High-temperature Pressurized Sintering of Yttria-stabilized Tetragonal Zirconia Powder Compacts by 28-GHz Microwave Irradiation

Abstract

The pressurized sintering behaviors of 3 mol% Y₂O₃ partially stabilized tetragonal ZrO₂ polycrystals (3Y-TZP) powder compacts by microwave irradiation (28-GHz) were investigated. The 3Y-TZP powder compacts were densified in a cylindrical closed α-Al₂O₃ die at constant external compression pressures (σ_ex) of 50–100 MPa and a temperature of 1273 K. To obtain adequate microwave permeability and block microwave irradiation, a die with thicknesses of 10 and 15 mm was used, respectively. The relative density (ρ_re) increased while the densification rate (ρ̇) decreased with increasing sintering times. Increasing σ_ex led to an increase in ρ_re and ρ̇. A stress exponent (n) was estimated from ρ̇ and the effective stress acting on the compact. Regardless of the die thickness, n values of 2 and 1 were respectively obtained in a low-stress region (i.e., densification rate was dependent on vacancy generation/annihilation) and a high-stress region (i.e., densification rate was dependent on vacancy flow). Thus, there was no difference in the densification rate-controlling mechanism by microwave irradiation. Regarding ρ̇ obtained by using the thin 10 mm die (microwave penetrating) and the thick 15 mm die (microwave blocking), both show similar ρ̇ values in the low-stress region, but the former shows a higher ρ̇ value than the latter in the high-stress region. These results indicate that microwave irradiation does not influence the rate of vacancy generation/annihilation, but promote vacancy flow.