Effect of diffusion barriers on steam oxidation properties and interface evolutions of Cr coating for zirconium alloy at 1200 °C

Abstract

Under the ATF design concept, Cr-based coating is the most promising zirconium alloy cladding protective coating in the case of PWR accidents. However, the primary factor affecting the oxidation resistance of Cr coatings is the interdiffusion of Cr and Zr. In this study, Cr coatings with three refractory metal interlayers of Nb, Mo, and Ta were deposited on zirconium alloy surfaces using closed-field unbalanced magnetron sputtering. The study investigated the effects of different interlayers on the microstructure, oxidation properties, and interface evolution process of Cr coatings under steam conditions at 1200 ℃, and compared them with Cr coatings without intermediate layers. The results indicate that the refractory metal interlayer influences the orientation and growth rate of Cr coating grains. The formation of a Laves phase mixed layer in the interlayer during oxidation effectively hinders the diffusion of O and Cr to the zirconium alloy matrix, enhancing its oxidation resistance. Furthermore, the diffusion rate and vacancy concentration of Mo and Ta elements to the Zr layer exceed that of diffusion to the Cr layer. During the cooling process, a precipitate phase forms in the Zr-4 matrix, while the Nb coating forms an infinite solid solution in the β-Zr.