Parameter optimization of HVOF sprayed nanostructured alumina and alumina–nickel composite coatings

The use of a dense ceramic layer as an environmental barrier is often limited due to the differences in coefficient of thermal expansion (CTE) between the coating and steel substrate and poor mechanical strength of the ceramic layer. Nanocrystalline composite materials have been recognized to have special mechanical properties, especially improved fracture toughness in bulk form. In this paper, efforts in transferring the same type of improvements into a thermal spray coating have been made. Development of high velocity oxy-fuel (HVOF) spraying of nanocrystalline Al2O3- and Al2O3–Ni-coatings, where up to ten percent of nickel has been added in order to toughen the coating, is described. Spray parameters were optimized for HV-2000 HVOF spraying system based on the on-line diagnostics and single splat studies. Parameters were selected aiming at different melting stages of the powder. The resulting microstructure of the coatings and effect of it on the coating properties is discussed.