Corrosion and tribocorrosion behaviour of cermet and cermet/nanoscale multilayer CrN/NbN coatings

The corrosion and tribocorrosion behaviour of thermally sprayed coatings and thermally sprayed/nanoscale multilayer CrN/NbN coatings deposited on steel specimens has been studied in 3.5% NaCl solutions. The thermally sprayed coatings are bulk HVOF deposits with nominal composition WC-12Co and Cr3C2-37WC-18Me, while the nanoscale multilayer deposit, topping the thermally sprayed ones, is a CrN/NbN coating, obtained by PVD magnetron sputtering technique. Combined wear-corrosion conditions have been achieved by sliding an alumina antagonist on the lateral surface of coated steel cylinders, during their exposure to the aggressive solution. Polarization curves recorded after 1h of immersion and polarization resistance values monitored during 3 days exposures showed that under free corrosion conditions Cr3C2-37WC-18Me always exhibits low corrosion rates and efficiently protects the underlying steel substrate from corrosion. Even on WC-12Co-coated specimens, low corrosion rates are measured at long immersion times. Both cermet coatings suffer severe corrosion attack under tribocorrosion conditions. On the contrary, the duplex nanoscale multilayer CrN/NbN /WC-12Co deposit affords very good corrosion resistance both in the absence and in the presence of wear, within the studied exposure period. SEM observations and EDS analyses have also been carried out to characterize the original powder and coating microstructures and to assess the damage extent at the end of the corrosion and tribocorrosion tests.