounds (TiN, PbO, Sb2O3, etc.) (Zabinski et al., 1992, 1995; Spalvins, 1984;Stupp, 1981; Hilton et al., 1992, 1998; Wahl et al., 1995; Lince et al., 1995). Tribological studies havedemonstrated that when doped properly and in the correct proportions, these dopants can substantiallyimprove the mechanical and tribological properties of MoS2 films. For example, Au-doped MoS2 filmswere shown to have more stable frictional traces and lower friction than undoped sputtered MoS2 films,as shown in Figure 22.4. Figure 22.5 compares the friction and wear performance of conventional MoS2

with that of Ti-doped MoS2 in increasingly humid air. Furthermore, doping of MoS2 with Pb, Ti, Ni, Fe,Au, and Sb2O3 resulted in film amorphization or densification and in reduction of the crystallite size,which in turn reduced the mean and variance of the friction coefficients and substantially increased theirwear lives (Zabinski et al., 1992, 1995; Wahl et al., 1995, 1999). The exact mechanisms responsible forlifetime improvements in doped MoS2 films are not yet fully understood. However, researchers havenoticed that doping generally results in preferential alignments of basal planes parallel to the substratesurface and thus lower susceptibility of MoS2 to oxidation or moisture-induced degradation. It is spec-ulated that such favorable alignment, together with increased resistance to oxidation, may have beenresponsible for increased wear life and lubricity.

Films with duplex and/or alternating layers of MoS2 and metals or hard nitrides have also beenproduced in recent years and used in a variety of applications (Hilton et al., 1992; Jayaram, 1995; Seitzmanet al., 1992). For example, MoS2 films prepared by RF magnetron sputtering on AISI 440C and 52100steels, and multilayer coatings of MoS2 with either nickel or Au-(20%)Pd metal interlayers (with layer

FIGURE 22.3 Friction performance and durability of sputtered and Ag+ ion-beam mixed MoS2 films on sapphiresubstrate.

thicknesses ranging from 0.2 to 1.0 nm) on silicon substrates, had very dense microstructures that insome cases exhibited significant orientation of MoS2 basal planes parallel to the substrate. Some of theoptimized films exhibited excellent endurance and friction coefficients of 0.05 to 0.08 in UHV (Hilton,1992).

Overall, these novel coating practices have led to favorable changes in crystallite size and film densityand reduced edge orientation in growing films, which in turn resulted in increased coating endurance.Some dopants (Pb, Ti, PbO) resulted in an amorphous microstructure but with no detrimental effecton the low-friction and wear behaviors of the films. In fact, despite the formation of an amorphousmicrostructure, significant increases in wear lives are attained with Pb- and Ti-doped films (Fox et al.,1999; Wahl et al., 1995, 1999). However, the mechanism(s) responsible for such remarkable performancehas not yet been resolved.

FIGURE 22.4 Effect of Au doping on friction behavior of sputtered MoS2 films. (From Spalvins, T. (1984), Frictionaland morphological properties of Au-MoS2 films sputtered from a compact target, Thin Solid Films, 118, 374-384.With permission.)

FIGURE 22.5 Friction performance of conventional and Ti-doped MoST films at different humidity levels. (Cour-tesy of Multi-arc, Inc.)

CO

EF

FIC

IEN

T O

F F

RIC

TIO

N, µ

.05

.04

.03

.02

.01

0 10000 20000 30000 40000 50000SLIDING DURATION, cycles

Au-MoS2

MoS2

00.00

0.05

0.10

0.20

0.15

0.25

0.30

20 40% Relative Humidity

Fri

ctio

n C

oeff

icie

nt (

µ)

60 80 100

Pin-On-DiscSubstrate: WCCounterpart: 6 mm dia. WC BallTrack Radius: 3.5 mmSpeed: 500 rpmLoad: 10 N

MoS2

MoST™

In recent years, researchers have demonstrated that low-friction surface films can be formed in situon the surfaces of Mo, W, and Mo- or W-containing metallic alloys by adding sulfur-bearing gases suchas H2S and SO2 into the test chamber (Singer et al., 1996a,b; Sawyer and Blanchet, 1999). In a modelexperiment run in high vacuum, where a small amount of H2S was admitted to maintain an S partialpressure of 13 Pa, Singer et al. (1996) recorded a friction coefficient of 0.01 on the resulting films on Mosubstrates.

Recently, WS2 was prepared as nanoparticles having structures similar to those of nested carbonfullerenes and nanotubes. Preliminary test results showed that these nanoparticles are highly effective inreducing friction and wear and do outperform the solid and thin film forms of WS2 and MoS2 whentested under the same test conditions (Rapoport et al., 1997). For the excellent durability and performanceof these nanoparticles, high chemical inertness and a hollow cage structure were proposed. Apparently,hollow structures are chemically very stable and do not interact with oxygen or water molecules in theenvironment. Because of their high rigidity, they impart high elasticity, which allows these particles toroll rather than slide.

22.2.1.2 Monochalcogenides

Sulfides and selenides of gallium and tin (i.e., GaS, GaSe, SnSe) have crystal structures that resemblethose of transition-metal dichalcogenides (i.e., MoS2, WS2, WSe2) which are well-known solid lubricants.Figure 22.6 shows the layered structure of GaSe. These solids are known as sandwich semiconductors in

FIGURE 22.6 (a) Crystal structure of GaSe, and (b) SEM photomicrograph of fractured GaSe pellet. (From Erdemir,A. (1994), Crystal chemistry and self-lubricating properties of monochalcogenides gallium selenide and tin selenide,Tribol. Trans., 37, 471-476.)

solid-state physics and have been studied extensively for their electrical and optical properties (Phillips,1969). Tin selenide represents a group of layered compounds that also comprise SnS and the sulfides andselenides of germanium, whereas GaSe belongs to a class that also includes the layered gallium sulfideand the sulfides and selenides of indium.

Using a pin-on-disk machine, Erdemir (1994) performed friction tests on large crystalline pieces andcompacts of GaSe and SnSe monochalcogenides against sapphire and 440C steel balls to assess theirlubricity. For the specific test conditions explored, friction coefficients of the sapphire/GaSe and sap-phire/SnSe pairs were ≈0.23 and ≈0.35, respectively. The friction coefficients of 440C pin/440C disk testpairs with GaSe and SnSe powders were ≈0.22 and ≈0.38, respectively. The friction data, together withthe crystal-chemical knowledge and electron microscopy evidence, supported the conclusion that thelubricity and self-lubricating mechanisms of these solids are closely related to their crystal chemistry andthe nature of their interlayer bonding.

In a series of earlier studies, Boes and Chamberlain (1968) and Gardos (1984) explored the tribologicaland thermal oxidation properties of composite lubricants consisting of indium/gallium and WSe2. Theprincipal goal of these studies was to achieve better oxidation resistance on WSe2 by alloying it with low-melting-point indium and gallium. Upon curing the composite mixture at high temperatures, the inves-tigators found that both indium and gallium underwent chemical reaction with WSe2 to form the selenidesof these metals. Further studies by Gardos (1984) demonstrated that, compared to the parent WSe2, thenew composite lubricant exhibited superior oxidation resistance over a wide range of ambient temper-ature. Also, the lubricating capability of this InSe/WSe2 composite was much superior to that of WSe2

alone, especially at elevated temperatures. Apparently, a protective film resulting mainly from the pref-erential oxidation of sub-stoichiometric indium selenides was primarily responsible for the superioroxidation resistance of this new lubricant. The protective film was thought to effectively shield thelubricating entities against oxidation.

As discussed later, chalcogenides owe their low-friction nature to their lamellar structures in whichstrongly bonded atoms form extensive rigid sheets (see Figures 22.1 and 22.6). In the cases of dichalco-genides such as MoS2 or MoSe2, the crystal structure is composed of a monolayer of Mo ions sandwichedbetween layers of S or Se ions. However, in the case of monochalcogenides such as GaS or GaSe, thecrystal structure is composed of double layers of Ga sandwiched between Se ions (Figure 22.6).

22.2.1.3 Graphite

Graphite is another classic example of lamellar solids that provides low friction and high wear resistanceto sliding surfaces. Because of its good lubricity, abundance, and low cost, it is used in many industrialapplications. Like diamond, graphite is a polymorph of carbon. Both occur naturally and are recoveredfrom deposits around the world; both can also be produced by synthetic means. Synthetic graphite isprimarily produced by heating petroleum coke to about 2700°C. Chemically, both graphite and diamondare the same, but differ totally in their structures and properties. For example, graphite is perhaps oneof the softest materials, while diamond is the hardest of all natural materials. Diamond has the highestthermal conductivity, whereas graphite is a relatively poor thermal conductor. However, graphite is agood electrical conductor, but diamond is an excellent electrical insulator. Graphite has a sheet-like crystalstructure (see Figure 22.1) in which all of the carbon atoms lie in a plane and are bonded only weaklyto the graphite sheets above and below. Each carbon atom in the plane joins to three neighboring carbonatoms at a 120° angle and at a distance of 0.1415 nm. The distance between atomic layers is 0.335 nmat room temperature, and the layers are held together by van der Waals forces.

