On the nature of phases in Al 2O 3 and Al 2O 3–SiC thermal spray coatings

Abstract

Nature of alumina phases in thermal sprayed Al 2O 3 and nanocomposite Al 2O 3–SiC coatings is investigated. The coatings were prepared using high velocity oxy fuel spraying (HVOF) and air plasma spraying (APS) techniques on mild steel substrates either with Ni5Al or Ni20Cr bond layer. Coating top layer thickness was 250 μm and 1 mm. The amount of SiC was varied from 2 to 8 mass%. Flat and roller geometries were used for substrates. While coatings on flat specimens were used for low temperature heat treatments (100 and 700 °C); peels (free forms) of roller specimens were used for high temperature (≥1000 °C) heat treatments. X-ray diffraction technique was used for phase characterisation. The coatings showed the presence of varied amounts of well-known alumina phases and the relatively new U phase. It was observed that depending upon the thermal energy of the spray process even smaller particles <20 μm, gave rise to (contrary to the earlier hypothesis) unmelted α-seeds (nuclei) and α-alumina nucleated in preference to γ-alumina. Presence of SiC (mostly inside alumina grains) in alumina nanocomposite coatings increased the amount of α-phase formation in the coatings. The transformation temperatures, of alumina polymorphs, of the thermally sprayed alumina coatings, are found to be in accordance with the dehydration of boehmite. The discrepancies (higher transformation temperatures) in the earlier literature on alumina sprayed coatings were due to the presence of impurities in the coating. SiC increased the transformation of metastable alumina polymorphs to stable α-phase by 200 °C. The alumina phases in HVOF Al 2O 3–SiC coatings undergo γ→δ→θ→α phase transformations on heating (1000–1400 °C) as observed during dehydration of boehmite. However, the transformation temperatures of metastable alumina phases in SiC containing nanocomposite coatings were 100–200 °C higher than the pure alumina coatings.