Load Dependence of Nanohardness in Nitrogen Ion Implanted Ti6Al4V Alloy and Fractal Characterization

Three different nitrogen ion doses were implanted into a Ti6Al4V alloy to improve its mechanical surface properties for the application of artificial joints. The titanium nitride phase and nitrogen element distribution profile were characterized with X-ray photoelectron spectroscopy (XPS). Nano-indentation tests were carried out on the surface of the Ti6Al4V alloy and implanted samples on a large scale of applied loads. The XPS analysis results indicate that nitrogen diffuses into the titanium alloy and forms a hard TiN layer on the Ti6Al4V alloy. The nanohardness results reveal that nitrogen ion implantation effectively enhances the surface hardness of Ti6Al4V. In addition, the nanohardness clearly reveals load dependence over a large segment of the applied loads. Thus a concept of nanohardness fractal dimension is first proposed and the dual fractal model can effectively describe nonlinear deformation in indentation areas on the Ti6Al4V surface. The fractal dimension shows a decreased trend in two regions of applied loads, indicating a decrease of the self-similarity complexity in surface indentation owing to an increase in nanohardness after nitrogen ion implantation.