Analysis of inclusion distributions in silicon carbide armor ceramics

It was determined that intrinsic microstructural defects (i.e. inclusions) are the preferential fragmentation path (initiation or propagation) for ballistically impacted SiC, and may contribute to variation in ballistic performance. Quasi-static and ballistic samples of SiC were studied and inclusions caused by common SiC sintering aids and/or impurities were identified. Ballistic rubble surfaces showed large inclusions of 10-400 micron size, while examination of polished cross-sections of the fragments showed only inclusions under 5 microns in size. The fact that large inclusions were found preferentially on rubble surfaces demonstrates a link between severe microstructural defects and the fragmentation of SiC armor.;Rubble of both a "good" and "bad" performing SiC target were examined. Inclusion size data was gathered and fit to a distribution function. A difference was observed between the targets. The "good" target had twice the density of inclusions on its rubble in the size range less than 30 microns. No significant difference between distributions was observed for inclusion sizes greater than 40 microns.;The "good" target fractured into an overall smaller fragment size distribution than the "bad" target, consistent with fragmentation at higher stresses. Literature suggests that the distribution of defects activated under dynamic conditions will be determined by the maximum stress reached locally in the target. On the basis of the defect distributions on its rubble, the "good" target appears to have withstood higher stresses. The fragment size distribution and inclusion size distribution on fragment surfaces both indicate higher stresses in the "good" target.;Why the "good" target withstood a greater stress than the "bad" target remains a subject for conjecture. It is speculated that the position of severe "anomalous" defects may be influencing the target's performance, but this currently cannot be demonstrated conclusively. Certainly, this research shows that inclusion defects are involved in the fragmentation process, with differences in the distributions on the rubble of the targets suggesting a role in ballistic performance.