NiTi shape memory alloy with enhanced wear performance by laser selective area nitriding for orthopaedic applications

In recent years, near-equiatomic NiTi alloy has been identified as a sound alternative to replace the conventional Ti6Al4V alloy as the next generation orthopaedic biomaterial because of its lower young modulus and unique shape memory effect. The potential problem of using NiTi alloy is the generation of Ni-rich debris when wear occurs. Surface treatment is therefore needed to improve the wear resistance in order to alleviate the impact of the wear. This paper details the surface treatment of NiTi by laser selective area nitriding for enhancing the wear resistance. This was done by a systematic two-step optimization approach: (1) selecting the appropriate set of laser parameters with an L9 Taguchi experiment to optimize the nitride properties and (2) identifying the optimized surface coverage ratio to maximize the wear resistance. The microstructure and surface profiles of the optimized nitride surface was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), and 3-D profile measurement, respectively. The wear resistance of nitrided surfaces with different coverage ratios were then evaluated using reciprocating wear testing against ultra-high-molecular-weight polyethylene (UHMWPE) in simulated body fluid, i.e., Hanks' solution.