High Temperature Oxidation Behavior of Zirconium Silicides and Their Coating by Laser Cladding on the Zircaloy-4 Tube

The Zr-Si system has been proposed as one of promising protective coating materials for accident tolerant fuel cladding. During the oxidation of zirconium silicides, ZrO₂ is formed above the zirconium silicide since its lower formation Gibbs free energy compared to SiO₂. After this oxidation the zirconium concentration in zirconium silicides is reduced and the formation of SiO₂ is favored under the ZrO₂. This inner layer of SiO₂ serves as a barrier to oxygen and prevents further oxidation. In this research, high-temperature oxidation behaviors of bulk zirconium silicides with different compositions were investigated. Coating of zirconium silicide on the surface of Zir-4 was performed using laser cladding method and its properties are also investigated. Samples were prepared by vacuum arc-melting. Table 1 presents ratio of zirconium to silicon of samples. After fabrication of samples, heat treatment was applied at 1000.deg.C for 20 hours in order to give homogeneity to sample and they were cut to 5 mm*5 mm*1 mm size. Samples were washed using ultrasonic cleaning in ethanol for 5 min, and then dried. Oxidation experiments in 1200.deg.C steam and Ar mixture with 10 ml/min flow rate were carried out with a TGA. The temperature of the samples was increased with 50.deg.C/min and only Ar was supplied while raising the temperature. Microstructure and crystal structure were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD) before and after oxidation. Figure. 1~ 3 are showing microstructure of fabricated samples. The concentrations of the different brightness areas were investigated using energy dispersive x-ray spectroscopy (EDS) and are shown in the figures. Applied laser power was 180 W and 0.3 mm hatching distance was used for coating. ZrSi₂ coating was best formed when the powder was mixed with Zr: Si = 1: 6 atomic ratios due to the difference in size between zirconium powder and silicon powder and existence of zirconium in substrate. Figure. 5 represents cross sectional image of zirconium silicides coating on the surface of Zir-4. ZrSi₂ was the main phase of the coating, and some pure Si and ZrSi were mixed.