Thermal shock resistance of TiN-, Cr-, and TiN/Cr-coated zirconium alloy

Abstract As a candidate technology for accident tolerant fuel (ATF), surface modified Zr cladding has captured the interest of an army of nuclear workers in the past few years. In this paper, three coatings (TiN, Cr, and TiN/Cr) were deposited on Zr-4 by direct current magnetron sputtering. The microstructural properties of the three coatings were detected by using scanning electron microscope (SEM) and X-ray diffraction (XRD). The thickness of coating was determined by examining the cross sectional SEM image. The adhesion properties of was characterized by using scratch testing. The thermal shock resistance (TSR) tests were carried out in three temperature variations, that is, △T = 800 °C, 1000 °C, and 1200 °C, respectively. After the TSR test, the surface morphology of the tested specimens was observed, XRD, SEM and energy dispersive spectrometer (EDS) were used to examine the microstructural properties of the tested specimens, and scratch testing was executed to determine the adhesion properties. The results show that TiN coating was partially peeled off and the inner zirconium alloy was oxidized. And the thickness of the generated ZrO2 layer is about 50 μm. The specimens coated with single Cr coating remained intact. The cross-sectional analysis result show that only about 30% of Cr coating is oxidized, and the residual Cr coating and inner zirconium alloy are not oxidized. It is mainly due to the dense and uniform structure of Cr2O3 formed by the oxidation of the outermost layer of Cr coating, which prevents the further penetration of oxygen into the interior and plays a very prominent protective role. For the bilayer TiN/Cr-coated specimens, the coefficient of thermal expansion (CTE) of intermediate TiN layer is different from that of zirconium alloy substrate. After TSR test, cracks and a large number of pores appear, and there are obvious gaps between the bulk oxidation products, which leads to poor protection of the internal materials. It indicates that the zirconium alloy coated with a single Cr coating has excellent TSR property.