Phase Evolution of Yttria-Stabilized Zirconia During Hybrid Microwave Sintering

Hybrid microwave (HMW) sintering of 7–8 wt% yttria-stabilized zirconia (8YSZ) powder was investigated. Commercial 8YSZ powders were pressed into green disks before they were later sintered using HMW heating at 1350 °C, 1400 °C, and 1450 °C for 2 h. For comparison, conventionally sintered samples were done at 1600 °C for 2 h. XRD, SEM, and thermal conductivity were employed to characterize the sintered samples using both methods. SEM micrographs confirmed the successful sintering of HMW samples without microcracks and at a much lower temperature as compared to the conventionally sintered sample, which showed a significant amount of microcracks. Significantly, microwaves suppressed the tetragonal (t) to monoclinic (m) transformation that often causes microcracks. Furthermore, the m-ZrO₂ phase amount in the HMW samples was decreasing with increasing the sintering temperature, as compared to the as-received powder. On the other hand, conventionally sintered samples showed a less significant reduction in the m-ZrO₂ phase. Moreover, as the temperature increases from 1350 to 1450 °C in the HMW samples, a new peak related to the cubic (c) phase is observed, which is not shown in the conventionally sintered samples. The thermal conductivities of the microwave-sintered samples were decreasing with the increase in the sintering temperature, which could be related to the c-ZrO₂ phase evaluation and the reduction in m-ZrO₂ phase content. This may be attributed to the generation of oxygen vacancies induced by microwaves. HMW is a promising sintering method, offering significant energy savings, crack-free microstructures, and potentially improved material performance.