Preparation and high temperature properties of Nb/Y2O3 coating on the pure Nb substrate by laser cladding

Abstract

In nuclear industry, the refractory metal of niobium (Nb) is an important candidate as the crucible substrate material for melting radioactive metals, such as uranium (U). However, since Nb readily reacts with molten metals at high temperatures, a protective coating is required. In this work, Nb/Y2O3 coatings were prepared on the pure Nb substrates by laser cladding with a dip coating technique. Phase microstructure, composition, thermal cycle durability, and corrosion resistance of the coating were investigated. The results show that dense Nb/Y2O3 coatings with a Ra value of 0.94 and an average bond strength of 73.1 MPa were obtained. The Y2O3 layer with thermochemical stability, located on the upper layer, was the main body of the coating. Also, few defects, such as microcracks, were observed in the coating. The Nb/Y2O3 coating reached 14 thermal cycles over a constant temperature of 1400 °C. The molten cerium (Ce) was selected for simulating corrosive environment. After corrosion for 4 h, the Nb substrate was severely damaged, and a corrosion reaction layer with a thickness of 18 ± 4 μm was formed between the Nb substrate and Ce. No corrosion layer was observed on the surface of Nb/Y2O3 coatings, indicating that the Y2O3 layer resulted in a considerable improvement in the corrosion resistance. During corrosion for 8 h, the accumulated thermal stress resulted in the crack propagation. Furthermore, the melt preferentially diffused towards the interior of the coating along the microcracks (as percolation channels) on the surface due to the capillary effect, aggravating the propagation of the crack tips. In addition, the melt performed poor wettability on the coating surface.