Nanoindentation hardness anisotropy of alumina crystal: A molecular dynamics study

Atomistic mechanisms of the initial stage of plasticity during nanoindentation are studied by molecular dynamics simulations for three surface orientations of alumina crystal. The simulations predict significant anisotropy and indentation depth dependence of the hardness value at the nanometer scale. The nanohardness anisotropy is found to arise from orientation-dependent dislocation activities. In the (0001) basal plane indentation, prism dislocations are emitted, followed by basal and pyramidal dislocations, to form massive subsurface defects; prism and pyramidal dislocations are emitted on (011¯0) and (21¯1¯0) prism plane indentations, respectively, to cause modest deformations. Stacking faults are also observed because these dislocations are extended to Shockley partial dislocations.