The effect of a tribo-modified surface layer on friction in elastomer contacts

Abstract

Friction between rubber and a counter surface has interested many researchers because of its huge practical importance. Rubber components are applied in various industrial applications such as tires, rubber seals, wiper blades, conveyor belts and syringes. The friction between a rubber surface in contact with a rigid surface is still not fully understood. The complexity lies partially in the viscoelastic nature of elastomers next to various parameters such as roughness, contact pressure and sliding velocity. In addition, several complex phenomena occur at the interface between rubber and the rough rigid body in contact, which can significantly influence the tribological behavior of the system. The contact and friction of rubber with a focus on the importance and influence of a tribo-modified surface layer is studied bith theoretically and experimentally in this work. Contact and friction models dealing with soft viscoelastic materials are briefly introduced and the complex aspects of the rubber friction problem are reviewed. To understand the dependence of rubber friction on various parameters, the rubber network, structure and morphology are described. The effect of different reinforcement fillers on the mechanical properties of the rubber is analyzed. The main components of the overall friction in elastomeric contacts are studied and the importance of each friction contributor as a function of the tribological conditions is investigated. It is shown that the shearing of a modified surface layer by a rigid counter surface does play an important role in the total friction. However, it has not been studied thoroughly, therefore this modified layer is researched extensively in this thesis. The contact and friction model of Persson is extended in such a way that it can model the contact and friction of a transversely isotropic viscoelastic solid, i.e. a fiber reinforced material, in contact with a rigid rough surface, and the results are validated experimentally. Based on AFM nanoindentations, the existence of a modified surface layer on top of rubber that was subjected to frictional energy is shown. The mechanical properties of the modified layer degrade as a function of tribological conditions when compared to the original surface. A physical model is developed which explains the modification due to mechanical degradation. In this model, the mechanical energy exerted on the bulk of the rubber as a function of the tribological conditions is related to the layer formation rate. The effect of wear is considered in the aforementioned model. It is emphasized that wear of the layer is as important as the formation and a high wear rate might even remove the tribologically modified layer. Finally, based on the formation and wear, the existence of a modified surface layer is discussed as a function of the energy input rate.