Effects of combustor rig exposure on a porous-matrix oxide composite

The present study explores the effects of exposure in a laboratory combustor on microstructural stability and property retention of an all-oxide fiber-reinforced ceramic composite. The material consists of a porous mullite-alumina matrix and Nextel 720 fibers in an eight-harness satin weave. To assess the effects of matrix strength, two matrix conditions are used, distinguished from one another by the amount of alumina added through precursor impregnation and pyrolysis (1.8% and 4.8%). In both cases, the dominant damage mode upon exposure involves interply delamination along the panel midptane. However, significant reductions in the rate and extent of cracking are obtained in the material with higher alumina content: a result of the higher delamination resistance. Mechanical tests performed on exposed specimens reveal a slight (10-20%) reduction in tensile strength along the fiber direction and a comparable increase in shear strength. These trends suggest some sintering of the matrix upon exposure. Examinations of fracture surfaces provide additional supporting evidence. Implications for long-term performance and strategies for imparting improvement in microstructural stability and delamination resistance are discussed.