Morphology and structure of YSZ powders: Comparison between xerogel and aerogel

Abstract

Yttria-stabilized zirconia (YSZ) fine powders were prepared via sol–gel route in order to shape thermal barrier coatings (TBCs) from these powders. The main objective is to develop new undirectional coatings to allow best thermo-mechanical accommodations compared to conventional processes. To reach this aim, powders have to be able first to be highly dispersed into a sol (non-agglomeration, high specific surface area, etc.) and second to crystallize in the required metastable phase t0. Two routes have been used to dry gels: the conventional way which consists of simple evaporation of the solvent is compared to drying in supercritical conditions. Both YSZ powders after calcination at 950 8C of xerogel (Ex-xero-YSZ powder) and aerogel (Ex-aero-YSZ powder) crystallize in the tetragonal form. N2 adsorption/desorption analysis of the Ex-xero-YSZ powder indicates an Sw of 2.8 m2/g. For the Ex-aero-YSZ powder, the Sw (26 m2/g) is much higher than of Ex-xero-YSZ, leading to a better sintering capability. This high Sw is correlated to the small crystallite size (26 nm) and the alveolar morphology of Ex-aero-YSZ powders compared to Exxero-powder (49 nm). By reducing particles size and increasing the Sw of the powders, supercritical drying appears as a promising way to prepare stable slurries or loaded sols from fine YSZ particles for TBC applications. Indeed, after preparing nanometric powders, they are dispersed into a sol before shaping on superalloys substrates. After thermal treatment at 950 8C for 2 h which corresponds to the working temperature of TBC, the final aim will be to prepare ceramic YSZ coatings.