Reseach About MoSi₂-SiC Composite Coating On Carbon Materials By Molten Salt Method In Situ Reaction

Carbon materials have many excellent properties, such as low density, low coefficient of linear expansion, excellent resistance to friction and wear performance, good thermal conductivity, etc. Carbon materials are widely used in aerospace, metallurgy, chemical industry and other fields. However, carbon materials started oxidation at400℃in the air, the structure and performance of the carbon materials is affected by oxidation, it limits its application as high temperature refractory materials. In order to solve the problems of high temperature oxidation, the MoSi2-SiC composite coating was prepared by salt pack cementation at about1000℃, then the oxidation resistance of the composite coating was studied in the air.Salt pack cementation used molten salt as heat medium and chemical reaction medium, making use of the ion migration in the molten salt system and high diffusion velocity and mixing of reactants in order to reduce the reaction temperature and reaction time significantly. The Mo2C coating was preparaed by secondary ammonium molybdate and KC1. It combined with the matrix well. There were no obvious penetrating cracks, thickness of the coating was about50μm. The coating was more compact with the higher concentration of secondary ammonium molybdate under the condition that the concentration of secondary ammonium molybdate was less than30%. If the concentration was more than30%, the density of the Mo2C coating was smaller. Then Si dissolved in the molten salt NaCl and NaF, reacting with Mo2C in order to obtain MoSi2-SiC composite coating. The MoSi2-SiC composite coating also combined with the matrix well. Thickness of the coating was about60μm. The coating was more compact with more contents of Si under the condition that the contents of Si was less than20%. If the contents were more than20%, the density of the MoSi2-SiC coating was smaller.Carbon materials with the MoSi2-SiC composite coating oxidated at different temperatures, after oxidation there were three layers:near the substrate was the M04.8S13C0.6transitional layer, middle was the MoSi2-SiC coating, the outside was SiO2coating. The weight after oxidation of graphite samples relative to the original weight was always on the increase, but increased weight gradually reduced. The weight increased0.24%after oxidation at1500℃for100h. The experimental results showed that graphite samples with MoSi2-SiC composite coatings had good oxidation resistance.The weight after oxidation of carbon/carbon samples relative to the original weight was always on the increase at900℃for20h, but at1300℃,1400℃and1500℃, increased weight gradually reduced.The weight reduced0.07%after oxidation at1500℃for10h. Results showed that the MoSi2-SiC composite coating achieved the purpose, but the oxidation resistance was worse than that of graphite, the oxidation resistance of carbon/carbon composites remains to be further improved.46figures,5tables,79references...