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 (∼20 ppm H 2 O/O 2 ) environments at 900°C and high‐water‐vapor (1% and 10% H 2 O/O 2 ) environments at 700° and 800°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 SiO 2 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 HBO 2 ( g ), H 3 BO 3 ( g ), and H 3 B 3 O 6 ( 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.