Contact Mechanics and Friction of Elastic Solids on Hard and Rough Substrates

This thesis presented an experimental study of contact mechanics and rubber friction. These are topics of huge importance in Nature and in technology. Despite its importance and the theoretical and experimental effort carried out, contact mechanics and rubber friction are still not well understood t day. The motivation of this work has been to compare the approach of Persson to contact mechanics and rubber friction with state-of-the-art theories and to test its accuracy by comparing the predictions with experimental results. The experiments described here not only address the two topics contact mechanics and rubber friction but also different applications that are of great importance in many technological systems. It is the first time that experimental data has been used to test the approach of Persson in detail. Very good agreement has been found with the Persson approach, while the predictions of the standard theories disagree even qualitatively. This is due to severe approximations made in the state-of-the-art theories, such as the neglect of longrange elastic deformations and the oversimplified description of surface roughness. Very good agreement has been found when the approach of Persson was applied to different problems involving contact mechanics, including the leak rate of seals or the squeeze-out of a fluid. There are no fitting parameters used in the analysis. Within the accuracy of the experiments, the theory on contact mechanics by Persson has been shown to work very well. In the last section of the thesis, the predictions of a rubber friction theory based on the contact mechanics approach are tested using a novel instrument that has been designed. The experimental data could be explained well by the theory if different energy dissipation mechanisms are taken into account and the rubber friction theory of Persson explains the results within the accuracy of the measurements. It is the only physical model that can presently be applied to rubber friction on hard and rough substrates. More work needs to be done in order to test the influence of temperature effects (flash temperature) on rubber friction during energy dissipation. This is crucial for technical problems, such as friction of a tyre or dynamic seals. Further study of rubber friction on different substrates, e.g. road surfaces and ground steel surfaces, is needed to test the theory in detail