Abstract: The surfaces of TC4 Ti-alloy,an advanced functional material in high-tech industries,were modified by acid/plasma etching and by hot-filament enhanced low-temperature plasma nitriding.The influence of the acid/plasma etching on the microstructures,interfacial adhesion and tribological properties of nitrided TC4 Ti-alloy was investigated with X-ray diffraction,scanning electron microscope,energy dispersive spectroscopy and conventional mechanical probes.The results show that the acid/plasma etching had a major impact.Specifically,the acid/plasma etching significantly improved the surface cleanliness,removed the oxide/passivation layer,increased the surface roughness and thickness of nitrided layer,activated the surface reaction,promoted the formation of N-containing solid solution and enhanced the interfacial adhesion.Most importantly,the 10 μm nitrided layer,on the TC4 alloy pre-etched by acid/plasma,exhibited a micro-hardness of 691 HV and a wear-rate 84% lower than that of TC4 Ti-alloy.