| This study focused on moisture and intermediate temperature effects on the embrittlement phenomenon and stress rupture life of the ceramic matrix composite (CMC) made of Sylramic(TM) fibers with an in-situ layer of boron nitride (Syl-iBN), boron nitride interphase (BN), and SiC matrix (Syl-iBN/BN/SiC). Stress rupture tests were performed at 550°C or 750°C with moisture contents of 0.0, 0.2, or 0.6 atm partial pressure of water vapor, pH 2O. The CMC stress rupture strengths at 100 hrs at 550°C with 0.0, 0.2, or 0.6 atm pH2O were 75%, 65% and 51% of the monotonic room temperature tensile strength, respectively. At 750°C, the corresponding strengths were 67%, 51%, and 49%, respectively. Field Emission Scanning Electron Microscopy (FESEM) analysis showed that the amount of pesting by glass formations increased with time, temperature, and pH2O leading to embrittlement. Total embrittlement times for 550°C were estimated to be greater than 63 hrs for 0.0 atm pH2O greater than 38 hrs for 0.2 atm pH 2O and between 8 and 71 hrs for 0.6 atm pH2O. Corresponding estimated embrittlement times for the 750°C were greater than 83 hrs, between 13 and 71 hrs, and between 1 and 6 hrs.; A time-dependent, phenomenological, Monte Carlo-type simulation of composite failure was developed. The simulated total embrittlement times for the 550°C cases were 300 hrs, 100 hrs, and 25 hrs for 0.0, 0.2, and 0.6 atm pH 2O, respectively. The corresponding embrittlement times for the 750°C cases were 300 hrs, 20 hrs, and 3 hrs. A detailed sensitivity analysis on the variables used in the model was conducted. The model was most sensitive to variation in the ultimate strength of the CMC at room temperature, the ultimate strength of the CMC at elevated temperature, and the reference strength of a fiber and it was least sensitive to variation in the modulus of elasticity of the matrix and fiber. The sensitivity analysis showed that the stress ruptures curves generated by variation in the total embrittlement time simulate the trends in the experimental data. This research showed that the degree of stress rupture strength degradation increases with temperature, moisture content level, and exposure time. |
