The formation of silicone polymer films on metal surfaces at high temperatures and their boundary lubricating properties

Abstract

In this paper we have studied the lubrication of heavily-loaded sliding copper contacts by a low viscosity (10 cSt) dimethylsilicone fluid containing a small quantity of a fatty acid surfactant (stearic acid). Effective lubrication is provided by an interfacial monomolecular film of coadsorbed dimethylsilicone and fatty acid molecules. This film continues to maintain low friction and wear with increasing temperature due to the reaction of the acid with the metal surfaces and the subsequent formation of the high-melting-point copper soap. At approx. 100°C, a further marked enhancement in lubrication is observed. Electrical resistance measurements show this to be due to the formation of a thin non-conducting interfacial film, which provides protection for the sliding surfaces up to temperatures in excess of 150°C. Infrared spectroscopic analysis has shown this film to consist of a high concentration of copper stearate embedded in a cross-linked polysiloxane matrix. The molecular structure of the metal soap is complex and possible interactions with the polysiloxane are considered. The results show that polysiloxane film formation first occurs at temperatures of the order of 95°C which is much lower than the bulk decomposition temperature of the dimethylsilicone fluid. Thermal decomposition occurs at these low temperatures due to the catalytic oxidative nature of metal oxide interfaces. A convenient method of analysing the observed variations in electrical resistance across the sliding contacts is described which provides an insight into the effects of increasing load, temperature, repeated traversals of the same wear groove, surface deformation and work hardening. Static electrical resistance measurements employing a delicately loaded gold wire probe show that the load-bearing properties of the film are dependent on the high concentration of copper stearate trapped at the metal interface. The polysiloxane network simply serves as a matrix and in the absence of the fatty acid is readily worn away by the sliding process. Experiments under controlled atmosphere confirm the role of oxygen and metal oxide in the formation of both the polysiloxane and the metal soap. The results emphasize the possible importance of such films in limiting the friction and wear of heavily-loaded metal surfaces at elevated temperatures.