In moist air, the friction coefficient of graphite varies from 0.07 to 0.15, depending on test conditions,sliding contact configuration, form of graphite used (powder, bulk, thin film, purity, crystallite orienta-tion), and test machine. The lowest friction coefficient of 0.01 was observed during a nanotribologyexperiment in which a W tip was slid against the cleaved graphite flakes (Mate, 1987). The dense andhighly oriented pyrolitic graphite (HOPG) performs extremely well in humid air, giving friction coeffi-cients of about 0.1. In dry air, inert atmospheres, or vacuum, graphite’s lubricity degrades rapidly, thefriction coefficient increases to as high as 0.5, and it wears out quickly. Experimental studies carried out

by research groups have confirmed that the lubricity of graphite is not due to its layered crystal structurealone, but depends strongly on the presence or absence of certain condensable vapors, water vapor beingone. Research has shown that only a small amount of condensable vapor is needed to improve the lubricityof graphite (Rowe, 1960). Certain vapors appear to be more effective than others. For example, a testrun by Savage (1948) showed that n-heptane and isopropanol are much more effective than water vaporin terms of increasing the lubricity of graphite. The beneficial effect of condensable vapors on the lubricityof graphite has been attributed to the saturation in its lattice of π-electrons, which otherwise make atomiclayers slide with difficulty. Figure 22.7 shows the relationship between wear rate and water vapor pressurefor graphite.

Graphite can provide lubrication up to about 500°C in open air, although friction tends to increaseas the temperature rises. At higher ambient temperatures, it begins to oxidize and lose its lubricity. Invacuum, the friction coefficient is initially high (i.e., 0.4), but decreases to about 0.2 at 1300°C. In mostsliding experiments, thin transfer films are formed on the surfaces of sliding counterfaces. These transferlayers are thought to be important for achieving longer wear life and possibly even lower friction. Whensmall amounts of sodium thiosulfate (Na2S2O3) or sodium molybdate (Na2MoO4) were added to graphiteto improve the transfer film forming behavior, researchers observed longer wear life and lower frictionagainst sliding steel counterfaces (Langlade et al., 1994). During these tests, transformations of thegraphite structure to a turbostatic phase was observed as a thin layer by means of electron microscopyand X-ray diffraction.

Graphite is inexpensive and readily available in various forms. It is resistant to both acids and bases.In practice, graphite is used in powder, colloidal dispersion, solid, and composite forms to combat frictionand wear. It is a key ingredient of electrical brushes used in many motors. It can be dispersed in water,solvents, oils, and greases to achieve better lubricity under extreme application conditions, such aslubrication of molds and dies in metal-forming, as well as flange faces of rails and railcar wheels. Graphiteis also used as a self-lubricating filler in various metal-, ceramic-, and polymer-matrix composites(Rohatgi et al., 1992; Prasad and McConnell, 1991; Gangopadhyay and Jahanmir, 1991). Carbon-graphitecomposites are rather common and widely used in various engine, aircraft, and seal applications.

FIGURE 22.7 Effect of water vapor pressure on wear rate of graphite. (From Savage, R.H. (1948), Graphitelubrication, J. Appl. Phys., 19, 1-10. With permission.)

Graphite fluoride is prepared by fluorinating graphite at stoichiometries from X = 0.3 to 1.1 in CFx.It is prepared by direct reaction of graphite with fluorine gas at controlled temperatures and pressuresand can be regarded as an intercalation compound of graphite. Fluorination increases the distancebetween atomic planes from about 0.34 nm to as high as 0.8 nm and, hence, results in easier shear andbetter lubricity (Fusaro and Sliney, 1970). It also causes basal planes of graphite to distort and lose theirplanar configuration. CFx is electrically insulating and nonwettable with water, but decomposes at about450°C.

Fluorination of graphite was shown by Fusaro and Sliney (1970) to substantially improve the lubricityand durability of this solid and make it less sensitive to variations in ambient humidity. Earlier studiesindicated that burnished CFx was capable of providing friction coefficients of 0.1 or less up to about480°C in open air. Compared to those of MoS2 and even HOPG under the same test conditions, suchfriction values were considerably lower. It is possible to prepare composite structures and resin-bondedfilms of CFx in order to achieve longer life; however, due to its high cost, CFx is rarely used by industry.

In a recent study, CFx was used as an additive to WS2 thin films to reduce their sensitivity to moisture(Zabinski et al., 1995). These films were produced on AISI 440C steel substrates by a pulsed laserdeposition method. Substrate temperature and CFx concentration were varied to control film micro-structure and chemistry. Tribological tests were conducted over a wide range of relative humidities (i.e.,<1 to 85% RH). Coatings with a low concentration of CFx exhibited ultra-low friction in dry air (frictioncoefficients <0.01), but the coefficients increased with increasing relative humidity. Films grown atelevated temperatures (300°C) or with higher concentrations of CFx showed insensitivity to humidity,but the friction coefficients were relatively high (0.04 in dry air).

22.2.1.3.1 Modern PracticesGraphitic lubricious precursors can also result from catalytic cracking of certain carbon-bearing gasesand can be used to lubricate surfaces, especially at high temperatures (Ashley, 1992; Lauer and Bunting,1988; Blanchet et al., 1994). This is done by injecting a stream of hydrocarbon-bearing gases into the testchamber where hot ceramic or metal surfaces are maintained and slid against one another. The hydro-carbons in the gas turn into a thin coating of graphite-like carbon that is responsible for lubrication.

In recent years, a few attempts were made to lubricate sliding surfaces by other carbon forms, such asbucky-balls (C60) (Bhushan et al., 1993) and hollow nanotubes of carbon (Falvo, 1999). In addition tothe powder form, sublimation or thermal evaporation methods were used to deposit C60 as stronglybonded and dense films on metallic and ceramic substrates. Depending on the form, density, and adhesionof these films to their substrates, friction coefficients of 0.15 to 0.5 were obtained. Ion irradiation of suchfilms with 2 MeV Ag+ and B+ ions at various doses resulted in partly crystalline to amorphous films thatwere able to provide friction coefficients of <0.1 (Bhattacharya et al., 1996).

Recently, a series of new boron-doped and partially graphitized carbon composites were developed toachieve better lubricity at elevated temperatures. Long-duration friction and wear tests were run as afunction of both increasing and decreasing temperatures to assess the durability and the friction andwear performance of the composites. As shown in Figure 22.8, the friction coefficients of the boron-doped carbon composite against a ceramic counterface were in the range of 0.05 to 0.1 at temperaturesup to 500°C. Based on analytical studies, it was concluded that the boron doping was essential forachieving higher oxidation resistance on these graphitic materials.

Vitreous or glassy carbon materials are made by pyrolysis of thermosetting polymers. Structurally,they are different from graphite, but the interatomic bonding and local arrangement at nanoscale aremore graphitic than diamond. They are extremely hard and, hence, more wear resistant than graphite,which is very soft. Just like graphite, the friction coefficient of glassy carbon shows high sensitivity torelative humidity of the test environment. Glassy carbon materials have very low fracture toughness, butcan be reinforced with metallic/nonmetallic fibers to achieve improved toughness. Copper-containingglassy carbon composites have high electrical conductivity and can be used for electrical contacts orbrushes (Burton and Burton, 1989).

Quite recently, researchers have produced highly disordered graphitic carbon layers on silicon carbideby reaction with chlorine and chlorine/hydrogen gas mixtures at 1000°C. The thickness of the graphiticlayer can vary from a few to 100 µm, depending on process time. When such a graphitized surface issubjected to sliding friction tests, very low (0.1 to 0.15) friction coefficients are achieved (Gogotsi et al.,1997). It is possible that such graphitic layers on rigid SiC substrates can be used to control friction andwear of microelectromechanical systems, sliding bearings (e.g., mechanical seals), electrical contacts, andbiomedical implants.

