Comparison of Whisker Growth Sites and Dislocation Etch Pits on Single-Crystal Sapphire

Abstract

It has been shown previously that alumina (sapphire) whiskers grown on single-crystal alumina grow coherently with and in the directions of screw dislocations in the substrate. A possible explanation is that the whiskers grow by the screw-dislocation mechanism at the site of emergent screw dislocations on the substrate surface. The present study was undertaken to determine if the whiskers grow at the sites of substrate dislocations as revealed by etching techniques. To do this, whisker growth sites were compared with the positions of dislocation etch pits. This was done for the prismatic [{112¯0}, <11¯00>] and basal [(0001), (112¯0)] slip systems on (112¯0), (11¯01), and (0001) crystal surfaces. The whiskers and etch pits did not form at the same places on the crystal surfaces. The reason may be that (1) the etch pits form at pure edge dislocations only, (2) surface nucleation generates screw dislocations at which the whiskers grow, or (3) screw dislocations play no part in the growth of these whiskers.