Chromium oxynitride thin films were deposited by radio-frequency (RF) reactive unbalanced magnetron sputtering at various O(2) flow rates onto Si(100) and glassy carbon substrates. The compositions of the thin films were analyzed by Rutherford backscattering spectroscopy. The thin films were found to contain up to 44 at.% oxygen. In Fourier-transform infrared spectra, a peak attributed to the Cr-N bond of Cr-N was observed, but no peak attributable to the Cr-O bond of Cr(2)O(3) was found. The textures of the thin films were observed by transmission electron microscopy, which revealed that samples had a columnar structure. The hardness of the thin films was measured by nanoindentation. The hardness increased from 20 GPa to a maximum value of 31 GPa with increasing oxygen content. (C) 2011 Elsevier B.V. All rights reserved.