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Characteristics: The properties
important in determining the suitability of a material for
use as a solid lubricant are discussed below.
(1) Crystal structure. Solid lubricants such
as graphite and MoS2 possess a lamellar crystal structure
with an inherently low shear strength. Although the lamellar
structure is very favorable for materials such as
lubricants, nonlamellar materials also provide satisfactory
lubrication.
(2) Thermal stability. Thermal stability is
very important since one of the most significant uses for
solid lubricants is in high temperature applications not
tolerated by other lubricants. Good thermal stability
ensures that the solid lubricant will not undergo
undesirable phase or structural changes at high or low
temperature extremes.
(3) Oxidation stability. The lubricant
should not undergo undesirable oxidative changes when used
within the applicable temperature range.
(4) Volatility. The lubricant should have a
low vapor pressure for the expected application at extreme
temperatures and in low-pressure conditions.
(5) Chemical reactivity. The lubricant
should form a strong, adherent film on the base material.
(6) Mobility. The life of solid films can
only be maintained if the film remains intact. Mobility of
adsorbates on the surfaces promotes self-healing and
prolongs the endurance of films.
(7) Melting point. If the melting point is
exceeded, the atomic bonds that maintain the molecular
structure are destroyed, rendering the lubricant
ineffective.
(8) Hardness. Some materials with suitable
characteristics, such as those already noted, have failed as
solid lubricants because of excessive hardness. A maximum
hardness of 5 on the Mohs’ scale appears to be the
practical limit for solid lubricants.
(9) Electrical conductivity. Certain
applications, such as sliding electric contacts, require
high electrical conductivity while other applications, such
as insulators making rubbing contact, require low
conductivity.
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