Evaluating the effect of oxygen content in BN interfacial coatings on the stability of SiC/BN/SiC composites

Abstract

Boron nitride was studied as a fiber-matrix interface coating for Nicalon/SiC composites. The effect of initial O-impurity content within the as-processed BN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. Two types of Nicalon/SiC composites were used for this study; one composite had a BN coating with I 1% (high-O BN) in the as-processed state. The high-O BN is actually most representative of BN coatings available commercially. The BN coatings in both the high-O and low-O BN containing composites were structurally similar. The samples used here were thinned to < 200 μm before oxidation and the final preparation for electron microscopy examination of the interface region was done after the reactions were completed. Thin samples were used to simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at temperatures as high as 950°C for times up to 400 h. After each oxidation experiment, the BN coatings were examined by TEM to quantify the extent of any reaction which occurred at either the fiber/BN and BN/SiC matrix interfaces. At 950°C for 100 h, there were no interface microstructural changes observed in the low-O BN but there was extensive silica formation at the fiber/BN interfaces in the high-O BN. After 400 h at 950°C, large voids formed at the fiber/BN interface in the high-O BN sample only. Oxygen present within the initial BN coating contributed significantly to the degradation of the interfacial properties of the composite. Several techniques, including transmission electron microscopy (TEM). Auger electron spectroscopy (AES ), energy-dispersive spectrometry (EDS). and electron energy-loss spectroscopy (EELS) were used to characterize changes in structure and chemistry of the fiber-matrix interface region and to elucidate and quantify composite degradation mechanisms.