On the friction mechanism of solid lubricants

It is shown that the coefficient of friction of polycrystalline graphite materials is governed by the crystallite size and adsorption heat of the molecules making up the lubricating layer. In the absence of a lubricating layer in the actual contact zones (in vacuum, dried gases, and in air at temperatures above 200°C), graphite loses its lubricity. To maintain low friction within 800°C, ultraphosphates are introduced into graphite pores; in heating, they release vapors of phosphoric anhydride to form a lubricating film. At 300°C, chalcogens MoS2, WS2, and MoSe2 release S and Se vapors that are adsorbed in contact and create a lubricating film. The capacity to release vapors is governed by the presence of solid solutions of chalcogens in addition to stoichiometric compounds.