Laser microstructuring of steel surfaces for tribological applications

Abstract

Laser technique is employed to improve substantially the tribological behavior of microstructured stainless steel surfaces. The laser-generated patterns consisting of precisely ablated microcraters were produced using a flash lamp pumped, Q-switched Nd:YAG laser. The rims of resolidified melt around the craters are typical features of laser-produced microstructures and the laser parameters were established where their extent was notably reduced. The remaining rims could easily be removed by gentle polishing. The laserstructured substrates were tribologically tested by a sliding tribometer under standard conditions. The test results show clearly that the microstructures improve the lifetime of the samples. PACS: 42.65.Cs; 79.60.DsThe microstructuring of solid surfaces modifies their friction and wear properties. At the present moment, experimental works [1,2] are focused on finding the effects of predetermined induced structures, while theoretical efforts [3,4] are beeing made to understand the mechanisms of this modification.For instance, an array of microholes induced on a friction surface can act like a lubricant reservoir and as a trap for abrasive particles -both effects improve the friction and wear properties -but the microholes can also generate hydrodynamic perturbations of the lubricating fluid -increasing the dynamic friction forces. The sizes and the distribution of the microholes are the key factors for determining which behavior will be dominant. Mainly depending on the target material and on efficiency considerations, several ways to generate such microstructures can be used, e.g. laser devices with various wavelengths, electronic guns, and chemical etching. The tool of choice for our purpose is the laser, which with its versatility can easily be adapted to a wide range of desired structure morphologies.A laser beam can either be focused on a solid surface producing removal of target material, or it can be expanded and an ablation mask used. The incident energy and the time of interaction between the laser radiation and the target surface determine [5] the phenomena that occur: a wide area of processes can be induced, from local heating to a very accurate removal of material without affecting the surrounding zones when ultrashort laser pulses are used. The use of ultrashort laser pulses in the femtosecond range was reported [6] to generate sharp microstructures in different materials; but these lasers, at the moment, lack the efficiency for the production of surface patterns consisting of millions of craters. Furthermore, they are expensive and not very suitable for an industrial environment as envisaged by a possible application resulting from this work. Use of the laser beamIn the presented work, a Q-switched Nd:YAG laser was used to induce microstructures on stainless steel samples (AISI 440C).The thermal diffusion length, L = (kτ p )/( c), is a significant parameter to describe in a first approximation the amplitude of the thermal phenomena that occur when a laser beam is focused on a solid surf...

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