Adherent amorphous hydrogenated carbon coatings on steel surfaces deposited by enhanced asymmetrical bipolar pulsed-DC PECVD method and hexane as precursor

• a-C:H can be deposited with good adhesion on steels by bipolar pulsed-DC PECVD.
• Using a Si interlayer adhesion increases considerably.
• Employing hexane as precursor gas allowed obtaining high deposition rates.
• The mechanical and tribological properties of the a-C:H coatings are very promising.

In order to overcome the high residual stress and low adherence of amorphous hydrogenated carbon (a-C:H) films on steel substrates, a thin amorphous silicon interlayer was deposited as an interface between the substrate and a-C:H films produced by using hexane vapor as a precursor gas. Amorphous silicon interlayer and a-C:H films were grown by employing a modified and asymmetrical bipolar pulsed-DC plasma enhanced chemical vapor deposition (PECVD) system, using silane and hexane atmospheres, respectively. The a-C:H films were analyzed according to their microstructure, and mechanical and tribological properties as a function of self-bias voltage. The chemical composition and hydrogen content of the a-C:H films were estimated by means of Raman scattering spectroscopy. The total stress was evaluated through the measurement of the substrate curvature, using a profilometer, while nanoindentation experiments helped determine the films' hardness. The friction coefficient and critical load were determined by using a tribometer. The corrosion resistance was evaluated by electrochemical potentiodynamic polarization techniques on a 3% solution of sodium chloride (NaCl). The results showed that the use of the amorphous silicon interlayer, deposited by low energy ion implantation, improved the a-C:H film deposition onto steel substrates, producing good adhesion, low compressive stress, and a high hardness. The composition, the microstructure, and the mechanical and tribological properties of the films were strongly dependent on the self-bias voltages. These results suggest that a combination of a modified pulsed-DC PECVD system and hexane as a precursor gas for growing a-C:H films may represent a good and new alternative for coating scaling for mechanical and tribological applications.