Lucas Frérot (Albert-Ludwigs-Universität, Freiburg) - Roles of roughness in tribology

Roughness is an inevitable part of virtually all tribological systems, and often dominates the frictional and wear responses of solid interfaces. The present seminar aims to picture selected effects of roughness on the friction response of fatty acid coatings and on the wear of elastoplastic materials.

 

We first discuss the role of roughness in frictional systems composed of self-assembled monolayers of stearic acid molecules. This fatty acid compound is often used in applications where reduced friction is necessary. Experiments conducted at LTDS by Dr. Juliette Cayer-Barrioz and her group have shown that these frictional layers exhibit rate-and-state frictional properties: the dynamic friction force depends on the sliding velocity, the static friction force depends on the resting time of the interface, and the interface has a memory length-scale characteristic of rate-and-state friction. We will show with nano-scale molecular dynamics that roughness plays a fundamental role in the emergence of these features, with a focus on frictional aging. 

 

We will also discuss some effects of roughness at continuum scales. Adhesive wear degrades surfaces by a fracture process: if the junction between two asperities is strong enough, a crack can form and detach a third-body particle. To study the effects of plasticity on the likelihood of nucleating cracks in a rough-surface contact, we will go throug the the development of an efficient numerical method for the modeling of elastic-plastic rough contact. Because self-affine surfaces have no separation of scales, very fine meshes at the contact interface are necessary. Fourier-based boundary integral approaches have proved superior to FEM in elastic contact, and we will show how they can efficiently be extended to elastic-plastic materials. This allows for a detailed study of the effects of plastic residual zones in adhesive wear.