Study On Preparation And Properties Of High Performance Reaction Bonded SiC Ceramic

Reaction bonded silicon carbide ceramic has been widely applied in nearly every industrial department of national economy since it has been born for more than half a century. At present, the performance of RBSC ceramic (d=3.1 g/cm3,σf= 1.2GPa) prepared from laboratory condition outclass industrial products. There are studies showing that the key of getting high performance RBSC ceramic is using superfine SiC particle which is easy to cause siliconizing obstruction and burst in sintering process. As a result, the preparation of higher performance RBSC ceramic and the analysis of forming mechanism of sintering defects have the important theories meaning and economic and social benefits.In this paper, the forming mechanism of defects of RBSC ceramic caused by impurity of SiC and structure of green body were studied. The high performance RBSC ceramic with fine grain structure has been prepared with methods of particle modification, reshaping, purification and in-situ coagulation casting. The main contents of this dissertation are listed as follows:Effects of forming mechanism of internal defects of RBSC ceramic prepared of 14μm (coarse grain)and 1.2μm(fine grain) SiC with different carbon density were studied. The residual stress of RBSC ceramic formed by residual Si were calculated by theoretical method. The results showed:the full-sintered RBSC ceramic prepared of fine SiC must be produced by lower carbon density of green body. The mechanism is that the finer capillary of green body will make siliconizing obstruction in sintering process. The result from stress calculation showed:the reason for the much bigger residual stress of RBSC ceramic is residual Si. The performance of RBSC ceramic was determined not only by the particle size of SiC but aslo by the scale of residual Si.The Gibbs free energy, initial temperrature, equilibrum constant and gas equilibrum partial pressure of reaction of reaction of impurity were calculated by energetics data. It was found that:the initial temperature of reaction of Al2O3 and C is 2031℃which have little effect in vacuum and low-temperature sintering process; the main impurity of SiC particle that influence the sintering process were Fe2O3 and SiO2; The equilibrum constant of the reaction of Fe2O3 and C, SiO2 and C was 8.486×10~11 and 4.216, respectively; the siliconizing obstruction prepared by Fe2O3 could be eliminated by insulation before sinter; The gas produced by the reaction of SiO2 and C was CO; The equilibrum partial pressure of CO could reach 3.69×103KPa when the temperature of internal green body was 1700℃; The CO gas excretion process appeared as bubbling and diffusion of O atom in liquid Si, which is the main cause of siliconizing obstruction and burst in sintering process.Effect of density, microstructure and bending strength of RBSC ceramic pre and post SiC particle purigying technology were researched. It was found that:the density of RBSC ceramic improved from 2.9834 g/cm3 to 3.0513 g/cm3 pre and post purifying technology, the bending strength of RBSC ceramic improved from 455MPa to 545MPa. As a result, the reduction of impurity of SiC particle could decrease gas production and siliconzing resistance in the sintering process and improve the density and bending strength of RBSC ceramic.The influence of slurry viscosity and bending strength of RBSC ceramic with reshaping technology were studied. The results showed:the viscosity of slurry made of slip casting decreased from 2412 mPa-s to 495 mPa-s (the rotational speed is 12 r.min-1) pre and post reshaping technology of 1.2μmSiC particle; the density of green body increased from 1.9260 g/cm3 to 1.9898 g/cm3; the bending strength of RBSC ceramic was improved from 526MPa to 578MPa. As a result, the reshaping technology could modify pore structure of green body and decrease the dimension of residual Si and improve the density and bending strength of RBSC ceramic.The purpose of modification of SiC and C was to meet the requirement of in-situ consolidation molding process. The influence of slurry viscosity with modification technology were stduied. The effect of density and bending strength of RBSC ceramic with different carbon density of green body were researched. The results indicate:the slurry viscosity decreased from 2188 mPa-s to 299 mPa-s pre and post the modification technology when the rotational speed was 12 r.min-1. The density and bending strength of RBSC ceramic were 3.0977 g/cm3 and 893MPa when the carbon density of green body was 0.8406 g/cm3, the distribution of residual Si of RBSC ceramic was uniform and the dimension of residual Si was less than lμm. Therefore, lower viscosity was the foundation of preparing green body with uniform structure, the uniform structure was the key factor of obtaining high performance RBSC ceramic.Effect of bending strength and grain size of RBSC ceramic with different sintering technology were studied. It was found that:the grain size was not grow up in vacuum sintering process, the bending strength of RBSC ceramic was 541MPa; the grain size was improved from 1.2μm to 5～7μm pre and post Ar atmosphere sintering process, the bending strength of RBSC ceramic was only 244MPa. As a result, the growth of grain size of RBSC ceramic was the main cause of bending strength reduction.