Abstract
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 ZrO2 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 cos2αsin2ψ 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 cos2αsin2ψ 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.
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