Study On Surface Modification,Strength And Toughness Of Reaction-bonded Silicon Carbide Ceramics

Silicon carbide（SiC）ceramics are widely used in aerospace,new energy,chemical industry,transportation and other fields because of their excellent characteristics such as high hardness,high high-temperature strength,low coefficient of thermal expansion,excellent thermal stability,good thermal shock resistance and chemical corrosion resistance.Among them,reaction-bonded SiC ceramics are especially suitable for preparing structural parts with large size,complex shape or high dimensional accuracy because of their characteristics of near net size and relatively low sintering temperature.However,reaction-bonded SiC ceramics are prone to problems such as insufficient or excessive silicon infiltration in the sintering process,resulting in easy deformation and cracking of sintered products.Meanwhile,the development of new technology also puts forward new requirements for traditional products.For example,the cathode material of lithium-ion battery needs to be synthesized at high temperature.However,the widely used structural parts such as SiC ceramic roller are easy to be eroded by lithium ion at high temperature,resulting in peeling and slag dropping,which not only reduces the service life of ceramic roller,but also affects the purity of cathode material.In this paper,the reasons for the deformation of reaction-bonded SiC ceramics in the preparation process and the reaction mechanism with lithium containing materials at high temperature were studied.SiC matrix composites with excellent mechanical properties,good lithium-ion corrosion resistance and small deformation were prepared by modifying reaction-bonded SiC ceramics at high temperature and introducing the second phase into the matrix as reinforcing material.Reaction-bonded SiC ceramics were modified by high-temperature treatment under a nitrogen atmosphere.The phase composition,mechanical properties,lithium-ion corrosion resistance and surface modification mechanism of reaction-bonded SiC ceramics at different nitriding temperatures were studied.The results showed that during the nitriding treatment,the free Si in the material reacted with N₂ and trace O2,and formed a dense protective layer on the surface of ceramic at above 1150℃.The main reactants were Si₃N₄,Si₂N₂O and SiO₂.According to the results of SEM,XRD and XPS,it was found that two morphologies of Si₃N₄ were formed on the ceramic surface.One was short columnar α-Si₃N₄ formed by the direct reaction of free Si and N₂,The other was fibrous β-Si₃N₄ formed by the reaction of SiO（g）with N₂.The mechanical properties and lithium-ion corrosion resistance of the modified SiC ceramics were measured.The results showed that the properties of the ceramics were the best when the nitriding treatment was carried out at 1350℃ for 2 h.At this time,the bending strength of the ceramic reached the maximum value of 425.39 MPa,which was 1.6 times that of the original sample,and the fracture toughness reached the maximum value of 4.47 Mpa·m^1/2,which was 1.2 times that of the original sample.Meanwhile,the lithium-ion corrosion resistance was significantly improved.Compared with the original SiC ceramics,there was almost no new phase on the surface of the modified samples after corrosion reaction.By introducing chopped carbon fiber（Cf）into SiC ceramic matrix,reaction-bonded Cf/SiC composites were prepared by extrusion molding.The phase composition,mechanical properties,lithium-ion corrosion resistance,and deformation of Cf/SiC composites with different addition of chopped Cf were studied.The results showed that when the content of chopped Cf was 3 wt%,the content of free Si in the composite was.the least.At this time,the bulk density of the ceramics reached the maximum value of 3.074 g/cm³,and the bending strength and hardness also reached the peak,which were 285.3 MPa and 29.5 GPA,respectively.When the content of chopped Cf was 1 wt%5 the fracture toughness of multiphase ceramics reached the maximum value of 4.8 MPa·m^1/2.By analyzing the corrosion resistance of Cf/SiC composites,the results showed that the corrosion resistance of the composites was the best when the content of chopped Cf was 3 wt%.Meanwhile,the effect of adding 3 wt%chopped Cf on the deformation of SiC ceramics was studied.The results showed that the deformation of the composites decreased significantly after adding chopped Cf,and the shrinkage in X-axis direction decreased from 0.3%to 0.23%,and that in Y-axis direction decreased from 0.42%to 0.26%.Reaction-bonded β-Si₃N₄/SiC composites were prepared by extrusion molding.The phase composition,mechanical properties,lithium-ion corrosion resistance,and deformation of βSi₃N₄/SiC composites with different content of β-Si₃N₄ were studied.The microstructure and phase composition of composites were analyzed by XRD and SEM.The results showed that when the content of β-Si₃N₄ was 3 wt%,the bulk density of multiphase ceramics can reach 3.101 g/cm³.At this time,the bending strength and hardness of the material were 300.7 MPa and 29.7 GPA,respectively.When the content of β-Si₃N₄ is 5 wt%,the fracture toughness of composites reached the maximum value of 4.7 MPa·m^1/2.By analyzing the corrosion resistance of β-Si₃N₄/SiC composites,the results showed that the corrosion resistance of the composites was the best when the content of β-Si₃N₄ was 3 wt%.At the same time,the effect of adding 3 wt%β-Si₃N₄ on the deformation of SiC ceramics was compared.The results showed that after adding 3 wt%β-Si₃N₄,the shrinkage of the composite in the X-axis direction decreased from 0.3%to 0.28%,and the shrinkage in the Y-axis direction decreased from 0.42%to 0.32%.In conclusion,the residual Si in reaction-bonded SiC ceramics is the main reason for the deformation,non-resistance to lithium-ion corrosion and poor mechanical properties of the products during sintering.The modification method of high-temperature treatment in the N₂ atmosphere and the addition of chopped Cf or β-Si₃N₄ can effectively improve the corrosion resistance and mechanical properties of reaction-bonded SiC products.The chopped Cf or βSi₃N₄ as reinforcing phase can also effectively reduce the deformation of reaction-bonded SiC.The reduction of residual Si and the introduction of various strengthening and toughening mechanisms are the main reasons for the improvement of SiC products.