Microstructural control of silicon carbide for ballistic applications

Silicon carbide (SiC), due to its low areal density and ability to stop tungsten carbide-cored bullets, is the ceramic armor of choice against heavy threats. Prochazka showed that SiC could be densified without using a liquid phase at about 2100°C using boron and carbon as additives, resulting in clean grain boundaries which give the material high hardness and excellent corrosion resistance. Well known liquid phase additives, such as yttrium aluminum garnet (YAG), Al, and Al-B-C, while lacking corrosion resistance, are all candidates for SiC-based armor and are known to increase fracture toughness. There is a hardness-toughness trade-off in silicon carbide that is related to fracture mode. The hardness of solid state SiC is essentially independent of grain size since it fractures transgranularly. While microcrack toughening can increase the toughness of SiC without decreasing hardness, additives can be used to engineer the amount of intergranular fracture and thereby increase the long-crack fracture toughness. Materials with single-edge precracked beam (SEPB) fracture toughness ranging between 2.5 and 8.5 MPam1 / 2 were prepared. To achieve high toughness it was necessary to have both intergranular fracture and elongated grains. Hardness decreased with increasing fracture toughness, independent of grain size for all but the finest grain sizes. Vickers hardness values generally decreased from 24 to 18 GPa as the toughness increased. Ballistic performance was independent of quasi-static fracture toughness and this parameter should not be pursued in making improved ceramic armor, at least at threat levels comparable to those tested. It was also shown that depth of penetration (DOP) was not a meaningful parameter for predicting the V 5 0 performance of advanced armor. DOP tests correlated with hardness measurements, which are not predictive of ballistic performance. Al was a better additive than Al-B-C or YAG for making advanced armor due to the low hot pressing temperature, better control of the microstructure and excellent ballistic performance. It can be made with a high Weibull modulus and appears to be an excellent choice when densifying SiC with applied pressure. A rich interplay in mechanical properties is available by understanding the roles of different additives in the densification process.