Microwave Sintering And Hybrid Braiding Structures Of PIP-SiC/SiC Composites

Continuous SiC fiber reinforced SiC ceramics matrix(SiC /SiC)composites have been considered to be one of the most promising materials in the domain of aerospace and nuclear energy,due to their notable advantages,such as high specific strength,high specific modulus,good toughness,excellent temperature and radiation resistance.Precursor infiltration and pyrolysis(PIP)is one of the most common processes to fabricate SiC/SiC composites,but the long process time and the high cost of raw materials are still the main factors to restrict the extensive applications of SiC/SiC composites fabricated by PIP process.In this thesis,on the one hand,replacing the conventional heating,the microwave heating with the characteristic of rapid sintering is applied to optimize the PIP process.The reduced process time and meanwhile the improved material properties are expected.On the other hand,the hybrid 3D-4d preforms can be fabricated with SiC fibers and high performance C fibers.The reduced raw material cost is expected.At last,the hybrid preforms are used to reinforce the SiC matrix composites fabricated via PIP process under the microwave heating.T he good compatibility between the microwave sintering and the hybrid braiding is expected.By combine the microwave sintering with 3D hybrid braiding,the time-saving and meanwhile the reduced costs of the raw materials can be realized.Firstly,the pyrolysis products of PCS,the tensile properties of SiC fibers and their microstructures under different microwave and conventional heating temperatures are studied.The results indicate that the atomic fractions of Si and C in the PCS pyrolysis products are much different between microwave and conventional heating,but the atomic fractions of O are nearly the same.The PCS pyrolysis products under the microwave heating contain more β-SiC micro-crystals with higher crystalline and less residual C than the conventional heating.The tensile strength and modulus of KD-I SiC fibers firstly increase and then decrease with the microwave and conventional treatment temperature increasing from 800°C to 1400°C.The tensile strength and modulus of KD-II SiC fibers decrease with the microwave treatment temperature increasing from 1100°C to 1400°C.The densification process,the flexural properties and the fracture behaviors of the SiC/SiC composites fabricated by different microwave and conventional sintering temperatures(800°C to 1100°C)are discussed.The results indicate that high sintering temperatures and low heating rates can bring about SiC /SiC composites w ith high densification degrees.Under the same sintering temperatures,the microwave heating sintered SiC/SiC composites have better flexural strength and toughness than the conventional heating sintered ones.The flexural strength and toughness of the SiC/SiC composites fabricated by the microwave heating of 1100°C are 551.5±51.7MPa and 19.2±1.7MPa·m1/2.The fracture behaviors of SiC/SiC composites are discussed from the point of the load-displacement curves and the microwave heating sintered SiC/SiC composites display more notable characteristic of ductile fracture than the conventional heating sintered ones.The lower flexural modulus of the microwave heating sintered SiC/SiC composites than the conventional heating sintered ones are explained from the point of the micro elastic modulus of the SiC fibers and SiC matrix,the interfacial bonding and the porosity of the fabricated SiC/SiC composites.After the annealing treatment,the residual tensile stress of the SiC fibers and SiC matrix is relieved and their micro elastic moduli are improved.Via calculating the nano scratch results,the residual tensile stress of the fibers and matrix of the microwave sintered SiC/SiC composites is higher than the conventional heating sintered ones.In spite of microwave or conventional heating,high sintering temperatures result in higher residual tensile stress.The impacts of the sintering time,the hot mould pressing pressures,the thickness of PyC interphase coatings and the SiC fiber reinforcement types on the densification process,the flexural properties and the microstructures of the SiC/SiC composites fabricated by PIP process via using microwave heating.The results indicate that the interphase bonding and the SiC fiber properties can be harmonized by changing the sintering time at each PIP cycle.The SiC /SiC composites fabricated with sintering time-1.0h at each PIP cycle have better mechanical properties than the other sintering time.The interphase bonding and the densification degrees of the fabricated SiC/SiC composites can be harmonized by changing the hot mould pressing pressures.Too high and too low mould pressing pressures both make against the flexural properties.The SiC/SiC composites fabricated assisted with hot mould pressing pressure-3MPa have better mechanical properties,whose flexural strength,modulus,toughness and density reach 492.7±36.8MPa,47.4±4.4GPa,23.4±1.5MPa·m1/2 and 2.16g·cm-3,respectively.The interphase bonding and the densificatio n degrees of the SiC/SiC composites fabricated assisted with hot mould pressing process can also be harmonized by the thickness of the PyC interphase coatings on the SiC fibers.The SiC/SiC composites coated with the PyC coating of 0.20μm have better mechanical properties,whose flexural strength,modulus and toughness reach 523.7±49.4MPa 、61.2±3.4GPa and 21.4±1.1MPa·m1/2,respectively.By using KD-II SiC fiber reinforcements with higher heat resistance,the flexural properties of the fabricated SiC /SiC composites can be improved greatly.The flexural strength,modulus and toughness of the KD-II/SiC composites sintered by the microwave heating of 1200°C reach 643.6±46.7MPa、76.9±9.5GPa and 31.3±0.7MPa·m1/2,respectively.Two kinds of 3D hybrid-braiding patterns based on the four-step braiding method are defined.The discussions about the entity simulation and the periodic structures of the defined hybrid-braiding preforms display their structural complexity.The occurrence probabilities of the 3D hybrid-braiding yarn shuttles were calculated by counting the occurrence times on every location of the machine bed within the periodic structure steps and dividing them.The examples demonstrated that,for most of the classifications,the averages of the probabilities have the tendency of approaching 1.0.Therefore,based on the additive property of the stiffness matrix,the defined 3D hybrid-braiding preforms can be simplified into two preforms braided with single kind of yarns for the sake of stiffness property prediction,which will provide forceful foundation for the SiC matrix composites reinforced by the hybrid fibers.Based on the 3-cell model and the average stiffness method,the impacts of the braiding angles and fiber volume fractions on the stiffness matrix of the SiC matrix composites reinforced by the hybrid fibers are calculated and the reasonability of the calculating results are verified.The hybrid 3D-4d preforms braided with KD-I SiC fibers and T300 C fibers are used to reinforce the SiC matrix composites fabricated by PIP process via using microwave heating and their longitudinal tensile properties are tested.The results indicate that the calculated and tested values of the longitudinal tensile moduli of the fabricated composites are not much different,proving the reliability of the 3-cell model and the written programs.The tensile properties of the hybrid fiber composites fall in between the C/SiC and SiC /SiC composites with the similar braiding parameters and the same fabricating routines,ind icating that the microwave sintering has good compatibility with the 3D hybrid braiding.