Surface Modification Of Carbon Fiber And Preparation And Properties Of CF/ZrC Porous Ceramic Composites

The development of modern aerospace technology has prompted the world's major military powers to strengthen the research and development of high-Machshund hypersonic vehicles.The development of hypersonic vehicles urgently requires the research and development of new thermal protection composites with lightweight,high reliability,high temperature resistance,anti-oxidation,and versatility.Traditional ultra-high temperature ceramic materials have low fracture toughness and thermal shock resistance.C/C and C/SiC composites have poor resistance to oxidation and ablation,and cannot meet the thermal protection requirements of future hypersonic vehicles.Continuous carbon fiber reinforced ultra-high temperature ceramic matrix composites are most likely to be the candidate material for thermal protection of next generation of hypersonic vehicles due to their high porosity,low density,outstanding mechanical properties,excellent thermal shock resistance and good high temperature stability.In the past half century,carbon fiber has been widely used as a reinforcement for resin and ceramic matrix composites by researchers due to its low density,high strength,excellent chemical stability,low thermal expansion and outstanding high temperature resistance.And carbon fiber reinforced composites are widely used in military and civilian fields such as aerospace,automobile manufacturing,and sporting goods because of their outstanding performance.However,carbon fiber always has intrinsic surface inertness,which will resulting in a weak interfacial bonding when combined with a ceramic matrix.And thus will limit the performance of the fiber reinforcement and affect the overall performance of the ceramic matrix composite.Meanwhile,the traditional ceramic matrix composites always have a single function,and the structure-function integration application is limited.In this paper,two kinds of one-dimensional nanomaterials,carbon nanowires?CNWs?and silicon carbide nanowires?SiCNWs?,were used to modify the carbon fibers,and two kinds of multi-scale enhancements,CNWs-CF and SiCNWs-CF,were prepared.Based on this,two kinds of multifunctional new porous ceramic matrix composites,CNWs-CF/ZrC and SiCNWs-CF/ZrC composites,with excellent high temperature resistance were prepared by precursor impregnation and pyrolysis method.The preparation process of two kinds of composites was studied in this paper.The effects of preparation methods and material composition on the mechanical properties,thermophysical properties and structural health monitoring performance of the two composites were investigated.The CNWs-CF multi-scale reinforcement was fabricated by growing carbon nanowires on the surface of carbon fibers through a plasma enhanced chemical vapor deposition?PECVD?process using CH₄ as the carbon source.The effects of different preparation conditions on the morphology and structure of CNWs were investigated.It is found that the optimum conditions for the growth of CNWs on the surface of carbon fiber by PECVD method are as follows:the catalyst is 0.05 mol/L acetone solution of Ni?NO₃?₂·6H₂O,the reaction gas is 40/10 sccm CH₄/H₂,the reaction temperature is 700°C and the RF time is30 min.The results show that carbon nanowires with a diameter of 80-120 nm and a length of 3-4?m were uniformly grown on the surface of carbon fiber along the radial direction of carbon fiber.The growth of carbon nanowires effectively increases the specific surface area and the surface roughness of the carbon fiber reinforcement,and improves the interfacial shear strength?IFSS?between the carbon fiber and the polymer matrix by 77%.Multi-scale SiCNWs-CF reinforcements were prepared by growing SiC nanowires on the surface of carbon fibers through a chemical vapor infiltration?CVI?process using methyltrichlorosilane?CH₃SiCl₃?as Si and C source.The controllable preparation process of SiC nanowires on the surface of carbon fibers was investigated.The optimum preparation conditions for the growth of SiC nanowires on the surface of carbon fiber by CVI technology are as follows:the catalyst is 0.05 mol/L acetone solution of Co?NO₃?₂·6H₂O,the reaction raw material is 80%CH₃SiCl₃/20%H₂,the reaction temperature is 1000°C,and the reaction time is 1 h.The results show that the SiC nanowires with a diameter of 80-100 nm and a length of about 10?m were uniformly grown on the surface of the carbon fiber.The growth of SiC nanowires on the surface of carbon fiber effectively enhances the mechanical connection between the carbon fiber and the ceramic matrix,and greatly increases the interfacial bonding strength between the carbon fiber and the ceramic matrix.CNWs-CF/ZrC ceramic matrix composites were prepared through PIP technique using carbon nanowires modified continuous carbon fiber as reinforcement.The reaction mechanism of the polymerization of ZrC ceramic precursors prepared from zirconium tetrachloride?ZrCl₄?,acetylacetone?CH₃COCH₂COCH₃?,methanol?CH₃OH?and 1,4-butanediol?C₄H₈?OH?₂?was studied.The thermal pyrolysis behavior of ZrC ceramic precursors was investigated and the results show that the ZrC ceramic precursor has four obvious weight loss steps during the heating process in an inert atmosphere.Pure phase ZrC ceramic with good crystallinity was prepared after thermal pyrolysis at 1600°C,and the ceramic yield was 38.72%.The changes of density,porosity and compressive strength of CNWs-CF/ZrC ceramic matrix composites prepared with different PIP cycles were studied.The compressive strength of CNWs-CF/ZrC composites prepared with six PIP cycles was 25.77±1.62 MPa.The effects of the introduction of CNWs on the thermal conductivity and structural health monitoring sensitivity of CNWs-CF/ZrC composites were mainly investigated.The results show that the thermal conductivity of the CNWs-CF/ZrC ceramic matrix composites is slightly higher than that of CF/ZrC ceramic matrix composites with the same preparation conditions.The coefficient of strain sensitivity of CNWs-CF/ZrC ceramic matrix composites is about 4.17 times of that of CF/ZrC ceramic matrix composites.SiCNWs-CF/ZrC ceramic matrix composites were prepared by PIP method with the SiCNWs-CF multi-scale structure as reinforcement.The effects of the growth of SiCNWs on the surface of carbon fiber on the microstructure and composition of SiCNWs-CF/ZrC composites were investigated.The results show that the introduction of the SiCNWs greatly increased the loading of ZrC ceramics on the surface of carbon fiber.The surface of SiCNWs-CF could be uniformly coated with a layer of ZrC ceramic with 6 PIP cycles.The density,porosity,compressive strength and room temperature thermal conductivity of the SiCNWs-CF/ZrC composites prepared with different PIP cycles were investigated.With six PIP cycles,the density of the SiCNWs-CF/ZrC composite was 1.85 g/cm³,the porosity of the composite was 68.13%,the compressive strength in z direction was23.64±1.25 MPa,and the compressive strength of SiCNWs-CF/ZrC composite in x/y direction was 13.02±1.23 MPa.The thermal conductivity at room temperature and the structural health monitoring sensitivity of CF/ZrC and SiCNWs-CF/ZrC composites were compared.The results show that the thermal conductivity of SiCNWs-CF/ZrC ceramic matrix composites is slightly higher than that of CF/ZrC composites.The introduction of SiCNWs on the surface of carbon fiber increased the structural health monitoring sensitivity of the SiCNWs-CF/ZrC composites by 14.27 times.