Surface modifications of TiN coating by pulsed TEA CO 2 and XeCl lasers

Abstract

Interactions of a transversely excited atmospheric (TEA) CO 2 laser and an excimer XeCl laser, pulse durations ∼2 μs (initial spike FWHM ∼100 ns) and ∼20 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steel AISI 316, were studied. Titanium nitride was surface modified by the laser beams, with an energy density of 20.0 J/cm 2 (TEA CO 2 laser) and 2.4 J/cm 2 (XeCl laser), respectively. The energy absorbed from the CO 2 laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer XeCl laser primarily leads to fast and intense target evaporation. The calculated maximum temperatures on the target surface were 3770 and 6300 K for the TEA CO 2 and XeCl lasers, respectively. It is assumed that the TEA CO 2 laser affects the target deeper, for a longer time than the XeCl laser. The effects of the XeCl laser are confined to a localized area, near target surface, within a short time period. Morphological modifications of the titanium nitride surface can be summarized as follows: (i) both lasers produced ablation of the TiN coating in the central zone of the irradiated area and creation of grainy structure with near homogeneous distribution; (ii) a hydrodynamic feature, like resolidified droplets of the material, appeared in the surrounding peripheral zone; (iii) the process of irradiation, in both cases, was accompanied by appearance of plasma in front of the target. Target color modifications upon laser irradiation indicate possible chemical changes, possibly oxidation.