Lifetime-applied stress response in air of a SiC-based Nicalon-fiber-reinforced composite with a carbon interfacial layer: Effects of temperature (300° to 1150°C)

The lifetimes in air as a function of applied flexure stress and temperature (300-1150°C) are described for a Si-O-C based (Nicalon) fiber plain-weave cloth reinforced SiC-matrix composite (∼7% closed porosity) with an ∼0.3 μm thick carbon interfacial layer. The measured lifetimes of both samples with and without an external SiC seal coating were similar and decreased with applied flexural stress (for stresses greater than ∼90 MPa) and with temperature. At temperatures of ≥600°C, the external CVD SiC coating had negligible effect on the lifetimes; however, at 425°C, a detectable improvement in the lifetime was observed with an external SiC coating. When the applied stress was decreased below an apparent "threshold stress" (e.g., ∼90 MPa) for tests conducted at temperatures ≤950°C, no failures were observed for times of ≥1000 h. Electron microscopy observations show that the interfacial carbon layer is progressively removed during tests at 425° and 600°C. In these cases, failure is associated with fiber failure and pull-out. At 950° and 1150°C, the carbon interface layer is eliminated and replaced by a thick silica layer due to the oxidation of the Nicalon fiber and the SiC matrix. This results in embrittling the composite.