Typically, the debonding and sliding interface enabling fiber pullout for SiC‐fiber‐reinforced SiC‐matrix composites with BN‐based interphases occurs between the fiber and the interphase. Recently, composites have been fabricated where interface debonding and sliding occur between the BN interphase and the matrix. This results in two major improvements in mechanical properties. First, significantly higher failure strains were attained due to the lower interfacial shear strength with no loss in ultimate strength properties of the composites. Second, significantly longer stress‐rupture times at higher stresses were observed in air at 815°3C. In addition, no loss in mechanical properties was observed for composites that did not possess a thin carbon layer between the fiber and the interphase when subjected to burner‐rig exposure. Two primary factors were hypothesized for the occurrence of debonding and sliding between the BN interphase and the SiC matrix: a weaker interface at the BN/matrix interface than the fiber/BN interface and a residual tensile/shear stress‐state at the BN/matrix interface of melt‐infiltrated composites. Also, the occurrence of outside debonding was believed to occur during composite fabrication, i.e., on cooldown after molten silicon infiltration.

Oxidation kinetics of Cu-8Cr-4Nb was investigated by TGA exposures between 500 and 900°C (at 25-50°C intervals) and the oxide scale morphologies examined by microscopy and micro-analysis. Because Cu-8Cr-4Nh is comprised of fine Cr2Nb precipitates in a Cu matrix, the results were interpreted by comparison with the behavior of copper (OFRC) and "NARloy-Z" (a rival candidate material for thmst cell liner applications in advanced rocket engines) under the same conditions. While NARloy-Z and Cu exhibited identical oxidation behavior, Cu-8Cr-4Nb differed markedly in several respects: below ~ 700°C its oxidation rates were significantly lower than those of Cu; At higher temperatures its oxidation rates fell into two categories: an initial rate exceeding that of Cu, and a terminal rate comparable to that of Cu. Differences in oxide morphologies paralleled the kinetic differences at higher temperature: While NARloy-Z and Cu produced a uniform oxide scale of CU20 inner layer and CuO outer layer, the inner (CU20) layer on Cu-8Cr-4Nb was stratified, with a highly porous/spongy inner stratum (responsible for the fast initial kinetics) and a dense/blocky outer stratum (corresponding to the slow terminal kinetics). Single and spinel oxides ofNb and Cr were found at the interface between the oxide scale and Cu-8Cr-4Nb substrate and it appears that these oxides were responsible for its suppressed oxidation rates at the intermediate temperatures. No difference was found between Cu-8Cr-4Nb oxidation in air and in oxygen at 1.0 atm.

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