Mitigating low-temperature hydrothermal degradation of 2 mol% yttria stabilised zirconia and of 3 mol% yttria stabilised zirconia/nickel oxide by calcium oxide co-doping and two-step sintering

Hydrothermal degradation deteriorates the fracture toughness and strength of tetragonal stabilised zirconia. The phase transformation to the monoclinic phase is particularly critical for materials with lower stabiliser content such as 2 mol% and 3 mol% yttria stabilised zirconia (2YSZ and 3YSZ). In this work, two routes in two-step sintering and co-doping with calcium oxide are analysed to mitigate the degradation. Both strategies show a reduction in average grain size of the 2YSZ while maintaining a comparable densification. The achieved smaller grains suppress the hydrothermal degradation rates. In addition, a mitigating effect beyond the reduction in grain size of YSZ is found for CaO doping. 1.6 mol% CaO co-doped 2YSZ shows less than 4% of monoclinic phase after 50 h in an autoclave with H₂O at 134 °C. Pure 2YSZ contrarily reaches 100% monoclinic phase after 20 h at the same conditions. A suppression of degradation by CaO doping was also observed for the composite of nickel oxide and 3 mol% yttria stabilised zirconia. Hence, CaO co-doping can be an interesting strategy to increase resistivity against hydrothermal degradation for both biomedical and renewable energy applications. The findings further outline a route to achieve tetragonal YSZ with lower yttria contents than 2 mol%.