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 c to a 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, singlecrystal ceramic substrates for a variety of applications. [ABSTRACT FROM AUTHOR]