The hardness of bulk crystals was investigated at 20 to 1400C by using the Vickers indentation technique. The hardness of AlN at room temperature was 17.7GPa. 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.

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