Preparation And Mechanical Properties Of Carbon Coated Silicon Carbide Nanowire Reinforced Silicon Carbide Ceramic Composites

Silicon Carbide(SiC)ceramics are covalently bonded ceramics with a lattice similar to diamond's orthotetrahedral structure,which gives it high strength,high thermal shock resistance,high abrasion resistance,low thermal expansion coefficient,etc.At the same time,SiC powder has a thin film of silicon dioxide(Si O₂)on the surface,which can prevent oxygen molecules from entering the ceramic,and therefore has strong antioxidant properties.Although its pyrometric properties are stronger than conventional high-temperature alloys,its density is only about half that of the alloy(3.21g/cm3),making it an ideal replacement material for high-temperature alloys for aerospace and nuclear applications.However,silicon carbide ceramics are poorly ductile and are extremely susceptible to brittle fracture under stress,leading to material failure,which greatly limits their range of applications.The introduction of a reinforced phase into the ceramic matrix is an effective way to improve the fracture toughness of the ceramic.As a one-dimensional material,silicon carbide nanowires(SiCNWs)possess not only superior properties similar to those of silicon carbide bulks,but also many properties unique to low-dimensional materials,making it an ideal reinforcer.However,since the silicon carbide nanowires belong to?-SiC and the silicon carbide matrix belongs to?-SiC,which are similar in nature,it is easy to make the interfacial bond between them too strong when sintering.This will not only cause damage to the silicon carbide nanowires,but also make the silicon carbide nanowires unable to absorb the fracture energy due to their inability to pull out and bridge when cracks occur,so how to control the strength of the interfacial bond between the two is an issue that needs to be solved.In this paper,silicon carbide nanowires were chosen to be used as reinforcement phase and introduced into the silicon carbide ceramic matrix to prepare a silicon carbide ceramic composite.The surface modified process of silicon carbide nanowires and the preparation process,mechanical properties and microstructure of silicon carbide ceramic composites were mainly studied.The results show that by coating the silicon carbide nanowires with a carbon coating,the interfacial binding between the nanowires and the substrate can be effectively avoided and the reinforcement and toughening effect of the silicon carbide nanowires on the silicon carbide substrate can be significantly improved.The main contents are as follows.(1)Using APTES to surface-modify the silicon carbide nanowires,and then the modified silicon carbide nanowires were mixed with a certain amount of resorcinol and formaldehyde(2:1molar ratio)in a solution of anhydrous ethanol and deionized water as solvent and stirred for a period of time,and a coating of phenolic resin was generated on the surface of the silicon carbide nanowires by in situ polymerization to obtain SiCNWs@Phenolic resin with a core-shell structure,which was then carbonized by high temperature under inert gas protection to obtain SiCNWs@C with a carbon coating on the surface.The thickness of the carbon coating can be controlled by adjusting the amount of resorcinol added to the solution.And thermogravimetric analysis can be used to characterize the carbon content of SiCNWs@C.(2)Silicon carbide ceramic bulks with different carbon coating thicknesses on the furface of SiC namowires was prepared by immobilizing the content of silicon carbide nanowires(5 wt.%)in the composite ceramic powder and changing the thickness of carbon coating on the surface of the nanowires.After testing and co MParison,it was found that when the silicon carbide nanowire surface was coated with a carbon coating of 120 nm,the composite ceramics had the best integrated mechanical properties,the MSP strength improved by 17%and the fracture toughness improved by18%co MPared to the composite ceramics without the carbon coating on the nanowire surface.The composite ceramics are all prepared by SPS process and the densities are all above 98%.(3)SiCNWs@C were prepared by selecting the optimal thickness of carbon coating explored in the previous step,and a series of composite ceramic powders were prepared by varying the net content of silicon carbide nanowires in the composite ceramic powder.The co MParison showed that when the net content of silicon carbide nanowires in the composite ceramics was 7 wt.%,the MSP strength and fracture toughness increased by 76%and 27.4%,respectively,co MPared to the substrate without added silicon carbide nanowires.