Improvement of wear resistance of Zircaloy-4 by nitrogen implantation

Nitrogen implantation process has been applied for improvement of wear resistance of Zircaloy-4 fuel cladding materials. Nitrogen was implanted at 120keV to a total dose range of $1\times 10^{17}$ions/$\textrm{cm}^2$ to $1\times 10^{18}$ions/$\textrm{cm}^2$ at various temperatures between $270^{\circ}C$ and $671^{\circ}C$. The microstructure changes by nitrogen implantation were analyzed by XRD and AES and wear behavior was evaluated by performing ball-on-disc type wear testing at various loads and sliding velocities under unlubricated condition. Nitrogen implantation produced ZrNx nitride above $3\times 10^{17}$ions/$\textrm{cm}^2$ as well as heavy dislocations, which resluted in an increase in microhardness of the implanted surface of up to 1400 $H_k$ from 200 $H_k$ of unimplanted substrate. Hardness was also found to be increased with increasing implantation temperature up to 1760 $H_k$ at $620^{\circ}C$. The wear resistance was greatly improved as total ion dose and implantation temperature increased. The effective enhancement of wear resistance at high dose and temperature is believed to be due to the significant hardening associated with high degree of precipitation of Zr nitrides and generation of prismatic dislocation loops.