The hardnesses of the opposite basal faces of 4H-type single crystals were measured at temperatures ranging from 25 to 200C. A marked hardness discrepancy was found between the Si-terminated (00•1) and C-terminated (00•¯1) faces of the polar crystal. Transmission electron microscopic investigation of dislocations in the plastic zone of the 1200C indentations showed that they lay predominantly on basal planes, parallel to the indented face, and that the extra half-planes of the non-screw dislocations originated from the indented face. It was also found that, when the (00•1) Si-terminated face was indented, the dislocations were either widely dissociated (with the width of the stacking-fault ribbon being much greater than the equilibrium value) or were single leading partials, with the corresponding trailing partials absent. In this case, all of the leading partials were found to have a Si core. On the other hand, the dislocations in the plastic zone of the C-terminated face were in the form of dissociated dislocations, with the width of the associated stacking-fault ribbons being appreciably less than the equilibrium value. The leading partials of these dissociated dislocations had a C core. The results indicated that the hardness of the polar basal faces of 4H-crystals at high temperatures was determined partly by the nature of the dislocation cores which were nucleated by the indentation process. It was suggested that this was due to the effect of the core upon the generation and glide of the leading partial dislocations.

X.J.Ning, N.Huvey, P.Pirouz: Journal of the American Ceramic Society, 1997, 80[7], 1645-52