Effect and mechanism of M2C carbides on sliding friction and wear behavior of M50 bearing steel

M50 steel is extensively used in aero-engine bearings due to its exceptional fatigue and wear resistance. M2C carbides are the primary hard phase in M50 bearing steel, significantly influencing its wear performance. This study systematically investigates the effect of M2C carbide content, size, aspect ratio, and distribution on the dry sliding friction and wear behavior of M50 steel by fabricating two distinct ingots to specifically explore the influence of carbide size. Dry sliding wear tests revealed that an increase in carbide size and distribution inhomogeneity resulted in higher friction coefficients and wear rates. Specifically, under a 10 N load, the friction coefficient of M50-1000 (0.75) was 13% higher, and its wear rate (1.52 μm3/(N·mm)) was 46% greater than that of M50-50. Multi-scale characterization showed that both the friction coefficient and wear rate increase with the size and uneven distribution of M2C carbides. Under dry sliding conditions, the presence of unusually large M2C carbides induces severe stress concentration due to the extreme hardness mismatch with the matrix (27.5 GPa vs. 15.6 GPa), leading to interfacial debonding, brittle fracture, and spalling. The resulting hard fragments trigger three-body abrasive wear, exacerbating plowing groove formation and accelerating material removal. This work provides crucial insights into the ''size-morphology-fracture-wear'' relationship of primary M2C carbides and emphasizes effective carbide control strategies to enhance the service life of M50 bearing steel in industrial applications.