Nondestructive evaluation of a high temperature ceramic matrix composite material
The purpose of this investigation was to nondestructively characterize a 2-D Nicalon-reinforced, CVI-SiC matrix composite before and after exposure to 1000, 1200, and 1400 °C in oxidizing atmospheres of air. Concurrently, the nondestructive testing methods used to detect thermally induced changes in the SiC/SiC composite were evaluated.
All of the NDT techniques, except film X-ray radiography, were sensitive to changes in the ceramic matrix composite (CMC) caused by thermal ageing. Surface and sub-surface scanning acoustic microscope (SAM) micrographs indicated that the CMC specimens were experiencing surface changes. The increase in surface reflectivity of the specimens after heat treatment was attributed to density and elastic property changes related to oxidation of the matrix, formation of silica on the fibers, and increased crystallinity of the Nicalon fibers. Acousto-ultrasonic results indicated that thermal exposure altered the ability of the CMC specimens to transmit ultrasonic energy. The ability to transmit energy was degraded or enhanced depending on ageing temperature and the presence or absence of a refractory coating. C-scans indicated that the heat-treated specimens attenuated/scattered more energy than the as-fabricated specimens. Through-transmission scans indicated that, after thermal exposure, the specimens that had received a refractory coating showed a greater increase in attenuation than the uncoated specimens.
Tensile tests performed at room temperature provided strength data and a means for monitoring acoustic emission. Optical micrographs showed void and silica formation around fibers and the development of microcracks within the matrix. Electron microprobe analysis verified that carbon and silicon were being lost and oxygen was being gained within the fiber-matrix interphase.