Impregnation molding of continuous fiber-reinforced ceramic-ceramic composites using preceramic polymers

A ceramic-ceramic composite processing method based on resin transfer molding of particle-filled preceramic polymers was proposed and a numerical investigation of the resin impregnation was performed. The study is intended to provide a better understanding of the particle filtration occurring during impregnation and the nonlinear relations between various processing parameters, so that by a proper process design, the particle filtration and hence microstructure heterogeneity can be minimized. The proposed process is based on the need to counteract the high porosity and cracks forming in the composite as a result of mass loss and densification in the polymer during conversion to ceramic. A formulation of the problem was accomplished through combining anisotropic porous flow theory with particle filtration. Physical models were incorporated for filtration coefficient and domain permeability, to include the effect of resulting nonhomogeneous particle distributions. Compression resin transfer molding was proposed as an alternative to conventional resin transfer molding for processing high fiber volume ceramic composites at lower process pressures. Computational analysis showed that compression resin transfer molding offers the opportunity for homogenization of particle distributions within the composite through manipulation of the flow path by proper design of the impregnation and compression stages. The flow length rather than the flow velocity was observed to be the dominating factor on amount of filtration when the filtration mechanism is governed by geometric effects. Due to the geometrical complexity of the flow configurations and the existence of a moving boundary, the computational technique of boundary-fitted coordinate systems encompassing numerical grid generation was employed for numerical solution. Stability analysis indicated that the filtration solution accuracy is very sensitive to a nondimensional parameter derived from the current formulation. Through a resin impregnation experiment for a complex mold geometry validated the neat resin impregnation model. Sample ceramic composite specimens showed that with particle fillers, the reimpregnation ceramic gain was less than that of a filler-free composite. However, the open porosities of filled composites were shown to be higher than that of the neat PCP composites, indicating a need for a more effective reinjection process for filled composites.