Laser Shock Peening of Gear Steels and Related Metallic Materials: Near-Surface States, Surface Integrity, and...

Contact fatigue, bending fatigue, and wear failures in gears are governed by the stress state, hardening gradient, microstructural stability, and surface topography within the near-surface and subsurface regions. For tooth flanks, rolling-sliding contact, asperity interaction, lubricant-film disturbance, and subsurface shear stress control micropitting, pitting, and spalling. For tooth-root fillets, local stress concentration and surface or near-surface defects dominate bending-fatigue crack initiation. Laser shock peening (LSP) introduces deep compressive residual stress, gradient hardening, and microstructural refinement, and is therefore relevant to gears when these effects are matched to the critical damage zones. This review examines LSP of gear steels and related load-bearing steels from the viewpoint of tooth-flank and tooth-root damage control. It links laser parameters, shock-induced plastic deformation, residual-stress depth, hardening response, surface roughness, and profile accuracy to bending fatigue, rolling contact fatigue, and wear behavior. LSP is most effective when the compressive residual-stress layer and hardened layer reach the contact- or bending-damage depth while lubrication compatibility, flank form accuracy, and subsequent finishing are preserved. Excessive pulse energy, overlap, or unstable absorbing/confining conditions may increase roughness, produce ablation or micropitting-sensitive defects, and compromise tooth profile accuracy. Thus, LSP for gears should be evaluated together with carburizing, nitriding, shot peening, surface rolling, polishing, coatings, and laser texturing rather than as an isolated treatment. The central task is to define gear-specific process windows that balance residual-stress depth, surface integrity, dimensional accuracy, and manufacturing repeatability.