Investigation On The Fabrication Technology,Microstructure And Mechanical Damage Mechanism Of C/C-SiC Composites Fabricated By Gaseous Silicon Infiltration

Carbon fiber reinforced carbon and silicon carbide composites(C/C-SiC),as advanced multifunctional ceramic matrix composites,have been widely applied in aerospace thermal structural systems and advanced braking systems and have great potential for development.In view of the shortcomings of the existing research on the preparation of C/C-SiC composites by Gaseous Silicon Infiltration(GSI),research on the key technological parameters of GSI and their effects on material composition and microstructure was carried out,and then by combining X-ray computed tomography and digital volume correlation to study the mechanical damage mode and mechanism,the relationship between microstructure and mechanical property of C/C-SiC composites with different structures was investigated.Research in this thesis enriches the theoretical basis of the preparation process of GSI and the means of studying the structure and mechanical damage of composite material and provides references for realizing the high-performance preparation of C/C-SiC composites.The main work of this paper can be summarized as follows:1.The effect of GSI temperature and time on the density,phase composition and microstructure of three-dimensional(3D)needle-punched C/C-SiC composites was explored.The results show that higher raction temperature(1700?)and longer time(2h)will be beneficial to increase the reaction degree of GSI.The changes of microstructure,SiC spacial distribution and pore size and distribution in C/C-SiC composites with the increasing of reaction time during GSI were investigated,which reveals the GSI densification process of 3D needled C/C-SiC composites,demonstrates the formation mechanism of SiC and enrichs the theoretical basis of GSI fabrication of C/C-SiC composites.GSI reaction of 3D needled C/C preforms mainly happen in the short-cut web region,and the C-Si reaction in GSI process can be divided into two stages:(i)Si vapor is absorbed on the surface of C to react and later the formed SiC wrap up the surface of the fiber as a thin layer;(ii)the reaction is controlled by the diffusion of C and Si through the SiC layer,and with the increase of the thickness of SiC layer and the decrease of reaction space,the reaction rate slows down gradually and stops,with the residual Si remains.Two types of SiC phases were observed:homogeneous and fine SiC layers on the surface of C fibers and polyhedral SiC grains with different sizes in the Si-SiC region.The 3Dstructure of SiC was extracted by combining X-ray tomography(XCT)with acid-dissolving etching method,which is spatially continuous in 2.0h GSI sample.2.The flexural,compressive and tensile properties of 3D needle-punched C/C-SiC composites were measured.The flexural strength and modulus of composites increase with the increase of GSI reaction time,reaching 185.4 MPa and 25.8GPa of 2.0h GSI sample in which the fracture surface has long fiber pull-out lengths,indicating a moderate interface.Needles will become the fragile part of crack propagation under bending loading.Compressive strength and modulus of 2.0h GSI samples in parallel direction and vertical direction are 269±4.8 MPa,21.1±0.7 GPa,272.9±3.7 MPa and14.1±0.6 GPa respectively.Tensile strengths of samples with needle direction as thickness and width directions are 109.4±5.2 MPa and 115.7±7.8 MPa respectively.A lot of fiber bundle pull-outs are observed in non-woven cloth while the fracture surface of short-cut web is even.By using XCT and DVC the compressive damage mode and mechanism of 2.0h GSI sample in three different directions were analysed,including the damage type and development,sample 3D deformation field and final failure pattern.Results show that DVC analysis can accurately measure the pattern,region and degree of inner damage,and needles can obviously restrict the development of damage under longitudinal compression.In-situ compression tests show that there are mainly six types of damage in specimen,including interface debonding and internal damage of 0~o and 90~o fiber bundles,matrix damage in short-cut web region and needle damage.3.3D five-directional C/C-SiC composites were fabricated by GSI,and the mechanical property and failure mode and mechanism were studied.By comparing with composites prepared by Precusor Impregnation and Pyrolysis(PIP),the effects of process types on the phase composition,microstructure and mechanical properties were investigated.The damage mode and mechanism of these two kinds of composites were directly and clearly explained by in-situ four point bending tests from the view of inner damage and its development.GSI sample has higher interfacial strength of fiber bundles thus cracks propagate almost directly through the fibre bundles and matrix,and because of the initial defects like the silicon corrosion of fiber and intra-bundle cracks composites have lower flexural strength(140±7 MPa).Cracks in PIP sample tend to propagate along the interface of matrix and fiber bundles,which has long crack path and high energy consumption,and accordingly PIP sample has higher flexural strength even it has lower density.DVC analysis also quantify the effect of inner damage to the elastic moduli of GSI and PIP samples during flexural loading,showing that elastic moduli in both two samples decrease from initial>100GPa to<40GPa at last.In in-situ compression tests,longitudinal compression samples form a connected network structure of cracks,including matrix cracks and intra-bundle and inter-bundle cracks,under capacity compression stress of 209 MPa and a top-down crack appear on one side of the sample,while catastrophic brittle fracture occur on the transverse sample after compression stress of 127 MPa,resulting in a main diagonal crack.The initial cracks within fibre bundles of GSI composite will expand and extend with increasing loads in bending test,but has less connection with the newly generated cracks.Similar initial cracks will propagate outside fiber bundle,forming interfacial and interbundle cracks in longitudinal compression case.4.The effect of architectures of C/C preforms on the phase composition and microstructure of C/C-SiC composites fabricated by GSI were studied.Loose and uniform pore structures with good continuity like the pore structure of short-cut needle felt will be benefical to the GSI reaction.More SiC will be generated in this kind of preforms and SiC has a spacial porous structure while in preforms with large fiber bundles the C-Si reaction mainly happen on the surfaces of fiber bundles and the SiC has a structure as thin shell.Results show that dense C/C-SiC composites with good mechanical properties can be prepared effectively by GSI,but details like C structure and process parameters need to be optimized to further improve composite property.Combined XCT and DVC analysis can be used effectively to study the damage mechanism,obtain 3D deformation,identify the onset of failure and even judge the flatness of sample.