22.2.1.4 Hexagonal Boron Nitride (HBN)

HBN is a synthetic solid lubricant with high refractory and lubricity qualities at elevated temperatures.Below 1000°C, oxidation is negligible. It is chemically inert and resists attack by molten metals, oxides,glasses, slags, and fused salts. Its crystal structure is similar to that of graphite as shown in Figure 22.1.The atomic planes are made of two-dimensional arrays of boron and nitrogen atoms, configured in ahoneycomb pattern (Rowe, 1960). As in graphite, the bonding between the atoms of HBN in each layeris covalent and very strong, while bonding between the layers is of the weak van der Waals type.

HBN is typically produced by reacting B2O3 with urea or ammonia gases at high temperatures. Unlikegraphite, HBN has a white color. Its cubic analog (i.e., cubic boron nitride) is like diamond and isextremely hard and resistant to wear. HBN is generally produced in powder form. Depending on man-ufacturing conditions, different grades (i.e., turbostatic, quasi-turbostatic, meso-graphitic, and graphitic)of HBN are obtained. In terms of lubrication performance, the graphitic grade provides the best results.Purity and powder size of final products can also affect lubrication performance. The presence of boronoxide in the structure or as a binder makes a significant difference in the tribological performance of HBN.

HBN can be compacted into dense, solid pieces or parts by hot-pressing and can also be prepared asa composite structure. It can be plasma-sprayed with other ceramics to obtain a self-lubricating coating.Recently, very fine particles of HBN were incorporated into electroplated Ni coatings to provide superiorfriction and wear properties under unlubricated and high-load, high-temperature sliding conditions(Funatani and Kurosawa, 1994; Pushpavanam and Natarajan, 1995). HBN can also be used as a self-lubricating phase in ceramic composites. Such materials will be very attractive for mechanical face sealapplications. In a recent study, Westergard et al. (1998) investigated the tribological performance ofSi3N4/SiC composites containing 0 to 8 wt% HBN. All specimens were produced by hot isostatic pressing.The results indicated that the presence of HBN in the composite body lowered the friction coefficientsof test pairs from a range of 0.4 to 0.9 to a range of 0.02 to 0.1. Analytical studies revealed that slidingsurfaces were covered by a thin, well-adhering tribofilm, which may have been responsible for theimproved tribological performance.

HBN has also been used as an additive in oils and greases. Recent tests by Kimura et al. (1999) showedthat addition of HBN in concentrations as little as 1 wt% results in an order of magnitude reduction in

FIGURE 22.8 Friction performance of boron-doped carbon composite at temperatures to 525°C.

wear of bearing steels sliding against each other in line contacts. At higher concentrations, the reductionin wear is even greater. Similar improvements in antiwear and antifriction properties were reported forHBN-containing greases by Denton and Fang (1995).

HBN has high thermal and chemical stability and does not appreciably oxidize up to about 1000°C.Typical friction coefficients of HBN in air are 0.2 to 0.3 up to about 700°C. It has been used as a releaseagent in metalworking operations involving high temperatures (Golubus, 1970). In high vacuum, HBNloses its lubricity. Buckley (1978) reported a friction coefficient of 1 for HBN sliding against itself inultrahigh vacuum. Much earlier tests by Deacon and Goodman (1958), Rowe (1960), and Haltner (1966)gave friction coefficients of 0.4 to 0.7 in high vacuum in the outgassed states. Admission of certain organicvapors into the test chamber reduced friction coefficients to the 0.2 level. Recent fundamental studies byMartin et al. (1992) in ultrahigh vacuum (10–8 Pa) and under partial pressures of CO, C3H8, H2O, airwith 50% humidity, N2, and O2 resulted in friction coefficients of 0.1 to 0.7; Table 22.3 summarizes theirexperimental results. These experiments further reinforced the initial assertion that HBN is not onlysimilar to graphite in its crystal structure, but also in its lubrication behavior (Rowe, 1960; Rabinowicz,1964). Thus, one can understand why HBN is often referred to as “white graphite.”

22.2.1.5 Boric Acid

Boric acid is a lamellar solid lubricant (Figure 22.9) with a crystal structure similar to those of graphiteand HBN (Erdemir, 1991). It has a triclinic unit cell in which boron, oxygen, and hydrogen atoms arearrayed to form extensive atomic layers parallel to the basal plane of the crystal (see Figure 22.1). Becauseof the triclinic crystal structure, the c-axis is inclined to the basal plane at an angle of 101° (see Figure 22.1).This inclination causes shifting of alternate layers along the c-axis. Bonding between the atoms lying on

TABLE 22.3 Effect of Various Gases at Various Pressures on Friction Behavior of HBN Sliding Against Itself

Environment Steady-state Friction Coefficient

UHV, 10–8 Pa 0.6–0.7CO, C3H8, H2O, air (50% RH); 10–3 Pa 0.6–0.7CO, N2, O2; 10 Pa 0.6–0.7Air (50% RH); 10 Pa 0.4C3H8; 10 Pa 0.4Air (50% RH); 105 Pa 0.2Air (50% RH), atmospheric pressure 0.1

Data from Martin, J.M., LeMogne, T., Chassagnette, C., and Gardos, M.N.(1992), Friction of hexagonal boron nitride in various environments, Tribol.Trans., 35, 463-472.

FIGURE 22.9 SEM photomicrograph of lamellar structure of boric acid.

the same plane is of the covalent/ionic and hydrogen type; the layers are 0.318 nm apart and held togetheronly by weak van der Waals forces.

Boric acid exists in two major crystalline forms: metaboric acid (H2O·B2O3 or HBO2) and orthoboricacid (3H2O·B2O3 or H3BO3). Furthermore, metaboric acid has been reported to crystallize in threedifferent forms: orthorhombic or α-metaboric acid, monoclinic or β-metaboric acid, and cubic orΓ-metaboric acid. Among these, orthoboric and orthorhombic metaboric acids exhibit layered-crystalstructures and thus can provide low friction. Orthoboric acid exists as a natural mineral known inmineralogy books as sassolite. It is stable up to about 170°C.

Due to its layered-crystal structure, H3BO3 is a self-lubricating solid. To demonstrate its lubricity,Erdemir (1991) performed extensive friction tests with solid compacts of H3BO3 on a pin-on-diskmachine. Cylindrical rods with a nominal diameter of 1.27 cm were compacted from 99.8 wt% H3BO3

powders by cold-pressing at about 35 MPa. To establish point contact during friction tests, one end ofthe rod-shaped compacts was finished with a hemispherical cap of 5-cm radius. Subsequently, the boricacid pin was attached to the pin holder of a pin-on-disk machine and rubbed against a 50-cm-diameterAISI 52100 steel disk.

The friction coefficient of the pin/disk pair described above was measured as a function of slidingdistance. The initial friction coefficient of this tribosystem was approximately 0.2; it then decreasedsteadily with distance and eventually reached a steady-state value of 0.1 after sliding about 20 m.

H3BO3 can spontaneously form on the surfaces of boron and B2O3 films. Erdemir et al. (1990b)investigated the formation and tribological characteristics of such boric acid films formed on the surfacesof vacuum-evaporated B2O3 layers. They found that H3BO3, which formed spontaneously on the surfacesof B2O3 coatings, is remarkably lubricious. For a sliding pair of sapphire ball/B2O3-coated Al2O3 disk,they reported friction coefficients ranging from 0.02 to 0.05 in open air with 50% relative humidity,depending on applied force. Figure 22.10 presents the friction coefficients of various balls sliding againsta B2O3-coated Al2O3 disk under different contact loads. The use of a harder, more rigid ball (e.g., sapphire)results in a lower friction coefficient because the true contact area between a hard, rigid ball will besmaller than that between a soft, less-rigid ball. Friction force, which is a product of the true contactarea multiplied by the shear strength of the contact interface, will be much lower when hard, rigid ballsare used in sliding contact.

Based on surface and structure analytical studies, it was concluded that low friction is a direct conse-quence of the layered crystal structure of H3BO3 films forming on the exposed surfaces of the B2O3 coatingby the spontaneous chemical reaction:

FIGURE 22.10 Variation of friction coefficient of boric acid films under different loads during sliding against steeland ceramic balls. (Adapted from Erdemir, A. (1991), Tribological properties of boric acid and boric-acid-formingsurfaces. I. Crystal chemistry and mechanism of self-lubrication of boric acid, Lubr. Eng., 47, 168-172.)

SAPPHIRE BALL ALUMINA BALL STEEL BALL

1N 2N 5N 10N

0.09

0.07

0.05

0.03

0.01

µ

The above reaction occurs naturally, and a thin layer of H3BO3 forms everywhere on the exposed surfaceof B2O3 coatings. Formation of such self-lubricating and self-replenishing films was later demonstratedon VB2, B4C, and borided steel surfaces, affording very low friction coefficients to sliding metal andceramic surfaces (Erdemir et al., 1991, 1996b,d, 1998, 1999; Bindal and Erdemir, 1996).

