Solid lubricants may be present in the friction area in forms of either dispersed particles or surface films:
Advantages of solid lubricants:
Disadvantages of solid lubricants:
The crystal lattice of these materials has a layered structure consisting of hexagonal rings forming thin parallel planes. Within the plane each atom is strongly bonded (covalent bonding to other atoms. The planes are bonded to each other by weak Van der Waals forces.
The layered structure allows sliding movement of the parallel planes. Weak bonding between the planes determines low shear strength and lubricating properties of the materials.
The most commonly used inorganic solid lubricants with lamellar structure are graphite, molybdenum disulphide (MoS2) and boron nitride (BN).
Other examples of such materials are sulphides, selenides and tellurides (chalcogenides) of molybdenum, tungsten, niobium, tantalum, titanium (eg. WS2, WS2, MoSe2, TaSe2, TiTe2), monochalcenides (GaS, GaSe, SnSe), chlorides of cadmium, cobalt, lead, cerium, zirconium (eg. CdCl2, CoCl2, PbCl2, CeF3, PbI2) and also some borates (eg. Na2B4O7) and sulfates (Ag2SO4).
Examples: B2O3, MoO2, ZnO, Re2O7, TiO2, CuO-MoO2, NiO-Mo2, PbO-B2O3, CuO-Re2O7.
Due to their low shear strength and high plasticity some soft metals possess lubrication properties: lead (Pb), tin (Sn), bismuth (Bi), indium (In), cadmium (Cd), silver (Ag).
Soft metals are used in pure form or as alloys, in form of coatings (Lead based engine bearing overlays, Tin based engine bearing overlays) or the second phase in Metal Matrix Composites (Copper based bearing materials, Aluminum based bearing materials). Coatings from soft metal lubricants are produced by the methods of Electroplating, Vapor deposition and Thermal spraying.
Composites containing soft metal lubricants are prepared by casting or sintering methods.
Soft metal are widely used as solid lubricants in Engine bearing materials.
Polytetrafluoroethylene (PTFE) and polychlorofluoroethylene are the typical examples of such materials.
The molecular structure of the materials consist of long-chain molecules parallel to each other. The bonding strength between the molecules is weak therefore they may slide past one other at low shear stresses.
The strength of the molecules along the chains is high due to strong bonding between the atoms within a molecule.
Such anisotropy of mechanical properties provides good lubrication properties of the materials.
Chain structure lubricants are used in form of coatings (films) applied on the substrates surfaces (Polymer based engine bearing overlays).
Soaps are metals (lithium, calcium, sodium, potassium) salts of fatty acids.
Soaps are prepared by chemical treating of oils and fats by strong alkaline solutions.
A soap molecule is composed of a long non-polar hydrocarbon tale, which is hydrophobic (repelled by water) and the salt polar end, which is hydrophilic (water soluble).
The soap molecules attached to the substrate surface provide good adhesion of the soap lubricant and low coefficient of friction.