High temperature stability and sintering resistance of Gd2O3-Yb2O3-Y2O3-ZrO2 (GYYZO) coating

Solid-solution agglomerated ZrO2-based powders co-doped with Gd2O3, Yb2O3 and Y2O3 rare earth oxides were synthesized by solid-phase sintering and spray drying. The corresponding GYYZO coating was prepared by using atmospheric plasma spraying, while the 8YSZ coating was also prepared as a control group for the high temperature performance investigation. The high temperature structure and phase stability as well as the sintering resistance of both coatings were investigated by SEM, XRD, DSC, EPMA and TEM observations. The results showed that the 8YSZ coating began to undergo a phase transition of tetragonal (t) phase to monoclinic (m) after 50 h heat treatment at 1400 °C, while the GYYZO coating remained free of phase transition even heat-treated for up to 200 h, demonstrating a better phase stability. In addition, as an indicator of sintering resistance of the coating, the porosity of the 8YSZ coating decreased up to 90.1 % after 200 h heat treatment, while that of the GYYZO coating decreased only 20.6 %. The superior high temperature performance of GYYZO coating can be explained by the pinning effect of nano-pores on grain boundaries. The formation of nano-pores was associated with the evolution of defect clusters in the coating, which were facilitated by the substitution defects and oxygen vacancies created by the GdYb co-doping. During the heating process, these clusters migrated along with the grain boundaries and formed pearl-string-like nano-pores at the grain boundaries. The eventual formation of nanopores not only further inhibits grain boundary migration to reduce sintering, but also prevents the segregation of Y elements to hinder t-phase to m-phase transition.