As described previously, H3BO3 crystallizes in a triclinic crystal structure essentially made of atomiclayers parallel to the basal plane. The atoms lying on each layer are closely packed and strongly bondedto each other. The bonds between the boron and oxygen atoms are mostly covalent, with some ioniccharacter. Hydrogen bonds strongly hold the planar boron/oxygen groups together. The atomic layersare widely spaced (e.g., 0.318 nm apart) and held together by weak forces (e.g., van der Waals). Becauseof the ionic character of interatomic bonds, boric acid can dissolve in water and some other solvents.With its layered crystal structure, H3BO3 resembles other layered solids well-known for their goodlubrication capabilities (e.g., MoS2, graphite, and HBN).

Erdemir (1990) proposed that under shear stresses, plate-like crystallites of H3BO3 can align themselvesparallel to the direction of relative motion. Once so aligned, they can slide over one another with relativeease and thus impart the low friction coefficients shown in Figure 22.10.

Boric acid films were shown to bond strongly to the surface of aluminum and its alloys and provideexcellent lubricity when used as a metal-forming lubricant (Erdemir and Fenske, 1998). Used as a fillerin polymers, boric acid and boron oxide can substantially lower friction and increase the wear resistanceof base polymers (Figure 22.11) (Erdemir, 1995). It was demonstrated that sub-micrometer size powdersof boric acid can be dispersed in oils and greases to impart better lubricity and extreme pressure capability(Erdemir, 1995, 2000b).

For applications at elevated temperatures, the use of H3BO3 is not recommended for several reasons.First, above about 170°C, H3BO3 tends to decompose and eventually turn into B2O3, thus losing its layeredcrystal structure and hence its lubricity. Second, at temperatures greater than about 450°C, B2O3 becomesliquid-like and tends to react with underlying substrates. For metals, the chemical reaction is negligible,

FIGURE 22.11 Friction performance of polyimide and boron oxide-filled polyimide.

1 2 3 2

45 1

2 3 2 3 3

298

B O coating H O moisture H BO

H kJ mol

( ) + ( ) →

= −∆ .

and low friction can be reinstated by viscous-flow lubrication. However, for ceramics — especially forthe oxides — the situation is different. Liquid B2O3 can react with these ceramics and lead to highcorrosive wear. Friction can also be very high because of the highly viscous nature of the reaction products.

22.3 Lubrication Mechanisms of Layered Solids

In general, the lubricity and durability of a solid lubricant are controlled by a mechanism that involvesinterfilm sliding, intrafilm flow, and film/substrate or interface slip, as illustrated in Figure 22.12. It hasbeen found that the lamellar solid lubricants discussed above provide lubrication by an interlayer shearmechanism, mainly because the crystal structures of these solids are such that while the atoms lying onthe same layer are closely packed and strongly bonded to each other, the layers themselves are relativelyfar apart and the forces that bond them (e.g., van der Waals) are weak (see Figure 22.1). Strong interatomicbonding and packing in each layer give these solids the very high in-plane strength that is essential forlonger wear life or reduced wear damage during sliding. When present on a sliding surface, crystallinelayers of these solids align themselves parallel to the direction of relative motion and slide over oneanother with relative ease to provide lubrication. Furthermore, the formation of a smooth transfer filmon the sliding surfaces of counterface materials is also important for long wear life and the accommodationof sliding velocity, as well as for dissipation of frictional energy.

Recent electron microscopy studies of H3BO3-lubricated rubbing surfaces of steel test pairs clearlyrevealed some plate-like crystallites exhibiting a preferred alignment parallel to the sliding direction(Figure 22.13). Similar observations were made by TEM on sputtered MoS2 films after sliding tests. Severalother studies used X-ray diffraction to further verify that indeed some crystalline orientation occurs onmost lamellar solids during sliding tests (Wahl et al., 1995, 1999; Martin et al., 1994; Moser and Levy,1993). Crystalline layers can be made of single or several atomic planes. For example, in graphite, H3BO3,and HBN, the layers are made of a single atomic plane; while in MoS2 and other transition-metaldichalcogenides, the layers consist of three atomic planes (see Figure 22.1); in monochalcogenides, thereare four atomic planes in each layer (see Figure 22.6).

FIGURE 22.12 Schematic representation of three ways by which sliding can be accommodated between an uncoatedand a coated surface. (From Singer, I.L. (1992), Solid Lubrication Processes, in Fundamentals of Friction: Macroscopicand Microscopic Processes, Singer, I.L. and Pollock, H.M. (Eds.), NATO-ASI Series, Vol. 220, Kluwer Academic, London,237-261. With permission.)

L INTRAFILM FLOW

L INTERFACE SLIDING

L INTERFILM SLIDING

V

V

V

V

The electronic states of atoms in each layer play a major role in the lubricity of each solid. Graphiteand HBN are similar in electronic states. The only major difference between the bonding configurationof these two solid lubricants is that the π-bonding and π-antibonding bands overlap weakly at theBrillouin-zone boundary in graphite, which is responsible for the fairly good electrical conductivity ofgraphite; whereas these bands are separated by an energy gap of several electron-volts in stoichiometricBN, causing it to be an insulator. In both cases, if residual π attractions between atomic layers are noteliminated or reduced, high friction and wear may result. In graphite, π-bond interactions are generallyreduced or eliminated by intercalation. Both donor (e.g., alkali metals) and acceptor (metal chlorides)type intercalants can be used for this purpose (Levy, 1979; Dresselhaus, 1996). As a result, the interlayershear properties of graphite are markedly improved. However, attempts to find effective intercalationspecies for HBN were mostly unsuccessful.

Within the layered-crystal structure of transition-metal dichalcogenides, metal atoms are sandwichedbetween the chalcogen atoms in a planar array of S-Mo-S, while the layers in monochalcogenides consistof four atomic layers. For example, each layer of GaSe consists of strongly bonded Ga and Se atoms inthe sequence Se-Ga-Ga-Se (see Figure 22.6). The Ga atoms are paired to form the two atomic planesinside, while the chalcogen atoms form the top and bottom planes. Note that the removal of one layerof Ga atoms in the GaSe crystal structure would have produced the exact crystal structure of MoS2, asillustrated in Figure 22.1(c); this suggests that mono- and dichalcogenides are indeed closely related. Theinteratomic bonding within the layers of mono- and dichalcogenides is strong and mainly of the covalenttype, whereas the bonding between adjacent layers is weak and of the van der Waals type. Within therigid two-dimensional layers of MoS2 crystals, the S atoms have a trigonal prismatic coordination aroundthe Mo atoms, while the Ga atoms in the GaSe structure are tetrahedrally coordinated.

Previous studies have clearly demonstrated that the lubricity of a solid lubricant is controlled by anumber of intrinsic and extrinsic parameters. For example, intrinsically, both graphite and MoS2 havelayered crystal structures, but the extent of their lubricity and durability is largely controlled by extrinsicfactors such as the presence or absence of vapors or gaseous species in the test environment. Graphitefunctions best in humid air, while MoS2 lubricates best in dry and vacuum environments. As mentionedearlier, the lubrication behavior of HBN and H3BO3 is similar to that of graphite. This contrastingbehavior of self-lubricating solids has been — and still is — the subject of numerous fundamental studies.Thus far, suggestions have been made that the enhanced lubricity of graphite and HBN in a humidenvironment may be related to the weakening effect of water molecules on the residual π-bonds betweenthe layers of their crystals. As for the poor lubricity of MoS2 in a humid environment, it has been suggestedthat water molecules react directly or indirectly with MoS2 and thus alter the interatomic array and

FIGURE 22.13 Physical evidence for preferred crystalline orientation and intercrystallite slip on boric acid-lubri-cated surface.

bonding, which in turn increase friction. When tested in open air, MoS2 and other dichalcogenides werefound to react with oxygen and form MoO3 or other types of complex oxides. It has been speculatedthat the chemical reactions leading to MoO3 formation occur predominantly at the prismatic edges wherereactive dangling bonds exist.

Another intrinsic parameter that can affect the lubricity of a layered solid is the interlayer-to-intralayerbond-length ratio. It has been reported that this ratio is a crude but revealing indicator of the lubricityof a lamellar solid (Jamison, 1972, 1978). In general, it was found that the greater the interlayer-to-intralayer bond-length ratio, the weaker the interlayer bonding with respect to the intralayer bonds, andthus the higher the lubricity. For MoS2 and GaSe crystals, the interlayer-to-intralayer bond-length ratiosare 1.5 and 1.6, respectively (Zallen and Slade, 1974).

