Abstract: The surfaces of 51 CrV4 steel,a spring material,were modified by ion nitriding.The influence of the nitriding temperature and time on the microstructures and tribological behavior of the nitrided layer was investigated with X-ray diffraction,scanning electron microscopy and conventional mechanical probes.The results show that the nitriding temperature and time had a major impact.For instance,as the nitriding temperature and time increased, the thickness of the bright,compact nitrided layer,comprising γ-Fe4N and CrN phases,increased;and nitrided for 20 h,the friction coefficient and wear-rate changed in a decrease-increase mode(V-shape curve) with an increase of the temperature.Nitrided at 450℃ for 20 h,the modified layer,with strong corrosion resistance and hardness up to 950 HV,had the smallest friction coefficient and the lowest wear-rate;however,a higher temperature and a longer time negatively affected the tribological properties because of the surface coarsening.