DEVELOPMENT OF NONDESTRUCTIVE CHARACTERIZATION TECHNOLOGIES FOR PROCESS CONTROL OF CERAMICS

Abstract Ceramic matrix composites using either continuous ceramic fibers or ceramic whiskers have been shown to have significantly higher fracture toughness than monolithic ceramics. High fracture toughness is necessary for ceramic applications in many advanced heat engines. However, in wear applications such as bearings (ball and roller), or in certain corrosive environments, monolithic ceramics have a significant role. Nondestructive characterization methods to measure different properties are therefore important for both types of materials. Mass-production methods such as slip casting and injection molding, for composites or monolithics, require high reliability; thus, the development of nondestructive characterization methods for process control can have a high payoff. In the case of injection molding, where relatively high loadings (>10wt.%) of polymeric binders are often used, knowledge of binder distribution is very important because nonuniformity can cause nonuniform final properties. For slip casting of whisker-type composites or cold-pressed whisker composites (with whisker loadings of 10-20 vol.%), the extent of possible whisker stratification should be known because of the impact on uniformity of mechanical properties. One method of producing continuous-fiber composites is chemical vapor infiltration (CVI). The production and process development in CVI requires knowledge of the as-infiltrated density distribution and the fiber orientation after infiltration. We have demonstrated on injection-molded Si3N4 with 10-15 wt.% binder that by appropriate use of high-gradient-field nuclear magnetic resonance (NMR) imaging and microfocus 3-D X-ray computed tomography (CI), the distribution of polymeric binders can be mapped to ±0.5 wt.%. In the case of SiC/SiC CVI continuous-fiber composites made with 0, ± -30 degree cloth layups, we have shown that density variations attributed to process conditions can be detected and that fiber orientations can be determined to better than ± 2 degrees by 3-D X-ray microfocus CT data, together with advanced image processing. All data were acquired on NMR and X-ray CT machines designed and built at Argonne National Laboratory.