Microstructure and Vacuum Tribological Properties of Commercial Titanium TA2 Treated by Plasma Nitriding

To investigate vacuum tribological properties of commercial titanium TA2 treated by plasma nitriding, optical microscope (OM), 3D profilometer, X-ray diffraction (XRD) and micro-hardness tester were used to characterize the microstructure, surface morphology, surface roughness, phase composition and micro-hardness of samples. Vacuum tribological properties of the original and the nitrided samples were tested by a vacuum tribometer, and then the morphology and composition of worn surface were investigated by scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The results show that a nitride layer consisting of hard phases TiN, Ti2N and alpha-Ti(N) is formed with a depth of 80 mu m. Besides, surface HV hardness is evidently improved from 1.40 GPa to 12.60 GPa after plasma nitriding; meanwhile the surface roughness, friction coefficient and wear resistance of commercial titanium are observably enhanced in atmosphere. Nonetheless, friction coefficient of samples tested in vacuum is decreased, and a milder wear is performed. In addition, the wear mechanism changes due to the formation of nitrided layer. In atmosphere, the wear mechanism of the original samples is the co-action of abrasive wear, plastic deformation and slight adhesive wear; nonetheless, the wear mechanism of the nitrided samples is severe plastic deformation, adhesive wear and material transfer. However, in vacuum severe adhesion wear, plastic deformation and mild abrasive wear occur on the surface of the original samples, while the worn surface of the nitrided samples tested in vacuum exhibits a typical abrasive wear feature.