Investigation Of PIP Process And Properties Of ZrB₂/High Thermal Conductivity Graphite Film Modified C/SiC Composite

Near-space aircraft in the extremely harsh environment of high temperature and high pressure,it is urgent to find an anti-ablative composite material with longer working life.Carbon fiber reinforced ultra-high temperature ceramic matrix composites overcome the shortcomings of traditional ceramic materials such as brittleness and thermal shock resistance,and have good application prospects in the field of spacecraft thermal protection systems.At the same time,the concept of heat conduction protection is proposed to improve the thermal conductivity of the surface of the material,quickly evacuate the temperature of the ablation point,and increase the working life of the material.In this thesis,C/SiC-ZrB2 composites were prepared by PIP process with ZrB2 ceramic precursor and PCS.The high thermal conductivity graphite film was used as the reinforcement,carbon fiber was used as the reinforcing toughening phase,and PCS was used as the matrix precursor to prepare high thermal conductivity C/SiC Composite.The cracking process and densification process of precursor ceramic transformation were studied.The microstructure,mechanical properties,thermophysical properties and ablation resistance of C/SiC-ZrB2 composites and the content of different high thermal conductivity graphite films were studied.The effect of ablation resistance on the microstructure,mechanical properties and ablation resistance of high thermal conductivity C/SiC composites was discussed.The main contents and conclusions are as follows:(1)The molecular structure of polycarbosilane(PCS)and ZrB2 precursors was determined,which provided a theoretical basis for the subsequent cracking process.The cracking process of PCS,ZrB2 precursors and mixed precursors was explored,and the effect of heating rate on the cracking process was explored.The study found that the higher the heating rate,the lower the ceramic yield,because the precursor molecules were not cracked into the amorphous state in situ before they were rearranged.At the same time,the viscosity of the PCS and ZrB2 precursors were found to be less than 100mp·s by viscosity test,which satisfied the PIP process requirements,thus further determining the process parameters of the C/SiC-ZrB2 composite prepared by PIP process.(2)SiC-ZrB2 multiphase ceramics were prepared by cleavage of mixed precursors obtained by the co-dissolution-inter-melting characteristics of ZrB2 precursors and PCS.During the cracking process,the ZrB2 precursor and PCS can undergo cross-linking reaction,which promotes both cracking and improves the ceramic yield.The ZrB2 precursor can inhibit the growth of SiC grains after PCS cracking.(3)The effects of PCS and ZrB2 precursors on the microstructure,flexural and anti-ablation performance of the C/SiC-ZrB2 composites were dicussed.The study found that the introduction of ZrB2 increased the density of the composite from 1.87g/cm3 to 2.08g/cm3,and the open porosity increased from 13.83%to 21.82%.This is because ZrB2 is soft and loose and has weak cohesive force,resulting in weak fiber and collective bonding,and the bending strength is also reduced from 323.3 MPa to 268.6 MPa.The introduction of a suitable ZrB2 content significantly improves the ablative resistance of the C/SiC composite because a suitable ZrB2 content forms a dense oxide layer of glassy SiO2 locked by granular ZrO2,which more effective protect internal material.Therefore,in order to prepare a C/SiC-ZrB2 composite with excellent properties,it is important to select suitable PCS and ZrB2 precursors.(4)The effects of high thermal conductivity graphite film content on the density,bending properties,thermal conductivity and anti-ablative properties of high thermal conductivity C/SiC composites were investigated.Studies have shown that as the high thermal conductivity graphite film increases,the density of the composite decreases and the open porosity increases.The reason is that the surface of the graphite film is too smooth and the surface does not bond well to the substrate.When stress is applied,delamination of the fiber and graphite film layers occurs,resulting in a decrease in the mechanical properties of the composite.Because of the high thermal conductivity of graphite itself,the thermal conductivity of composites increases with increasing graphite film content.When the high thermal conductivity C/SiC composite was subjected to a plasma ablation test at 1500? for 600 s,it was found that the ablation rate of the material line gradually decreased to 0.18?m/s withthe increase of graphite film content,and the ablation resistance was improved.Analysis reasons:The thermal conductivity of the surface of the material increases,the stagnation point temperature is lowered,and the ablation resistance of the composite material is improved,which provides an idea for the material of the spalling heat protection structural component.