Femtosecond Laser-Induced Micro/Nano Structures on GCr15 Bearing Steel Surfaces and Their Friction-Reducing and Anti-Wear Performance

To improve the lubricating and friction-reducing performance of GCr15 bearing steel surfaces, a femtosecond laser was used to fabricate hierarchical micro-/nano-composite structures. The effects of these structures on surface morphology, hardness, oleophilicity, and tribological performance were investigated. The results show that after femtosecond laser treatment, the sample surfaces exhibit micrometer–scale papillary structures, groove structures, nano-scale ripples with characteristics of laser-induced periodic surface structures (LIPSS), while surface roughness increases significantly. As the laser power increases from 1.15 W to 1.38 W, the surface micro-/nano-structures develop more fully, and the hardness increases by approximately 4.4 % (F1) and 8.3 % (F2) compared with the untreated sample. Static oil contact angle measurements reveal that the laser-treated surfaces show significantly enhanced oleophilicity; the F2 sample exhibits an apparent oil contact angle of 3.9° at 10 s after droplet deposition under the present measurement conditions. The reduction in apparent oil contact angle is attributed to the synergistic effects of hierarchical surface morphology, multi-scale capillary effects, and possible modifications of surface chemical composition, rather than a strict equilibrium wetting state. Friction and wear tests demonstrate that the femtosecond laser-textured surfaces exhibit improved friction-reduction and wear-resistance performance under both dry friction and oil-lubricated conditions. Compared with the untreated sample, under dry friction the friction coefficient and wear rate are reduced by up to 23.0 % and 64.1 %, respectively; under oil lubrication, the corresponding reductions are up to 52.6 % and 66.3 %. Analysis of the friction-reduction and anti-wear mechanisms indicates that under dry friction, the micro-/nano-composite structures contribute to reducing abrasive ploughing and adhesive peeling by decreasing the effective contact area, accommodating wear debris, and benefiting from the increased surface hardness, thereby alleviating wear. Under oil lubrication, the enhanced oleophilic behaviour and hierarchical surface structures facilitate oil spreading, infiltration, and retention within textured regions, contributing to improved lubrication performance and reduced friction coefficient and wear rate. This study provides a feasible processing strategy and theoretical reference for femtosecond laser-based functional modification of GCr15 bearing steel surfaces.