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

Yttria-stabilized zirconia (YSZ) fine powders were prepared via sol-get 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 t'. 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 degrees C of xerogel (Ex-xero-YSZ powder) and aerogel (Ex-aero-YSZ powder) crystallize in the tetragonal form. N(2) adsorption/desorption analysis of the Ex-xero-YSZ powder indicates an S(w) of 2.8 m(2)/g. For the Ex-aero-YSZ powder, the S(w) (26 m(2)/g) is much higher than of Ex-xero-YSZ, leading to a better sintering capability. This high S, is correlated to the small crystallite size (26 nm) and the alveolar morphology of Ex-aero-YSZ powders compared to Ex-xero-powder (49 nm). By reducing particles size and increasing the S(w) 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 degrees C for 2 It which corresponds to the working temperature of TBC, the final aim will be to prepare ceramic YSZ coatings. (c) 2009 Elsevier Ltd and Techna Group S.r.l. All rights reserved.