Measurement of normal contact stiffness of fractal rough surfaces

TL;DR

This study experimentally examines how surface roughness and fractal dimension influence the normal contact stiffness of aluminium surfaces, revealing strong correlations with RMS slope and fractal dimension, but weaker with roughness.

Contribution

It provides new experimental data linking surface fractality and roughness to contact stiffness, highlighting the importance of fractal dimension and RMS slope.

Findings

Contact stiffness correlates strongly with RMS slope (~90%)

Fractal dimension significantly influences contact stiffness

Weak correlation between contact stiffness and RMS roughness (57%)

Abstract

We investigate the effects of roughness and fractality on the normal contact stiffness of rough surfaces. Samples of isotropically roughened aluminium surfaces are considered. The roughness and fractal dimension were altered through blasting using different sized particles. Subsequently, surface mechanical attrition treatment (SMAT) was applied to the surfaces in order to modify the surface at the microscale. The surface topology was characterised by interferometry based profilometry. The normal contact stiffness was measured through nanoindentation with a flat tip utilising the partial unloading method. We focus on establishing the relationships between surface stiffness and roughness, combined with the effects of fractal dimension. The experimental results, for a wide range of surfaces, showed that the measured contact stiffness depended very closely on surfaces' root mean squared (RMS) slope and their fractal dimension, with correlation coefficients of around 90\%, whilst a relatively weak correlation coefficient of 57\% was found between the contact stiffness and RMS roughness.