OXIDATIVE AND HYDROLYTIC STABILITY OF BORON-NITRIDE - A NEW APPROACH TO IMPROVING THE OXIDATION RESISTANCE OF CARBONACEOUS STRUCTURES

Boron nitride (BN), derived from borazine oligomers, was examined as a means of enhancing the oxidation resistance of carbon fiber-carbon matrix composites (C-C). To clarify the widely held impression that BN suffers from a hydrolytic instability, a systematic investigation of the accelerated hydrolysis of BN at 700 degrees C was carried out. These results showed that the stability to moisture was directly related to the degree of crystalline perfection. A similar argument was found to hold with respect to resistance of BN toward air oxidation. Based on these data, processing conditions were designed to yield a very stable BN with an interlayer spacing of d(002) = 3.35 Angstrom. A number of different schemes were employed to test the ability of BN to impart high-temperature oxidation stability to carbon fiber composites. These included: BN coatings on C-C composites, infiltration of BN into porous C-C, BN as the matrix in a carbon fiber lay-up (C-BN), and BN fiber-BN matrix composites (BN-BN). In all cases, incorporation of BN was shown to improve the oxidation resistance, with the BN-BN composite exhibiting the highest stability in air.