Table 3
PROPERTIES REQUIRED BY SOLID LUBRICANTS
To Provide Low-Friction and Wear
Thin films Self-lubricating materials
Good film-forming ability (powders) Ability to form transfer films
Ductility Low-moderate elastic modulus
Good adhesion to substrate Adequate strength for required load capacity
Film continuity
Low-Shear Strength
General
High thermal/oxidative/hydrolytic stabilities
High softening/melting points
Chemically inert
High-thermal conducltvity/diffusivity
Corrosion protection of substrate
Appropriate electrical conductivity
No abrasive impurities
Low toxicity/environmental compatibility
Low-thermal expansion
electrical contacts, it is being increasingly supplanted by MoS
2
for three reasons. First is
the wide variability in graphites from different sources; MoS
2
quality is more rigidly con-
trolled by specifications. Second, the low friction of MoS
2
does not depend on adsorbed
vapors and is, in general, lower in vacuum than in air. Finally, the load-carrying capacity
of MoS

2
, with real promise appear to be sulfides and
selenides of Mo, Ta, W, and Nb. Since these are synthesized directly from the elements,
the compositions are not always stoichiometric and the crystal structure not wholly hexagonal.
Some compounds are nevertheless superior to MoS
2
in two main areas. TaS
2
, TaSe
2
, and
WS
2
have greater oxidation stability while TaS
2
, TaSe
2
, and NbSe
2
have much greater
electrical conductivity.
12
Experimental determinations of frictional properties and endurance
of surface films are somewhat conflicting, but no synthetic dichalcogenides appear to be
consistently superior to MoS
2
. Together with uncertainties about composition and high
expense, this has precluded their widespread use. WS
2
and NbSe

and effective lubrication occurs in both
vacuum
17
and inert gases.
18
Comparisons with MoS
2
, however, are less favorable; (CF
x
)
n
is
variously reported as being superior
14
or not,
15
depending on the lest method and on the
formulation of the lubricant film. As a very general summary, (CF
x
)
n
appears to offer little
over MoS
2
in most applications.
A number of other lamellar solids with crystal structures of the CdI
2
or CdCl
2
type also

2
also provide effective lubri-
cation in the range 250 to 1000°C; high friction (f>0.3) below 150°C can be partially
alleviated by the addition of Ag.
20
A series of metal oxides, tungstates, and molybdates also
show promise as high-temperature lubricants, with reasonably low-friction coefficients at
700°C, e.g., f ~
_
0.2 (MoO
3
, K
2
MoO
4
) and f ~
_
0.3 (Co
2
O
3
, NiMoO
4
). None, however, are
effective at room temperature.
Synthetic mixed metal sulfides, e.g., AsSbS
4
,Ce
2
(MoS

temperatures or to applications where sliding is limited, e.g., rolling element bearings. Ag-
Pd films have been used at temperatures up to 1000°C, and Pb films have been very successful
for long-term rolling bearing lubrication in space mechanisms.
23
Au is also of interest in
the latter application, but test results have proved extremely variable. Vacuum sputtering
and ion-plating permit close control of film composition and thickness and can provide
outstanding adhesion to the substrate. Optimum film thickness for maximum wear life is
generally very similar to that required to give minimum coefficient of friction, 0.1 to 1 µm.
Diffusion Coatings
An alternative to deposition of a surface film for reducing friction and wear of metals is
the thermal diffusion of foreign atoms into a surface. Some commonly available treatments
of this type, fisted in Table 4, have different objectives: to increase wear resistance by
increasing surface hardness (C,N in steels), to produce a low-shear strength surface to inhibit
scuffing or seizure (S in steels), or to provide either of the above in conjunction with increased
corrosion-resistance (Sn-Cu in steels).
Analogous to diffusion treatments, although not involving high bulk temperatures, is the
recently developed “ion-implantation” in which surfaces are bombarded with ions of the
element of interest accelerated to high energies. The surface usually increases in hardness
and also develops a compressive stress which improves fatigue resistance. Although depth
of penetration is small, ~
_
100 nm or less, beneficial effects on wear appear to persist long
after removal of material to this depth.
24
274 CRC Handbook of Lubrication
Table 4
SOME SURFACE TREATMENTS TO REDUCE FRICTION
AND/OR WEAR OF METALS
Diffusion Treatments

