The formation of gradient nanostructured medium carbon steel via mild, moderate, and severe ultrasonic nanocrystal surface modification options: Assessment on wear and friction performance

In this study, the effect of UNSM applied under different static loads on the microstructure and friction-wear performance were detected. A significant correlation was noticed between the increase of the static load and nanocrystalline layer thickness. Both nanocrystallization layer and deformation depth increased significantly after UNSM. The grain size was measured under 500 nm for M series and 100 nm for O and S series of UNSM static loads. Surface integrity improved remarkably after mild (M) and moderate (O) UNSM (Ra values are 0.25 mu m and 0.7 mu m, respectively). The roughness was achieved approximately under 1 mu m for M and O types and 2 mu m for S types of operations. A 65% increase in hardness emerged by severe UNSM with a surface hardness of approximately 375-430 HV. UNSM demonstrated remarkable results on friction-wear performance by providing higher microhardness and residual compressive stress improvements with lower surface roughness.