The nuclear fuel cladding tube, as a critical component in nuclear reactors, has its performance directly linked to the safety and efficiency of nuclear energy, as any degradation or failure in these tubes could lead to catastrophic consequences, including radioactive material release and reactor shutdown. Against this backdrop, Cr/CrN coatings have emerged as a focal point in surface modification research for cladding tubes, owing to their excellent wear and corrosion resistance properties, which are particularly vital in environments characterized by high temperatures, neutron irradiation, and chemically aggressive coolants. However, the optimization of their wear resistance to accommodate harsher operational environments, such as those encountered during loss-of-coolant accidents or prolonged exposure to corrosive species, has remained an unresolved challenge. The work aims to introduce an innovative LSP technique to enhance the wear resistance of Cr/CrN composite coatings and explore the effect of varying laser energies on the microstructure and fretting wear behavior of the coatings, addressing a critical gap in current surface engineering strategies for nuclear applications. In the study, Cr/CrN composite coatings were firstly deposited on the surface of zirconium alloy with multi-arc ion plating technology, a method known for its ability to produce dense, adherent films with excellent interfacial bonding strength. Subsequently, advanced analytical tools such as XRD for phase identification, SEM for microstructural observation, and EDS for compositional analysis were employed to comprehensively characterize the Cr/CrN coatings treated with different laser energies. These techniques enabled a detailed understanding of how LSP affected the microstructure, phase composition, and elemental distribution of the coatings. The laser shocking induced microcracks on the cross sections of the coatings, with the number of cracks increasing with the laser energy, which necessitated careful control of process parameters to avoid compromising structural integrity. Compared to the untreated substrate, the laser-peened coatings exhibited significant improvements in hardness, residual compressive stress, and grain size. Specifically, the nano-hardness increased from 5.81 GPa to 7.76 GPa, the residual stress rose from ‒462.5 MPa to ‒561.3 MPa, and the average grain size of the near-surface coatings was refined from 30.26 nm to 27.06 nm, all of which contributed to enhanced wear resistance and crack closure under service conditions. Regarding the assessment of wear performance, systematic testing was conducted with a custom-designed fretting wear tester, which simulated the relative motion between cladding tubes and fuel pellets under realistic operating conditions. The results revealed that laser treatment did not alter the fretting wear mechanism of the Cr/CrN composite coatings, which was primarily dominated by adhesive wear, accompanied by oxidative wear and abrasive wear. This suggested that LSP primarily enhanced the resistance of the material to these wear modes rather than changing their fundamental nature. Notably, when the laser energy was controlled at 60 mJ, the coating achieved the lowest wear volume and wear rate, representing reductions of 31.87% and 32.71%, respectively, highlighting the importance of precise energy control in LSP for optimal performance, compared to the untreated substrate. Nevertheless, excessively high laser energies (e.g., 100 mJ) triggered shock bending effects, leading to bent protrusions and crack propagation on the coating, which instead diminished the wear resistance of the material, underscoring the need for energy thresholds to avoid detrimental effects in nuclear applications. In summary, this study confirms the effectiveness of appropriate laser energies in enhancing the wear resistance of Cr/CrN composite coatings, while revealing the potential damaging effects of excessively high energies on the coating. These findings provide crucial technical support and theoretical basis for the further optimization and application of Cr/CrN coatings in nuclear fuel cladding tubes, ultimately contributing to the safety and reliability of nuclear energy systems.
Key words
Cr/CrN composite coatings /
laser shock peening /
microstructure /
fretting wear /
wear mechanisms
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Funding
The Opening Foundation from Key Laboratory of Materials in Dynamic Extremes
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