Effect of laser radiation wavelength on the structure and functional properties of TiNi alloy during UV laser treatment

Introduction. The widespread use of TiNi-based functional alloys in medicine requires targeted management of their surface properties, such as wettability and biocompatibility. One of the promising methods for surface modification is laser treatment, especially in the UV range of the spectrum. The efficiency of UV laser treatment is due to the high photon energy, strong absorption by metals, and the shallow depth of the thermal effect zone. The purpose of this work is to investigate the effect of UV laser radiation wavelength (266 and 355 nm) on the structural and phase state, chemical composition, and wettability of the TiNi alloy surface, with the goal of subsequently controlling the material's functional properties. Materials and research methods. TiNi surface modification was performed using a pulsed Nd:YAG laser operating at wavelengths of 266 and 355 nm in ambient air. The modified surfaces were analyzed by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Microstructure, elemental composition, and phase composition were analyzed by X-ray diffraction (XRD). Wettability was estimated using the sessile drop method. The free surface energy, along with its dispersive and polar components, was then calculated from the contact angle data using the OWRK method. Results and discussion. UV laser treatment, varying parameters such as laser radiation wavelength and scanning speed, was found to induce changes in the morphology, elemental composition, phase composition of the surface layer of TiNi alloy samples, and their surface properties. Following UV laser treatment at wavelengths of 266 and 355 nm and low scanning speeds (V = 200 and 500 µm/s), single microcracks or microcrack networks resulting from thermal exposure were observed on the specimen surfaces. The oxygen content on the TiNi surface increased by a factor of 5 to 18 compared to the initial state after UV laser treatment. Furthermore, the phase composition of the TiNi alloy underwent noticeable changes, with titanium oxide phases being detected on the surface after laser exposure. The higher-energy photons (λ = 266 nm) resulted in a more pronounced change in the surface morphology and properties of TiNi compared to the 355 nm radiation under identical treating conditions. UV laser treatment significantly increased the surface hydrophilicity: the contact angle decreased from ≈75° in the initial state to ≈25° and ≈11° after treatment with 355 and 266 nm radiation wavelength, respectively. Additionally, an increase in the free surface energy of the TiNi specimens was observed, primarily due to a significant increase in the polar component.