Tribological research aims at extending an application’s period of usage and at saving resources by the improvement of the frictional behaviour of contacts. Our research on this topic is performed in close cooperation with Dr.-Ing. Andrea Codrignani from the Department of Microsystems Engineering – IMTEK at the University of Freiburg. The current goal is the deeper understanding of how to apply surface textures to reduce the friction in lubricated contacts.

Microtexturing of a surface can be an efficient tool for this purpose. In recent years more and more attention is focused on surface patterns made by simple
shapes such as dimples. If well designed, this kind of texture is able to reduce the friction coefficient dramatically, especially in the so called mixed lubrication regime where most of the everyday life devices are designed.

This research is also based on the precious collaboration with the Institute of Applied Materials which provides experimental results for the validation of the numerical approach. In the reference experiments, a small metal part (called pin) is indented on a rotating disc in order to analyze the friction coefficient obtained as the ratio between friction force and normal force. It is experimentally proved that this friction coefficient can be considerably reduced by introducing surface textures like the one in figure 1.

 Figure 1. Left side: experimental setup for the investigation of the friction coeficient of a small metal part (Pin) which is indented on the above rotating disc. Right side: magnification of the surface of the Pin, the surface texture is made by small spherical dimples.

Our numerical investigation is mainly focused to the reproduction of the experimental results and to the optimization of the surface textures.

This research is based on the development of both full CFD approach, by means of the compressible Navier-Stokes equations, and also on the development of simplified tribological models such as the Reynolds equation. It is important to mention that both these two numerical approaches can take into account all the relevant fluid dynamic phenomena such as cavitation, compressibility and the flow-roughness mutual influence.

Furthermore a contact mechanics solver is coupled with the above solvers in order to provide the detailed description of the whole Thermo-Elasto-Hydrodynamic-Dynamic Problem.

Figure 2. Left side: pressure distribution on the upper surface of the pin, in the second half of the surface the pressure is below the cavitation pressure. Right side: pressure distribution of a single dimple placed in positive pressure region of the pin.