The Microstructure Construction And Properties Of Multidimensional C_f/ZrB₂-SiC Composites

The nose cones and leading edges of hypersonic aircrafts would face the high-temperature exceeding 1800? and large temperature gradient as well as strong oxidizing environments during fighting,pushing an urgent need of long-term ultra-high temperature non-ablation thermal protection material to maintain the dimension and maneuverability as well as precision strike performance of the aircraft.Traditional refractory metals,graphite,C/C and C/SiC composites cannot meet the requirements of thermal protection,and the UHTCs have been identified as potential candidates owing to their high melting point and superior mechanical properties as well as excellent ablation resistance.However,the low fracture toughness and poor thermal shock resistance of UHTCs limited their engineering applications,and the improvement in reliability of UHTCs toughened by traditional reinforcements have tended to their limitation.The carbon fiber is considered as the most potential and effective reinforcement for UHTCs owing to their excellent mechanical properties at room and high temperature.Although the introduction of short-chopped carbon fiber could improve the fracture toughness of UHTCs,its toughening effect is limited,in which it is hard to solve the intrinsic brittleness of ceramics.The introduction of continuous carbon fibers would greatly improve the failure strain and work of fracture of UHTCs,completely change the brittle fracture failure mode and greatly improve the reliability of UHTCs.However,the non-uniform compounding between continuous carbon fiber and UHTCs and the degradation of carbon fiber are the core issues of continuous carbon fiber reinforced ultra-high temperature ceramic composite(UHTCMCs).In this thesis,firstly,the slurry brushing technique was exploited to achieve the uniform compounding between carbon fiber bundles and UHTC matrix,and the fabrication of 2D C_f/ZrB₂-SiC composite was obtained by low-temperature hot pressing with the assistance of high sintering activity of nanosized ZrB₂ powder.However,the2D C_f/ZrB₂-SiC composite adopted a laminated structure,which was easy to delaminate and crack under the load of stress,resulting in a poor application reliability.Subsequently,a 3D carbon fiber braid was used to replace 2D carbon fiber bundles,and the 3D C_f/ZrB₂-SiC composite was achieved by the vibration-assisted slurry impregnation and hot pressing.The mechanical properties and thermal shock resistance as well as oxidation ablation resistance of 3D C_f/ZrB₂-SiC composite were investigated systematically.It was worth noting that the hot pressing technique would inevitably lead to fiber deformation or warpage in z plane of 3D carbon fiber braid,hindering the densification process and reducing its overall performance.Inspired by the concept of architecture,the vibration-assisted slurry injection/vacuum impregnation approach was used to achieve the uniform filling of the UHTC particles into the 3D carbon fiber preform,and the pressureless densification of 3D C_f/ZrB₂-SiC composite was achieved based on the low-temperature PIP process.The combined fabrication processes could overcome the problem of non-uniform compounding between UHTC matrix and 3D fiber preform,solve the issue of fiber warpage and deformation in z plane of 3D fiber preform,and inhibit the degradation of carbon fiber,laying the foundation for engineering application of 3D C_f/ZrB₂-SiC composites.The low-temperature densification of 2D C_f/ZrB₂-SiC(0/90°)composite was achieved by nano slurry brashing combined with hot pressing,and the effect of carbon fiber heat treatment temperature on the microstructure and mechanical properties of 2D C_f/ZrB₂-SiC composite was investigated.The maximum of fracture toughness(6.15±0.12 MPa·m^1/2)and work of fracture(1368 J/m²)of 2D C_f-500?/ZrB₂-SiC composite were achieved when carbon fiber was pretreated at 500?.The enhancement in mechanical properties was attributed to the fact that carbon fiber heat treatment could reduce the surface reactivity of the carbon fiber,weaken the fiber/ceramic interface bonding force and promote the toughening mechanisms such as fiber bridging and fiber pull-out.Moreover,the carbon fiber pretreatment could significantly improve the thermal shock resistance,leading to a high critical thermal shock temperature difference of 764? for 2D C_f-500?/ZrB₂-SiC composite.The 2D C_f-500?/ZrB₂-SiC composite maintained its dimensional shape after 1950?/1000s oxyacetylene testing,but exfoliation of oxide layer was detected on the surface of oxyacetylene scour area owing to its layered structure,which was hard to meet the engineering application requirements for ablative thermal protection materials.The high-performance 3D C_f/ZrB₂-SiC composite was successfully obtained by the vibration-assisted slurry impregnation combined with low-temperature hot pressing(VSI+HP).The fracture toughness and work of fracture of 3D C_f/ZrB₂-SiC(VSI+HP)composite were approached to 5.34±0.13 MPa·m^1/2 and 1104 J/m²,respectively,which were 25%and 90%higher than those of C_f/ZrB₂-SiC(SI+HP)composite prepared by traditional slurry impregnation and hot pressing.The enhancement in mechanical properties was attributed to the coupling effect of multiple mechanisms such as crack deflection,crack bifurcation,fiber bridging and fiber pull-out.The critical thermal shock temperature difference of 3D C_f/ZrB₂-SiC(VSI+HP)composite was approached to 788? after water quenching thermal shock testing,which was higher than that of traditional UHTCs(?Tc?500?).The 3D C_f/ZrB₂-SiC(VSI+HP)composite remained intact after 1500?/1h static oxidation,and the surface was covered by a dense Si O₂glass layer,exhibiting an excellent oxidation resistance.Moreover,3D C_f/ZrB₂-SiC(VSI+HP)composite remained structure intact after 1800?/1000s oxyacetylene testing,and the linear ablation rate was as low as-1.70×10^-4 mm/s.The pinning effect of Zr O₂ particle on the molten Si O₂ phase relieved the volatilization of Si O₂ oxide layer to avoid further oxidative ablation,and the ablation mechanism of 3D C_f/ZrB₂-SiC composite was revealed.The vibration-assisted slurry injection and vacuum impregnation combined with polymer infiltration and low-temperature pyrolysis processes were used to achieve introduction of high ceramic content into 3D carbon fiber preform and pressureless sintering preparation of 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite.The 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite exhibited excellent room and high temperature mechanical properties,in which the room flexural strength and work of fracture were approached to 385±74 MPa and 3723 J/m²,respectively,and the high temperature flexural strength at 1400? and 1800? was 420±28 MPa and 129±31 MPa,respectively.The excellent mechanical properties were attributed to the coupling effect of multiple toughening mechanisms such as crack deflection,crack bifurcation,fiber bridging,fiber bundles pull-out and interface debonding.The critical thermal shock temperature difference(?T)of 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite was as high as 962?,combining with 70%residual strength retention rate,indicating the improvement in damage tolerance of ZrB₂-based UHTCs.The anti-oxidation and ablative properties of 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite were evaluated.3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite exhibited non-ablative property after static oxidation at 1400? and 1500?,which the surface of composite was covered by the dense Si O₂ oxide layer.The 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite remained intact after oxyacetylene testing under 2250? with60s,and the surface was covered with Si O₂-coated Zr O₂ core-shell structure,leading to a low linear ablation rate of-2.33×10^-3 mm/s.After oxyacetylene testing at 2250? with 360s,the overall oxide layer on the surface of 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite was peeled off,resulting in an increased linear rate of-3.06×10^-4 mm/s,but the overall dimension and shape of the composite remained.Moreover,the surface of composite below the exfoliated oxide layer was covered by porous Zr O₂ layer,and the growth of Zr O₂ gain could fill the gaps between particles and pores left by fiber oxidation,which suppressed internal 3D C_f/ZrB₂-SiC(VSI+VI+PIP)composite from further oxidative ablation.