Modifications of phases, microstructure and hardness of Ni-based alloy plasma coatings due to thermal treatment

Abstract

This article presents an experimental study of nickel based plasma sprayed surface coatings obtained using different processing and heat treatment conditions. In as-coated state the coatings are multilayered and contain structural components in varied shapes. The influence of the plasma spraying regime on the character and the amount of the porosity of the layers was analysed. The phase composition of the layers consisted of both amorphous state and crystalline fcc phases in ratios, which depended on the processing parameters. When the coating was subjected to a linear-rate secondary heating the amorphous phase decomposed in the temperature interval 500–700 °C into the phases Ni 4Si, Ni 3Si and Ni 3B. No significant changes in the morphology of the microstructure were observed during the heating to temperatures up to 700 °C. Further increase of the temperature resulted in numerous reversible and non-reversible phase transformations, which were found to depend on the temperature of heating. The microstructure of the coatings became more homogeneous due to the heat treatment. The hardness of the plasma coated layers increased during secondary heating to 700 °C and thereafter decreased upon further increase of the temperature.