the polyimides.
27
Their maximum useful temperature for long-term use, ~
_
300°C, exceed
that of PTFE but the frictional properties are inferior, f ~
_
0.13 to 0.3.
By far the greatest use of polymers in solid lubrication is in self-lubricating composites
as direct replacements for lubricated metals.
28,29
Of the hundreds of polymers commercially
available, the few finding widespread use as self-lubricating materials are listed in Table 5.
Reinforcing fibers, fillers, and additives commonly incorporated to improve particular prop-
Volume II275
Table 5
PLASTICS AND FILLERS FOR SELF-LUBRICATING COMPOSITES
Copyright © 1983 CRC Press LLC
erties are also given. PTFE almost invariably requires reinforcement when used in bulk as
it is extremely susceptible to viscoelastic deformation under load. Reinforcements are also
commonly used with some thermosetting resins, e.g., phenolics, to increase toughness.
Friction and wear properties of the latter are improved by addition of lamellar solids such
as MoS
2
, or PTFE powder or flock. Some additives can also be multifunctional. A good
example is graphite which, particularly in fiber form, not only reduces friction and wear
but also increases the strength, stiffness and thermal conductivity of polymer composites.
FRICTION AND WEAR TESTING
Three separate objectives are involved in performance-testing solid lubricants and self-
lubricating materials: provision of design data, selection or development of materials, and

of the film is subjected.
31
Even in very carefully controlled conditions, repeat determinations
of wear life can show considerable scatter. With the Timken apparatus, Figure 1e, scatter
in wear life determinations can exceed ±100%. With Falex tests, Figure 1e, scatter is
usually less than ±50%. Falex tests are commonly incorporated into specification require-
ments for thin film lubricants.
The four-ball machine, Figure la, is widely used for evaluating solid lubricant additives
in oils; the pin/disc and pin/ring arrangements (Figures 1b to d) are used for wear testing
self-lubricating composites as well as thin film lubricants; reciprocating line-contact arrange-
ments (Figure 1d) show promise for wear testing thin, self-lubricating, bearing-liner ma-
terials;
32
the press-fit test (Figure 1h) is used for dry powders and rubbed films and the
journal and thrust-bearing configurations (Figures 1f and g) simulate bearing applications
for both thin films and self-lubricating composites.
OPERATIONALPERFORMANCE
Thin Film Lubricants
Rubbed Films
The simplest way to coat a solid lubricant on a metal surface is by burnishing of dry
powder (MoS
2
, graphite, etc.) with a soft tissue. MoS
2
films produced in this way range
from 0.1 to 10 µm thick, depending on rubbing time. Film thickness also increases with
increasing humidity.
33
Bonding of lamellar solids to the substrate appears to involve three
mechanisms: (1) particles can be physically trapped within surface depresssions, (2) crys-

is accelerated by
moisture, and after prolonged storage of powder in air at room temperature, MoO
3
, adsorbed
H
2
O, and H
2
SO
4
can all be present as surface contaminants. For this reason, pH limits of
aqueous extracts from MoS
2
powder are required by most specifications,
38
or a direct cor-
rosion test.
39
MoS
2
powder is commonly protected against oxidation during storage either
by adsorption of long chain organic inhibitors or by enclosure in an inert gas atmosphere.
Bonded Coatings
To overcome the dependence of burnished film thickness on relative humidity, and to
obtain greater film thickness and wear lives, lamellar solids are often incorporated within a
synthetic resin binder to produce a “bonded coating”. An enormous number of coating
formulations has been developed
40
and some of the more widely used constituents are listed
in Table 6. MoS

