Influence of micro-tension on fiber orientation and crystallization in continuous polymer-derived boron nitride fiber

The continuous boron nitride (BN) fiber exhibits exceptional properties such as high-temperature resistance, excellent wave transparency, and resistance to ablation. However, ceramic BN fiber fabricated from polymerderived ceramics (PDCs) routes often suffers from low strength, which is closely related to its orientation degree and crystallization. Utilizing stretching can improve the crystallization orientation of fiber during the heat treatment process, we present a novel approach to address this issue in this study. Graphite rings are employed to provide continuous micro-tension by restricting the shrinkage of BN fiber during the polymer-to-ceramic conversion. The micro-tension level is adjusted by the diameter variation of the graphite rings. A series of analyses reveal a significant enhancement in the orientation degree of the ceramic BN fiber, transforming it from disordered to highly oriented (with omega value of 0.84). The orientation factor (f) grows by nearly 20%. Furthermore, the crystallization increases from 81.13% to 90.72%. The ratio of pore area to cross-section area of ceramic BN fibers decline from 0.63% to 0.19%. Correspondingly, the bulk density is increased by nearly 44%. Consequently, the strength and Young's modulus of the fiber are enhanced by approximately 3 and 10 times, respectively.