HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY OF THE PLANAR DEFECTSTRUCTURE OF HEXAGONAL BORON-NITRIDE

Basal plane stacking disorder, delamination cracks, misorientation ban ds and low angle (0001) twist boundaries are observed by high resoluti on transmission electron microscopy in small, highly defective, boron nitride inclusions introduced unintentionally in silicon nitride-silic on carbide composites during the fabrication process. The delamination cracks are produced as a consequence of the magnitude of the thermal stresses introduced during cooling from the consolidation temperature and the layered nature of the hexagonal boron nitride crystal structur e. Detailed consideration of the misorientation bands shows that, whil e the majority can be safely described as kink bands, there are a numb er of examples where the geometry is strikingly similar to the geometr y of deformation twinning in hexagonal boron nitride.