Zr alloys are widely used as fuel rod cladding materials in nuclear reactors because of their small neutron absorption cross-section, high strength at high temperatures, and strong corrosion resistance. With the new iteration of nuclear power technology, the traditional Zr alloy cladding tube has exposed serious defects in safety performance, and the safety of nuclear fuel assemblies has been severely challenged. Currently, the exploration and development of new ATF materials have focused on nuclear reactor safety. The physical and chemical properties of the substrate can be improved by spraying a wear-resistant coating on the Zr alloy surface. The Cr coating has excellent high-temperature wear resistance and is currently the most important coating material. However, there are few reports on the fretting wear properties of Cr-coated Zr alloy cladding tubes and dimple structures that are in actual use in nuclear reactors. In this paper, the effect of tangential displacement on the fretting wear mechanism and wear properties of Cr-coated Zr-1Nb alloy coated tubes are investigated at room temperature. Scanning electron microscopy and energy-dispersive spectrometry were used to observe the surface morphology and elemental composition of the wear marks, respectively. The wear volume and maximum wear depth of the wear marks were measured using a white light interferometer. The results showed that the surface material underwent repeated alternating loads, resulting in cracks within the microstructure that eventually expanded to form wear debris. The tangential displacement amplitude is the key factor affecting fretting wear. As the amplitude increased, the wear debris was discharged more easily, the direct contact area between the samples increased, the sliding zone increased, and the fretting running status changed from a mixed fretting regime to a gross slip regime. This resulted in more evident delamination wear, oxidation wear, and increased surface stress, which led to the intensification of wear. When the tangential displacement is small, the Cr-coated Zr-1Nb alloy cladding tube only comes into contact with the dimple ends, wear debris is difficult to discharge, and its presence changes the friction from two-body contact to three-body contact. A large amount of wear debris accumulated between the samples and underwent repeated grinding and oxidation sintering, forming a third body layer. This layer can be regarded as a solid lubricant that prevents direct contact with the sample surface and further wear damage. When the displacement is large, the wear debris is easily discharged; therefore, less wear debris accumulates on the surface. The Cr-coated Zr-1Nb alloy cladding tube and dimple primarily contact each other through two-body contact; however, there is also three-body contact in the central region of the wear mark. Excessive shear stress and strain on the sample surface caused material detachment or delamination. In all experiments, the surface of the Cr-coated Zr-1Nb alloy cladding tube exhibited an evident abrasive adhesion phenomenon. However, with the increase in tangential displacement, the relative slip distance between the samples increased, and the debris was more easily discharged during the fretting process; thus, the accumulation and adhesion phenomenon of the debris was gradually weakened. Because there was a phenomenon of stress concentration at the contact edge area between the contact surfaces of the specimen, wear preferentially occurred in the contact edge area. It can be observed from the micromorphology of the wear surface that its characteristics are material accumulation, lamellar stripping, and microcracks in local areas. Therefore, the wear mechanism of the Cr-coated Zr-1Nb alloy cladding tube was primarily adhesive and peeling wear, and the elemental composition of the wear surface indicated that the O element entered the contact area during the test and oxidized wear.
