On the nature of phases in Al2O3 and Al2O3-SiC thermal spray coatings

Nature of alumina phases in thermal sprayed Al2O3 and nanocomposite Al2O3-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 mum and I rum. 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 degreesC); peels (free forms) of roller specimens were used for high temperature (greater than or equal to1000 degreesC) 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 mum, gave rise to (contrary to the earlier hypothesis) unmelted alpha-seeds (nuclei) and alpha-alumina nucleated in preference to gamma-alumina. Presence of SiC (mostly inside alumina grains) in alumina nanocomposite coatings increased the amount of a-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 alpha-phase by 200 degreesC. The alumina phases in HVOF Al2O3-SiC coatings undergo alpha --> delta --> theta --> alpha phase transformations on heating (1000-1400 degreesC) as observed during dehydration of boehmite. However, the transformation temperatures of metastable alumina phases in SiC containing nanocomposite coatings were 100-200 degreesC hi,,her than the pure alumina coatings. (C) 2004 Published by Elsevier Ltd and Techna Group S.r.l.