Processing of nanocrystalline 8 mol% yttria-stabilized zirconia by conventional, microwave-assisted and two-step sintering

Manufacturing of near full dense (>97%) 8 mol% yttria-stabilized zirconia (8YSZ) nanopowder (15–33 nm) compacts was manipulated using conventional sintering (CS), two-step sintering (TSS) and microwave-assisted sintering methods. Microwave firing was performed via two different heating rates, i.e. 5 and 50 °C min−1. Although, the lower rate microwave sintering (LMS) was found to yield the higher densities at lower temperatures, this regime ultimately did not provide higher final densities compared to the other methods. The higher rate microwave sintering (HMS) on the other hand managed to suppress the accelerated grain growth and resulted to a finer microstructure (0.9 μm) than LMS (2.35 μm) and CS (2.14 μm). In spite of the great capability of TSS method in fabricating the specimens with ultra-fine grains (0.29 μm), microstructural inhomogeneity and the long total sintering time (>20 h) in comparison with HMS (29 min) set restrictions on the application of TSS method. Based on the effect of grain size on the mechanical properties of ceramics, the specimens produced by TSS exhibited higher fracture toughness (3.16 ± 0.06 MPa m1/2) than those obtained from CS (1.61 ± 0.07 MPa m1/2) and LMS (1.9 ± 0.09 MPa m1/2), due to their finer grain size. The proximity in the fracture toughness values of TSS and HMS (3.17 ± 0.10 MPa m1/2) samples stems from the higher microstructural homogeneity caused by HMS, while having a larger grain size.