High-temperature oxidation of boron nitride: II, boron nitride layers in composites

The oxidation of BN composite interphases was examined with a series of model materials, Oxidation was examined in both low-water-vapor (similar to 20 ppm H2O/O-2) environments at 900 degrees C and high-water-vapor (1% and 10% H2O/O-2) environments at 700 degrees and 800 degrees C, The low-water-vapor case was explored with layered BN/SiC materials. This case was dominated by borosilicate glass formation, and the 20 ppm water vapor gradually removed the boron from the glass, leaving a larger amount of SiO2 than would be expected from simple SiC oxidation, Layered SiC/BN/SiC materials were also used to study low-water-vapor oxidation effects within the composite. The high-water-vapor case was explored with SiC/BN/SiC minicomposites, and it was dominated by volatilization of BN as HBO2(g), H3BO3(g), and H3B3O6(g), A model for recession of the BN fiber coating was developed based on the gas-phase diffusion of these species out of the annular region around the SiC fiber and concurrent sealing of this annular region by oxidation.