The aim of the p roject is to develop a modelling of the chemical vapour infiltration (CVI) processing of SiC ceramic matrix composites (CMC). The research could result: (i) in a better knowledge of the mechanisms involved in the deposition of a SiC ceramic within a porous substrate and (ii) in a forecast of the kinetics of densification and infiltration homogeneity as well as the chemical composition of the ceramic, as a function of the experimental parameters. achievements to date: The first part of the research dealt with the mechanisms of deposition on flat substrates in the Si-C-H-Cl system. The growth rates were investigated in both laboratories under complementary conditions. These CVD kinetics were successively modelled, under both hypotheses of physical and chemical limitations. In the first case, the study of momentum and mass transports in the reactional area showed, by computing gaseous species concentration and deposition thickness profiles, that the growth rate is: (i) not influenced by total pressure and hardly by tempeature; (ii) raised by increasing the total flow rate and by decreasing the dilution ratio. In the second limitation, the theoretical approach, based on thermodynamics and considerations of adsorption phenomena, resulted in two kinetic laws related to two chemical mechanisms depending on the experimental conditions. The second part of the programme was to define, prepare and characterize the model porous substrates. The ICT samples, including several straight cylindrical pores, were prepared either by drilling holes in resin samples subsequently carbonized (diameters > 0.1 mm) or by removing by oxydation the fibres of a composite made of carbon fibres reinforced PCS-based SiC. The LCTS samples were obtained from carbon- core SiC CVD-filaments oxydized at 800C which resulted in straight cylindrical tyubes (diameters of 37 m).
Preliminary experiments indicated that although silicon carbide growth rates at low temperatures and with known parameters were understood, the chemical processes involved at higher temperatures did not meet expectations. The chemical vapour infiltration (CVI) of straight cylindrical pores was analysed in 2 sets of experiments that are both based on the mass balance equations arising from the competition between the mass transport (Fick and Knudsen diffusion) and the chemical reactions that occur during movement within pores.
The 2 areas of research made similar predictions for the effect of parameters such as temperature, total pressure and geometry of the pore, and investigated:
the ratio between the rate of chemical reaction and the rate of diffusion (defined as the Damkohler number) and whether it determines the uniformity of concentration and deposit profiles;
the conservation equations for the various entities in the system, modelling the diameter variations over time and with regard to the position in the pore.
Analytical techniques used for solving the mass balance equation were found to match situations where wide pores were to be found. The numerical method used was found to give satisfactory results for narrow pores.
Validation was done using scanning electron microscopy on prepared porous substrates (pore sizes of 6 um to 1000 um) created by mechanical drilling and controlled oxidation of carbon fibre in silicon carbide substrates. From these results the researchers were able to determine that the analytical technique could be applied in situations where the pore sizes are broadly distributed. In these circumstances, the conditions met with after CVI processes matched the predictions when modelling the infiltration kinetics of silicon carbide whiskers.
AMONG THE TECHNIQUES PRESENTLY KNOWN FOR THE PROCESSING OF CERAMIC MATRIX COMPOSITES (CMC), THE CHEMICAL VAPOR INFILTRATION (CVI)TECHNIQUE IS UNDOUBTEDLY THAT WHICH IS THE MOST FLEXIBLE AND LEADS TO THE HIGHEST PERFORMANCES.HOWEVER, IT IS A SLOW PROCESS. ITS INDUSTRIAL DEVELOPMENT REQUIRES AN EFFORT OF FUNDAMENTAL RESEARCH, PARTICULARLY IN THE FIELD OF MODELLING (IN ORDER TO SHORTEN THE DENSIFICATION DURATION, TO IMPROVE THE MATERIAL QUALITY AND, THUS, THE PERFOMANCES-COST RATIO.
THE PROGRAMME COULD RESULT :
1- FROM A FUNDAMENTAL POINT OF VIEW, IN A BETTER KNOWLEDGE OF THE CHEMICAL MECHANISMS INVOLVED IN THE DEPOSITION OF A CERAMIC MATERIAL WITHIN A POROUS SUBSTRATE AND,
2- FROM AN ECONOMICAL POINT OF VIEW, IN GUIDE-LINES THAT COULD BE USED BY INDUSTRY TO OPTIMIZE THE CVI-PROCESSING OF CERAMIC COMPOSITES WITH A VIEW TO SHORTEN DENSIFICATION TIMES, TO IMPROVE MATERIAL QUALITY AND TO REDUCE PROCESSING COSTS.
Funding SchemeCSC - Cost-sharing contracts