| Niobium and niobium alloys are the most potential candidates as high temperature structure materials for aircraft and aerospace engines, weapons thruster, nuclear reactor, submarines and gas turbine enginery, owing to their good room and high temperature mechanical strength, high melting point, low density, and excellent processability. However, one of the main drawback of Niobium and niobium alloys is the poor oxidation resistance, Nb2O5formed in the oxidation process has no protective ability and abscises easily. In order to solve their high temperature oxidation resistance behavior, the silicide coating was prepared on Nb521alloy surface by glow plasma surface metallurgy (GPSM) technique. The isothermal anti-oxidation properties were tested in static air at1100℃,1200℃,1300℃,1400℃and1500℃,respectively.The microstructure, chemical composition, phases distribution and properties of the coating and the relationship between these features and the oxidation resistance were analyzed by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and oxidation resistance measurement. The oxidation mechanism of the different coatings was discussed based on the investigation of the experimental results, and the following main conclusions can be drawn:Fe-Cr-Mo-Si and MoSi2coating were fabricated successfully using GPSM technique.The structure of the coating is columnar crystal which is composed of deposition layer and diffusion layer. There is no significant interface between diffusion layer and substrate, and alloying elements content decreases in gradient along the distance from surface to the center.Kinetic curves of oxidation of Fe-Cr-Mo-Si and MoSi2coating were studied. The results show that the growth kinetic fit well to parabolic law.The two types of coating are all can improve the anti-oxidation properties.The oxidation behavior of the two coatings was compared, and it was found that MoSi2coatings have better oxidation resistance than Fe-Cr-Mo-Si coating. The excellent oxidation resistance of MoSi2comes from continuous SiO2glass film formed at high temperature on its surface, which has lower oxygen diffusion rate and self-healing function that could protect the material from further oxidation.Some defects such as micro-cracks are usually caused by different coefficient of thermal expansion between substrate and coatings during depositing and oxidation. The defects accelerate oxidation process and reduce the oxidation resistance of coatings. |
