Creep behavior in interlaminar shear of Nextel™720/alumina ceramic composite at elevated temperature in air and in steam

The creep behavior in interlaminar shear of an oxide–oxide ceramic matrix composite (CMC) was evaluated at 1200 °C in laboratory air and in steam using double-notch shear test specimens. 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 interlaminar shear properties were measured. The creep behavior was examined for interlaminar shear stresses in the 4–6.5 MPa range. Primary and secondary creep regimes were observed in all tests conducted in air. In steam, the composite exhibited primary, secondary and tertiary creep. In air, creep run-out defined as 100 h at creep stress was achieved in all tests. In the presence of steam, creep performance deteriorated rapidly and run-out was achieved only at 4 MPa (∼50% of the interlaminar shear strength at 1200 °C). The retained properties of all specimens that achieved run-out were characterized. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degradation appears to be the cause of reduced creep lifetimes in steam.