Study On Phase Composition,Microstructure And Properties Of Bauxite-based Homogeneous Mullite-SiC-O'-siAlON Composite Materials

Bauxite is China's strategic resource.Synthesis of high performance bauxite-based mullite provides an effective way for comprehensive utilization of abundant bauxite resources in China.However,the synthesized homogeneous bauxite-based mullite has the disadvantages of poor toughness,low high temperature strength,poor thermal shock resistance and weak corrosion resistance during applications.The introduction of SiC into mullite materials can make up for shortcomings of bauxie-based mullite materials.However,the present methods by adding SiC or through carbothermal reduction in-situ formation of SiC have some shortcomings,such as uneven mechanical mixing,difficult to sinter or complex process,high cost and difficult to industrialize.Therefore,in this study,a novel method has been adopted to prepare bauxite based mullite-SiC-O'-SiAlON composites at high temperature in carbon embedded condition using the synthesized homogeneous bauxite-based mullite aggregates and fine powder,silicon powder,and phenolic resin as binder.Using the plasticity of Si at low temperature increases the density of green body,and Si reacts CO,N₂ to in-situ formation whisker-like SiC and O'-Si Al ON at high temperature to prepare mullite-SiC-O'-SiAlONcomposites.The novel method is simple,low cost and suitable for production mullite-SiC-O'-SiAlONcomposites in large scale.The effects of various amount of Si,temperature and additives?Al,Zn and SiC?on the composition,microstructure and properties of the prepared composites have been studied.The catalytic mechanism of the trace impurities in mullite for Si reaction and growth of SiC whiskers,growth mechanism of SiC and O'-Si Al ON in complicated systems,and the strengthening and toughening mechanism of whisker-like SiC and O'-Si Al ON in the composites have been also discussed.The research results are as follows.The incorporated Si in bauxite-based mullite will react with C,CO and N₂ to form non-oxide SiC,O'-Si Al ON whiskers in carbon embedded condition at high temperature,thus the mullite-SiC-O'-SiAlON composites are prepared.The SiC,O'-Si Al ON whiskers fill in pores and directly bond with mullite leading to the dense in structure,high strength and good sintering properties of the composites.The trace impurities of Fe₂O₃ and Ti O₂ in mullite can promote the reactions of Si and catalyze the growth of whisker-like SiC and O'-Si Al ON without other additional catalyst,thus forming self-catalysis in the material.The growth mechanisms of SiC and O'-Si Al ON whiskers are governed by VS and VLS.On the basis of introducing Si powder,further adding appropriate amount of Al,Zn and SiC powder?1-2%?can promote Si reaction to form whisker-like SiC and O'-Si Al ON,and improve the density and strength of the composites.The reason is that Al,Zn and SiC additive can increase the gas pressure in the inner of sample at high temperature and promote gas phase transmission and reaction.But the excessive additives can increase the vapor pressure in the sample and more gaseous substances discharing,resulting in the loosening structure of the composite material and reducing its sintering performance.The bauxite-based mullite-SiC-O'-SiAlON composite has high strength at high temperature and excellent thermal shock resistance.The high temperature strength and thermal shock resistance of the composite are more than 10 times and 2 times higher than that of mullite brick.The optimum addition of Si powder is 10%.The high strength of the composite is attributed to the direct bond between SiC,O'-Si Al ON and mullite,which play the role of pinning and anchoring effects.The excellent thermal shock resistance of the composite is due to SiC and O'-Si Al ON whiskers form an interlocking network structure,which consumes a lot of fracture work and has a remarkable toughening effect when SiC and O'-Si Al ON whiskers are bridged,pulled-out and cracks deflected.On the basis of introducing Si powder,proper amount of Al powder was introduced.Except whisker-like SiC and O'-Si Al ON,needle-like Al N is formed in the material.The introduction of Al contributes to the growth of non-oxide crystals.The enhancement effect is remarkable,and the high temperature strength increases 80%.The introduction of a small amount of Zn?<1%?has little effect on the high temperature mechanical properties of the composites.Excess Zn powder can degrade the high temperature properties of the material,because Zn escapes in the gaseous state and destroys the material structure.With the introduction of 1-5%SiC powder,the high temperature strength properties of the material have slightly change,and thermal shock resistance of the material has been significantly improved,which is mainly due to the increase of the total SiC content.Bauxite-based mullite brick has poor alkali resistance,and the composite material prepared by introducing Si powder has good alkali resistance.Compared with bauxite-based mullite material,the weight gain and strength of the composite materials decreased after alkali corrosion at 1100?and 1300?.The structure of erosion layer is compact,SiC and O'-Si Al ON still exist in the sample,and its morphology is the same as before alkali erosion experiment.After adding Al,Zn and SiC powder,the composite materials show good alkali resistance.The introduction of proper amount of Al and Zn powder can further improve the alkali resistance of the composite materials,while the alkali resistance of the composite materials is slightly reduced after the introduction of SiC.The process of alkali erosion is as follows:in the alkali medium with high activity,mullite,Sialon and SiC react with CO to form corundum and quartz phase first,then K or K₂O react with corundum,quartz or directly with mullite to form potassium nepheline,leucite and high potassium glass phase,and then the composite material is eroded.The mechanism of improving the alkali resistance of the composite material is that the SiC and O'-Si Al ON in the sample strengthen the structure and the potassium glass phase formed after the alkali erosion of the system makes the structure of the material compact,which prevents the alkali from further entering the sample.Bauxite-based mullite-SiC-O'-SiAlON composite has excellent oxidation resistance,and the oxidation nature of the composite belong to protective oxidation,i.e.when oxidized,O'-Si Al ON and SiC in the composite material are oxidized firstly on the surface of the material,and the oxidized product Si O₂ forms a glass film with impurities in mullite,the pores are sealed by the glass film and prevents oxygen from entering the inner of the sample through pores.When the oxidized products Si O₂ and Al₂O₃ react with each other,mullite-containing glass film can be formed.The oxidation resistance of the composite can be improved by introducing appropriate amount of Al,Zn and SiC powders.