22
Argonne National Laboratory
22.1 Introduction
General Characteristics of Solid Lubricants • New Products,
Practices, and Approaches in Solid Lubrication
22.2 Classification of Solid Lubricants
Lamellar Solid Lubricants
22.3 Lubrication Mechanisms of Layered Solids
22.4 High-Temperature Solid Lubricants
Lubricious Oxides, Fluorides, and Sulfates • Composites •
New Approaches to Solid Lubrication at High Temperatures
22.5 Self-Lubricating Composites
Traditional Materials • New Self-lubricating Composite
Coatings and Structures
22.6 Soft Metals
22.7 Polymers
22.8 Summary and Future Directions
In most tribological applications, liquid or grease lubricants are used to combat friction and wear; but when 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 lubricants
are summarized in Table 22.1
. A combination of solid and liquid lubrication is also feasible and may
have a beneficial synergistic effect on the friction and wear performance of sliding surfaces. Solid lubricants can be dispersed in water, oils, and greases to achieve improved friction and wear properties under conditions 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 sliding surfaces. 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 National
Laboratory (“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 license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.
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TABLE 22.1
Comparison of Solid and Liquid Lubricants in Tribological Applications
Application Environment and/or Condition
Vacuum
Pressure
Temperature
Electrical conductivity
Radiation
Wear
Friction
Thermal conductivity and heat dissipation capability
Storage
Solid Lubricants
Some solids (i.e., transition-metal dichalcogenides) lubricate extremely well in high vacuum, have very low vapor pressure
Can endure extreme pressures
Relatively insensitive; can function at very low and high temperatures; low heat generation due to shear
Some provide excellent electrical conductivity
Relatively insensitive to nuclear radiation
Provide excellent wear performance or durability at slow speeds and under fretting conditions; lifetime is determined by lubricant film thickness and wear rate
Extremely low friction coefficients are feasible
Excellent for metallic lubricants; poor for most inorganic or layered solids
Liquid and Grease Lubricants
Most liquids evaporate, but perfluoropolyalkylethers (PFPE) and polyalfaolefins (PAO) have good durability
May not support extreme pressures without additives
May solidify at low temperatures and decompose or oxidize at high temperatures; heat generation varies with viscosity
Mostly insulating
May degrade or decompose over time
Provide marginal performance and durability at slow speeds and under fretting conditions; need additives for boundary lubrication
Depends on viscosity, boundary films, and temperature
Good
Hygiene
Compatibility with tribological surfaces
Resistance to aqueous and chemically aggressive environments
Can be stored for very long times (dichalcogenides are sensitive to humidity and oxygen)
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
Compatible with hard-to-lubricate surfaces (i.e., Al,
Ti, stainless steels, and ceramics)
Relatively insensitive to aqueous environments, chemical solvents, fuels, certain acids and bases
May evaporate, drain, creep, or migrate during storage
May release hazardous emissions; liquid lubricants may spill or drip and contaminate environment; fire hazard with certain oils and greases
Not suitable for use on non-ferrous or ceramic surfaces
May be affected or altered by acidic and other aqueous environments 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 layered crystal structure. A few others (e.g., soft metals, polytetrafluoroethylene, polyimide, certain oxides and rare-earth fluorides, diamond and diamond-like carbons, fullerenes) can also provide lubrication although 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). Because a special chapter ( see Chapter 24) in this Handbook is devoted to the friction and wear behavior of diamond 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 MX
2
(where M is molybdenum, tungsten, or niobium, and X
is sulfur, selenium, or tellurium). Figure 22.1
shows the layered crystal structures of these solids. Certain
monochalcogenides (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 carry away heat from sliding interfaces.
2. Depending on test environment and contact conditions, their friction coefficients may be high or fluctuate significantly.
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Carbon
3.35 A
°
(a)
Sulfur
Molybdenum
2.96 A
°
(b)
Oxygen
Boron
Hydrogen
3.18 A
°
(c) (d)
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.
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 some abrasive, nonlubricious by-products.
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Well-known solid lubricants (graphite, HBN, and transition-metal dichalcogenides) owe their lubricity
to a unique layered structure. As illustrated in Figure 22.1
, the crystal structures of these solids are such
that while the atoms lying on the same layer are closely packed and strongly bonded to each other, the layers 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 of relative 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 this mechanism is largely responsible for low friction and is essential for long wear life, a favorable crystal structure in itself is not sufficient for effective lubrication. The presence or absence of certain chemical adsorbates is also needed for providing easy shear in most solids. For example, moisture or some other
In contrast, MoS
2
and other transition-metal dichalchogenides work best in vacuum or dry running conditions, 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 are typically in the range of 0.002 to 0.05 in vacuum or dry and inert atmospheres, but increase rapidly to
0.2 in humid air. It is generally agreed that no solid can provide very low friction and wear, regardless of test environment and/or conditions.
Soft metallic lubricants have crystal structures with multiple slip planes and do not work-harden appreciably during sliding contact. Dislocations and point defects generated during shear deformation are rapidly nullified by the frictional heat produced during sliding contact. Most high-temperature solid lubricants rely on thermal softening and/or limited chemical reaction with sliding surfaces that make them shear with relative ease; whereas self-lubricating polymers consist of long molecular chains with high chemical inertness and/or very low surface energy, making them non-stick or largely insensitive to chemical bonding.
Ambient temperature has a strong influence on the lubricity of solid lubricants. Graphite can provide lubrication up to 400°C, while HBN can withstand temperatures up to 1000°C. Most transition metal dichalcogenides tend to oxidize at elevated temperatures, and thus lose their lubricity. MoS
2
can provide lubrication up to 400°C, while WS
2
endures up to 500°C (Sliney, 1982). In general, those with higher oxidation resistance or chemical/structural stability perform the best at elevated temperatures. Oxideand fluoride-based solid lubricants (e.g., CaF
2
, BaF
2
, PbO, and B
2
O
3
) (Sliney, 1993), as well as some soft metals (e.g., Ag, Au), function quite well at elevated temperatures (Erdemir et al., 1990c; Erdemir and
Erck, 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 and resist oxidation.
Solid lubricants can be applied to a tribological surface in a variety of forms. The oldest and simplest method 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 great success (Heshmat and Heshmat, 1999; and Higgs et al., 1999). Certain solid lubricants have been blended in an aerosol carrier and sprayed directly onto the surfaces to be lubricated. Powders of solid lubricants can be strongly bonded to a surface by appropriate adhesives and epoxy resins to provide longer wear life (Gresham, 1997). They can also be dispersed or impregnated into a composite structure. Certain solids (e.g., HBN and boric acid) have been mixed with oils and greases in powder form to achieve improved 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 powders or 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 microstructure, uniform thickness, and long wear life (Spalvins, 1969, 1971, 1980; Erdemir, 1993). Ion-beam deposition and mixing can also be used to enhance the durability of solid lubricant coatings (Bhattacharya
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et al., 1993; Erck et al., 1992). However, the lifetimes of most solid lubricants are still limited because of the finite lubricant film thickness. To increase their durability, a self-replenishment or resupply mechanism is needed but very difficult.
In recent years, several new lubricants and modern lubrication concepts have been introduced to achieve better lubricity and longer wear life in demanding tribological applications. Some of the traditional solid lubricants were prepared in the forms of metal, ceramic, and polymer-matrix composites and used successfully 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 cracking of carbon-bearing gases were also shown to provide good lubricity at elevated temperatures (Lauer and
Bunting 1988; Blanchet et al., 1994). Recent developments in PVD and CVD deposition technologies have led to the synthesis of a new generation of adaptive, self-lubricating coatings with composite or multilayer architectures (Jayaram et al., 1995; Zabinski et al., 1992, 1995; Voevodin et al., 1999). These exotic architectures, based on layers of a self-lubricating dichalcogenide (e.g., MoS
2
, WS
2
, etc.) and a soft metallic or hard ceramic layer, were shown to work extremely well under demanding tribological conditions. Multifunctional nanocomposite films (consisting mainly of MoS
2
and Ti) have also been produced by magnetron sputtering and are quite hard, moisture insensitive, and self-lubricating, thus raising the 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 also made their way into the commercial marketplace and have been meeting the ever-increasing performance demands of more severe applications.
Recently, carbon and WS
2
were prepared in the form of hollow nanotubes and demonstrated to provide high 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 insensitive to variations in ambient pressure, environment, and temperature (Zabinski, 1997). A series of adaptive lubrication strategies has also been introduced in recent years and shown to be effective over a wide range of temperatures and pressures (Walck, 1997). Minute oxygen deficiency or sub-stoichiometry in rutile 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 alkaline halides (i.e., CaF
2
, BaF
2
) as the self-lubricating entities and CrC and/or Cr
2
O
3
as the wear-resisting entities were also shown to provide excellent lubrication over a wide temperature range (DellaCorte, 1998;
DellaCorte and Fellenstein, 1997). Furthermore, H
3
BO
3
powders, films forming on B
2
O
3
, and B
4
C coatings were 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., Cs
2
MoO
2
S
2
, ZnMoO
2
S
2
), and a few other alkali metals were found to be effective in controlling friction and wear at elevated temperatures (King, 1990).
These solids possess a lamellar structure like MoS
2
but can endure much higher temperatures than MoS
2
.
Furthermore, certain complex oxides and oxide-fluorides (i.e., ZnO/SnO/SrF
2
, NiO/BaTiO
3
,
MgO/ZnO/CaF
2
, NiO/SrF
2
) 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, and mechanistic understanding of lubricious oxides used to combat friction and wear at elevated temperatures. Based on this approach, one can predict the shear rheology and hence lubricity of an oxide or oxide mixture at elevated temperatures.
