Laser surface alloying of NiTi shape memory alloy with Mo for hardness improvement and reduction of Ni²⁺ion release

Interests in applying NiTi shape memory alloys as metallic implant materials have grown immensely in recent years due to their unique shape memory, super-elastic and corrosion resistance properties. However, concern has been raised for its long-term applications inside the human body because of the risk of release of Ni2+ions. The sources of Ni2+ion are the free nickel elements at the surface and the results of fretting wear and corrosion of the metal systems. This paper aims at investigating the feasibility of using molybdenum to alloy with the free Ni on the surface of NiTi alloys and in the mean time improving the surface hardness of NiTi. To introduce Mo into the surface of NiTi metallurgically, laser surface alloying is one of the most feasible route. The mechanical properties of the alloyed surface were examined. A single point wear test was used to study the wear resistance of the samples. It was found that the microhardness and the wear resistance of the laser-alloyed surface were better than the untreated ones. The Ni2+ion release rate of the laser surface alloying specimen was found to be ten times lower than the untreated one. The corrosion rate of the alloyed surface was an order of magnitude lower than that of the untreated sample.