Properties of Silicon Carbide Fiber-Reinforced Silicon Nitride Matrix Composites

The mechanical properties of NASA Lewis Research Center developed SiCRBSN composites and their thermal and environmental stability have been studied. The composites consist of nearly 30 vol of aligned 142 um diameter chemically vapor-deposited SiC fibers in a relatively porous silicon nitride matrix. In the as-fabricated condition, the unidirectional and two-dimensional composites exhibited metal-like stress-strain behavior, graceful failure, and showed improved properties when compared with unreinforced matrix of comparable density. Furthermore, the measured room temperature tensile properties were relatively independent of tested volume and were unaffected by artificial notches normal to the loading direction or by thermal shocking from temperatures up to 1100C. The four point bend strength data measured as a function of temperature to 1400C in air showed that as-fabricated strength was maintained to 1200C. At 1400C, however, nearly 15 percent loss in strength was observed. Measurement of room temperature tensile strength after 100 hr exposure at temperatures to 1400C in a nitrogen environment indicated no loss from the as- fabricated composite strength.