Laser colouring on titanium alloys: characterisation and potential applications

Oxides of titanium exhibit vivid colours that can be generated naturally or manipulated through controlled oxidation processes. The application of a laser beam for colouring titanium permits flexible manipulation of the oxidized geometry with high spatial resolution. The laser-based procedure can be applied in an ambient atmosphere to generate long-lasting coloured marks. Today, these properties are largely exploited in artistic applications such as jewellery, eyewear frames, watch components, and, in the industrial sector, for marking logos on car body-shells. Moreover, this technique has the potential for widespread adoption in the optoelectronics industry for applications such as projectors. In order to fulfil this potential in a manufacturing environment the process must have a high degree of controllability, which can be achieved through a better understanding of its underlying mechanisms. A key area of limited understanding regarding the laser colouring of titanium alloys is the magnitude of influence on resulting colour of both composition and light interference phenomena within laser induced surface layers. In this paper, a deep investigation of the state of art and of its multiples theories has been conducted and an experimental study into the effect of laser process parameters on the colouring of commercially pure titanium has been conducted using a nanosecond pulsed Ytterbium fibre laser, operating at a wavelength of 1064 nm. Colour analysis was conducted using a dual-beam spectrophotometer whilst compositional analysis was performed by X-ray photoelectron spectroscopy. Additionally, the surface was explored by optical and scanning electronic microscope and its roughness was profiled using white light interferometry. The findings of this work provide support for the hypothesis of a single analytical model describing the underlying phenomena involved in the selective reflection of specific colours by a metal oxide; through the analysis of experimental data and analytical modelling it has been shown that interference is the dominant colouring mechanism and that it is possible to control the colour produced. The detailed investigation of metal oxide properties presented in this work has resulted in the identification of applications that can benefit from this uniquely controllable metal colouring technique