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A novel low temperature and green salt bath nitriding of TC4 titanium alloy was developed by adding a certain amount of K2SO4 in the KNO3 and NaNO3 salt bath. The results show that compared with as-received TC4 alloy, the surface hardness and tribological properties of the nitrided titanium alloy were greatly improved. Furthermore, based on the theoretical and experimental analysis of the effects of the K2SO4 on the inhibition of the cyanide in the nitriding salt bath and tribological properties of the nitrided samples, wonderful discoveries were obtained. The tribological properties of the nitrided samples were dominated by not only the surface hardness but also the subsurface hardness. Moreover, there exists an optimum value for K2SO4 addition (about 1 wt-%): K2SO4 can not only effectively inhibit cyanide production but also cause the hardness in the nitride layer subsurface to further be improved, resulting in the further enhancement of the tribological properties.
Zhu et al. (2021) studied this question.