Computer Simulation And Validation For ICVI Process Of Porous Particle Preform

Si₃N₄ can be used in high temperature and high pressure due to its excellence properties such as hard wearing, heat resisting, corrosion resisting, and high stabilization of chemical. However the inadequate toughness limits its application. Fiber, whisker, and particle are usually used to solve this problem at present. Fracture of fiber composites are similar with that of metals, they have thick skin to crack and super reliability to be unable to occur tragedy. Particle composite is of isotropy and low cost, it is supplement to fiber composites. The microstructure of ceramic matrix composite produced by isothermal chemical vapor infiltration (ICVI) can be designed. The ICVI process is very complicated, and has a long cycle. The dimension of the particles is related to the pore structure, and then affects the infiltration. Adopt the proper means to set up the mathematics model of ICVI course, simulate the course of the densification of ICVI can guide the preparation of particle composite effectively.In this thesis, we studied the pore structure of Si₃N_4p/Si₃N₄ particle perform, and a model of spherical pore was built. Approved by the experiment the model can attribute the process of the densification of the ICVI effectively, and optimized the dimension of the particle in the prilling processing. Main research contents and conclusion are as follows:(1) Microstructure of preform by granulation and forming were observed in SEM. Two kinds of holes are found in the preform: holes about several micrometers among the particles and holes about several millimeters among the conglomerations, which are similar with fiber preform, but more complicated.(2) Borrowing ideas from the geometric model of fiber preform, this paper build a mass transportation model of spherical pore. And use finite difference method simulated the densification of the ICVI processing of the Si₃N_4p/Si₃N₄ particle composite. Time of the infiltration, structure of the poles and the variation of thedensity were predicted in the simulation, according to the experiment it shows that: the results of the simulation are similar with the results of the experiment. This model can attribute the ICVI processing of the Si3N4p/Si3N4 particle composite very well.(3) In the calculation ,Genetic Algorithm and the GA toolbox in Matlab were used to optimize the dimension of the particle in prilling processing. It shows that:when the diameter of particle is 6 urn and the dimension of prilling is 1.9mm, thedensification is the minimum;the best value of T is 1000K while P is 1000.2Pa.