Measurement of the elastic modulus of a plasma-sprayed thermal barrier coating using spherical indentation

Failure in plasma-sprayed thermal barrier coatings systems mostly takes place in the ceramic topcoat or at the interface between the topcoat and the bondcoat. The failure normally occurs by spallation of the topcoat at shutdown operations from high temperatures where compressive thermal mismatch stresses are induced in the topcoat. In order to analyse the residual stresses, knowledge about the elastic modulus for the different components in the coating system is required. In this work, a spherical indentation method has been used for room-temperature measurement of the elastic modulus of both the topcoat (300-μm-thick air-plasma sprayed ZrO2–8wt.%Y2O3) and the bondcoat (150-μm-thick air-plasma sprayed Ni–23Co–17Cr–12Al–0.5Y). This method gives the compressive modulus, which for the topcoat may be more adequate as it has different modulus in tensile and compressive stress states due to its microstructure. Measurements were made on specimens in as-coated condition and after heat treatment (1500 h at 1000 °C). A significant increase in the elastic modulus of the bondcoat was observed as a result from the heat treatment, from 137 to 226 GPa, which is explained by the internal oxidation at the initial weak inter-splat regions. A sintering effect and a change in the elastic modulus were also observed for the topcoat as a result of the heat-treatment, where the elastic modulus increased from 38 to 60 GPa.