Properties of Zr–ZrC–ZrC/DLC gradient films on TiNi alloy by the PIIID technique combined with PECVD

The Zr–ZrC–ZrC/DLC gradient composite films were prepared on TiNi alloy by the techniques combined plasma immersion ion implantation and deposition (PIIID) and plasma enhanced chemical vapor deposition (PECVD). With this method, the Zr–ZrC intermixed layers can be obtained by the ion implantation and deposition before the deposition of the ZrC/DLC composite film. In our study, an optimal gradient composite film has been deposited on the NiTi alloys by optimizing the process parameters for implantation and deposition. The surface topography was observed through AFM and the influence of the deposition voltage on the surface topography of the film was investigated. XPS results indicate that on the outmost layer, the Zr ions are mixed with the DLC film and form ZrC phase, the binding energy of C 1s and the composition concentration of ZrC depend heavily on the bias voltage. With the increase of bias voltage, the content of ZrC and the ratio of sp3/sp2 firstly increases, reaching a maximum value at 200 V, and then decreases. The nano-indentation and friction experiments indicate that the gradient composite film at 200 V has a higher hardness and lower friction coefficient compared with that of the bare NiTi alloy. The microscratch curve tests indicate that gradient composite films have an excellent bonding property comparing to undoped DLC film. Based on the electrochemical measurement and ion releasing tests, we have found that the gradient composite films exhibit better corrosion resistance property and higher depression ability for the Ni ion releasing from the NiTi substrate in the Hank's solution at 37°C.