METAL-SURFACE NITRIDING BY LASER-INDUCED PLASMA

We study a nitriding technique of metals by means of laser induced pla sma. The synthesized layers are composed of a nitrogen concentration g radient over several mu m depth, and are expected to be useful for tri bological applications with no adhesion problem. The nitriding method is tested on the synthesis of titanium nitride which is a well-known c ompound, obtained at present by many deposition and diffusion techniqu es. In the method of interest, a laser beam is focused on a titanium t arget in a nitrogen atmosphere, leading to the creation of a plasma ov er the metal surface. In order to understand the layer formation, it i s necessary to characterize the plasma as well as the surface that it has been in contact with, Progressive nitrogen incorporation in the ti tanium lattice and TiN synthesis are studied by characterizing samples prepared with increasing laser shot number (100-4000). The role of th e laser wavelength is also inspected by comparing layers obtained with two kinds of pulsed lasers: a transversal-excited-atmospheric-pressur e-CO2 laser (lambda = 10.6 mu m) and a XeCl excimer laser (lambda=308 nn). Simulations of the target temperature rise under laser irradiatio n are performed, which evidence differences in the initial laser/mater ial interaction (material heated thickness, heating time duration, etc .) depending on the laser features (wavelength and pulse time duration ). Results from plasma characterization also point out that the plasma composition and propagation mode depend on the laser wavelength. Corr elation of these results with those obtained from layer analyses shows at first the important role played by the plasma in the nitrogen inco rporation. Its presence is necessary and allows N-2 dissociation and a better energy coupling with the target. Second, it appears that the n itrogen diffusion governs the nitriding process. The study of the meta l nitriding efficiency, depending on the laser used, allows us to expl ain the differences observed in the layer features as purity, thicknes s, and surface morphology. (C) 1996 American Institute of Physics.