Current-carrying tribological behaviour of ta-C coatings with Zr–Cu–Si single- and multi-element doping

The wear behaviour of tetrahedral amorphous carbon (ta-C) coatings under current-carrying sliding was investigated as a function of dopant chemistry, comparing single-doped (Zr, Cu, Si) and multi-element doped (ZrCuSi) systems. Sliding tests conducted at applied currents up to 1500 mA revealed pronounced differences in friction evolution, wear response, and interfacial stability, which were analyzed using an energy–based wear framework. Single-doped coatings exhibited current-sensitive behaviour, characterized by rising coefficients of friction and transitions into higher wear regimes, whereas the multi-doped ta-C:ZrCuSi coating maintained low friction and a stable wear response across the investigated energy range. Single-doped coatings formed discontinuous and unstable surface layers characterized by fragmented debris, Fe–O–rich transfer films, or direct exposure of the ta-C base layer. In contrast, sliding against ta-C:ZrCuSi led to the formation of a persistent, coating-derived tribolayer within the wear track and on the counterface, enriched in Zr–Cu–Si–O species, indicating sliding against a chemically stabilized tribolayer rather than repeated oxidation of the counterface. Raman spectroscopy identified confined sp 2 -rich carbon regions within this tribolayer and revealed vibrational modes characteristic of monoclinic ZrO 2 , consistent with the presence of a structurally stable oxide phase. These sp 2 -rich regions contribute to localized shear accommodation but, by themselves, do not account for the low friction observed. X-ray photoelectron spectroscopy confirmed that zirconium in ta-C:ZrCuSi remained in the Zr 4+ oxidation state after sliding, supporting the chemical stability of the zirconium oxide phase identified by Raman spectroscopy. Together, these results show that improved wear resistance and low friction under current-carrying sliding are associated with the formation of a persistent tribolayer that accommodates dissipated energy at the sliding interface. The combined use of energy-based wear analysis with spatially resolved chemical and bonding-state characterization provides a systematic basis for interpreting such behaviour in ta-C-based coatings, while the absence of such a stabilizing tribolayer is associated with separation of the doped surface layer from the underlying ta-C base layer.