FABRICATION AND PROPERTIES OF CERAMIC COMPOSITES WITH A BORON-NITRIDEMATRIX

Boron nitride (BN) matrix composites reinforced by a number of differe nt ceramic fibers have been prepared using a low-viscosity, borazine o ligomer which converts in very high yield to a stable BN matrix when h eated to 1200 degrees C. Fibers including Nicalon (SLC), FP (Al2O3), S umica and Nextel 440 (Al2O3-SiO2) were evaluated. The Nicalon/BN and S umica/BN composites displayed good flexural strengths of 380 and 420 M Pa, respectively, and modulus values in both cases of 80 GPa. On the o ther hand, FP/BN and Nextel/BN composites exhibited very brittle behav ior. Nicalon fiber with a carbon coating as a buffer barrier improved the strength by 30%, with a large amount of fiber pullout from the BN matrix. In all cases except for Nicalon, the composites showed low die lectric constant and loss.