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-tris(methylamino)-2,4,6-trimethylborazine ([(CH3H)NBNCH3](3)) used as a boron nitride precursor leads to an amorphous BN preceramic when pyrolyzed up to 1000 degrees C. Further annealing at 1700 degrees C is required to obtain a product free from organic residues and to achieve the crystallization of h-BN. A microcomposite consisting of a h-BN matrix reinforced with carbon fibers has been obtained by pyrolyzing bundles 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%. Compact h-BN coatings with a thickness of 4-10 mu m have also been deposited by an impregnation-pyrolysis treatment on graphite substrates. Penetration of the precursor into the open micropores of the substrates to a depth of about 5 mu m resulted in strong BN-graphite interfacial bonding. Oxidation tests conducted on these coated graphite substrates have revealed the efficiency of the h-BN coating for protecting graphite. Compared with uncoated specimens, the oxidation resistance of BN-coated substrates was 10 times higher at 650 degrees C and 5 times higher at 750 degrees C.