John and Zabinski (1999) investigated the lubrication properties of some sulfate-based (i.e., CaSO
4
,
BaSO
4
, and SrSO
4
) coatings at high temperatures as a potential replacement for alkaline halides (i.e.,
CaF
2
, BaF
2
). They found that these sulfates became highly lubricious at 600°C and were able to provide friction coefficients of
≈
0.15 to sliding surfaces. Structural studies revealed the presence of a carbonate crystal structure, along with a sulfate crystal structure after testing. The carbonate crystal structure
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consisted of alternating layers of alkali-earth atoms and carbonate ions. It was proposed that such a layered structure may have been responsible for the low-friction nature of these sulfates at high temperatures. The new lubricants and lubrication approaches mentioned above are some of the most notable developments in recent years and certainly have the potential to overcome difficult lubrication problems that may arise in the future.
In this chapter, solids with self-lubricating capabilities are reviewed first and classified on the basis of their crystal structures, chemistry, and operational limits. A summary of the recent understanding of the lubrication mechanisms of both traditional and new solid lubricants is presented next. Then, the present state-of-the-art in advanced solid lubrication methods and practices is provided. Particular emphasis is placed on the synthesis and/or applications of solid lubricant films on tribological surfaces by means of advanced surface engineering processes such as ion-beam-assisted deposition, ion-beam mixing, and unbalanced magnetron sputtering. Traditional and new applications for self-lubricating composite solid lubricants are also emphasized. This chapter primarily focuses on developments evolved during the last decade 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 structures and those that provide lubrication at high temperatures. Soft metals and polymers are briefly discussed because there are several excellent articles providing in-depth information on the properties and applications 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).
Solid lubricants can be categorized into several subclasses. Table 22.2
provides such a classification based
on the chemistry, crystal structure, and lubricity of the most widely used and recently developed solid
lubricants. As can be realized from Table 22.2
, the range of friction coefficients is rather large for a given
solid lubricant. This is mainly because friction is very sensitive to test environment, condition, and/or configuration. Ambient temperature and the type of counterface materials can also make a big difference in 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 major role. For example, the wide range of friction coefficients for MoS
2
(i.e., 0.002 to 0.25) stems from several factors affecting its shear rheology and hence frictional properties. These factors include film microstructure and chemistry, test environment, ambient temperature, contact pressure, film thickness, stoichiometry, and purity. Deposition and/or lubricant application methods can also play a major role in frictional performance of MoS
2
films. Due to their porous, columnar structure, MoS
2
films deposited by conventional sputtering methods tend to exhibit higher friction and shorter wear lives than films produced by more robust ion-beam-assisted deposition and closed-field unbalanced magnetron sputtering techniques.
The MoS
2
films deposited by these advanced physical vapor deposition methods can have near-perfect stoichiometry, purity, and basal plane orientation parallel to the substrate surfaces. These highly optimized 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 over
broad test conditions, temperatures, and environments. Each lubricant listed in Table 22.2
functions
rather nicely under certain test conditions, but not under all conditions. Researchers have mixed two or more of these lubricants to broaden the operational range, but in most cases the improvements were either transitory or short-lived.
Lamellar or layered solid lubricants are the class that is most studied by scientists and widely used by industry. Among the best-known examples are transition-metal dichalcogenides (e.g., MoS
2
), graphite,
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TABLE 22.2
Solid Materials with Self-lubricating Capability
Classification Key Examples
Lamellar solids
Soft metals
Mixed oxides
Single oxides
Halides and sulfates of alkaline earth metals
Carbon-based solids
Ag
Pb
Au
In
MoS
2
WS
2
HBN
Graphite
Graphite fluoride
H
3
BO
3
GaSe, GaS, SnSe
Sn
CuO–Re
2
O
7
CuO–MoO
3
PbO–B
2
O
3
CoO–MoO
3
Cs
2
O–MoO
3
NiO–MoO
3
Cs
2
O–SiO
2
B
2
O
3
Re
2
O
7
MoO
3
TiO
2
(sub-stoichiometric)
ZnO
CaF
2,
BaF
2
, SrF
2
CaSO
4
, BaSO
4
, SrSO
4
Diamond
Organic materials/polymers
Diamond-like carbon
Glassy carbon
Hollow carbon nanotubes
Fullerenes
Carbon-carbon and carbon-graphite-based composites
Zinc stearite
Waxes
Soaps
Bulk or thick-film (>50 µm)
PTFE
Metal-, polymer-, and ceramic-matrix composites consisting composites of graphite, WS
2
, MoS
2
, Ag, CaF
2
, BaF
2
, etc.
Thin-film (<50 µm) composites Electroplated Ni and Cr films consisting of PTFE, graphite, diamond, B
4
C, etc., particles as lubricants
Nanocomposite or multilayer coatings consisting of MoS
2
, 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 of test conditions, environments, and temperatures. The objective here is to show how friction varies depending on test conditions, as well as from one solid to another.
Typical Range of
Friction Coefficient a
0.18
0.3–0.2
0.1
0.15–0.6
0.2
0.2
0.1
0.1–0.6
0.2–0.4
0.15–0.2
0.02–1
0.003–0.5
0.15
—
0.15
0.05–0.3
0.1–0.2
0.2–0.4
0.15–0.25
0.04–0.15
0.05–0.4
0.002–0.25
0.01–0.2
0.150–0.7
0.07–0.5
0.05–0.15
0.02–0.2
0.15–0.25
0.2–0.35
0.15–0.2
0.2–0.3
0.15–0.25
0.2
0.3–0.1
0.35–0.2
0.2–0.1
0.47–0.2
HBN, and H
3
BO
3
. MoS
2
, graphite, and boric acid are natural minerals, extracted from deposits around the world. Other lamellar solids, such as WS
2
, fluorinated graphite, and transition-metal diselenides and ditellurides, are synthetic and are used at much smaller scales than graphite, HBN, and MoS
2
. MoS
2
and
WS
2
are well-suited for aerospace and cryogenic applications, while HBN is preferred for lubrication at elevated temperatures. HBN is widely used as a release agent in high-temperature metal-forming operations. Graphite and H
3
BO
3
work extremely well in moist air. The lubricity of graphite persists up to
400°C, while H
3
BO
3
begins to decompose at about 170°C. These two solids do not provide lubrication in dry or vacuum environments. Graphite fluoride is produced by the fluorination of graphite. This
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process increases the spacing between the carbon-carbon layers in graphite from about 0.34 nm to values as high as 0.8 nm, resulting in easier shear and hence better lubricity, even in dry environments. Among the lamellar solids, MoS
2
and WS
2
have the best overall load-carrying capacity as thin films on rigid substrates.
Most lamellar solids have good wetting capability or chemical affinity for ferrous surfaces. On a rough or porous sliding surface, they fill in the valleys between asperities and/or pores, thus providing a smoother surface finish and better support. When applied properly, these solids can also withstand extreme contact pressures without being squeezed out of the load-bearing surfaces. WS
2 is preferred over
MoS
2
when applications involve relatively higher temperatures. However, WS
2
is a synthetic lubricant and thus is expensive. Selenides of W, Nb, and Mo can provide even higher temperature capabilities than their sulfide analogs, but they too are expensive and are used on much smaller scales. Certain selenides and tellurides (e.g., V, Nb) provide excellent electrical conductivity.
22.2.1.1 Transition-Metal Dichalcogenides
Transition-metal dichalcogenides, MX
2
(where M is Mo, W, Nb, Ta, etc., and X is sulfur, selenium, or tellurium), 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-suited for cryogenic applications. MoS
2
and WS
2
are the best-known examples and the most widely used dichalcogenides. MoS
2
is a natural mineral known as molybdenite, whereas WS
2
and other dichalcogenides are man-made and therefore expensive. The hardness values of these solids on the Mohs scale are 1.5 to 2 and their specific gravities lie between 4.7 and 5.5. They are chemically stable and resist attack by most acids, except aqua regia and hot and highly concentrated HCl, H
2
SO
4
, and HNO
3
. At room temperature 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). Oxidation of MoS
2
does not begin until the temperature reaches about 375°C. At approximately 500°C, rapid oxidation begins and MoO
3
and SO
2
are produced. The thermal and oxidative stability of WS
2
is better than that of MoS
2
(Sliney, 1982).
22.2.1.1.1
Preparation and Uses of Dichalcogenides
MoS
2
and other dichalcogenides are applied on tribological surfaces as thin, strongly bonded solid films providing 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, and film thickness, the friction coefficients of MoS
2
and other dichalcogenides vary considerably. In moist air, the lifetimes of lubricant films are rather short and typical values for friction coefficients are 0.05 to
0.25. 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 MoS
2
last much longer, but their friction coefficients are generally high (Gresham, 1977).
The advanced physical vapor deposition (PVD) methods used in the deposition of high-quality MoS
2 films include magnetron sputtering (Spalvins, 1969, 1971, 1980; Stupp, 1981), ion-beam-assisted deposition (IBAD) (Bolster, 1991; Wahl et al., 1995; Seitzman et al., 1995; Dunn et al., 1998), and ion-beam mixing (Kobs et al., 1986; Bhattacharya et al., 1993; Rai, 1997). A pulsed laser deposition (PLD) method can also be used to deposit high-quality MoS
2
and other composite films with excellent tribological performance (Zabinski, 1992; Prasad, 1995). Sputtering has been and is still the most widely used method.
