Cyclic Oxidation Behavior and Microstructure of Nanocrystalline Ni–20Cr–4Al Coating

Abstract

The cyclic oxidation behavior and microstructure of a nanocrystalline Ni–20Cr–4Al coating have been investigated. Cyclic oxidation tests were conducted on uncoated and coated samples at peak temperatures of 750 and 1010 °C for up to 2070 thermal cycles between the peak and room temperatures. The results showed that a dense Al2O3 scale formed on the coated samples during thermal cycling to both peak temperatures. Evidence of internal oxidation of the coating was observed only on the samples exposed to 1472 one-hour thermal cycles at the peak temperature of 1010 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. Thermal exposure led to depletion of Al from the coating and grain coarsening within the coating. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are believed to be related to the fine-grained structure of the coating.