Temperature Effects on Friction and Elevated Temperature Behavior of Base Oil-Additive Combinations Under Boundary Lubricated Conditions

Lubricant effectiveness is critical to the overall operational success of engine systems and components, particularly when boundary lubricated conditions occur in the contacting regions. In this investigation, several material and base oil/additive combinations were studied under low-speed boundary lubricated conditions in the ball-on-flat configuration, under predominantly bulk elastic Hertzian contact stresses. The purpose of the tests was to investigate the thermal behavior of base oil/additive mixtures through controlled heat input and performance monitoring as indicated by measured friction coefficient. In doing so, the sliding friction coefficients were sensitive to the lubricant mixture and evolving boundary film generated at the interface, minimizing contributions from plastic deformation and asperity flash temperature generation. The steels used were AISI 304 stainless, 1095 high-carbon, and 52100. The base oils used were mineral oil and poly-alpha-olefin (PAO) synthetic oil. The additives (1% by mass) were zinc dithiophosphate (ZDP) primary alcohol, overbased magnesium and calcium sulfonates, and stearic acid. The results indicated that the friction behavior is strongly dependent on the specific steel, base oil, and additive combination used during testing. In addition, some of the results were interpreted based on discussions presented in the literature.