Influence of laser wavelength on the modification of friction between 100Cr6 steel and polytetrafluoroethylene by femtosecond laser-induced periodic surface structures

The authors report on surface modification by laser-induced periodic surface structures of different periodicities and modulation depths to modify dry and lubricated tribological properties. Using 220 fs laser pulses in the infrared (λ1=1030nm), visible (λ2=515nm), and ultraviolet (λ3=343nm) spectral regions, the authors periodically structure two-dimensional areas with periodicities in the range between 300 and 950 nm and modulation depths between 30 and 180 nm, respectively. The coefficient of friction is measured by performing a linear reciprocating ball-on-disk test with polytetrafluoroethylene balls on such structured 100Cr6 bearing steel. This configuration reveals a modified friction behavior using load forces between 50 and 1000 mN and a translation speed of 4 mm/s. In general, the results show an increased coefficient of friction in the presence of the applied periodic structures. In particular, in the case of dry environmental conditions, laser-induced periodic surface structures with an increasing spatial period lead to an increase in the coefficient of friction. In addition, for a sliding direction perpendicular to the alignment of the periodic structures, a higher coefficient of friction is found in comparison to a parallel movement. While in the case of lubricated friction, an increased coefficient of friction is also found; yet, it reveals a less pronounced dependency of the sliding direction as compared to dry conditions. For lubricated linear reciprocating movements, the coefficient of friction decreases under increasing load forces, which results in a load depended friction reduction.