Microstructure and properties of novel wear and corrosion resistant CrON/NbON nano-scale multilayer coatings

CrN/NbN nano-scale multilayer coatings have found use in a number of commercial applications where wear and corrosion resistance determine the service life of the components. To further improve their performance a novel CrON/NbON topcoat has been developed. The coatings were deposited using an industrial Hauzer HTC 1000/4 UBM-ABS coating machine utilising the Arc Bond Sputtering method where cathodic arc metal ion etching is used to prepare the interface prior to coating deposition by unbalanced magnetron (UBM) sputtering. The oxynitride process was performed in mixture of dry air and argon at bias voltages varying from UB=−75 V to −120 V at total pressures during deposition from 3.5*10−1 Pa to 4.9*10−1 Pa. The thickness of the oxynitride film varied between 1.6 and 2.3 μm, while the total coating thickness varied between 4.6 and 5.3 μm. The XTEM investigation revealed that the microstructure of the oxynitride layer was dense columnar with a pronounced nano-scale multilayer architecture. By X-ray diffraction (XRD), the coatings were identified as crystalline with mixed texture. As the pressure during the oxynitride deposition stage was increased the crystal structure of the top layer became increasingly amorphous/nano-crystalline. An increase in the bias voltage also caused a shift from {100} texture towards {111} texture. The best performing oxynitride coatings had similar low sliding wear rates as the reference standard coating without affecting their corrosion resistance. However, the wear rate against a 100Cr6 counter body was reduced by a factor of 10 and the friction coefficient from 0.57 to 0.49. The wear rate of both the coating and the counter body was reduced as the bias voltage was increased, while increasing the deposition pressure had adverse effects on the tribological properties. The wear behaviour can be related to the special nano-scale multilayer structure of the oxynitride layer as coating with best tribological properties exhibits a pronounced nano-scale multilayer structure parallel to the coating surface.