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Emergence of Surface Roughness in Shaping, Finishing and Wear Processes

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

"Rough surfaces are everywhere in our environment. While we may perceive some as smooth when we look at them or touch them, there is hardly any natural or artificial surface that is not rough at some scale. The size of features varies drastically, from micrometer-sized asperities and cracks on polished surfaces of household devices, through millimeter scale bumps and pores on construction materials, up to several hundred meters high crags and crevasses in mountain ranges on the surface of the earth. A remarkable feature is that rough surfaces are often self-affine, meaning that if we zoom in on a part of the surface, we see similar features as on the full surface, but at a different scale. Self-affine scaling has been observed from atoms to mountains, spanning 15 orders of magnitude in length.

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."
Computer simulations were carried out to determine whether plastic deformation alone could cause self-affine roughening of a body with an initially flat surface. The Molecular Dynamics (MD) simulation technique was used for this purpose. In MD, all atoms in the body are simulated. Quantum mechanical effects are neglected. Instead, the motion of the atoms is assumed to follow classical mechanics, like the motion of Billiard balls. This approximation allows to simulate systems with millions of atoms. Indeed, such a large number of atoms was required to analyze the scaling properties of the surface over a representative range of scales.

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.
The molecular calculations carried out within this project shed a novel light on the processes through which roughness emerges. Future work will focus on using more generic computational techniques to understand the formation of roughness at larger scales. Continuum mechanical models open the opportunity to selectively tune individual properties of the material under investigation to understand the effect of these properties on the structure of surface roughness.
Rough surface of an initially flat gold crystal after deformation