Résumé
A series of experimental boron nitride (BN) fibers were prepared from a Balkylaminoborazine-based polymer (=poly[(methylamino)borazine]) according to the Polymer-Derived Ceramic (PDC) route. The polymer was melt-spun in N2 into mono- and multifilaments, prior the curing of the resulting green fibers in NH3 at 400DGC and the subsequent pyrolysis of the as-cured fibers in NH3 (1000DGC), then N2 up to 1800DGC to generate BN fibers. It was established that the melt-spinning operation could deliver BN fibers with different microtexture/microstructure, and therefore with different mechanical behavior depending on the spinning parameters. The relationship between mechanical properties and fiber microtexture/microstructure could be studied in the present paper. Melt-spinning operation into multifilaments provided either low-modulus fibers with a featureless cross-sectional microtexture as glassy-like materials or fibers with a microtextural skin-core heterogeneity decreasing the fiber strength. The disordered microstructure of low-modulus BN fibers consisted of disoriented nanosized grains mixed in an amorphous matrix. In contrast, the melt-spinning operation into monofilament produced high-modulus fibers with a coarse-grained microtexture. In such samples, the extended grains were ordered along the fiber-axis increasing the crack propagation along the cleavage basal planes, and therefore decreasing the failure strain of fibers.