| Nb-based alloys are important high temperature structural materials due to their high melting point, good properties of high temperature strength and corrosion resistance. But the applicability is limited by their poor oxidation properties. In order to solve this problem, the silicide was used as the high temperature oxidation resistance protective coating for Nb-based alloys. The modified Si-Cr-Fe coating by Ge added was prepared by second slurry-firing. The ultra-high temperature oxidation resistance MoSi2 coating was also produced by slurry-firing technology. The static and dynamic anti-oxidation properties were tested at various temperature 1200℃, 1400℃, 1600℃. The structure, composites and phase distribution of the coating and the relationship between these features and the oxidation resistance of the coating were investigated by SEM, EDS, XRD and Microhardness Analysis. A reactive diffusion model of interface between the MoSi2 coating and the Nb substrate was established. The characteristic and the disciplinarian were explored by compare and analysis of the collected data. Through the study, the following main conclusions can be drawn.The second sully-firing technology can improve the uniformity of the structure and the compaction of the surface on coating, widen the transitional layer, and improve the oxidation resistance properties. The modified element Ge mostly concentrated in the body of the coating and the interfacial layer. It can improve the anti-oxidation properties, and effectively stop the extension of the crack in coating. A multi-layer structure can be found in coating. The microhardness was gradually decreased through the cross-section on the coating, and a larger change was found on the interface between the various layers. The good anti-oxidation properties were decided by the integrative effect of the surfaced status, structures of the coating, and the thermal expansion coefficient mismatch.The high temperature oxidation resistance property of MoSi2 coating is better than the modified Si-Cr-Fe coating. Both of the two coatings can selective oxidation to form oxide glass with the self-healing function on the surface. The impurity, which existed in slurry, destroyed theintegrality of the surface, and reduced the anti-oxidation properties of the coating. The interface between the coating and the substrate was metallurgic combination, and a transitional layer was formed by diffusion. The composite multi-layer structure improved the oxidation resistance properties of the coating. As further interdiffusion of the element, which between the coating and the substrate, a large number of cavities were formed and perforated transversely. The transverse cracks can also be formed by thermal stress. The coating was broken through the interface between the body of the coating and the transitional layer.Three hypotheses were proposed: 1) the diffusion is very slow, and the whole process was controlled by the diffused law; 2) the diffusion should belong to body diffusion, but the short distanced diffusion; 3) the reaction between the reactive phases should be completed instantly. In other words, the quasi- equilibrium status should be always maintained in the reactive interface. A reactive diffusion model of interface between the MoSi2 coating and the Nb substrate was established based on the Nb-Si binary phase diagram and the experimental results. The calculated numerical values were compared with the experimental results, and the factors of the according error were analyzed. Several referenced opinions were proposed for study the high temperature oxidation resistance protective coating of Nb-based alloys further. |
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