In situ molten phase-assisted self-healing for maintaining fiber morphology during conversion from melamine diborate to boron nitride

C(3)N(6)H(6)·2H(3)BO(3) (M·2B) is a highly promising precursor of boron nitride (BN) fibers due to its eco-friendly and low-cost fabrication. However, it is still unclear why the fibers can maintain their morphology in spite of drastic weight loss (nearly 80 wt%) during M·2B-to-BN pyrolysis. Herein, an interesting cracking and self-healing behavior of the heated M·2B fibers was observed at initial pyrolysis. In situ formed molten boron oxide (B(2)O(3)) was figured out to be the healing agent for the cracks and subsequently merged into the continuous matrix enclosing melamine/melem molecules, which subsequently acted as a nitrogen source. The B(2)O(3) matrix helped to keep the fiber morphology undamaged under the second weight-loss stage in the pyrolysis process. This strategy of taking advantage of the in situ formed molten phase for healing cracks offers detailed guidance to prepare defect-free M·2B-derived BN fibers and would be significant in defect repair for other ceramics.