Influence of laser-induced discrete hardening units on gear surface fatigue resistance Ⅱ: Distribution-dependent fatigue damage and contact behavior via equivalent cylindrical rolling contact

The discrete laser spot-matrix hardening (DLSH) process is an innovative eco-friendly surface strengthening technique that enhances the surface strength of materials while preserving their inherent toughness. This unique configuration effectively reduces maximum contact stress while mitigating tooth surface fatigue damage through stress redistribution mechanisms ‎[1,2]. However, due to the intricate interface contact dynamics during meshing, it is necessary to elucidate the influence of hard-phase distribution parameters on fatigue damage progression and contact stress redistribution. Therefore, an equivalent rolling disk was employed to investigate the influence of hardened unit distribution patterns on rolling contact fatigue (RCF) behavior at the pitch circle position through coupled experimental and finite element analysis. The result showed that the rolling contact wear volumes were reduced by 41.9 % and 55.4 % for the "large-sparse" and "small-dense" hardened unit distributions, respectively, compared to the untreated samples, which significantly enhanced the contact fatigue wear resistance of the cylindrical surfaces. It was also observed that the hardened units exerted a distinct spatial blocking effect on interfacial plastic flow, with the plastic flow layer depths being reduced by over 60 % and 83 % for the two configurations, respectively. The originally uniform contact stress distribution was discretized, leading to the formation of localized stress concentrations within the hardened unit regions while the contact stress in the soft matrix was reduced, inducing a periodically fluctuating and locally concentrated contact stress field. During different stages of rolling contact, a "pinning effect" was induced, which simultaneously protected the substrate material and impeded crack propagation.