Computational Simulation Of ICVI Process Of Silicon Carbide Ceramic Matrix Composites

Silicon carbide ceramic matrix composites (CMC-SiC) have been designed anddeveloped as promising thermostructural materials in aeronautics and astronautics fieldsdue to their excellent performances. The isothermal chemical vapor infiltration (ICVI)process is widely used in fabrication of CMC-SiC. Besides the elaborate experimentalwork, modeling and numerical simulation have been regarded as powerful tools fordeep understanding of ICVI process. It is logical to develop mathematical models basedon experimental knowledge and physicochemical phenomena in ICVI process toprovide insight for optimization of ICVI process as well as valuable guidelines forfuture experimental research. In this thesis, mathematical models of ICVI process ofCMC-SiC were firstly proposed and then solved by finite element method tocomputationally re-appear the ICV1 process of CMC-SiC in various conditions. Themain contents and conclusions are as follows:1. A two-dimensional mathematical model was proposed to describe thephysicochemical phenomena in porous preform during the ICVI process for fabricationof CMC-SiC and the infiltration induced structural changes of preform is depicted by amodel called搈 ultimodal porosity model? The above mathematical model was thenimplemented by finite element method to numerically simulate the densificationbehavior of CMC-SiC component of small scale thruster liner for solid rocket engine.The calculation results show that three different stages exist in ICVI process ofCMC-SiC, that is, micro-pores infiltration dominated stage, mixture dominated stageand macro-pores infiltration dominated stage. The correspondences between calculatedresults and experimental data imply that this mathematical model is reasonable andfeasible to characterize the ICVI process of CMC-SiC.2. A two-dimensional mathematical model for fabrication of CMC-SiC in ahot-wall ICVI reactor was developed. Transport phenomena of momentum, energy andmass in conjunction with infiltration induced changes of preform structure were takeninto account. The integrated model was solved by finite element method to numericallysimulate flow field, temperature field, concentration field and densification behavior ofCMC-SiC, which laid solid foundations for deep understanding of ICVI process ofCMC-SiC.3. The mathematical model for fabrication of CMC-SiC in a hot-wall ICVI reactor was rationally simplified on the basis of quantitatively calculation, that is: (1)the forced convection in porous preform should be neglected due to its trivial effects onICVI process; (2) Navier-Stokes equations are not suggested and momentumconservation equations are preferable to represent momentum transport in the freemedia of ICVI reactor for fabrication of CMC-SiC; (3) the reactor wall reaction shouldbe neglected due to its trivial effects on ICVI process; (4) Simplification of reactorconfiguration by neglecting expansion zone of reactor is reasonable and acceptable fornumerical simulation of ICVI process of CMC-SiC.4. The dependence of ICVI process of CMC-SiC on operating conditions (such astemperature, pressure and reagent flux, etc.), reactor dimensions (such as the diameterof reactor, the diameter of reactor inlet, etc.), preform structure(such as initial porosityof preform) and the location of preform in reactor were systematically investigated bycomputational simulation of the above developed mathematical models. The calculationresults show that the reasonable operating conditions for ICVI process of CMC-SiC areas follows: temperature of 900～1100℃, total gas pressure of 5000～8000Pa and MTSflux of 10～50 sccm; the higher the initial porosity of preform, the longer the terminatingtime of ICVI process, the higher the final density of preform and the better theinfiltration uniformity; inlet dimension of reactor has little influence on the ICVIprocess of CMC-SiC; the optimal reactor diameter is 90mm for the preform consideredin this thesis; the rational location of preform is from 0.05m to 0.24m.5. The optimization of SiC whisker preform was implemented using the abovedeveloped mathematical models. The optimized dimension of SiC whisker aggregatewas given on the basis of numerical simulation of ICVI process of mini SiC_w/SiCcomposites and the subsequent SiC_w/SiC components. The calculation results show thatthe reasonable diameter of SiC whisker aggregate is from 0.3mm to 0.5mm.6. Computational simulation of simultaneous densification of multiple preformsin an ICVI reactor was implemented using the above developed mathematical modelsand the effects of the ratio between preforms volume and reactor volume on ICVIprocess of CMC-SiC was studied. The calculation results show that the reasonable ratiobetween preforms volume and reactor volume should be less than 11.25% for thepreform and reactor considered in this thesis, that is, the number of preforms in reactorshould be less than five.7. The above developed mathematical models were proposed to systematicallyanalyze the gas transport in ICVI process of the complicated-shaped components suchas full-size small scale nozzle, extended part of large scale nozzle, large scale thruster chamber for ramjet and large scale nose cone, etc. The flow field and concentration filedof ICVI reactor with large scale nose cone were then optimized.