<title>Overview of progress in strengthening sapphire at elevated temperature</title>

Abstract

Loss of mechanical strength at high temperature limits the thermal shock resistance of sapphire IR windows and domes. The critical weakness is loss of compressive strength along the c-axis of the crystal. Compression causes twinning in the crystal. When twins intersect, a fracture forms at the intersection. The fracture initiates mechanical failure if sufficient tensile stress is present. Twinning is a property of the perfect crystal. Strategies for increasing the high- temperature compressive strength of sapphire are based on introducing crystal defects to inhibit twinning. Doping with Mg<SUP>2+</SUP> or Ti<SUP>4+</SUP> can double the compressive strength at 600 degrees C. Ion implantation increases the tensile strength by a factor of 2 at 300 degrees C. Placing a thin sheet of graphite between sapphire and the test fixture in mechanical tests apparently reduces contact compressive stresses and increases the apparent strength of sapphire in many kinds of mechanical tests at 600 degrees C.