Critical States of Titanium-Doped Metal–Carbon Coatings under Sliding Friction Conditions

We have studied diamond-like carbon coatings produced by vacuum ion-plasma deposition. The coatings were doped with Ti ions via arc evaporation of a titanium cathode in parallel with laser arc evaporation of carbon from a graphite cathode. The titanium-doped diamond-like coatings were grown on 40CrNi steel substrates with a sorbite structure. The Ti content of the metal–carbon coatings thus prepared was in the range 1.5–5.5 wt %. The coating thickness was widely varied in our experiments, in the range δ = 0.5–4.5 μm. The effect of the parameter δ on tribological characteristics of the coatings was one of the key aspects of this research. The structure of the coatings was studied by high-resolution electron microscopy and their mechanical properties were studied using continuous indentation. The tribological properties of the coatings were assessed in ball-on-disk sliding friction tests on a tribometer at friction loads in the range F = 1–10 N. In analyzing the experimental data obtained, we used a computational analysis model which allowed us to construct critical state diagrams of various materials and coatings. The results demonstrate that such diagrams can be used for reliably predicting the minimum coating thickness δmin needed for preventing premature failure of coatings in tribological tests. In addition, titanium doping of diamond-like carbon coatings (in the range of Ti concentrations studied) was shown to help stabilize the coating thickness: no spontaneous cracking of the coatings was detected at thicknesses of up to δ = 4.5 μm, and their microstructure remained sufficiently uniform, without any signs of formation of secondary carbide phases.