Ultra High Temperature Ceramic Modified Carbon/Carbon Composites Prepared by Reactive Melt Infilation Method

Author:Liu Yue

Supervisor:fu qian gang

Database:Doctor

Degree Year:2018

Download:20

Pages:155

Size:14172K

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Carbon/carbon(C/C)composites have low density,high specific strength,high specific modulus,and excellent mechanical properties at high temperature,and so on,which are considered as one of important candidates for the thermal protection system and dynamical system of advanced aerospace aircraft.The application of C/C composites in aerospace field is severely restrict due to their high oxygen sensitivity.In order to develop the application field of C/C composites in high temperature environment,the Ultra High Temperature Ceramics(UHTC)were introduced into C/C composites by using reactive melt infiltration(RMI)and isothermal chemical vapor deposition(ICVI).Using 2D needle punched carbon fabric as reinforcement,natural gas as precursor of pyrocarbon(PyC),SiC,SiC-ZrB2 as ceramic matrix,the C/C-SiC,C/C-SiC-ZrB2 composites were prepared by RMI combined with ICVI.In order to improve the fracture tolerance and strength of C/C-SiC-ZrB2 composites,SiC nanowire reinforced C/C-SiC-ZrB2 composites were obtained via simple in-situ growth of SiC nanowires directly in the porous preform before reactive metal infiltration of SiC-ZrB2 di-phase ceramic matrix.The flexural strength of the composites was carried out by a three-point bending test.The specific heat capacity and thermal conductivity of the composites were performed by laser scattering method.Ablation properties of C/C and ceramic modified C/C composites were tested in oxyacetylene torch and a small solid rocket motor.The phase and composition of samples before and after ablation test were identified by X-ray diffraction(XRD).The microstructure and morphology of ablation surface of the composites were characterized by scanning electron microscopy(SEM)combined with energy dispersive spectroscopy(EDS).The main contents and results are listed as follows:The C/C composites with different densities were prepared by ICVI.The holes in the carbon fabric were filled with PyC,the density of C/C composites was increased with the extension of time.During RMI process,the low density of porous C/C composites has larger porosity,which is beneficial to the infiltration of high temperature melt.The pre-deposited PyC reacted with high temperature melt and generated ceramic matrix in the modified C/C composites.The density of modified C/C composites was linearly decreased with the increasement of the density of C/C composites.The improvement of heat treatment temperature and holding time is advantageous to the high temperature melts infiltration.The temperature is too high,resulting in the erosion of carbon fiber,decreasing the mechanical properties of composites.As the extension of holding time,the infiltrated depth increased gradually.However,the increment of infiltrated depth was not obvious after the holding time more than90 min.The good comprehensive performance of modified C/C composite was prepared by using porous C/C composites with density of 1.201.35 g/cm3 under the temperature of20002100℃and holing time of 90120 min.SiC and SiC-ZrB2 modified C/C composites were prepared by changing the heat treatment temperature and holding time.The flexural strength of the modified composites was carried out by a three-point bending test.The flexural strength of C/C-SiC and C/C-SiC-ZrB2 composites is 105±10 MPa and 110±8 MPa,respectively.Compared to pure C/C composites,their values is increased by 31%and 37%,respectively.To improve the mechanical properties of C/C-SiC-ZrB2 composites,in-situ grown SiC nanowires prepared by sol-gel carbothermal reduction were introduced into porous C/C composites in advance of preparation of SiC-ZrB2 ceramic matrix in this work.The SiC-ZrB2 ceramic matrix was fabricated by RMI process.The flexural strength and fracture toughness of SiCNW-reinforced C/C-SiC-ZrB2 composites were 135±12MPa and 7.75±0.96 MPa·m1/2.In order to simulate the ablation behavior of the wedge components in high temperature environment,such as wing leading edge,nose,engine jet vane,the C/C,C/C-SiC,C/C-SiC-ZrB2 and SiCNW-reinforced C/C-SiC-ZrB2 composites were machined into wedge specimen,and their ablation resistance were tested by oxyacetylene torch.Results show that the linear ablation rate and mass linear ablation rate of modified C/C composites were lower than that of pure C/C composites.Compared with pure C/C composites,the mass ablation rate of C/C,C/C-SiC,C/C-SiC-ZrB2 and SiC nanowire reinforced C/C-SiC-ZrB2 composites was decreased by15.4%,48.9%and 69.1%,respectively;the corresponding linear ablation rate was decreased by 18.7%,68.8%and 82.9%.During ablation process,the SiO2 glass layer was blown by the oxyacetylene torch,the carbon fiber and matrix were exposed to high temperature combustion gas,leading to destruction of the main structure of CS composites.ZrO2-SiO2 layer could effectively resist the erosion of oxyacetylene.The larger thermal gradient and stress was generated during ablation test,resulting in the formation of defect and desquamation of ZrO2-SiO2 layer near the defect.After introducing SiC nanowire into C/C-SiC-ZrB2 composites,the mechanical properties of composites were significantly increased,resulting in an excellent configuration stability.In the solid rocket motor plume environment,C/C,C/C-SiC,C/C-SiC-ZrB2 and SiC nanowire reinforced C/C-SiC-ZrB2 composites were suffered seriously ablation in high temperature environment for 4.5 s.Their linear ablation rate were 5.65 mm/s,5.55 mm/s,5.11mm/s and 5.01 mm/s,respectively.During ablation process,the composites were suffered from thermochemical ablation,which directly weakened the bonding strength at the interface between the fiber and matrix,leading to the decreasing their strength.Subsequently,carbon fiber and matrix were striped by solid rocket motor plume that contained a mass of Al2O3particles,which has high temperature and kinetic energy.Therefore,the synergistic process of thermochemical ablation and mechanical erosion resulted in the large recession of the composites.