The Study On Strengthening-Toughening And Ablation-Oxidation Resistance Of C_f/ZrC-SiC Composites

Ultra-high temperature ceramics(UHTCs)and their composites have extremely high melting point(>3000?)and excellent oxidation ablation resistance,which are considered as important candidate materials for thermal protection system of hypersonic vehicle.The carbon fiber has supreme mechanical properties,therefore“combination of UHTCs and carbon fiber”has gradually become the current development trend and research hot-spot at home and abroad.It is a key scientific problem to realize the synergistic effect of strengthening,toughening and oxidation resistance of carbon fiber reinforced UHTCs.The most prominent and critical issues are listed as follow:the existing preparation methods are easy to cause structural damage and performance degeneration to carbon fiber;the introduction of UHTCs is difficult to achieve large-scale on-demand control of composition and content;problems in realizing the double continuity distribution of UHTCs phase and carbon fiber;the formed oxidation layer and matrix materials have poor binding.Based on the above scientific problems,this dissertation studies the strengthening,toughening properties,and oxidation ablation resistance of C_f/ZrC-SiC composites.The microstructure and toughening properties of C_f/ZrC-SiC composites prepared by hot pressing are investigated.A novel method for introducing ceramics with high-frequency vibration-assisted slurry injection is proposed.The preparation parameters of 3D C_f/ZrC-SiC are reduced from>1900°C/high pressure to 1300°C/without pressure by creative means of the impregnation and pyrolysis of liquid SiC precursors.The microstructure,toughening,thermal shock resistance and oxidation ablation resistance of 3D PyC-C_f/ZrC-SiC prepared at low temperature without compression are analyzed.The microstructure and toughening properties of ZrC-SiC,short chopped C_f/ZrC-SiC,lamellar C_f/ZrC-SiC and 3D C_f/ZrC-SiC prepared by hot pressing were studied.Decoupling C_f/ZrC-SiC composites,the sintering densification behavior of C_f/ZrC-SiC prepared by hot pressing sintering are analyzed.It is revealed that the introduction of SiC improves the density,flexural strength,and fracture toughness of ZrC-SiC ceramic by inhibiting the growth of ZrC grains and enhancing the sintering activity.The short chopped C_f/ZrC-SiC composites are manufactured by ball-mill mixing and spark plasma hot pressing sintering.The pull-out length of the short cut carbon fiber is limited,so the increase of fracture toughness is not obvious(4.65±0.32MPa?m^1/2).The layered C_f/ZrC-SiC composites are prepared by rolling-hot pressing sintering,which increase the length of fiber embedded in ceramic matrix,the length of fiber pulling out and fiber bridging are increased obviously(>50?m),and the fracture toughness along the fiber arrangement direction is increased to 5?6MPa·m^1/2.However,due to the lack of carbon fiber along the z direction,the performance of the composite along z direction is restricted(fracture toughness is 4.31±0.24MPa m^1/2).The 3D C_f/ZrC-SiC composites are prepared by slurry impregnation and hot pressing.The main microstructure damage forms and mechanism of fiber chemical corrosion(particle corrosion and surface corrosion),interface particle infiltration and fiber bending deflection of z direction are revealed.It is proposed that the preparation conditions for inhibiting the fiber microstructure damage and improving the toughening properties of composites should be<1500?/without pressure.The microstructure and toughening properties of C_f/ZrC-SiC composites prepared by low temperature without pressure are investigated.A novel method for introducing ceramics with high-frequency vibration-assisted slurry injection is proposed.The ceramic is directly injected inside carbon fiber braid via form of powder to achieve high content of ceramic components(>30vol.%),which is more than 50%higher than that of vibration-free assistance.Based on the introduction of ZrC ceramics by vibration assisted grouting,the traditional hot-pressing sintering preparation method of UHTCs composites is innovated by means of creative means of the impregnation and pyrolysis of the liquid SiC precursors.The preparation conditions are reduced from 1900?/high pressure to1300?/no pressure.The mechanism of low temperature pressureless sintering of 3D C_f/ZrC-SiC composite are revealed.With its excellent sintering activity and mechanical properties,in-situ cracked nano SiC ceramic can effectively fill the gap between ZrC ceramic particles and carbon fiber braid in low temperature environment,achieving high-strength bonding between ZrC UHTC particles and between ZrC UHTC and carbon fiber.Under the condition without interfacial layer protection,the low temperature pressureless preparation method effectively suppresses the structural damage of carbon fibers.In the form of fiber extraction number(bunches)or fiber pull-out length(>100?m)both have absolute advantages.The fracture work(3046J/m²)and fracture toughness(11.12±1.25MPa?m^1/2)obtain level ascension compared with other UHTC composites.At the same time,the flexural strength and fracture strain are up to 313MPa and 1.35%,respectively,which solves the problem of intrinsic brittleness of UHTCs.The mechanical-thermal shock resistance and oxidation ablation properties of 3D PyC-C_f/ZrC-SiC composites prepared at low temperature without pressure are investigated.Under the influence of the PyC weak interface layer,the failure strain increases from 1.35%to 1.5?2%,and the fracture toughness is further improved(13.05±1.72MPa?m^1/2),while the flexural strength of the composite performs good(251±27MPa).The thermal shock resistance of 3D PyC-C_f/ZrC-SiC composites under different temperature conditions from room temperature to 1200?is characterized.The maximum critical thermal shock temperature difference is approached to 875?.Compared with traditional ceramic materials,it has been improved more than 3 times.The static oxidation behavior and dynamic oxidation ablation properties of 3D PyC-C_f/ZrC-SiC composite are studied.After the natural adsorption and 1300?without pressure pyrolysis of Zr B₂ ceramic slurry mixed with liquid phase SiC ceramic precursor,the surface densification of 3D PyC-C_f/ZrC-SiC composite is achieved via forming in situ“pinning”with the pores.It exhibits near-zero ablation in the oxyacetylene evaluation environment above 1800?/300s,which achieves the synergy between toughening and anti-oxidant in a real sense,laying a theoretical foundation and technical support for the engineering application of UHTCs.