Integrative design of novel Mo-Si-based alloys and protective coatings for high temperature structural applications

• Applicants: Professor Dr.-Ing. Hans Jürgen Christ, since 8/2021; Professor Dr.-Ing. Martin Heilmaier; Professor Dr. Uwe Schulz
• Subject Area: Coating and Surface Technology ; Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 386184182

Mo-silicide based alloys with high Ti-concentrations are a new class of high temperature materials with promising properties. In particular, these multiphase alloys are more creep resistant than cur-rently used Ni base single crystals and exhibit good oxidation resistance at temperatures beyond 1000°C. At moderate temperatures, however, they suffer from pesting and the intrinsic oxidation resistance is low. The overarching goal of this proposal is still the integrative development of a techni-cally relevant Mo-Si-B-Ti alloy combined with an adapted coating system which provides oxidation and corrosion protection in dry and wet environments in a temperature range between 600 and 1300°C. In the first part of this project, we developed eutectic/eutectoid Mo-Si-Ti alloys which exhibit a good compromise between creep and oxidation resistance. The improvement of the oxidation behavior relies on the one hand on increasing the intrinsic oxidation resistance by CALPHAD based alloy design and, on the other hand by extrinsic protection due to novel silica based coatings. Oxidation of the new alloys in wet conditions is significantly accelerated as compared to dry atmosphere which is accom-panied by a change from nearly parabolic to linear rate law. This can be rationalized by the faster O- and Ti-diffusion. The Si-coatings provide good oxidation protection for 300h at 1200°C in air by the formation of a slow growing SiO2-barrier. For better compatibility between the alloys and the coating, adapted bond coats were developed, which identify themselves by reduced phase reactions at inter-faces and interdiffusion. In the present proposal we target two main research areas: first, the oxidation and creep behavior of the alloys will be investigated. We will address (i) the contribution of the single phases to oxidation, (ii) the differences in oxidation mechanisms under dry and wet conditions and (iii) the underlying creep mechanisms. As neither intrinsically formed oxides nor simple coatings will guarantee sufficient long-term protection in water vapor environment, the material system will be additionally protected by Environmental Barrier Coatings (EBCs). Hence, second, regarding the development of EBC as well as the characterization of the oxidation and creep behavior of coated alloys the following aspects will be in focus: (i) stability of coating systems in wet atmosphere, (ii) compatibility between base alloys and the respective layers of the EBC-systems. These issues will be resolved by a combined ap-proach utilizing thermodynamic modelling and experiments. Aided by the developed simulation tool coupling thermodynamics and kinetics, we will identify which combination of thermally grown layers and deposited coatings will enable optimal protection against dry and wet atmospheres. Finally, it will be investigated whether these coating systems are mechanically compatible to the base alloys under superimposed creep or temperature change loading conditions.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Alexander Kauffmann
• Ehemalige Antragstellerin: Professorin Dr.-Ing. Bronislava Gorr, until 8/2021