炭／炭复合材料SiC／Mo（Six，Al1-x）2涂层形成机理与抗高温氧化性能

采用高温包渗技术在炭／炭复合材料表面制备了SiC／Mo（Six，Al1-x）2复合涂层，采用两步反应法研究了复合涂层的生成机理。发现复合涂层是由Si、Al2O3、SiC、MoSi2原始粉末材料与基体炭材料经过复杂化学反应生成的SiC、Mo（SixAl1-x）2以及微量Mo4．8Si3C0．6固溶体组成。在较低温度下（〈1750℃），单质硅与基体碳的液-固相反应，经过2小时后可以在炭／炭复合材料表面和内部孔隙表面生成致密的SiC过渡涂层；在较高温度下（≤2000℃），SiC、Al2O3和MoSi2间的反应较为复杂，其主要过程为SiC与Al2O3间生成液体硅、液体铝和气态SiO、Al2O的多相反应，该反应生成的液体铝能够与MoSi2颗粒发生置换反应，生成熔点降低的Mo（Six，Al1-x）2转移涂层；同时，生成的液体硅与CO反应生成晶须状β—SiC，并与Mo（Six，Al1-x）2形成增强型复合涂层。本文还研究了过量单质Si和SiC对Mo（Six，Al1-x）2的还原反应，化学反应推论与实验结果相吻合。以新提出的涂层生成机理为指导，以粉末原料质量组成为Si10％，Al2O3 10％，SiC54％和MoSi226％时所制得了致密并且无粘结的复合涂层材料，并研究了封孔处理后复合材料的抗氧化性能。

MECHANISM OF SiC/Mo(Six,Al1-x)2 COMPOSITE COATINGS FORMATION ON C/C COMPOSITE AND ITS OXIDATION RESISTANCE

SiC/MoSi2 dense coating can provide protection against high temperatures oxidation for C/C composite. Since Mo（Six,Al1-x）2 exhibits more excellent oxidation resistant properties compared to MoSi2,a series of composite coatings of SiC/Mo （Six,Al1-x）2 were prepared in the study with multi--phase reaction techniques on C/C composite. The mechanism of the coating formation was studied based on the method of two--step chemical reactions,and aided by analysis of coatings phase and composition. It was found that the major components of the coating were SiC and Mo（Six,Al1-x）2 with a trace of Mo4.8Si3C0.6 solid solution, which were formed from complex chemical reactions among carbon, silicon, alumina, SiC, and MoSi2 reactants. Therefore,SiC and Al2O3 are not only sintering resistant materials but also reactive agents contributing to the formation of coatings ,which are not understood before. According to the study ,mechanism of the composite coating formation followed two--step reactions. Namely ,the inner layer of dense SiC was formed from a reaction between liquid silicon with the matrix carbon under 1 750℃,which was a silicon erosion of carbon on both the surface and some inner pores of C/C composite. After the depletion of silicon and meanwhile buffer layer SiC formation,very complex chemical reactions occurred as the temperature increased up to 2 000℃ among SiC,MoSi2,and Al2O3. According to the results of chemical thermodynamic and elementary analysis,it was found that the major reaction occurred between SiC and Al2O3,resulting in the formation of liquid Si,Al,gaseous SiO and AlzO. The new formed Al reacted rapidly with MoSi2 powder, leading to the formation of Mo （Six,Al1-x）2 coating with decreased melting points compared with MoSi2. The new formed Si reacted with CO,leading to the formation of whisker--like beta--SiC phase, which combined with Mo （Six,Al1-x）2,and leading to the formation of the composite coating. According to the above new mechanism, Mo4.8Si3C0.6 was formed from a solid pha