Abstract :
Alumina phases formed in Al2O3 coatings and nanocomposite coatings of Al2O3–SiC, Al2O3–Mo were investigated. The coatings were prepared using high velocity oxy fuel spraying (HVOF) and air plasma spraying (APS) techniques on mild steel substrate either with Ni5Al or Ni20Cr bond layer. The amount of SiC and Mo were varied from 2 to 8 and 12 to 22 mass%, respectively. X-ray diffraction technique was used for phase characterisation. All the coatings showed the presence of varied amounts of well-known alumina phases, α- and γ-Al2O3, in the as-sprayed condition and after heat treatment up to 700 °C. In addition, a new highly oriented phase, referred to as U phase and characterised by an orthorhombic unit cell with a=8.644 Å, b=8.286 Å, c=4.922 Å, was observed in all the alumina-based coatings. In as-sprayed condition, the U phase appeared in all the coatings prepared with Ni5Al bond layer and not in others. The new phase developed in other coatings, without bond layer or with Ni20Cr bond layer, under favourable energetics, i.e., after mild heat treatment at temperature as low as 100 °C and even due to mechanical stresses induced during cutting or peeling.