Recently, closed-field unbalanced magnetron sputtering of MoS
2
has become very popular and is highly effective in many tribological applications, including metal-forming and -cutting operations (Fox et al.,
1999). Fi l 2.2 compares the endurance lives of MoS
2
films prepared by various methods.
MoS
2
films may also contain lattice and volume defects (i.e., large voids or porosities) in their microstructures (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 the deposition chamber or introduced during film deposition. They may also come from the source or target material used during deposition. Depending on the level of contaminants, resultant MoS
2
films may show
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Maximum Thrust Bearing Endurance of *1 µ m* MoS
2
Coatings burnished dc sputtered rf sputtered
NRL S-modulated
OSMC pure
OSMC AuPd
NRL Pb-alloyed
0 2 4 6 8
Revolutions to Failure
(Millions)
10 12
FIGURE 22.2
Endurance lives of MoS
2
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 Desposition., Naval Research Laboratory, NRL/MR/6170-94-7633.) significant differences in their tribological properties (Suzuki, 1998). Crystalline films with porous columnar structures tend to wear out rather quickly. Tilting or bending of columnar grains with poor cohesion during sliding results in fracture of the top portions of each column; the remaining lower part is smeared on the surface and the basal planes of the MoS
2
crystal are eventually oriented parallel to the sliding surfaces (Spalvins, 1980; Hilton, 1991). The rate and degree of reorientation appear to depend on the initial microstructure. Recent studies have indicated that films with dense morphology, preferred basal orientation, 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 MoS
2
film (Donnet,
1993; Martin et al., 1993). These super-low friction coefficients are obtained in ultrahigh vacuum and are attributed to a combination of perfect basal orientation of the MoS
2
layers and to the absence of any adsorbed species or contaminants on sliding surfaces.
22.2.1.1.2
Modern Practices
Increasing demand for higher performance, longer wear life, and better efficiency in advanced mechanical systems that depend primarily on solid lubrication for safe operation has intensified interest in new and exotic lubrication practices in recent years. One of the major reasons for this interest was that conventional lubrication practices could no longer meet the performance and durability needs of advanced mechanical systems. Most studies have concentrated on MoS
2
and have resulted in a better understanding of the friction and wear mechanisms of this solid lubricant. Such mechanistic understanding is, in turn, used to develop the new and better lubrication practices that are in wide use by industry today.
With the advanced PVD methods mentioned earlier, MoS
2
films can be grown at subzero, room, or elevated temperatures. At lower deposition temperatures or under high-energy ion bombardment, one can obtain amorphous and sub-stoichiometric films with relatively poor tribological properties. During sliding or upon annealing, the crystallinity, and hence the lubricity, of MoS
2
may be restored (Zabinski et al., 1994). Ion-beam mixing of sputtered MoS
2 or WS
2
films (50 to 70 nm thick) with sapphire, Si
3
N
4, and ZrO
2
substrates can also result in an amorphous microstructure with a sub-stoichiometry of MoS
1.8
.
In these studies, 2-MeV Ag + ions at 5 × 10 15 cm –2 dose were used. During tribological tests in dry N
2
,
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FIGURE 22.3
substrate.
Friction performance and durability of sputtered and Ag + ion-beam mixed MoS
2
films on sapphire 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 those of as-sputtered films on all ceramic surfaces studied. The improvements in wear lives were correlated
with a significant improvement in film/substrate adhesion (Bhattacharya et al., 1993). Figure 22.3
shows
the friction performance and durability of sputtered and Ag + ion-beam mixed MoS
2
films on sapphire substrates. Similar improvements in wear lives of WS
2
film were found after ion-beam mixing (Rai et al.,
1997).
Recently, researchers have developed novel means to dope MoS
2
films with certain metals (e.g., Au,
Ni, Ti, Pb, C, etc.) and compounds (TiN, PbO, Sb
2
O
3
, 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 have demonstrated that when doped properly and in the correct proportions, these dopants can substantially improve the mechanical and tribological properties of MoS
2
films. For example, Au-doped MoS
2
films were shown to have more stable frictional traces and lower friction than undoped sputtered MoS
2
films,
as shown in Figure 22.4
.
Figure 22.5
compares the friction and wear performance of conventional MoS
2 with that of Ti-doped MoS
2
in increasingly humid air. Furthermore, doping of MoS
2
with Pb, Ti, Ni, Fe,
Au, and Sb
2
O
3
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 their wear lives (Zabinski et al., 1992, 1995; Wahl et al., 1995, 1999). The exact mechanisms responsible for lifetime improvements in doped MoS
2
films are not yet fully understood. However, researchers have noticed that doping generally results in preferential alignments of basal planes parallel to the substrate surface and thus lower susceptibility of MoS
2
to oxidation or moisture-induced degradation. It is speculated that such favorable alignment, together with increased resistance to oxidation, may have been responsible for increased wear life and lubricity.
Films with duplex and/or alternating layers of MoS
2
and metals or hard nitrides have also been produced in recent years and used in a variety of applications (Hilton et al., 1992; Jayaram, 1995; Seitzman et al., 1992). For example, MoS
2 films prepared by RF magnetron sputtering on AISI 440C and 52100 steels, and multilayer coatings of MoS
2
with either nickel or Au-(20%)Pd metal interlayers (with layer
© 2001 by CRC Press LLC

.05
.04
.03
.02
MoS
2
Au-MoS
2
.01
0 10000 20000 30000
SLIDING DURATION, cycles
40000 50000
FIGURE 22.4
Effect of Au doping on friction behavior of sputtered MoS
2
films. (From Spalvins, T. (1984), Frictional and morphological properties of Au-MoS
2
films sputtered from a compact target, Thin Solid Films, 118, 374-384.
With permission.)
0.30
0.25
Pin-On-Disc
Substrate: WC
Counterpart: 6 mm dia. WC Ball
Track Radius: 3.5 mm
Speed: 500 rpm
Load: 10 N
0.20
0.15
0.10
0.05
MoS
2
MoST ™
0.00
0 20 40 60
% Relative Humidity
80 100
FIGURE 22.5
Friction performance of conventional and Ti-doped MoST films at different humidity levels. (Courtesy of Multi-arc, Inc.) thicknesses ranging from 0.2 to 1.0 nm) on silicon substrates, had very dense microstructures that in some cases exhibited significant orientation of MoS
2
basal planes parallel to the substrate. Some of the optimized 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 density and 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 effect on the low-friction and wear behaviors of the films. In fact, despite the formation of an amorphous microstructure, 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 performance has not yet been resolved.
© 2001 by CRC Press LLC

In recent years, researchers have demonstrated that low-friction surface films can be formed in situ on the surfaces of Mo, W, and Mo- or W-containing metallic alloys by adding sulfur-bearing gases such as H
2
S and SO
2 into the test chamber (Singer et al., 1996a,b; Sawyer and Blanchet, 1999). In a model experiment run in high vacuum, where a small amount of H
2
S was admitted to maintain an S partial pressure of 13 Pa, Singer et al. (1996) recorded a friction coefficient of 0.01 on the resulting films on Mo substrates.
Recently, WS
2
was prepared as nanoparticles having structures similar to those of nested carbon fullerenes and nanotubes. Preliminary test results showed that these nanoparticles are highly effective in reducing friction and wear and do outperform the solid and thin film forms of WS
2
and MoS
2
when tested under the same test conditions (Rapoport et al., 1997). For the excellent durability and performance of 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 the environment. Because of their high rigidity, they impart high elasticity, which allows these particles to roll rather than slide.
22.2.1.2 Monochalcogenides
Sulfides and selenides of gallium and tin (i.e., GaS, GaSe, SnSe) have crystal structures that resemble those of transition-metal dichalcogenides (i.e., MoS
2
, WS
2,
WSe
2
) 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.)
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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 and selenides of germanium, whereas GaSe belongs to a class that also includes the layered gallium sulfide and the sulfides and selenides of indium.
Using a pin-on-disk machine, Erdemir (1994) performed friction tests on large crystalline pieces and compacts of GaSe and SnSe monochalcogenides against sapphire and 440C steel balls to assess their lubricity. For the specific test conditions explored, friction coefficients of the sapphire/GaSe and sapphire/SnSe pairs were
≈
0.23 and
≈
0.35, respectively. The friction coefficients of 440C pin/440C disk test pairs with GaSe and SnSe powders were
≈
0.22 and
≈
0.38, respectively. The friction data, together with the crystal-chemical knowledge and electron microscopy evidence, supported the conclusion that the lubricity and self-lubricating mechanisms of these solids are closely related to their crystal chemistry and the nature of their interlayer bonding.
In a series of earlier studies, Boes and Chamberlain (1968) and Gardos (1984) explored the tribological and thermal oxidation properties of composite lubricants consisting of indium/gallium and WSe
2
. The principal goal of these studies was to achieve better oxidation resistance on WSe
2
by alloying it with lowmelting-point indium and gallium. Upon curing the composite mixture at high temperatures, the investigators found that both indium and gallium underwent chemical reaction with WSe
2
to form the selenides of these metals. Further studies by Gardos (1984) demonstrated that, compared to the parent WSe
2
, the new composite lubricant exhibited superior oxidation resistance over a wide range of ambient temperature. Also, the lubricating capability of this InSe/WSe
2
composite was much superior to that of WSe
2 alone, especially at elevated temperatures. Apparently, a protective film resulting mainly from the preferential oxidation of sub-stoichiometric indium selenides was primarily responsible for the superior oxidation resistance of this new lubricant. The protective film was thought to effectively shield the lubricating entities against oxidation.
