| Two kinds of different surface coatings, PyC-SiC/SiO2 coating and Al2O3-SiO2-TiO2 coating, on three-dimensional braided carbon fiber were coated by precursor impregnation and pyrolysis(PIP) and by sol-gel technology. After that the 3D-Cf/SiC composites with the two different interphases were all fabricated by the PIP process. The effect of deposition conditions, including immension-pyrolysis cycle, immension time and heat treatment temperature, on microstructures and phase composition of the coating, and the tensile strength and the oxidation resistant properties of the carbon fiber were investigated.And the influence of interphases under different conditions on microstructure and mechanical properties of 3D-Cf/SiC composites were also investigated by means of XRD,SEM, as well as three point bending test. The main results are as follows:(1) In this work, a double-layer oxidation resistant coating, PyC as the bottom layer and SiC/SiO2 as the top layer, on 3D braided carbon fiber has been prepared successfully by a PIP method. The influence of phenol-formaldehyde resin concentration and heat treatment temperature on the PyC coating was considered. The best quality of PyC coating obtained was use of the precursor consisting of 5 wt% phenol-formaldehyde resin in acetone at 750°C. And the effects of MTES concentration, immension-pyrolysis cycle and heat treatment temperature on the SiC/SiO2 coating were studied. Results showed that the best quality of SiC/SiO2 coating obtained was use of the precursor consisting of 10 wt%PCS with 5 wt% MTES in DVB after 2 immension–pyrolysis cycles at 1400°C. The oxidation resistance of carbon fibers was improved obviously by the PyC-SiC/SiO2 coatings. Tensile strength measurement revealed that the PyC-SiC/SiO2 coated carbon fiber maintained 87.7 % of its original strength.(2) In this work, the Al2O3-Si O2-TiO2 coatings on the surface of 3D braided carbon fiber have been prepared successfully by a simple sol-gel process. The factors of immension-pyrolysis cycle, immension time and heat treatment temperature on the microstructures and phase composition of the coating were investigated. The resultsshowed that the good morphology of the coating obtained was use of heat treatment at750°C for 2 immension–pyrolysis cycles by the immersion time of 40 min per cycle. The oxidation resistance of carbon fibers was improved by the Al2O3-SiO2-TiO2 coatings.Tensile strength measurement revealed that the Al2O3-SiO2-TiO2 coated carbon fiber maintained 96.8 % of its original strength when the thickness of the coating was 470 nm.(3) The 3D-Cf/SiC composites with the two different interphases have been successfully fabricated by the PIP progress. The factors of the different interphase or the same interphase prepared under different conditions were all influenced the mechanical properties of the 3D-Cf/SiC composites. The carbon fiber covered with relatively better coating used in 3D-Cf/SiC composites also made the composites have excellent mechanical properties. And the tensile strength of the 3D-Cf/SiC composites with PyC-SiC/SiO2 interphase was 247 MPa compared with that of with Al2O3-SiO2-TiO2 interphase of 303 MPa achieved.(4) Compared with the brittle fracture of the 3D-Cf/Si C composites without interphase,the composites with both of the different interphases have the characteristics of standard toughened fracture behavior, and the coatings coated on the surface of carbon fiber play a better role in the fracture process of composite materials.(5) By comparing the two kinds of different interphaces on the preparation and propertites, the results showed that, compared to the PyC-SiC/SiO2 coating prepared by the precursor impregnation and pyrolysis method with lower preparation period and higner oxidation resistance, the Al2O3-SiO2-TiO2 coating prepared by the sol-gel method have lower cost, higher rentension rate for the carbon fiber in the aspect of tensile strength and higher mechanical properties for the composite materials. |
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