Microstructure And Thermal-shock Performance Of ZrB₂-based Ultra High Temperature Ceramics

In recent years, with the rapid development of the aerospace industry, hypersonic weapons with main technological character of hypersonic, long-time service, high mobility and precise strike from long distance have been drawn much attention all over the world. It is a great challenge for the thermal protection materials used in weapon furnish demanding hypersonic (Mach>5) and long time service, which demands the material must possess the essential performances of limiting temperature and durability, resistance of oxidation and ablation, resistance of thermal shock and light-weight strengthening and toughening. Therefore, the development of novel high temperature structure materials applied in mal-conditions becomes urgent.At present, the research on ultra high temperature ceramics (UHTCs), which can be competent for extreme conditions, is mainly focused on transition metal boride and carbide. Of these UHTCs, ZrB₂ has been attracted much attention for materials for structural applications for their unique combination of relative lower density, high melting point, high hardness, good conductivity and so on. Further, studies of many researchers show that ZrB₂-SiC based ceramics have better combination properties than pure ZrB₂. But the thermal shock resistance and the failure evaluation of the material under the coupling thermo-elasticity are the main questions to further investigate.Considering the lower toughness of ZrB₂-SiC based ceramics, 10vol.%AlN is added which can improve the sinterability and density notably so that the flexural strength , toughness and heat capability are increased obviously, but the hardness, thermal conductivity and thermal diffusion are decreased. For the same components of ZrB₂-SiC-AlN ceramics, initial finer powder can further improve the sinterability, hardness, flexural strength and toughness on condition that the thermophysical properties (heat capability, thermal conductivity and coefficient of thermal expansion) are maintained. Meanwhile, due to the great improvement of toughness of ZSA-2 ceramic, the crack growth resistance KR of ZSA-2 ceramics is higher than ZSA-1 ceramics. But the KR of ZSA-2 decreases more steeper than ZSA-1 while crack length is greater than 0.4mm, which concludes that the finer grain ceramic has a more notable resistance to crack growth in small crack size. The crystal structure, interface structure and its orientation relationship of ZS and ZSA ceramics are analyzed detailedly using HRTEM. The interface structure in UHTCs has a great influence on the high temperature performance. It is concluded that SiC polytype is mainly present as 6H structure in these ceramics, and many defects and stacking faults can be found in SiC. There also exist many edge-type dislocations and other defects in ZrB₂ grains. The interfaces among component phases are very pure, and there have been no chemical reaction, interfacial amorphous layer or transition layer are found at the interfaces. No evident orientation relationships are observed between the interfaces of ZrB₂/SiC and ZrB₂/AlN. The crystals are prone to grow along the directions of AlN [001] // SiC [010] after a tiny adjusting. In addition, according to the prior analysis, the main sintering process of ZS and ZSA is affirmed to be solid phase sintering. The process of sintering densification is principally closeness, rearrangement, bonding and plastic deformation of the power particles as well as crystal glide and mass transfer by plastic creep flow.The thermal shock resistance of two kinds of ZrB₂-SiC-AlN ceramics is investigated by quenching-strength method. It is concluded that the critical temperature difference of ZSA-2 is greater than that of ZSA-1 owing to the improvement of toughness of ZSA-2. It is also found that the influence of thermal shock on toughness is not significant. The experimental and theoretical researches on samples with different thickness indicate that the critical temperature difference for cracking decreases with increasing of the thickness. The thicker of the thickness, the more evident of this tendency. However, the effect of thickness on critical temperature difference becomes smaller when the thickness enlarges to some extent. Moreover, the thermal shock resistances of ZSA-1 and ZSA-2 ceramics are evaluated by indentation-quench method, which characterizes the thermal shock resistance via the percentage crack growth versus quenching temperature difference. It is shown that no obvious crack growth is observed when temperature difference (ΔT) is low enough. In a mediumΔT interval (160⁴00℃) the crack extends stably with increasingΔTs. When theΔT is greater than or equal to 400℃, some of cracks grow unstably. And then a corresponding theoretical model is established to analyze this process which is in good agreement with the experimental results. Considering the crack-healing process of the indented crack, the oxidation of ZrB₂-SiC-AlN ceramic with indented cracks is researched at 1000℃for different time, and the influence of the oxidation time on residual strength is also evaluated. It is found that the effect of oxidation on strength is particularly obvious. Even oxidation for little time, the strength can increase notably due to the bluntness of crack tip during oxidation. The superficial oxidation layer becomes thicker and denser with the increasing oxidation time so that the strength becomes higher. The surface oxidation layer is the densest and the strength gets maximum after oxidation for 60min. After that, the surface oxidation layer becomes loose, and a looser transition layer is also observed, so the strength of the samples oxidized decreases with the increasing oxidation time.Further, one dimension and three dimensions thermal shocks of ZSA ceramics are analyzed by finite element method, and the distributions of temperature and thermal stress are evaluated. The initial crack shape is determined by the cross-section micrograph of the indented pre-crack. According to this, the finite element model is established, afterward J-integral and stress intensity factor K is calculated. The critical temperature difference for crack propagation is determined via the J-criterion and K-criterion which is in good agreement with the experimental results. Furthermore, based on the cohesive zone model, the crack growth of pre-crack under different thermal shock temperature differences is simulated and the percentage crack growth is calculated. The depth that thermal stress can work at different thermal shock temperature difference is estimated. According to these'depths', a dimensionless parameter for characterizing the thermal stress influence-depth is introduced. Then the results of finite element analysis (FEA) are modified using this parameter. Comparing with the experimental results, the errors of modified FEA results are acceptable.According to actual crack shape, the model is established to analyze the crack growth in ZSA ceramics under the loading cases of thermal shock and coupling thermal shock with tension and shear. Crack growth patterns for different orientations of interior cracks are calculated under various loading cases. Therefore, the most primary crack growth pattern is determined under each loading case. Moreover, crack propagation patterns of surface pre-crack in material included second phase particles are simulated under the loading cases of thermal shock, coupling thermal shock with tension and shear. It is included that the crack propagates linearly under the great thermal stress, and the crack traverses SiC particles under coupling thermal shock- tension stress while the crack grows mainly along the grain boundary of SiC under the loading case of coupling thermal shock and shear stress.