Study On Zirconia-Corundum-Mullite Composite Ceramics For High Temperature

Structural ceramics,which are known as high temperature resistance,corrosion resistance and high strength,have a wide range of applications in chemical industry,kiln,aerospace,defense and other fields.Coal series kaolin is a unique hard kaolin resource of our country with high purity（kaolinite content of more than 90%）,low cost and rich reserves,which can be used as a high quality ceramic raw material.In this study,mullite was in-situ synthesized to improve the thermal shock resistance and high temperature deformation resistance of corundum.Firstly,different content of mullite combined with corundum was synthesized and physical properties,thermal shock resistance and high temperature resistance were also studied as the function of mullite content.Then partially stabilized zirconia was added to reinforce the corundum-mullite composite ceramics.Finally,SiC was added to improve the thermal conductivity as well as thermal shock resistance of zirconia-corundum-mullite composite ceramics,and the resistant to oxidation of the samples was studied.The relationships between preparation process,composition and microstructure with properties of the composite ceramics were characterized by XRD,SEM and EPMA.The main research results are listed as follows:（1）Corundum-mullite composite ceramics were in-situ prepared by reaction sintering coal series kaolin andα-Al₂O₃.The optimum sintering temperature of series A was 1620°C.After the thermal shock and pyroplasticity experiments,it was found that the A3 sample（coal kaolin 37.69%,mountain aluminumα-Al₂O₃ 62.31%）had better comprehensive performance.After 30 times thermal shock（room temperature to 1100°C）,the bending strength increased 7.7%compared with that before thermal shock test,and the pyroplastic index（PI）was as low as 6.93×10^-66 mm^-1 after fired at1620°C.As seen from the microstructure of the sample,the shape of mullite was predominantly short columnar,bulk,and corundum growing with mullite.The grains of the composite ceramics combined tightly to ensure the samples had good bending strength.The thermal shock resistance was good for the phase compositions were unchanged before and after thermal shock test.（2）PSZ was added to improve the thermal shock resistance of the corundum-mullite composite ceramics on the basis of A3（coal kaolin 37.69%,mountain aluminumα-Al₂O₃ 62.31%）.The results showed that the martensitic transformation of PSZ could significantly improve the bending strength of the sample,and the Y₂O₃ introduced by PSZ could lower the sintering temperature of the sample and promote the densification process.B2 sample（PSZ addition amount of 10%）fired at 1560°C had the best comprehensive properties,which the Wa,Pa,D,and bending strength were 2.88%,8.84%,3.07 g·cm^-3 and 161.15 MPa,respectively.After thermal shock the bending strength was improved 12.96%and the pyroplastic index was 4.10×10^-66 mm^-1 after fired at 1560°C.The zirconia was distributed between the grains of corundum and mullite seen from SEM.The phase compositions were unchanged after thermal shock,which indicated that a good high temperature resistance the composite ceramics possess.（3）In order to improve the thermal properties of the samples,different amounts of SiC were added on the basis of B2 samples,and buried sintering method was employed to firing the samples.The results showed that the density of samples by buried sintering method was low,for the SiO₂ in the sample could react with the graphite to produce CO or SiO gas,which was not conducive to the densification of the sample.The comprehensive properties of C2 was the best among all the C series samples,and the Wa,Pa,D and bending strength were 12.21%,29.63%,2.43 g/cm^-3and 116.49 MPa,respectively.The SiO₂ produced by oxidation of SiC during thermal shock test could react with corundum and mullite was obtained to improve the bending strength.The SiC was encapsulated by corundum and mullite,which gave the samples good oxidation resistance.The oxidative weight gain was 20.1925mg·cm^-2,and the oxidation rate constant was 3.2895 mg²·cm^-4·h^-1.The phase compositions of the samples before thermal shock were mullite,corundum,m-ZrO₂,t-ZrO₂ and SiC.α-SiO₂ appeared in the C1^C4 samples after 30 times thermal shock cycles,which indicating that some of the SiC was oxidized during the thermal shock.After 100 h oxidation,the ZrO₂ and SiO₂ in the sample were converted to ZrSiO₄ and the diffraction peaks of ZrO₂ were not detected.