Investigation Of C/C-SiC-ZrB₂ Composites For Anti-ablation

Investigation of ultra high temperature material which can endure temperature over 2000 oC has been a key technique for the Thermal Protection System and power system of new aerospace aircraft. Carbon/carbon（C/C） composite is one of the most promising candidates for the application. A lot of researches have been performed to improve the ablation resistance of C/C composite by introducing Ultra High Temperature Ceramics（UHTC） into it. However, how SiC works on the ablation of C/C and UHTC modified C/C composites has not been clarified, and effect of distribution and structure of introduced ceramics on the ablation property of C/C composites are rarely reported now. To solve the above problems, ablation behavior of C/C composites modified by ZrB₂ and SiC under oxyacetylene torch were studied in the present work.Using 2D needle punched carbon fabric as reinforcement, natural gas as precursor of pyrocarbon, polycarbosilane and organic ZrB₂ as precursor of introduced ceramics, SiC, ZrB₂ and ZrB₂-SiC modified C/C composites were prepared by method of precursor infiltration and pyrolysis（PIP） combined with thermal gradient chemical vapor infiltration（TCVI）. Three-point bending test was performed to evaluate the mechanical property of prepared composites. Specific heat capacity and thermal conductivity of the composites were determined by laser scattering method. Ablation under oxyacetylene torch in different heat fluxes, single-cyclic modes and different loading time were studied to understand the ablation mechanism of the modified C/C composites. The phase analysis before and after ablation were conducted by XRD. Morphology and chemical composition were investigated by SEM combined with EDS. The surface temperature during ablation was measured by an infrared thermometer. The oxidation and mechanical erosion during ablation were discussed depending on the surface temperature and peeling off of ceramic particles. The main contents and results are listed as follows:Two C/C-SiC composites whose SiC particles deposited at non-woven and web layer respectively were prepared by changing the parameters of PIP and TCVI. Effect of addition and distribution of SiC on the three-point bending behavior, thermal conductivity and ablation property of C/C composites were studied. Compared to C/C composites, C/C-SiC composites containing non-woven layer distributed SiC have lower bending strength for the oxidized carbon fibers during pyrolysis of precursor, higher bending modulus and similar thermal conductivity, C/C-SiC composites containing web layer distributed SiC have higher bending strength, same bending modulus but decreased fracture elongation and thermal conductivity. In heat flux of 2.38MW/m2, the ablation of SiC is slower than pyrocarbon and its oxidation product could infiltrate defects, thus C/C-SiC has better ablation resistance than C/C composites. In heat flux of 4.18MW/m2, the high surface temperature over 2000 oC accelerates the vaporizing of SiO2 and causes the faster consumption of SiC than pyrocabron, finally results in higher ablation rates of C/C-SiC than C/C composites. The effect of SiC on the ablation in different heat flux of C/C composites is more obvious when SiC located in web layer than SiC in non-woven layer for the distinct content of the two layers in composites.Effect of SiC addition on the property of C/C-ZrB₂ composites was studied by introducing ZrB₂ and ZrB₂-SiC to the interface of fiber and matrix. With rising of heat flux, ablation rates of C/C-ZrB₂-SiC increase faster than that of C/C-ZrB₂ composites as the result of vapor pressure of SiO2 versus temperature. In a certain heat flux, the surface temperature is mainly controlled by specific heat capacity and thermal conductivity of composites, while accumulations of ZrO2 and SiO2 are also work on that. In heat flux of 2.38 MW/m2, SiC could improve the cyclic ablation resistance of C/C-ZrB₂ composites. Besides, ablation in 2.38 MW/m2 for different time indicated that SiC could reduce the surface temperature and form a SiO2 rich sub layer in short term ablation. With prolonging of ablation, SiC loses its effect gradually, the surface temperature and ablation rates of the two composites tend to be the same.ZrB₂ and ZrB₂-SiC were introduced into pyrocarbon matrix of C/C composites. Interface and matrix distribution were compared. ZrB₂ and ZrB₂-SiC in matrix present gathered state. Composites containing matrix distributed ZrB₂ and ZrB₂-SiC have higher three-point bending strength as the result of fiber protection by pyrocarbon, lower thermal conductivity caused by new interface and defects induced by ZrB₂ and ZrB₂-SiC. After high temperature heat treatment, for composites containing matrix distributed ZrB₂ and ZrB₂-SiC, four cracks come into being, bending strength and modulus decrease, thermal conductivity rises, ablation in 2.38 MW/m2 is promoted and in 4.18 MW/m2 is weakened. Sealing of cracks and local thermal stress are the main reasons for the above ablation behavior. As a result of mechanical erosion, interface distributed ZrB₂ and ZrB₂-SiC are more effective than matrix distributed ones in improving of ablation resistance.C/C-ZrB₂-SiC composites containing isolated and coupled ZrB₂-SiC particles were prepared. The two composites have similar three-point bending behavior while composites containing isolated ZrB₂-SiC particles have lower thermal conductivity. For coupled ZrB₂-SiC particles, the fast ablation of SiC in high temperature accelerates peeling off of ZrB₂. Thus composites containing isolated ZrB₂-SiC particles have better ablation resistance.C/C-ZrB₂-SiC composites containing uniformly distributed and layer segregated ZrB₂ and SiC were prepared. ZrB₂ exist as particles and SiC as relative continuous phase. More ZrB₂ in non-woven layer is beneficial for the three-point bending strength and modulus while reduces the fracture elongation. Ablation in different heat flux indicated that more SiC in non-woven layer is helpful to ablation resistance. The defects induced by ZrB₂ and SiC at high temperature and the structure of ZrB₂-SiC are the main reason for the different ablation of the three kinds of composites.