As discussed later, chalcogenides owe their low-friction nature to their lamellar structures in which strongly bonded atoms form extensive rigid sheets ( see
Figures 22.1
and
22.6
). In the cases of dichalco-
genides such as MoS
2
or MoSe
2
, the crystal structure is composed of a monolayer of Mo ions sandwiched between layers of S or Se ions. However, in the case of monochalcogenides such as GaS or GaSe, the
crystal 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 resistance to sliding surfaces. Because of its good lubricity, abundance, and low cost, it is used in many industrial applications. Like diamond, graphite is a polymorph of carbon. Both occur naturally and are recovered from deposits around the world; both can also be produced by synthetic means. Synthetic graphite is primarily produced by heating petroleum coke to about 2700°C. Chemically, both graphite and diamond are the same, but differ totally in their structures and properties. For example, graphite is perhaps one of the softest materials, while diamond is the hardest of all natural materials. Diamond has the highest thermal conductivity, whereas graphite is a relatively poor thermal conductor. However, graphite is a good electrical conductor, but diamond is an excellent electrical insulator. Graphite has a sheet-like crystal structure ( see
Figure 22.1
) in which all of the carbon atoms lie in a plane and are bonded only weakly
to the graphite sheets above and below. Each carbon atom in the plane joins to three neighboring carbon atoms at a 120° angle and at a distance of 0.1415 nm. The distance between atomic layers is 0.335 nm at 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 orientation), and test machine. The lowest friction coefficient of 0.01 was observed during a nanotribology experiment in which a W tip was slid against the cleaved graphite flakes (Mate, 1987). The dense and highly oriented pyrolitic graphite (HOPG) performs extremely well in humid air, giving friction coefficients of about 0.1. In dry air, inert atmospheres, or vacuum, graphite’s lubricity degrades rapidly, the friction coefficient increases to as high as 0.5, and it wears out quickly. Experimental studies carried out
© 2001 by CRC Press LLC

FIGURE 22.7
Effect of water vapor pressure on wear rate of graphite. (From Savage, R.H. (1948), Graphite lubrication, J. Appl. Phys., 19, 1-10. With permission.) by research groups have confirmed that the lubricity of graphite is not due to its layered crystal structure alone, but depends strongly on the presence or absence of certain condensable vapors, water vapor being one. Research has shown that only a small amount of condensable vapor is needed to improve the lubricity of graphite (Rowe, 1960). Certain vapors appear to be more effective than others. For example, a test run by Savage (1948) showed that n -heptane and isopropanol are much more effective than water vapor in terms of increasing the lubricity of graphite. The beneficial effect of condensable vapors on the lubricity of graphite has been attributed to the saturation in its lattice of π -electrons, which otherwise make atomic
layers slide with difficulty. Figure 22.7
shows the relationship between wear rate and water vapor pressure
for graphite.
Graphite can provide lubrication up to about 500°C in open air, although friction tends to increase as the temperature rises. At higher ambient temperatures, it begins to oxidize and lose its lubricity. In vacuum, the friction coefficient is initially high (i.e., 0.4), but decreases to about 0.2 at 1300°C. In most sliding experiments, thin transfer films are formed on the surfaces of sliding counterfaces. These transfer layers are thought to be important for achieving longer wear life and possibly even lower friction. When small amounts of sodium thiosulfate (Na
2
S
2
O
3
) or sodium molybdate (Na
2
MoO
4
) were added to graphite to improve the transfer film forming behavior, researchers observed longer wear life and lower friction against sliding steel counterfaces (Langlade et al., 1994). During these tests, transformations of the graphite structure to a turbostatic phase was observed as a thin layer by means of electron microscopy and 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 friction and 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 as lubrication of molds and dies in metal-forming, as well as flange faces of rails and railcar wheels. Graphite is 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-graphite composites are rather common and widely used in various engine, aircraft, and seal applications.
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Graphite fluoride is prepared by fluorinating graphite at stoichiometries from X = 0.3 to 1.1 in CF x
.
It is prepared by direct reaction of graphite with fluorine gas at controlled temperatures and pressures and can be regarded as an intercalation compound of graphite. Fluorination increases the distance between atomic planes from about 0.34 nm to as high as 0.8 nm and, hence, results in easier shear and better lubricity (Fusaro and Sliney, 1970). It also causes basal planes of graphite to distort and lose their planar configuration. CF x
is electrically insulating and nonwettable with water, but decomposes at about
450°C.
Fluorination of graphite was shown by Fusaro and Sliney (1970) to substantially improve the lubricity and durability of this solid and make it less sensitive to variations in ambient humidity. Earlier studies indicated that burnished CF x
was capable of providing friction coefficients of 0.1 or less up to about
480°C in open air. Compared to those of MoS
2
and even HOPG under the same test conditions, such friction values were considerably lower. It is possible to prepare composite structures and resin-bonded films of CF x
in order to achieve longer life; however, due to its high cost, CF x
is rarely used by industry.
In a recent study, CF x
was used as an additive to WS
2
thin films to reduce their sensitivity to moisture
(Zabinski et al., 1995). These films were produced on AISI 440C steel substrates by a pulsed laser deposition method. Substrate temperature and CF x
concentration were varied to control film microstructure 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 CF x exhibited ultra-low friction in dry air (friction coefficients <0.01), but the coefficients increased with increasing relative humidity. Films grown at elevated temperatures (300°C) or with higher concentrations of CF x
showed insensitivity to humidity, but the friction coefficients were relatively high (0.04 in dry air).
22.2.1.3.1
Modern Practices
Graphitic lubricious precursors can also result from catalytic cracking of certain carbon-bearing gases and 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 test chamber where hot ceramic or metal surfaces are maintained and slid against one another. The hydrocarbons 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 as bucky-balls (C
60
) (Bhushan et al., 1993) and hollow nanotubes of carbon (Falvo, 1999). In addition to the powder form, sublimation or thermal evaporation methods were used to deposit C
60
as strongly bonded and dense films on metallic and ceramic substrates. Depending on the form, density, and adhesion of these films to their substrates, friction coefficients of 0.15 to 0.5 were obtained. Ion irradiation of such films with 2 MeV Ag + and B + ions at various doses resulted in partly crystalline to amorphous films that were 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 to achieve better lubricity at elevated temperatures. Long-duration friction and wear tests were run as a function of both increasing and decreasing temperatures to assess the durability and the friction and
wear 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 temperatures up to 500°C. Based on analytical studies, it was concluded that the boron doping was essential for achieving 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 are more 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 to relative humidity of the test environment. Glassy carbon materials have very low fracture toughness, but can be reinforced with metallic/nonmetallic fibers to achieve improved toughness. Copper-containing glassy carbon composites have high electrical conductivity and can be used for electrical contacts or brushes (Burton and Burton, 1989).
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FIGURE 22.8
Friction performance of boron-doped carbon composite at temperatures to 525°C.
Quite recently, researchers have produced highly disordered graphitic carbon layers on silicon carbide by reaction with chlorine and chlorine/hydrogen gas mixtures at 1000°C. The thickness of the graphitic layer can vary from a few to 100 µm, depending on process time. When such a graphitized surface is subjected 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 and wear of microelectromechanical systems, sliding bearings (e.g., mechanical seals), electrical contacts, and biomedical 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 a honeycomb pattern (Rowe, 1960). As in graphite, the bonding between the atoms of HBN in each layer is covalent and very strong, while bonding between the layers is of the weak van der Waals type.
HBN is typically produced by reacting B
2
O
3
with urea or ammonia gases at high temperatures. Unlike graphite, HBN has a white color. Its cubic analog (i.e., cubic boron nitride) is like diamond and is extremely hard and resistant to wear. HBN is generally produced in powder form. Depending on manufacturing 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 boron oxide 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 as a 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 superior friction 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 selflubricating phase in ceramic composites. Such materials will be very attractive for mechanical face seal applications. In a recent study, Westergard et al. (1998) investigated the tribological performance of
Si
3
N
4
/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 coefficients of test pairs from a range of 0.4 to 0.9 to a range of 0.02 to 0.1. Analytical studies revealed that sliding surfaces were covered by a thin, well-adhering tribofilm, which may have been responsible for the improved tribological performance.
HBN has also been used as an additive in oils and greases. Recent tests by Kimura et al. (1999) showed that addition of HBN in concentrations as little as 1 wt% results in an order of magnitude reduction in
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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
CO, C
3
H
8
, H
2
O, air (50% RH); 10 –3 Pa
CO, N
2
, O
2
; 10 Pa
Air (50% RH); 10 Pa
C
3
H
8
; 10 Pa
Air (50% RH); 10 5 Pa
Air (50% RH), atmospheric pressure
0.6–0.7
0.6–0.7
0.6–0.7
0.4
0.4
0.2
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.
wear of bearing steels sliding against each other in line contacts. At higher concentrations, the reduction in wear is even greater. Similar improvements in antiwear and antifriction properties were reported for
HBN-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 release agent in metalworking operations involving high temperatures (Golubus, 1970). In high vacuum, HBN loses its lubricity. Buckley (1978) reported a friction coefficient of 1 for HBN sliding against itself in ultrahigh 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 organic vapors into the test chamber reduced friction coefficients to the 0.2 level. Recent fundamental studies by
Martin et al. (1992) in ultrahigh vacuum (10 –8 Pa) and under partial pressures of CO, C
3
H
8
, H
2
O, air with 50% humidity, N
2
, and O
2
resulted in friction coefficients of 0.1 to 0.7; Table 22.3
summarizes their
experimental results. These experiments further reinforced the initial assertion that HBN is not only similar 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 graphite
and HBN (Erdemir, 1991). It has a triclinic unit cell in which boron, oxygen, and hydrogen atoms are arrayed to form extensive atomic layers parallel to the basal plane of the crystal ( see
Figure 22.1
). Because
of 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
FIGURE 22.9
SEM photomicrograph of lamellar structure of boric acid.
