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

## TL;DR

This study compares titanium-doped and titanium-silver co-doped diamond-like carbon films, showing how co-doping affects surface properties like hardness, roughness, and wettability.

## Contribution

The novelty lies in demonstrating how Ti/Ag co-doping improves DLC film properties over Ti-doping alone, especially at low to moderate concentrations.

## Key findings

- Ti/Ag-DLC films showed lower graphitization and reduced surface oxidation compared to Ti-DLC films.
- Ti/Ag-DLC films reduced the coefficient of friction by up to 2-fold under low normal loads.
- Co-doping with Ti and Ag allowed tunable control over hardness, roughness, and wettability.

## Abstract

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.