Boron nitride matrices and coatings from boryl borazine molecular precursors

Thermal treatment of 2,4,6-[(NHPri)(2)B(NPri)](3)B3N3H3 1 in vacuo led to a polyborazine 2 in which the borazine rings are mainly linked by -N-B-N- br idges. Pyrolysis of 2 up to 1400 degrees C resulted in the formation of hig h crystallinity hexagonal boron nitride which has been characterised by ESC A, powder X-ray diffraction and FTIR spectroscopy. Precursor 2 has been use d to prepare carbon fibres/BN microcomposites by dipping bundles of fibres into a solution of 2 followed by pyrolysis up to 1200 degrees C. Optical mi crographs and SEM analysis have shown that the BN matrices were compact and displayed good adherence properties onto graphite substrates. By contrast, BN coatings obtained from a polymer derived from B(NHPri)(3) displayed poo r adherence properties. These results are related to the hydrocarbon chain ratio of each preceramic polymer. The effectiveness of the BN protective co atings in preventing oxidation has been measured by isothermal gravimetry i n air. As expected, by considering the quality of the coatings, those obtai ned from 2 showed the most interesting properties.