Oxidation of Cr2AlC coatings in the temperature range of 1230 to 1410 °C

Abstract

The isothermal oxidation behavior of Cr2AlC coatings on alumina substrates was investigated in the temperature range of 1230 to 1410°C. The structure, surface morphology, microstructure evolution and chemistry of the reaction products have been investigated. In the investigated temperature range, the Cr2AlC films form a dense continuous oxide scale consisting of α-Al2O3 on Cr carbides. The oxidation rates determined by thermo gravimetric analysis (TGA) were parabolic, indicating that diffusion through the scale is the rate limiting mechanism. The activation energy for oxidation was determined to be 348kJmol−1 and the parabolic rate constant at 1230°C was 7.1×10−10kg2m−4s−1. Hence, the oxidation behavior is comparable to NiAl in the temperature range and time intervals investigated. With increasing oxidation time voids form at the interface between oxide and Cr carbides and the amount of Cr7C3 increases at the expense of Cr3C2. Based on our thermodynamic calculations the oxygen partial pressure below the oxide scale increases as Al is depleted and Cr carbides oxidize, resulting in CO gas- and Cr2O3-formation. The formation of gas may together with the depletion of Al and Cr lead to the significant void formation observed in the Cr carbide interlayer. Observation of both Cr carbide precipitates and the formation of (Al,Cr)2O3 solid solution support this notion. For comparison bulk Cr2AlC was oxidized. It is argued that the absence of pores in oxidized bulk Cr2AlC is due to the considerably larger amount of Al available.