Magnetron sputtering of titanium carbonitride nanocomposite coatings - Does the choice of carbon source affect film properties?

We report on a study of titanium carbonitride coatings, with potential applications as protective coatings on bipolar plates in fuel cells. Two series of Ti-C-N coatings with a carbon concentration varying between 8 and 34 at.% were deposited by magnetron sputtering, using a graphite target or methane gas as carbon source. Characterisation with X-ray diffraction, ion beam analysis, Raman spectroscopy and electron microscopy shows that the coatings consist of a crystalline titanium carbonitride phase and an amorphous carbon tissue phase: nc-Ti(C,N)/a-C(:N). It was found that the mechanical and electrical properties are primarily dependent on the carbon content and not the choice of carbon source. An increase in C content leads to a decreasing crystallite size and an increasing amount of amorphous carbon. Thus, the phase content and microstructure and thereby the properties are controlled by the carbon content, making the nc-Ti(C,N)/a-C(:N) coatings highly tuneable. Depending on C content hardness of 11-38 GPa and resistivity of 150-623 mu ohm cm was observed. Additionally, the coatings were found to exhibit a contact resistance against silver that was 10 times lower than that of a stainless steel reference. This makes titanium carbonitride nanocomposite coatings promising candidates for the use on bipolar plates in fuel cells.