Mechanism of mechanical failure of sapphire at high temperature

The strength of sapphire decreases more rapidly with increasing temperature than does the strength of polycrystalline alumina and many other ceramics. Twinning on the rhombohedral plane (1102) at elevated temperature induced by compression along the crystallographic c-axis [0001] appears to initiate failure and accounts for the decreased strength. The tensile strength of sapphire along the (alpha) - [1120] or c-axes is constant to within approximately 30% between 20 degree(s) and 800 degree(s)C. Compressive strength along the (alpha) -axis is also constant to within approximately 20%. However, compressive strength along the c-axis falls by > 95% (from 2000 MPa to less than 100 MPa) between 20 degree(s) and 800 degree(s)C.