Mechanical Properties of Single‐Crystal α‐Si ₃ N ₄

Single crystals of α‐Si 3 N 4 several millimeters in diameter and several millimeters long have been grown by chemical vapor deposition. Some of the microstructural and mechanical properties have been evaluated using X‐ray diffracometry, optical and transmission electron microscopy, and high‐temperature microhardness testing. The crystallographic growth direction determines the quality of the crystals, including the density of internal microcracks and the nature and quantity of special boundaries. The measurement of crack lengths associated with microindentations has shown that cleavage in α‐Si 3 N 4 is relatively isotropic. Finally, indentation fracture toughness values agree well with theoretical predictions based on recent bond‐energy calculations.