Effect of Different Laser Energy Nitriding on the Fretting Wear Performance of Zr Alloy

The zirconium (Zr) alloy fuel cladding is one of the key structural components of a nuclear reactor and the first and most important line of defense for accommodating fission products. During the operation of nuclear reactors, Zr alloy fuel cladding is subjected to extreme harsh environments, such as high temperature, high pressure and high flow rate for a long period of time. The wear and corrosion resistance of Zr alloys is important for the safe operation of nuclear reactors. Surface modification can effectively improve the corrosion and wear resistance of fuel cladding. Compared with coating technology, nitriding technology does not have problems for bonding between the coating and the substrate. Current research on surface nitriding of Zr alloys mainly focuses on plasma nitriding and ion implantation techniques. Research on laser nitriding of Zr alloy surfaces and their fretting wear characteristics is scarce. In this study, the surface of Zr alloy was treated with laser nitriding at different laser energies. The microstructure of Zr alloy treated with different laser energies and its fretting wear performance were studied. The results showed that after nitriding with different laser energies, the surface of the Zr alloy showed a typical molten state after melting, vaporizing and cooling under the thermal effect of the laser, and this state was more obvious with the increase of the laser energy. At the same time, doping of N atoms and formation of the ZrN phase led to different cooling rates in the molten zone that produced large tensile stresses after cooling. This led to cracks on the surface of Zr alloys after laser nitriding at different energies, and the crack density increased with increasing laser energy. This also led to an increase in the surface roughness of the Zr alloy with increasing laser energy after laser nitriding treatment. Due to the presence of water in the industrial nitrogen, nitrides were generated on the surface of the sample along with some oxides. When the laser energy was 100 mJ, there was no ZrN generation, and N existed mainly as a diffusion layer within the Zr alloy substrate. ZrN generated when the laser energy reached 200 mJ and above, which increased with the increase of laser energy. Due to the generation of ZrN phase and the presence of some oxides, the surface Vickers hardness of Zr alloys after laser nitriding treatment at different energies increased by 37.5% compared to Zr alloys. After laser nitriding treatment, the wear mechanism of Zr alloys changed. For the untreated Zr alloys, the wear mechanism was dominated by delamination and spalling wear, accompanied by oxidative and abrasive wear. The phenomenon of delamination and peeling decreased with the increase of laser energy. Wear mechanisms changed to predominantly abrasive wear with oxidative wear and delamination spalling. The wear volume of sample nitriding with laser energy 400 mJ was reduced by 46.5% compared with that of untreated Zr alloy.