Effects of steam environment on creep behavior of Nextel™720/alumina ceramic composite at elevated temperature

The tensile creep behavior of an oxide-oxide continuous fiber ceramic composite (CFCC) was investigated at 1000 and 1100 °C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (Nextel™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. The influence of the loading rate on tensile stress-strain response and on tensile properties at 1100 and 1200 °C in steam was also explored. A change in loading rate by four orders or magnitude had a profound effect on tensile strength. Tensile creep behavior was examined for creep stresses in the 80-150 MPa range. Primary and secondary creep regimes were observed in creep tests at all temperatures investigated. Creep rates increased with increasing temperature and creep stress. The presence of steam accelerated creep rates and dramatically reduced creep life. The detrimental effects of steam become progressively more pronounced with increasing temperature. At 1000 °C, creep run-out defined as 100 h at creep stress was achieved in all tests. At 1100 °C, creep run-out was achieved in all tests in air and only in the 100 MPa test in steam. The residual strength and modulus of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated.