Oxidation and tribo-oxidation of nanocomposite Cr-Si-N coatings deposited by a hybrid arc/magnetron process

CrN, Cr2N, CrN + Si (6 at.%) and Cr2N + Si (3 at.%) thin films were deposited on M2 steel by a hybrid process in which Cr was arc-evaporated while Si was magnetron sputtered. The aim is to determine in both cases the role of Si addition on functional properties: oxidation and/or wear resistance. X-ray diffraction and high-resolution transmission electron microscopy showed that the nanocomposite structure was only obtained for CrN + Si, whereas for Cr2N + Si films, silicon would be in solid solution. A large amorphization of the intergranular regions is then observed, avoiding the formation of a SiNx matrix. Hardness measurements confirmed that adding Si in CrN improved the mechanical properties, a phenomenon which is not observed in the case of Cr2N + Si. Isothermal and dynamic oxidations were assessed by thermogravimetric analysis. No significant improvement was evidenced between Cr2N and Cr2N + Si. On the contrary, oxidation resistance of CrN was found to be greatly enhanced by the nanocomposite structure, with a kinetic rate drastically reduced and a temperature of oxidation delayed of about 150 degrees C. Wear resistance was investigated through an original tribological test combining simultaneous harmful effects of a severe wear performed in an oxidative environment In such aggressive conditions, Cr2N-based coatings evidenced a brittle character associated to a high propensity to flaking. On the contrary, balls coated with CrN-based films withstand this test, for which the beneficial influence of silicon enrichment is again established. A quantitative approach of wear indicates specific wear energy three times higher than that of CrN at room temperature, and still twice at 150 and 300 degrees C. (C) 2009 Elsevier B.V. All rights reserved.