Film Thickness in Highly Loaded Contacts - Influence of Various Base Oils and VI-Improvers

The objective of the research project was the investigation of lubricant film formation in elastohydrodynamic contacts for oils of different origin (paraffinic, naphthenic, polyalphaolefin) and various types of VI-improvers (polymethacrylate, olefin copolymer, styrene butadiene copolymer). An essential part of the research was to find out whether VI-improvers maintain their thickening effect under contact conditions of high pressure (p=10,000 bar) and temperature (ϑ=50-90 °C), high shear rate (γ̇=10⁶ s^-1) and short contact time, all these conditions occurring simultaneously. A twin disk machine was used to determine values of film thickness in a line contact. For these measurements, an electrical capacitance method was used. In addition to a thermal viscosity loss, high shear rate γ̇ leads for VI improved oils to temporary viscosity loss and thus reduced film thickness. Additional permanent viscosity loss due to shear thinning reduces measured film thickness further. Lubrication with non-Newtonian fluids results in higher film thickness compared to their base oil but, even for pure rolling, in thinner films as a Newtonian fluids of same nominal viscosity.