Résumé
Boron nitride (BN) fibers were efficiently prepared from a B-aminoborazine-based polymer according to the Polymer-Derived Ceramic (PDC) route via melt-spinning and heat-treatments up to 1800 degrees C in a controlled atmosphere. The microtextural and microstructural changes in the material during the polymer-to-ceramic conversion were investigated by means of electron microscopy and XRD observations. The microtexture of the fibers was featureless as glassy-Re materials when fibers were exposed at temperatures below 1400 degrees C. Mechanical properties of such amorphous fibers were poor at these temperatures. Upon further heating to 1500 degrees C, the microstructure changed from disordered nanocrystals embedded into an amorphous phase to a turbostratic phase. This amorphous-to-crystalline transition was accompanied with a large increase in the mechanical properties. At 1800 degrees C, the microtexture of the fibers was coarse-grained and was correlated to the identification of a "meso-hexagonal" BN phase with basal layers almost aligned along the fiber-axis in the material. Polycrystalline BN fibers exhibited high mechanical properties (sigma = 1.4 GPa, E = 360 GPa) after curing of the polymer fibers at 400 degrees C and subsequent pyrolysis of cured fibers at 1800 degrees C.