摘要
The objectives of this study were to investigate the incipient oxidation behavior of ZrO₂-coated Zircaloy-4 (Zry-4), and evaluate the feasibility of producing ZrO₂ protective layer by vacuum annealing of ZrN coating. ZrO₂ coatings with tetragonal (t) and monoclinic (m) dominant phases were deposited on Zry-4 using unbalanced magnetron sputtering. The wettability tests and the widths of interdiffusion zones at film/substrate interface indicated the ZrO₂ coatings having good intrinsic adhesion on the Zry-4 substrate. The oxidation behavior of the coated specimens and blank Zry-4 substrate were evaluated by thermogravimetric analysis performed at 700, 800 and 900 °C in argon atmosphere. After oxidation, X-ray diffraction (XRD) patterns revealed that both thermally formed ZrO₂ and the deposited ZrO 2 possessed an m-ZrO₂ dominant structure. The weight gain at 700 °C indicated no significant difference in the oxidation behavior between uncoated and ZrO₂-coated Zry-4, and the oxidation kinetics obeyed the parabolic law. However, the oxidation kinetics above 800 °C deviated from the parabolic law, where m-ZrO₂-coated Zry-4 showed the least weight gain at 800 °C, and t-ZrO₂-coated Zry-4 displayed a better oxidation resistance at 900 °C. Oxide nodules were observed on the uncoated Zry-4 specimens after oxidation; in contrast, the ZrO₂ coatings, both monoclinic and tetragonal phases, could act as a barrier to prevent the formation of oxide nodules. The azimuthal XRD cos 2 αsin 2 ψ technique was used to measure residual stress of t-ZrO₂ and m-ZrO₂ phases and reveal the distribution of each phase in the specimens. The results indicated that the oxide phases were not uniformly distributed in the specimens. The azimuthal XRD cos 2 αsin 2 ψ technique can be used as a convenient and nondestructive method to examine the uniformity of the phase distribution in a multi-phase oxide layer. The experimental results revealed that producing a ZrO₂ coating on Zry-4 by vacuum annealing of ZrN-coated Zry-4 was infeasible.