BORON-NITRIDE MATRICES AND COATINGS OBTAINED FROM TRIS(METHYLAMINO)BORANE - APPLICATION TO THE PROTECTION OF GRAPHITE AGAINST OXIDATION

Tris(methylamino)borane (B(CH3NH)(3)) partially converted into 1,3,5-t ris(methylamino)-2,4,6-trimethylborazine ([(CH3H)NBNCH3](3)) used as a boron nitride precursor leads to an amorphous BN preceramic when pyro lyzed up to 1000 degrees C. Further annealing at 1700 degrees C is req uired to obtain a product free from organic residues and to achieve th e crystallization of h-BN. A microcomposite consisting of a h-BN matri x reinforced with carbon fibers has been obtained by pyrolyzing bundle s of fibers previously dipped into the precursor. Characterization of this microcomposite has shown that the BN matrix was compact and free of bubbles, although the theoretical ceramic yield was only 26%. Compa ct h-BN coatings with a thickness of 4-10 mu m have also been deposite d by an impregnation-pyrolysis treatment on graphite substrates. Penet ration of the precursor into the open micropores of the substrates to a depth of about 5 mu m resulted in strong BN-graphite interfacial bon ding. Oxidation tests conducted on these coated graphite substrates ha ve revealed the efficiency of the h-BN coating for protecting graphite . Compared with uncoated specimens, the oxidation resistance of BN-coa ted substrates was 10 times higher at 650 degrees C and 5 times higher at 750 degrees C.