high stresses the reverse may occur.
41
Sliding speed usually has little effect on either friction
or wear until it becomes so high that frictional heating begins to soften or degrade organic
resin binders. The most important variable is temperature. With organic binders, wear life
tends to decrease with increasing temperature but with inorganic binders the converse is
sometimes observed because of low-temperature brittleness. Probably best all-round per-
formance over the widest temperature range is given by formulations incorporating high-
temperature resin binders such as polyimides. Binder properties may also affect the way in
which wear life depends on relative humidity.
Significant reductions in both wear life and load-carrying capacity of solid lubricant films
occur in the presence of conventional oils.
42
In some cases the reduction in performance is
a consequence of the resin binder being attacked by certain fluids, e.g., acrylics by chlorinated
organic solvents. More generally, fluids tend to cause adhesion failures at the substrate
interface and also impede reaggregation of lubricant debris produced during wear. Despite
these reductions in performance, some MoS
2
-bonded coatings persist sufficiently long in the
presence of oils to facilitate running-in,
43
and to reduce tool wear during machining operations.
44
The most promising high-temperature coatings are those incorporating CaF
2
/BaF
2
eutectic.
These may be applied by spraying from dispersions, followed by fusing at around 1000°C,

undergo large increases in wear above critical loads and speeds as a consequence of surface
melting. Effects on thermosetting resins are less dramatic because oxidative degradation,
leading to surface embrittlement, is a function of exposure time as well as temperature.
Thermal conductivity of the counterface is also relevant and at high sliding speeds can
become more important than the conductivity of the polymer composite itself. Limiting
speeds for polymers sliding against themselves are, in general, several hundred times lower
than those for polymers sliding against metals.
48
Wear rates of polymer composites depend strongly on the surface roughness of metal
counterfaces. In early stages of sliding, wear rate varies typically with initial Ra roughness
raised to a power of 2 to 4;
49
for this reason smooth counterfaces are always recommended
for applications such as dry bearings. During running-in, however, the initial counterface
roughness is frequently reduced, either by transfer of the polymer and/or fillers or by
polishing/abrasive action of fillers, leading to a reduction in wear rate. Steady-state roughness
and steady-state rate of wear depend both on the composite composition and on relative
hardness of the fillers and counterface.
50
Relationships between steady-state rate of wear
and initial counterface roughness thus become very variable and examples are shown in
Figure 2. Although an optimum counterface roughness for minimum wear is sometimes
suggested, experimental results are conflicting.
For PTFE composites and other polymers incorporating solid lubricants which rely on
transfer film formation on the counterface to achieve low wear, wear behavior is strongly
influenced by environmental factors. Relative humidity is particularly important and in-
creasing humidity can either reduce or increase wear depending on the type of filler; there
are no systematic trends.
51
Liquid water, however, increases wear by inhibiting transfer film

A great deal of effort has been devoted to material combinations and/or composite fab-
rication to obtain both low friction and wear. Incorporation of PTFE in lamellar solid-metal
composites appears to facilitate transfer film formation, and carbides in Ta-Mo-MoS
2
improve
strength.
57
Fabrication techniques use conventional powder metallurgy, infiltration of porous
metals, electrochemical codeposition, plasma spraying, and machining of holes or recesses
280 CRC Handbook of Lubrication
Copyright © 1983 CRC Press LLC
Volume II 283
Table 9
CLASSIFICATION OF CARBONS AND GRAFITES
Copyright © 1983 CRC Press LLC
cermets over ceramics are greater toughness and ductility, but the metal content, usually Co
or Ni, reduces the maximum temperature.
Few general guidelines are available to predict the wear behavior of ceramics, particularly
coatings where properties depend as much on method of deposition as on composition.
Friction coefficients tend to be very variable but can be as low as 0.2 to 0.25 at high
temperatures, e.g., Cr
2
C
3
-Ni-Cr or Cr
2
O
3
sliding against themselves.
5

