MD simulation of indentation and scratching of single crystal aluminum

Molecular Dynamics (MD) simulations of indentation and scratching have been conducted on single crystal aluminum in various crystal orientations and directions of scratching to investigate the anisotropy in hardness and friction. Depending on the crystal orientation, the atoms near the surface are found to be disturbed to different degrees due to repulsive forces between them as the indenter approaches the workmaterial. The hardness is found to increase significantly as the indentation depth is reduced to atomic dimensions. The calculated values of hardness are found to be an order of magnitude higher (and close to theoretical strength) than the corresponding engineering values which can be expected considering the size effect possible at indentation depths of a few nanometers or less. It thus appears that at very low depths of indentation (or nanoindentation), the plastic deformation underneath the indenter is governed by the theoretical yield strength of the material. The anisotropy in hardness and friction coefficient of single crystal aluminum with different crystal orientations and scratch directions is found to be in the range of 29%, which is close to the value of its anisotropy in the elastic range (21.9%) (stiffest in 〈111〉 and least stiff in 〈100〉) [R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 4th edn., Wiley, 1996, p. 14]. A similar observation was made in a recent investigation on the nanometric cutting of single crystal aluminum [R. Komanduri, N. Chandrasekaran, L.M. Raff, M.D. Simulation of Nanometric Cutting of Single Crystal Aluminum-Effect of Crystal Orientation and Direction of Cutting, 1998, accepted for publication in Wear]. Among the orientations investigated, hardness is maximum in (001)[100] and minimum in (012̄)[221]. Friction coefficient values are found to be higher (0.6–0.9) with the maximum along (001)[1̄10] and minimum along (110)[1̄10]. The [1̄10] scratch direction represents the close packed direction for aluminum. The minimum and the maximum scratch hardness are observed with (111)[1̄10] and (111)[2̄11] crystal orientations. Although, similarities are found between nanoindentation and scratching, and nanometric cutting, the rake angle effect is found to be dominated by the large negative rake angle presented by the indenter in the former case.