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 propose an experimental investigation of the mechanical response of SrTiO3 substrates probed by a commercial nanoindentor. Load-displacement curves have been acquired respectively on well-defined crystalline surfaces and on mechanically lapped surfaces. We observe the first-loading cycles to be considerably affected by surface roughness whenever the penetration depth is kept below the interface width. The obtained results are analyzed within an elasto-plastic deformation model for fractal surfaces and a picture is developed to describe the deformation process with respect to surface roughness and structural parameters.

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