Fabrication and Ablation Mechanism of Non-woven Cloth Needled C/C Throat Composites

Author:Li Yan

Supervisor:cui hong


Degree Year:2018





Non-woven cloth needled preform overcome the disconnected shortcomings of fibers between the 2D carbon cloth,and have the advantages of low cost,easy industrialization,suitable for various sizes,various shapes and good mechanical properties,etc.Thus it have been widely used as the expansion section,extension cone of nozzle and carbon brake disk.In order to apply this type of needled preform as the preform of solid rocket motor nozzle throat successfully,theoretical foundation for its application and the densification method of non-woven cloth needled C/C composites were studied.The microstructure and thermomechanical properties were characterized.Moreover,via small engine test and full-scale engine test at the service condition,the ablation behavior and ablation mechanism of non-woven cloth needled material were determined to provide a theoretical basis for its reliability.The main research works and results are as follows:The proper temperature field inside the preform is the precondition for the effective densification of thermal gradient chemical vapor infiltration(CVI).In this work,the thermal gradient CVI furnace temperature field is simulated and analyzed.Based on the internal temperature distribution of the preform at the beginning and end of deposition stages combined with the thermal gradient CVI densification model,the partition CVI process scheme for the thick wall preform was designed.The partition CVI process can be conducted by changing the temperature control mode,the densification behavior of two temperature controlling ways,namely keeping constant temperature of the inner wall or outer wall of the samples,were studied.The results indicated that,compared to the outer wall temperature controlling,the composite,prepared by the inner wall temperature controlling way,owned higher temperature and appropriator temperature gradient during densification,which was consistent well with the partition densification process scheme.Moreover,the inner wall temperature controlling way enabled the large-scale C/C composites densifying from inner side to outside wall,achieved uniform density distribution.The densification efficiency was improved by 73.8%than the material prepared by the current technology.Under the way of the inner wall temperature controlling,the densification behavior of needled preform was studied.The results showed that,compared to the integral felt preform,the inner space of the non-woven colth layer is straight and evenly distributed,which was more helpful for the deep penetration of the carbon source gas.With the same densification time,the density of needled preform can reach to 1.27g/cm3,and the internal density was evenly distributed.Three kinds of thick wall non-woven cloth needled C/C throat composites,namely IR-C/C,IP-C/C,and P-C/C respectively,were prepared by means of CVI and resin impregnation/carbonization(PIC),CVI and pitch impregnation/high pressure carbonization(HPIC),HPIC.Their thermomechanical properties showed that the needled materials prepared by the first two techniques meet the requirements of the throat material.The IP-C/C composites had excellent performance,the Z-direction tensile strength at room temperature and 2800℃reached to 24.7MPa and 52.9MPa,respectively,which were higher than needled intergral felt C/C throat composites used widely at the present.At the same time,thermal stability of IP-C/C were outstanding,the Z-direction thermal expansion coefficient at 1000℃was only 1.4×10-6/℃,and Z-direction thermal conductivity coefficient appeared to be 81.9 W/(m·K).The performance of the preform and final heat treatment process had great influence on IP-C/C composites.When the Z-direction tensile strength of preform rose from 0.0194MPa to 0.0628MPa,the Z-direction tensile strength of IP-C/C increased from 16.1MPa to 31.5MPa,and Z-direction compressive strength increased slightly.After the final heat treatment,the mechanical properties of the materials declined,the more raise of the heat treatment temperature,the more reduction of the performance.Nevertheless,the thermal stability of needled C/C composites increased.The ablation performance of needled C/C throat material under the service condition was characterized by means of small engine and full-scale engine.The results showed that the average linear ablation rate of needled throat composites were low,ablation surface was uniform and smooth.The average ablation rate of IP-C/C was only 0.056mm/s,which was 21%lower than that of IR-C/C.After the ground fire test,the structure of IP-C/C was complete.As for IR-C/C,due to high oxidative activity,the ablation surface was rough,there were small cracks existed,and the thermal shock resistance was low,but it was still higher than integral felt C/C throat composites used widely at the present,and that the preparation process was simple,the production cost was low.