Fretting wear and friction reduction of CP titanium and Ti-6Al-4V alloy by ultrasonic nanocrystalline surface modification

► Surface hardness was increased by about 25% after the UNSM treatment. ► By UNSM treatment, grain size was decreased from initial size. ► Compressive residual stress was increased after the UNSM treatment. ► The UNSM-treated specimens showed better frictional properties. ► The UNSM-treated specimens showed better fretting wear resistance.