Magnetron sputter deposition of zirconium-silicide coating for mitigating high temperature oxidation of zirconium-alloy

Abstract

The air oxidation behavior of zirconium-silicide coatings for three stoichiometries, namely, Zr2Si, ZrSi, and ZrSi2, at 700 degrees C has been investigated. These three coatings were deposited on a zirconium-alloy substrate using a magnetron sputter process at a low temperature. Argon gas pressure was observed to have a profound effect on the coating microstructure, with lower pressures favoring a denser and more protective microstructure. Coatings of ZrSi2 stoichiometry clearly showed superior oxidation resistance presumably due to the formation of a thin protective oxide layer, consisting of nanociystalline SiO2 and ZrSiO4 in amorphous Zr-Si-O matrix. The thermal stability of the coatings was evaluated by annealing in an argon environment, and this also assisted in eliciting the effects of oxidation-induced inward Si migration. Thicker coatings of ZrSi2 were prepared and evaluated for oxidation resistance at 700 degrees C for longer exposure times, as well as at 1000 degrees C and 1200 degrees C. Once again the thin oxide layer provided for significant oxidation resistance. Pre-oxidizing the samples at 700 degrees C prior to 1000 degrees C and 1200 degrees C oxidation tests substantially reduced the extent of oxidation. Insights into the fundamental mechanisms of the oxidation behavior of zirconium-sificide coatings were obtained using a combination of scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy techniques. One potential application of these coatings is to enhance the oxidation resistance of zirconium-alloy fuel cladding in light water reactors under normal and accident conditions. (C) 2017 Elsevier B.V. All rights reserved.