Carbide-mediated martensite nanostructuring enabling tribological performance improvement under laser shock peening

Laser shock peening (LSP) is widely utilized to improve the near-surface strength and wear resistance of martensitic steels. However, how hard carbides leverage their mechanical heterogeneity with the martensitic matrix to respond to extreme shock loading, dissipate energy, and participate in shock-induced deformation remains unclear; In particular, how carbide damage and interfacial accommodation under different shock energies mediate martensitic evolution and govern the tribological response remains insufficiently understood. This study takes M50 bearing steel as a representative carbide-bearing martensitic steel and investigates the effects of LSP pulse energy (30, 50, 70, and 90 mJ) on near-surface microstructural evolution and tribological performance, establishing an energy-dependent processing-structure-property relationship. The results show that LSP activates the carbide–matrix mechanical mismatch under high-strain-rate loading. Carbides act as heterogeneous deformation centers, where plastic accommodation driven by interfacial mismatch and strain gradients promotes dislocation rearrangement, subgrain boundary formation, and martensitic nanostructuring. Meanwhile, short-range redistribution of carbon (C) and chromium (Cr) at the interfaces contributes to the stabilization of refined structures. With increasing LSP energy, the microstructural strengthening and tribological responses exhibit a non-monotonic trend: At low energies, carbide-mediated refinement and residual-stress retention are limited, whereas excessive energy induces severe carbide cracking and interfacial damage, interrupting strain-accommodation pathways and weakening near-surface strengthening. The optimal condition is achieved at 70 mJ, where the resulting compressive residual stress field, with a surface value of −547.00 MPa and retention to a depth of 200 μm, together with the nanostructured near-surface layer, reduces the average coefficient of friction (COF) by 35.77% and wear loss by 83.72%. The wear mechanism changes from severe abrasive wear to a mild wear mode dominated by oxidative wear with slight adhesion. These findings indicate that LSP parameter design for carbide-bearing martensitic steels should balance residual-stress strengthening, carbide-assisted martensitic nanostructuring, and controlled carbide damage, rather than simply pursuing higher surface hardness or higher laser energy input.