MICROPLASTICITY DURING HIGH-TEMPERATURE INDENTATION AND THE PEIERLS POTENTIAL IN SAPPHIRE (ALPHA-AL2O3) SINGLE-CRYSTALS

The temperature dependence of the Vickers hardness is measured on the basal (0001) plane of single crystal alpha-Al2O3 (sapphire) from room temperature to 1273 K. The plastic zone surrounding the indents is investigated using selective chemical polishing and etching, optical microscopy, and transmission electron microscopy (TEM). Indentation was accompanied by three competitive damage processes: fracture, twinning, and dislocation plasticity. At room temperature, cracking predominates, and the presence of dislocations and/or twins could be revealed only by TEM. At intermediate temperatures (673 to 1073 K), extensive rhombohedral twinning is observed, while at higher temperatures, prismatic slip bands on {1120BAR} dominates the microstructure. TEM observations reveal that the dislocation substructure at the vicinity of the indents consists of fairly straight dislocations lying in basal and/or prism planes and aligned along the [1010BAR] and [1120BAR directions. The details of the glide dissociation of perfect [1010BAR] screw dislocations into three collinear 1/3 [1010BAR] partials on the prism plane, the mechanism of the microplasticity of sapphire single crystals, and details of the Peierls potential for alpha-Al2O3 are discussed.