Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 degrees C at a heating rate of 10 degrees C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 degrees C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 pm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 degrees C, attributed mainly to the enhance densification of the sintered bodies.