Phase field modeling of diamond-silicon carbide ceramic composites with tertiary grain boundary phases

Phase field simulations are used to study effects of microstructure features on overall strength and ductility of polycrystalline ceramic composites. The material of present interest is a diamond-silicon carbide (SiC) blend with a grain boundary (GB) phase consisting of much smaller SiC grains, graphitic inclusions, and porosity. Homogenized properties—elastic moduli and surface energy—used in the phase field representation of the GB phase are obtained from a novel approach involving bounds from elastic homogenization theory. Extensive parametric calculations on synthetic microstructures with smoothed polyhedral grains are used to deduce trends in structure-property-performance relations. Results suggest that peak strength, for a fixed fraction of GB phase, can be increased by the following prescriptions: increasing bulk diamond content, reducing porosity, reducing graphite content, and distributing any unavoidable defects uniformly rather than randomly. For the assumed constitutive behavior of the GB phase, graphite appears less deleterious than pores of the same average volume fraction, and median defect concentration appears to be more important than randomization of distributions at low volume fractions. More complex interrelations among microstructures, model features, and material responses are also revealed, for example effects of anisotropic fracture, inelastic dilatation associated with crack opening, and twin variant selection in the SiC phase.