The presence or absence of electrostatic attractions between the layers of mono- and dichalcogenidesconstitutes yet another intrinsic parameter that can affect the lubricity of these solids. For example, ofthe numerous metal dichalcogenides, only a few can impart low friction to sliding tribological interfaces.Previous research has shown that despite their layered crystal structures, NbS2, TiS2, VS2, TaS2, etc., arenot as lubricious as MoS2 or WS2 (Clauss, 1972; Jamison, 1978). Based on the molecular orbital andvalence bond theories, Jamison (1972, 1978) proposed the following explanation for the poor lubricatingperformance of NbS2, TiS2, VS2, TaS2, etc. In the layered crystal structure of these solids, there is a regionof negative electrical charge that not only concentrates above the chalcogen atoms of a given layer, butalso extends well into the pockets between the chalcogen atoms of neighboring layers. Because the bottomsof the pockets are positively charged (due to the exposed ion cores of the surrounding atoms), anelectrostatic attraction exists between the layers, making the layers of these solids shear with difficulty.

As for the excellent solid-lubricating capacities of MoS2 and WS2, the region of negative electricalcharge is contained within the layers. Thus, the surfaces of the chalcogen atoms are positively charged,creating an electrostatic repulsion between the layers and making interlayer slippage exceedingly easy(Jamison, 1978). Relatively greater interlayer separation in MoS2 and WS2 crystals is thought to resultfrom the same electrostatic repulsion between successive layers.

In general, previous studies have clearly demonstrated that the friction and wear performance of solidlubricants are strongly affected by both the intrinsic (crystal-specific) and extrinsic (operating-environ-ment-specific) factors. Therefore, no single solid lubricant can provide low friction and wear in allenvironments. Furthermore, not all layered solids are good solid lubricants. The type and magnitude ofinterlayer bonds are also important.

22.4 High-Temperature Solid Lubricants

For applications in open air and at temperatures above 500°C, most of the lamellar solids mentionedabove lose their lubricity and become useless. Furthermore, most sliding interfaces (including metalsand non-oxide ceramics) become oxidized (Quinn and Winer, 1985). Thin oxide films that form on thesliding surfaces may, in turn, dominate the friction and wear behavior of these interfaces. In particular,wear debris particles trapped at sliding interfaces could be very abrasive and cause high wear. If the slidingbodies differ chemically or if there is a third or fourth body at the sliding interface, two or more oxidesmay form on the sliding surface and control friction and wear. In past years, significant research has beencarried out to study the shear rheology of such oxides and to formulate alloys or composite structuresthat can lead to the formation of oxides with very low shear strength (Peterson et al., 1994). These areoften referred to as lubricious oxides.

22.4.1 Lubricious Oxides, Fluorides, and Sulfates

Certain oxides (e.g., Re2O7, MoO3, PbO, B2O3, NiO, etc.), fluorides (e.g., CaF2, BaF2, SrF2, LiF, and MgF2),and sulfates (e.g., CaSO4, BaSO4, and SrSO4) become soft and highly shearable at elevated temperaturesand hence can be used as lubricants (Sliney et al., 1965; Sliney, 1969; John and Zabinski, 1999). Whenapplied as thin or thick coatings (by means of PVD, plasma spraying, fusion bonding, etc.), these solids

can provide acceptable levels of friction coefficients and long wear life. They can also be mixed with othersolid lubricants to obtain lubrication over much wider temperature ranges. Major drawbacks of oxide-based lubricants are that they are inherently brittle and thus may fracture easily and wear out quickly.Furthermore, most oxide-based lubricants do not provide lubrication down to room temperature. Poten-tial applications for lubricious oxides include high-temperature seals, bearings, and gears, valves andvalve seats, variable stator vanes, and foil bearings.

Recent systematic studies have demonstrated that the oxides of Re, Ti, Ni, W, Mo, Zn, V, B, etc., becomehighly lubricious and can provide fairly low friction at elevated temperatures (Kanakia et al., 1984;Kanakia and Peterson, 1987; Peterson et al., 1960, 1982, 1994). Mixed oxides (e.g., CuO-Re2O7, CuO-MoO3, PbO-B2O3, PbO-MoO3, CoO-MoO3, Cs2O-MoO3, NiO-MoO3) can also provide wider operationalranges and can be prepared as alloys or composite structures to provide longer durability. The lubriciouslayers that form by oxidation of alloy surfaces are very desirable and exceptionally advantageous whencompared with the solid lubricant coatings with finite lifetimes. At high temperatures, as the oxide layeris depleted from the surface by wear, the alloying ingredients diffuse toward the surface where the oxygenpotential is higher; they oxidize again to replenish the consumed lubricious layers that have low shearstrength and/or surface energy to decrease friction (Peterson et al., 1982, 1994). Figure 22.14 shows thefrictional performance of CuO-MoO3 at different temperatures (Wahl et al., 1997).

In a series of fundamental studies, Gardos (1988) demonstrated that at a very narrow range of anionvacancies and at high temperatures, crystalline TiO2 (rutile) and rutile-forming surfaces can provide verylow friction coefficients to sliding tribological interfaces. Further work by Gardos (1993) and Woydt et al.(1999) demonstrated the formation of Magneli phases on sliding surfaces containing titanium-basedalloys and compounds. Their findings suggested that Magneli phases are principally the result of tribo-oxidation and that once formed, they can dominate the tribological behavior of sliding ceramic interfaces,mainly because of their unique shear properties. However, TiOx-based solid lubricants have not yet foundwide use, mainly because of the difficulty in achieving and maintaining the very narrow range of oxidestoichiometry needed for good lubricity.

A new breed of lubricious zinc oxide films was recently synthesized by pulsed-laser deposition, andtheir tribological properties were explored over a wide range of test conditions (Zabinski et al., 1997).The stoichiometry and microstructure of these films were found to have profound effects on lubricityand were controlled by adjusting substrate temperature and oxygen partial pressure during deposition.Zinc oxide films with oxygen deficiency and nanoscale structure were found to provide low frictioncoefficients and long wear lives at room temperature. However, as the chemical stoichiometry and crystalstructure approached those of the bulk zinc oxide, the tribological properties and load/speed sensitivity

FIGURE 22.14 Effect of test temperature on friction coefficient of an ion-beam-deposited Cu-Mo film; lowerfriction coefficients at high temperatures are attributed to formation of CuO-MoO3 films. (From Wahl, K.J., Seitzman,L.E., Bolster, R.N., Singer I.L., and Peterson, M.B. (1997), Ion-beam deposited Cu-Mo coatings as high temperaturesolid lubricants, Surf. Coat. Technol., 89, 245-251. With permission.)

0.6

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00 1000 2000 3000

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of the films degraded. Figure 22.15 shows the variation of friction coefficient of a pulsed-laser-depositedZnO film during sliding against an AISI 440C steel ball.

22.4.2 Composites

Plasma-sprayed self-lubricating composites and adaptive lubricants were recently engineered to combatfriction and wear problems at high temperatures. The composite coatings consist of silver and alkalinehalides (i.e., CaF2, BaF2) as the self-lubricating entities and chrome carbide and/or oxide as the wear-resisting entities (DellaCorte and Sliney, 1987, 1990; Sliney, 1993; DellaCorte and Fellenstein, 1997).Thick plasma sprayed coatings (0.1 to 0.2 mm) and bulk powder metallurgy composite forms of thesesolid lubricants provide friction coefficients ranging from 0.2 to 0.5, depending on ambient temperature,load, and speed. Over the years, these solid lubricants have been highly optimized and carefully formulatedand the latest formulations are capable of providing lubrication over much broader temperature rangesthan their earlier versions. Figure 22.16 shows the friction performance of PS-304 (consisting of 20 wt%Cr2O3, 10 wt% Ag, 10 wt% BaF2/CaF2 eutectic composition, and NiCr as the binder) against an aluminaball at temperatures up to 870°C. Recent studies have also demonstrated that these lubricants are very

FIGURE 22.15 Variation of friction coefficient of pulsed-laser deposited ZnO film with number of sliding cycles.(From Zabinski, J.S., Saunders, J.H., Nainaparampil, J., and Prasad, S.V. (2000), Lubrication using a microstructurallyengineered oxide: performance and mechanisms, Tribol. Lett., 103-116. With permission.)

FIGURE 22.16 Friction performance of PS-304 self-lubricating composite coating at temperatures to 870°C.

Cycles

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suitable for high-speed sliding bearing surfaces and provide excellent durability and frictional perfor-mance, especially when used in foil bearing applications (DellaCorte, 1998).

