Microstructures and wear resistance of laser surface alloyed NiTi with high Mo concentration

Nickel Titanium alloys are a unique class of materials which exhibit shape memory and superelastic properties. The interest in this material for medical applications, such as surgical implants, has been steadily growing in recent years. In the present study, NiTi (55%Ni-45%Ti) SMA were surface alloyed with molybdenum using a continuous wave Nd-YAG laser in Nitrogen shielding gas atmosphere, aiming at improving the surface hardness and in the mean time, enhancing its corrosion resistance. By optimizing the process parameters, an alloyed surface is obtained which has 300Hv increase in hardness as compared with the as received sample. A relatively high Mo content, with average of 30% Mo, in the alloyed zone is achieved for better surface properties. The crosssections of the laser-treated specimens were examined by optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffractometry (XRD). Vickers hardness test recorded a significant improvement in the surface hardness of specimen. Polarization curves consistently showed that the corrosion resistance was enhanced after laser surface alloying with Mo as compared with the untreated NiTi substrate.