High temperature phase changes and oxidation behavior of Cr–Si–N coatings

Composition, structure and morphology of Cr–Si–N coatings deposited by cathodic arc ion-plating on cemented carbide substrates were studied. A systematic variation of the deposition parameters resulted in relative Si atomic concentrations Si/(Cr + Si) within 0 and 15at.%, which affect
significantly the properties of the coatings, their phase stability and oxidation resistance. All Cr–Si–N coatings consist of nanocrystalline CrN grains with diminishing crystallite sizes at increased Si content. The microstructure of the samples, observed by scanning electron microscopy, is columnar
for the coatings with low Si concentration, and becomes denser for specimens with higher Si content. The hardness and Young's modulus of the coatings increase with increasing Si concentration up to a maximum value of 26GPa and 430GPa, respectively, for the Cr0.94Si0.06N coatings, followed by a progressive decrease. X-ray powder diffraction studies were performed in situ up to 1000°C in vacuum and in air. Annealing in vacuum resulted in the decomposition of CrN into Cr2N and N2 and the subsequent oxidation. The annealing performed in air proved an excellent oxidation resistance of the coatings, which strongly depends on their composition and morphology. The best oxidation resistance was obtained for the hardest samples with intermediate Si concentration, for which the formation of Cr2O3 has not been observed even at 1000°C. Recrystallization, which occurs at elevated temperatures both in vacuum and in the air, becomes significant above approximately 800°C.