The hardness of bulk crystals was investigated at 20 to 1400C by using the Vickers indentation technique. The hardnesses of GaN and AlN at room temperature were 10.8 and 17.7GPa, respectively. With increasing temperature, the hardness showed a gradual decrease and then a steep decrease from around 1000C; which indicated the beginning of macroscopic dislocation motion and plastic deformation. On the basis of the observed temperature dependence of the yield stress, the activation energy for dislocation motion at high temperatures in GaN was deduced.

High-Temperature Strength of III-V Nitride Crystals. I.Yonenaga: Journal of Physics - Condensed Matter, 2002, 14[48], 12947-51