Highly-loaded lubricated rolling-sliding contacts, which occur in gears, are generally known as elastohydrodynamically (EHL) or thermal elastohydrodynamically lubricated (TEHL) contacts (Figure 1). The elastic deformation of the rolling-sliding elements allows the transmission of Hertzian pressures of over 2000 N/mm² (20.000 bar) at lubricant film thicknesses of < 1 µm. The transmitted shear stress can even exceed the shear strength of aluminum.

Basic research on film thickness formation in TEHL contacts is carried out at optical EHL tribometers. Based on chromatic interferometry the local film thickness in TEHL contacts is visualized and quantitatively evaluated (Figure 2). Depending on load, temperature and velocity, film thickness formation and friction behavior of lubricants, surfaces and materials can be evaluated.

Basic research on rolling and sliding contacts of bearings and gears is frequently carried out at twin disk test rigs, which allow continuous variation of slide-to-roll ratios for different loads and temperatures. Thereby contact-integral measurements of the coefficient of friction and lubricant film thickness as well as contact-local measurements of the pressure, temperature and lubricant film thickness by means of thin film sensors (Figure 3) can be conducted. Tribofilms, which form by interaction of the lubricant and ambient medium in tribological contacts, are systematically investigated in long-term collaboration with research partners by latest surface analyses techniques (Figure 4). At the FZG, five disk test rigs are available. An optical tribometer is available for essential research topics, with which the formation and characteristics of lubricating films can be investigated with contact resolution and used for validating TEHL simulations.

Tribosimulations offer the possibility to dissolve the very small scales in space and time of TEHL contacts. Thereby, at least the areas of fluid dynamics and contact mechanics have to be solved in parallel. By taking into account measured lubricant properties, the physically based simulation models are suitable to calculate inter alia the temperature and friction behavior along the path of contact of a DLC coated spur gear (Figure 5). Further optimization of tribosimulations is an important subject of research at FZG.