Influence of the substrate pre-treatment on the mechanical and corrosion response of multilayer DLC coatings

This work evaluates the mechanical, tribological and corrosion response of single-layer and multilayer DLC films deposited on top of martensitic stainless steel, which was previously plasma nitrided. The efficiency of plasma nitriding as pre-treatment was evaluated using a control group of samples, which was only coated under the same conditions. The pre-treated samples showed a better adhesion of both single and multi-layer DLC films, showing no signs of spallation after applying up to 50 N load during the scratch test. Fretting wear tests (ball on disk) with applied loads 40-70 N were used to evaluate the mechanical response of the different systems. The best response to fretting wear was given by the coatings deposited onto the pre-treated stainless steel substrate, showing a good mechanical stability without cracking up to 70 N load. Additionally, the corrosion resistance evaluated using step-increasing bias potentiostatic tests, turned out to be acceptable when using plasma nitriding as a pre-treatment. On the contrary, the DLC coatings on top of non-pre-treated substrates failed at a potential 400 mV lower than the 1DLC or 5DLC + N samples. Finally, it is seen that combining nitriding with DLC in the form of multilayers constituted the best protective system, since it can delay the formation of pathways between the corrosive solution and the substrate, until 2000 V bias and 3 hours duration of the experiment.