Research On Design, Preparation And Properties Of Near-Zero Expansion C/SiC Composites

Carbon fibers reinforced silicon carbide ceramic matrix composites(C/SiC) hasgreat advantages in the fabrication of space opto-mechanical structures for its excellentcharacterized as lightweight、high mechanical strength、low thermal expansion andthermal shock resistance. Thermal expansion is one key issue for opto-mechanicalstructure to keep sensitivity sharp. Fabricating near-zero expansion C/SiC composites isa method to solve this problem and become one of hotspot in many countries. Thisdissertation mainly focuses on the research of design、preparation and properties ofnear-zero expansion C/SiC composites which fabricated by precursor infiltration andpyrolysis (PIP) process. Structure of near-zero expansion C/SiC composites weredesigned and analyzed by finite element method. The effect of carbon fibers braidedstructure and fibers bundle specification and PIP cycles on the longitudinal andtransversal thermal expansion of C/SiC composites were researched. The influence ofimitate using environment experiment and matrix modified phase on thermal expansionof C/SiC composites has been studied. Near-zero expansion C/SiC composites withcomprehensive properties were obtained.Different micro unit cell models of the carbon fibers performs’ structure werestudied in this paper. Fibers’ directions of2.5D,3D3d,3D4d,3D5d,3D6d braidedstructure were analyzed, and finite element models were established by the ANSYSsoftware. Finite element results showed that the types of the braided structure causedgreat influence to the thermal expansion of C/SiC composites.3D5d braided structureC/SiC composites have the lowest thermal expansion and can realize near-zeroexpansion in one direction.Relations of thermal expansion of different braided structure C/SiC composites totemperature have been investigated. Longitudinal thermal expansion of1D C/SiCcomposites was controlled by negative thermal expansion of carbon fiber in axialdirection. Transversal thermal expansion of1D C/SiC composites was controlled byradial expansion of carbon fibers. Braided structure of carbon fibers effected thermalexpansion of C/SiC composites, coefficient of thermal expansion(CTE) of3D5d C/SiCcomposites in longitudinal direction was the lowest of all the braided structurecomposites with the value of0.19×10-6/K, while3D4d was the highest and the CTEvalue was1.14×10-6/K. Transversal CET of3D3d C/SiC composites was the lowestwith the value of0.59×10-6/K, while3D5d was the highest and the CTE value was2.51×10-6/K.Relations of CTE and carbon fibers bundle of3D5d C/SiC composites have beeninvestigated. Longitudinal CTE of3D5d C/SiC composites with B fifth fibers bundlewas lower than that with A fifth fibers bundle, but transversal CTE is higher. Longitudinal CTE of3D5d C/SiC composites with D braided fibers bundle was thesame with C braided fibers bundle, but transversal CTE is higher also.Relations of thermal expansion and PIP cycles of C/SiC composites have beenstudied. Longitudinal and transversal CTE rise at14~16cycles and descend at16~20cycles. The changing of CTE was mainly due to the competing of the rising of SiCvolume fraction and reducing of porosity and residual stress.The effect of mechanics shake experiment, moisture absorption experiment andthermal shock conditions on3D5d C/SiC composites have been researched. It wasshown that mechanics shake condition made CTE of the composites rose for residualstress relaxing of interface, and the room temperature CTE was changed to0.39×10-6/Kfrom0.19×10-6/K. The influence of moisture absorption and thermal shock condition tothe composites were negligible.The technology of matrix modified phase controlling C/SiC composites has beenstudied. Adding PIP-C in C/SiC composites changed the individual constituents ofmatrix. Longitudinal CTE of C/SiC composites changed from-0.12×10-6/℃to-0.6×10-6/℃with1cycle to4cycles of PIP-C process, transversal CTE of C/C-SiCcomposites was about2.0×10-6/℃and were not obviously changed with the rising ofPIP-C cycles.