Sol-gel-derived single-crystal alumina coatings with vermicular structure

Alumina sols were spin-coated on {0001} (c-plane) sapphire substrates. Heat treatment in air at temperatures varying between 1100° and 1400°C resulted in epitaxial conversion of the coating to α-alumina. Seeded single-crystal conversion could be achieved at temperatures as low as 1025°C at longer annealing times (18 h). The converted coatings were crack free, and exhibited a porous, vermicular microstructure that was attributed to the negative volume change during the γ to α alumina phase change, coupled with the volume constraint of the underlying sapphire substrate. Isolated regions of the coating that had delaminated from the substrate did not convert to {0001} sapphire, but instead remained polycrystalline after heat treatment. Single-crystal islands that were not c-plane oriented were occasionally observed at shorter times. It is suggested that such regions resulted from the random nucleation of α-grains with a more rapid growth rate in the direction perpendicular to the substrate. The epitaxial conversion of alumina sol-gel coatings provides a potentially convenient method for generating patterned, single-crystal ceramic substrates for a variety of applications.