Plasma spraying of Al2O3–WO3 composite coatings

Al2O3–WO3 oxide composites are of considerable interest as solid acid catalysts in the petrochemical industry. Typically they consist of a monolayer of WO3 dispersed over a high surface area alumina support. The aim of this work was to form a single layer Al2O3–WO3 composite coating by plasma spraying. Ammonium metatungstate was dispersed through porous alumina particles and transformed to WO3 by heat treatment. WO3 doped powders were plasma sprayed using “low” and “high” enthalpy plasma systems. Plasma spraying led to dramatic changes in the WO3 oxide to form metallic tungsten, a metastable Al2O3–W solid solution and the mixed oxide Al2(WO4)3. The mechanisms accounting for these transformations are discussed.