Thin films of zirconia-carbon (ZrO2-x-C) were deposited by RF magnetron sputtering, on both polycarbonate and cemented-WC substrates. Two separate targets of monoclinic ZrO2 and graphite were used and a composition range was selected from 0 to similar to 30 at.% carbon. The C-containing films resulted in a composite structure, which consisted of a zirconia phase coexisting with an amorphous carbon phase. A tetragonal zirconia phase was found in films containing 10-30 at.% carbon, while films of pure zirconia grew in the monoclinic phase. The tetragonal phase of zirconia coincided with the presence of compressive internal stresses in the films, which increased with increasing C content. As a consequence of the microstructure induced by the presence of carbon, the ZrO2-x-C films resulted in a stronger surface hardening of the polymeric substrates than the pure zirconia films did.