Progress in the synthesis of Al- and Cr-based sesquioxide coatings for protective applications

The challenge of synthesizing protective alumina coatings in corundum structure at moderate temperatures inspired the development of novel strategies of both thin film processing and microstructural thin film design. While the growth of pure corundum coatings is still a domain of chemical vapor deposition, new concepts developed in physical vapor deposition addressed the growth of advanced solid solutions, nanocomposites, or multilayers. The system Al-Cr-O has received huge attention as thin films of the type (Al$_{1−x}$Cr$_x$)$_2$O$_3$ could be grown at substrate temperatures even below 500 °C in laboratory model experiments. These coatings offer particular potential for applications that require chemical inertness, thermal stability, mechanical strength, and excellent tribological properties at elevated temperatures. Mastering its low temperature deposition is one of the major aims of industrial research, as it would also enable the protection of temperature-sensitive components and consequently path the way for novel, yet not realizable applications. This work summarizes the state of the art and recent progress in the development and synthesis of such sesquioxide protective coatings prepared by physical vapor deposition. General coherencies are described for (Al$_{1−x}$Cr$_x$)$_2$O$_3$ coatings. Topics covered are important growth parameters (i.e., the oxygen gas flow and the substrate bias), the chemical composition and its impact on alloying concepts applicable to (Al$_{1−x}$Crx)$_2$O$_3$ coatings, specific architectural coating designs, and relevant properties such as the thermal stability and material response upon annealing in oxidative environments. In addition, a brief outlook into the development of other Cr-based sesquioxide thin films, (Cr,Zr)$_2$O$_3$ and (Cr,V)$_2$O$_3$, is given