Oxidation, mutual diffusion and crack behaviors of high-throughput prepared CrAl coating on Zry-4 in steam at 1200 °C

Abstract

The compositionally graded CrAl coating was deposited on Zry-4 substrates by high-throughput magnetron sputtering. The oxidation, mutual diffusion and crack behaviors of CrAl coating in steam at 1200 °C exhibited strongly dependence on the initial Al concentration ($N_{\mathrm{Al}}^0$). As $N_{\mathrm{Al}}^0$ increased, the surface oxide transformed from the single-layer Cr₂O₃ to the bi-layer Cr₂O₃/α-Al₂O₃, and subsequently to the single-layer α-Al₂O₃. The transition point from the internal to the external oxidation was determined as 18.0 at% Al. The dependence of the thickness of the surface oxide layer and interdiffusion layers (IDLs) on $N_{\mathrm{Al}}^0$ was demonstrated. The theoretical model of interface diffusion based on the Fick’s second law was proposed. The Al reservoir was directly affected by the inward diffusion of Al rather than the oxidation process until the Al content decreased to a critical level. The cracks were caused by the stress when the α-Al₂O₃ formed internally, while they were a consequence of the aggregation of pores when the α-Al₂O₃ formed externally. Cracks and pores accumulated the ingress of O and outward diffusion of Zr, resulting in the failure of the CrAl coating