Multi-scale Simulation Study On Nanocomposite Ceramic Tool Materials

Ceramic tool material has been one of the most important cutting tools because of its high hardness, favorable high-temperature hardness and good wear resistance. However, the fracture toughness of ceramics is low which limits the application of ceramic tools. The appearance of nanocomposite ceramic tool materials can substantially solve the problem of low fracture toughness of ceramic materials. The macro mechanical properties of materials are governed by their microstructures, it is very meaningful to simulate the micro structure of nanocomposite ceramic tool materials by coupling macro-scale and micro-scale simulation methods and study the relationship between microstructure and mechanical properties.In this paper, the grain growth theory for ceramic tool materials during fabrication and Monte Carlo simulation algorithm have been analyzed. The Monte Carlo Potts model for microstructure evolution in two-phase nanocomposite ceramic tool materials has been established. Based on Microsoft Visual C++6.0 compiler, the simulation program has been developed using C++language and the microstructure evolution has been simulated and verified by experiments.The effect of nanoparticles with various size and area fraction on microstructure evolution of nanocomposite ceramic tools has been simulated. It is shown that the nanoparticles have inhibition effect on matrix grain growth and refine the ceramic matrix grain. Large-sized nanoparticles have weaker pinning effect than the small one for a specific area fraction, and nanoparticles with high area fraction have stronger pinning effect on matrix for a given grain size. The microstructure of nanocomposite ceramic obtained from simulation is intra/intergranular-type, and the positions of nanoparticles depend on the size and area fraction of nano-phase. The small-sized nanoparticles are easier to enter into the matrix grains, whereas the large one prefers to locate on the grain boundaries. When the area fraction is lower, the total pinning force is weaker, as a result the nanoparticle is easier to be entrapped inside the matrix grains.The effect of grain boundary energy ratio on microstructure evolution of nanocomposite ceramic tools has been simulated. It is shown that the competition between the boundary energy and the interfacial energy affects the microstructure and grain growth of nanocomposite ceramic tool materials. When the interfacial energy is higher than the grain boundary energy, the nanoparticles may enter into the matrix grains and form an intra/intergranular-type nanocomposite ceramic material and the nanoparticles have a stronger pinning effect on matrix grain growth. When the interfacial energy is lower than the grain boundary energy, the nanoparticles mainly locate at the grain boundaries and form an intergranular-type nanocomposite ceramic material.The effect of initial matrix grain size on microstructure evolution of nanocomposite ceramic tools has been simulated. It is shown that the initial matrix grain size affects the microstructure type and grain growth process. For a given grain size and area fraction of the nano-phase, the larger the initial matrix grains, the larger the final grain size is, but the lower the grain growth rate is. There are less nanoparticles entering into the matrix grains, and it is more difficult to obtain an intragranular-type microstructure.The relationship between fabrication temperature and grain growth velocity, the relationship between fabrication pressure and grain growth velocity, and the relationship between simulation time and real duration time have been establishied. These three relationships have been incorporated into simulation program. The microstructure of nanocomposite ceramic tool materials has been simulated allowing for the fabrication parameters. It is shown that the average grain radius increases with an increment in simulation time, fabrication temperature and fabrication pressure. But the effect of fabrication temperature on grain growth is bigger than that of fabrication pressure.The Monte Carlo Potts model for microstructure evolution in nanocomposite ceramic tool material containing pores has been set up and programmed. The effect of nanoparticles and pores on the microstructure evolution, grain growth and densification has been simulated. It is shown that the nanoparticles inhibit grain growth, and the pores affect grain growth and densification. The inhibition by pores is very obviously, the grain growth is slower at a lower density and vice versa. The densification of nanocomposite ceramic tool material increases with an increment in simulation time, and the grain growth restrains densification.The microscale finite element simulation model for nanocomposite ceramic tool materials has been established by coupling Monte Carlo algorithm with finite element method. The residual thermal stress and the stress generated by applied load has been simulated, and the coupling simulation at macro-scale and micro-scale is accomplished.The residual thermal stress of Al2O3/SiC nanocomposite ceramic tool materials has been simulated. The results show that there are mainly residual compressive stresses in nano SiC particles, and there are not only residual tensile stresses but also compressive stresses and shearing stresses in matrix. The magnitude and distribution of residual thermal stress is closely related to the content and size of nano-phase and the size of matrix-phase. The maximum residual tensile stress increases with an increment in the size and content of nano-phase, and the maximum residual compressive stress increases at first and then decreases with the content of nano-phase.The stress field of Al2O3/SiC nanocomposite ceramic tool materials has been simulated under uniaxial pressure. The results show that the residual thermal stress can cause bigger compressive stress field which is helpful to close the crack tip and result in toughening effect.The toughening mechanism by residual thermal stress in nanocomposite ceramic tool materials has been analyzed by using the simulation results of finite element method. The internal residual compressive stress can close crack tip, and the partial tensile stress around nano particles can induce intragranular failure, make crack branch and deflect, to improve the fracture toughness of nanocomposite ceramic tool materials. In order to reduce the distribution scope and magnitude of tensile stress, the nano-phase content and size as well as the initial matrix grain size should be controlled.A kind of nanocomposite ceramic tool material Al2O3/SiC has been developed to verify the simulated results. The characteristics of the micro structure of the developed material Al2O3/SiC are in well agreement with those of the simulated microstructure. Therefore the established models and the simulation results in the dissertation are convinced.