Most of the natural or engineered surfaces are not perfect, meaning they are rough. The surface roughness was found across many scales: from atomic to tectonic. The earth looks smooth from space, but someone standing at the foot of the Himalayas would see a totally different picture. A silicon wafer used for chip production looks and feels ideally smooth, but at the atomic scale it is rough (with roughness ~1 nm).
Already since the 1950s the impact of surface roughness on friction, adhesion and wear phenomena was recognized. Roughness is an input in all of the contact models dealing with the contact of rough surfaces, thus one cannot underestimate the importance of this topic for tribologists.
The surfaces are often fractals, however, the reason for the origin of this self-affinity is not known. Since the self-affinity is common for the surfaces from atomic to tectonic scales, the mechanism responsible for the formation of the surface roughness must be the same. This is surprising, since the mountains are formed due to tectonic motion of soil, while steel ball bearing surfaces undergo various surface finishing treatments. Yet, all the surfaces deform.
The researchers from the Simulation group at the Department of Microsystems Engineering at the University of Freiburg addressed this mystery in their recent work. They have used Molecular Dynamics simulations to observe the evolution of surface roughness at the atomic scale. They have used a simple biaxial compression of 3 different systems: a homogeneous single-crystal , a crystal with stoichiometric disorder, high-entropy alloy , and amorphous . These materials are known to exhibit different molecular deformation mechanisms.
The results of the research indicate that despite all the differences between the materials, the deformation leaded to the generation of self-affine surfaces for all of them. The reason for such behavior is attributed to the statistical nature of plastic deformation. And even though plastic deformation occurs via different mechanisms in these materials, the resultant self-affine surface is common for all of the materials. Thus, formation of the self-affine surface roughness is independent on the deformation mechanism. At least at the small scales.
Based on the simulation results authors argue that the same could hold for larger scales as well, since the previous observations show self-affinity of the surfaces from atomic to tectonic scales.
The results of this work lead to a better understanding of the surface roughness formation and evolution and hopefully can lead to methods of controlling surface roughness. This is key in optimizing and controlling other tribological phenomena, such as friction and wear.
Further information: Adam R. Hinkle et al. The emergence of small-scale self-affine surface roughness from deformation, Science Advances (2020). DOI: 10.1126/sciadv.aax0847