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0.09
0.07
0.05
0.03
0.01
1N 2N 5N 10N
SAPPHIRE BALL ALUMINA BALL STEEL BALL
FIGURE 22.10
Variation of friction coefficient of boric acid films under different loads during sliding against steel and ceramic balls. (Adapted from Erdemir, A. (1991), Tribological properties of boric acid and boric-acid-forming surfaces. I. Crystal chemistry and mechanism of self-lubrication of boric acid, Lubr. Eng., 47, 168-172.) the same plane is of the covalent/ionic and hydrogen type; the layers are 0.318 nm apart and held together only by weak van der Waals forces.
Boric acid exists in two major crystalline forms: metaboric acid (H
2
O·B
2
O
3
or HBO
2
) and orthoboric acid (3H
2
O·B
2
O
3
or H
3
BO
3)
. Furthermore, metaboric acid has been reported to crystallize in three different forms: orthorhombic or α -metaboric acid, monoclinic or β -metaboric acid, and cubic or
Γ -metaboric acid. Among these, orthoboric and orthorhombic metaboric acids exhibit layered-crystal structures and thus can provide low friction. Orthoboric acid exists as a natural mineral known in mineralogy books as sassolite. It is stable up to about 170°C.
Due to its layered-crystal structure, H
3
BO
3
is a self-lubricating solid. To demonstrate its lubricity,
Erdemir (1991) performed extensive friction tests with solid compacts of H
3
BO
3
on a pin-on-disk machine. Cylindrical rods with a nominal diameter of 1.27 cm were compacted from 99.8 wt% H
3
BO
3 powders by cold-pressing at about 35 MPa. To establish point contact during friction tests, one end of the rod-shaped compacts was finished with a hemispherical cap of 5-cm radius. Subsequently, the boric acid pin was attached to the pin holder of a pin-on-disk machine and rubbed against a 50-cm-diameter
AISI 52100 steel disk.
The friction coefficient of the pin/disk pair described above was measured as a function of sliding distance. The initial friction coefficient of this tribosystem was approximately 0.2; it then decreased steadily with distance and eventually reached a steady-state value of 0.1 after sliding about 20 m.
H
3
BO
3
can spontaneously form on the surfaces of boron and B
2
O
3
films. Erdemir et al. (1990b) investigated the formation and tribological characteristics of such boric acid films formed on the surfaces of vacuum-evaporated B
2
O
3
layers. They found that H
3
BO
3
, which formed spontaneously on the surfaces of B
2
O
3
coatings, is remarkably lubricious. For a sliding pair of sapphire ball/B
2
O
3
-coated Al
2
O
3
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 against
a B
2
O
3
-coated Al
2
O
3
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 be smaller than that between a soft, less-rigid ball. Friction force, which is a product of the true contact area multiplied by the shear strength of the contact interface, will be much lower when hard, rigid balls are used in sliding contact.
Based on surface and structure analytical studies, it was concluded that low friction is a direct consequence of the layered crystal structure of H
3
BO
3
films forming on the exposed surfaces of the B
2
O
3
coating by the spontaneous chemical reaction:
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1 2 B O coating
2 3
( )
3 2 H O moisture
2
)
H BO
3 3
H
298
.
kJ mol
The above reaction occurs naturally, and a thin layer of H
3
BO
3
forms everywhere on the exposed surface of B
2
O
3 coatings. Formation of such self-lubricating and self-replenishing films was later demonstrated on VB
2
, B
4
C, and borided steel surfaces, affording very low friction coefficients to sliding metal and ceramic surfaces (Erdemir et al., 1991, 1996b,d, 1998, 1999; Bindal and Erdemir, 1996).
As described previously, H
3
BO
3
crystallizes in a triclinic crystal structure essentially made of atomic layers parallel to the basal plane. The atoms lying on each layer are closely packed and strongly bonded to each other. The bonds between the boron and oxygen atoms are mostly covalent, with some ionic character. Hydrogen bonds strongly hold the planar boron/oxygen groups together. The atomic layers are widely spaced (e.g., 0.318 nm apart) and held together by weak forces (e.g., van der Waals). Because of the ionic character of interatomic bonds, boric acid can dissolve in water and some other solvents.
With its layered crystal structure, H
3
BO
3
resembles other layered solids well-known for their good lubrication capabilities (e.g., MoS
2
, graphite, and HBN).
Erdemir (1990) proposed that under shear stresses, plate-like crystallites of H
3
BO
3
can align themselves parallel to the direction of relative motion. Once so aligned, they can slide over one another with relative
ease 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 provide excellent lubricity when used as a metal-forming lubricant (Erdemir and Fenske, 1998). Used as a filler in polymers, boric acid and boron oxide can substantially lower friction and increase the wear resistance
of base polymers ( Figure 22.11
) (Erdemir, 1995). It was demonstrated that sub-micrometer size powders
of 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 H
3
BO
3
is not recommended for several reasons.
First, above about 170°C, H
3
BO
3
tends to decompose and eventually turn into B
2
O
3
, thus losing its layered crystal structure and hence its lubricity. Second, at temperatures greater than about 450°C, B
2
O
3
becomes liquid-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.
© 2001 by CRC Press LLC

L INTRAFILM FLOW
V
L INTERFACE SLIDING
V
L INTERFILM SLIDING
FIGURE 22.12
Schematic representation of three ways by which sliding can be accommodated between an uncoated and a coated surface. (From Singer, I.L. (1992), Solid Lubrication Processes, in Fundamentals of Friction: Macroscopic and Microscopic Processes, Singer, I.L. and Pollock, H.M. (Eds.), NATO-ASI Series, Vol. 220, Kluwer Academic, London,
237-261. With permission.) and low friction can be reinstated by viscous-flow lubrication. However, for ceramics — especially for the oxides — the situation is different. Liquid B
2
O
3
can react with these ceramics and lead to high corrosive wear. Friction can also be very high because of the highly viscous nature of the reaction products.
In general, the lubricity and durability of a solid lubricant are controlled by a mechanism that involves
interfilm sliding, intrafilm flow, and film/substrate or interface slip, as illustrated in Figure 22.12
. It has
been found that the lamellar solid lubricants discussed above provide lubrication by an interlayer shear mechanism, mainly because the crystal structures of these solids are such that while the atoms lying on the same layer are closely packed and strongly bonded to each other, the layers themselves are relatively far apart and the forces that bond them (e.g., van der Waals) are weak ( see
Figure 22.1
). Strong interatomic
bonding and packing in each layer give these solids the very high in-plane strength that is essential for longer wear life or reduced wear damage during sliding. When present on a sliding surface, crystalline layers of these solids align themselves parallel to the direction of relative motion and slide over one another with relative ease to provide lubrication. Furthermore, the formation of a smooth transfer film on the sliding surfaces of counterface materials is also important for long wear life and the accommodation of sliding velocity, as well as for dissipation of frictional energy.
Recent electron microscopy studies of H
3
BO
3
-lubricated rubbing surfaces of steel test pairs clearly revealed some plate-like crystallites exhibiting a preferred alignment parallel to the sliding direction
( Figure 22.13
). Similar observations were made by TEM on sputtered MoS
2
films after sliding tests. Several other studies used X-ray diffraction to further verify that indeed some crystalline orientation occurs on most 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, H
3
BO
3
, and HBN, the layers are made of a single atomic plane; while in MoS
2
and other transition-metal dichalcogenides, the layers consist of three atomic planes ( see
Figure 22.1
); in monochalcogenides, there
are four atomic planes in each layer ( see
Figure 22.6
).
© 2001 by CRC Press LLC

FIGURE 22.13
Physical evidence for preferred crystalline orientation and intercrystallite slip on boric acid-lubricated surface.
The electronic states of atoms in each layer play a major role in the lubricity of each solid. Graphite and HBN are similar in electronic states. The only major difference between the bonding configuration of these two solid lubricants is that the π -bonding and π -antibonding bands overlap weakly at the
Brillouin-zone boundary in graphite, which is responsible for the fairly good electrical conductivity of graphite; whereas these bands are separated by an energy gap of several electron-volts in stoichiometric
BN, causing it to be an insulator. In both cases, if residual π attractions between atomic layers are not eliminated or reduced, high friction and wear may result. In graphite, π -bond interactions are generally reduced 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 interlayer shear properties of graphite are markedly improved. However, attempts to find effective intercalation species for HBN were mostly unsuccessful.
