Title: Thermal and environmental barrier coating system concepts for 1316°C
Abstract: SiC-based composites have begun to be used for non-stressed applications in hot sections of aero-gas turbine engines. Chemical reaction between residual oxygen and water vapor in the gas stream and the SiC components results in the formation of SiO2 (s) and CO (g). Exposure of this to the hot environment is accompanied by water vapor volatilization of the SiO2 to form Si(OH)4 (g), resulting in recession of the composite and must therefore be protected by environmental barrier coating (EBC) systems, whose function it is to reduce or eliminate the flux of oxidizing species to the composite surface.
An EBC system of interest is the silicon-Yb2Si2O7 (YbDS) system, which has shown to survive thermal and steam cycling up to 2,000 hours at 1316°C. At this time a ~3µm thick SiO2 thermally grown oxide (TGO) grows in between the silicon and YbDS layers, which endures significant tensile elastic strain during thermal cycling, resulting in cracking and eventual failure of the coating. Furthermore, volatility of SiO2 in the form of Si(OH)4 from the YbDS layer due to interaction with steam at 1316°C reduces it to Yb2SiO5 (YbMS). This leaves behind a porous layer with a mismatching coefficient of thermal expansion (CTE), resulting in cracking and flaking of the top of the EBC system.
This proposal discusses an effort to reduce the propensity for failure of these type of silicon-YbDS coating systems by:
- Deposition of a reactive HfO2 layer using EB-CPD in between the silicon and YbDS layers to transform the CTE mismatched SiO2 TGO in a CTE matching HfSiO4 layer
- Deposition of a porous, steam impervious, HfO2-based thermal barrier coating layer using EB-DVD on the YbDS layer to reduce the loss of SiO2
- Synthesis of a new multi-component rare earth disilicate concept that lowers the thermal conductivity of the YbDS layer closer to 1 W/m K while maintaining CTE requirements for these coating systems
James Fitz-Gerald, MSE, Chair
Haydn Wadley, MSE, Advisor
Elizabeth Opila, MSE
Prasanna Balachandran, MSE/MAE
Patrick Hopkins, MAE/MSE
All interested persons are invited to attend.