ENVIRONMENTAL DURABILITY OF BN-BASED INTERPHASEST(FOR SiC_f/SiC_m COMPOSITES) IN H_2O CONTAINING ATMOSPHERES AT INTERMEDIATE TEMPERATURES

摘要

The effect of H_2O containing environments on the stability of BN interphases was assessed for several BN-based interphases fabricated by different routes. Hi-Nicalon or Nicalon fiber, CVI SiC matrix minicomposites with BN interphases that were deposited at different temperatures (1050 to 1800 deg C) or doped with Si were subjected to ends-on oxidation in a variety of H_2O/O-2 containing environments at intermediate temperatures (700-800 deg C) and times (l-100h). The samples were observed qualitatively to determine if the BN interphase had volatilized, formed a glass, or remained intact. Several samples were longitudinally polished to determine the distances of BN recession. It was found that the higher the BN processing temperature the more durable the BN in the H-2O containing environments. Si-doping the 1400 deg C BN resulted in about the same durability as the 1800 deg C BN.