Evolution of structural features and mechanical properties during the conversion of poly[(methylamino)borazine] fibers into boron nitride fibers

Poly[(methylamino)borazine] (PolyMAB) green fibers of a mean diameter of 15 mum have been pyrolyzed under ammonia up to 1000degreesC and heat treated under nitrogen up to 2000degreesC to prepare boron nitride (BN) fibers. During the polymer-to-ceramic conversion,the mechanical properties of the green fibers increase within the 25-400degreesC temperature range owing to the formation of a preceramic material and remain almost constant up to 1000degreesC. Both the crystallinity and the mechanical properties slightly increase within the 1000-1400degreesC range, in association with the consolidation of the fused-B3N3 basal planes. A rapid increase in tensile strength (a R) and elastic modulus (Young's modulus E) is observed in relation with crystallization of the BN phase for fibers treated between 1400degreesC and 1800degreesC. At 2000degreesC, meso-hexagonal BN fibers of 7.5 mum in diameter are finally obtained, displaying values of sigma(R) = 1.480 GPa and E = 365 GPa. The obtention of both high mechanical properties and fine diameter for the as-prepared BN fibers is a consequence of the stretching of the green fibers on a spool which is used during their conversion into ceramic. (C) 2004 Elsevier Inc. All rights reserved.