EFFECT OF C₂H₂ FLOW RATE ON THE DEPOSITION OF TI-TIC-TIC/DLC GRADIENT NANO-COMPOSITE FILM ON NITI ALLOY

Ti-TiC-TiC/DLC gradient nano-composite films have been prepared on the NiTi substrates by the technique of plasma immersion ion implantation and deposition (PIIID) combining with plasma-enhanced chemical vapor deposition (PECVD). The influence of C₂H₂ flow rate ranging from 30sccm to 50sccm on the chemical structure, microstructure, mechanical properties and corrosion resistance of resulting thin films are investigated by XPS, XRD, friction coefficient test, nano-indentation, electrochemical corrosion test and atomic absorption spectrometry. XPS and XRD results indicate that on the outmost layer, the Ti ions are mixed with the DLC film and form TiC phase, the binding energy of C 1s and the composition concentration of TiC depend heavily on the C₂H₂ flow rate. With the increase of C₂H₂ flow rate, the content of TiC and the ratio of carbon sp³/sp² decreases. The nano-indentation and friction experiments indicate that the gradient composite film at 30 sccm has a higher hardness and lower friction coefficient compared with that of the bare TiNi alloy. The microscratch curve tests indicate that Ti-TiC-TiC/DLC gradient composite films have an excellent bonding property with the substrate. Based on the electrochemical measurement and ion releasing tests, we have found that the Ti-TiC-TiC/DLC 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.