Comparative studies on the thermal stability and corrosion resistance of CrN, CrSiN, and CrSiN∕AlN coatings

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Abstract
In this work, three kinds of Cr-based nitride coatings such as monolithic CrN, CrSiN coatings, and multilayered CrSiN∕AlN coating with bilayer period of 3.0nm were deposited on both Si (100) wafer and AISI H13 steel substrates by unbalanced magnetron sputtering. Thermal stability of these coatings was evaluated by annealing the coatings at temperatures between 600 and 1000°C for 30min in air. In addition, the corrosion behaviors of these coatings were investigated by potentiodynamic polarization tests in a deaerated 3.5wt.% NaCl solution at 40°C. Results from annealing test show the monolithic CrN and CrSiN coatings were completely oxidized after annealed at 800 and 900°C, and their cross sectional images and atomic force microscopy showed a loose and very porous morphology due to the oxidation. Also, the hardness values of the monolithic CrN and CrSiN coatings were decreased significantly from 22 and 27GPa to 8 and 14GPa, respectively. However, the multilayered CrSiN∕AlN coating still exhibited a dense microstructure without visible change after annealed at 1000°C, and moreover, the relatively high hardness of 25GPa was maintained. The superior thermal stability of the CrSiN∕AlN multilayer coating could be attributed to the formation of the dense and stable oxidation barrier consisted of the Al2O3, Cr2O3, and amorphous SiO2 phases near the surface region, which retard the diffusion of oxygen into the coating. In the potentiodynamic polarization test results, it was found that the significantly improved corrosion resistance of the multilayered CrSiN∕AlN coating was observed in comparison with those from the monolithic CrN and CrSiN coatings, and its corrosion current density (icorr) and protective efficiency were measured to be approximately 4.21μA∕cm2 and 95%, respectively.

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