Corrosion behaviour of TiN/a-C superhard nanocomposite coatings prepared by a reactive DC magnetron sputtering process

Nanocomposite coatings of TiNla-C were prepared on tool steel substrates using a multitarget reactive DC magnetrons puttering process at various TiN layer thicknesses (0.6-2.8 nm). The a-C layer thickness was approximately 0.45 nm. Structural characterisation of the coatings was done by X-ray diffraction (XRD). Incorporation of an a-C phase in TiN matrix reduced crystallite size of the coatings, as revealed by XRD and atomic force microscopy. XRD data showed that the nanocomposite coatings exhibited {111} texture and the average crystallite size was ca. 7.5-9.0 nm. Nanoindentation data showed that 1.5 mum thick nanocomposite coatings exhibited a maximum hardness of 5100 kg mm-1. The potentiodynamic polarisation of 1.5 mum thick coatings in 0.5 M HCl solution indicated that the nanocomposite coatings exhibited superior corrosion protection of the tool steel substrate as compared to the single layer TiN coatings of similar thicknesses. Enhancement in the corrosion behaviour of the nanocomposite coatings has been attributed to small crystallite size and dense microstructure. Potentiodynamic polarisation studies conducted on ca. 100 not thick nanocomposite coatings revealed that for a given a-C layer thickness the corrosion current decreased with a decrease in TiN layer thickness. This was supported by scanning electron microscopy (SEM) studies on the corroded samples. The SEM micrographs showed that density and diameter of the corrosion pits were smaller for nanocomposite coatings as compared to single layer TiN coatings of similar thicknesses.