Optimization Design For C/SiC Composites

With the development of computer science and artificial intelligence techniques, materials design is becoming one of the three main parts of materials science and engineering (MSE) with materials experiments and their theories. Carbon fiber reinforced Silicon Carbide matrix composite (C/SiC), which is used for the hot parts of the aeroengines, are investigated to realize its virtual design in this paper. Because of the severe and complex service conditions with elevated temperature in the aeroengines, the computer simulation theories and techniques were investigated mainly in two aspects: the environmental properties evolution of the material, and the chemical vapor infiltration (CVI) and the chemical vapor deposition (CVD) processing. A software system for materials design is developed, which can realize the closed loop and multi-scale materials design. Simulation results were proved with corresponding experiments. Both materials processing and service information can be displayed with the software system, which shortens the period and reduces the cost for materials processing and environmental tests. And the new type of materials for aeroengines can be prompted based upon the above information. Materials design theory and methods are developed finally. The main results are listed below:1. New principles and models of materials design have been brought forward, which include two basic elements, environment and microstructure. These principles and models provide a mathematical definition for the closed loop design. And a new concept of "order" has been put forward to quantitatively define the length scale of the multi-scale design. Two difficult problems in materials design, closed loop design and multi-scale design, can be described with above mathematical models.2. Both macroscopic and microscopic process or phenomena are involved during the manufacture of C/SiC, such as position optimization for components and flow field simulation in macroscale, densification process of fibrous perform in mesoscale, and products calculations in microscale etc. Genetic Algorithms (GAs), Finite Element Method, boundary layer theory, two-step-three-stage approach, quantum chemistry and thermodynamics such theories or mathematical methods have been applied to investigate the macroscopic and microscopic process or phenomena during manufacturing the C/SiC composites and their components. And the optimal CVI/CVD processing parameters were obtained.1) In order to properly use the space of the reactor, position optimization forcomponents in the chemical vapor infiltration (CVI) reactor was realized with GAs. Flow field simulation results showed that the laminar boundary layer on the leeward side of a component was thicker than that on the windward side of the components. The thickness difference can be decreased by decreasing the gas flux. Components with uniform surface deposition quality were attained by changing the attitude of the components at times during the CVD processing.2) There are two-steps deposition of pyrolytic carbon (PyC) interphase and SiC matrix and three-stages densifications of the pores among single fibers by depositing both PyC interphase and SiC matrix, and the densification of the pores among fiber tows by depositing SiC matrix. A new mathematical technique, a two-step-three-stage approach, was proposed to model the densification of a carbon fiber preform with PyC and SiC matrix during CVI processing.3) Based on CVI densification kinetics analyses, optimal processing parameters for SiC matrix are T=1223~1273 K and P=4~8 kPa. In above case, 25 mmx25 mm is an upper limit of the preform section size if the maximum porosity gradient in the preform is not larger than 20%. Based on both of the thermodynamics calculation of the processing products and the CVI densification kinetics analyses, the optimal processing parameters of CVD SiC are MTS=60ml-min"1, H2=300 ml-min"1, Ar=200 ml-min"1, T=1223~1273K and P=5 kPa.4) Based on thermodynamics calculation, the optimal processing parameters of CVD SiC coating are MTS=60 ml-min"1, H2=300ml-min"', Ar=200 ml-min'1, T=l 200-1400K, and P=5 kPa. Above results accord with the practical processing parameters.5) G3MP2, an accurate quantum chemistry calculation, is applied to obtain thethermochemical data of A,G°. A,G° of MTS(g) (CH3SiCl3) in the 3rd edition ofJANAF thermochemical tables were corrected and the calculated value is proved in the latest 4th edition of NIST-JANAF thermochemical tables. It is proved that the thermodynamics calculation is available and reliable base on the accurate quantum chemistry calculation.3. Both macroscopic and microscopic process or phenomena are involved during the service of C/SiC, such as the fracture behaviors in macroscale and the service products calculations in microscale etc. Factorization method for failure mechanism analysis, Adaptive Neural Fuzzy Inference System (ANFIS), quantum chemistry and thermodynamics such theories or mathematical methods have been applied toinvestigate the macroscopic and microscopic process or phenomena during the service of C/SiC. And the microstructure design criteria were developed based above environmental performance evaluation results.1) Active oxidation of SiC at the elevated temperature is the dominant failure mechanism of C/SiC. If the concentration of the wet oxygen is fixed, the active oxidation of SiC at the elevated temperature can be weakened by designing a proper volume fraction of the preform's porosity. If the volume fraction of the preform's porosity is fixed, the active oxidation of SiC at the elevated temperature also can be weakened by designing a proper concentration of the wet oxygen.2) Under the accelerate environmental conditions(Po2=8 kPa, Ph2o=15 kPa, Psait=lOO ppm, 10 h, T=400~1500°C), the oxidation and corrosion resistance of C/SiC is improved by increasing the SiC coating up to 4 layers, about 80 microns, (weight change<0.5%)04. A software package that has the features in both the scientific and practice viewpoints has been developed. This package can realize the multi-scale and closed loop materials design for C/SiC, help us to design microstructure and processing parameters to meet the requirements of environmental performance, and validate the designing results based on the virtual environmental performance simulation.