Medium-entropy alloys have emerged as promising candidates for advanced engineering applications, yet the underlying friction mechanisms of NiCoFe-based MEAs remain unclear. In this study, molecular dynamics simulations were employed to systematically investigate the tribological properties and deformation mechanisms of single-crystal and polycrystalline Ni37Co30Fe33 MEAs. The effects of penetration depth and sliding velocity were thoroughly examined. The results indicate that the polycrystalline alloy exhibits a higher friction coefficient than the single crystal due to the obstruction effect of grain boundaries on dislocation motion. In the single crystal, plastic deformation is dominated by the generation of stacking faults, Lomer–Cottrell locks, and prismatic dislocation loops (PDLs), with high shear strain concentrating near these defects. For the polycrystal, dislocation propagation is significantly hindered at grain boundaries, leading to stress concentration and a notable inhibitory effect on plastic deformation. Increasing the penetration depth enhances the frictional force, normal force, and friction coefficient, primarily attributed to the intensified plastic deformation, increased number of HCP-structured atoms, and elevated dislocation density, which trigger a significant rise in local deformation resistance. Conversely, the frictional force shows negligible sensitivity to sliding velocity, while the normal force increases slightly, resulting in a decreasing trend in the friction coefficient. Higher sliding velocities promote the nucleation of PDLs but reduce the total dislocation density, causing atomic pile-up to localize near the scratch track. This study reveals, from the atomic scale, the evolution laws of the friction performance of the Ni37Co30Fe33 medium-entropy alloy under different friction depths and sliding velocities, and provides insights at the microscopic scale.
