| Ceramic-matrix composites (CMCs) are an important class of composite materials that are being considered for a broad range of aerospace components that need to withstand elevated temperatures. CMCs offer significant potential for raising the thrust-to-weight ratio of gas turbine engines by tailoring directions of high specific reliability using design-based fiber architecture.; Reactive melt infiltration is a process used to manufacture silicon carbide fiber reinforced silicon carbide (SiC/SiC) composites. This method of making SiC/SiC composites is advantageous since it is fast and net shapes can be obtained. The present stage of research on reactive infiltration is primarily experimental, wherein complete infiltration is rarely achieved and unreacted silicon still remains in the composite. Towards this end, this thesis deals with the modeling aspect of reactive melt infiltration and estimates permeability as a function of time for a preform geometry of hexagonally packed array of cylindrical (carbon-coated silicon carbide) fibers.; The modeling of the complete process has been structured into two stages, namely, micro and macro modeling. At the micro level, spherical and cylindrical geometries are considered for the reinforcements coated with carbon and solutions to the problem of coupled heat and mass transfer with chemical reaction and volume expansion are derived and solved numerically. Finite element analysis is used to qualitatively study the change in porosity in a representative volume element (RVE) with moving boundaries. The porosity of an RVE is calculated as a function of time from the growth of the SiC layer on the carbon-coated reinforcement. The permeability of the preform is then estimated as a function of the porosity in the macro model.; The thickness of the reaction product (SiC) layer forming the matrix in the composite is determined in terms of physical parameters (diffusivity of the reactants Si and carbon, temperature) and non-dimensional physical quantities (Sherwood number, Nusselt number, Thiele modulus). It is found that one of the dominant factors influencing the permeability is the initial temperature of the preform or more specifically that of the solid reactant, carbon. Also, the rate at which the permeability decreases is directly related to the physical process variables during infiltration. |
