Characteristics of Solid Lubricants

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Characteristics of Solid Lubricants

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.