A metallurgical zirconium nitride (ZrN) layer was fabricated using glow metallurgy using nitriding with zirconiuming prior treatment of the Ti6Al4V alloy. The microstructure, composition and microhardness of the corresponding layer were studied. The influence of this treatment on fretting wear (FW) and fretting fatigue (FF) behavior of the Ti6Al4V alloy was studied. The composite layer consisted of an 8-μm-thick ZrN compound layer and a 50-μm-thick nitrogen-rich Zr–Ti solid solution layer. The surface microhardness of the composite layer is 1775 HK0.1. A gradient in cross-sectional microhardness distribution exists in the layer. The plasma ZrN metallurgical layer improves the FW resistance of the Ti6Al4V alloy, but reduces the base FF resistance. This occurs because the improvement in surface hardness results in lowering of the toughness and increasing in the notch sensitivity. Compared with shot peening treatment, plasma ZrN metallurgy and shot peening composite treatment improves the FW resistance and enhances the FF resistance of the Ti6Al4V alloy. This is attributed to the introduction of a compressive stress field. The combination of toughness, strength, FW resistance and fatigue resistance enhance the FF resistance for titanium alloy.
This work reported a comparison between the wear and fretting fatigue (FF) behaviors of a Cr-alloyed layer and a Cr-Ti solid-solution layer. The hardness and toughness of both layers were evaluated to support this comparison. The results showed that the Cr-alloyed layer had high surface hardness but poor toughness, while the Cr-Ti solid-solution layer had excellent toughness but low hardness. The FF properties of the modified Ti6Al4V alloy depended on the trade-off between two factors: wear resistance and fatigue resistance. Although the Cr-alloyed layer could effectively resist the wear in fretting areas, its poor toughness caused the fatigue resistance to drop sharply and hence led to a premature failure in FF test. Due to the relatively good fatigue resistance, the Cr-Ti solid-solution layer had slightly higher FF life than that of the Cr-alloyed layer; however, its low hardness resulted in severe wear in correspondence with the fretting area and thus a failure to improve the FF properties of Ti6Al4V alloy. When combined with shot peening post-treatment, the FF life of both layers increased by about three times compared to that of the Ti6Al4V alloy. A further study showed that the poor toughness or low hardness still exerted negative influence on combination-treated samples.
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