68
Unfortunately, a similar approach is not yet available for self-lubricating
components other than dry bearings, e.g., gears, seals, or thin-film solid lubricant coatings.
Dispersions in Oils and Greases
Graphite and MoS are extensively used as additives in conventional oils and greases to
reduce friction and wear when full-film hydrodynamic or elastohydrodynamic lubrication
cannot be achieved. The concentrations added vary widely, from 0.1 to 60% by weight, the
higher values producing pastes used primarily for component assembly purposes. Relevant
specifications are listed in Table 13. Numerous rig tests have demonstrated that MoS
2
can
provide increases in load-carrying capacity, reductions in wear, and increased life of rolling
bearings. The optimum concentrations depend on the type of carrier fluid and the sliding
conditions but are typically around 3% by weight in oils and 20% by weight in greases.
Automotive experience has confirmed the beneficial effects of MoS
2
additions to oils in
reducing both wear and fuel consumption (friction).
69
Two cautionary comments are in order.
First, detergent additives in automotive oils can inhibit the wear-reducing ability of MoS
2
and graphite, and some anti-wear additives can even increase wear rates slightly.
70
Second,
solid lubricant additions can affect the oxidation stability of oils and greases, and this may
influence the concentration of oxidation inhibitors required; smaller particles have a greater
effect on oxidation stability than larger ones.
The influence of solid lubricant particle size on performance in oils and greases is con-
fused.

greases, which can provide effective lubrication in oxidizing environments over a wide
temperature range.
74
Typical applications are in rocket motors and space components.
REFERENCES
1. Campbell, W. E., Solid lubricants, in Boundary Lubrication: An Appraisal of World Literature, Ling, F.
F., Klaus, E. E., and Fein, R. S., Eds., American Society of Mechanical Engineers, New York, 1969,
197.
2. Lansdown, A. R., Molybdenum disulphide: a survey of the present state of the art, Swansea Tribol. Cent.
Rep., 74, 279, 1974.
3. Pratt, G. C., Plastic-based bearings, in Lubrication and Lubricants, Braithewaite, E. R., Ed., Elsevier,
Amsterdam, 1967, 377.
4. Claus, F. J., Solid Lubricants and Self-Lubricating Solids, Academic Press, New York, 1972.
5. Lancaster, J. K., Dry bearings: a survey of materials and factors affecting their performance, Tribology,
6, 219, 1973.
6. Lancaster, J. K., Friction and wear (of polymers), in Polymer Science, Jenkins, A. D., Ed., North Holland,
Amsterdam, 1972, 960.
7. Tabor, D., Friction, adhesion and boundary lubrication of polymers, in Advances in Polymer Friction and
Wear, Lee, L H., Ed., Plenum Press, New York, 1974, 1.
8. Roselman, I. C. and Tabor, D., The friction of carbon fibres, J. Phys. D., 9, 2517, 1976.
9. Peterson, M. B. and Johnson, R. L., Friction Studies of Graphite and Mixtures of Graphite With Several
Metallic Oxides and Salts at Temperatures to 1000°F, TN-3657, National Aeronautics and Space Admin-
istration, Washington, D.C., 1956.
10. Grattan, P. A. and Lancaster, J. K., Abrasion by lamellar solid lubricants. Wear, 10, 453, 1967.
11. Giltrow, J. P. and Lancaster, J. K., The role of impurities in the abrasiveness of MoS
2
, Wear, 20, 137,
1972.
12. Magie, P. M., A review of the properties and potentials of the new heavy metal derivative solid lubricants,
Lubr. Eng., 22, 262, 1966.

