Crystallinity, Crystalline Quality, and Microstructural Ordering in Boron Nitride Fibers

Abstract : A qualitative and quantitative analysis of the microtexture/microstructure in several experimental boron nitride fibers was investigated by X‐ray diffraction and electron microscopy according to their mechanical properties. Young's modulus was controlled by grain orientation. The disordered microstructure of low modulus fibers consisted of amorphous regions and nanometer‐sized crystallites arranged in a wide range of orientation. In contrast, high modulus samples showed a coarse‐grained microtexture in which crystallites were ordered in a meso‐hexagonal arrangement along the fiber ‐axis. Cracks were easily propagated as crystallinity increased, leading to decreased failure strain. The strength‐microstructure relationship was obscured by the fiber porosity and defects.