Effect of femtosecond laser power and overlap ratio on surface roughness parameters, contact angle, and tribological properties of the textured SKD 61 tool steel with oil lubrication

This study prepared samples with femtosecond(fs)-laser-induced periodic surface structures (LIPSSs) controlled by laser power (or peak fluence, F₀) and overlap ratio (OR) to improve the friction and wear behavior of the SKD 61 tool steel sliding against a hard Si₃N₄ ceramic counterface in oil lubrications. Pin-on-plate (pin: Si₃N₄; plate: SKD 61 steel) tribological tests were conducted with the reciprocating motion perpendicular to the laser scanning direction and having an incline angle (40°∼50°) with respect to the LIPSSs. The contact angles (θ) formed on the textured surfaces were measured for an oil lubricant. The (θ)_oil results were found to have their correlation with the areal surface roughness (S_a), skewness (Sk), and kurtosis (Ku) of the textured surfaces produced at various F₀ and OR values. The mean friction coefficient (μ) and wear loss (W) of the steel specimens in oil lubrications are reduced by decreasing contact angle, (θ)_oil. The combined effect of F₀ and OR on the (θ)_oil and the (θ)_oil effect on μ and W are thus linked together, and the correlation provides an efficient and convenient way in the choices of laser operating conditions with the minimum μ and W. The minimum values of μ (=0.0089) and W (=2.124 × 10⁴ μm³) were obtained with OR = 50% and a laser power of 17 mW.