EFFECTS OF ENVIRONMENT ON CREEP BEHAVIOR OF AN OXIDE-OXIDE CERAMIC COMPOSITE WITH +- 45 DEG FIBER ORIENTATION AT 1200 DEG C

摘要

The tensile creep behavior of an oxide-oxide continuous fiber ceramic composite (CFCC) with +- 45 deg fiber orientation was investigated at 1200 deg C in laboratory air, in steam and in argon. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (Nextel~(TM)720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The tensile stress-strain behavior was investigated and the tensile properties measured at 1200 deg C. The elastic modulus was 45 GPa, UTS was 55 MPa, and failure strain was 0.27 percent. Tensile creep behavior was examined for creep stresses in the 15-45 MPa range. Primary and secondary creep regimes were observed in all tests. Tertiary creep was observed at 45 MPa in air and at stress levels >= 40 MPa, in steam and argon environments. For creep stress levels <=35 MPa, creep run-out (set to 100 h) was achieved in all test environments. At creep stresses > 35 MPa, creep performance was best in laboratory air and worst in argon. The presence of either steam or argon accelerated creep rates and significantly reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degradation appears to be the cause of early failures in argon.