Research On Ablation Properties Of Carbon/carbon Composites For SRM Throat

Nozzle is one of the key components in Solid Rocket Motor （SRM） and throat is the most important part in nozzles for it works in the most serious conditions. With the development of SRM for more loads, huger propulsion, elevated temperature and farther shot, its working conditions becomes more and more severe. The requirements for nozzle materials are accordingly increased. Carbon/carbon （C/C） composites have superior characteristics of low density, high strength, excellent ablation property, good thermal shock resistance, designable characteristic and so on. These outstanding properties make them the essential candidate materials for high temperature applications, which is especially true for SRM throats.With the application background of C/C composites for SRM throat, influences of ablation conditions, reinforcements as well as types and structure of carbon matrix on ablation properties of C/C composites were investigated after arc stagnation ablation test and the ablation mechanism were discussed;Combine with preparation of SRM throats, ablation morphilogies of C/C throats were observed and their ablation mechanisms were discussed; Furthermore, C/C composites added with refractory carbide or with SiC coating were prepared, and their ablation behaviors and properties were analysized; In addition, the common used methods for testing ablation rates of C/C composites were evaluated. Effects of ablation conditions （stagnation pressure, enthalpy, and ablating time） on ablation properties of C/C composites were investigated. The results show that around 3000℃, the higher of the enthalpy （temperature）, the more serious ablation of the composites; The longer the ablating time, the much loss of the ablation quantity; But among them, stagnation pressure has greater influence on the ablation property of C/C composites.The ablation properties of C/C composites were investigated with arc stagnation ablation test.The results show that ablation property of C/C composites of parallel ablation or with rough laminar carbon is respectively better than that of vertical ablation or with smooth laminar carbon. vertical ablation; In addition, the more of pyrolytic carbon or the increasing numbers of pierced carbon fibers, the better of ablation property.Ablation mechanism of C/C composites was discussed and ablation models were built. The ablation process of C/C composites is controlled by chemical erosion and mechanical denudation; and macro morphology is mainly caused by mechanical denudation, while micro morphology is mainly caused by chemical erosion. Ablation always tends to occur at interfaces, defects and pores and furtherly spread. Thus make the gaps at the interfaces, defects and pores enlarged. In addition, newly born cracks formed by thermal stress and inherent cracks will extend and transfix. Under the shearing force and swirl force of the ablation gas or thermal stress, the surfaces of C/C composites would be blown away and form orderly new ablation section cause mechanical denudation.Combining the development for SRM throat, C/C composite throats were discussed tothgether with their ablation mechianism. C/C throats with needled integrated carbon fiber performs have lower ablation rates and uniformed ablation surfaces. Because there are only a few fibers in Z direction, strength in Z direction is low, which is eay to cause a few cracks along carbon fiber layers at throat or the places close to throat; C/C throats with carbon fiber pierced performs have good thermal properties, mechanical properties, ablation properties, and small anisotropy; Besides, with the increasing of pressure in combustion chamber, ablation rate of C/C throat will increase. The ablation process of C/C throats includes chemical erosion and mechanical denudation. Erosion of particles will reinforce the transmition of heat, change or even destroy the structure of C/C composites and accelerate the chmical erosion process.Changes and function of WC in C/C composites during the ablation process were analysized. Not only macro morphology but also micro morphology of C/C composites with WC were quite different from those without; the ablation process of C/C composites with WC includes: oxidation of carbon fibers, carbon matrices and WC, melting of WC and WO₃, together with denudation of WC, WO₃ and C/C composites. Effect of SiC coating on ablation property of C/C composites was found. During the ablation process, SiC will be oxided and SiO₂ will be melt, therefore, SiC coating can low down the ablation rates of C/C composites and ameliorate their ablation surfaces.The testing methods for ablation properties of C/C composites were compared. The plasma ablation method is easy to operated, but its ablation conditions are quite different from real throat; Furthermore, the ablation rates are too small and the repetition testing results are bad, so it can only test ablation properties in a qualitative way; In arc stagnation ablation test, stagnation temperature, pressure and ablation time can be adjusted, its ablation rates are moderate and the ablation results can be repeated, thus it can model real throats better. In addition, compared with SRM fire test, it is much easier and cost much less.Those results will have meaningful worthiness for further development and applications of C/C composites with lower ablation rates.