Effects of frequency and environment on fatigue behavior of an oxide–oxide ceramic composite at 1200 °C

The effect of frequency on fatigue behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite–alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension–tension fatigue tests were performed at frequencies of 0.1 and 10 Hz for fatigue stresses ranging from 75 to 170 MPa. Fatigue run-out was defined as 105 cycles at the frequency of 0.1 Hz and as 106 cycles at the frequency of 10 Hz. The CFCC exhibited excellent fatigue resistance in laboratory air. The fatigue limit was 170 MPa (88% UTS at 1200 °C). The material retained 100% of its tensile strength. Presence of steam significantly degraded the fatigue performance, with the degradation being most pronounced at 0.1 Hz. Composite microstructure, as well as damage and failure mechanisms were investigated. Examination of fracture surfaces revealed higher degrees of fiber pull-out in specimens tested at 10 Hz, indicating weakening of the fiber/matrix interface. A qualitative spectral analysis showed evidence of silicon species migration from the fiber to the matrix.