Infiltration and diffusion of carbon into polycrystalline yttria (Y2O3) under high pressure and high temperature

Interactions between polycrystalline yttria (Y2O3) and graphitic carbon during high-pressure high-temperature processing can take two main forms: (i) major carbon ingress into fractured grain boundaries of Y2O3 by an infiltration mechanism, and (ii) minor carbon ingress into intact grain boundaries of Y2O3 by a diffusion mechanism. In the latter case, when grain coarsening occurs, excess carbon particles form at grain boundaries, since there is little or no solubility of carbon in the Y2O3 grains themselves, irrespective of crystal structure. Segregation of carbon to grain boundaries can reduce grain-boundary strength. For example, high-purity fine-grained Y2O3, which is originally characterized by transgranular fracture, displays intergranular fracture after carbon ingress into it.