Thermal shock resistance and thermal-mechanical processing of sapphire

The finite elements method was applied to calculate transient thermal stresses in sapphire with taking into account the crystalʹs anisotropy, scale factor, temperature function of heat transfer and other properties. The sapphire fracture behaviour as compared to ceramic polycrystalline materials under heat load, differs due to two factors: (a) twinning type strain at rather low test temperatures <200  °C and (b) fracture toughness anisotropy. The cracking behaviour and thermal shock resistance (TSR) of the crystal against its dimensions, orientation and the heat load type was explained using the force fracture mechanics and the twinning and cracking features in the tensile and compressive stress fields. The influence of residual stresses after stress relaxation was studied. A possibility to increase the strength by 2–3 times and TSR of sapphire using a thermomechanical process was shown. This enables thermal stress generation under high-rate heatup to 1500–2000 °C, thus lowering the stress concentration on defects through local stress relaxation.