Plasma Spray – Physical Vapor Deposition of Thermal and Environmental Protection Systems

Thermal barrier coatings (TBCs) have been a longstanding requirement for Ni-based superalloy turbine engine components to maintain high engine efficiencies.  However, due to the thermal limits that these metal-based systems have reached, ceramic matrix composite (CMC) materials have been gathering increased interest due to their enhanced thermomechanical stability and lower densities than superalloys.  However, due to the deleterious formation of silicon hydroxide species, these materials undergo significant recession due to the presence of water vapor in combustion environments.  Therefore, environmental barrier coatings (EBCs) are necessary to protect the underlying silicon-based ceramic.  Although rare earth silicate systems have demonstrated protection as EBCs for SiC/SiC materials, their thermal stability is significantly lower than the CMC substrate, and only marginally better than the metallic-based systems they are set to replace.  In order to raise the temperature capability and the erosion resistance of these materials, a T/EBC system was developed and deposited using Plasma Spray- Physical Vapor Deposition (PS-PVD) technology.  PS-PVD processing fills the gap between conventional thermal spray processes and vapor phase methods, as coatings can be deposited as either thin, dense coatings or vapor-deposited columnar microstructures by changing the processing conditions.  A dense rare earth silicate coating was deposited as an environmental barrier and a columnar low thermal conductivity (k) doped zirconia layer was applied on top to increase the temperature stability and enhance the erosion resistance.  Coatings were evaluated for their durability in water vapor and thermal conductivity using laser testing.  Scanning electron microscopy and X-ray diffraction were used to characterize the coating morphology and thickness, as well as phase structure and stability.