HIGH-PERFORMANCE BORON-NITRIDE FIBERS FROM POLY(BORAZINE) PRECERAMICS

Boron-nitride (BN) fiber has been fabricated by utilizing the preceramic method. A melt-processable polymer precursor, poly(aminoborazinyl), was readily prepared by thermal condensation of B,B,B-tri(methylamino)borazine (MAB) in the presence of 10 wt% of 1-aminododecane (LA). It was melt-spun into thin fiber with an ordinary ram extruder. The spinnability of the fiber was excellent, in spite of the low molecular weight (c. 570) of the precursor, because it has a structural similarity with melt-spinnable carbon pitch. On spinning the fiber surface was slightly hydrolyzed in atmosphere and the fiber became infusible. The fiber was then directly pyrolyzed up to 1000-degrees-C in an ammonia gas flow and was further sintered up to 1800-degrees-C in a nitrogen flow. The strength of the sintered BN fiber was found to increase above 1400-degrees-C and to reach 1 GPa at 1800-degrees-C. It was therefore confirmed that the BN fiber is free from the serious degradation above 1400-degrees-C in inert atmosphere where most of the common ceramic fibers are known to lose their original strength.