Thermal stability and mechanical properties of fluorinated diamond-like carbon coatings

Diamond-like carbon (DLC) coatings with variable fluorine content were produced by a plasma assisted chemical vapour deposition (PACVD) technique, based on a remote plasma configuration. The feed gas consisted of mixtures of C2H2 and CF4 and the fluorine percentage in the coatings was in the range of 0–36 at.%. Competition between fluorine and hydrogen in defining the coating composition is discussed. Microindentation experiments were performed to evaluate hardness and elastic modulus, which were found to decrease as the fluorine content increases. A similar trend was shown by the internal stress of the coatings. Adhesion to the substrate was assessed by scratch tests and high critical load was achieved with moderate fluorine content (12 at.%). The counteracting effects of internal stress and interface toughness may explain this behaviour.The thermal stability of the coatings was investigated by performing annealing treatments in a vacuum furnace at 500 °C for variable times. All tested samples maintained their integrity at the end of the heat treatments, but the fluorine content and hardness decreased as the annealing time increased. Coatings with low fluorine content retained good hardness after being annealed for 57.6 ks (16 h).

► Fluorinated DLC coatings were deposited by a remote plasma PACVD technique.
► High hardness and low internal stress are achieved with low fluorine content.
► Moderate fluorine addition improves the adhesion to the substrate.
► Upon heating, fluorine is partially released and hardness decreases.
► Good hardness is retained after annealing at 500 °C for 16 h.