Within the layered-crystal structure of transition-metal dichalcogenides, metal atoms are sandwiched between the chalcogen atoms in a planar array of S-Mo-S, while the layers in monochalcogenides consist of four atomic layers. For example, each layer of GaSe consists of strongly bonded Ga and Se atoms in the sequence Se-Ga-Ga-Se ( see
Figure 22.6
). The Ga atoms are paired to form the two atomic planes
inside, while the chalcogen atoms form the top and bottom planes. Note that the removal of one layer of Ga atoms in the GaSe crystal structure would have produced the exact crystal structure of MoS
2
, as
illustrated in Figure 22.1(c) ; this suggests that mono- and dichalcogenides are indeed closely related. The
interatomic bonding within the layers of mono- and dichalcogenides is strong and mainly of the covalent type, whereas the bonding between adjacent layers is weak and of the van der Waals type. Within the rigid two-dimensional layers of MoS
2
crystals, the S atoms have a trigonal prismatic coordination around the 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 a number of intrinsic and extrinsic parameters. For example, intrinsically, both graphite and MoS
2
have layered crystal structures, but the extent of their lubricity and durability is largely controlled by extrinsic factors such as the presence or absence of vapors or gaseous species in the test environment. Graphite functions best in humid air, while MoS
2
lubricates best in dry and vacuum environments. As mentioned earlier, the lubrication behavior of HBN and H
3
BO
3
is similar to that of graphite. This contrasting behavior 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 humid environment may be related to the weakening effect of water molecules on the residual π -bonds between the layers of their crystals. As for the poor lubricity of MoS
2
in a humid environment, it has been suggested that water molecules react directly or indirectly with MoS
2
and thus alter the interatomic array and
© 2001 by CRC Press LLC

bonding, which in turn increase friction. When tested in open air, MoS
2
and other dichalcogenides were found to react with oxygen and form MoO
3
or other types of complex oxides. It has been speculated that the chemical reactions leading to MoO
3
formation occur predominantly at the prismatic edges where reactive dangling bonds exist.
Another intrinsic parameter that can affect the lubricity of a layered solid is the interlayer-to-intralayer bond-length ratio. It has been reported that this ratio is a crude but revealing indicator of the lubricity of a lamellar solid (Jamison, 1972, 1978). In general, it was found that the greater the interlayer-tointralayer bond-length ratio, the weaker the interlayer bonding with respect to the intralayer bonds, and thus the higher the lubricity. For MoS
2
and GaSe crystals, the interlayer-to-intralayer bond-length ratios are 1.5 and 1.6, respectively (Zallen and Slade, 1974).
The presence or absence of electrostatic attractions between the layers of mono- and dichalcogenides constitutes yet another intrinsic parameter that can affect the lubricity of these solids. For example, of the numerous metal dichalcogenides, only a few can impart low friction to sliding tribological interfaces.
Previous research has shown that despite their layered crystal structures, NbS
2
, TiS
2
, VS
2
, TaS
2
, etc., are not as lubricious as MoS
2
or WS
2
(Clauss, 1972; Jamison, 1978). Based on the molecular orbital and valence bond theories, Jamison (1972, 1978) proposed the following explanation for the poor lubricating performance of NbS
2
, TiS
2
, VS
2
, TaS
2
, etc. In the layered crystal structure of these solids, there is a region of negative electrical charge that not only concentrates above the chalcogen atoms of a given layer, but also extends well into the pockets between the chalcogen atoms of neighboring layers. Because the bottoms of the pockets are positively charged (due to the exposed ion cores of the surrounding atoms), an electrostatic attraction exists between the layers, making the layers of these solids shear with difficulty.
As for the excellent solid-lubricating capacities of MoS
2
and WS
2
, the region of negative electrical charge 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 MoS
2
and WS
2
crystals is thought to result from the same electrostatic repulsion between successive layers.
In general, previous studies have clearly demonstrated that the friction and wear performance of solid lubricants are strongly affected by both the intrinsic (crystal-specific) and extrinsic (operating-environment-specific) factors. Therefore, no single solid lubricant can provide low friction and wear in all environments. Furthermore, not all layered solids are good solid lubricants. The type and magnitude of interlayer bonds are also important.
For applications in open air and at temperatures above 500°C, most of the lamellar solids mentioned above lose their lubricity and become useless. Furthermore, most sliding interfaces (including metals and non-oxide ceramics) become oxidized (Quinn and Winer, 1985). Thin oxide films that form on the sliding 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 sliding bodies differ chemically or if there is a third or fourth body at the sliding interface, two or more oxides may form on the sliding surface and control friction and wear. In past years, significant research has been carried out to study the shear rheology of such oxides and to formulate alloys or composite structures that can lead to the formation of oxides with very low shear strength (Peterson et al., 1994). These are often referred to as lubricious oxides.
Certain oxides (e.g., Re
2
O
7
, MoO
3
, PbO, B
2
O
3
, NiO, etc.), fluorides (e.g., CaF
2
, BaF
2
, SrF
2
, LiF, and MgF
2
), and sulfates (e.g., CaSO
4
, BaSO
4
, and SrSO
4
) become soft and highly shearable at elevated temperatures and hence can be used as lubricants (Sliney et al., 1965; Sliney, 1969; John and Zabinski, 1999). When applied as thin or thick coatings (by means of PVD, plasma spraying, fusion bonding, etc.), these solids
© 2001 by CRC Press LLC

0.6
0.5
0.4
0.3
300 C
25 C
0.2
0.1
0
0
1.3 kg
600 C
1000
Sliding Cycle
2000 3000
FIGURE 22.14
Effect of test temperature on friction coefficient of an ion-beam-deposited Cu-Mo film; lower friction coefficients at high temperatures are attributed to formation of CuO-MoO
3
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 temperature solid lubricants, Surf. Coat. Technol., 89, 245-251. With permission.) can provide acceptable levels of friction coefficients and long wear life. They can also be mixed with other solid lubricants to obtain lubrication over much wider temperature ranges. Major drawbacks of oxidebased 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. Potential applications for lubricious oxides include high-temperature seals, bearings, and gears, valves and valve 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., become highly 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-Re
2
O
7
, CuO-
MoO
3
, PbO-B
2
O
3
, PbO-MoO
3
, CoO-MoO
3
, Cs
2
O-MoO
3
, NiO-MoO
3
) can also provide wider operational ranges and can be prepared as alloys or composite structures to provide longer durability. The lubricious layers that form by oxidation of alloy surfaces are very desirable and exceptionally advantageous when compared with the solid lubricant coatings with finite lifetimes. At high temperatures, as the oxide layer is depleted from the surface by wear, the alloying ingredients diffuse toward the surface where the oxygen potential is higher; they oxidize again to replenish the consumed lubricious layers that have low shear
strength and/or surface energy to decrease friction (Peterson et al., 1982, 1994). Figure 22.14
shows the
frictional performance of CuO-MoO
3
at different temperatures (Wahl et al., 1997).
In a series of fundamental studies, Gardos (1988) demonstrated that at a very narrow range of anion vacancies and at high temperatures, crystalline TiO
2
(rutile) and rutile-forming surfaces can provide very low 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-based alloys and compounds. Their findings suggested that Magneli phases are principally the result of tribooxidation and that once formed, they can dominate the tribological behavior of sliding ceramic interfaces, mainly because of their unique shear properties. However, TiO x
-based solid lubricants have not yet found wide use, mainly because of the difficulty in achieving and maintaining the very narrow range of oxide stoichiometry needed for good lubricity.
A new breed of lubricious zinc oxide films was recently synthesized by pulsed-laser deposition, and their 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 lubricity and 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 friction coefficients and long wear lives at room temperature. However, as the chemical stoichiometry and crystal structure approached those of the bulk zinc oxide, the tribological properties and load/speed sensitivity
© 2001 by CRC Press LLC

1.00
0.80
0.60
µ
0.40
0.20
Cycles
Cycles
1x10
6
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 microstructurally engineered oxide: performance and mechanisms, Tribol. Lett., 103-116. With permission.)
of the films degraded. Figure 22.15
shows the variation of friction coefficient of a pulsed-laser-deposited
ZnO film during sliding against an AISI 440C steel ball.
Plasma-sprayed self-lubricating composites and adaptive lubricants were recently engineered to combat friction and wear problems at high temperatures. The composite coatings consist of silver and alkaline halides (i.e., CaF
2
, BaF
2
) as the self-lubricating entities and chrome carbide and/or oxide as the wearresisting 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 these solid 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 formulated and the latest formulations are capable of providing lubrication over much broader temperature ranges
than their earlier versions. Figure 22.16
shows the friction performance of PS-304 (consisting of 20 wt%
Cr
2
O
3
, 10 wt% Ag, 10 wt% BaF
2
/CaF
2
eutectic composition, and NiCr as the binder) against an alumina ball at temperatures up to 870°C. Recent studies have also demonstrated that these lubricants are very
FIGURE 22.16
Friction performance of PS-304 self-lubricating composite coating at temperatures to 870°C.
© 2001 by CRC Press LLC

suitable for high-speed sliding bearing surfaces and provide excellent durability and frictional performance, especially when used in foil bearing applications (DellaCorte, 1998).
To achieve low friction from room temperature to very high temperature, a series of adaptive solid lubricants has recently been developed. A good adaptive lubricant is made of several ingredients that provide low friction at low temperatures, and as the temperature increases, these lubricious ingredients react with each other and/or oxygen in air to form a high-temperature solid lubricant phase providing low friction. The lubricating entities in this case were selected from those metals that can react with the environment to form the kind of lubricious layers needed (Wlack et al., 1997; Zabinski et al., 1992). One problem is that the oxidation is not reversible, so when the temperature returns to low values, the friction may increase. To solve this problem, researchers have used very thin diffusion-barrier layers to limit the extent of oxidation to the very top surface rather than to the bulk or over a thick layer. Another approach was to use capsules of high-temperature adaptive lubricants in a low-temperature matrix. While the lowtemperature matrix provides lubricity at lower temperatures, the capsules with a protective shell on the surface react with oxygen and become lubricious, thus providing the needed level of lubricity.
