Study On Microstructure And Properties Of Ni₅Al-Al₂O₃ Gradient Coating By Air Plasma Spray

This paper used the air plasma spray to prepare the Ni5Al-Al2O3 gradient coating on304 stainless steel surfaces by Ni5 Al and Al2O3 powders. Prepared the gradient coating with 600A/650A/700 A spray current and studied in the microstructure of Ni5Al-Al2O3 gradient coating by Scanning Electron Microscopy(SEM) and X-ray Diffraction(XRD).Compared the microhardness, the bonding strength between coating and substrate, the wear resistance of Ni5Al-Al2O3 gradient coating which deposited by different spray current.Tested the polarization curves and studied the high temperature stability and the oxidation resistance of gradient coating.The results show that the Ni5Al-Al2O3 gradient coating has typical lamellar structure and the coating contains a certain amount of porosity and some powders without fusion.The Ni5Al-Al2O3 gradient coating combines with the stainless steel in the shape of‘anchors’ with mechanical and metallurgical combination, without crack appears. The fracture morphology of gradient coating has mixed-fracture characteristics. The Fe-Ni compound generates between Ni5Al-Al2O3 gradient coating and stainless steel. There is some element diffusion between them. With the increase of spraying current, the spraying powders melt more completely. The amount of unmelted powders and porosity has decreased gradually. The lowest porosity is 5.99%, when the spraying current is 700 A.The bonding strength of Ni5Al-Al2O3 gradient coating is 14.1MPa, 15.1MPa and16.3MPa, when the spraying current is 600 A, 650 A and 700 A. After 12 hours’ oxidation by800°C, the Ni5Al-Al2O3 gradient coating has uniform organization without significant changes. With the increase of spraying current, the density, microhardness and the wear resistance of Ni5Al-Al2O3 gradient coating has increased and the porosity has decreased.The increase of oxidation weight has gradually reduced.The thermal shock times of gradient coating are between 50 to 60 times and it increases slightly with the increase of spraying current. The corrosion potential of gradient coating is higher than the stainless steel and the corrosion current density was less than the stainless steel. The gradient coating which is prepared by 600 A had the highest corrosion potential(-0.62V) and the lowest corrosion current density(0.018mA·cm-2). There are some differences in the corrosion resistance of the three different gradient coatings. The reason for the differences may be caused by the porosity of the gradient coating.The results show that, with the increase of plasma spraying current, the structure and properties of Ni5Al-Al2O3 gradient coatings have improved and enhanced. There are certain theoretical significance and good application prospects for the surface modificationof stainless steel.