To achieve low friction from room temperature to very high temperature, a series of adaptive solidlubricants has recently been developed. A good adaptive lubricant is made of several ingredients thatprovide low friction at low temperatures, and as the temperature increases, these lubricious ingredientsreact with each other and/or oxygen in air to form a high-temperature solid lubricant phase providinglow friction. The lubricating entities in this case were selected from those metals that can react with theenvironment to form the kind of lubricious layers needed (Wlack et al., 1997; Zabinski et al., 1992). Oneproblem is that the oxidation is not reversible, so when the temperature returns to low values, the frictionmay increase. To solve this problem, researchers have used very thin diffusion-barrier layers to limit theextent of oxidation to the very top surface rather than to the bulk or over a thick layer. Another approachwas to use capsules of high-temperature adaptive lubricants in a low-temperature matrix. While the low-temperature matrix provides lubricity at lower temperatures, the capsules with a protective shell on thesurface react with oxygen and become lubricious, thus providing the needed level of lubricity.

22.4.3 New Approaches to Solid Lubrication at High Temperatures

Recently, a crystal-chemical approach was introduced by Erdemir (2000a) to classify lubricious oxideson the basis of lubrication performance and operational limits. This approach was proposed to serve asa guide for determining the kind(s) of lubricious oxides needed on a sliding surface at high temperatures.Apparently, the crystal chemistry of certain oxides that form on sliding surfaces relates strongly to theirshear rheology and hence their lubricity at high temperatures.

The principle of the crystal-chemical approach is based essentially on the ionic potential of an oxideand is defined as γ = Z/r, where Z is the cationic charge and r is the radius of the cation. Erdemir (1999)proposed that using this principle, one can establish model relationship(s) between the quantum-chem-ical characteristics and the lubricity of oxides at high temperatures. Specifically, it is possible to establisha correlation between the ionic potential or the cationic field strength of an oxide and its shear rheology,and hence its lubricity.

Apparently, ionic potential controls several key physical and chemical phenomena in oxides. In general,the higher the ionic potential, the greater the extent of screening of a cation in an oxide by surroundinganions such as B2O3 or Re2O7. Oxides with highly screened cations are generally soft and their meltingpoints are low. Their cations are well-separated and completely screened by anions; hence, they havelittle or no chemical interaction with other cations in the system. Most of their bonding is with sur-rounding anions. Conversely, oxides with lower cationic field strengths or ionic potentials (e.g., Al2O3,ZrO2, MgO, and ThO2) are very strong, stiff, and difficult to shear, even at high temperatures, becausetheir cations interact with each other and form strong bonds.

The crystal-chemical approach can be used to predict the extent of adhesive interactions between twoor more oxides at a sliding interface; hence, it can be used to predict frictional performance. Extensiveresearch by previous investigators has already identified several lubricious oxides that afford fairly low(≈0.2) friction coefficients at elevated temperatures (Kanakia et al., 1984; Kanakia and Peterson, 1987;Peterson et al., 1960, 1982, 1994). Some of these oxides and their friction coefficients at high temperaturesare shown in Figure 22.17. As can be deduced from this figure, the higher the ionic potential, the lowerthe friction coefficient. This means that oxides with higher ionic potentials appear to shear more easilyand thus exhibit lower friction at high temperatures. As mentioned earlier, the higher the ionic potential,the greater the screening of a cation in an oxide by surrounding anions. The highly screened cations inan oxide will interact very little with other cations in their surroundings, and this will allow them toshear more easily at elevated temperatures.

In most tribological situations, two or more dissimilar solid bodies may be rubbing against each other,and often the sliding surfaces are covered by more than one kind of oxide. The crystal-chemical approachintroduced in this chapter can also be used to predict the lubricity of such complex binary oxide systems.Specifically, crystal chemistry can be used to estimate the solubility, chemical reactivity, number of

compounds formed, and eutectic temperature or lowering of the melting point of an oxide when a secondoxide is present. For example, the eutectic temperature and compound-forming tendencies of two oxidesare closely related to the cationic field strengths or ionic potentials of the involved elemental species. Theability of an oxide to dissolve in or react with other oxides or to form complex oxides is estimated fromthe difference in relative ionic potentials of the oxides in the system. In general, the greater the differencein ionic potential, the lower the eutectic temperature and the greater the tendency to form complex oxides.

Figure 22.18 shows several cases in which two oxides (CuO-Re2O7, CuO-MoO3, PbO-B2O3, PbO-MoO3,CoO-MoO3, and NiO-MoO3, etc.) were either present at or purposely introduced to the sliding interfacesto achieve low friction at high temperatures. Most of these data were extracted from papers and progressreports authored by Peterson et al. (Peterson, 1987; Peterson et al., 1960, 1982, 1994), who have hadextensive experience with lubricious oxides. Nickel-based superalloys, because of their relevance to high-temperature applications, were used as substrates in most of their studies. The specially formulated nickelalloys contained Ti, Ta, W, Re, B, and Mo as potential lubricious oxide formers. Note that the scatter inthe friction values shown in Figures 22.17 and 22.18 is large; this is not unusual in the field of tribologybecause test machines, conditions, or parameters vary greatly from study to study.

Recently, Cs-based oxides were reported to be very promising for lubricating Si-based ceramic com-ponents at high temperatures. At 600°C, 0.02 to 0.1 friction coefficients have been reported for Cs2O-lubricated Si3N4 ceramics (Strong and Zabinski, 1999). During sliding at high temperature, a mixed oxidelayer consisting of Cs2O and SiO2 was found and believed to be responsible for low friction. As can beseen from Figure 22.18, such a combination would result in a large difference in the ionic potentials ofthese two oxides.

From Figure 22.18, it can be seen that as the difference in ionic potential increases, the lubricity of theoxide species also increases. There are two fundamental reasons for this phenomenon. One is that as thedifference in ionic potential increases, the ability of oxides to form a low-melting-point or readilyshearable compound improves; hence, oxides tend to exhibit lower hardness and shear strength at elevatedtemperatures because the anions are able to better shield or screen the cations and thus make them lesslikely to interact with neighboring cations. The second reason for the phenomenon is that the ability oraffinity of ionic species to form highly stable compounds (that exert very little chemical or electrostatic

FIGURE 22.17 Relationship between ionic potentials and friction coefficients of single oxides. (Adapted fromErdemir, A. (2000a), A crystal chemical approach to lubrication by solid oxides, Tribol. Lett., 8, 97-102.)

attraction) improves as the difference in ionic potential increases. Lower attraction between slidingsurfaces means lower adhesive forces across the sliding contact interfaces, and hence lower friction(Erdemir, 2000a).

22.5 Self-Lubricating Composites

22.5.1 Traditional Materials

Self-lubricating composites have been available for a long time and are used rather extensively by industryto combat friction and wear in a variety of sliding, rolling, and rotating bearing applications. They aregenerally prepared by dispersing appropriate amounts of a self-lubricating solid (as fillers, preferably inpowder form) with a polymer, metal, or ceramic matrix. With powder metallurgy techniques, fillers andmatrix materials can be thoroughly mixed, compacted, and then sintered (if necessary) to obtain thedesired shape. They can also be extruded, rolled, or hot/cold-pressed into useful shapes. Recently,compositionally and functionally gradient self-lubricating composite structures were also manufacturedand offered for industrial use. While the core is made of nearly pure matrix material to provide highstrength, hardness, and toughness, the near-surface regions where sliding will occur are enriched in self-lubricating powders to achieve lubricity. Composite structures prepared in this fashion are used for awide range of tribological applications, such as bushings, bearings, and a variety of gears and tractiondevices. For example, copper-graphite and silver-graphite composites are used in electrical brushes andcontact strips, while aluminum-graphite composites are well-suited for bearings, pistons, and cylinderliners in engines and a host of other mechanical systems (Kumar and Sudarshan, 1996).

Recent tribological studies have demonstrated that when mixed at correct concentrations with optimalparticle sizes, self-lubricating filler materials can have a substantial beneficial impact on the mechanicaland tribological properties of matrix materials. For example, it was shown that graphite, MoS2, and boricacid fillers tend to increase the wear resistance of nylon and polytetrafluoroethylene (PTFE)-type poly-mers (Blanchet and Kennedy, 1992; Fusaro, 1990). Aluminum-graphite composites exhibit excellentlubricity, durability, and resistance to galling under both dry and lubricated conditions (Rohatgi et al.,

FIGURE 22.18 Relationship between friction coefficient and difference in ionic potentials of double oxides. (FromErdemir, A. (1999), A crystal chemical approach to lubrication by solid oxides, Tribol. Lett., 8, 97-102.)

