A Simple Analysis of Texture-Induced Friction Reduction Based on Surface Roughness Ratio

Abstract

The effect of surface texture on friction reduction under fluid lubrication has been broadly acknowledged in the tribology community. However, the lack of understanding of the underlying mechanisms remains a challenge for the advancement of textured enhanced lubrication. Numerous models have been proposed, but they are almost all based on the hydrodynamic effect alone and have proven cumbersome, system specific and unreflective of the beneficial secondary lubrication provided by the residual lubricants within the texture. This paper presents a simple analysis of texture induced friction reduction based on the actual liquid-solid interface area and the secondary lubrication hypothesis. A simple model based on the surface roughness ratio (the ratio between the actual and projected solid surface area) of the textured surface was proposed which 1) is simple, intuitive, quantitative and sensitive to texture shape and area fraction; 2) directly reflects proposed secondary lubrication mechanisms; 3) reflects the general data trend in the collected literature. By focusing on the variations of key texture parameters, the proposed model combined with a sampling of independent studies in literature has demonstrated that 1) the effect of increased pit depth-to-diameter ratio (d/D) on friction reduction is most significant between 0.01 and 0.2; 2) further increase in d/D only marginally affects the friction coefficient; 3) texture's area fraction plays a much weaker role than the depth/diameter ratio in friction reduction. By quantitatively isolating the secondary lubrication effects, this model may help to link disparate studies in the literature while providing defensible quantitative insights into texture induced lubrication mechanisms.