Optimisation of carbon implantation pre-treatments on the adhesion strength of amorphous carbon coatings on AISI 440C steel substrates

The effect of carbon ion implantation pre-treatment conditions (implantation energy and substrate etched depth) on the adhesion strength of amorphous carbon coatings deposited onto hardened AISI 440C steel substrates was investigated. Based on the scratch test and the X-ray photoelectron spectroscopy analysis, the most influential factor on the adhesion strength was found to be the carbon concentration on the outmost surface of the implanted steel substrates. A carbon concentration of 20–30 at.% led to the highest carbide concentration and thus the highest adhesion strength between the coatings and the substrates. By optimising the implantation energy and the etched depth to expose the highest carbide bonds on the steel substrates, well-adherent carbon coatings of different thickness (0.5–2.0 μm) could be obtained. The carbon coatings possessed a high scratch resistance with a critical load over 70 N even at a coating thickness of 2.3 μm. In addition, the coatings also exhibited a good tribological performance in the ball-on-disk wear test with friction coefficients below 0.1, which was maintained constantly over 1×106 rotation cycles (a sliding distance of 18.8 km) of unlubricated sliding against a WC–6%Co ball.