Nondestructive evaluation of ceramic preforms

In order to identify and eliminate flawed ceramic preforms prior to metal infiltration, several approaches for nondestructive evaluation were studied. The techniques chosen include immersion ultrasonics, scanning acoustic microscopy (SAM), and radiography. Undesirable irregularities in the preforms include cracks, voids, density gradients, and shot. The latter represents spherically shaped particles of alumina, with diameters that may exceed ten times that of the fibers, and that are incorporated into the porous fiber preform. It is generally thought that shot with sizes greater than 100 μm affects the mechanical properties of the resulting metal matrix composite. With the aid of SAM and scanning electron microscopy (SEM), shot in near-surface areas could be evaluated. Cracks and voids could also be located using SAM and SEM. On a larger scale, the ultrasonic immersion equipment was capable of detecting and quantifying density gradients, which were verified using radiographic techniques. Examples will be given to illustrate the sensitivity and resolution of the various techniques with regard to examination of the flawed preforms. [Work supported by Center for Advanced Materials, Pennsylvania State University.]