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

Basal plane stacking disorder, delamination cracks, misorientation bands and low angle (0001) twist boundaries are observed by high resolution transmission electron microscopy in small, highly defective, boron nitride inclusions introduced unintentionally in silicon nitride-silicon 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 structure. Detailed consideration of the misorientation bands shows that, while the majority can be safely described as kink bands, there are a number of examples where the geometry is strikingly similar to the geometry of deformation twinning in hexagonal boron nitride.