Computational Simulation Of Micro-structural Evolution For Chemical Vapor Infiltration Process Of Ceramic Matrix Composites

Compared with conventional processing techniques, chemical vapor infiltration(CVI) can obtain composite materials with excellent mechanical and thermal properties,so it is widely used in fabrication of CMC-SiC. However, the major drawback of thismethod is the very low infiltration rate ralted to the competition between the depositionreaction and the diffusion of the gaseous products, and then CVI process usually requirelong processing time, therefore are highly inefficient and leads to high product cost.Besides the elaborate experimental work, modeling and numerical simulation provideinsight for optimization of CVI process as well as valuable guidelines for futureexperimental research, and have been regarded as powerful tools for deep understandingof CVI process. However, in most simulation works, the structural features of thedual-scale pores and the evolution of gas and thermal transport properties withdensification degree have not been considered accuratelly, which affect the reliabilityand usability of the simulation.In this thesis, level set method was proposed to describe the evolution of themicro-structures during the CVI process, mathematical models predicting the structuralevolvement, gas transport property and thermal transport property for CVI process wereproposed and then used the finite element method to computationally simulate thedensification behaviour of the CVI process of CMC-SiC at multi-scale. The maincontents and conclusions are as follows:1. Modeling the pore structure evolution at bundle scale: according to the structuralproperties of the micro-pores inside the bundles and the macro-pores between thebundles, level set method in conjunction with steady state diffusion equation wasproposed to describe the evolution of the micro-pores and macro-pores during the CVIprocess. The above mathematical models were then implemented by finite elementmethod to numerically simulate the evolution of the pore structures. The effects of thegeometry of preform on the structural evolvement had been investigated. Based on thenumerical results, an analytical model to depict the structural evolvement had beendeveloped. The results indicate:(1) The micro-pores starts to be sealed when thevolmue fraction of the micro-pore inside the bundle is less than22%;(2) Themacro-pores starts to be sealed when the macro-poeosity is less than18%;(3) Thestructural evolvement of the micro-pores is mainly affected by the aspect ratio of thetow cross section and the initial pore volmue fraction of the the bundle;(4) The structural evolvement of the macro-pores is mainly affected by the ratio of the gapwidth to the tow width between adjacent tows along the in-plane direction.2. Modeling the gas transfer at preform scale: according to the structural propertiesof the preform, level set, mass conservation and momentum conservation equationswere coupled and then a mathematical model were developed to estimate the gastransport properties of the composites during CVI process. The above mathematicalmodel was then implemented by finite element method to numerically simulate theevolution of gas transport properties with the structural evolvement. Based on thenumerical results, an analytical model to predict the gas transport properties had beendeveloped. The results indicate:(1) The gas transport properties of2D wovencomposites along the through-thickness direction are an order of magnitude larger thanthe ones along the in-plane direction, the former is mainly affected by the ratio of thegap width to the tow width between adjacent tows and the aspect ratio of the tow crosssection, the latter is mainly affected by the ratio of the gap width to the tow widthbetween adjacent tows;(2) The effect of residual porosity on gas transport propertiesshould be considered;(3) The structure of the preform plays more influence onpermeability than diffusivity.3. Modeling the thermal transfer at preform scale: according to the structuralproperties of the preform, level set and energy conservation equations were coupled andthen a mathematical model were developed to estimate the thermal transport propertiesof the composites during CVI process. The above mathematical model was thenimplemented by finite element method to numerically simulate the evolution of thermaltransport properties with the structural evolvement. Based on the numerical results, ananalytical model to predict the thermal transport properties had been developed. Theresults indicate:(1) The thermal conductivity of the composites is highly affected by themicro-porosity, macro-porosity and the preform structure;(2) The actual thermalconductivity of the CVI-CMC is highly lower by the residual pores;(3) The actualthermal conductivity of CVI-CMC is not only related to the geometry, the thermalconductivity of its constitute, but also to the interface between each constitute andmatrix cracking.4. Modeling the densification behaviour at reactor scale: a two-dimensionalmathematical model was proposed to describe the structural changes of the micro-poresand macro-pores of the composites during the ICVI process. The above mathematicalmodel was then implemented by finite element method to numerically simulate thedensification behavior of CMC-SiC component in fiber and bundle scale, which was an important extension of the multi-scale simulation of ICVI process. The results indicate:(1) The comparison of the simulation with experimental data indicate that When theCVI process is performed at the relatively low temperatures (~1000℃) and underrelatively low pressure (P<15kPa), the reaction follows the first order. HCl mainlysuppress the decomposition of MTS in the gas phase, and plays little effect on thesurface reaction;(2) Temperature is the most important process parameter of controllingthe uniformity of the deposit, lower the tempertature benefits the uniformity of thedeposit, but makes the a longger time of processing; lower the pressure improves theuniformity of the deposition on the macro-pores, but plays no influence on themicro-pores; the mole fraction of MTS plays little influence on uniformity of thedeposit;(3) Increasing the radius of the fiber and decreasing the thickness of the towimprove the uniformity of the deposition on the micro-pores, while increasing thethickness of the preform and decreasing the thickness of the tow are bad for theuniformity of the deposition on the macro-pores;(4) At5kPa, the critical temperaturesfor deposition on the micro-pores and macro-pores are1000℃and1050℃, respectively;(5) Three different stages exist in ICVI process: micro-pores on the surface of thepreform infiltration dominated stage, macro-pores on the surface and micro-pores insidethe preform infiltration simultaneously dominated stage and macro-pores infiltrationdominated stage, the density gradient generates at the first stage, decreases at the secondstage and almost remains constant at the third stage;(6) The best strategy of temperaturefor ICVI process is,950~1000℃at the first70hours, and increase the temperature to1100℃.