Fatigue behavior of an advanced melt-infiltrated SiC/SiC composite at 1200°C in air and in steam

The tension-tension fatigue behavior of an advanced melt-infiltrated (MI) silicon carbide/silicon carbide (SiC/SiC) ceramic matrix composite (CMC) was investigated at 1200 degrees Celsius in air and in steam. The MI composite consisted of ten 0/90 plies of Hi-Nicalon-S SiC fibers woven in a five harness satin weave (5HSW), a CVI BN interphase to provide a weak fiber-matrix interphase, and a CVI SiC matrix layer followed by infiltration with SiC particulate slurry and molten silicon. Nine tensile specimens of the material were used in a pilot investigation of mechanical performance at elevated temperature. One specimen was used in a monotonic tensile test at 1200 degrees Celsius in air to determine tensile properties. The remaining specimens were tested in tension-tension fatigue; four were tested in air and four in steam. Tests were performed at 1200 degrees Celsius at 1.0 Hz with a stress ratio of minimum to maximum stress of R=0.05. Maximum stress ranged from 120 to 160 MPa in air and from 100 to 120 MPa in steam. Fatigue run-out was defined as 2 x 105 cycles. All specimens that achieved fatigue run-out were tested in tension to failure to evaluate retained strength and modulus. Strain accumulation and changes in modulus with cycles were measured to qualitatively evaluate damage development. The fatigue life was shorter in steam with a fatigue limit between 110 and 120 MPa. The fatigue limit in air was between 120 and 130 MPa. Specimens that achieved fatigue run-out retained 83-97 percent of their tensile strength. Post-test microstructure of the composite was examined in order to study damage and failure mechanisms