Carbon nitride (CNx) hard coatings, prepared by unbalanced dc magnetron sputtering of carbon in an argon / nitrogen atmosphere, have been studied using Auger electron spectroscopy and x-ray photoemission spectroscopy. The films were deposited on silicon substrates at about 250 degreesC and reached a saturation value of the nitrogen to carbon ratio of 0.17. This value was increased by the presence of chromium at the interface layer to the substrate., The in-depth composition of the CD, hard coatings was found to be homogeneous, even after heat treatment up to 800 degreesC in ultrahigh vacuum (LTHV). However, the UHV heat treatment resulted in diffusion processes and chemical interactions at the interface to the substrate.