Mechanism of directional cracking of Ti,C:sapphire crystal

Abstract

Directional cracking in Ti,C:sapphire crystals grown along [1120] by Vertical Bridgman method often occurs in the cutting and processing process. In this work, we discuss the characteristic and mechanism of directional cracking of Ti,C:sapphire, and find that directional cracking originates from (1100) lattice plane and spreads along [0001] orientation. Through the Crystalmaker Simulation software, we find that atomic arrangement on (1100) lattice plane is the most sparse and adjacent atomic spacing is the largest along vertical [0001] direction, so in the system (1100) [0001] of lattice has a minimum cracking strength. Irregular carbon inclusions in the cracked Ti,C:sapphire are observed with optical microscopy, scanning electron microscopy (SEM), and X-ray diffractometry. These inclusions cause great internal stress in the cooling process due to thermal expansion mismatch and cracking originating from and spreading in the weak system (1100) [0001] of lattice. As a consequence, macroscopic directional cracking is observed in the Ti,C:sapphire. The study has important theoretical and practical significance for growing high-quality Ti,C:sapphire crystal.