Strength of thermally exposed alumina fibers Part I Single filament behavior

The strength of a continuous-fiber-reinforced ceramic composite is directly related to the strength distribution of the reinforcements. Therefore, it is essential to understand how processing and service conditions affect the filament strength distribution. The objective of this paper was to determine the strength distribution of Nextel™ 610 alumina filaments, a potential reinforcement in oxide-oxide ceramic composites, and to characterize the defect population as a function of exposure temperature. The filament strength was measured in single filament tests and decreased as the exposure temperature increased. The defect population was quantified via fractography, and eight distinct defect types were identified. Grain growth and filament-to-filament sintering were identified as critical microstructural changes during the heat treatments, and both affected the development of grain boundary grooves, either from thermal etching or from the breaking of the filament-to-filament sintering bonds. The initial crack path was intergranular but quickly changed to a mixture of transgranular and intergranular fracture. During catastrophic crack growth, the crack path changed back to intergranular. The critical crack size, as calculated from linear elastic fracture mechanics, was found in many cases to be greater than the size of the strength limiting defect suggesting the presence of subcritical cracking.