Tension-Compression Fatigue of Hi-Nicalon/SiC Ceramic Matrix Composite at 1200 C in Air and Steam

The tension-compression fatigue of a Hi-Nicalon /Silicon Carbide ceramic matrix composite (CMC) was studied at 1200°C in laboratory air and in steam environment. The CMC investigated in this effort consisted of an oxidation inhibited HyprSiC matrix reinforced with laminated woven Hi-Nicalon fibers. Fiber preforms had pyrolytic carbon fiber coating with boron carbide overlay applied were then densified with the HyprSiC oxidation inhibited matrix through chemical vapor infiltration (CVI). The tensile stressstrain behavior was evaluated and the tensile properties measured at 1200°C. Tension-compression fatigue tests were conducted at a frequency of 1.0 Hz with a ratio of minimum stress to maximum stress of R = -1.0. Tension-compression fatigue behavior was studied for fatigue stresses ranging from 80 to 300 MPa in air and in steam. Fatigue run-out was defined as 2x105 cycles. Fatigue limit (based on a run-out condition of 2x105 cycles) was 80 MPa in air and in steam. The presence of steam decreased the fatigue life of specimens tested above the fatigue limit. Specimens that achieved fatigue run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength. Reduction in tensile modulus was 68% while the change in compressive modulus was negligible.