Preparation of Ni-Cr composite coatings on tungsten by molten salt thermal diffusion method

BackgroundTungsten is a commonly used shielding material in 90Sr radioisotope thermoelectric generators; however, its resistance to high-temperature oxidation is relatively poor.PurposeThis study aims to enhance the oxidation resistance of tungsten in high-temperature conditions.MethodsThe molten salt thermal diffusion method was employed to prepare a nickel-chromium composite coating on tungsten. The coatings produced at different molten salt temperatures were characterized using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Additionally, cyclic oxidation tests and isothermal oxidation tests were conducted to evaluate the oxidation resistance and spallation resistance of the coatings. The adhesion strength between the coatings and the substrate was also assessed using the indentation method.ResultsThe XRD results indicate that the primary phase of the coatings is γ-(Ni,Cr). The SEM and EDS results indicate that the coating surface is blocky and that the impurity phosphorus (P) element gradually disappears when the molten salt temperature is 950 ℃. Oxidation tests demonstrate that the coating sample prepared at 950 ℃ exhibits excellent oxidation resistance and anti-spallation properties. Further characterization by SEM, XRD, and EDS reveals that a dense layer of Cr2O3 forms on the surface of the oxidized sample while oxygen only exists on the coating surface and does not diffuse into the substrate. Adhesion tests confirm that the coating prepared at 950 ℃ possesses the highest adhesion and exhibits a fine crystalline structure.ConclusionsResults of this study show that a temperature of 950 ℃ is optimal for preparing nickel-chromium coatings using the molten salt diffusion technique.