A study of microstructural and electrochemical properties of ultra-thin DLC coatings on AlTiC substrates deposited using the ion beam technique

Microstructural and electrochemical characterization of diamond like carbon (DLC) ion beam-deposited on AlTiC (70 wt% Al2O3 + 30 wt% TiC) substrate has been carried out. Tapping mode atomic force microscopy (AFM) imaging showed that the island-like topography of DLC-coated substrates is similar to the un-coated one, indicating the uniform coverage of DLC without visible pinholes. Confocal micro-Raman analysis demonstrated that the total Raman intensity, as well as the I-D/I-G ratio, increases with the coating thickness. Electrochemical impedance spectra showed that with the increasing DLC coating thickness, a transition from one-time constant response to two-time constant response occurred when the coating thickness equals 5 nm (IS2), indicating the existence of micro-defects in the coatings which are invisible for AFM. More detailed analysis using the equivalent circuit model revealed that the charge transfer resistance (R-ct) at electrolyte/substrate interface and the resistance (R-p) related to DLC coatings increase significantly with the coating thickness, while the double-layer capacitance (C-dl) and the capacitance (C-co) of DLC coatings decrease dramatically. All these phenomena can be interpreted in terms of the evolution of the subsurface diamondlike phase (sp(3)-bond) and the reduction of micro-defects in the DLC coatings with the growing film. As a result, an increase in the corrosion potential (E-c) with the DLC coating thickness was also detected using the Tafel technique. In consequence, the DLC coatings can improve significantly the anti-corrosion properties of AlTiC substrates when the coating thickness is more than a few tens of nanometres. (C) 1999 Elsevier Science S.A. All rights reserved.