Orientation dependent crack patterns in alumina films on NiAl single crystals due to spherical indentation

Spherical indentations on (100), (110) and (111) oriented NiAl at room temperature caused anisotropic surface topographies around the indents which revealed the highly anisotropic plastic deformation of NiAl. No cracks were observed at the surface of non-oxidized samples. For oxidized samples, radial and circumferential-like cracks in the alumina film around the indent indicated circumferential and radial tensile strains at the NiAl surface depending on the in-plane crystal direction. Unlike the case of isotropic plastic deformation, on (100) NiAl, a sinking-in of the surface at the periphery of the indent was connected with circumferential tensile strains in special directions as indicated by the appearance of radial cracks.