Creep of Hi-Nicalon S Fiber Tows at Elevated Temperature in Air and in Steam

Structural aerospace components require materials that have superior long-term mechanical properties and can withstand severe environmental conditions, such as high temperatures, high pressures and moisture. Ceramic-matrix composites (CMCs) are capable of maintaining excellent strength and creep resistance at high temperatures, which makes them attractive candidate materials for aerospace applications, particularly in propulsion components. Silicon Carbide (SiC) ceramic fibers have been used as constituent materials in CMCs, although oxidation of the SiC to SiO2 has been a known fiber degradation mechanism. Recently developed near stoichiometric Hi-Nicalon-S fibers have shown significant improvements in thermo-chemical stability. Creep of the Hi-Nicalon-S fibers at elevated temperature in air and in inert gas environments has been examined. However performance of these new fibers at elevated temperatures in steam environments has not been studied thoroughly. The objective of this thesis is to investigate creep of near stoichiometric Hi-Nicalon-S SiC fiber tows at elevated temperatures in air and in steam. The creep response of Hi-Nicalon-S SiC fiber tows was investigated at 800?C, 900?C, 1000?C and 1100?C in laboratory air and in steam. The creep stresses ranged from 154 MPa to 1250 MPa. Creep run-out was defined as 100 h at creep stress. The presence of steam degraded the creep performance of the fiber tows at all temperatures. However, the negative effects of steam became less pronounced as the temperature increased. Less degradation due to steam at higher temperature is attributed to the transition from passive oxidation at 800?C-1000?C to active oxidation at 1100?C of the Hi-Nicalon-S SiC fibers