Fabrication And Properties Of SiC Nanofibers Reinforced SiC Ceramic By DIW And RMI

As an important thermal structural material,silicon carbide（SiC）ceramics have a wide application prospect in aerospace,energy,transportation and other fields.However,the traditional molding methods can not realize the structure of complex ceramic parts.In recent years,the technology of 3D printing SiC ceramics with special-shaped structure has been developed rapidly.Among them,direct writing molding technology has the advantages of simple and fast,is the mainstream technology of SiC ceramic 3D printing.How to improve the strength and toughness of3D printed SiC ceramics is of great significance for the wide application of special-shaped SiC ceramics,which has attracted the attention of researchers.In order to improve the strength and toughness of 3D printing SiC ceramics,this thesis through the study of high solid content of ceramic slurry components,the introduction of SiC nanofibers(SiC_nf),combined with direct writing molding 3D printing technology and reactive melting process to prepare SiC ceramics.By means of rheometer,SEM,XRD,universal testing machine and other equipment,the preparation and rheological properties of direct ink writing SiC slurry,the microstructure and mechanical properties of direct ink writing green bodies and reactive melt infiltration SiC ceramics were investigated.The main research results are as follows:（1）Firstly,the water based SiC ceramic slurry suitable for direct ink writing with high solid content were prepared.The effects of three dispersants（polyethylene glycol（PEG）,tetramethylammonium hydroxide（TMAH）and poly ethylenimine（PEI））on the rheological properties of ceramic slurry with solid content of 35 vol%were discussed.When the storage modulus of SiC ceramic slurry with PEG as the dispersant was the largest which is 2.9×10⁴ Pa,the viscoelasticity of the slurry was the best.The ceramic slurry with PEG as dispersing agent has more suitable rheological properties for direct ink writing.（2）By adjusting the density of particles in the green body with different grain composition,the mechanical properties and microstructure of the ceramics after reactive melt infiltration are greatly affected.As the proportion of fine SiC particles in the slurry increases gradually,the particles in the slurry are packed more closely,and the bulk density of the green body is improved.When the ratio of fine SiC particles to coarse SiC particles（SiC0.8:SiC10）is 1:9,the volume density of ceramic materials is the highest and the apparent porosity is the lowest.With the increase of the proportion of fine SiC particles,the fracture toughness of ceramic materials shows an increasing trend.When the ratio of SiC0.8:SiC10 is 5:5,The fracture toughness of ceramic material reaches the maximum value of 3.63 MPa·m^1/2.（3）The effects of different carbon black additions on the properties and microstructure of ceramic slurry and sintered ceramics were studied.With the increase of carbon black content,the viscosity of the slurry was greatly improved,and all the slurry show shear thinning phenomenon,and too much carbon black will cause the shear stress corresponding to the flow point of the slurry is too high,which leads to the slurry is difficult to flow,and prevents the normal extrusion of the slurry.With the increase of carbon black content,the bulk density,flexural strength and fracture toughness of SiC ceramics firstly increase and then decrease.When the carbon black content is 2 vol%,the flexural strength of SiC ceramic samples reaches the maximum value of 236.3 MPa.When the carbon black content is 3 vol%,the flexural strength of SiC ceramic samples reaches the maximum value of 236.3 MPa.The fracture toughness of SiC ceramic material reaches the maximum value of 3.82 MPa·m^1/2.（4）The SiC_nf/SiC composites were prepared by direct ink writing and reactive melt infiltration using SiC_nf as reinforcement phase.With the increase of SiC_nf content,the bulk density of the green body decreases,the apparent porosity increases,and the agglomeration of small particles in the green body decreases.With the increase of SiC_nf content,the bulk density of SiC ceramic materials first increases slowly,and then decreases slightly.When the content of SiC_nf is 4 vol%,the flexural strength and fracture toughness of ceramic materials reach the maximum value of 279.8 MPa and4.26 MPa·m^1/2,respectively.Compared with the sample without SiC_nf,their increases were 18.5%and 22.4%respectively.（5）The preparation of resin based ceramic slurry using phenolic resin as carbon source was studied.SiC ceramics were obtained through direct ink writing,curing,pyrolysis and reactive melt infiltration.With the increase of the concentration of phenolic resin,the bulk density of green body also increases,the shrinkage rate of curing and cracking decreases,and the flexural strength of sintered body increases first and then slowly decreases.With the increase of SiC_nf content,the volume density of ceramic preform decreases gradually,the flexural strength of sintered body increases first and then decreases,and the fracture toughness increases first and then stabilizes.When the content of SiC_nf is 4 vol%,the flexural strength and fracture toughness of SiC_nf/SiC composites reach the maximum.Which is 234 MPa and 3.64 MPa·m^1/2,respectively.To sum up,based on the principle of 3D printing technology of direct ink writing,this paper prepared a silicon carbide slurry with good dispersion and suitable for extrusion molding by selecting the optimal type and proportion of raw materials,and at the same time in the extrusion process,the slurry is easier to form.The SiC ceramic products with special-shaped structure were obtained by the process of reactive melt infiltration.Finally,by printing some SiC ceramics with typical structure,the writability,stability and net forming ability of the whole process are verified.Finally,the feasibility of the process to print SiC ceramics with complex structure was proved.