Surface modifications of TiN coating by the pulsed TEA CO2 and KrCl laser

Interaction of a transversely excited atmospheric (TEA) CO2 and excimer-KrCl laser, pulse duration ∼2 μs (FWHM of initial spike ∼100 ns) and ∼12 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steels, was studied. The results have shown that the titanium nitride was surface modified by the laser beam, with an energy density (ED) of 43.0 J/cm2 (TEA CO2 laser system) and 3.3 J/cm2 (KrCl system), respectively. The energy absorbed from the CO2 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-KrCl laser primarily leads to a fast and intense target evaporation. Calculated maximal temperatures, on the target surface, were 3940 and 8000 K for the TEA CO2 and the KrCl lasers, respectively. It is assumed that TEA CO2 laser affects the target deeper, during a longer time than the KrCl laser. The effects of the KrCl laser are confined to a localized area, near target surface, in the course of short time. The following morphological changes of titanium nitride surface were observed: (i) both types of laser caused ablation of the TiN coating in the central zone of the irradiated area, (ii) appearance of a hydrodynamic feature like resolidified droplets of material in the surrounding outer zone (cracking) in the case of the TEA CO2 laser, and (iii) appearance of wave-like microstructures (consisting of periodic parallel fringes with a period of about 20 microns) in the case of KrCl laser. Formation of the wave-like microstructures is discussed in the context of laser-induced periodic surface structure (LIPSS) effects and diffraction phenomena. The process of the CO2 and KrCl laser interaction with titanium nitride were typically accompanied by plasma formation. Plasma appeared above the target after about 40 and 1 laser pulse, respectively