Effect of He ion irradiation on the microstructure of t′ phase yttria-stabilized zirconia ceramic coatings

Yttria-stabilized zirconia (YSZ) films with tetragonal (t′) phase was prepared on China Low Activation Martensitic steel (CLAMs) substrates by supersonic atmospheric plasma spraying (SAPS) with ZrO2-8 wt% Y2O3 nano-powders as feedstock. This study deals with the subject of the structural modifications and phase transformations in t′ phase YSZ coatings induced by implantation of low-energy (60 keV) He ions. It is found that the phase transformation in t′ phase YSZ coatings during helium ion-irradiation as a function of fluence can be divided into a two-stage process based on the microstructural evolution as follows: (i) In the case of He ions at 0.5 × 1017 cm2 (∼1.5 dpa), no obvious volume swelling can be detected, suggesting the irradiation-induced defects is primarily associated with a positive relaxation volume. Therefore, only t′ → t phase transformation was observed for the samples implanted at a fluence of 0.5 × 1017 cm−2. (ii) While increasing the fluence from 0.5 × 1017 cm−2 to 2 × 1017 cm−2, an increase in the defect concentrations can be observed, leading to an exponential growth in volume swelling from 3.2% to 9.7% observed. These high- density extended defects can induce a plastic deformation in the lattice structure, resulting significant lateral compressive stress. Correspondingly, the t′ → t + c phase transformation was detected for the samples implanted at the fluence of 2 × 1017 cm−2. At a fluence of 10 × 1017 cm−2, the formation of the He bubbles, and ellipsoidal cavities can be observed by TEM, inducing the further volume swelling. Meanwhile, the intensity of (4 0 0)c peak increases while fluence increase from 2 × 1017 cm−2 to 10 × 1017 cm−2. Based on the experimental results, a reasonable mechanism for the phase transformation of t′ phase YSZ samples during He ions irradiation is proposed.