Thermal and Environmental Barrier Coating Concepts for Silicon-based Ceramic Matrix Composites

SiC-based composites have been implemented for use in 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 gas environment is accompanied by water vapor volatilization of the SiO2 to form Si(OH)4 (g), resulting in recession of the composite. It must therefore be protected by an environmental barrier coating (EBC) system, whose function 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 ß-cristobalite 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, reducing the effective thickness of the protective EBC layer.

In this dissertation, two concepts are explored to reduce the growth of the ß-cristobalite SiO2 TGO. First, the TGO is directly transformed into a more thermo-mechanically compatible HfSiO4 material. This concept is divided into three parts, where first the hafnon reaction kinetics and mechanisms are explored by annealing of a partially consolidated cristobalite + HfO2 powder composite system. Then, the dynamic formation of hafnon through oxidation of silicon and subsequent reaction of SiO2 with HfO2 is explored by oxidation of a silicon + HfO2 powder composite system. Finally, a thin HfO2 layer is deposited on the conventional silicon bond coat, and covered with a YbDS EBC layer. The silicon/HfO2/YbDS system was subjected to a high temperature (1316°C) steam environment and thermally cycled up to 1000 1-hour cycles. This showed a reduction in SiO2 TGO thickness of at least half.

The second concept explores the design of thermal management concepts in EBC systems. During the use of EBC systems in gas turbines, the SiC composite is usually internally cooled, creating a thermal gradient across the coating thickness. In contrast, the EBC systems and the substrates they are deposited on are usually subjected to the same temperature, without the use of a thermal gradient. With thermal management in mind, this concept was divided into two parts. First, a monoclinic HfO2 thermal barrier coating (TBC) layer was deposited on the silicon/YbDS system using either atmospheric plasma spray (APS), or electron-beam directed vapor deposition (EB-DVD), and subsequently subjected to a high temperature (1316°C) steam environment with thermal cycling, in order to investigate the challenges of depositing a TBC onto an EBC system. The thermo-mechanical behavior during deposition and during thermal cycling has been investigated, as well as the thermo-chemical compatibility between the m-HfO2 TBC and YbDS EBC, revealing an adverse reaction to form a Yb2O3-stabilized HfO2 reaction layer between the EBC and TBC layer.

The second part of the thermal management concept explored the reduction of the thermal conductivity of the YbDS EBC by the addition of multiple components to YbDS in order to reduce the thermal conductivity while maintaining a CTE close to that of SiC and silicon. Two systems were explored, based on the Yb2Si2O7-Gd2Si2O7 and Yb2Si2O7-La2Si2O7 binary systems, adding up to a total of five components (i.e. Y2Si2O7, Lu2Si2O7, and Er2Si2O7) to these systems in equimolar ratios. Two systems ((Yb0.2Y0.2Lu0.2Gd0.2)2Si2O7 and (Yb0.25Y0.25Lu0.25Er0.25Gd0.25)2Si2O7) were found to attain a single γ-phase structure with a thermal conductivity <2 W m-1 K-1 and CTEs close to that of SiC and silicon, providing a promising route for the future development of combined thermal and environmental barrier coating (T-EBC) systems.