Lubrication by lamellar solids

Abstract

An experimental study has been made of the friction properties of graphite, molybdenum disulphide, boron nitride and talc. The formation of surface layers of the lamellar solids on platinum, and the friction of these layers at elevated temperatures in air have been examined. There is evidence that the frictional behaviour of the solids is dominated by the forces acting between separate crystallites, cleavage of individual crystallites being of secondary importance. The structure of the lamellar solids gives rise to plate-like crystallites, and a large proportion of the surface area is composed of faces with relatively low surface energy, with a small proportion of high-energy edge surface. The edges can react with gases to give a surface of relatively low surface energy, and the adhesion between the crystallites is then small, giving the solids low friction properties. In general, the removal of adsorbed gases increases the adhesion between the crystallites (particularly at the edges) so that the friction increases. This is observed for the removal of physically adsorbed volatiles from the partly ionic boron nitride and talc, and for the removal of the chemically bound carbon oxides from graphite. Molybdenum disulphide behaves differently, for with this solid the presence of adsorbed water promotes hydrogen bonding between the crystallites and thereby increases the adhesion between them . Other workers have shown that the friction of outgassed graphite in vacuo decreases reversibly at high temperatures. A similar behaviour has been shown to occur for boron nitride in air at temperatures below that at which it oxidizes rapidly, and suggests its use as a high temperature lubricant. The decrease in friction is caused by a gradual weakening of the intercrystallite bonding as the temperature is increased. Small quantities of bulk impurities can have a large influence on the intercrystallite bonding. It is believed that the impurity responsible in the case of boron nitride is boric oxide, because it melts at a temperature close to that at which the reversible decrease in friction occurs. Thermogravimetric analysis has been used to show that when the materials undergo a chemical change such as rapid oxidation they no longer give a low friction even when present in excess on the surface. Electron diffraction studies show that on rubbing in air the lamellar solids tend to form oriented layers on them etal surface, so that the plate-like crystallites lie flat. The orientation does not cause the low friction, but the low adhesion between the crystallites allows them to become oriented in their most favourable position, and independently causes the friction to be low. Reflexion electron micrographs show that the lubricant is occluded within crevices in the surface. The micrographs of the tracks formed during lubricated sliding show that the metal is deformed plastically, but that failure occurs mainly or entirely within the lubricant. An important factor in the formation of satisfactory lubricant layers appears to be the hardness of the lubricant relative to that of the metal, as the lamellar solid may protect the metal by becoming embedded in the surface.