Behavior of Nicalon™-fiber-reinforced glass-matrix composites under thermal cycling conditions

The damage evolution of commercially available SiC Nicalon((TM))-fiber-reinforced glass-matrix composites under thermal cycling conditions in oxidizing atmosphere has been investigated. The samples were alternated quickly between high-temperature (T = 700 degrees C) and room-temperature air for different numbers of cycles. Thermal aging experiments were also conducted by exposing the samples in air at 700 degrees C for long periods of up to 250 h. Both destructive and non-destructive measurement techniques were employed to characterize the samples and to detect differences in behavior for the various thermal loading conditions. The flexural strength and Young's modulus decreased, while the internal friction increased with increasing numbers of cycles. Material degradation was attributed to phenomena related to viscous flow of the glass matrix, and to oxidation of the fiber, which occurred as a consequence of the extended exposures at high temperatures. The microstructural damage observed includes porosity formation (cavitation) within the matrix and at the fiber/matrix interfaces. The experimental results also suggest degradation of the in situ fiber strength due to fiber surface oxidation and damage, fiber displacement and consequent fiber-to-fiber contact. (C) 1998 Elsevier Science Ltd. All rights reserved.