Comparative Study of Titanium-Doped and Titanium–Silver Co-Doped Diamond-Like Carbon Films

Hydrogen-free diamond-like carbon (DLC) films were deposited by magnetron sputtering and doped with titanium (Ti-DLC) and codoped with titanium and silver (Ti/Ag-DLC, 80/20 at. % TiAg target). Ti loadings of 0.3–1.8 at. % produced only modest roughness changes (R_q ≈ 1.8–2.3 nm) and a slight increase in I_D/I_G and sp²/sp³ ratios, though the D-band down-shifted markedly. Ti/Ag codoped DLC films contained 1.0–6.9 at. % total metal, while the surface was enriched in Ag according to X-ray photoelectron spectroscopy. Except for the highest doped film, Ti/Ag-DLC showed lower graphitization than the Ti-DLC films prepared under identical conditions. R_q increased to 3.9 nm for the Ti/Ag-DLC films, reaching the highest value at the lowest Ti/Ag content. The presence of Ag also diminished surface oxidation and reduced oxygen concentration at low doping levels. Ti doping and Ti/Ag codoping of DLC films reduced the coefficient of friction by up to 2-fold when normal loads of 1–10 nN were used. Nanoindentation tests revealed that both Ti-DLC and Ti/Ag-DLC films show their greatest hardness loss at the lowest dopant concentrations. Water contact angles for Ti-DLC films changed nonmonotonically but became slightly less hydrophilic (∼69°) compared to undoped DLC. Ti/Ag-DLC films first became more wettable (59.9°) and then recovered to 68.5° as metal content increased. OWRK calculations showed a dopant-induced decline in total surface free energy, which was driven by reductions in dispersive components. Collectively, these data indicate that the Ti/Ag codoping offers a tunable balance of hardness, roughness, and wettability, combining the benefits of Ti with the advantages of Ag when applied at low to moderate concentrations.