Realizing near-full density monophasic tetragonal 1.5-mol% yttria-stabilized zirconia ceramics via current-ramp flash sintering

This study demonstrates the successful fabrication of near-full density monophasic tetragonal 1.5-mol% yttria-stabilized zirconia (1.5YSZ) ceramics using current-ramp flash (CRF) sintering technique. The process involved regulating Joule heating in the samples by fine-tuning the input power through an alternating current field (nominal current density: 50 mA·mm⁻²) within a 3-min timeframe at a furnace temperature of 1100°C. This approach effectively enhanced grain size uniformity, which is crucial for preventing sample cracking associated with spontaneous tetragonal-to-monoclinic (T→M) phase transformation, thereby promoting densification with average relative densities exceeding 99%. The highest average fracture toughness of the 1.5YSZ samples was measured at 9.2 MPa·m^0.5 using the standardized single-edge pre-cracked beam method. This toughness is approximately double that of commonly used 3YSZ samples produced by CRF sintering, all of which exhibited comparable average grain sizes and relative densities. Additionally, the 1.5YSZ samples demonstrated nearly identical resistance to low-temperature degradation (LTD) compared to the 3YSZ samples after accelerated hydrothermal aging at 140°C for 15 h, roughly equivalent to 60 years at 37°C. The reduced yttria concentration in 1.5YSZ facilitates T→M phase transformation at lower stress thresholds, enhancing toughness through increased crack shielding from a larger volume fraction of transformed grains. Furthermore, the uniform yttria distribution in 1.5YSZ, revealed by scanning transmission electron microscopy, compensates for the reduced tetragonal phase stability and contributes to improved LTD resistance. Notably, these exceptional properties were achieved at a furnace temperature 300°C lower and with a sintering duration several hours shorter than those of conventionally-sintered counterparts.