Vacuum Tribological Properties of Titanium with a Nanocrystalline Surface Layer

In order to enhance the tribological properties of titanium (Ti), a gradient structure with a nanocrystalline surface layer was formed on the surface of the UNS R50400 (Gr2/TA2/CP3) Ti discs by means of ultrasonic surface rolling processing (USRP). The microstructure and microhardness of the USRP and untreated Ti were investigated. The tribological properties of the USRP Ti under both vacuum and atmospheric conditions were examined in comparison with the untreated Ti using a ball-on-disc vacuum tribometer. The results showed that an ultrafine-grained (UFG) deformation layer with a thickness of 200 μm was produced on the USRP Ti, and its mean surface grain size was refined to about 38 nm from 45 μm. The surface hardness of the USRP Ti is 310 HV, which is about 2.3 times that of the untreated Ti. Additionally, the USRP Ti possesses a lower friction coefficient and higher wear resistance under vacuum conditions than untreated Ti. Under atmospheric conditions, USRP Ti shows more extensive oxidation wear and milder abrasive wear compared with the untreated Ti. Under vacuum conditions, because of the formation of the UFG deformation layer, USRP Ti exhibits a significant decrease in plastic deformation and adhesive wear compared with the untreated Ti.