| Ultra-high temperature ceramics(ZrB2、ZrC、HfB2 and HfC)have been the most promising candidate materials for the nose tip and sharp leading edge of new hypersonic aircraft,due to its excellent properties such as low density,high melting point,high thermal conductivity,high hardness,chemical inertness and good oxidation resistance.At present,UHTCs ceramics can be divided into:monolithic UHTCs,fiber reinforced UHTC matrix composites and laminated UHTCs.Monolithic UHTCs have good ablation resistance,but intrinsic brittleness leads to poor thermal shock resistance,which easily leads to catastrophic failure of materials,limiting the practical application in high temperature environment.Fiber reinforced ultra-high temperature ceramic matrix composites have excellent toughness and thermal shock resistance,but the low density affects its ablation resistance.Laminated ceramic materials,which own unique laminated structure,designability and structure controllability in and between layers,have received extensive attention.It provides an effective technical approach to improve fracture toughness and thermal shock resistance of ultra-high temperature ceramics while maintaining high temperature and ablative resistance.Therefore,ZrB2-SiCw laminated ceramics were prepared by introducing BN,Ti,Nb and Ti-Nb-Ti sandwich interlayers in this experiment,using tape casting combined with spark plasma sintering.In this paper,the effects of different microstructures on the mechanical properties of laminated ceramics were studied,and the interface micromorphology and phase evolution rule under different preparation processes were analyzed to reveal the failure mechanism of materials under different interlayer types.The thermal shock resistance of laminated ceramics was studied by quenchingstrengthening and indentation-quench method and the influence of different interlayer on the thermal shock resistance of laminated ceramics was analyzed.The applicability of two methods is compared,and the approach to improve the thermal shock resistance of laminated ceramics are given.The main research contents and results are as follows:(1)ZrB2-SiCw/BN laminated ceramics were prepared by a weak BN ceramic interlayer.The relationship between the preparation process,microstructure and mechanical properties was studied,and the influence of structural parameters on the residual stress and apparent fracture toughness of materials was analyzed.The results show that the optimized values of the sintering process are 1700℃ and 30 MPa.The laminated structure and the multi-scale toughening mechanism of SiC whiskers are introduced to make the material have non-catastrophic fracture behavior.Compared with monolithic ceramics,the mode of crack propagation is abundant,the consumption of fracture energy increases,and the fracture toughness increases significantly.The flexure strength and fracture toughness of the samples were 381 MPa and 13.31 MPa·m1/2,respectively.With the gradual increase of structural parameters,the residual stress difference in the layer decreases and the stress distribution is uniform.The values of the thickness ratio,layers,and the modulus ratio is 6,7,9,respectively,the apparent fracture toughness of the material reaches the maximum value.(2)ZrB2-SiCw/Ti laminated ceramics were prepared by introducing high activity Ti metal interlayer.The effect of preparation technology on the in-situ reaction of Ti layer was studied,and the relationship between microstructure evolution and mechanical behavior was analyzed.The results show that the metal layer react with the ceramic matrix in-situ forming a three-layered TiB2-TiBw-Ti structure at the interface,which improves the interface bonding strength.Meanwhile,the pull-out of TiBw and the ductile fracture mode of residual Ti will significantly improve the fracture toughness of the material and achieve the effect of strengthening and toughening.As the sintering temperature increases,the element diffusion rate,the in-situ reaction at the interface,the degree of ceramization of the Ti layer,and the reaction products increase,the fracture mode of the sample gradually changes from delamination to deflection.The element diffusion distance increases with the increase of holding time.The atomic flow is not uniform owing to the blocking effect of TiCx,resulting in the generation of the kirkendall void at the interface,which seriously damages the mechanical properties of the sample.(3)ZrB2-SiCw/Nb laminated ceramics were prepared by introducing high melting point Nb metal interlayer.The influence of preparation technology on the in-situ reaction of Nb layer interface was studied.The results show that NbB2 and Nb5Si3 are generated by the interface reaction,which alleviates the residual thermal stress at the matrix/interlayer interface and improves the interlayer interface bonding.The tearing ridges and dimples produced by the fracture of Nb effectively blunted the crack tip,increased the crack propagation resistance and improved the fracture toughness of the sample.Compared with the highly active Ti layer,Nb with large atomic radius and low diffusion rate makes the slow in-situ reaction at the interface.With the increase of sintering temperature and holding time,the material shows a stable phase change in interlayer,small increase of layer thickness and low degree of ceramics,while the flexure strength and fracture toughness of the sample do not change much and the stepshaped fracture behavior is obvious.(4)ZrB2-SiCw/Ti-Nb-Ti laminated ceramics were prepared by introducing Ti-NbTi interlayer.The interfacial phase composition was determined by SEM and TEM,the in-situ reaction mechanism was revealed,and the mechanical response of different phases was studied.The relationship between interfacial phase and mechanical properties was illustrated.The results show that element diffusion results in the formation of TiB2,(Ti,Nb)C whisker and(Ti,Nb)ss,which reduces the thermal expansion mismatch between ceramic and metal and alleviates the damage of thermal stress to the material.TiB2 and(Ti,Nb)C induced the crack to produce deflection and bifurcation,while(Ti,Nb)ss,residual Ti and Nb reduced the stress at the crack tip and caused the crack bridging phenomenon.The synergistic effect of the in-layer material on the shielding and buffering of the crack tip made the fracture toughness as high as 15.18 MPa m1/2,which increased by 107%compared with the monolithic ceramic.(5)Quenching-strengthening and Indentation-quench method were used to study the thermal shock resistance of ZrB2-SiCw laminated ceramics.Compared the different test methods applicability and laminated ceramic thermal shock performance difference,the thermal shock resistance of laminated ceramic was analyzed in theory and the laminated ceramic thermal shock performance improvement is given.The results show that ZrB2-SiCw/Nb sample has the best thermal shock resistance in quenchingstrengthening method,and the value of ΔTc is 726℃.Based on the theory of thermal shock resistance of monolithic ceramics,the calculation formula of laminated ceramicsΔTc is given to qualitatively understand the thermal shock resistance of laminated ceramics.In the indentation-quench method,ZrB2-SiCw/Ti and ZrB2-SiCw/Ti-Nb-Ti have the best thermal shock resistance,the ΔTc of both samples is 726℃.Based on the stress intensity of the crack tip and compared with KIC,the ΔTc of laminated ceramic was obtained.The high-performance thermal shock resistant laminated ceramics can be achieved by microstructural control,interfacial stress and phase regulation,and optimization of structural parameters. |
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