The effect of Si incorporation on the corrosion resistance of a-C:H:SiOx coatings

Abstract

Diamond-like carbon coatings become very popular materials mainly because of their excellent properties such as low coefficient of friction, high hardness, and good anti-wear and corrosion characteristics. In the literature more and more common reports of excellent properties of carbon coatings doped with silicon can be found. Incorporation of silicon has a beneficial effect on the anti-corrosion behavior of DLC films, mainly related to the formation of a passive layer (SiOx) at the electrode/electrolyte interface. There are also reports, where authors claim, that this beneficial effect of silicon results from a decreased level of the residual stress of the coating. Therefore, the main purpose of this work was to determine the corrosion properties of a-C:H:SiOx films characterized by different level of the residual stress and Si/O concentration ratio. The coatings with thickness of 1μm were grown onto commercial AISI 316L stainless steel substrates using RF PACVD method under negative self-bias of 400 and 800V from gas mixtures composed of CH4 and HMDSO (hexamethyldisiloxane) with different flow ratios. Prior to the investigation the surface topography and chemical structure of a-C:H:SiOx samples were determined using AFM and Raman spectroscopy, respectively. Corrosion performance was measured by the detection of corrosion potential in open circuit, determination of polarization resistance according to Stern–Geary method and the registration of potentiodynamic characteristics in wide range of anodic polarization. The changes on sample surfaces after the corrosion tests were analyzed by scanning electron microscopy. The results showed the varied nature of corrosion parameters depending on the applied negative self-bias potential. A positive effect of SiOx incorporation on the corrosion characteristics of a-C:H:SiOx coatings can be observed, however, the dependence is not linear throughout the whole range of Si/O concentrations.