Behaviour of Lubricants in the Mixed Elastohydrodynamic Regime
A range of lubricant base fluids have been chosen and their film-forming properties measured in the mixed elastohydrodynamic regime in pure rolling conditions. The Dowson-Hamrock elastohydrodynamic film thickness equation was found to remain valid over a large film thickness range for the tested base fluids. For hexadecane, the film thickness equation remains valid down to a film thickness of about 0.5 nm, a size equivalent to a molecular monolayer on each solid surface. However thicker films than predicted by elastohydrodynamic theory were found for all the other tested fluids at low speeds in the very thin film regime. This effect can be interpreted as boundary film formation by the fluids. The origins of these boundary films are still being explored but may be due to adsorption of molecules of the fluid on the solid surfaces and/or enhancement of viscosity due to the presence of a solid surface. This study has shown that full film lubrication is still possible for smooth surfaces in pure rolling in the very thin film regime where the film thickness is far smaller than the composite surface roughness.