Surface coatings to improve oxidation and wear resistance for components are state of technology since many years. However, a continuous effort in research of new coating material is present, which show an optimal protection and thus lead to an optimization of processes e.g. concerning the efficiency of nozzles. The MAX-phase Cr2AlC offers great potential as oxidation and wear coating material, due to the combination of ceramic and metallic properties with excellent oxidation resistance.
The aim of the investigation was the characterization of oxidation behavior and phase evolution of Cr2AlC ARC-PVD-thin films at 700 °C in air as well as in vacuum from 30 °C to 900 °C. Heat treatments were performed under isothermal conditions up to 207 h and samples were investigated at various time steps. The oxidation behavior was characterized by mass change over time measurements. REM, EDX and XRD were used for analysis of the microstructural evolution. A main issue was the interdiffusion at high temperature between coating and substrate. For better understanding of those processes, in situ XRD measurements under vacuum were carried out between 30 °C and 900 °C.
The results indicated that the mass change due to oxidation can be described by a power law. The corresponding power law exponent was about 0.27, which is significantly lower as reported in a few other papers for higher temperatures. During oxidation interdiffusion between Cr2AlC and the IN718 substrate at the coating-substrate interface as well as the formation of an aluminum oxide layer at the surface lead to significant aluminum depletion. Thus Cr2AlC degradation and Cr23C6 formation occurs. The interdiffusion proceeded in both ways. Accordingly nickel and the formation of a nickel-containing phase could be verified in the coating via EDX and XRD. In situ XRD under vacuum in the range of 30 °C – 900 °C approved the fast Cr2AlC degradation through interdiffusion. According to the XRD measurements a 4 μm thick coating did not contain any Cr2AlC after heat treatment at 900°C for 2 h.
Besides the Cr2AlC coating degradation structural changes in form of large pores were observed. These structural weakening can lead to film failure. However these changes in the investigated time are mainly a result of interdiffusion.