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In some high-temperature applications, it is desirable to deliver the lubricant as a thin liquid film without recirculation. For these conditions, the buildup of the deposit on the hot surfaces outside the actual bearing contact area can be a problem. In this study, a mathematical model is developed which incorporates several chemical and physical processes which are taking place when a thin liquid film of lubricant is converted to a deposit at elevated temperatures. Specifically, the model utilizes kinetic studies of the degradation of tricresyl phosphate and the evaporation of this lubricant at modest temperatures, <280°C, and modest levels of TCP degradation to predict the final or terminal amount of deposit which will be produced over a broad range of temperature. The key to this model is incorporating the influence of the evaporation of the liquid lubricant on the diffusion of oxygen in the gas phase. The diffusion of the evaporated lubricant molecules away from the liquid phase inhibits the diffusion of the oxygen in the gas phase. Consequently, the oxidation of the liquid lubricant is significantly reduced by the unavailability of oxygen even when the liquid film is exposed to a moving stream of air.

Gel permeation chromatography is shown to be a valuable technique for analysis of the oxidation and thermal degradation products of various types of esters. Esters potentially useful as synthetic lubricant basestocks, such as tridecyl pelargonate (a mono-ester), di-2-ethylhexyl sebacate (a diester) and trimethylolpropane triheptanoate (a neopentyl polyolester) form products of up to 50 000 molecular weight when oxidized at high temperatures. The oxidation products have been separated by molecular weight using preparative gel permeation chromatography (GPC) and then have been analyzed spectrographically. The polymeric products of oxidation show similar chemical bond structure for the various types of esters studied. The importance of oxygen and metal catalysis on product formation is demonstrated. The influence of steel surfaces on the products of oxidation suggests a tribochemical reaction resulting in the ultimate formation of a "friction polymer."