Abstract
The thermal conductivity of yttria-stabilized zirconia (YSZ) thermal barrier coatings increases with high-temperature aging. This common observation has been attributed to the densification of the coatings as porosity sinters out and pores and cracks spheroidize to minimize their surface energy. We show that the thermal conductivity of fully-dense 3mol.% Y2O3 stabilized zirconia (3YSZ) also increases with high-temperature aging, indicating that densification and pore shape changes alone are not responsible for all the observed increase in thermal conductivity of coatings. Instead, there are also increases due to a combination of phase separation and grain growth. The increase in thermal conductivity can be described by a Larson–Miller parameter. It is also found that the increase in thermal conductivity with aging is greatest when measured at room temperature and decreases with increasing measurement temperature. Measured at 1000°C, the thermal conductivity of zirconia is almost temperature independent and the changes in thermal conductivity with aging are less than 15%, even after aging for 50h at 1400°C.
Highlights
It is standard practice to coat the hottest sections of gas turbines, including blades and vanes, with a thermal barrier coating (TBC) so that higher turbine inlet temperatures and, higher engine efficiencies can be achieved [1]
We study the effect of high temperature aging on the thermal conductivity of fully dense, nanocrystalline, tetragonal 3 mol% Y2O3‐stabilized ZrO2 (3YSZ), a readily available commercial composition close to that used in TBCs
While this value is significantly smaller than the activation energy for yttrium diffusion in zirconia 405 kJ/mol [11] it is approximately the same as the activation energy of oxygen diffusion in yttria‐stabilized zirconia (YSZ) (96 kJ/mol) [12]
Summary
It is standard practice to coat the hottest sections of gas turbines, including blades and vanes, with a thermal barrier coating (TBC) so that higher turbine inlet temperatures and, higher engine efficiencies can be achieved [1]. Analysis of our own measurements on the effect of thermal cycling on thermal conductivity of an EB‐PVD 7YSZ [9] using effective medium theory [10] and expressed in the finite element results presented by Lu et al [8] suggests that the densification and porosity evolution are not sufficient to describe the observed conductivity increases. This suggests that the thermal conductivity of the zirconia material itself changes upon high‐temperature aging
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