Free-electron laser surface processing of titanium in nitrogen atmosphere

Surface treatment by means of pulsed laser beams in reactive atmospheres is an attractive technique to enhance the surface features, such as corrosion, wear resistances and hardness. Among all laser types, the free-electron laser (FEL) is relatively new for materials processing, its main peculiarity being the versatile pulse structure, with high repetition rates (the so-called macropulse). We have employed the FEL at the Jefferson Lab (Virginia, USA) to irradiate pure titanium substrates in nitrogen atmosphere. The influence of various experimental parameters (macropulse duration, spot overlap and laser fluence) on the nitrogen incorporation and the resulting microstructures has been investigated. It will be shown that the laser treatment is not only a successful way to form titanium nitride, but also that a proper set of the experimental parameters can efficiently control the growth mechanism and the crystallographic texture of the resulting nitride phase, leading to the synthesis of highly oriented δ-TiN(2 0 0) layers. As it will be shown, the surface features strongly depends on the irradiation conditions: in particular a very smooth, crack-free surface can be obtained with small spot overlap and long macropulses (1000 μs), while the roughness increases with large overlap and short macropulses (<250 μs). Accordingly, the degree of crystallographic texture is higher in the samples with the smother surface. Although the nitrogen content at the surface is rather independent of the irradiation conditions (being always close to 50 at.%), the hardness profiles of the TiN coating is influenced by the laser spot overlap and consequently by nitrogen gradient.