Effect of Microstructure on the Mechanical Behavior of Continuous‐Fiber‐Reinforced Ceramic‐Matrix Composites

Abstract

The mechanical behavior of a unidirectional continuous‐fiber ceramic‐matrix composite (CFCMC) was correlated with matrix‐rich channels in the microstructure. A large population of CFCMCs was prepared via alkoxide infiltration, which incorporated either uncoated Nicalon fibers (64 samples) or BN‐coated fibers (118 samples). No structure/property correlation was observed for the uncoated composites, because of the uniformity of the microstructure. For the BN‐coated composites, both the flexure strength and work of fracture (WOF) were correlated with oriented matrix‐rich channels. The channels were <90 μm wide and were distributed throughout the cross section. The BN‐coated CFCMCs exhibited laminate‐like behavior: the strength was statistically higher when the channels were aligned parallel with the applied load and the WOF was statistically higher when the channels were perpendicular to the load. Grouping the specimens on the basis of channel orientation, relative to applied load, reduced the variance in strength and WOF. This categorization also resulted in consistent, predictable failure behavior. This observation implies that prior CFCMC data that do not consider microstructure orientation may show wider scatter in mechanical behavior than is warranted.