Understanding the wear behavior and mechanism of gradient nanostructured M50 bearing steel through nanoscratching tests

A gradient nanostructured M50 bearing steel with gradient structural size, carbides, and dislocation density was successfully fabricated by ultrasonic shot peening (USP) technology at room temperature. The friction behaviour and mechanism of gradient nanostructure M50 steel were investigated by using nanoscratch technology under various normal applied loads and depths. Under low normal applied load, gradient samples at ~5 μm depth exhibited a 26.5% maximum reduction in wear rate compared to the tempered sample; under high normal applied load, wear rates for samples at ~100 μm depth exhibited a maximum reduction of 44.6%. At low normal applied load, the wear mechanism involves plowing; while at high normal applied load, the wear mechanism shifts to cutting. Under low normal applied load, wear resistance correlates positively with hardness and negatively with structural size. Under high normal applied load, the increase in hardness of the martensite matrix and the partial decomposition of coarse spherical carbides enable the carbides on the surface of the USPed sample to withstand higher shear stress and stronger stress concentration, preventing cracking of the matrix and carbide edges, and spalling of carbides. High dislocation density (resulting in residual compressive stress) will slow down or inhibit the generation and expansion of cracks during the scratching process, potentially explaining why samples at ~100 μm depths exhibit superior wear resistance.