Study Of Preparation And Properties Of SiOC Ceramic And SiC Nanowires In-situ Modified Carbon/Carbon Composites

Carbon fiber reinforced pyrolytic carbon matrix(C/C)composites play an irreplaceable role in the field of aerospace materials due to their unique properties such as high temperature resistance(up to 3000?),low specific gravity,excellent mechanical properties at high temperature,erosion resistance and wear resistance.Because of its simple structure and easy densification in the later stage,2 D carbon fiber preforms are commonly used to reinforce C/C composites.However,the reinforcement between fiber and fiber laminates,between fiber bundles and fiber bundles,and the gap between fiber braiding and other parts are relatively weak,so destructive cracks propagate in this area and finally make the material invalid.It is an effective method to improve the interlaminar mechanical properties of C/C composites by introducing nano-reinforcing phases into the fibers to control the micro-structure of the materials.The nano-reinfored phases and fiber reinforced phases can enhance the pyrolytic carbon matrix at multi-scale and then change its macro-properties to meet the specific environmental requirements.In this paper,sol-gel impregnation,Chemical Vapor Infiltration(CVI)reaction and thermal carbon reduction were used to synthesize SiC nanowires in the composites without introducing any catalyst.The in-situ multi-scale reinforced toughened C/C composites were achieved.In the process of nanowire formation,SiOC ceramics mesophase should be synthesized first.SiOC ceramics phase has unique rod structure and can also strengthen and toughen C/C composites.SiOC ceramic mesophase modified C/C composites are called SiOC-C/C composites.SiC-C/CI and SiC-C/CII composites can be synthesized under two different conditions by changing the process.Firstly,the growth process of SiOC ceramics mesophase and SiC nanowires was optimized to obtain uniform and stable reinforcing phase of SiOC and SiC nanowires.The microstructure,phase composition and chemical bonding of SiOC ceramics mesophase and SiC nanowires were systematically analyzed,and the growth mechanism was studied.The microstructures,chemical bonding and mechanical properties(including compression,bending and interlaminar shear)of SiOC-C/C composites,SiC-C/CI composites and SiC-C/CII composites were studied.The strengthening and toughening mechanisms of SiOC ceramics and SiC nanowire reinforced phases were revealed.The thermophysical properties of SiC-C/CI composites and SiC-C/CII composites,including thermal weight loss,thermal conductivity and thermal expansion,were studied.The modification mechanism of SiC nanowires was discussed.The main research contents and achievements are as follows:(1)Using silicon in silica gel as Sisource and methane gas deposited carbon as C source,ceramic SiOC phase was prepared inside the carbon felt by sol-gel impregnation and CVI reaction,SiOC ceramic with a rodlike structure in the inner of C/C composites.SiOC amorphous ceramics mesophase is not only essential for the formation of SiC nanowires,but also can be used as reinforcing phase to modify C/C composites.C/C composites can be enhanced by SiOC in-situ modified.Introducing SiOC ceramic can increase the brittleness of materials,enhance the cohesion of carbon matrix,absorb a lot of energy during failure,and increase the hardness of materials,thus can improve the mechanical properties of C/C composites.(2)The mechanical properties of SiOC-C_PyC/C composites modified by SiOC after pre-deposition of pyrolytic carbon layer are obviously improved.Compared with C/C composites,the compressive strength of out-of-plane and in-plane,the three-point bending strength and the interlaminar shear strength are increased to 158%,117%,119%and 92%,respectively.The compressive modulus ut-of-plane and in-plane and the three-point bending modulus are increased to 214%,137%and 185%,respectively.(3)SiC nanowires were synthesized in situ without any catalyst using SiOC amorphous ceramic mesophase as reactant and 2 D carbon felt as a three-dimensional support structure.The shape of SiC nanowires was columnar and beaded,with a diameter of about 100-200 nm and a length ranging from several microns to tens of microns.The crystal type of nanowires is3C-SiC in cubic system.SiC nanowires can not grow uniformly and steadily without SiOC mesophase or carbon felt to provide growth space.The formation of SiC nanowires is mainly due to the carbothermal reduction reaction.The modification of nanowires by O element can make the nanowires have a larger aspect ratio.(4)SiC-C/CI composites can be obtained by densifying PyC matrix after in-situ growth SiC nanowires in carbon felt.SiC nanowires distribute irregular and multidirectional in the matrix layer,and the distribution of SiC nanowires in on-woven layer is lesser than in needling felt layer.After densifying,PyC matrix is deposited layer by layer around the SiC nanowires in a coarsening manner.SiC nanowires can achieve multi-scale in-situ strengthening and toughening in C/C composites.Compared with C/C composites,the mechanical properties of with SiC nanowires were significantly improved.Compared with C/C composites,SiC-C/CI composites of out-of-plane and in-plane compressive strength,three-point bending,as well as interlaminar shear strength have 227%,109%,130%and190%improvements,respectively.SiCNW-C/C composites also had 49%,111%and 42%in the out-of-plane,in-plane compressive modulus and three-point bending modulus respectively.The indentation modulus increases by 90%.(5)The strengthening and toughening mechanisms of SiC nanowires in SiC-C/CI composites include:(a)SiC nanowires deposited around annularly by PyC distribute in the composites without direction,so SiC nanowires can be used as the second reinforcing phase except fibers to reinforce and toughen C/C composites;(b)Some SiC nanowires grow outward with a radial shape perpendicular to fibers,which not only can effectively improve the interface bonding between carbon fibers and PyC matrix and but also repair annular cracks;(c)SiC nanowires joining the PyC and PyC layers can reduce the defects between pyrolytic carbon layers,and increase the cohesion of pyrolytic carbon.(6)SiC-C/CII composites which were in-situ modified by SiC nanowires in another way were prepared by SiOC-C/C composites after heating treatment at 1600?.Unlike SiC-C/CI composites,SiC nanowires are linearly distributed in the voids and defects of the composites,SiC nanowires distributed in the pyrolytic carbon matrix with a three-dimensional network structure.SiC nanowires modify C/C composites by homogenizing internal defects,reducing pore size and filling vacancies.Compared with C/C composites,the thermal conductivity of increased significantly and the thermal expansion coefficient reduced.