The effect of coating thickness on the thermal conductivity of EB-PVD PYSZ thermal barrier coatings

The effect of coating thickness on thermal conductivity of electron beam physical vapor deposited (EB-PVD) partially Y2O3 stabilized ZrO2 (PYSZ) thermal barrier coatings (TBCs) was investigated. Two-layer samples, PYSZ coating deposited on nickel-base superalloy IN625 substrate with a 50 μm NiCoCrAlY bond coat, as well as free-standing PYSZ coatings and quasi-free-standing PYSZ coatings (PYSZ on transparent sapphire) were included in the study. Fracture surface and surface morphology of the coated samples were examined by scanning electron microscopy. Thermal diffusivity measurements for determining thermal conductivity were made from room temperature up to 1150 °C using the laser flash technique. The thermal conductivity of EB-PVD PYSZ coatings strongly depends on the coating thickness with lower values for thin TBCs. The columnar microstructure of EB-PVD ceramic coatings characterized by a fine grained inner zone and a coarse grained outer zone causes this thickness effect. The changes in thermal conductivity of EB-PVD PYSZ coatings are explained by a model with different thermal resistors attributed to the two zones.