Study Of Microstructure And Properties Of Plasma-sprayed NiCr-Cr₃C₂ Coatings Both Before And After Remelting

Plasma spraying is one of the most commonly used thermal spray techniques for ceramic coatings on metal surface. It has the advantages of quick deposition velocity and high production efficiency. However, some defects, such as porosity, micro cracks, lamellar structure were existed in plasma sprayed coating unavoidably. The bonding state between the coating and substrate is a kind of mechanical combination. Remelting was applied to reduce these problems, by which excellent properties of ceramic coatings could be obtained.The influence of process parameters of laser and plasma remelting on the microstructure and performance of plasma-sprayed (PS) NiCr-Cr₃C₂ coatings was investigated by orthogonal experiment design. The microstructure and properties of plasma-sprayed NiCr-Cr₃C₂ coatings, both before and after remelting of the coatings, were investigated.Microstructure and phase composition of the coatings were studied by means of X-ray Diffusion (XRD), Optical Microscopy (OM), Scan Electron Microscope (SEM) and Energy Disperse Spectroscopy (EDS). The corrosion resistance of the coatings was tested by salt fog test (NSS) and electrochemical experiment. The conclusions were stated as below:The result showed that the plasma sprayed coating had a lot of defects. The microstructure of the coating were improved by remelting.The remelting could eliminate the lamellar structural properties of plasma sprayed coating and reform a continuous and compact coating. Compared with the plasma-sprayed coating, the phases of the laser remelting and the plasma coatings were changed. The effects of the remelting parameters on the plasma-sprayed coatings were studied. The optimizing parameter of laser remelting were Power 600W, Scanning velocity 1.5m/min and Moving distance 2.1mm.That of plasma remelting were Current 60A, Velocity 35cm/min. The corrosion resistance of coatings was checked by salt fog test and electrochemical test. There were corrosion producets at the plasma-sprayed coatings after 6h, the remelting coatings were glossy after 188h. The results showed that the corrosion resistance of remelting coating had been greatly improved than those of plasma-sprayed. The reason might be that the compact coating was obtained by remelting. It could prevent the electrolyte to penetrate into the inner part of coating. The relations of processing parameter and electrochemical corrosion resistance were showed by Tafel polarization curve.Remelting improved the electrochemical corrosion resistance, especially laser remelting.