Laser nitriding of metals

Laser nitriding can be described as the irradiation of metal surfaces by short laser pulses in nitrogen containing atmospheres. This may lead to a strong take-up of nitrogen into the metal and nitride formation which can improve the metal’s surface properties, e.g. the hardness or the corrosion and wear resistance. Here, the laser nitriding of iron, carbon steel, stainless steel, and aluminum was investigated employing a combination of complementary methods. Ion beam analysis (Rutherford Backscattering Spectroscopy and Resonant Nuclear Reaction Analysis) was employed for element and isotope profiling. Mossbauer spectroscopy and X-ray diffraction were used for phase analysis. Surface profilometry, optical and electron microscopy revealed the surface topography and morphology obtained after laser nitriding. Microhardness measurements by the nanoindentation technique characterized the mechanical surface properties obtained by the treatment. By this combination of methods it became possible to resolve the influence of the treatment parameters (laser fluence, number of pulses, spot size, spatial intensity distribution, and gas pressure) in different materials treated (iron, carbon steels and stainless steel). It is shown that laser nitriding is a complex process, composed of several superimposed effects. Laser heating, melting and evaporation in combination with plasma formation and the generation of laser-supported absorption waves are the essentials of the process. Pressure- and plasma-enhanced dissolution and diffusion of nitrogen in combination with macroscopic material transport (piston effect, convection, fall-out) are further important effects determining the results. Additional marker experiments and laser treatments in isotopically enriched nitrogen atmospheres allowed to analyze these effects and to develop scenarios for the nitriding process and the material transport mechanisms. A simulation of the nitrogen depth profiles for the single spot irradiations was derived, whose results are in good agreement with the experimentally observed profiles.