Research On CNTs/rGO Toughened SiC Ceramic Matrix Composites For 3D Printing Technology

Reactive-bonded sintered silicon carbide ceramic(RB-SiC)has excellent mechanical and physical properties such as high hardness,high temperature resistance,corrosion resistance and oxidation resistance.As an ideal structural material,it is widely used in vehicle machinery,microelectronics,chemical industry,aerospace and biomedicine.However,its defects such as large brittleness,insufficient toughness and poor reliability seriously hinder the application of silicon carbide ceramic.In addition,the traditional forming method of silicon carbide ceramics is mainly injection molding that requires specific mold,the preparation ability for complex shape and structural ceramics is deficient,which also brings difficulties to the wide application of silicon carbide ceramics.Therefore,in order to improve the bending strength and fracture toughness of SiC ceramics,one-dimensional carbon nanotubes(CNTs)and two-dimensional reduced graphene oxide(rGO)were introduced as the second phase reinforcements to improve the mechanical properties of SiC ceramics,CNTs/rGO toughened SiC ceramic matrix composites were prepared.In order to improve the forming ability of silicon carbide ceramics,this paper introduces the slurry direct writing ceramic 3D printing technology as the forming method of composite materials.Based on the above two aspects,the research of CNTs/rGO toughened SiC ceramic matrix composites for 3D printing technology has been explored and determined in this project.The specific research contents are as follows:(1)The green preparation process of SiC ceramic matrix composites was studied.Due to the direct-writing molding method of the slurry,in order to meet the printing requirements,the rheological properties of the ceramic slurry were first studied.Through the discussion of the type and concentration of the binder,the content of the dispersant,the particle size ratio of the silicon carbide particles and other factors,the 2.5wt%concentration of sodium alginate solution was determined as the binder,the 1wt%powder mass of polyethylene glycol 400 was determined as the dispersant,silicon carbide particles with a particle size ratio of 7:3 are used as raw materials to produce ceramic slurry with shear thinning characteristics that meets the requirements of direct writing molding.The ceramic slurry that meets the printing requirements is input into the printing equipment.In order to ensure the printing accuracy,the 3D printing process of ceramics is studied.Through the discussion of factors such as extrusion ratio,printing resolution,printing speed and filling rate,the 3D printing scheme with 100%extrusion ratio,0.6mm nozzle diameter,0.24mm layer thickness,30mm/s printing speed and 80%filling rate as the printing parameters is finally determined.Through the above two studies,ceramic green bodies with average error of X,Y and Z direction less than 0.15 mm were successfully prepared.(2)The sintering process and matrix properties of SiC ceramic matrix composites were studied.Due to the use of reactive sintering,the degumming and sintering parameters of the green body were studied.The final appropriate reactive sintering scheme was determined by discussing the degumming and sintering temperature(800℃ and 1600℃),heating rate,particle size(4mm)and content of silicon powder(0.95 times)and other parameters.The physical and mechanical properties of pure silicon carbide ceramics prepared by the above reactive sintering scheme were studied,the influence of particle size ratio of different size silicon carbide particles on the properties of the test piece was discussed.When the particle size ratio of large and small silicon carbide particles was 7:3,the compactness was the best,the density was 3.028g/cm3,the relative density was 97.45%.The mechanical properties are excellent which the hardness reaches 2561.4HV,the bending strength reaches 232.7MPa,the fracture toughness reaches 3.37MPa·m1/2.The microstructure is analyzed,which lays a foundation for the later study of the toughening effect of the reinforcement.(3)The effect of CNTs and rGO toughened SiC ceramic matrix composites was studied.Because CNTs and rGO are agglomerated,they are easily eroded by high temperature melting and molten silicon during reaction sintering,the dispersion and coating process of reinforcements are discussed respectively.The dispersed and coated reinforcement was introduced into the composite,the physical and mechanical properties of the composite were discussed and the microstructure of the composite was analyzed to explain the macroscopic performance.The results showed that the mechanical properties of the composite were the best when the amount of carbon nanotube reinforcement was 3wt%,in which the bending strength reached 3 1 0.5MPa,which was 33.4%higher than the pure silicon carbide matrix.The fracture toughness reaches 4.37MPa·m1/2,which was 30%higher than the pure silicon carbide matrix.When the amount of reduced graphene oxide added to the reinforcement is 5wt%,the mechanical properties of the composite are the best.The bending strength reaches 302.7MPa,which is 30%higher than the pure silicon carbide matrix,the fracture toughness reaches 4.11MPa·m1/2,which is 21.9%higher than the pure silicon carbide matrix.The common toughened mechanism of the composite was analyzed by microstructure.The fracture,pull-out,bridging and guided crack deflection of the reinforcement in the matrix made the overall mechanical properties of the composite significantly improved.