Heat treatment of plasma-sprayed Al2O3 and Al2O3–WO3 coatings between 500 and 1000 °C

The changes in phase composition and microstructure of Al2O3 and Al2O3–WO3 plasma sprayed coatings were characterised as a function of heat treatment in air at 500–1000 °C. The Al2O3 sample was dominated by γ-Al2O3 which transformed to δ-Al2O3 with heat treatment above 900 °C. No changes in the sample microstructure were observed. The Al2O3–WO3 sample had a heterogeneous phase microstructure ranging from WO3 rich splats to Al2O3 splats containing tungsten in solid solution or as metallic particles. Heat treatment at 500 °C formed lattice-like WO3 rich surface features which transformed to Al2(WO4)3 at 700 °C. Higher temperatures led to loss of tungsten through WO3 vaporisation.