Evaluation of fatigue strength of tungsten-containing DLC coating on NiTi shape memory alloy

NiTi shape memory alloy (SMA) is expected as medical devises utilizing the shape memory effect. However, release of the Ni ions from the SMA into the human body is one of the significant issues to be solved because Ni ions cause the allergetic reaction. Diamond-like carbon (DLC) coating is one of the promising materials that can prevent the release of such ions. To use DLC as coating materials onto NiTi, fatigue strength to endure cyclic bending of the SMA substrate is required. Although conventional DLC coating is very brittle and show week adhesive strength to metallic substrate, the addition of metal into a DLC matrix is one of the possibility to get good adhesiveness and bending flexibility. In this study, the fatigue strength of tungsten-containing DLC coating on SMA plate was investigated. DLC was deposited on a SMA plate by radio-frequency chemical vapor deposition (RFCVD) and also co-sputtering of metal target. The fatigue strength of DLC has been characterized by applying 1% cyclic bending strain. The surfaces of the coatings were observed after bending test using a scanning electron microscope (SEM), a optical microscope and an atomic force microscope (AFM). In the result, no cracks and interface delamination were obsearved up to 10⁵ cycles.