Microstructure and properties of Ni-Al coatings obtained by conventional and high-velocity atmospheric plasma spraying

Abstract

In this paper, we report on the microstructure and properties study of the Ni-Al system (75/25 at. %) coatings obtained by atmospheric plasma spraying (APS) and high-velocity plasma spraying (HV-APS) methods which differ in the speed of the sprayed particles. The structure and phase composition were investigated using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray phase analysis (XRD). The paper presents the measurements results of porosity and microhardness, as well as the resistance of coatings to high-temperature oxidation. Additionally, we propose a phase transformations model that explains the difference in structure formation. For HV-APS and APS coatings, the difference in structure is shown to be due to different residence conditions of the sprayed particles in the plasma jet. Particles in the HV-APS coatings were completely melted, while a significant proportion of unmelted particles was observed in the APS coatings. HV-APS coatings have better high-temperature oxidation resistance compared to APS coatings, which is explained by a decrease in their porosity to ∼2 %.