Uniaxial Pressing Induced Unusual Phase and Microstructural Evolutions in 20CaSZ Ceramics

Abstract

Although, uniaxial cold pressing of green ceramic bodies is generally considered as a mundane (or innocuous) processing step, for the current work, it resulted in ‘peculiar’ phase evolutions in 20 mol.% CaO-doped zirconia (20CaSZ) system. While the 20CaSZ powders display a phase-pure cubicity (till 1400 °C), the sintered ceramic displayed a mixed phase (orthorhombic and cubic). Such a phase evolution is not supported by thermodynamic phase diagram involving CaO and ZrO₂. The proportion of the mixed phases increased with the green compaction pressure (from 2 to 35% by volume) and displayed a ‘core-shell’-type evolution of mixed phases while the core depicted a cubic-ZrO₂ phase. Induced by an applied pressure gradient, the evolution of the unexpected orthorhombic CaZrO₃ phase was triggered by an abnormal segregation of Ca²⁺ dopants (in the green pellet itself). The orthorhombic CaZrO₃ phase caused a volume expansion of ~100% (unit cell volume: cubic phase ~134.77 Å³ vs orthorhombic phase ~257.86 ų) and a density difference of ~130% (cubic ZrO₂: 5.41 g/cm³ vs orthorhombic CaZrO₃: 4.169 g/cm³), leading to unique 'grain-bursting' features. The present work points toward the sensitivity of CaO dopants with pressing and associated ceramic processing operations.