Insights into the issues during hot‐pressing and vacuum sintering of 10 mol.% CaO doped zirconia ceramics

Abstract

A vacuum environment is usually preferred for reducing the dwell time and sintering temperature for doped zirconia ceramics. However, for the present work with 10 mol.% CaO‐doped ZrO2 (10CaSZ) ceramics, both hot‐pressing and vacuum sintering techniques yielded ceramics with open pores (ρ ∼ 93%) and significant amounts of the deleterious monoclinic phase (50%). This is in stark contrast with the conventionally sintered highly dense (>99%) 10CaSZ ceramics, where only tetragonal and cubic phases were noted. Probing via electron paramagnetic resonance (EPR) depicted the trapping of oxygen vacancies by vacancies generated in vacuum at high temperature. A possible change in the local coordination (of ions) and the subsequent chemical environment was confirmed by the observed asymmetric signal shape and broader linewidth of the EPR signals. Potential loss of oxygen and deviation from stoichiometry (for dopant) was confirmed via EPR analysis along with the support of energy dispersive spectra analysis. Despite no noticeable shift in the XRD pattern for Ca2+ depletion, the observed compositional gradient differences in the calcium elemental mapping suggested dopant segregation at the grain boundaries. The conclusions from the present work point toward specificity of a given materials system for its beneficial densification under hot pressing or vacuum sintering.