Laser surface micro-texturing to enhance the frictional behavior of lubricated steel

Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided.