Gelcasting And Mechanical Properties Of Pressureless Sintered Silicon Carbide Ceramics Based On Isobam

Pressureless sintered silicon carbide ceramics?SiC?have excellent physical and chemical properties such as high hardness,high strength at high temperature,high thermal conductivity,good chemical stability,and have been widely used as wear resistant,high temperature resistant,corrosion resistant,heat transfer or resistant.Typical molding methods for pressureless sintered silicon carbide ceramics in practical applications include dry compression molding,isostatic pressing molding,casting,extrusion molding.However,dry pressure and isostatic pressing are not conducive to the preparation of complex shape samples.Casting has shortcomings such as long molding cycle,large drying shrinkage and uneven distribution of green body density,which reduce the reliability of the material.Extrusion molding is mostly used for the preparation of pipes.Gelcasting can avoid the deficiencies of the above-mentioned molding methods.The core problems in the process include the preparation of a high solid content slurry and the development of a novel gel system.At present,the preparation of atmospheric pressure cemented silicon carbide slurry uses submicron SiC powder as the main raw material.Because submicron powder has small particle size,high specific surface area and large inter particle force,the trend of agglomeration in the slurry is obvious.The volume solid content was not over 50vol.%under the pouring conditions.The gel system of pressureless sintered silicon carbide ceramics is mainly acrylamide system,which has the disadvantages of high toxicity and high dosage of organic additives.In this work,the non-toxic,low-volume Isobam gel system was applied to the gelcasting of dense SiC ceramics,and the ultra-high solid content SiC slurry was prepared by the powder surface modification cooperative particle packing of fine and coarse graded SiC powders.This work focuses on studying the rheological properties of the grain composition slurry,the microstructure of the sintered body,and the influence of mechanical properties.In the process of optimizing the gradation parameters of SiC particles sintered at atmospheric pressure,the ceramic grain transform from coarse columnar grains to uniform equiaxed grains as the content of micron-sized coarse powder increases.Adding 100wt.%sub-micrometer Si C powders,the phenomenon of abnormal grain growth is obvious.The small crystal grain length is only³?m and the large crystal grain length is more than 500?m.With 65wt.%of 4.6?m powder added,the grain size of the prepared SiC ceramic material is between 5-30?m and grain growth is equiaxed grains with particle size of¹2?m.The flexural strength and fracture toughness were 440±25MPa and 4.92±0.24MPa·m¹/2/2 respectively,which were improved by 14.1%and 17.1%compared with SiC ceramics added with 100wt.%submicron powder.In the preparation of the silicon carbide slurry,tetramethylammonium hydroxide was used as a dispersant for the SiC powder and the B₄C powder,and the carbon black particles were modified with the polyvinyl pyrrolidone to prepare a solid content of 55vol.%slurry.The addition amount of Isobam was as low as0.3wt.%and the solidification of SiC slurry containing gel system was achieved.After sintered,SiC ceramic with a relative density of 95.7%is obtained.The experimental results show that the introduction of 65wt.%4.6?m micron coarse powder,SiC slurry solid content increased from 50vol.%to 55vol.%,sintered to obtain a density of 95%of silicon carbide ceramics.Grain composition effectively suppresses the abnormal grain growth in the Si C ceramics.With the increase of the coarse powder content,the grain size changes from the coarse columnar grains to the short axis grains.Without adding any micron-sized SiC powders,the phenomenon of abnormal grain growth is obvious.The sizes of different crystal grains are very different.The small grain length is only³?m,and the large grain length exceeds500?m,while after adding 65wt.%of 4.6?m powder,the average grain size of the ceramic material is about 20?m.The fracture toughness of the material is4.55±0.27MPa·m^1/2,which is increased by 8.3%compared with the fracture toughness(4.20±0.12MPa·m^1/2)of the silicon carbide ceramic without added coarse powder,and the hardness increases to 24.3±0.2GPa.