Multi-million atom molecular dynamics simulations of tensile testing have been performed on nc-SiC. Reduction of grain size promotes simultaneous enhancement of ductility, toughness, and strength. Simulations show that the nc-SiC fails by intergranular fracture preceded by atomic level necking. Atomic diffusion can prevent premature cavitation and failure, and therefore it sets an upper limit on high strain-rate deformations of ceramics. We report a non-diffusional mechanism for suppressing premature cavitation, which is based on unconstrained plastic flow at grain boundaries. In addition, based on the composite's rule of mixture, we estimate Young's modulus of random high-angle grain boundaries in nc-SiC to be about 130 GPa. The effect of temperature and strain rate on mechanical properties is studied.