Prepare And Properties Study Of Vairable Density Preform C/C Composite Materials

The carbon fiber reinforced-carbon composites（carbon/carbon composites）has beenwidely used in in the applications of Aeronautics and Space because of its distinctiveperformance. To prepare C/C composites, we should prepare strengthen material(preform)with carbon fibers first, and then is the densification process with carbon matrix filling pore ofthe preform. The Chemical Vapor Infiltration（CVI）process is more and more popular in use.The preform is the framework of C/C composites. It can not only determine the structure andshape of C/C composites, but also affect their property.At present, the production and application fields of needled preform are more and morewidely, but in actual application of general needled preform there is still insufficient, the maininsignificance is in the densification process, the surface pores of preform are easy to jam,leading to longer material preparation period and higher production cost.Therefore, on thebasis of understanding the the effect of preform pore structure to precast gas mass transfer inthe CVI process and the composite material performance, the design and preparation of newtype of needle preform structure is important to increase the densification rate, reduce theproduction cost and improve the composites properties.This study design and prepare two kinds of new type of carbon fiber needled preform.Comparing the general preform, two layers-variable density preform and three layers-variabledensity preform (The preform and the composites which were made by these preform werereplaced by H, B1and B2respectively), examine the structure and the pore size distributionof these preform. Prepare C/C composites by CVI process, and the effect of the pore structureof preform on the process of densification and the microstructure of matrix were studied.At the same time, flexural properties, Compression performance, graphitization degree andoxidation performance were tested to study the effect of the structure of preform on themechanical properties graphitization degree and oxidation of C/C composite. The maincontents and conclusions of this work are as follows:(1) In three kinds of preform, the outer structure of preform H is the most closely, and itsinner structure is relatively looser. the upper structure of preform B1is relatively looser, andits lower structure id more closely. The outer structure of preform B2is relatively looser, andits inner structure is more closely. In preform H, more than eighty percent of the aperture isabout10microns. In the upper layer of preform B1, more than ninety percent of the apertureis concentrated in the10microns to50microns, and in the lower layer, more than sixtypercent of the pore diameter is less than10microns and relatively concentrated distribution.In the outer layer of preform B2, more than ninety percent of the aperture focusing on15to40microns, while in the outer layer, the majority of the aperture is smaller than10micrometers and relatively concentrated distribution.(2) The preform density and structure are the main factors of deciding the CVI rate andfinally composite material density. With the same preform density, composite material B2getthe highest density, composite material B1get the second, and the density of compositematerial H is the lowest. Achieving the same density, preform B2takes the shortest time, and preform H takes the longest time. With the same structure, the deposition rate reduce alongwith the increase of preform density, and over time, the gap first increases then decreases. Thepreform with bigger density produce the composite material with smaller eventually density.(3) The microstructure of pyrolytic carbon has no direct relationship with the porestructure of the preform. In composite material B2, the obturator is less. In composite materialB1, there is some small obturator in the lower layer. Composite material H have the worstelectrode position effect, and the obturator is larger and more.(4) Composite materials prepared from variable density preforms have better mechanicalproperties, and compared with composite material B1, composite material B2has moreobvious influence on the material mechanical properties. In the compression test, when theinitial density of preform is0.45g/cm3, composite B2has the most obvious influence on thecompression properties, and when the initial density of preform is0.40g/cm3, composite B1has the most obvious influence on the compression properties. In the bending test, when theinitial density of preform is0.50g/cm3, composite B2has the most obvious influence on thebending properties, when the initial density of preform is0.55g/cm3, composite B1has themost obvious influence on the bending properties.(5) In the oxidation test, the oxidation loss rate of the three types of materials increasewhen the temperature rise. Composite materials prepared with different structures havedifferent oxidation resistance. Composite B2has the minimum the oxidation loss rate atdifferent temperatures, followed by the composite material B1, and the oxidation loss rate ofmaterial H is the biggest. This shows that the antioxidant properties of composite material B2is the best, while that of composite H is the worst.