Effects of Environment on Creep Behavior of Nextel 720/Alumina-Mullite Cermaic Composite at 1200-C

The creep behavior of an oxide-oxide ceramic matrix composite (CMC) was investigated at 1200-C in laboratory air, in steam and in argon. The composite consisted of a porous aluminamullite matrix reinforced with lminated, woven mullite/alumina (Nextel/720) fibers. The composite had no fiber coating and reliedon its porous alumina/mullite matrix for flaw tolerance. Tensile stress-strain behavior was investigated and the tensile properties were measured at 1200C in laboratory air. Tensile creep behavior of the CMCs was examined for creep stress level of 73,91,1114 and 136 MPa. Creep run-out, set to 100h, was achieved for stress levelslt; 91 MPa in air. The presence of steam or argon accelerated the creep rates the N720/AM composite. Optical and scanning electron microscope (SEM) micrographs were used to examine fracture surfaces and to evaluate failure mechanisms. Fracture surfaces of the N720/AM composite were predominately planar. Limited areas of short fiber pull-out were observed for specimens tested at low creep stress levels in air.