Laser surface microstructuring to improve tribological systems

The controlled laser patterning of solid surfaces improves their wear properties: laser generated microcraters of defined dimensions and morphology can act as lubricant reservoirs and as traps for wear particles. For generating such microstructures different techniques are possible; however laser ablation has the advantage of a great versatility, since it can be adapted to produce a wide range of structures crater arrangements. We have generated microstructures with ps- and ns- pulses on TiN coated steel, uncoated steel and on uncoated steel with was subsequently coated with TiCN. Laser patterning of metals with pulses in the 100ns range is very effective however the process is governed by local melting and vaporization. The melt ejection yields the formation of resolidified droplets and rims on the target surface. This undesired artifacts can be removed by gentle polishing. Furthermore, a recast layer of increased hardness and brittleness of about 1µm thickness which homogeneously covers the crater walls is observed. Tribological tests were performed using the ball-on-disk method. The lifetime of the structured samples, defined as the sliding distance after which the friction coefficient showed an abrupt increase, was found to be significantly enhanced for all structured surfaces. The enhancement ranged from a 30% lifetime increase when TiN coatings were directly structured to an increase of over 10 times for structured hard metal which was subsequently coated with TiCN. To evaluate the possibility of circumventing some of the drawbacks of laser ablation in the ns regime, mainly the melt rims on the surface and the recast layer film on the crater walls of the single microholes the quality of microstructures produced in various metals with pulses of femtosecond duration was studied. To that end the evolution of the ablated depth over a large number of incident femtosecond laser pulses and the occurrence of structure and hardness changes in the immediate vicinity of the laser induced craters was analyzed for two sorts of steel and for hard metal. The analysis evidenced changes in the crystalline structure of the target materials in the fluence regime above 2J/cm2 but only minor alterations up to this fluence. Hardness measurements were performed on the cross-section surfaces in points situated in the immediate vicinity of the laser induced pores. Affected zones in the material surrounding few pores induced in the fs- regime were found and in such zones a significant hardness increasing was evidenced.