Influence of gas pressure on the low-temperature plasma nitriding of surface-nanocrystallined TC4 titanium alloy

The influence of gas pressure on plasma nitriding of surface nanocrystalline TC4 titanium alloy prepared by shot peening (SP) was investigated. SP treatment under 0.3 MPa-25 min (Almen intensity is 0.32 mm) before plasma nitriding effectively refined the surface grain size of TC4 alloy to about 42 nm, which helped to sufficiently lower the nitriding temperature of TC4 to 500 °C. After nitriding treated under different gas pressures from 50 Pa to 300 Pa, nitriding gas pressure was found to has an optimal value at 200 Pa with which a thickest nitriding layer of 1.34 μm and highest hardness of 1569 HV was achieved at 500 °C for 20 h. Furthermore, the influence of gas pressure on the formation and migration of active nitride particles was revealed to be related with their energy and efficiency. Higher nitriding pressure would produce more active nitride particles but consume more energy during collision which made active nitride particles difficult to transit through the nitriding environment to land on the specimen. Balanced between these two aspects, 200 Pa was found to form more active nitride particles as well as maintain a high level energy. Meanwhile, SPed specimen presented a thicker nitriding layer than coarse-grained specimen. So in this study, we combine the promotion effect of surface nanocrystalline and the optimization of nitriding pressure to comprehensively improve the plasma nitriding efficiency.