Simulation Study On Nano-scale Interfacial Behavior And Micro-scale Fracture Behavior Of Ceramic Tool Materials

Improving the fracture toughness has been one of the important research subject in the study field of ceramic tool materials. The mechanical properties of ceramic tool materials in macro-scale are determined by microstructure. And the essential reason for the brittleness of ceramic tool materials lies in the chemical bond properties in atomic scale. In this paper, the microstructure of ceramic tool materials in nano-scale and micro-scale are simulated based on multi-scale simulation theory. This is very meaningful to the research and development of ceramic tool materials.The interface simulation model of Al2O3(012)/SiC(310) is established and the analysis of electronic density, density of states and population is also conducted. It is concluded that Al-C bond and O-Si bond which are located at grain boundary primarily contribute to the interface bonding strength and creep resistance. And the contribution order is Al-C>O-Si. In the interface model, the bonding strength of Si-C in SiC grain is the strongest and the average bonding effect of Al-C and Si-O located at grain boundary is higher than that of Al-O located in Al2O3grain.The interface molecular dynamics simulation models of single-phase Al2O3and multi-phase Al2O3/SiC are built. The interface energies for all interface models are calculated based on molecular dynamics method and the interface bonding strength and crack propagation resistance are compared. The interface energies for single-phase Al2O3and multi-phase Al2O3/SiC are passed on to the micro-scale fracture behavior simulation model as the fracture energy of Al2O3grain boundary and Al2O3/SiC interface respectively. The interface energy variation and diffusion behavior of the Al2O3(012)/SiC(011) interface model during the sintering process are studied. The interface energy increases firstly until reaches its maximum at the temperature of1500K and then decreases. Diffusion coefficients of atoms show a first increasing then decreasing tendency as the temperature goes up. The relationship of diffusion coefficients for each kind of atoms at the same temperature is C>Si>O>Al.Cohesive model is established by embedding cohesive elements with fracture criteria into Voronoi tessellation model which is representative of the microstructure of ceramic tool materials. The crack growth simulation is carried out on microstructures of single-phase ceramic tool materials without pores, single-phase ceramic tool materials with pores and nanocomposite ceramic tool materials respectively.The influence of grain size on crack path is studied in single-phase materials without pores. It is concluded that only intergranular fracture arises in single-phase materials. In microstructure with smaller grains, the crack path is more zigzag, more cracks deflect and more fracture energy is dissipated. Microstructure with smaller grains possess higher critical fracture energy release rate. Thus it is beneficial to the fracture toughness improvement by means of reducing the grain size. Besides, it is found that stress concentration effect of pores along grain boundary increases the probability of grain boundary fracture based on the simulation results of single-phase materials with pores. Pores will reduce the fracture surface energy of ceramic tool materials.The influence of microstructural type, nanoparticle size and nanoparticle volume content on micro-scale fracture behavior of nanocomposite ceramic tool materials is studied. The simulation results show that the nanoparticles have changed the fracture pattern from intergranular mode in single-phase materials to intergranular-transgranular-mixed mode. It is mainly the nanoparticles along grain boundary that have an impact on the fracture pattern change in nanocomposite ceramic tool materials. Microstructure with smaller nanoparticles, in which there are more nanoparticles dispersed along matrix grain boundary, has higher fracture toughness. Microstructure with a nanoparticle volume content of15%has the most obvious transgranular fracture phenomenon and highest critical fracture energy release rate.The effect of the fracture energy ratio among nanoparticle/matrix interface, matrix grain and matrix grain boundary on the fracture pattern of nanocomposite ceramic tool materials is studied. It shows the magnitude relationship of critical fracture energy release rate is strong interface>medium interface>weak interface. And strong interface is good for enhancing the fracture toughness of material.The distribution of residual stress in the microstructure of nanocomposite ceramic tool materials is simulated and fracture behavior of microstructure under applied load together with internal residual stress is also modeled as well. For intracrystalline microstructure, nanoparticles in matrix have no effect on intergranular crack propagation and fracture pattern when not allowing for residual stress. While allowing for the residual stress, the residual tensile stress in matrix grains will make the transgranular fracture more obvious. For intercrystalline microstructure, the strong bonding strength of nanoparticle and matrix grain leads to transgranular fracture when not allowing for residual stress. If allowing for the residual stress, the radial compressive stress promotes the interface bonding strength of nanoparticle and matrix grain and enhances the pinning effect of nanoparticles on crack. While the tangential tensile stress and external stress overlap, it will raise the tendency of nanoparticles fracture.The distribution of residual stress in the microstructure of nanocomposite ceramic tool materials with different nanoparticle volume content is simulated. As the nanoparticle volume content goes up, the compressive and tensile stress distribution domain around nanoparticles gradually rise while the tensile stress field increases more obviously. The average tensile stress grows constantly when the nanoparticle volume content grows. While the average compressive stress shows a firstly increase then decrease tendency as the nanoparticle volume content rises. While allowing for the residual stress, transgranular fracture is enhanced by the residual tensile stress in matrix grains and the critical fracture energy release rate is higher. The critical fracture energy release rate firstly increases until reaches its maximum at15%and then decreases as the nanoparticle volume content goes up. The microcrack initiation in the microstructure containing residual stress is simulated. It is found that the smaller is nanoparticle, the shorter is microcrack. The microcrack density grows with an increase in nanoparticle volume content. For intercrystalline microstructure, microcracks initiate from nanoparticles along grain boundaries. While microcrack initiates from nanoparticles in matrix grains for intracrystalline microstructure..The applied load is applied to intercrystalline and intracrystalline microstructure models with microcracks to model the fracture process. It is shown that the existence of microcracks will decrease the actual fracture surface area during the process of crack propagation. This will reduce the fracture surface energy and is not beneficial to the fracture toughness promotion.The microstructure models of single-phase Al2O3ceramic tool materials with different number of grains are established and the fracture simulation is conducted under the applied load. As the crack path grows from microscale to macroscale, the grain-bridging phenomenon appears and the critical fracture energy release rate gradually rises. The fracture toughness of single-phase Al2O3ceramic tool materials in macro-scale is predicted based on simulation results.