Raman spectroscopy study of C/Cr coatings deposited by the combined steered cathodic ARC/unbalanced magnetron sputtering technique

C/Cr coatings were prepared by the combined steered cathodic arc/unbalanced magnetron sputtering technique with a wide range of bias voltages, Ub from −65 to −350 V. The paper focuses on the microstructure evolution and phase transformations in the coatings and their influence on the tribological performance of C/Cr coatings as a function of the bias voltage. Raman spectroscopy on the surface of the as deposited coatings and in the wear track generated in pin-on-disc tests has been carried out in conjunction with TEM and SEM studies of coatings microstructure and XPS analysis of the chemical bonding in the films. The Raman spectra from the surface of the C/Cr coatings consists of two main Raman features on a strong photoluminescence background: broad bands around ∼1380 cm−1 and ∼1572 cm−1, which are designated to D (disorder) and G (graphitic) peaks, respectively. The broad D and G bands further confirm the TEM-selected area diffraction (SAD) analysis, which shows an amorphous character of the coating. The downshift of the G-band by ∼2–10 cm−1 from the planar graphite (1582 cm−1) is ascribed as the shell curvature of the carbon plane, which could be due to the presence of Cr. We assume the D-band position has a close dependence on the stress in the coating, which results in shifting of the D-band to higher wave number as the stress in the coatings increases. The Raman spectra show better resolution of the D and G peaks for samples deposited at low bias voltage (Ub = −65 to Ub = −75 V). However, these peaks become more diffused and difficult to resolve, with a much lower intensity as the applied bias voltage increases and completely disappears at Ub = −350 V, which suggests formation of metal carbides in the coating. This interpretation was further confirmed by the XRD, TEM and SAD pattern observations. The ID/IG ratio correlates well with the friction behaviour of the films. The friction coefficient decreases and reaches its minimum of μ = 0.16 at Ub = −95 V due to the increase in the sp2 content in the film, which is accompanied by a maximum ID/IG ratio of 7.15.