An ultrahigh vacuum investigation of the chemistry and tribology of extreme pressure lubricant films.

摘要

Studies on the chemistry of extreme-pressure (EP) additives have revealed that the additives thermally decompose at the hot interfaces encountered during EP conditions to deposit lubricant films. The nature of these films depends on the additives and experimental conditions so that chlorinated hydrocarbons thermally decompose to form a layer of FeCl2 or Fe3C, and sulfur-containing molecules react to form FeS or a carbide. The goal of this dissertation is to make the link between the surface chemistry occurring during EP lubrication and the tribological properties of the resulting films. Three related studies were performed for this goal in an ultrahigh vacuum to provide well-characterized samples and to avoid any air contamination.;Tribological studies on inorganic halide films deposited on pure metal substrates. This model is simple but provides rich information that helps the understanding of more complicated, reactively formed lubricant films. Briefly, we have found that: (1) The complete coverage of the substrate by a monolayer of a halide results in a large friction coefficient reduction. The limiting friction coefficient, achieved by the completion of the monolayer, depends linearly on the hardness of the film material. The friction coefficient obeys Amontons' law and so is independent of the normal load. (2) A second regime is found where the friction coefficient increases with increasing film thickness, but still obeys Amontons' law. This is due to the increased contact area between the rough tribopin and the growing film. The frictional behavior of this regime is analyzed using a modified Greenwood-Williamson theory. (3) A third regime is identified when the film thickness is larger than the tribopin roughness, where the friction coefficient mu ∝ 1/√load. In this case, the load is distributed over the whole surface so that the interface behaves elastically.;Tribological properties of reactively formed lubricant films using CCl4 and iron. Similar friction coefficients are found for evaporated and reactively formed thin FeCl, films. Frictional behavior differences between evaporated and reactively formed films are explained by the layer-by-layer film growth and carbide formation for the latter case.;Surface and tribological chemistry of tributyl phosphite and phosphate. Tributyl phosphite and phosphate decompose on iron oxide to ultimately deposit phosphorus and carbon. A depth profile reveals that the carbon is located predominantly at the surface, while the phosphorus is distributed relatively uniformly throughout the oxide film. This implies that the tribological film formed in these reactions comprises a hard film produced by rapid diffusion of POx fragments into the oxide, covered by a low shear strength graphitic film.