Cr-coated cladding, as the most promising accident-resistant cladding material, has received extensive attention and research. Focusing on the main failure mechanism of fretting erosion damage of fuel rod cladding, the influence of the rotational contact angle between Cr-coated zirconium alloy cladding and the stiffness of the positioning frame on the fretting erosion behavior of Cr-coated zirconium alloy cladding is studied. A 15 μm thick Cr coating is prepared on the surface of the zirconium alloy according to the multi-arc ion plating method. Fretting wear tests under different rotational contact angles are conducted with a fretting wear testing device under high-temperature and high-pressure water chemical conditions. The grid dimple is Zr-Sn series zirconium alloy material. The average thickness of the Cr coating is approximately 14.86 μm, and the hardness and roughness of the surface are 285.00HV0.2 and 241.30 nm respectively, which are significantly improved compared with the zirconium alloy substrate. After the tests, the samples are characterized with a metallographic microscope, a field emission scanning electron microscope (FE-SEM), an energy dispersive spectrometer (EDS), and an optical 3D surface profiler to examine the wear mark morphology, microstructure, and elemental distribution. The study finds that under different rotational contact angles, the wear marks on the Cr-coated surface are all elongated, and the erosion at both ends and the middle part of the contact is more severe. Due to the grid dimple surface having a certain curvature, as the rotational contact angle increases, the initial contact area decreases, and the length of the wear marks gradually decreases. As the contact area decreases, the contact stress increases, leading to the accumulation of wear debris during the fretting process, which causes the wear area to increase. The wear area exhibits a trend of first decreasing and then increasing. When the rotational contact angle is 4°, the wear area on the Cr-coated zirconium alloy cladding surface is the smallest, at 12×104 μm2. When the rotational contact angle is 8°, the wear area is the largest, at 69×104 μm2. Since the hardness of the Cr coating is significantly greater than that of the zirconium alloy material, the Cr coating almost does not wear during the fretting process. Instead, the Zr-Sn series zirconium alloy material experiences layer peeling and transfers to the Cr-coated surface, where it oxidizes and forms a wear layer. As the rotational contact angle increases, the initial contact area decreases, and the normal stress on the contact area increases, leading to more severe adhesive marks on the Cr-coated surface. The wear debris particles gradually become finer under the combined effects of high contact stress and shear stress, and the thickness of the oxide layer decreases. The wear mechanism shifts from a combination of adhesive wear and oxidative wear to predominantly adhesive wear. With the increase of the rotational contact angle, more delamination cracks appear on the oxide grinding chip layer accumulated on the surface of the Cr coating, causing local peeling of the grinding chip layer and subsequently forming peeling pits. Under the action of large contact stress and shear stress, the grinding chip particles gradually refine, and the thickness of the oxide layer shows a decreasing trend. The rotational contact angle influences the initial contact area and contact stress, thereby affecting the wear behavior and mechanism of the Cr-coated zirconium alloy cladding. The Cr coating effectively enhances the wear resistance of the zirconium alloy.
Key words
Cr-coated zirconium alloy cladding /
fretting wear /
rotational contact angle /
wear mechanism
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Funding
Sichuan Science and Technology Project (2022JDRC0099)
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