Study On Fabrication Process And Properties Of C/C Composites Via Multi-physics Fields CVI

C/C composites are possessed with excellent performances including high-temperature mechanical and friction properties,and widely applied in aviation and aerospace industries.Chemical vapor infiltration(CVI)is the most competitive strategy to fabricate the composites.The major efforts in present work are directed to shorten the production cycle and lower the cost. Through experimental investigations and theoretical analyses,the main factors affecting the preparation process of C/C composites via multiphysics-field CVI were optimized,and also both the deposition mechanism of pyrocarbon and material properties were investigated in this work.The following are the main contents and conclusions.1.A fast optimization technique was put forward,based on the results of orthogonal test and regression design.Aimed at improving material performance and lowering production cost,C₃H₆ and LPG were selected as carbon precursor and H₂ as carrier gas.Orthogonal test and regression design were performed respectively.Experimental results indicate that temperature is the utmost important factor,and then pressure.With regarding to the density and graphitization degree,we obtained their optimal parameters and further forecast formula via regression analysis.Experimental evidence demonstrates the validity and correction of both orthogonal test and regression forecast formula.Under the optimized conditions,C/C composites can be prepared with a density as high as 1.69 g·cm³ and a graphitization degree higher than 73%.The as-prepared composites also exhibit uniform density distribution.2.In our work,a novel morphology of pyrocarbon was observed,which has many cones,tubes and nodules on the surface.The pyrocarbon shows weak Malta cross under PLM and good graphitization degree up to 76.8% after a treatment at 2300℃.EDS and XRD measurement shows the absence of catalyst.SEM image demonstrates that the pyrocarbon structure is straight locally,whereas flexural at a long range.This novel pyrocarbon is names as DOT STRUCTURE in this study.Its formation mechanism is proposed as following in the multiphysics field,supersaturated deposition blocks as droplets float in the gas surrounding and surface diffusion occurs when the droplets are absorbed on the surface of deposition region.H₂ derived from C(H)desorption and that leads to the formation of hollow nodules in the inner parts of liquid drop while tubes or cones in the outer parts.Consequently,the rapid deposition of pyrocarbon on them causes the various morphologies.3.Pyrocarbon in different structure can be obtained while fabricating C/C composites via multiphysics-field CVI.Kinetics study reveals that C₃H₆ has apparent activation energy of 64.725 kJmol^-1in multiphysics field,lower than the reported values in ICVI(151 kJmol^-1)and FCVI(88.343 kJmol^-1).The integrated effects from temperature,concentration,chemical and electromagnetic fields are suggested to be the reasons why the activation energy is depressed.During the preparation process,H flows to the regions with higher temperature and combines with active sites,which elevates the supersaturation degree.H₂ also suppresses the formation of soot in the vapor. Static magnetic field near the surface increases the velocity of surface diffusion,which is beneficial for the directional orientation of surface deposits along the magnetic direction and as a result improving the microstructure property.While the charged deposition blocks in the vapor drift to the deposition surface,owing to the gradient electromagnetic field,which results in a higher deposition rate and efficiency.4.Based on the experimental evidence,the deposition mechanism of pyrocarbon is described as homogeneous vapor reactions gradually lead to the appearance of linear hydrocarbon,aromatics or PAHs,viscous droplets and soots.Pyrocarbon of various structures is presented after surface deposition. SL structure pyrocarbon is obtained predominantly in the product mostly derived from linear hydrocarbon,while DOT STRUCTURE with reproducibility dominates in the product resulted from surface absorption, diffusion and deposition of viscous droplets.5.The C/C composites fabricated by multiphysics field CVI or CVI plus resin impregnation have flexure strength 213.07MPa and 215.8 MPa respectively,and flexure modulus 25.5GPa and 28.8 GPa respectively.The parallel ILSS is 18.2 MPa and 22 MPa respectively,while the perpendicular compressive strength is 173MPa and 208.7MPa respectively.Malta cross under PLM indicates that thick pyrocarbon deposited at net layer and banded structure pyrocarbon in the bundle fibers account for the higher mechanical properties. 6.Friction performance of the prepared C/C composites was tested under simulating normal aircraft braking conditions.Excellent braking curve was observed without any tail.The steady coefficient is larger than 0.7 in all the tested samples,which implies that multi-physics field CVI is capable of fabricating C/C composites used as braking products.Simulation analysis about the thermal field under braking shows the drift of high-temperature zone from outer to inner,which is a result of rapid thermal diffusion while braking velocity slowing down.The discrepancy between simulation and experiment curves maybe originate from the oxidation contribution which isn't considered in the simulation.