Periodic Reporting for period 2 - ShapingRoughness (Emergence of Surface Roughness in Shaping, Finishing and Wear Processes)
Reporting period: 2019-08-01 to 2021-01-31
Roughness is important because it affects the interaction between surfaces. Roughness limits the area of intimate atomic contact between two objects to the highest protrusions and therefore a small fraction of the apparent area of the surface. This controls surface forces, such as contact stiffness, adhesion (""stickiness"") and friction. Roughness also influences the life-time of a mechanical device: Wear occurs primarily at asperity contacts and fatigue cracks often nucleate at rough interfaces. Finishing surfaces by polishing, lapping or grinding is therefore used to tune function and reliability of a mechanical device, but it can be responsible for a substantial part of the manufacturing cost of a system. In small-scale, micro- and nanodevices, finishing is often not possible, holding-back potential applications of miniaturized components.
Since most of the surfaces around us appear self-affine, this opens the question how this self-affinity emerges. One plausible origin of self-affine roughness is plastic (irreversible) deformation during formation and processing of the body. Necessarily, as a body is deformed the surface of this body is deformed, too, and therefore stores the spatial signature of the microscopic mechanisms of deformation. It could then be spatial correlations in these deformation mechanisms which lead to a self-affine topography.
The central goal of this project is to investigate this connection between plastic deformation and roughening. Computer simulations that are idealized version of our reality are carried out to determine whether plastic deformation alone could cause self-affine roughening of a body with an initially flat surface. This project will use different technique, such as atomic-scale simulations and continuum solid mechanics calculations to tackle this problem at different scales. The outcome of these calculation will lead to an understanding of the basic process that then allow identification of strategies to control surface-roughness - and thereby the functional properties affected by it."
The simulations indeed showed the emergence of self-affine roughness on initially flat surface. Detailed investigation of the processes revealed that the full subsurface deformation field has a self-affine structure. The self-affine character of the surface roughness is therefore indeed a fingerprint of subsurface deformation processes. The detailed topography evolution data from these simulations was used to build a simple analytical model that could be used by scientist or engineers to predict surface roughness from a knowledge the near-surface strain that a material has experienced.
These calculations were carried out for different materials, from perfectly crystalline metallic crystals to fully disordered metallic glasses, with identical results. Self-affine scaling emerged in all cases and seems indeed to be a universal phenomenon.