Role of grain constraint on the martensitic transformation in ceria-doped zirconia

Zirconia polycrystals have historically suffered from catastrophic cracking during the tetragonal-monoclinic martensitic transformation. Recently, transformation-induced cracking has been avoided by doping to achieve crystallographic compatibility between the transforming phases. However, these materials showed depressed transformation temperatures and incomplete transformation, the causes for which are yet unknown. In this work, we probe these phenomena by performing a comparative study of sintered pellets and powders. We characterize the thermally induced transformation in a series of ZrO₂-CeO₂ compositions by in situ diffraction and calorimetry and develop a thermodynamic model of the system. In compositions exhibiting reduced cracking, we find that transformation temperatures are depressed in pellets but not in powders. Correspondingly, we measure significant compressive strains in pellets consistent with thermodynamically expected transformation temperature depression, demonstrating the influence of grain constraint and the resultant pressure build-up. However, we find that both pellets and powders exhibit incomplete transformation. In pellets, this is attributed to early exhaustion of autocatalysis caused by grain constraint, whereas in powders, this is attributed to insufficient driving force for distributed heterogeneous nucleation.