Control of laser-gas-material interactions to enhance the surface properties of NiTi for orthopaedic applications

In this study, the surfaces of martensitic NiTi alloy were modified by open-air fibre laser treatment under Ar shielding, with the aims of enhancing surface hardness and antibacterial performance for orthopaedic applications. Surface properties of laser-treated NiTi were characterized by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), optical microscopy (OM), Vickers micro-hardness, X-ray diffraction (XRD), white-light interferometry (WLI), and atomic force microscopy (AFM). Bacterial coverage on the laser-treated NiTi after cultured with Staphylococcus aureus (S. aureus) for 24 h was analysed by fluorescence microscopy. The processing parameters were categorized into two groups: lower and higher power groups of 40 W and 50 W, respectively. In each group, the laser power was kept to the same value and the stand-off distance (SD, the distance between the tip of laser nozzle and surface of substrate) was varied between three values, namely 1.5 mm, 3.0 mm and 4.5 mm. Depending on laser-gas-material interactions (via the control of laser power and SD), surface defects, e.g. porosity (in the remelted zone) and droplets (on the top surface) can arise. The findings indicated that presence of droplets is a main contributor in influencing bacterial adherence on the laser-treated surfaces. The larger and the higher the droplet size (range from 15.1 to 84.0 μm²) and concentration (range from 48.8 to 1472.3 number/mm²), the more likely bacteria adhere to the surfaces. It is the first study to report on how the surface defects of NiTi can be minimized by careful control of laser-gas-material interactions. The optimum surface conditions can be obtained with the laser power of 40 W and SD of 4.5 mm, resulting in an increased hardness from 323.2 ± 4.2 HV (BM) to 426.6 ± 19.9 HV with absence of both porosity and droplets, as well as decreased bacterial adherence from 0.79 ± 0.3% (BM) to 0.12 ± 0.04%.