Phase and fracture toughness evolution during isothermal annealing of spark plasma sintered zirconia co-doped with Yb, Gd and Nd oxides

Dense pellet of rare-earth oxide (5YbO1.5 + 1.5GdO1.5 + 1.5NdO1.5) stabilized zirconia, a candidate thermal barrier composition, was prepared by spark plasma sintering. Isothermal annealing was carried out at 1250 °C up to 192 h. X-ray diffraction and Raman spectroscopy were used to characterize the phases. Cubic precipitates formed in the initial tetragonal phase from 6 h, with volume fraction of 22.31% after 192 h. 15.35 vol.% of monoclinic phase was observed in XRD at 192 h along with cubic and two tetragonal phases. Indentation toughness increased by 49% from 155 ± 24 J m−2 at 0 h to 231 ± 71 J m−2 at 6 h of annealing and it decreased towards the initial value upon further annealing to 192 h. Ferroelastic toughening dominates until transformation toughening ensues at 192 h. The toughness is consistently higher than for singly doped, sintered t′ zirconia pellets that are reported in the literature.