Emergence of surface and interface structure from friction, fracture and deformation
- James Kermode (University of Warwick, United Kingdom)
- Lars Pastewka (University of Freiburg, Germany)
- Gianpietro Moras (Fraunhofer Institute for Mechanics of Materials IWM, Germany)
- Alessandro De Vita (King's College London and University of Trieste, United Kingdom)
Surfaces and interfaces are the fingerprint of a material. They often have a topographic and morphological structure that control their properties: for example, a grain boundary can be “locally” doped to control how difficult it is for a dislocation to cross it; the roughness found on most engineering surfaces signals a fatigue strength decrease of the component, and it controls whether its surface can be wet or whether it is adhesive.
Structure is also an analytical tool: understanding the relation between past deformation and surface or interface structure can help characterise mechanical processes simply by looking at post mortem materials, commonly employed for example in fractography or to estimate subsurface strain in tribology by quantifying the deformation of grains. This is often much easier and more cost effective than performing in situ experiments, frequently hindered by buried interfaces at which deformation takes place.
The topographic and morphological structure of surfaces and interfaces are hence emergent properties that are typically not systematically controlled. Few of these processes are understood at the fundamental level, often because they involve interactions spanning multiple length scales. Identifying how to deform a material in order to obtain the desired structure promises a new level of control in materials processing.
The general topic of surface and interface structure will be sub-divided into four categories:
1. Understanding the structure of fracture surfaces. Fracture surfaces have been extensively studied in the literature  but most studies are descriptive, noting self-affine fractal properties of the emerging roughness. In addition to roughness that is found on disordered materials, crystals often show distinct geometric surface features such as ridges  or Wallner lines . These features can be controlled by dopant chemistry  or environmental conditions [5,6].
2. Emergence of surface roughness during plastic deformation. Roughness often emerges when materials are deformed plastically [7–9], e.g. in rolling or grinding processes. This is well-known in the production engineering community but little is known about the underlying material processes. There has been recent progress in using finite-element modelling , molecular dynamics  and statistical plasticity models  to study the emergence of surface roughness, but no unifying picture has emerged.
3. Evolution of interface morphology during frictional loading. Frictional loading can induce fracture or plastic deformation , but complex loading geometries lead to inhomogeneous distribution of stress that give rise to complex subsurface deformation patterns. For example, near surface grains can refine or coarsen (often accompanied by roughening of the surface) , nanolaminates intermix to form vortices reminiscent of Kelvin-Helmholtz instabilities  or crystalline material form metastable glasses [16,17].
4. Structure from elastic instabilities. The study of structure formation by elastic instabilities, such as wrinkling (e.g. in skin ), folding  or crumpling (e.g. in thin sheets ) has progressed much further than the material science questions listed above, partially because it is supported by a much larger scientific community.
This workshop will bring together theoreticians and experimentalists working on the emergence of surface and interface structure from mechanical processes, such as friction, fracture and deformation. We would like to point out that this is a small and emergent field that nevertheless has relevance to a variety of communities in science and engineering. To the best of our knowledge, there are individual disconnected scientific studies on aspects of this, but before this workshop we are unaware of any concerted effort to bring together researchers in this field.
Special focus will therefore be given to identifying the most relevant open problems, particularly those which can be attacked by combining techniques existing in the diverse literature. Ample time will be allocated to discussing how to combine the different available modelling techniques, and integrate them with high-precision experimental techniques. This is expected to naturally lead to new links/collaborations.
Workshop topics will be aligned with the four areas of work identified in the description section above, each of which has corresponding opportunities:
1. Understanding the structure of fracture surfaces. Only very few of the phenomena identified above have been quantitatively explained by physical models. We will explore topics such as how local heterogeneities influence crack propagation and the resulting fracture surface morphology.
2. Emergence of surface roughness during plastic deformation is an example where deep understanding is lacking. We will explore the conflict between the traditional materials science view that grain anisotropy is the cause of roughness during deformation [21–24] and the observation that grain anisotropy does not explain the roughness found at subgrain scales in many experiments [7–9].
3. Evolution of interface morphology during frictional loading. Surfaces are well-document to evolve, even roughen , during frictional loading. The subsurface region often experiences severe deformation, and this can give rise to complex deformation patterns, such as vortices . Neither phenomenon has received a consistent explanation and the connection between both has not yet been explored.
4. Structure from elastic instabilities. We will invite select speakers from this relatively advanced but disjunct field to identify synergistic ideas and methods relevant to areas 1-3 above.
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