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COATINGS (EBC, TBC, ETBC)

The R&D activities in the field of protective coatings for fibre-reinforced ceramics like WHIPOX comprise the development of coating materials, coating technologies and coating performance testing.

Development of Coating Materials

The application of ceramic materials in high-velocity hot-gas streams is affected by corrosive degradation. For example non-oxide materials like silicon carbide SiC decompose in oxygen atmosphere at temperatures above 1000°C. However, even all-oxide ceramics may suffer hot-corrosion caused by hot water vapor present as main combustion product. In particular silica-containing ceramics are severely attacked by water vapor at temperatures of more than 1300°C, which is true for highly porous WHIPOX-type composites with fibres and matrices based on aluminosilicates (mullite). Therefore research focuses on highly corrosion resistant materials that are suitable as environmental barrier coatings (EBCs). A second issue for materials development is the protection of the composite against thermal overload. The maximum application temperature of the composites can be increased if an additional high-temperature resistant thermal barrier coating (TBC) is applied. Environmental and thermal barrier coatings (ETBC) provide environmental and thermal protection. Coatings based on alumina and zirconia are relevant since they exhibit thermal and chemical stability along with an inevitable thermodynamic stability against WHIPOX-type composites.

Coating technology

A fundamental aspect of coating development is the use of a technically and economically favorable coating technique. The coating properties strongly depend on the employed coating method and are basically controlled by the coating microstructure. This behavior is observed even for identical coating materials. In order to define optimized coating techniques, the Institute for Materials research follows different routes, also in cooperation with external partners. This comprises

• Chemical methods (Dip-coating, painting, sol-gel)
• Chemical vapor deposition (CVD)
• Electron-beam physical vapor deposition (EB-PVD)
• Thermal spraying (atmposphere-pressure plasma spraying (APS), vacuum plasma spraying (VPS)

Coating performance testing

Testing of coatings includes gas permeability, thermal shock and corrosion resistance and thermomechanical stability. These investigations are performed in special testing facilities, which were partly designed and assembled in-house. Gas permeability can be measured up to 10-bar pressure. In a high-temperature water-vapor corrosion facility the samples can be exposed to streaming water vapor (up to 10m/s) at temperatures up to 1400°C. Special testing campaigns are performed in cooperation with other DLR Institutes. In a model-burning chamber at the DLR-Institute of Propulsion Technology coated WHIPOY samples are exposed to thermochemical conditions, which simulate the actual conditions in a real gas turbine. During a simulated atmosphere re-entry coated WHIPOX samples are exposed to a supersonic flow field consisting of partially ionized air, which leads to a very high thermal load and severe corrosive attack. These simulations are performed in the arc-heated wind tunnel of the DLR-Institute of Aerodynamics and Flow Technolog.