Carbon layers formed on steel and Ti alloys after ion implantation of C+ at very high doses

Ion implantation is a useful technique to tailor surface properties of steel and Ti alloys. In particular, very high dose C⁺ implantation (in the range of 10¹⁸ ions cm⁻²) offers the possibility of forming carbon layers without a sharp interface with the substrate material. In this study, ion implantation of carbon doses up to 8×10¹⁸ ions cm⁻² has been performed on 440C martensitic stainless steel and Ti6Al4V substrates under similar conditions and tribological and surface analysis results have been compared. Surface hardening occurred for all ion implantation conditions up to doses of 10¹⁸ ions cm⁻²1, 2, 3. Higher doses resulted in a different behaviour for both materials. The stainless steel showed a softening while a twofold hardness increase was maintained in the Ti alloy. Nevertheless, at the higher implanted dose a decrease in hardness was also observed in the Ti alloy. Small area XPS analyses were performed to evaluate the chemical states after ion implantation and establish a relationship with the observed surface hardening. Depth profile XPS analyses showed that for a dose of 4×10¹⁸ ions cm⁻² a carbon layer (with concentration over 85% at. C) was formed in the near surface region for both materials.