Study On High Silicon Coating By Dc-plasma Jet Surface Metallurgy

On the background of lots of researches and theoretical analyses, on the basis of the principle of alloy powder design, Fe-Si and Fe-Si-B high Si coating alloy powders were designed. By means of seeding powder synchronically, high Si coating has been prepared on the substrate of the Q235 steel by high energy plasma beam.The process of DC-plasma jet surface metallurgy has been discussed, which is a quick and non-equilibrium solidification process. With the movement of plasma torch, melting and freezing occur simultaneously, and the process of surface metallurgy can be regarded as a phasing and continuous reaction process. The solidification mechanism, characteristics and influence on structure of DC-plasma jet surface metallurgy have been analyzed in full.Microstructure, chemical composition and phase composition of the Fe-Si and Fe-Si-B high Si coating have been studied in detail using scanning electron microscope (SEM) equipped with energy dispersive spectrum (EDS), and X-ray diffraction (XRD). Microhardness, soft magnetic properties of the Fe-Si and Fe-Si-B high Si coating were measured and analyzed by microscopic hardness meter and vibrating sample magnetometer (VSM), soft magnetic properties and the relation to microstructure and alloy elements content were also analyzed. At last the high temperature oxidation-resisting properties of Fe-Si-B high Si coating were analyzed by TGA/DSC1. Temperature-programmed oxidation behavior and oxidation mechanism of the coating at 100℃-1200℃in air were preliminary studied in the paper.Results show that there is no obvious transition from Fe-Si high Si coating to the substrate, while there is typical white planar crystal structure from Fe-Si-B high Si coating to the substrate. With the distance increasing from the substrate, the crystal structure changes from bulky branches in the bottom of the coating to regular dendrites in the middle and fine dendrites near the surface. The coating is comprised ofα-Fe,Fe3Si,Fe5Si3,FeSi2,FeB,Fe5SiB2. The Si content, B content(<3.8%)of the coating is higher, the microstructure of the coating is finer, microhardness is higher and soft magnetic properties are better. Microhardness of Fe-Si-B high Si coating increases greatly than the substrate, and microhardness distribution from the coating to the substrate is in agreement with Si content distribution. Sample9, containing Si 15%, B3.8%is the best. Its top coating microhardness is the highest (926HV0.3), its coercive force is only 0.031 KOe, magnetic saturation is 101emu/g and residual magnetic saturation is 0.76emu/g, rectangular ratio is 0.008. Sample9 and 10 begin to oxidize strongly from 880℃,962℃respectly.