Developing a nanostructured surface layer on AISI 316 stainless steel by ultrasonic surface nanocrystallization and evaluating its tribological properties

Austenitic stainless steels have received a lot of attention for a wide range of applications, including petrochemical, automotive industry, building architecture, and bioengineering; however, their poor mechanical properties such as high wear rate are always challenging. In this research, ultrasonic surface nanocrystallization treatment (USNT) has been used for surface modification and improvement of tribological characteristics of AISI 316 stainless steel. For this purpose, 375 N accompanied with 20 kHz vibration frequency was applied on the surface to provide static and dynamic forces simultaneously, increase dislocations density, and fabricate a nanocrystalline surface. After this process, the untreated and USN-treated specimens were characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction analysis, Vickers microhardness measurements, and surface profilometry. Also, a reciprocating pin-on-plate test was used to evaluate the tribological features of the surface. After the wear test, the untreated sample showed approximately twice weight loss, in comparison with the USN-treated specimen. The friction coefficient during 500 m sliding was reduced from around 1 (for the untreated specimen) to 0.4 after USNT. Also, the SEM investigations showed that the abrasive wear is decreased after USNT. The improved tribological properties of AISI 316 in this research was attributed to the residual compressive stress, nanocrystallization, strain-induced phase transformation, microhardness enhancement, and surface roughness reduction after USNT.