13 - Effects of environment on creep behavior of three oxide–oxide ceramic matrix composites at 1200 °C

Abstract

Oxide–oxide ceramic matrix composites (CMCs) are among leading candidate materials for use in high-temperature structural applications, especially in components requiring a long service life in oxidizing environments. High-temperature mechanical behavior and properties of three oxide–oxide composites of alumina and mullite are discussed. The effects of steam on high-temperature creep performance of the CMCs are evaluated. The composites consist of a porous oxide matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, have no interface between the fiber and matrix, and rely on the porous matrix for flaw tolerance. The matrix materials were alumina, aluminosilicate, and alumina-mullite. The tensile stress–strain behavior was investigated, and the tensile properties measured at 1200°C. The tensile creep behavior of the three oxide–oxide CMCs was investigated at 1200°C in laboratory air, in steam, and in argon. The presence of steam accelerated creep rates and reduced creep lifetimes of the three CMCs. In the case of the composite with the aluminosilicate matrix, no-load exposure in steam at 1200°C caused severe degradation of tensile strength. Examination of the fracture surfaces points to matrix densification and subsequent loss of matrix porosity as the key mechanisms behind the degradation of mechanical properties and performance at elevated temperature in steam.