Structure and oxidation behavior of compositionally gradient CrNx coatings prepared using arc ion plating

Abstract

Functionally gradient coatings are coating systems which are used to increase performances of high temperature components in the hot gas turbine section of gas turbine engines, diesel engines, in aerospace and aircraft applications. These coatings consist of a transition from the metallic bond layer to cermet, and then to the ceramic layer. In this study, compositionally gradient CrNx coatings were fabricated using arc ion plating technique with N2 flux rate increasing from 0 to 120SCCM while film thickness builds up. Oxidation was carried out in air at evaluated temperatures ranging from 500°C up to 700°C for 2h. The original and the oxidized samples were characterized by glow discharge optical emission spectrometry (GDOES), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), and X-ray diffraction (XRD). GDOES results show that nitrogen content increases gradually from 0 to around 0.5 outward along film thickness. TEM results show that N2 flow rate change during deposition results in phase changes in order of Cr, Cr+Cr2N, Cr2N, Cr2N+CrN, and CrN, respectively, outward along depth direction. The microstructure shows an apparent columnar feature for the coatings deposited without substrate bias, and becomes denser for specimens fabricated with increasing the bias voltage. At 500°C, the CrNx coatings show no apparent oxidation phenomena; when exposed to higher temperature, e.g. 600°C and 700°C, oxidation occurs with the formation of Cr2O3 phase. The CrNx coatings deposited at all other biases but −50V, show good adhesive property upon high-temperature oxidations.