1992). When the graphite content in aluminum-matrix composites exceeds ≈20 vol%, the friction coef-ficient approaches that of pure graphite and becomes highly independent of the matrix alloy. Aluminum-WS2 composites were also found to be very effective in reducing galling and in providing excellent lubricityand durability, especially in high-vacuum environments (Prasad and Mecklenburg, 1994). The presenceof WS2 particles in the matrix results in significantly increased resistance to seizure and enables thecomposite body to operate under very high loads without galling.

Recent studies concluded that improved tribological behavior was mainly due to the formation of athin transfer layer on the sliding surfaces of counterface materials. In the case of polymers, a significantincrease in mechanical strength was also observed and thought to be responsible for high wear resistance.It was found that, initially, transfer films were not present but formed as a result of surface wear andsubsurface deformation. They are continuously replenished by embedded graphite particles dispersed inthe matrix (Rohatgi et al., 1992).

In addition to metal-matrix composites, a series of self-lubricating polymer and ceramic matrixcomposites have also been developed, tested, and offered for industrial use in recent years (Gangopadhyayand Jahanmir, 1991; Prasad and Mecklenburg, 1994; Fredrich et al., 1995). These composites are emergingas an important class of tribological materials, offering new means to combat friction, wear, and gallingunder extreme conditions. In a recent study, tribological properties of fine-grain alumina (20%)-graphitecomposites were explored as potential candidates for advance sealing applications. Pin-on-disk wear testsshowed that friction coefficients can be reduced from 0.5 for alumina-on-alumina to ≈0.25 for alumina-graphite composite (Yu and Kellett, 1996).

In another study, ceramic-matrix composites were fabricated by drilling a series of small holes inalumina and silicon nitride ceramics and then filling the holes with NiCl2-intercalated graphite underhigh pressure. Addition of graphite to silicon nitride considerably reduced the friction coefficient, butthe alumina-graphite composites exhibited only a marginal reduction in friction coefficient comparedto that of the alumina. The reduction in friction coefficient for silicon nitride-graphite composite canbe explained by the formation of transfer films consisting of a mixture of materials from both contactingsurfaces. However, for the alumina-graphite composites, the graphite regions were completely coveredwith steel wear particles, inhibiting the formation of graphite-containing transfer films (Gangopadhyayand Jahanmir, 1991).

Mixing of Sb2O3 with MoS2 was shown to act synergistically to improve the friction and wear behaviorof MoS2. Specifically, the tribological behavior improves because only the thin layers of MoS2 residingon top are exposed to the environment, while the MoS2 at the bottom is protected against thermal orenvironmental degradation by Sb2O3, which also acts as a beneficial support for MoS2. The proposedmechanism suggests that composite structures containing Sb2O3 were also found to be more resistant totribo-oxidation than was pure MoS2 alone (Zabinski et al., 1993).

22.5.2 New Self-lubricating Composite Coatings and Structures

Recent advances in PVD and CVD technologies have led to the development of a new generation of self-lubricating nanocomposite films and multilayer coatings. One such film is based on the MoS2 and Tisystem and is produced by closed-field unbalanced magnetron sputtering. This film is much harder andmore wear-resistant than conventional MoS2 coatings, yet it still has the low friction characteristics ofconventional MoS2 films. Its friction coefficient against a steel ball is ≈0.02 in humid air and <0.01 indry N2. The Vickers hardness value could be >1500 HV (Teer et al., 1997; Fox et al., 1999). Furthermore,this coating is not greatly affected by moisture in the test environment. It is proposed for use in a varietyof dry sliding and machining applications (e.g., milling, drilling, tapping, cold-forming dies and punches,stamping, bearings, and gears for aerospace and vacuum applications).

New coating architectures based on layers of a self-lubricating solid (e.g., MoS2, WS2, etc.) and a metal,ceramic, or hard metal nitride or carbide (i.e, Ti, TiN, TiC, Pb, PbO, ZnO, Sb2O3) were also producedin recent years and were shown to work extremely well under demanding tribological conditions. Thesecoatings can be prepared by co-sputtering of MoS2 and TiN or TiB2 targets, or a single target composed

of TiN and MoS2. The resultant coatings may consist of distinct TiN and MoSx phases in the form of ananodispersive system. The hardness of these coatings could be as high as 20 GPa, while their frictioncoefficients are generally low (i.e., ≈0.1), even in open-air environments. Because of high hardness andlow friction, they can be used in both sliding and machining applications (Gilmore et al., 1998a,b).Recently, researchers have also produced multilayers of MoSX/Pb and MoSX/Ti (with individual layerthickness in the 4 to 100 nm range) by magnetron sputtering at room temperature. Sliding wear tests in50% relative humidity showed great improvements in wear life over that of pure MoSx coatings (Sim-monds et al., 1998). The three-dimensional design of adaptive coatings based on a multicomponentMoS2/TiC/DLC coating architecture resulted in improved tribological properties over broad ranges ofenvironmental humidity and other test parameters (Voevodin et al., 1998). While the hard coatings ofTiC and DLC provided high strength and resistance to wear, solid lubricants DLC and MoS2 providedlow friction at the sliding surface. The coating had friction coefficients of 0.15 in humid air and 0.02 indry nitrogen, thus increasing the prospects for use in space mechanisms.

PbO/MoS2 and ZnO/WS2 nanocomposite films were also produced in recent years and tested for theirlubricity and durability in a variety of environments. The volume fraction of WS2 decreased withincreasing depth from the surface (Wlack et al., 1994). Composite films perform significantly betterduring tribotesting than films composed entirely of MoS2 or PbO and ZnO. In addition, the compositefilms demonstrate the properties of “adaptive” lubricants. MoS2 provides lubrication at room tempera-ture; however, when the films are exposed to oxidizing environments at elevated temperatures, they adaptby forming PbMoO4. This compound has been noted to display lubricant properties at high temperature.Thus, there is significant potential for tailoring film compositions so that the components react to producelubricious wear debris and lubrication over extended temperature ranges (Wlack et al., 1997; Zabinskiet al., 1992).

Electroless nickel, chromium, nickel-phosphorus coatings containing small amounts of graphite, MoS2,PTFE, and diamond particles were also developed in recent years and used to achieve relatively thickfilms with self-lubricating properties. The deposition of MoS2 containing Ni-P composite coatings (con-taining ≈3 wt% MoS2) resulted in significant improvements in wear resistance and reduced the frictioncoefficient of the base Ni coatings (Moonir-Vaghefi et al., 1997).

22.6 Soft Metals

Mainly because of their low shear strengths and rapid recovery as well as recrystallization, certain puremetals (e.g., In, Sn, Pb, Ag, Au, Pt, Sn, etc.) can provide low friction on sliding surfaces (Wells and DeWet, 1988; Sherbiney and Halling, 1977). They are used chiefly as solid lubricants because the attractiveproperties they combine are unavailable in other solid lubricants. For example, in addition to its softnature, silver has excellent electrical and thermal conductivity, oxidation resistance, good transfer-film-forming tendency, and a relatively high melting point; thus, it has been commercially used to lubricatethe high-speed ball bearings of rotating anode X-ray tubes for many years. The Mohs hardness values ofsoft metals are generally between 1 and 3. Reported friction coefficients of soft metals range from 0.1 to0.4, depending on the metal and test conditions. Pb, In, and Sn provide better lubricity at room temperaturethan Ag, Au, and Pt. At elevated temperatures, Pb, Sn, and In melt and undergo oxidation. Ag, Au, andPt have fairly high melting points, do not oxidize appreciably, and hence are preferred for high-temperaturelubrication purposes (Erdemir and Erck, 1996; Maillat et al., 1993; Seki et al., 1995). Au remains in metallicform regardless of the temperature, while Ag2O decomposes as the temperature increases and Pt oxidizesonly slightly. Bronze and babbitts prepared by alloying some of these soft metals with Al, Zn, Cu, havebeen used as bushings, bearings, and other tribological applications for a number of years.

Soft metals are generally produced as thin films on surfaces to be lubricated. Simple electroplatingand vacuum evaporation can be used to deposit most of these metals as self-lubricating films, but denseand highly adherent films are produced by ion plating, sputtering, or ion-beam-assisted depositiontechniques (Erdemir et al., 1990a). Film-to-substrate adhesion is extremely critical for achieving long

wear life or durability, especially on the surfaces of ceramic tribomaterials (Spalvins and Sliney, 1994;Spalvins, 1998). The thickness of the soft metallic films also plays a major role in both friction and wear.The lowest friction coefficients and wear rates are usually obtained with thinner films (i.e., 0.5 to 1 µmthick) and under higher contact pressures (Dayson, 1971; El-Sherbiny and Salem, 1986). Figure 22.19shows the variation of friction coefficient of an indium film with thickness (Sherbiney and Halling, 1977).However, too thin a film tends to wear out quickly. Also, the friction coefficients of most soft metals tendto decrease as the ambient temperature increases, mainly because of additional softening and rapidrecovery from strain hardening. Thick films result in large contact areas and hence high friction.

