The Evolution of Tribomaterial During Sliding: A Brief Introduction
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- Rigney, D.A. & Karthikeyan, S. Tribol Lett (2010) 39: 3. doi:10.1007/s11249-009-9498-3
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This brief introductory article summarizes key findings from experiments and from computer simulations concerning the dramatic changes that commonly occur adjacent to sliding interfaces. We conclude that a wide range of observed features depends on a few basic processes (plastic deformation, interactions with the environment (including the counterface) and mechanical mixing) and that sliding leads to flow patterns similar to those expected in fluid flow.
It is now widely recognized that sliding dramatically changes the material adjacent to the sliding interface. The modified material, simply called ‘tribomaterial’ here, has been given many other names, including the following: amorphous layer, Beilby layer, transfer layer, fragmented layer, highly deformed layer, glaze layer, white-etching layer, nanocrystal layer, third body  and mechanically mixed material. Sliding commonly produces tribomaterial that is both structurally and chemically different from the bulk material [2, 3]. The development of this tribomaterial influences both friction and wear and suggests that simple models, e.g., familiar adhesion, delamination, fatigue and oxidation models, are not adequate for understanding and controlling sliding behavior.
Experimental observations during and after sliding, combined with computer simulations [2–16], show why our understanding of sliding processes has been elusive. In contrast to abrasion, which can be described in terms of geometry, relative hardness, indentation and microcutting , sliding commonly involves all of the following: large plastic strains and strain gradients, high strain rates and strain rate gradients, mechanical mixing of components from both contacting solids and from the environment, and various recovery processes, some of which may occur after sliding has ceased [3, 14, 16]. Sliding can drive the affected material very far from equilibrium, allowing levels of solubility and the appearance of phases not expected from experience with systems that are closer to equilibrium [2, 18]. Similar processes and products have been observed when mechanical alloying occurs in high-energy ball mills .
A wide range of observed features depends on a few basic processes: plastic deformation, interactions with the environment (including the counterface) and mechanical mixing. These processes are not adequately incorporated in traditional models [19, 20] of friction and wear. The composition and properties of the mixed material can vary widely for different materials and sliding conditions, so there can be a broad range of sliding behavior despite the involvement of the same basic processes. Special cases, e.g., effects of phase transformations, particle cracking, degradation of lubricants, etc., contribute to sliding behavior, but within the same broad framework.
Molecular dynamics (MD) simulations suggest for both crystalline and amorphous materials that sliding leads to flow patterns similar to those expected in fluid flow [6–16]. Mixing occurs when a Kelvin–Helmholtz shear instability [21, 22] leads to vorticity, and the size scale of the vortices is similar to that of grain sizes in nanocrystals. This correlation suggests that vorticity drives mechanical mixing and is at least partially responsible for the development of nanocrystalline material during sliding and during other processes involving severe plastic deformation. Recent results suggest that the formation of nanocrystals may be influenced by vorticity-driven dynamic recrystallization . The disappearance of markers is also associated with vorticity. The simulations show dramatic rearrangements of structure when the normal load is removed or when sliding ceases [14, 16]. These observations raise important questions about conclusions based on even the most careful post-test observations of tribomaterial.
In all cases reported, the tribomaterial that develops during sliding is clearly different from the bulk material in the contacting materials. Therefore, a focus on the tribomaterial and its properties will be needed to develop friction and wear models that are physically reasonable and ultimately useful. In the case of wear models, the fracture characteristics of the tribomaterial need to be incorporated.
The authors are pleased to acknowledge the contributions of J.E. Hammerberg (Los Alamos National Laboratory), M.L. Falk (University of Michigan and Johns Hopkins University, W.K. Kim (University of Michigan), W. Windl (Materials Science and Engineering (MSE), The Ohio State University (OSU)) and recent members of the tribology research group in MSE at OSU, especially X.Y. Fu, T. Kasai, J.H. Wu, H.J. Kim and A. Emge. We are also grateful to the following research sponsors: The National Science Foundation (NSF), U. S. Civilian Research and Development Foundation (CRDF), Dayton Area Graduate Studies Institute (DAGSI), U. S. Department of Energy (DOE/NNSA/SSAA), Los Alamos National Laboratory, Ohio Supercomputer Center and the Michigan Center for Parallel Computing.