Study On The Thermal Radiation Mechanisms And Properties Of C/SiC Composites

Carbon fiber reinforced silicon carbide composites (C/SiC) exhibit excellentproperties in many aspects, such as low density, high-temperature resistance, oxygenresistance and non-catastrophic failure. So it has be widely used in the high temperaturefield, such as aeronautics and astronautics fields. The high temperature environmentsmake the thermal radiation become the main way for the heat transfer between C/SiCcomposites and outer space. There are few studies about the thermal radiation propertyof C/SiC composites, and the relationship between thermal radiation and microstructureof C/SiC is not clear. Therefore, investigating the transfer mechanism of the thermalradiation in C/SiC composites with multielement and multilayer, establishing theinteraction mechanism between environmental control factors, microstructure andthermal radiation, and getting the regulation mechanism of thermal radiation propertyof C/SiC composites are very important guiding meaning for application andperformance evaluation of C/SiC composites.The thermal radiation property of C/SiC composites fabricated by chemical vaporinfiltration method were studied in the thesis. Combined with the basic theory ofthermal radiation, the effects of structure evolution of constituent and C/SiC compositeson the thermal radiation property were tested and investigated. The interactionmechanism between environmental factors, microstructure of composites and thermalradiation was got through analyzing the relationship of the three factors. The effects ofmatrix modification by introducing the modified materials on the thermal radiationproperty of C/SiC composites were also investigated. The main contents and results areas follows:1) The relationship between microstructure and thermal radiation property of Cphase was investigated. The results show that:(1) The appearance of crystallinegraphites on samples surface increased thermal radiation property of graphite materials.(2) The thermal radiation property of carbon fiber increased with increasingtemperature in1000oC~1600oC, and the fluctuation of the spectral emissivity was alsoenhanced.(3) The heat treatment at1600oC and1800oC enhanced the fluctuation ofspectral emissivity of carbon fibers with wavelengths. Meanwhile, the total emissivityfalled by2.71%(1600oC-HT) and9.94%(1600oC-HT).(4) PyC coating weanked thefluctuation of spectral emissivity of carbon fibers with wavelengths and decreased totalemissivity by30%. Increasing PyC coating thickness from0.5μm to1.0μm had limited effect on thermal radiation property of carbon fibers.(5) CNTs coating increasedthermal radiation property of carbon fibers by35%during the testing temperature range.2) The relationship between microstructure and thermal radiation property of Cphase was investigated. The results showed that:(1) The theoretical infrared emissivityof SiC was calculated by the Lorentz model with single oscillator, and there was aReststrahlen band during10~14μm.(2) The spectral emissivity of CVD SiC presentedobvious Reststrahlen band of SiC than that of HP SiC. The total emissivity of CVD SiCwas better than that of HP SiC below1300oC.(3) When the SiC coating thickness waslimited, the thermal radiation property of SiC coating decreased with the increasingsurface roughness of substrate.(4) The shape of SiC power and SiC whisker greatlychanged the spectral emissivity of ZrB2-SiC ceramics. The thermal radiation propertyof the sample made by SiC whisker presented obvious Reststrahlen band and was betterthan that of the ones made by SiC power.3) Effects of preform architectures, coating condition and densification on thermalradiation property of C/SiC composites were investigated. The results showed that:(1)The thermal radiation property of3D C/SiC was better than that of2D C/SiC due to theexcellent heat transfer ability in3D preform.(2) SiC coating greatly changed thespectral emissivity of C/SiC composites, but had little effect on the total emissivity ofsamples.(3) As the SiC coating thickness increasing from20μm to100μm, the thermalradiation property of C/SiC composites increased excepting the decrease at40μm.Meanwhile, the spectral emissivity of samples presented obvious Reststrahlen bandwith increasing coating thickness.(4) The thermal radiation property of C/SiCcomposites reduced with the decrease in the surface roughness of samples. Increasingtemperature enhanced the effect of surface morphology on thermal radiation of samples(5) As the densification increasing, the thermal radiation property of C/SiC compositesincreased, and the greatest growing rate appeared when SiC content was over50%.4) Effects of microstructure damage introduced by heat treatment, air oxidation andwater-oxygen corrosion on the thermal radiation property of C/SiC composites wasinvestigated. The results showed that:(1) The heat treatment conducted at1600oC and1800oC had a little influence on the thermal radiation property of C/SiC composites.When the heat treatment temperature was at2000oC, the growth of β-SiC crystal grainand the increasing α-SiC content increased the thermal radiation property of C/SiCcomposites.(2) Oxidizing at700oC had little effect on the spectral emissivity of C/SiCcomposites, but increased the total emissivity of sample. Oxidizing at1000oC caused little changes in the thermal radiation property of samples. Oxidizing at1300oCincreased the thermal radiation property of samples and the effect was enhanced byprolonging oxidation time.(3) Corrosion at700oC and1000oC had little effect on thespectral emissivity of C/SiC composites, but decreased thermal radiation of samples.The decrease enlarged with oxidation time prolonging and test temperature increasing.Corrosion at700oC greatly changed the spectral emissivity of C/SiC composites andthe reduction of total emissivity was over the other corroded samples.5) The influence of modification substance and SiC coating on the thermal radiationproperty of C/SiC composites was studied. The results showed that:(1) Theintroduction of Si phase and Si-B-C-N ceramics into C/SiC matrix could improve thethermal radiation property of C/SiC composites. The growing rates were50.00%and6.00%at1000oC,60.00%and16.67%at1600oC. C phase greatly increased thermalradiation of samples.(2) The introduction of C phase and Si-C-N ceramics into C/SiCmatrix could decrease the thermal radiation property of C/SiC composites below1400oC. The reduction rates were16.00%and12.00%at1000oC. C phase greatly decreasedthermal radiation of samples.(3) After depositing SiC coating, the spectral emissivityof C/SiC composites with modification substance presented the obvious Reststrahlenband during10~14μm. The modification substance could still affect the thermalradiation property of samples because of the controlled position of the substrate.