This work presents the structural evolution of nanocomposite CrCx/a-C:H coatings prepared by unbalanced magnetron sputtering of a metallic Cr target in Ar+CH4 glow discharges using low negative dc bias voltages. Raman spectroscopy and x-ray photoelectron spectroscopy were used to characterize the phase composition and the chemical bonding in the films deposited at different experimental conditions. The results were correlated to the chemical composition obtained by elastic recoil detection analysis. The coating microstructure was investigated on selected samples by high-resolution transmission electron microscopy combined with electron energy-loss spectroscopy analysis. The nanocomposite coatings can be divided into hard CrCx dominated films, when prepared at low CH4 partial pressure to total pressure (p(t)) ratios (p(CH4)/p(t)< 0.4), and into low-friction a-C:H dominated films, when prepared at p(CH4)/p(t) > 0.4. The structure of the low-friction a-C: H dominated coatings consists of 2 - 10 nm sized fee CrC crystallites embedded in a Cr containing a-C: H matrix. (c) 2006 American Vacuum Society.