A fractal approach to the contact mechanics of randomly rough surfaces

The non-stationary character of roughness is a widely recognized property of surface morphology and suggests modelling several solid surfaces by fractal geometry. In the field of contact mechanics this demands for novel investigations attempting to clarify the role of multi-scale roughness during physical contact. Here we review the results we recently obtained in the characterization of the contact mechanics of randomly rough surfaces by depth-sensing indentation. Experiments were conducted on self-affine organic thin films, load-displacement curves being acquired by an atomic force microscope equipped with custom-designed tips. We observed the first-loading cycle to be considerably affected by surface roughness. Plastic failure was found to dominate incipient contact while contact stiffness increased on decreasing fractal dimension and roughness. Our findings suggest fractal parameters to drive contact mechanics whenever the indentation depth is kept below the interface width.