Microstructural Features And Dislocations on Thermally Etched Sapphire Surfaces

Basal, prism and rhombohedral-plane specimens cut and polished from flame-grown sapphire boules developed fine surface textures during heat-treatment in the range 1700° to 1900°C. Microscopic examinations of such surfaces revealed low-angle grain boundaries, dislocations, and other crystal imperfections. Thermally etched textures apparently were generated by three overlapping processes, namely annealing, etching, and decoration. Thermal etching gave evidence of high-temperature relaxation of residual strain energy introduced at low temperature (analogous to cold work in metals), including that introduced by fracture, microindentation, and cutting and polishing during surface preparation.

Single crystals of Al2O3 (sapphire) were grown from PbO-PbF2 and MoO3-PbF2 fluxes; they varied from flat plates (PbF2-rich melts) to equidimensional crystals (PbO- or MoO3-rich melts). The primary growth planes are basal, first-order rhombohedral, and second-order rhombohedral. The habit change is interpreted on the basis of F- contamination and Pb2+ surface adsorption. Possible ion species in the melts and their relative importance on crystal growth from these systems are discussed.


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