The combination of very high thermal conductivity with low shear strength and chemical inertnessmakes silver and gold coatings ideal for applications involving high frictional or ambient heating, suchas sliding ceramic interfaces. As can be seen in Figure 22.20, thin Ag films can lower wear rates of zirconiaballs and disks by factors of 2 to 3 orders of magnitude. Reduction in wear is more dramatic at highersliding speeds. This is mainly because of the fact that zirconia has a very poor thermal conductivity, andthus suffers severe thermomechanical wear at high sliding velocities. However, when a highly thermally

FIGURE 22.19 Variation of friction coefficient of indium films as a function of film thickness. (From Sherbiney,M.A. and Halling, J. (1977), Friction and wear of ion-plated soft metallic films, Wear, 45, 211-220. With permission.)

FIGURE 22.20 Wear performance of uncoated and silver-coated zirconia (calcia-partially stabilized) test pairs at slidingvelocities up to 2 m/s. (Adapted from Erdemir, A., Busch, D.E., Erck, R.A., Fenske, G.R., and Lee, R.H. (1991b), Ion-beam-assisted deposition of silver films on zirconia ceramics for improved tribological behavior, Lubr. Eng., 47, 863-867.)

conductive film like silver is present at the sliding interface, the wear rate decreases dramatically, mainlybecause frictional heat is dissipated rapidly from the sliding interface. The low friction coefficient of silveralso helps in reduced frictional heating. Figure 22.21 shows the condition of a wear track formed on asilver-coated zirconia disk. Overall, the film is still intact; only the tips of substrate asperities are exposed,but the base zirconia is well-protected against wear. Figure 22.21 also reveals some physical evidence forshear deformation experienced by soft silver film during contact sliding.

Silver is used as a lubricant in X-ray tubes, certain satellite parts, ball bearings, bolts, and other slidingparts in nuclear reactors. When applied as a dense and adherent coating on the surfaces of thesecomponents, it can effectively dissipate frictional heat that can otherwise cause thermomechanical andtribochemical wear. Used on ceramic surfaces, it shears easily, thereby reducing the friction and micro-fracture-induced wear of the sliding ceramic surfaces (Erdemir et al., 1990a, 1991). Silver and other softmetallic coatings can also protect the sliding surfaces against environmental and/or tribochemical deg-radation under dry and oil-lubricated sliding contact conditions (Ajayi et al., 1993; Erdemir et al., 1992;DellaCorte et al., 1988). Under lubricated sliding conditions, thin silver films were extremely effective inreducing friction and wear at temperatures up to 300°C (Ajayi et al., 1993, 1994; Erdemir et al., 1992,1996a). One of the major shortcomings of metallic solid lubricants is that most of them react with sulfurand chlorine (if present in the operating environment) and may undergo rapid corrosive wear.

22.7 Polymers

Polymers in various forms are widely used in tribology. They are lightweight, relatively inexpensive, andeasy to fabricate. They can easily be blended with other solids to make self-lubricating composite struc-tures. Certain polymers (polytetrafluoroethylene [PTFE], polyimide, nylon, ultra-high-molecular-weightpolyethylene [UHMWPE], etc.) are self-lubricating when used in both the bulk and thin-film forms, oras binders for other solid lubricants (Lancaster, 1984; Fusaro, 1988, 1990; Gresham, 1994; Jamison, 1994).

Coatings can be produced on a tribological surface by first spraying or sprinkling the powders, thenconsolidating and curing them at high temperatures. The most common polymer-based solid lubricantis PTFE, which is widely known as Teflon (an E.I. DuPont de Nemours Co. trade name). It is a “nonstick”surfacing agent used in cookware, seals, and gaskets to facilitate release. It is also used in various otherforms (powder, composite, colloidal dispersion in oils and greases) to achieve low friction. Its frictioncoefficient ranges from 0.04 to 0.2, depending on test conditions. PTFE can be used at temperatures upto about 250°C. Polyimide and its coatings can also provide low friction. It can also be composited witha self-lubricating inorganic filler to enhance its mechanical and tribological properties, especially at

FIGURE 22.21 Physical condition of a wear track formed on silver-coated zirconia disk during sliding againstzirconia ball.

elevated temperatures (Fusaro and Sliney, 1973; Blanchet and Kennedy, 1992). Fine PTFE powders havealso been used as additives in various oils and as thickeners in greases (Willson, 1992).

UHMWPE is another popular polymer used widely in total joint replacements (Kurtz et al., 1999).Because of the very long molecules and highly entangled molecular chains, it provides better wearresistance than PTFE. However, wear of this polymer still poses a major obstacle for the longevity of thetotal joint replacements. Recent efforts to solve these problems have increased interest in the structure,morphology, and mechanical properties of the UHMWPE and in various surface and structural treatmentprocesses (such as crosslinking, carbon-fiber reinforcing, recrystallization). It was reported thatcrosslinked UHMWPE has much better wear properties and thus is a promising alternative for total jointreplacements. There are several excellent review articles and book chapters devoted to the tribologicaluses of UHMWPE and other polymers in various applications. It is impossible to cover all of them here,but readers can refer to the references provided here for further information (Wang et al., 1995; Briscoe,1990; Zhang, 1997; Bahadur and Gong, 1992; Friedrich et al., 1995).

22.8 Summary and Future Directions

This chapter further demonstrates that solid lubricants have much to offer for demanding tribologicalapplications. Their use in advanced tribosystems is expected to increase in the near future, mainly becausethe operating conditions of future tribosystems are becoming more and more demanding. One majorproblem is that there exists no such lubricant that is capable of providing reasonably low and consistentfriction coefficients over broad test conditions, temperatures, and environments. The results of previousstudies demonstrate that the performance of layered solid lubricants are very much dependent ontribological and environmental conditions. For example, the lubricity of transition-metal dichalcogenidesis adversely affected by moisture, while graphite depends on moisture for good lubricity. Layered solidlubricants can be doped or intercalated with a number of metals and compounds to achieve lessersensitivity to ambient humidity and temperature.

Nowadays, most solid lubricants are produced as thin solid films on sliding surfaces. They are alsoused as fillers in self-lubricating metallic, ceramic, and polymeric composites. In most cases, a transferfilm is found on the sliding surfaces. In general, formation of such a film at sliding interfaces seems tobe key to achieving low friction and long wear lives in most solid-lubricated surfaces. For solid lubricantfilms, strong adhesion is key for long service life. Modern sputtering techniques and ion-beam processesare quite capable of imparting strong adhesion between solid lubricant films and their substrates. Ion-beam mixing of conventional solid lubricants, such as MoS2, with ceramics is also feasible and appearspromising for severe tribological applications.

For materials with poor thermal conductivity, Ag and Au films combining high thermal conductivitywith low shear strength and good chemical inertness should be considered. Silver is primarily used as alubricant in ball bearings of rotating anode X-ray tubes. A unique solid lubricant, boric acid, which formsnaturally on the surfaces of boric oxide- and boron-containing ceramics, has recently been discovered.It was shown that this lubricant can impart remarkably low friction coefficients (e.g., 0.02) to slidinginterfaces in moist environments where MoS2 is known to be ineffective.

For applications involving high temperatures, most layered solid lubricants appear ineffective. Acombination of solid and liquid lubrication may provide short-term solutions to this problem; but fora long-term solution, the development of effective lubricious oxides, fluorides, and other compounds isessential. Recently, a crystal-chemical approach was introduced to classify lubricious oxides on the basisof their lubrication performance and operational limits at high temperatures. This approach may serveas a basis for determining the kind(s) of lubricious oxides needed on a sliding surface at high temperatures.Lubrication from vapor phases and by catalytic cracking of carbonaceous gases also appears promising.Recently, sulfates of Ca, Ba, and Sr were shown to provide quite a low friction coefficient at hightemperatures. A series of adaptive lubrication strategies was also introduced in recent years and shownto be effective in achieving lubrication at broader temperature ranges.

Certain polymers are also used as solid lubricants because the attractive properties they combine areunavailable in other solid lubricants. Polymers are particularly favored for applications where cost, weight,corrosion, and biocompatibility are the major considerations. In short, solid lubricants have been aroundfor a long time and they have been meeting some very important and critical tribological needs. Theyare expected to be in high demand for many more years to come.

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