Optical characterization of laser coloured titanium under different processing atmospheres

In this work, laser colouring of commercially pure Ti (grade II) samples under three different gas atmospheres is shown and analysed. The laser colouring process was carried out by means of a ns-pulsed Yb-fiber laser operating at a wavelength of 1064 nm with a pulse duration of 250 ns, while the process gas composition was controlled by mixing an inert gas, i.e. argon, with oxygen in different proportions; the results are compared with the same processing in ambient air atmosphere. Colour appearance based on the interference phenomenon in a thin oxide film is explained with a simple analytical model. Hemispherical reflectance spectra of coloured specimens were measured in the visible wavelength range, and, from them, chromaticity values were calculated. The results showed that the employed gas mixtures increased the oxide layer thickness, allowing for a more rapid colour transformation. The annulled wavelength, expressed by the lowest reflectance wavelength, moved from near-ultraviolet region towards near-infrared with increased fluence, as expected from the interference model. Oxide thicknesses of specimens with different colours were measured to validate the analytical model, which was found to predict in a satisfactory manner the lowest reflectivity wavelengths in the visible spectrum.