Recently, a crystal-chemical approach was introduced by Erdemir (2000a) to classify lubricious oxides on the basis of lubrication performance and operational limits. This approach was proposed to serve as a 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 their shear rheology and hence their lubricity at high temperatures.
The principle of the crystal-chemical approach is based essentially on the ionic potential of an oxide and 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-chemical characteristics and the lubricity of oxides at high temperatures. Specifically, it is possible to establish a 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 surrounding anions such as B
2
O
3
or Re
2
O
7
. Oxides with highly screened cations are generally soft and their melting points are low. Their cations are well-separated and completely screened by anions; hence, they have little or no chemical interaction with other cations in the system. Most of their bonding is with surrounding anions. Conversely, oxides with lower cationic field strengths or ionic potentials (e.g., Al
2
O
3
,
ZrO
2,
MgO, and ThO
2
) are very strong, stiff, and difficult to shear, even at high temperatures, because their cations interact with each other and form strong bonds.
The crystal-chemical approach can be used to predict the extent of adhesive interactions between two or more oxides at a sliding interface; hence, it can be used to predict frictional performance. Extensive research 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 temperatures
are shown in Figure 22.17
. As can be deduced from this figure, the higher the ionic potential, the lower
the friction coefficient. This means that oxides with higher ionic potentials appear to shear more easily and 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 in an oxide will interact very little with other cations in their surroundings, and this will allow them to shear 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 approach introduced 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
© 2001 by CRC Press LLC

FIGURE 22.17
Relationship between ionic potentials and friction coefficients of single oxides. (Adapted from
Erdemir, A. (2000a), A crystal chemical approach to lubrication by solid oxides, Tribol. Lett., 8, 97-102.) compounds formed, and eutectic temperature or lowering of the melting point of an oxide when a second oxide is present. For example, the eutectic temperature and compound-forming tendencies of two oxides are closely related to the cationic field strengths or ionic potentials of the involved elemental species. The ability of an oxide to dissolve in or react with other oxides or to form complex oxides is estimated from the difference in relative ionic potentials of the oxides in the system. In general, the greater the difference in 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-Re
2
O
7
, CuO-MoO
3
, PbO-B
2
O
3
, PbO-MoO
3
,
CoO-MoO
3
, and NiO-MoO
3
, etc.) were either present at or purposely introduced to the sliding interfaces to achieve low friction at high temperatures. Most of these data were extracted from papers and progress reports authored by Peterson et al. (Peterson, 1987; Peterson et al., 1960, 1982, 1994), who have had extensive experience with lubricious oxides. Nickel-based superalloys, because of their relevance to hightemperature applications, were used as substrates in most of their studies. The specially formulated nickel alloys contained Ti, Ta, W, Re, B, and Mo as potential lubricious oxide formers. Note that the scatter in
the friction values shown in Figures 22.17 and
22.18
is large; this is not unusual in the field of tribology
because 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 components at high temperatures. At 600°C, 0.02 to 0.1 friction coefficients have been reported for Cs
2
Olubricated Si
3
N
4
ceramics (Strong and Zabinski, 1999). During sliding at high temperature, a mixed oxide layer consisting of Cs
2
O and SiO
2
was found and believed to be responsible for low friction. As can be
seen from Figure 22.18
, such a combination would result in a large difference in the ionic potentials of
these two oxides.
From Figure 22.18
, it can be seen that as the difference in ionic potential increases, the lubricity of the
oxide species also increases. There are two fundamental reasons for this phenomenon. One is that as the difference in ionic potential increases, the ability of oxides to form a low-melting-point or readily shearable compound improves; hence, oxides tend to exhibit lower hardness and shear strength at elevated temperatures because the anions are able to better shield or screen the cations and thus make them less likely to interact with neighboring cations. The second reason for the phenomenon is that the ability or affinity of ionic species to form highly stable compounds (that exert very little chemical or electrostatic
© 2001 by CRC Press LLC

FIGURE 22.18
Relationship between friction coefficient and difference in ionic potentials of double oxides. (From
Erdemir, A. (1999), 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 sliding surfaces means lower adhesive forces across the sliding contact interfaces, and hence lower friction
(Erdemir, 2000a).
Self-lubricating composites have been available for a long time and are used rather extensively by industry to combat friction and wear in a variety of sliding, rolling, and rotating bearing applications. They are generally prepared by dispersing appropriate amounts of a self-lubricating solid (as fillers, preferably in powder form) with a polymer, metal, or ceramic matrix. With powder metallurgy techniques, fillers and matrix materials can be thoroughly mixed, compacted, and then sintered (if necessary) to obtain the desired 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 manufactured and offered for industrial use. While the core is made of nearly pure matrix material to provide high strength, hardness, and toughness, the near-surface regions where sliding will occur are enriched in selflubricating powders to achieve lubricity. Composite structures prepared in this fashion are used for a wide range of tribological applications, such as bushings, bearings, and a variety of gears and traction devices. For example, copper-graphite and silver-graphite composites are used in electrical brushes and contact strips, while aluminum-graphite composites are well-suited for bearings, pistons, and cylinder liners 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 optimal particle sizes, self-lubricating filler materials can have a substantial beneficial impact on the mechanical and tribological properties of matrix materials. For example, it was shown that graphite, MoS
2
, and boric acid fillers tend to increase the wear resistance of nylon and polytetrafluoroethylene (PTFE)-type polymers (Blanchet and Kennedy, 1992; Fusaro, 1990). Aluminum-graphite composites exhibit excellent lubricity, durability, and resistance to galling under both dry and lubricated conditions (Rohatgi et al.,
© 2001 by CRC Press LLC

1992). When the graphite content in aluminum-matrix composites exceeds
≈
20 vol%, the friction coefficient approaches that of pure graphite and becomes highly independent of the matrix alloy. Aluminum-
WS
2
composites were also found to be very effective in reducing galling and in providing excellent lubricity and durability, especially in high-vacuum environments (Prasad and Mecklenburg, 1994). The presence of WS
2
particles in the matrix results in significantly increased resistance to seizure and enables the composite body to operate under very high loads without galling.
Recent studies concluded that improved tribological behavior was mainly due to the formation of a thin transfer layer on the sliding surfaces of counterface materials. In the case of polymers, a significant increase 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 and subsurface deformation. They are continuously replenished by embedded graphite particles dispersed in the matrix (Rohatgi et al., 1992).
In addition to metal-matrix composites, a series of self-lubricating polymer and ceramic matrix composites have also been developed, tested, and offered for industrial use in recent years (Gangopadhyay and Jahanmir, 1991; Prasad and Mecklenburg, 1994; Fredrich et al., 1995). These composites are emerging as an important class of tribological materials, offering new means to combat friction, wear, and galling under extreme conditions. In a recent study, tribological properties of fine-grain alumina (20%)-graphite composites were explored as potential candidates for advance sealing applications. Pin-on-disk wear tests showed that friction coefficients can be reduced from 0.5 for alumina-on-alumina to
≈
0.25 for aluminagraphite composite (Yu and Kellett, 1996).
In another study, ceramic-matrix composites were fabricated by drilling a series of small holes in alumina and silicon nitride ceramics and then filling the holes with NiCl
2
-intercalated graphite under high pressure. Addition of graphite to silicon nitride considerably reduced the friction coefficient, but the alumina-graphite composites exhibited only a marginal reduction in friction coefficient compared to that of the alumina. The reduction in friction coefficient for silicon nitride-graphite composite can be explained by the formation of transfer films consisting of a mixture of materials from both contacting surfaces. However, for the alumina-graphite composites, the graphite regions were completely covered with steel wear particles, inhibiting the formation of graphite-containing transfer films (Gangopadhyay and Jahanmir, 1991).
Mixing of Sb
2
O
3
with MoS
2
was shown to act synergistically to improve the friction and wear behavior of MoS
2
. Specifically, the tribological behavior improves because only the thin layers of MoS
2
residing on top are exposed to the environment, while the MoS
2
at the bottom is protected against thermal or environmental degradation by Sb
2
O
3
, which also acts as a beneficial support for MoS
2.
The proposed mechanism suggests that composite structures containing Sb
2
O
3
were also found to be more resistant to tribo-oxidation than was pure MoS
2
alone (Zabinski et al., 1993).
Recent advances in PVD and CVD technologies have led to the development of a new generation of selflubricating nanocomposite films and multilayer coatings. One such film is based on the MoS
2
and Ti system and is produced by closed-field unbalanced magnetron sputtering. This film is much harder and more wear-resistant than conventional MoS conventional MoS
2
2 coatings, yet it still has the low friction characteristics of
films. Its friction coefficient against a steel ball is
≈
0.02 in humid air and <0.01 in dry N
2
. 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 variety of 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., MoS
2
, WS
2
, etc.) and a metal, ceramic, or hard metal nitride or carbide (i.e, Ti, TiN, TiC, Pb, PbO, ZnO, Sb
2
O
3
) were also produced in recent years and were shown to work extremely well under demanding tribological conditions. These coatings can be prepared by co-sputtering of MoS
2
and TiN or TiB
2
targets, or a single target composed
© 2001 by CRC Press LLC

of TiN and MoS
2
. The resultant coatings may consist of distinct TiN and MoS x
phases in the form of a nanodispersive system. The hardness of these coatings could be as high as 20 GPa, while their friction coefficients are generally low (i.e.,
≈
0.1), even in open-air environments. Because of high hardness and low friction, they can be used in both sliding and machining applications (Gilmore et al., 1998a,b).
