Tribological contact properties are influenced by the plastic response of the material. A better understanding of the evolution of the plasticity in the vicinity of the contact area is clearly desirable. Therefore, a three dimensional Discrete Dislocation Dynamics tool  is adapted for contact problems with a spherical tip.
In a first step, indentation simulations were performed to study the dislocation multiplication during contact formation. In experiments (e.g. ), prismatic and helical prismatic dislocation structures are formed during indentation. Discrete Dislocation Dynamics simulations show that these dislocations do not need to be generated by high stresses directly at the indenter, as suggested by some atomistic simulations, but can be created from a single pre-existing dislocation underneath the indent .
In a second steep, simulations of sliding tips were performed. These simulations allow to investigate the dislocation microstructure evolution underneath the moving contact. Due to the complex stress field of the indenter, dislocation are trapped underneath and transported with the moving indenter tip. Simulations show how crystallographic orientation of the surface and sliding direction change the resulting dislocation structure and surface topography. Grain boundaries acting as dislocation obstacles trap part of the dislocation structure
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 S. Graca, P. A. Carvalho, R. Colaco, Journal of Physics D: Applied Physics 44 (33) (2011) 335402
 J. Gagel, D. Weygand, P. Gumbsch, submitted