Radiograph and ultrasound detect defects early

Ceramic processing is a multistep operation starting with raw materials and proceeding to green forming, densifying and machining. Two of the most commonly used processes are injection molding and pressure casting. Structural flaws--such as density gradients, inclusions, cracks, voids, agglomerates and subsurface residual stresses--are unavoidably introduced during the multistep operations. The flaws produce unreliable materials that must be rejected. Radiographic and ultrasonic techniques have been successful in detecting defects during early stages of ceramic processing and in the final product. NDE techniques have helped establish a cause-and-effect relationship between process parameters and resulting defects. This information has been used to improve process conditions, minimize part rejection rate and reduce fabrication cost. This paper describes the use of radiography and ultrasonic testing on Si[sub 3]N[sub 4] and SiC. The methods were used to detect iron inclusions in the starting powders, the formation of defects, ceramic densification, to monitoring thermal mismatch effects, and development of cracks during machining.