Frictional properties of multi-asperity surfaces at the nanoscale
RR Santhapuram and AK Nair, COMPUTATIONAL MATERIALS SCIENCE, 136, 253-263 (2017).
Asperities are considered as unevenness of surfaces, or surface roughness. Surfaces that are finely polished are still considered uneven at the nanoscale. This unevenness of surface reduces the actual contact area when two surfaces come into contact. Understanding surface asperities are very important because the friction and wear properties of two materials depend on the nanoscale contact between the material surfaces. Many experimental studies have concluded that surface texture can help improve contact characteristics and reduce the frictional forces between surfaces. We use molecular dynamics simulations to study the frictional and mechanical response of an aluminium surface with cylindrical and spherical asperities that resemble true surfaces. Nanoindentation and scratch tests are carried out using different indenter radii on spherical and cylindrical asperities, and the results are compared to surfaces without asperities. When comparing the spherical and cylindrical asperities, we observe that the coefficient of friction (COF) is lower for spherical asperity surfaces, if the indenter radius is less than or equal to 4 nm, and the COF is lower for cylindrical asperity surfaces, if the indenter radius is greater than or equal to 5 nm. Finally, the COF decreases with increasing indenter radius for the surface geometries studied here. The atomic mechanisms corresponding to the observed frictional response of the surfaces are explained by dislocation nucleation and propagation in the system. These studies could perhaps be used to guide experiments to design multi- asperity surfaces for tribological applications. (C) 2017 Elsevier B.V. All rights reserved.
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