Spallation of SiC bicrystals with a symmetric tilt grain boundary

In cold-pressed or hot-pressed SiC ceramics, grain boundary volume fraction can be significant depending on grain size. Ultrafine-grained SiC with 100–500 nm grain size typically has 2–10 vol% grain boundaries. Coarser-grained SiC with a few micros grain size usually has about 1 vol% grain boundaries. Grain boundaries are weakly bonded or have impurities and often act as weak zones for the initiation and propagation of spallation damage. Under high strain rates, voids are more likely to nucleate at grain boundaries due to the structural discontinuities and stress concentration. Different grain boundary types are expected to have varying resistance to spallation. But the understanding of spallation resistance and how it is related to grain boundary microstructures are still not very clear.
Recently, we have extensively studied SiC bicrystals under shock loading conditions using large scale atomistic simulations. The bicrystal setup can isolate the effects of a single grain boundary, allowing us to focus on its interaction with shock waves without the complexity of polycrystalline structures. Our studies focus on symmetric tilt grain boundaries with a wide range of misorientations. Simulations results indicate that spallation strength and fracture energy are strongly affected by the grain boundary microstructure, esp. the excess free volume. Grain boundary energy also plays a considerable role. A general trend is low-angle grain boundaries tend to have higher spallation strengths.