Yttrium silicates are promising materials for improved oxidation and erosion protection for carbon fiber-reinforced composites. A two-layer coating system of low-pressure plasma-sprayed yttrium, silicate on chemical vapor deposition-SiC-precoated C/C-SiC was tested under atmospheric re-entry conditions simulated within a plasma wind tunnel test facility. The thermal expansion behavior Of Y2SiO5 and Y2Si2O7 was investigated. The chemical compatibility with and without increasing oxygen partial pressure at the interface of the two-layer system was calculated by the CALPHAD method. The calculations were compared with experimental results. Furthermore, a thermodynamic explanation is presented to understand and predict the observed coating failure mechanism, identified as blister formation.