Characterization of the arc ion-plated CrN coatings oxidized at elevated temperatures

Abstract

Microstructure and chemistry of the arc ion-plated CrN coatings oxidized in air at temperatures ranging from 300 to 800 °C for 60 min were analyzed by x-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and Auger electron spectroscopy (AES). The CrN coatings were prepared by cathodic arc ion plating deposition on a type 304 stainless steel with a Cr interlayer. The XRD result shows that oxidation of the CrN-coated steel above 500 °C produces two new phases, Cr2O3 and β-Cr2N, and the amount of both phases increases with the oxidation temperature. Cross-section TEM shows three distinct regions including the steel substrate, the Cr interlayer, and the CrN coating in the as-deposited specimen, in which the CrN layer exhibits a columnar structure and preferred orientation. Oxidation of the CrN-coated steel at high temperatures produces an oxide layer, Cr2O3, on the coating surface, and the underlayer is a mixture of CrN and β-Cr2N phases. Unlike the as-deposited specimen, the dual phase layer in the oxidized specimens has an equiaxed grain structure and the average grain size of the layer increases with the oxidation temperature. In addition, pronounced grain growth in the dual phase layer near the coating surface is observed in the specimen heat-treated at 800 °C. Elemental analyses of the CrN coating near the free surface by EELS and AES reveal that the O/N ratio of the coating and the thickness of the oxide layer increase with the oxidation temperature.