Tribocorrosion behaviour of DLC-coated 316L stainless steel

Abstract

In the present work, we systematically investigate the tribocorrosion behaviour of diamond-like carbon (DLC) films on 316L stainless steel substrates in the context of their biomedical applications. Two different bond layers at the interface were particularly studied, namely the plasma nitrided layer and the plasma deposited amorphous hydrogenated silicon nitride (a-SiN x :H). Tribocorrosion tests were performed using a ball-on-flat tribometer where the sliding contact is fully immersed in NaCl 1 wt.% solution. The sample was connected to a potentiostat: it served as a working electrode and its open circuit potential (OCP) was monitored before, during, and after sliding. Electrochemical impedance spectroscopy (EIS) was applied to characterize the electrochemical behaviour of the surfaces before and after rubbing. The OCP measured during sliding was shown to depend on the properties of the protective layer; a decrease in the OCP indicates delamination of the protective layer and subsequent exposure of the substrate to the electrolyte. We found that the DLC coating with the nitrided bond layer delaminated from the wear track within 50 cycles of sliding, while it resisted the entire tribocorrosion test (1800 cycles) without failure when the a-SiN x :H bond layer was applied. The EIS results are interpreted in terms of appropriate equivalent circuits. It is shown that the a-SiN x :H bond layer significantly increases the corrosion resistance by acting as a corrosion barrier, while the DLC coating assures high wear resistance and low friction. The polarization resistance of DLC-coated 316L with the a-SiN x :H bond layer was found to be 3.76 GΩ cm 2 compared to 27.5 MΩ cm 2 for the same DLC coating without a-SiN x :H.