The phase stability of t-ZrO₂ realized by grain size at cryogenic temperature in ZrO₂/TiO₂ composite

In this work, the trade-off between transformation toughening of ZrO2/TiO2 ceramic coatings and low temperature (293.15 K similar to 203.15 K) was achieved by adjusting the grain size of t-ZrO2 (d(t-ZrO2)). A negative correlation between d(t-ZrO2) and phase transition temperature (M-s) was also established based on strain energy and chemical free energy in ZrO2/TiO2 ceramic coatings. Interestingly, t-ZrO2 grains lost toughening effects when the d(t-ZrO2) reaches the critical size (20 nm) even under cooling and stress. This is attributed to the number of dislocations per unit volume increases 1.5 times with the d(t-ZrO2) decreases from 40 nm to 20 nm, which aggravates the lattice distortion and dislocation tangle and leads to the fracture toughness increases by 32.5% at 203.15 K. Therefore, the interfacial cohesion of the semi-coherent interface between (0 1 1) t-ZrO2 and (1 1 0) TiO2 was enhanced and t-ZrO2 was completely stabilized when d(t-ZrO2) reaches critical size. This work elucidates the effect of size effect on M-s and provides a reference for transformation toughening of ceramic coatings at cryogenic temperature.