Subsurface Dynamic Deformation and Nano-structural Evolution in 40Cr Steel Under Dry Sliding Wear

Dry sliding tribological properties of normalized 40Cr steel pins against quenched and tempered GCr15 discs were investigated under various sliding distances through disc-on-pin contact configuration. Microstructural feature evolution and strain-hardening behavior of worn subsurface in 40Cr pins were systematically analyzed using scanning electron microscopy and transmission electron microscopy. It is found that based on the variation of wear rate, three stages can be divided throughout the friction process: initial wear, slow wear and stable wear. The corresponding microstructures beneath contact surface are subjected to significant strain-hardening effect and structural rearrangement. Nano-grains of 40–100 nm are produced in the topmost subsurface layer and the nanocrystallization mechanism during friction process is further elucidated. Pronounced vortex structures are generated in the localized subsurface zones due to high strain localization and shear instability. With the prolongation of sliding distances, vortex structures suffer dynamic embrittlement and exfoliation from the wear surface as a result of periodically excessive hardening effect, leading to the high wear rate in stable wear stage.