Metastable phase formation in plasma-sprayed ZrO₂ (Y₂O₃)-Al₂O₃

Rapidly solidified ZrO₂ (Y₂O₃)-Al₂O₃ powders were prepared by melting fine-particle aggregates in a high-enthalpy plasma flame and then rapidly quenching them in cold water or on a copper chill plate. To ensure complete melting and homogenization of all the particles before quenching, the water-quenching, the water-quenching treatment was often repeated two or even three times. The resulting melt-quenched powders and splats displayed a variety of metastable structures, depending on composition and cooling rate. ZrO₂-rich material developed an extended solid solution phase, whereas eutectic material formed a nanofibrous or amorphous structure. Under high cooling rate conditions, the ZrO₂-rich material developed a nanocomposite structure (t-ZrO₂ + α-Al₂O) directly by melt-quenching, whereas, more typically, such a structure was developed only after post-annealing of the as-quenched metastable material.