CRYSTALLIZATION OF CALCIUM HEXALUMINATE ON BASAL ALUMINA

The epitactic growth of calcium hexaluminate (CA(6)) on the basal plan e of alumina was observed during an investigation of the interaction b etween silicate glass films deposited on single-crystal alumina (alpha -Al2O3). Two distinctly different orientation relationships occurred. The first can be described as perfect hexagon-on-hexagon epitaxy with (11 (2) over bar 0)(CA6)parallel to(10 (1) over bar 0)(Al2O3) and (000 1)(Al2O3)parallel to(0001)(CA6). This relationship has 1.1% lattice mi sfit in the substrate plane; the misfit is accommodated at the interfa ce by a hexagonal network of misfit dislocations. In the second relati onship the CA(6) grains are rotated about the substrate normal and can be explained by the coincident-site lattice model for phase boundarie s. Both orientation relationships emphasize the influence of substrate crystallography on the crystallization process.