Micro-dimpled surface by ultrasonic nanocrystal surface modification and its tribological effects

► EBSD micrographs show that more surface plastic deformation is observed in the surface layer of the UNSM-treated surface comparing to the untreated surface. ► The key point for realizing the surface nanocrystallization of a bulk material is to introduce interfaces into the surface layers, so that its microstructure is transformed into nano-sized crystallites. ► UNSM is an effective technology to realize the surface nanocrystallization on metallic materials. It is well known that dimpled surface can improve tribological characteristics. The fundamental aim of the present research is to demonstrate the process of making micro-dimples by ultrasonic nanocrystalline surface modification (UNSM) technology and to evaluate its effects on tribological characteristics. Ball-on-disk test was performed in order to reveal the influence of mechanism and micro-dimples. UNSM-treated surface is compared to ground surface and friction coefficient of the UNSM-treated surface has been reduced by about 25%, which is a significant phenomenon in improving tribological properties of mating surfaces. Wear volume loss of UNSM-treated surface has also been reduced by about 60%. Scanning electron microscopy (SEM) and surface roughness profile measurement were utilized to study the worn surface.