The Preparation Of Niobium Alloy Silicide Coating And Their Modified And Research

Niobium alloys have been applied successfully in aerospace and nuclear industries due to their low density, high melting point, high strength and good deformation ability. However, poor oxidation resistance of niobium alloys seriously restricts its engineering applications. The protective coatings are the most effective way to protect niobium alloys from oxidation, and silicide coatings are the optimal selection among all of the coatings due to the good oxidation resistance at high temperature.In this paper, Si-Mo coating and CeO2modified coating were prepared by complex packing cementation method, also prepared Si-Ti-Cr and its modified coating by slurry fusing method. We explored the impact of the addition of rare earth oxide CeO2on morphology and performance of Si-Mo coating,then analysised mechanism of CeO2, the growth mechanism of Si-Mo coating, high temperature oxidation behavior and phase evolution with using of XRD, SEM, TEM, EPMA and EDS. In contrast of the surface, cross-sectional structure, phase composition and distribution of Si-Ti-Cr and its modified coating before and after high temperature oxidation. Also,discussed the impact of Al, W, rare earth oxide Y2O3of high temperature oxidation resistance of Si-Ti-Cr coating. The interface reactions between coating and niobium alloys were investigated. The kinetic equations of the coating were established.The conclusions are as follows:(1) Preparation of better performance Si-Ti-Cr coating by complex packing cementation method and optimized the ball milling time of Mo powder. After ball milling20h, the Mo powder particles was small and dispersed, which was good to the coating and sintering of Si-Ti-Cr coating, and it was uniform, compactfewer defects, also,the average of binding was about46.6MPa. The coating structure from the outside in order of the MoSi2main layer, microporous band, NbSi2layers, Nb5Si3diffusion layer. The initial stage of oxidation, the coating mainly occurred the selective oxidation of Si element and SiO2, MoSi2decomposed, from outside to inside followed by SiO2N MoSi2、(Mo,Nb)5Si3、 NbSi2and Nb5Si3layer. Oxidation late, NbSi2was complete decomposition and Mo involved in the oxidation reaction, then the rate of MoSi2consumption was gradually accelerated, also the coating oxidation rate was accelerated. (2) CeO2modified Si-Mo coating on niobium alloy matrix was prepared by composite pack cementation. The surface of the coating was roughness, uniformity and the particles was small. Also, the addition of CeO2had not changed niobium alloy Si-Mo coating structure. But could promote the formation of small grains in the level of micro and nano, which caused the coating densification. At the same time, CeO2produced reactive element which speed up the silicification rate, then promoted the formation of Si-Mo coating and shorten the pack cementation time nearly20%. However, the addition of CeO2promoted the adsorption of carbon atmosphere, then, increased the carbon content in the coating.(3) The process optimization of Si-Cr-Ti modified coating by slurry fusing method as follows:while sintering at1420℃for0.5h, the coating was thickness, uniform and the coating binding was50.1Mpa. The Si-Cr-Ti and its modification coating from outer to inner were MeSi2loose layer, two intertwined main laye NbSi2and MeSi2, Me5Si3diffusion layer(Me, refers to the coating and the substrate metal elements). Nb-Ti-Al alloy and Si-Cr-Ti coating generated SiO2, Al2O3and TiO2and Cr2O3composite oxide at1400℃sintering for11h. And MeSi2-layer becomed thin, while Me5Si3layer becomed thick.(4) The coating cross-section organization of Si-Cr-Ti-W was similar to the coating of the Si-Cr-Ti, which enriched in the surface of the coating to form a rough island-like organization. And the oxidation experiments showed that the addition of W could improve the heat resistance of the coating, then improve the coating oxidation. The coating surface of Si-Cr-Ti-Al-Y2O3was smooth and dense. Y2O3dispersed in the coating which promoted the formation of grain and increased the density of the coating. Al element enriched in the outer and generated Al4Si3structure,then after oxidation get SiO2film contained Al2O3, finally significantly improved the oxidation resistance.(5) Nb-Ti-Al alloy was mainly generated TiNb2O7、 TiO2、Al2O3composite oxide at1400℃within the time frame1-11h,while Nb521alloy was mainly generated Nb2O5. The two alloys, high temperature protection capability were limited. The oxidation parabolic rate constant dclined three orders or more when Nb alloy substratecoated Si-Mo, Si-Cr-Ti modified coating. It indicated that the silicide and its modified coatings had good high temperature oxidation protection ability. At the basis of the Wagner oxidation theory, the relevant oxidation model had been simplified. In the study, the weight gain of the kinetic equation of the Nb alloy matrix and the silicide coating oxidation could be expressed as follows:...