Effect of fiber coating on lifetime of Nicalon fiber-silicon carbide composites in air

Studies of the lifetime of Nicalon fiber-reinforced SiC matrix composites subjected to flexural stresses were conducted at temperatures of 600 and 950°C in air. The objective of this study was to evaluate the effectiveness of selected fiber coatings; i.e., carbon, boron-doped carbon (B-doped C), and boron nitride (BN), on the time-to-failure of Nicalon-SiC composites. Results at 600 and 950°C showed that the composite having a B-doped C fiber coating exhibited lifetimes that depend upon stress and temperature as found earlier for a composite with a carbon fiber coating. However, the replacement of the undoped carbon with a B-doped C fiber coating increased the lifetime by 10–100-fold and the apparent fatigue limit by a factor of two (to ≈150 MPa). On the other hand, the lifetime of composites at 950°C with a BN fiber coating was insensitive to the applied stress for levels up to ≈200 MPa (fatigue limit level) and was up to a thousand-fold longer than those composites having either B-doped C or undoped carbon fiber coatings. However, at 600°C, the composite with a BN fiber coating did exhibit a stress-dependent lifetime with an apparent fatigue limit of ≈150 MPa. The lifetime-limiting process in both of these composites is due to the progressive oxidation of the interface region and formation of boron-containing silicate glass, which forms a strong interfacial bond between fiber and matrix, (thus, high debond stress and interfacial shear stress), resulting in embrittlement of the composites.