Which asperity scales matter for true contact area? A multi-scale and statistical investigation

The true contact area between two surfaces is only a small fraction of the apparent macroscopic contact area; it governs many interfacial properties such as friction and contact resistance and depends sensitively on roughness. However, for real-world multi-scale surface topography, it is not clear which size scales of roughness govern the true contact area. This study investigates true contact area for a real-world surface that has been characterized across all scales from Angstroms to centimeters. Elastic and elastic-plastic contact is investigated using both a multiscale framework and a statistical roughness model. The multiscale method is a rough-surface contact-modeling technique based on Archard's stacked scales from a spectrum of the surfaces, which has shown promise when compared to previous experimental and numerical results. In contrast, statistical models assume that the asperities follow a defined height distribution and are in contact when taller than the mean surface separation. The results show that even the smallest scales can have a significant influence on the contact area, especially when the contact is elastic. However, when the contact is elastic-plastic, the influence of smaller scales can be limited depending on the character of the roughness. For self-similar, fractal-like roughness across some scales, the pressure tends to saturate at those scales. This work also explores the inclusion of scale-dependent yield strength. Both the multiscale and statistical models predict that the inclusion of scale-dependent strength causes the predicted contact area of the elastic-plastic models to come into closer agreement with that of the elastic model, especially when a wider range of size scales are included. In addition, both types of models predict that below a certain scale, smaller asperities flatten under contact pressure and will no longer influence the predicted contact area. Taken together, this work helps to guide the accurate modeling of rough-surface contact, and provides insights into which scales can be modified to improve performance in manufactured components. •Many scales of surface roughness influences the prediction of real contact area by statistical and stacked multiscale models.•Small scales of roughness decrease the real contact area, although several models predict there is a limiting threshold.•Including plasticity reduces the influence of smaller scales of roughness on the real area of contact.•Stacked multiscale models