Research On Andalusite Based Composite Ceramics For High Temperature

High temperature structural ceramics have wide applications,not only for the long-term modern industrial productions and new energy technologies,but also for the short-term military and aerospace technologies.Because of the wide sintering temperature range and the small firing shrinkage,andalusite can be applied for the preparation of high temperature structural ceramics.In the temperature range of 1100^oC¹600 ^oC,andalusite can be converted into mullite and the excessive silica rich liquid phase,which is harmful to the mechanical properties of ceramics.As it is known that Al2O3 addition can consume the excessive silica rich liquid phase by second mullitization,this study is aimed at improving the mechanical properties of ceramics by reducing the content of silica rich liquid phase with the help of Al₂O₃addition.In order to meet the application requirements of high temperature structural ceramics,the composite ceramics were prepared from andalusite,ZrO₂,SiC and?-Al₂O₃ by in-situ synthesized technology in this study.Effects of ZrO₂,SiC,?-Al₂O₃and sintering temperature on water absorption,apparent porosity,bulk density,bending strength,high temperature plastic deformation,micro-structure and phase composition of the samples were investigated.The relationships between preparation techniques,compositions,performances and micro-structure were investigated by using modern testing technologies,including XRF,XRD,SEM and EDS.The methods and mechanism to improve the thermal shock resistance,density and thermal conductivity of the andalusite based composite ceramics were discussed.Corundum-andalusite based composite ceramics have high hardness,small plastic deformation and other excellent performances at high temperature.Corundum-andalusite based composite ceramics were fabricated from andalusite and?-Al₂O₃ by in-situ synthesis technology.Effects of mullite/corundum ratio and sintering temperatures on physical performance,thermal shock resistance,micro-structure and phase compositions of the samples were investigated.Results indicated that the in-situ synthesized mullite from andalusite combined with corundum satisfactorily,which significantly improved the thermal shock resistance as no crack formed after 30 cycles of thermal shock?1100 ^oC-room temperature,air cooling?.Samples A4?andalusite:37.31 wt%,?-Al₂O₃:62.69 wt%,TiO₂ in addition:1 wt%,mullite:corundum=6:4 in wt%?achieved the optimum properties when sintering at 1650 ^oC,which were listed as follows:Wa of 0.15%,Pa of 0.42%,and D of 3.21 g/cm³,bending strength of 117.32 MPa.The phase compositions of the sintered samples before and after thermal shock tests were mullite and corundum constantly.The fracture modes of the crystals were trans-granular and inter-granular fractures,which could endow the samples with high thermal shock resistance.To enhance the bending strength of the composite ceramics,the experiments of series C were conducted by adding ZrO₂ on the basis of A4 at the sintering temperature range of 1500-1650 ^oC.The relationship among the phase compositions,processing technologies,struture and properties of zirconia-corundum-andalusite based composite ceramics was studied.The enhancement mechanism of ZrO₂ was revealed.Results showed that the high temperature plastic deformation of sample C2?contained 10%ZrO₂?sintered at 1600 ^oC was equal to sample C0 sintered at 1650 ^oC,The value of bending strength increased by 17.84%comparing with sample C0sintered at 1650 ^oC?which had the highest bending strength of 117.32 MPa?.As cooling from high temperature to room temperature,t-ZrO2 converted into m-ZrO2,whose volume expansion could compensate the residual stress,thus increasing the bending strength of samples.Therefore,the loss rate of bending strength of samples C2 after thermal shock tests was only 2%.Besides,the phase compositions of the sintered samples C2 were corundum,mullite,t-ZrO2 and m-ZrO2,mullite and corundum,whose grains interconnected with each other.Proper growth and uniform distribution of crystals were observed throughout the composites.Meanwhile,zirconia particles were attached to mullite and corundum grains.After 30 cycles of thermal shock,the combination of crystals weakened,which decreased bending strength.The distribution of ZrO₂ in sample C2 sintered at 1600 ^oC became inter-granular.Thermal conductivity is an important performance for andalusite based composite ceramics.Based on C2?andalusite is 33.58%,?-Al₂O₃ is 56.42%,ZrO₂10%,TiO₂ 1%?,formulae D1-D4 were designed with 10%,20%,30%,40%SiC addition,respectively.Effects of SiC addition and sintering temperatures on the water absorption,porosity,bulk density,bending strength,phase composition and microstructure of the sample were analyzed.Results showed that the best formula was D3 sintered at 1420 ^oC?33.58%andalusite,56.42%?-Al₂O₃,10%ZrO₂,30%SiC addition,1%TiO₂ addition?with Wa of 10.69%,Pa of 26.62%,D of 2.49 g/cm³,bending strength of 80.57 MPa.After 30 cycles of thermal shock,the bending strength of D3 was 81.89 MPa,showing that D3 had good thermal shock resistance.XRD analysis showed that the phase compositions of the sintered samples were corundum,mullite,m-ZrO₂ and SiC.SEM analysis indicated the grains with proper growth were distributed uniformly throughout the composites,with zirconia dispersion among matrix consisted by mullite and silicon carbide.During the processes of thermal shock,the surface oxidation of samples changed the stress state of the surface,and finally avoids the thermal shock fracture of samples.It is showed that the phase composition of samples D3 after 100 h oxidation were mullite,corundum,SiC,ZrSiO₄ and?-SiO₂.Sample D3 showed good oxidation resistance as SiC phase still existed after 100 h oxidation and the appearance of the samples was not changed except for the color.The good oxidation resistance could be attributed to the generation of compact SiO₂ film which was obtained by the SiC oxidization.