Advanced ceramic materials (e.g. Si 3 N 4 , zr0 2 ' SiC, Al 2 0 3 ) are being developed for high temperature applications in advanced heat engines and high temperature heat recovery systems [1]. Although fracture toughness has been a constant problem, advanced ceramics are now being developed with fracture toughnesses close to those of metals [2] . Small size flaws (10-200 ~m), small non-uniformities in density distributions (0.1-2%) present as long-range density gradients, and porous regions

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... ich can be seen as localized areas of slightly lower density, are critical in most ceramics. The need to detect these small flaws is causing a significant effort to be devoted towards nondestructive evaluation. Detection of "defects" such as those noted in engineering ceramics has presented problems for conventional non-destructive evaluat ion methods l3j.