Recently, researchers have also produced multilayers of MoS
X
/Pb and MoS
X
/Ti (with individual layer thickness in the 4 to 100 nm range) by magnetron sputtering at room temperature. Sliding wear tests in
50% relative humidity showed great improvements in wear life over that of pure MoS x
coatings (Simmonds et al., 1998). The three-dimensional design of adaptive coatings based on a multicomponent
MoS
2
/TiC/DLC coating architecture resulted in improved tribological properties over broad ranges of environmental humidity and other test parameters (Voevodin et al., 1998). While the hard coatings of
TiC and DLC provided high strength and resistance to wear, solid lubricants DLC and MoS
2
provided low friction at the sliding surface. The coating had friction coefficients of 0.15 in humid air and 0.02 in dry nitrogen, thus increasing the prospects for use in space mechanisms.
PbO/MoS
2
and ZnO/WS
2 nanocomposite films were also produced in recent years and tested for their lubricity and durability in a variety of environments. The volume fraction of WS
2
decreased with increasing depth from the surface (Wlack et al., 1994). Composite films perform significantly better during tribotesting than films composed entirely of MoS
2
or PbO and ZnO. In addition, the composite films demonstrate the properties of “adaptive” lubricants. MoS
2 provides lubrication at room temperature; however, when the films are exposed to oxidizing environments at elevated temperatures, they adapt by forming PbMoO
4
. 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 produce lubricious wear debris and lubrication over extended temperature ranges (Wlack et al., 1997; Zabinski et al., 1992).
Electroless nickel, chromium, nickel-phosphorus coatings containing small amounts of graphite, MoS
2
,
PTFE, and diamond particles were also developed in recent years and used to achieve relatively thick films with self-lubricating properties. The deposition of MoS
2
containing Ni-P composite coatings (containing
≈
3 wt% MoS
2
) resulted in significant improvements in wear resistance and reduced the friction coefficient of the base Ni coatings (Moonir-Vaghefi et al., 1997).
Mainly because of their low shear strengths and rapid recovery as well as recrystallization, certain pure metals (e.g., In, Sn, Pb, Ag, Au, Pt, Sn, etc.) can provide low friction on sliding surfaces (Wells and De
Wet, 1988; Sherbiney and Halling, 1977). They are used chiefly as solid lubricants because the attractive properties they combine are unavailable in other solid lubricants. For example, in addition to its soft nature, silver has excellent electrical and thermal conductivity, oxidation resistance, good transfer-filmforming tendency, and a relatively high melting point; thus, it has been commercially used to lubricate the high-speed ball bearings of rotating anode X-ray tubes for many years. The Mohs hardness values of soft metals are generally between 1 and 3. Reported friction coefficients of soft metals range from 0.1 to
0.4, depending on the metal and test conditions. Pb, In, and Sn provide better lubricity at room temperature than Ag, Au, and Pt. At elevated temperatures, Pb, Sn, and In melt and undergo oxidation. Ag, Au, and
Pt have fairly high melting points, do not oxidize appreciably, and hence are preferred for high-temperature lubrication purposes (Erdemir and Erck, 1996; Maillat et al., 1993; Seki et al., 1995). Au remains in metallic form regardless of the temperature, while Ag
2
O decomposes as the temperature increases and Pt oxidizes only slightly. Bronze and babbitts prepared by alloying some of these soft metals with Al, Zn, Cu, have been 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 electroplating and vacuum evaporation can be used to deposit most of these metals as self-lubricating films, but dense and highly adherent films are produced by ion plating, sputtering, or ion-beam-assisted deposition techniques (Erdemir et al., 1990a). Film-to-substrate adhesion is extremely critical for achieving long
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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.) 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 µm
thick) and under higher contact pressures (Dayson, 1971; El-Sherbiny and Salem, 1986). Figure 22.19
shows 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 tend to decrease as the ambient temperature increases, mainly because of additional softening and rapid recovery 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 inertness makes silver and gold coatings ideal for applications involving high frictional or ambient heating, such
as sliding ceramic interfaces. As can be seen in Figure 22.20
, thin Ag films can lower wear rates of zirconia
balls and disks by factors of 2 to 3 orders of magnitude. Reduction in wear is more dramatic at higher sliding speeds. This is mainly because of the fact that zirconia has a very poor thermal conductivity, and thus suffers severe thermomechanical wear at high sliding velocities. However, when a highly thermally
FIGURE 22.20
Wear performance of uncoated and silver-coated zirconia (calcia-partially stabilized) test pairs at sliding velocities up to 2 m/s. (Adapted from Erdemir, A., Busch, D.E., Erck, R.A., Fenske, G.R., and Lee, R.H. (1991b), Ionbeam-assisted deposition of silver films on zirconia ceramics for improved tribological behavior, Lubr. Eng., 47, 863-867.)
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FIGURE 22.21
Physical condition of a wear track formed on silver-coated zirconia disk during sliding against zirconia ball.
conductive film like silver is present at the sliding interface, the wear rate decreases dramatically, mainly because frictional heat is dissipated rapidly from the sliding interface. The low friction coefficient of silver
also helps in reduced frictional heating. Figure 22.21
shows the condition of a wear track formed on a
silver-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 for
shear 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 sliding parts in nuclear reactors. When applied as a dense and adherent coating on the surfaces of these components, it can effectively dissipate frictional heat that can otherwise cause thermomechanical and tribochemical wear. Used on ceramic surfaces, it shears easily, thereby reducing the friction and microfracture-induced wear of the sliding ceramic surfaces (Erdemir et al., 1990a, 1991). Silver and other soft metallic coatings can also protect the sliding surfaces against environmental and/or tribochemical degradation 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 in reducing 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 sulfur and chlorine (if present in the operating environment) and may undergo rapid corrosive wear.
Polymers in various forms are widely used in tribology. They are lightweight, relatively inexpensive, and easy to fabricate. They can easily be blended with other solids to make self-lubricating composite structures. Certain polymers (polytetrafluoroethylene [PTFE], polyimide, nylon, ultra-high-molecular-weight polyethylene [UHMWPE], etc.) are self-lubricating when used in both the bulk and thin-film forms, or as 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, then consolidating and curing them at high temperatures. The most common polymer-based solid lubricant is 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 other forms (powder, composite, colloidal dispersion in oils and greases) to achieve low friction. Its friction coefficient ranges from 0.04 to 0.2, depending on test conditions. PTFE can be used at temperatures up to about 250°C. Polyimide and its coatings can also provide low friction. It can also be composited with a self-lubricating inorganic filler to enhance its mechanical and tribological properties, especially at
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elevated temperatures (Fusaro and Sliney, 1973; Blanchet and Kennedy, 1992). Fine PTFE powders have also 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 wear resistance than PTFE. However, wear of this polymer still poses a major obstacle for the longevity of the total 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 treatment processes (such as crosslinking, carbon-fiber reinforcing, recrystallization). It was reported that crosslinked UHMWPE has much better wear properties and thus is a promising alternative for total joint replacements.
There are several excellent review articles and book chapters devoted to the tribological uses 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).
This chapter further demonstrates that solid lubricants have much to offer for demanding tribological applications. Their use in advanced tribosystems is expected to increase in the near future, mainly because the operating conditions of future tribosystems are becoming more and more demanding. One major problem is that there exists no such lubricant that is capable of providing reasonably low and consistent friction coefficients over broad test conditions, temperatures, and environments. The results of previous studies demonstrate that the performance of layered solid lubricants are very much dependent on tribological and environmental conditions. For example, the lubricity of transition-metal dichalcogenides is adversely affected by moisture, while graphite depends on moisture for good lubricity. Layered solid lubricants can be doped or intercalated with a number of metals and compounds to achieve lesser sensitivity to ambient humidity and temperature.
Nowadays, most solid lubricants are produced as thin solid films on sliding surfaces. They are also used as fillers in self-lubricating metallic, ceramic, and polymeric composites. In most cases, a transfer film is found on the sliding surfaces. In general, formation of such a film at sliding interfaces seems to be key to achieving low friction and long wear lives in most solid-lubricated surfaces. For solid lubricant films, strong adhesion is key for long service life. Modern sputtering techniques and ion-beam processes are quite capable of imparting strong adhesion between solid lubricant films and their substrates. Ionbeam mixing of conventional solid lubricants, such as MoS
2
, with ceramics is also feasible and appears promising for severe tribological applications.
For materials with poor thermal conductivity, Ag and Au films combining high thermal conductivity with low shear strength and good chemical inertness should be considered. Silver is primarily used as a lubricant in ball bearings of rotating anode X-ray tubes. A unique solid lubricant, boric acid, which forms naturally 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 sliding interfaces in moist environments where MoS
2
is known to be ineffective.
For applications involving high temperatures, most layered solid lubricants appear ineffective. A combination of solid and liquid lubrication may provide short-term solutions to this problem; but for a long-term solution, the development of effective lubricious oxides, fluorides, and other compounds is essential. Recently, a crystal-chemical approach was introduced to classify lubricious oxides on the basis of their lubrication performance and operational limits at high temperatures. This approach may serve as 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 high temperatures. A series of adaptive lubrication strategies was also introduced in recent years and shown to be effective in achieving lubrication at broader temperature ranges.
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Certain polymers are also used as solid lubricants because the attractive properties they combine are unavailable 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 around for a long time and they have been meeting some very important and critical tribological needs. They are expected to be in high demand for many more years to come.
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