In order to limit global warming to less than 2 °C compared to the pre-industrial level, the International Energy Agency (IEA) estimated that CO₂ emissions have to be reduced by 50% in 2050 relative to 2009. At the same time, worldwide demand for air travel is expected to at least double over the next twenty years, causing a significant increase in carbon dioxide emissions even under most optimistic scenarios. Therefore, all types of gas turbines will play an important role for energy supply in the foreseeable future. Increasing the working temperature of gas turbines is thermodynamically the most important way to increase their efficiency and to reduce their CO₂ emissions. But the working temperature of nickel and cobalt superalloys of about 1100 ° C can no longer be increased significantly. Materials that are suitable for the next generation of gas turbines are ceramics such as Si₃N₄, SiC and SiC/SiC, which are able to withstand temperatures of up to 1400 °C. However, when natural gas and future fuels such as green hydrogen and ammonia are burned, moisture is generated, which destroys these ceramics. Therefore, high temperature stable environmental barrier coatings are needed to protect ceramics and are thus a key factor for their use.
In recent years we developed an easy-to-apply coating system that protects such non-oxide ceramics very effectively under extremely harsh conditions found in stationary turbines and aircraft engines. In contrast to expensive conventional coating methods like physical vapor deposition and plasma spraying for the processing of thermal and environmental barrier coating, our coating system based on the oligosilazane Durazane 1800 and the active fillers Si and Yb₂O₃ was applied via spraying onto a silicon nitride substrate. This approach enables the coating of complex geometries as required for components of gas turbines.
Pyrolysis of the coating at 1415 °C for 5 h in air led to the quantitative in situ formation of β-Yb₂Si₂O₇ during but with 4.5 vol% residual open porosity. Therefore, the application of a bond-coat composed of the silazane Durazane 2250 and Si powder was necessary to protect the Si3N4 substrate from oxidation during pyrolysis and densification of the top-coat at 1415 °C for 5 h in air. The resulting double-layer coating system exhibited a dense microstructure with a total thickness of 68 µm, excellent adhesion strength (36.9 ± 6.2 MPa), hardness (6.9 ± 1.6 GPa) and scratch resistance (28 N). The remarkable protection potential was confirmed after 15 thermal cycles between 1200 and 20 °C by quenching in water with only minor signs of spallation, and hot gas corrosion at 1200 °C for 200 h (p = 1 atm, pH₂O = 0.15 atm, v = 100 m s^-1), reducing mass loss in 96% compared to uncoated Si₃N₄ substrates. The excellent thermomechanical properties and the corrosion resistance of the in situ generated Yb₂Si₂O₇ coating system in extreme combustion environments confirm its high potential for protecting Si₃N₄, SiC and SiC/SiC ceramics in the next generation of gas turbines.