Thermal stability and oxidation properties of magnetron sputtered diamond-like carbon and its nanocomposite coatings

Diamond-like carbon or amorphous carbon (a-C) and its nanocomposite nc-TiC/a-C(Al) coatings have been exposed to temperatures up to 600 °C for one hour. Hardness and oxidation resistance have been studied using nanoindentation, Raman Spectroscopy, X-ray Diffraction and X-ray Photoelectron Spectroscopy. Pure a-C becomes thermally unstable at temperatures higher than 300 °C when coating hardness drastically drops with temperature because of graphitization that causes increase of sp² and decrease of sp³ bonding structure. nc-TiC/a-C(Al) is stable up to 600 °C. Its hardness is not sensitive to temperature because the hardness mainly comes from the TiC nanocrystallites rather than sp³ bonding. As such, the coating hardness experiences only a slight drop after annealing at 600 °C. The oxidation resistance of nc-TiC/a-C(Al) nanocomposite is much better than that of the pure a-C. Even after annealing in air at 600 °C for 1 h, nc-TiC/a-C(Al) coating still maintains its original thickness (while the pure a-C loses 60% of it).