Excessive recession caused by rapidly flowing hot-gases can impede the application of ceramic materials. For example non-oxide ceramic materials such as silicon carbide suffer from oxidation at temperatures above 1000°C in oxygen containing environments. However, also a priori oxidation-resistant oxide ceramics are prone to recession in harsh environments. A key issue is hot water vapor being a main product generated during combustion of hydrocarbon fuels. In particular a lot of silica-rich oxide ceramics suffer from water-vapor thermo-chemical attack beyond 1000°C, an effect being highly critical for oxide ceramic matrix composites (CMC) such as WHIPOX if their reinforcing ceramic fibers are consisting of aluminiumsilicate phases such as Mullite.
A possible solution for the corrosion problem are environmental barrier coatings (EBC) which are highly thermochemical stable and thus provide high corrosion resistance; for example inside of a combustion chamber of a gas turbine engine. A further key aspect which may hinder application of oxide CMC is thermal overload, eventually resulting in loss of mechanical strength and embrittlement. Thermal barrier coatings (TBC) with low heat conductivity offer potential for increased peak application temperatures of oxide CMC.
EBC and TBC on the basis of alumina, yttria and zirconia are being developed in particular of WHIPOX-type oxide CMC. Such coatings exhibit high erosion resistance and meet requirements for high-temperature applications i.e. thermal and chemical stability against rapidly flowing hot-gases as well as stability versus oxide CMC base materials.
The development of coatings for oxide CMC is not limited to thermal and environmental barrier coatings: application of oxide CMC in space and hypersonic flight require adapted thermo-optical materials properties such as infrared absorbance or emittance. WHIPOX-type oxide CMC tiles have been developed as part of the thermal protection system (TPS) of DLR’s vehicle SHEFEX II (Sharp Edge Flight Experiment). Frictional heat generated during hypersonic flight has to be re-emitted by TPS tiles as thermal radiation (“radiation cooling“). In order to achieve a suitable, increased emittance the standard “white” WHIPOX-CMC were “blackened” by means of black coating materials / pigments. Since “blackened” oxide CMC also show high absorbance for sunlight such modified CMC offer potential as high-temperature and oxidation-resistant absorber structures for solar-thermal applications.
The selection of a viable coating technology is crucial for development of protective coatings. Coating technologies typically produce substantially different coating microstructures and properties even for similar coating materials. Not all methods allow for coating of complex shaped components. In order to find the best match of component, coating materials and technology we assess, also in co-operation with external partners, the most conventional methods to process ceramic coatings:
• Liquid phase processes (dip-coating, sol-gel)
• Electron-beam physical vapor deposition (EB-PVD)
• Magnetron-, Gas-flow-sputtering
• Thermal spraying ( atmosphere pressure ,vacuum-plasmaspraying)
• Chemical vapor deposition (CVD)