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

Boron nitride was studied as a fiber-matrix interface coating for Nicalon(T M)/SiC composites. The effect of initial O-impurity content within the as-p rocessed BN coatings on the long-term interface stability was investigated at elevated temperatures in flowing oxygen. Two types of Nicalon(TM)/SiC co mposites were used for this study; one composite had a BN coating with < 2% oxygen (low-O BN) and another composite had BN with an oxygen concentratio n > 11% (high-O BN) in the as-processed state. The high-O BN is actually mo st representative of BN coatings available commercially. The BN coatings in both the high-O and low-O BN containing composites were structurally simil ar. The samples used here were thinned to < 200 mu m before oxidation and t he final preparation for electron microscopy examination of the interface r egion was done after the reactions were completed. Thin samples were used t o simulate maximum corrosion effects that would occur at the surface of an actual part during service. Ech sample was exposed to flowing oxygen at tem peratures as high as 950 degrees C for times up to 400 h. After each oxidat ion 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 in terfaces. At 950 degrees C for 100 h, there were no interface microstructur al changes observed in the low-O BN but there was extensive silica formatio n at the fiber/BN interfaces in the high-O BN. After 400 h at 950 degrees 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 te chniques, including transmission electron microscopy (TEM), Auger electron spectroscopy (AES), energy-dispersive spectrometry (EDS), and electron ener gy-loss spectroscopy (EELS) were used to characterize changes in structure and chemistry of the fiber-matrix interface region and to elucidate and qua ntify composite degradation mechanisms. (C) 1999 Elsevier Science Ltd, All rights reserved.