Processing And Properties Study Of 3D Needled C/C-SiC Composites By Gaseous Silicon Infiltration

With high specific strength,modulus,high thermal conductivity,low coefficient of thermal expansion?CTE?and excellent friction and wear properties,C/C-SiC composites have been widely applied as structural materials in the field of aerospace and high performance brake devices.Gaseous silicon infiltration?GSI?,known as an important method for preparing C/C-SiC composites,has a series of advantages such as high efficiency,low production cost and the ability to obtain nearly zero-porosity materials.Compared with liquid silicon infiltration?LSI?,GSI has deeper permeation depth and can be controlled more easily,so it is a good way to prepare C/C-SiC.However,the study on GSI is rare,and the effect of CVI C content,fiber volume fraction and properties of pyrolysis carbon on the properties of C/C-SiC composites has not been studied systematically.Therefore,the materials database for C/C-SiC needs to be further improved.Herein,this article focus on the study of processing and properties of 3D needled C/C-SiC composites by GSIThe effect of CVI C content on the phase composition,microstructure and properties of C/C-SiC composites was studied.The C/C preforms?30% fiber volume fraction?,had the density between 0.822g/cm³ and 1.363g/cm³,and were used to fabricate the C/C-SiC composites.Their mechanical properties increase first,then decrease with the growth of CVI C content.Too low CVI C content resulted in the corrosion of fiber by Si,while too high CVI C content made channel blocks for Si infiltration and lead to insufficient reaction.Specifically,C/C-SiC composite with the density of 1.085g/cm³ has the highest flexure strength,flexure modulus and fracture toughness of 308.31 MPa,31.71 GPa and 11.36MPa·m^1/2,respectively.Because of the effects of phase content and microstructure,thermal conductivity and CTE decrease with increase of preform densities.The effect of fiber volume fraction on phase composition,microstructure and properties of C/C-SiC composites by GSI was investigated.For the preform having density around 1.0¹.1g/cm³,its mechanical properties increase with the fiber volume contentranging from 10% to 30%,but the thermal conductivity and CTE have no significant changes.C/C-SiC composite with 10% fiber volume fraction has the lowest flexure strength,elastic modulus and fracture toughness of 154.7MPa ? 14.6GPa and5.7MPa·m^1/2,respectively,while has the highest thermal conductivity and CTE of 31.857W/?mK?and 4.475×10-6/K.As to the morphology of the obtained sic grains,it consists of C-SiC interfacial nanometer grain,and SiC-Si interfacial micrometer grain which is away from the carbon source.The Closed pores of composites include small pores between fiber bundles and big pores between web layers.In addition,the SI-C reaction in the GSI process will be inhibited for the blockling effect of the produced sic and the final thickness of Si C layer is about 5?m.The influence of the thickness of the CVD C coating on the mechanical properties of C/C-SiC composites was also studied.It can be found that too thin CVD C coating?about 200nm?would lead to fiber corrosionby gaseous Si seriously,but too thick CVD C coating?about 2?m?would make preform with too much higher density,inhibitting the infiltration of si vapor.It turns out that the preform with 30 vol.%has a density of ?1.1g/cm³,and the CVD C with the thickness of about 1?m is favourable for the GSI process.Finally,the effect of preforms with different carbon matrix on the structure and property of C/C-SiC composites was comparatively studied.For C/C preforms prepared from phenolic resin andfuran resin,the Py C is agglomerated after GSI,thus the reaction is insufficient,leading to the larger porosityand the poorer mechanical properties.The C/C-Si Ccompositesprepared from phenolic resin andfuran resin have a flexuralstrength of 279 MPa and239MPa,respectively.In order to improve SiC content and reduce Si content,the SiC was introduced into the composites by infiltration and pyrolysis of PCS,however,themechanical properties?flexure strength: 283MPa?does not increase visibly due to the lower continuityof Si C product after GSI process.