| Carbon fiber reinforced carbon matrix?C/C?and carbon fiber reinforced carbon-silicon carbide matrix?C/C–SiC?compoistes have been attracted considerable attention in aeronautical and astronautical fields,automobile and new power plants due to their advantages including low density,high specific strength,high temperature resistance,good wear resistance and low thermal expansion coefficients.As strategic materials concerning national security,the demands in various industrial applications and service environment for the properties of C/C and C/C–SiC composites are somewhat different.Therefore,it is necessary to tailor the microstructure of the composites to satisfy the specific requirements.As well known,constructing multilayered structure is an effective method to improve the properties of the composites.In this thesis,multilayer-textured C/C composites and carbon fiber reinforced multilayered pyrocarbon-silicon carbide??PyC–SiC?n?matrix?C/?PyC–SiC?n?composites were fabricated by isothermal chemical vapor infiltration?ICVI?.Effects of introduction and thickness of high-textured?HT?PyC layer on mechanical properties of C/C composites were analyzed.Then the effects of PyC–SiC sequences number?n=1,2 and 4?on matrix microstructure,mechanical properties,internal friction,thermophysical properties and electromagnetic interference?EMI?shielding performance of C/?PyC–SiC?n composites were also studied.The relationships between the mechanical and physical properties and matrix microstructure were explained in detail.The main contents and conclusions are listed as following:Four kinds of C/C composites with various textures and different thickness of HT PyC layer were prepared from the preforms by ICVI.The results show that with the increase of HT layer thickness from 0 to 9.4?m,the flexural strength firstly increases from 97.5 to 116.5MPa and then decreases slightly to 107.3 MPa;the ductility factor increases from 0%to19.8%.It is concluded that the introduction of multilayered texture could improve the mechanical properties of C/C composites,which is attributed to multiple crack deflection and interfacial sliding between different textured PyC layers and between sub-layers within HT layer.Moreover,the increase in thickness of HT layer improves the plasticity of the composites in association with a relatively high flexural strength.Therefore,the mechanical properties and fracture behaviors of C/C composites can be tailored by controlling texture types and thickness of high-textured pyrocarbon layer reasonably and appropriately.Three kinds of C/?PyC–SiC?n composites were prepared by deposition of 1 PyC–SiC sequence??PyC–SiC?1?,2 PyC–SiC sequences??PyC–SiC?2?and 4 PyC–SiC sequences??PyC–SiC?4?inside preforms via ICVI.The results show that the three composites have a two-layer,four-layer and eight-layer structure around the carbon fibers,in which PyC layers and?-SiC layers alternately deposite and parallel to each other.In some regions,partial delamination between PyC layers and SiC layers occur.It is found that SiC layers with large-faceted crystals deposit on the oriented PyC layers resulting in flat interfaces of PyC/SiC,while the opposite deposition sequence leads to uneven interfaces of SiC/PyC.The mechanical properties of C/?PyC–SiC?n composites with different sequences number were characterized by three-point bending tests.The results show that all C/?PyC–SiC?n composites exhibit pseudo-plastic fracture behavior in a step-like mode instead of catastrophic failure.With increasing the number of sequences?n?,flexural strength,fracture toughness and specific strength of the composites increase from 121±17 to 193±18 MPa,from 3.0±0.1 to 4.2±0.3 MPa·m1/2 and from 90.3 to 141.9 kN·m/kg,respectively.The enhanced mechanical properties of C/?PyC–SiC?n composites are attributed to the increasing number of interfaces,supplying more channels for crack deflection and propagation,which is favorable to more fracture energy dissipation.Effects of PyC–SiC sequences?n?on the internal friction of C/?PyC–SiC?n composites were studied.The results present that with increasing the number of sequences?n?,the internal friction increases due to the enhancement of interfacial internal friction.The internal friction of C/?PyC–SiC?n composites increases with the increase of frequency related to thermoelastic mechanism.Effect of temperature on internal friction behaviours is attributed to combined effects of carbon fibres,PyC–SiC matrices and interfaces between fibers and PyC,adjoining PyC and SiC layers.It is also found that internal friction is sensitive to microstructural defects induced by damage.These results indicate that internal friction is an effective and efficient method to characterize the structural evolution and internal damage of C/?PyC–SiC?n composites non-destructively.The thermal expansion behaviors in the temperature range of 8002300°C and thermal conductivity from room temperature to 1900°C of C/?PyC–SiC?n composites with various microstructures were investigated.The results exhibit that the coefficients of thermal expansion?CTEs?and thermal conductivity of the composites are determined by combined action of carbon fibers,PyC–SiC matrix and the interfaces.In the temperature range of8002300°C,CTEs of the three composites are?5.46.5?×10-6/K,?4.75.8?×10-6/K and?3.65.2?×10-6/K.With increasing PyC–SiC sequences number?n?,the decrease of CTEs is due to the increase of interfacial delamination,which could provide room for thermal expansion.Between room temperature and 1900°C,the thermal conductivity of C/?PyC–SiC?n composites decreases from?8.723.9?to?5.512.5?W/?m·K?with the increase of sequences number,which are attributed to the enhancement of phonon-interface scattering resulted from the increasing number of interfaces.Modified parallel and series models considering the interfacial thermal resistance are proposed to elaborate thermal conductivity of the composites,which is in accordance with the experimental results.The electrical conductivity and EMI shielding performance of three kinds of C/?PyC–SiC?n composites were investigated.The results show that the electrical conductivity of C/?PyC–SiC?n composites increases from 8.7 S/cm to 18.4 S/cm with increasing the number of sequences?n?.Meanwhile,owing to the increasing electrical conductivity and polarization of the multilayered matrix,the total shielding effectiveness?SE?and specific EMI SE of the composites increases from 34.4 to 41.7 dB and from 25.7 to 30.7 dB·cm3/g in X-band,respectively.C/?PyC–SiC?n composites exhibit relatively high specific strength and specific EMI SE,which indicates the great potential for various applications as lightweight and high-performance structure and shielding materials. |
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