22. Stott, F. H., Lin, D. S., Wood, G. C., and Stevenson, C. W., The tribological behavior of nickel and
nickel-chromium alloys at temperatures from 20° to 800°C, Wear, 36, 147, 1976.
23. Todd, M. J. and Bentall, R. H., Lead film lubrication in vacuum, Proc. ASLE 2nd Int. Conf. Solid Lubr.,
SP-6, American Society of Lubrication Engineers, Park Ridge, III., 1978, 1948.
24. Dearnaley, G. and Hartley, N. E. W., Ion implantation of engineering materials, Proc. Conf. Ion Plating
and Allied Techniques, CEP Consultants Ltd., Edinburgh, 1977, 187.
25. Pooley, C. M. and Tabor, D., Friction and molecular structure: the behavior of some thermoplastics,
Proc. R. Soc. London Ser. A, 239, 251, 1972.
26. Spalvins, T., Sputtering technology in solid film lubrication, Proc, ASLE 2nd Int. Conf. on Solid Lubr.,
SP-6, American Society of Lubrication Engineers, Park Ridge, III., 1978, 109.
27. Fusaro, R. L., Friction and Wear Life Properties of Polyimide Thin Films, TN-D-6914, National Aero-
nautics and Space Administration, Washington, D.C., 1972.
28. Brydson, J. A., Plastic Materials, 3rd. ed., Butterworths, London, 1975.
29. Theberge, J. E., Properties of internally lubricated, glass-fortified thermoplastics for gears and bearings,
Proc. ASLE Int. Conf. Solid Lubr., SP-3, American Society of Lubrication Engineers, Park Ridge, III.,
1971, 106.
30. Benzing, R. J., Goldblatt, I., Hopkins, V., Jamison, W., Mecklenburg, K., and Peterson, M., Friction
and Wear Devices, 2nd ed., American Society of Lubrication Engineers, Park Ridge, III., 1976.
31. McCain, J. W., A theory and tester measurement correlation about MoS
2
dry film lubricant wear, SAMPE
J., February/March, 1970, 17.
32. Lancaster, J. K., Accelerated wear testing of PTFE composite bearing materials, Tribal. Int., 12, 65,
1979.
33. Johnston, R. R. M. and Moore, A. J. W., The burnishing of molybdenum disulphide on to metal surfaces,
Wear, 19, 329, 1972.
34. Stupian, G. W., Feuerstein, S., Chase, A. B., and Slade, R. A., Adhesion of MoS
2
powder burnished
on to metal substrates, J. Vac. Sci. Technol., 13, 684, 1976.

Mech. Eng., 183 (3P)), 98, 1969.
50. Lancaster, J. K., Polymer-based bearing materials: the role of fillers and fibre reinforcement, Tribology,
5, 249, 1972.
51. Arkles, B. C, Gerakaris, S., and Goodhue, R., Wear characteristics of fluoropolymer composites.
Advances in Polymer Friction and Wear, Plenum Press, New York, 1974, 663.
Volume II 289
Copyright © 1983 CRC Press LLC
52. Evans, D. C., Fluid-polymer interactions in relation to wear, Proc. 3rd Leeds-Lyon Symp. Wear of Non-
Metallic Materials, Mechanical Engineering Publication, London 1978, 47.
53. Ikeda, H., Piastic-Based Anti-Friction Materials, Japanese Patent, 75101441, 1975.
54. Williams, F, J., Teflon airframe bearings — their advantages and limitations, SAMPE Quart., 8, 30, 1977.
55. Sitch, D., Self-lubricating rolling element bearings with PTFE-composite cages, Tribology, 6, 262, 1973.
56. Anon., Self-Lubricating Bearings — A Performance Guide, U.K. Natl. Center of Tribology, Risley, War-
rington, 1977.
57. McConnell, B. D. and Mecklenburg, K. R., Solid lubricant compacts — an approach to long-term
lubrication in space, 76-AM-2E-1, ASLE Trans., 1976, preprint.
58. Gardos, M. N., Some Topographical and Tribological Characteristics of a CaF
2
/BaF
2
, Eutectic-Containing
Porous Nichrome Alloy Self-Lubricating Composite, 74LC-2C-2, ASLE Trans., 1974, preprint.
59. Sliney, H. E., Wide-Temperature-Spectrum Self-Lubricating Coatings Prepared by Plasma Spraying, TM-
79113, National Aeronautics and Space Administration, Washington, D.C., 1979.
60. Paxton, R. R., Carbon and graphite materials for seals, bearings, and brushes, Electrochem. Tech., 5,
1974, 1967.
61. Strugala, E. W., The nature and cause of seal carbon blistering, Lubr. Eng., 28, 333, 1972.
62. McKee, D. W., Savage, R. H., and Gunnoe, G., Chemical factors in carbon brush wear, Wear, 22, 193,
1972.
63. Giltrow, J. P., The influence of temperature on the wear of carbon fibre-reinforced resins, ASLE Trans.,

