Effects of 3-d surface topography on the ehl film thickness and film breakdown

A systematic analysis is carried out to study the effect of 3-D surface roughness on the film thickness and film breakdown in micro-elastohydrodynamic lubrication (EHL). First, the analysis focuses on the global behavior of EHL film thickness. For simplicity and without loss of generality, a 3-D sinusoidal rough surface in contact with a smooth surface is analyzed, assuming isothermal conditions. Heavily loaded contacts are examined using Hertzian peak pressures up to 2.5 GPa. Two wavelengths are separately used to generate the roughness pattern along the direction of surface motion (i.e. the longitudinal wavelength). Each simulates the surface topography with long or short wavelength. The roughness pattern perpendicular to the direction of surface motion is described by the transverse wavelength which is set independently. The average value of EHL film thickness in the Hertzian region is calculated for various transverse wavelengths of surface roughness and for four values of slide-lo-roll ratio. The analysis predicts a rapid reduction of the film for small values of the transverse wavelength, suggesting a possible global breakdown of the EHL film. This possible breakdown is shown to be mainly caused by the local side flow of the lubricant around asperities in a narrow region of the Hertzian inlet and is worse under higher sliding conditions. Next, the local behavior of EHL film is analyzed. Multi-sinusoidal wave functions with various wavelengths and amplitudes are used to simulate contacts formed between two surfaces with random roughness. Thermal effects are also included in the analysis. Results obtained with a slide-to-roll ratio of 0.5 show an event of local film breakdown, leading to asperity collisions inside the Hertzian contact region. This film breakdown is caused by the formation and subsequent growth of a locally cavilated region as the two rough surfaces slide against each other. More extensive studies are needed to gain a further understanding of the complex time-dependent events leading to local EHL film breakdown.