Oxidation of graphite-like carbon films with different microhardness and density

Evolution of microhardness, internal stress and thickness of three types of carbon (a-C) films (#1–3) with predominantly sp2 bonds with different microhardness (H=60, 24 and 20 GPa) and density after annealing in air for 1 h at temperatures of 100, 200, 300, 350 and 400 °C was investigated. The stable behavior of these quantities up to 400 °C demonstrated only superhard films from set #1. The films from set #2 (H=24 GPa) showed the noticeable decrease in these values after annealing at 350 °C. The largest decrease of all the measured values after annealing at 350 °C showed the films from set #3. The films from series #2 and 3 fully vaporized after annealing at 400 °C. A considerable decrease has been observed not only in film thickness, but also in the density of films from set #3 after annealing at temperatures more than 300 °C, that indicates a modification of the film structure. Significant penetration of oxygen into film body without a loss of film thickness was observed in the range of 250–300 °C only for film #3. It was registered by the increase of resistivity, D band intensity in Raman spectra and sharp increase of the IR transmittance. Absorption bands at approximately 1750 and 1610 cm−1 which correspond to the vibrations of CO and CC appear in these IR spectra and simultaneously the G band in Raman spectra shifted to 1605 cm−1. The film obtained has a graphite-like structure and its composition corresponds approximately to formula CO0.15CO0.18. This structure is very unstable and very quickly vaporized starting from 350 °C.