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 d irections of scratching to investigate the anisotropy in hardness and frict ion. Depending on the crystal orientation, the atoms near the surface are f ound to be disturbed to different degrees due to repulsive forces between t hem as the indenter approaches the workmaterial. The hardness is found to i ncrease significantly as the indentation depth is reduced to atomic dimensi ons. The calculated values of hardness are found to be an order of magnitud e higher (and close to theoretical strength) than the corresponding enginee ring values which can be expected considering the size effect possible at i ndentation depths of a few nanometers or less. It thus appears that at very low depths of indentation (or nanoindentation), the plastic deformation un derneath the indenter is governed by the theoretical yield strength of the material. The anisotropy in hardness and friction coefficient of single cry stal aluminum with different crystal orientations and st:ratch directions i s found to be in the range of 29%, which is close to the value of its aniso tropy in the elastic range (21.9%) (stiffest in (111) and least stiff in (1 00)) [R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Mat erials, 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 C utting of Single Crystal Aluminum-Effect of Crystal Orientation and Directi on of Cutting, 1998, accepted for publication in Wear]. Among the orientati ons investigated, hardness is maximum in (001)[100] and minimum in (01 (2) over bar)[221]. Friction coefficient values are found to be higher (0.6-0.9 ) with the maximum along (001)[(1) over bar 10] and minimum along (110)[(1) over bar 10]. The [(1) over bar 10] scratch direction represents the close packed direction for aluminum. The minimum and the maximum scratch hardnes s are observed with (111)[(1) over bar 10] and (111)[(2) over bar 11] cryst al orientations. Although, similarities are found between nanoindentation a nd scratching, and nanometric cutting, the rake angle effect is found to be dominated by the large negative rake angle presented by the indenter in th e former case. (C) 2000 Elsevier Science S.A. All rights reserved.