Thermal Properties and Microstructures Analysis of YSZ and YSZ-Al2O3 Thermal Barrier Coatings

Abstract

Yttria-stabilized zirconia (YSZ), 15 wt.%Al2O3-doped YSZ (YSZ-15 wt.%Al2O3), and 35 wt.%Al2O3-doped YSZ (YSZ-35 wt.%Al2O3) coatings were prepared by atmospheric plasma spraying. Composition and microstructure of the coatings were characterized. Moreover, the effect of composition and microstructure on the thermal conductivities and thermal expansion coefficients (TECs) of the coatings was investigated. The YSZ-15 wt.%Al2O3 coatings possess much lower thermal conductivity compared to YSZ coating which is attributed to the smaller grains, more oxygen vacancies and the large atomic weight difference between Al and Zr. The mentioned factors improve the phonon scattering thereby reducing the thermal conductivity. The lower TECs of YSZ-15 wt.%Al2O3 coatings are attributed to the smaller lattice parameters due to the substitutional Al3+ doping at Zr4+ sites. The decrease in lattice parameter values increases the crystal lattice energy. And the TEC is inversely proportional to the lattice energy. Moreover, the TEC of the YSZ-35 wt.%Al2O3 coating is much lower than the YSZ coating, which is due to the precipitation of Al2O3 and the formation of amorphous phases. Compared to ZrO2 phase, the Al2O3 phase possesses lower TECs values. The amorphous phase can account for larger expansion tolerance compared to the corresponding crystalline phase due to its loose atomic arrangement.