Adhesion and Chemical Composition of Carbon Coatings Produced in Low-Temperature RFCVD Processes on Substrates of 35CrMnSi5-5-4 Steel with Ultra-Fine Grain Bainitic and Martensitic Structure

The objective of this study is to investigate the feasibility of producing a carbon coating through the Radio Frequency Chemical Vapor Deposition (RFCVD) process on steel substrates that have undergone isothermal and martensitic hardening, without significantly altering their structure, which opens the possibility of applying such coatings to ultra-fine grained bainitic substrates. Coating procedures were carried out within a temperature range of 50–250 °C. Dilatometric measurements confirmed the stability of the ultra-fine grain bainitic structure, whereas the martensitic structure exhibited marginal tempering within this temperature range of coating deposition. Results indicate that the coatings’ nanohardness increases from about 10 GPa in temperature of 50 °C to 12 GPa in case of the ultra-fine grain bainitic substrate, and 14 GPa in case of the martensitic substrate in temperature of 230 °C. Complete delamination occurs at approximately 9N for the martensitic substrate and 6N for the ultra-fine grain bainitic substrate. As the RFCVD process temperature increases from 50 °C to 250 °C, the delamination threshold decreases to 0N, resulting in spontaneous delamination upon the first contact with the indenter. Analysis using XPS and Raman spectroscopy revealed that the carbon coatings predominantly possess an amorphous structure, comprising approximately 25–37 % sp3-type bonds, which showed an increase with the rise in RFCVD process temperature.