A number of ceramics such as silicon nitride, and zirconia are being considered for high temperature structural applications. The primary problem with these ceramics is their wide fracture strength variability. In consequence, non-destructive evaluation techniques are required to ensure their reliable use. The brittle nature of ceramics inhibits the strain energy release at flaws by plastic deformation. As a result, critical flaw size in these materials is small. For example, flaws in the size range of 20–100 µm are considered as “critical” in silicon nitride for engine applications. Surface cracks are particularly important since they are the major source of failure in ceramics (1). These cracks are generated during machining operations and usually consist of arrays of semi-elliptical cracks with random inclination to the surface, but a preferred alignment parallel to the direction of motion of the abrading particles (2).