The in situ growth of Er2O3 coatings on V–4Cr–4Ti in liquid lithium

The in situ Er2O3 insulating coating is under development for self-cooled Li/V-alloy type fusion blankets. A previous study showed formation and long-term stability of the coating by exposing V–4Cr–4Ti substrate to liquid Li doped with Er. In this study, the mechanisms for nucleation and growth of the coating are investigated. By oxidation and annealing, oxygen was charged into the substrate to form a Ti–O net phase as oxygen source to the coating. The surface layer formed on V–4Cr–4Ti consists of two sub-layers, an insulating Er2O3 coating and an intermediate layer of mixed ErN and Er–V–O. The measured growth rate shows logarithmic kinetics with high exponent (n ≈ 3 or 4) at 873 and 923 K, suggesting that the rate of growth to Er2O3 coating should be very low. The growth was accelerated suddenly at 973 K resulting in a low exponent (n ≈ 2) that almost obeys a parabolic law. A phenomenological model was proposed to describe the mechanism of coating, which showed the growth of the Er2O3 coating is controlled by the diffusion of oxygen and delivery of erbium to interface between V-alloys substrate and liquid Li.