Study On Fabrication And Properties Of SiC Hollow Spheres

Low density and high intensity are the eternal theme pursued by people,especially for space applications of new materials.The porous SiC ceramic prepared by SiC hollow spheres has become a new type of lightweight space light machine structure materials,due to its excellent performance in dimensional stability,light weight,high strength,high temperature resistance,wear resistance,corrosion resistance,oxidation resistance,high thermal conductivity and low expansion coefficient.However,the preparation of SiC hollow spherical material is difficult,and it is hard to sinter and compact the SiC itself.So far,there are few reports about the preparation of SiC hollow spheres in our country.Not only the molding process is complicated,but also the prepared SiC hollow spheres can not meet the practical application requirements,which have limited its application.In this paper,SiC hollow spheres were prepared by molten salt method and template method.And then,PIP,CVI and PIP-CVI method were adopted to obtain coposites with high strength and good mechanical properties.The enhanced SiC hollow spheres were added into the SiC ceramics by GC-GSI,and the porous SiC ceramics with excellent properties were successfully obtained.The principle and technological conditions of SiC hollow spheres prepared by molten salt method and template method were studied.A new method for preparing SiC hollow spheres was obtained.The principle of the molten salt method is that the molten salt provides a liquid environment for the reaction,and the reaction of silicon and carbon in the molten state allows the silicon-carbon reaction to proceed in a shorter time and at a lower temperature.The effect of carbon-silicon ratio on the composition of SiC hollow spheres was studied.The technological conditions of carbon hollow ratio of 3:1and reaction temperature of 1400?were determined.The SiC hollow spheres with an average diameter of 1.69mm and a thickness of 40?m ball.The template method uses flour and carbon black to prepare a carbon template ball.A SiC hollow sphere with a diameter of 4.56 mm and a thickness of 0.61 mm is prepared under a mass ratio of SiC powder to carbon template ball of 4:3.The template method has the advantage of good sphericity and high ball-obtainable rate.Considering low strength and poor mechanical properties of SiC hollow spheres prepared by the template method,PIP,CVI and PIP-CVI were used to enhance SiC hollow spheres and the specific process parameters were determined,respectively.After one cycle PIP,SiC hollow ball density reached 1.409g/cm³,and spherical shell porosity was 0.67%.The deposition temperature of CVI has an important effect on the microstructure of SiC hollow spheres.When the deposition temperature was 950?,a large number of small SiC particles were formed on the surface of the hollow spheres.When the deposition temperature is 1050?,the surface of the hollow SiC spheres was covered by spherical SiC particles.The CVI enhanced SiC hollow spheres under the conditions of deposition temperature of 1050?and deposition time of 30h was carried out.The density of SiC hollow spheres increased to 1.401g/cm³,and the internal porosity of spheres was 0.9%.PIP-CVI combined process effectively combines the advantages of PIP process and CVI process to prepare SiC hollow spheres with excellent mechanical properties.At the same time,the surface of SiC hollow spheres was dense and delamination can be obviously seen at the cross section.The outer layer was CVI Continuous deposition of SiC layer,and the inner layer of was PIP SiC hollow ball base.The two layers were connected closely together and improved the performance of SiC hollow spheres.Comparative analysis of the composition of the three SiC hollow spheres under high temperature conditions,weight loss and mechanical properties.PIP-CVI process of SiC hollow spheres was chosen as the enhancement process.The main components of SiC hollow spheres prepared under the three process conditions were?-SiC and?-SiC.And residual C was also formed in CVI SiC hollow spheres due to the high temperature deposition,which promoted the formation of intermediate product of MTS.Of the C-containing intermediate is cleaved to produce free C.The SiC hollow spheres prepared under the three process conditions have the following performance rules:the maximum compressive stress and the fracture energy increased with the increase of the diameter of SiC hollow spheres.While the compressive modulus decreases,and the compressive strength remains unchanged.The maximum compressive stress reached161.67N,the fracture energy was 6.62?10^-3J,the compressive strength was 20.79MPa,and the compressive modulus was 5.86GPa.The technological conditions of porous SiC ceramics with SiC hollow spheres added by gel-casting and gas-phase osmosis were investigated.Porous SiC ceramics with good properties were prepared.Carbon black was more difficult to disperse in the ceramic slurry,carbon black dispersant PVP had a good dispersion effect.In the dry process,the weight loss rate was 10.2%and the volumetric shrinkage rate was 2.2%.Molding was good and stable.The porous ceramic after silicon carbide infiltration is dense,the interface between the hollow sphere and the ceramic substrate is clear,the density is 2.19g/cm³ and the porosity is 2.2%.Its mechanical properties were characterized.The compressive strength of porous SiC ceramics was 147.4MPa and the compressive modulus was 15.67GPa.Afterwards,the porous SiC ceramics could be applied to the preparation of lightweight space SiC mirror through the sandwich structure design.