The effectiveness of various combinations of borate-, sulfur-, and phosphorus-containing additives blended in gear oil to form antiwear tribofilms on steel surfaces sliding in the boundary lubrication regime was investigated experimentally. The formation of protective tribofilms and their tribochemical activity in the temperature range of 32–100°C were analyzed in terms of coefficient of friction and contact voltage results. The antiwear performance obtained with each blend was evaluated by wear volume calculations based on surface profilometry measurements. Scanning electron microscopy studies provided insight into the dominant wear mechanisms at various temperatures. The tribological performance of the tribofilms and associated wear mechanisms were found to depend strongly on the type of additive(s), tribofilm composition, and temperature. This study has revealed that sulfide-dominated tribofilms produce lower friction, whereas borate- and phosphate-dominated tribofilms are more effective in increasing the wear resistance of the steel surfaces.
Keywords: Additives, Boundary Lubrication, Friction, Antiwear Tribofilms, Wear Mechanisms