Influence of the spatial laser intensity distribution on laser nitriding of iron

Laser nitriding of iron and other metals is governed by the complicated int erplay of the laser-plasma-solid interactions which lead to a superposition of several mechanisms. This work reports on the drastic influence of the s patial laser intensity distribution on the nitriding process. The effects o f the lateral laser intensity on the nitrogen lateral and depth profiles, t he phase formation, the surface topology, and the microhardness are reveale d by resonant nuclear reaction analysis, Mossbauer spectroscopy, surface pr ofilometry, and nanoindentation. Homogeneous laser beams lead to a strong r eduction or almost the absence of the piston mechanism, thus confining the nitriding and the transport processes to the laser spot and avoiding the fa llout. The details are discussed in relation to the results obtained for th e raw-beam irradiations. Much higher nitrogen saturation concentrations can be achieved with a homogenized beam, but the surface hardness and the hard ening depth are lower than in the case of irradiation with the raw beam. Th e epsilon nitride is only found for raw-beam irradiation. (C) 1999 American Institute of Physics. [S0021-8979(99)02213-6].