Effects of base oil composition in a variety of mixed lubrication conditions were studied by sliding experiments. Mineral base oils as well as polar and non-polar synthetic base oils were tested. In the region of transition from hydrodynamic to mixed lubrication, oils of non-polar types with higher pressure viscosity coefficient produced thicker films and higher friction. They were expected to form solid-like films that prevented asperity contacts. These films broke down under much severer conditions from the mixed to the boundary lubrication regimes, where oils of polar molecule type produced better boundary films by adsorption. Tests were also conducted with oils containing tricresylphosphate. The additive in the solidification type oils effectively formed surface films, while it did not in the adsorption type base oils, probably because they prevented the additive from adsorbing on the surface. It was also found that higher pressure viscosity coefficient allowed less tricresylphosphate to react on the surfaces.
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