C / C Composites Of Mosi <sub> 2 </ Sub> / Sic High Temperature Anti-oxidation Coating

The anti-oxidation of carbon/carbon composite is the key point of the study field about the composite. In the paper, the preparation of MoSi2/SiC system high temperature anti-oxidation coating was probed into by the complex process of burying and sealing, and then the formation mechanism, structure and framework characteristic , anti-oxidation property and laws of the coating were researched.The optimizing infiltration compositions and preparation process parameter of the coatings were obtained on the basic of the experimental investigation. Two-stage holding process in the preparation of coating and sealing in the surface of the coating have been proposed. The optimizing composites were 50%MoSi2+40%SiC+10%Al2O3 and 50%MoSi2+ 20% SiC+20%Si+10%Al2O3 at weight percent respectively. So, MoSi2/SiC composite coating with excellent anti-oxidation property can be prepared by the process.It was found that: the composite gradient coating with suitable framework, which is SiC transiting layer, SiC compacting layer,MoSi2/SiC double phase layer and outer layer containing MoSi2 mainly in the inner to outer of the coating ,is prepared by the two-stage burying process. As the holding time at high temperature enlongs, the thickness of the coating increases, for example, the thickness of 480 m holding for 6 hours at 1950 C,and the thin layer coating MoSi2 mainly in the surface of the coating is continuous. The bond between the coating and the carbon/carbon composite is mainly metallurgical bond, and mechanical bond, so the bonding strength is high. The surface of the coating is sealed by Si(OC2H5)4,and the SiO2 in gel can fill the cracks and cover the coating. The oxidation of specimen without sealed coating is losing weight, and that of the specimen with seal coating is increasing weigh after given oxidation time in 1200-1500 C air.According to thermodynamics analyses, the oxidation of the coatingcan be in (1) Si (s) +O2 (g) =SiO2 (s) (2) SiC (s) +2O2 (g) =SiO2(s) +CO2 (g) (3) 5MoSi2 (s) +7O2 (g) =Mo5Si3 (s) +7SiO2 (s) ,andMoSi2 is oxidized easier then Sic, and oxidation outcome is SiO2 in the MoSi2/SiC coating. As the oxidizing temperature is elevated, the oxidation weight loss rate of the coating without sealing in 1200-1 500 C air.To sum up, the optimizing infiltration compositions and process are:(l) 50MoSi2+20%Si+20%SiC+10%Al2O3 , holding 2 hours at 1700 Cand holding 6 hours at 1850 C in Ar protecting atmosphere;(2) 50MoSi2+ 40%SiC+10%Al2O3, holding 2 hours at 1700 C and holding 6 hours at 1950 C in Ar protecting atmosphere, and then the coating is sealed by Si(OC2H5)4.The best anti-oxidation effect is 1.2802% increasing weight after oxidation for 52 hours in 1500 C air.