Effects of viscosity and sliding speed on boundary slippage in thin film hydrodynamic lubrication

The paper presents an experimental study of the effects of two parameters: lubricant viscosity and sliding speed on boundary slippage in thin film hydrodynamic lubrication using a slider-on-disc tester. Two sets of experiments were purposely derived. One used the same types of lubricants of various viscosities with an oleophobic sliding surface. The other one adopted a base oil with additives of different concentrations of C6F13COOH, which provided different affinity to the surface. Viscosity effect was evaluated by the former, while the speed effect was illustrated from results of both tests. Lubricant film thickness was measured via optical interferometry under different speed/load conditions. A boundary yield stress slip model was adopted to construct film thickness-boundary yield stress charts for various running conditions. The boundary yield stresses corresponding to the measured film thickness under the specified running conditions were obtained from the specific chart. It was found that the boundary yield stress increased linearly with lubricant viscosity and sliding speed. The findings can be explained efficiently by the slip model proposed by Spikes and Granick in 2003 [1].