Research On Thermal Shock And Ablation Resistance Of High Thermal Conductivity Carbon/Carbon Composites

The high thermal conductivity carbon/carbon(HTC-C/C)composites are applied in the thermal protection structure of aircraft to achieve the heat-dissipation under cyclic heat-generation and balance the thermal-load under high-density heat flow since they own the effective heat-transfer performance and the excellent mechanical properties at high temperatures,so it is of importance to study their thermal shock and ablation resistance to improve the service reliability of the thermal protection system(TPS).In this work,the unidirectional HTC-C/C composites were prepared.HRSEM,HRTEM,micron-indentation test results,internal friction,Raman spectra,flexural strength,thermal conductivity,etc.,were obtained to characterize the microstructure,interface,internal defects,carbon structure,mechanical and thermal conductive properties.The thermal shock stability and anti-ablation performance of HTC-C/C composites were studied.The effects of heat treatment or nanophase(carbon nanotube,CNT and/or carbon nanosheet,CNS)modification on the microstructure,mechanical and thermal conductive properties,thermal shock and ablation resistance of HTC-C/C composites were investigated.The main research contents and conclusions are listed as follows:The isothermal chemical vapor infiltration(ICVI)was used to prepare the unidirectional HTC-C/C composites,and then they were heat-treated at 2450℃.(I)Effects of heat treatment on the microstructure and performances of the composites were investigated.After heat treatment,the interface bonding was weakened and the internal defects were produced more;the flexural strength was improved by 19.1%due to the weakened interface conducive to the fiber pull-out.Meanwhile,the matrix presented medium or high texture and the microcrystalline structure of the fiber and matrix was more complete,causing the axial thermal conductivity increased by 59.2%.(II)Influences of heat treatment on thermal shock resistance of HTC-C/C composites were analyzed.For the untreated composites,with more thermal shock times,the flexural strength and axial thermal conductivity increased at first,attributed to the released residual stress and the improved carbon structure of matrix respectively;and then they changed to decrease,owing to the alternating thermal stress causing the severe structure damage.As for the heat-treated composites,the structure sensitivity to the alternating stress declined;the strength maintained slightly increased and the axial thermal conductivity continued to decrease;while their mechanical and thermal conductive performances were still much greater than those of the untreated samples.Besides,the ablation resistance of the unidirectional HTC-C/C composites related with their fiber orientations was studied.When the fiber axis was parallel to the flame,the ablation damage became more serious as the ablation process went on since more pores were exposed to the flame,facilitating the flame to deepen-in;it could be called the "pore-accelerating-ablation" mechanism.When the fiber axis was vertical to the flame,more carbon fibers were exposed to the flame as the ablation process continued,increasing the heat dissipation capacity and reducing the structure damage;it is called the "fiber-suppressing-ablation" mechanism.Electrophoretic deposition(EPD)was used to decorate CNT onto the mesophase pitch(MP)-based carbon fibers and then the CNT-modified HTC-C/C composites were prepared.The interface bonding was greatly improved and the amount of defects became less;the flexural strength increased due to the strong interface;the axial thermal conductivity declined owing to the greater phonon scattering at interface.With more thermal shock times,the strength increased since the partial debonding at interface released the residual stress and the moderately weakened interface was conducive to the pull-out of fibers and CNT.The axial thermal conductivity increased at first,related to the improvement of carbon structure of CNT and Py C;while it changed to decrease with more severe fiber damage and more matrix defects.By introducing CNT,the anti-ablation performance of the composites was greatly improved,and the difference in the ablation resistance due to the different fiber orientations was significantly reduced.The chemical vapor deposition(CVD)and EPD were used to introduce CNS and CNT into the interface and matrix respectively;then the CNS-CNT synergistically-modified HTC-C/C composites were prepared.By strengthening the interface and matrix,the flexural strength of the composites was increased by 78.2%.The axial thermal conductivity was improved by 27.9%,since CNS at interface favored the heat transfer from the matrix to the fiber and CNT between Py C layers contributed to the heat conduction in the matrix.The mass ablation rate of the composites decreased by 81.8%,compared with that of the unmodified ones;this was not only related with the strengthened interface and matrix better resisting the mechanical denudation,but also depended on the weakened oxidative erosion caused by the decreased surface temperature and the less structure defects.A triple-layer TPS was prepared and the effects of such design on temperature decrease and structure protection during the ablation process were investigated.Compared with TPS made of PAN-C/C composites and carbon foam,the triple-layer TPS presented better anti-ablation performance with the mass ablation rate dropped by 90.0%,owing to the greater heat-transfer capability to reduce the temperature and more excellent oxidation resistance of HTC-C/C composites.Compared with TPS composed of HTC-C/C and PAN-C/C composites,the back surface temperature of the triple-layer TPS was lowered by ～700℃,attributed to the good thermal insulation effect of carbon foam.