LIFETIME-APPLIED STRESS-RESPONSE IN AIR OF A SIC-BASED NICALON-FIBER-REINFORCED COMPOSITE WITH A CARBON INTERFACIAL LAYER - EFFECTS OF TEMPERATURE (300-DEGREES TO 1150-DEGREES-C)

The lifetimes in air as a function of applied flexure stress and tempe rature (300-1150 degrees C) are described for a Si-O-C based (Nicalon) fiber plain-weave cloth reinforced SiC-matrix composite (similar to 7 % closed porosity) with an similar to 0.3 mu m thick carbon interfacia l layer. The measured lifetimes of both samples with and without an ex ternal SiC seal coating were similar and decreased with applied flexur al stress (for stresses greater than -90 MPa) and with temperature. At temperatures of greater than or equal to 600 degrees C, the external CVD SIC coating had negligible effect on the lifetimes; however, at 42 5 degrees C, a detectable improvement in the lifetime was observed wit h an external SiC coating. When the applied stress was decreased below an apparent ''threshold stress'' (e.g., similar to 90 MPa) for tests conducted at temperatures less than or equal to 950 degrees C, no fail ures were observed for times of greater than or equal to 1000 h, Elect ron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425 degrees and 600 degrees C, I n these cases, failure is associated with fiber failure and pullout. A t 950 degrees and 1150 degrees C, the carbon interface layer is elimin ated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the comp osite.