Preparation and Cold Sintering Properties of Metastable Nanocrystalline Zirconia Flake Powders

Although some ceramic materials are prepared via cold sintering, the relative density of zirconia as an important structural ceramic is still low. To improve the relative density of cold sintered zirconia ceramics, metastable nanocrystal zirconia flakes were used as a starting powder. ZrO₂ sheet containing 56% tetragonal phase was prepared with graphene oxide as a template and trihydroxymethyl aminomethane as a precipitator. The influence of sintering additives, temperature, pressure and liquid content on the relative density and microstructure of ZrO₂ was investigated. The results show that the relative density of approximately 70% is achieved under the condition of cold sintering with HCl as a sintering additive, 300 ℃, 500 MPa and liquid content of 20% (in mass fraction), which is much higher than the previously reported relative density of cold sintered zirconia. The increase of relative density is mainly due to the phase transformation of the sintered metastable ZrO₂ and the particle rearrangement and solution-precipitation of the flake powder. The Young modulus and flexural strength of cold-sintered ZrO₂ ceramics are 24 GPa and 20 MPa, respectively, due to the decrease of porosity.