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

Abstract

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 Cr x ) 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.

Quo Vadis: AlCr-Based Coatings in Industrial Applications

“…It was shown that oxynitride and oxide coatings (alpha and cubic crystal modifications) have a potential for use in various tribological applications and for oxidation protection [166][167][168][169][170], and moreover, even for use in solar absorbers [171]. Both vacuum arc deposition and various sputtering methods, including HiPIMS processes and hybrid processes (HiPIMS plus DC magnetron sputtering), were applied to investigate coatings of the types (AlCr)ON and (AlCr)O [23,[172][173][174][175].…”

“…However, at a high Al-content, the tendency towards the formation of dual-phase compositions containing metastable phase fractions has been observed, which may negatively influence the mechanical properties and performance. The authors concluded that the cathodic arc evaporation of (Al,Cr) 2 O 3 -based coatings is very complex when the microstructure of the arc cathode used is included as a factor [174].…”

Improvement of the Thermal Stability of Zircon by Formation of the Solid Solution of Cr in Zircon

“…Researches [42][43][44][45] have shown that a small amount of Ce 4+ , Pr 3+ , Nd 3+ , V 5+ , and Y 3+ can be incorporated into the zircon structure as a solid solution. Some researchers [30,31,37,46,47] studied the effect of Cr 2 O 3 addition to the ZrO 2 and reported that a small amount of chromia dissolves into the zirconia especially at high temperatures (1600°C). It was mentioned that a secondary phase was formed at the samples with 4 wt.% of chromia at all sintering temperatures.…”