Increasing interest in and application of gas bearings requires knowledge of a number of
gas properties which are not as readily available as the properties of common liquid lubricants.
This is particularly true in process fluid lubrication where gases other than air are involved.
This section provides as much as possible of the information required in the design of a
wide variety of gas bearings. Some brief background is followed by property data and by
discussions on a number of typical applications.
NATURE OF A GAS
In the gaseous state of matter, individual atoms or molecules are in constant motion and
are separated from each other by distances of several times their diameter. The gas particles
collide with each other frequently and travel in straight lines between collisions. The average
velocity of the particles is an expression of the gas temperature, increasing with temperature.
When a gas particle hits a solid surface and bounces off, the change in momentum of the
particle exerts a force on the surface. The sum of the countless surface collisions is the
pressure the gas exerts on the surface. If one of a pair of parallel surfaces is moving, it will
impart an additional component of velocity to each gas particle hitting it. This additional
velocity is transmitted to other particles in the course of collisions and eventually to the
other surface. The result is a force on the other surface expressed as the product of the area
of the surface, the rate of shear, and the viscosity. The rate of shear is defined as the velocity
difference between the surfaces divided by the distance between them.
If a volume of gas is compressed, more particles must hit each unit of surface, and the
pressure increases. If the temperature is increased, average particle velocity is increased,
momentum change in each surface collision increases, and again the pressure is increased.
This behavior is expressed in the “perfect gas” law PV = nRT where R is the “gas constant”
and n the mass (moles) of the volume of gas involved.
Almost all gases are “perfect” at low pressures, usually one atmosphere or less. Deviation
occurs at very low pressures when not enough particles are present to provide many collisions
between impacts with the surface. When the pressure is high, the gas particles are forced
more closely together, molecular attractions between particles begin to exert an influence,
and deviations from the perfect gas law are observed.
Mixtures of gases behave as if each were alone in the total volume. Each exerts a partial

p
and C
v
, is the energy required to raise the temperature of a unit quantity
of gas by one degree. When the process is carried out at constant pressure, the quantity is
C
p
. At constant volume, the quantity is C
v
. The units are kilo-Joules per kilogram per degree.
Sonic velocity—Φ, is the speed with which a pressure wave is transmitted through a gas.
Since an increase in pressure can be transmitted only through particle collisions, the speed
of transmission will be related to the particle velocities, and hence to gas temperature.
Mean free path—λ, is the average distance traveled by a gas particle between collisions.
This quantity is of interest in bearings operating with very close clearances or at very low
pressures where the mean free path approaches the surface separation distance. Its calculation
is treated in a following section.
Equation of state— Relates the physical properties of a perfect gas to each other and to
the quantity of gas present. It is PV=nRT, where Vis the volume in m
3
occupied by n
kg-mol of gas at an absolute temperature T. Gas constant R =8.3143 kJ/kg-mol·K.
PHYSICALDATA
Data in Table 1 are abstracted from an extensive listing of thermophysical properties of
liquids and gases.
1
The first three columns give the common name of the gas, its chemical
formula, and its molecular weight. Column four gives the boiling point in K at a pressure
of 760 mmHg or 1.01 bar. Also, given are specific volume in m
3

)
n
(1)
292 CRC Handbook of Lubrication
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Copyright © 1983 CRC Press LLC