Multi-scale Analysis Of The Strength Of ZrB₂-SiC Ceramic Composite

With the rapid development of aerospace technology,the ultra-high temperature materials used in various key parts such as the aircraft nose cone,wing leading edge and the engine hot end,are mainly concentrated in ultra-high temperature ceramic materials composed of boride and carbide.Among the ultra-high temperature ceramic materials,Zr B₂-based ultra-high temperature ceramic materials have attracted extensive attention in high-temperature structural ceramic materials due to their high melting point,high strength,high hardness,good electrical and thermal conductivity and corrosion resistance.However,the brittleness of ceramic materials severely limits its application.Therefore,the strengthening and toughening of ceramic materials has become a key issue for research.Many researches have been done to change the microstructure of the material by changing the preparation process of Zr B₂ ceramic materials and adding different phases to the ceramic to enhance the toughening effect.For multiphase materials,the mechanical properties and failure mechanism are not only related to their macroscopic properties,but also closely related to the microscopic information such as the properties of the constituent phases,morphology,distribution and interface characteristics.The fracture of materials is a process involving the initiation of microscopic cracks,the development at mesoscale and ultimate macro damage.In order to optimize and make better use of multiphase composite materials,it is necessary to grasp the influence of the microstructure of materials on their macroscopic mechanical properties,and to develop multi-scale research methods.This thesis takes ZrB₂-SiC ceramic material as the research object.The intergranular damage and strength of ZrB₂-SiC ceramic composites are analyzed and calculated at multiple scales from the intermolecular forces of the materials,microscopic crack propagation behavior and macroscopic mechanical properties.The hierarchical multi-scale method is used to apply low-scale calculation results as high-scale input to study the relationship between the microstructure of multiphase ceramic materials and its macroscopic strength.First,the tensile strength of the ZrB₂-SiC interface was calculated using molecular dynamics methods at the atomic scale.The Tersoff potential function and the mixing rule were used in the molecular dynamics calculation to simulate the interaction between atoms in the system.The elastic constants and lattice constants of Zr B₂ single crystal and Si C single crystal were calculated by first-principles method to verify the accuracy of the Tersoff potential function and its potential parameters.The tensile strength and modulus of the ZrB₂-SiC interface model were obtained by molecular dynamics calculation,which provided a basis for calculating the grain boundary strength of Zr B 2-Si C at a higher scale.During the preparation of polycrystalline materials,defects are easily generated at the grain boundaries to affect their strength.The strength of the ZrB₂-SiC grain boundary and the crack propagation at the grain boundary were simulated by a cohesive element model at single crystal scale.The p orosity and microcracks were set at the grain boundary,and the ZrB₂-SiC grain boundary properties calculated by molecular dynamics were taken as the constitutive relationship of the cohesive element at the grain boundary.The effects of density,microcrack and microcrack distribution on the mechanical properties of ZrB₂-SiC grain boundaries were investigated.The tensile properties of ZrB₂-SiC grain boundaries are degraded due to the presence of intercrystalline voids and microcracks.The microcrack distribution affects the stress concentration factor during the tensile process of the ZrB₂-SiC grain boundary model,which affects its strength.The effect of ZrB₂-SiC microstructure modeled at the mesoscopic scale by image method and Voronoi polygon method on its mechanical properties was studied.The grain boundary properties with different length microcracks and different densities were introduced into the grain boundary cohesive element of ZrB₂-SiC microstructure.Firstly,the effect of residual thermal stress on the mechanical properties of ZrB₂-SiC microstructure model was studied.The existence of residual thermal stress affects the stress distribution inside the microstructure,which makes the strength of the model slightly improved.The tensile strength of the model increases as the density increases and the length of the microcrack decreases.Therefore,when the microstructure of the material is constant,the strength of the material depends on the strength of the grain boundary.When the grain boundary strength is constant,the grain size has little effect on the tensile strength of the model.For microstructures with defects,the tensile strength of the model decreases with increasing grain size,except for individual grain size.On the one hand,Si C particles promote the formation of surrounding cracks in the microstructure of ZrB₂-SiC ceramics,and at the same time hinder the crack propagation,so that the Si C content has little effect on the tensile strength of the model.The macroscopic mechanical properties of ZrB₂-SiC composites with different microstructures were obtained by homogenizing the results calculated at the microscopic scale.The macroscopic tensile strength and flexural strength of the material decrease with decreasing density.And the crack initiates at the lower strength element in the stress concentration zone.The grain size has little effect on the macroscopic tensile strength and flexural strength of the material.As the grain size increases,the macroscopic tensile modulus of the m aterial increases.The effects of grain size,microscopic defects and Si C content on the macroscopic mechanical properties of ZrB₂-SiC materials are consistent with the effects on the microstructure model.Although the macroscopic mechanical properties of the material are the same as the microstructure strength,they reflect the combined effects of various microscopic strengths.Moreover,when the distribution of material element property is different,the macroscopic tensile and bending strength of the material are greatly different,which reflects the dispersion of the mechanical properties of the composite material due to the uneven microstructure.In the past,the researches on the mechanical properties of ZrB₂-SiC ceramic composites were mainly tried by experimental means.In this thesis,the influence of the microstructure of ZrB₂-SiC ceramic composite on its macroscopic mechanical properties is studied by means of calculation methods,and some factors affecting the mechanical properties are obtained,w hich has guiding significance for material design.