Characterisation of ultra-thin DLC coatings by SEM/EDX, AFM and electrochemical techniques

Characterisation of DLC coatings ion beam-deposited on to AlTiC substrate (70 wt% Al2O3 + 30 wt% TiC) has been carried out. AFM and SEM analysis showed that the island-like topography of DLC-coated AlTiC substrates is very similar to that of uncoated substrate, implying the full coverage of coatings without visible defects. Moreover, the island height was found to increase on average either with film thickness or deposition voltage due to the possible preferential growth of DLC on TiC and the possible preferential sputter etching of Al2O3 by energetic ions at the initial film growth stage when deposited at increased voltages. Electrochemical measurements showed that coating with relatively thick DLC films (> 100 nm) at an ion beam deposition voltage of 200 V can enhance significantly the impedance of sample/solution interface so as to increase the corrosion potential of the substrate. For a given film thickness, however, the impedance as well as the corrosion potential increase with deposition voltage up to 200 V and decrease thereafter. This is attributed mainly to the kinetic energy-dependent microstructure of both DLC and the coating/substrate interface. Consequently, deposition of DLC under proper conditions can improve dramatically the corrosion resistance of AlTiC when used in a corrosive environment. (C) 1998 Elsevier Science S.A.