Abstract
In order to apply silicon carbide (SiC) fiber reinforced SiC matrix (SiC/SiC) composite, which has light weight and superior heat resistance, for high-pressure turbine materials for aircraft engines, development of environmental barrier coatings (EBC) for SiC/SiC composite is essential. EBC is fabricated by depositing several layers on SiC/SiC composite and cooling it to room temperature. During the cooling process, the temperature gradient appears to occur due to the difference of thermal conductivities of EBC layers and substrate. The purpose of this study is to establish a theoretical framework for predicting energy release rate (ERR) for interface crack initiation due to thermal stress, taking into account the temperature gradient in an EBC model (Yb2SiO5 : Yb2Si2O7 = 100:0, 80:20, 50:50, 20:80, 0:100/Mullite/Si-based bond coat/CVD-SiC). We conducted heat-transfer finite element method (FEM) analysis to obtain the temperature distribution in the EBC model during the cooling process. The result reveals that salient temperature gradient does not appear in the EBC layers and substrate. Thus, we can regard the temperature in the EBC as uniform. Thermal stress FEM analysis was also conducted to calculate ERRs for interface crack initiation due to thermal stress in EBC during the cooling process. We compared ERRs obtained by the FEM analysis and a simple theory for interface crack where ERR is estimated as the sum of nominal strain energies in the coating layers multiplied by a dimensionless factor. We found that the dimensionless factor is no longer a constant but is determined by thicknesses of Mullite, Si-based bond coat and CVD-SiC layers.
