SYNTHESIS AND PROPERTIES OF IN-SITU SI3N4-REINFORCED BAO-CENTER-DOT-AL2O3-CENTER-DOT-2SIO(2) CERAMIC-MATRIX COMPOSITES

Silicon nitride (94.5% alpha, 5.5% beta), BaCO3, Al2O3, and SiO2 powde rs were mixed and pressureless sintered to produce a ceramic matrix co mposite consisting of 30 vol% barium aluminosilicate (BaO . Al2O3 . 2S iO(2) or BAS) matrix reinforced with in situ grown whiskers of beta-Si 3N4. In situ X-ray studies of the reactions indicated that BaCO3 decom poses first to yield BaO which reacts with SiO2 to yield a series of b arium silicates which then react with Al2O3 between 950 and 1300 degre es C to yield hexacelsian BAS. The sintering times were varied in orde r to develop a material system that combines the favourable properties of BAS with the high strength of Si3N4. in situ high-temperature X-ra y studies after composite processing did not reveal any changes in the BAS or Si3N4 UP to temperatures of 1300 degrees C. Dilatometry studie s of the sintered composite indicated a low-temperature transformation between 230 and 260 degrees C with the temperature of transformation and volume change associated with the hexagonal to orthorhombic transf ormation decreasing with an increase of sintering time. Room- and high -temperature (1400 degrees C) strengths were evaluated using four-poin t bend flexural tests. Composites exhibited near theoretical densities and an increase in flexural strength that was primarily dependent on the higher alpha- to beta-Si3N4 transformation.