Fabrication And Performance Evaluation Of Al₂TiO₅/Al₂O₃ Composite Ceramics And Graded Materials

Aluminum titanate (Al2TiO5) has many excellent properties such as low thermal expansion coefficient,thermal conduction coefficient, high melting point, endured corrosion and thermal resistance, so it is one of the best materials with low thermal expansion coefficient resisting high temperature. For the sake of two disadvantages of Al2TiO5 that its mechanical strength is low and in the middle temperature range it can decompose into Al2O3 and TiO2 easily, the further application for Al2TiO5 is restricted. In this dissertation, MgTi2O5 and Fe2TiO5 are introduced to form solid solution to restrain the decomposition. Al2TiO5/Al2O3 composites are fabricated to offer not only improved strength but also excellent thermal shock-resistance. Based on the above research, the graded structure for Al2TiO5/Al2O3 is put forward for the first·time through structure design and thermal stress calculation, which can achieve the application in thermal shock resistance.Firstly, the thermal dynamics of Al2TiO5 powder synthesis and the crystal lattice parameters of Al2TiO5 solid solutions are calculated. The phase composition and thermal stability of Al2TiO5 solid solution, as well as the relative density, phase composition and microstructure of aluminum titanate Al2TiO5 ceramics, are characterized. The mechanical properties, thermal properties and thermal shock resistance are also tested. The results show that the additive MgO and Fe2O3 can effectively restrain the decomposition of Al2TiO5 and promote the sintering, which enhances the density of Al2TiO5. Al2TiO5 becomes more and more stable in the middle temperature range with-the increase of the solid solution content. When the content of MgTi2O5 and Fe2TiO5 is equal or greater than 5 mol% and 4 mol%, Al2TiO5 doesn't decompose after being kept at 1100℃for 50 hours. Moreover, the relative density of Al2TiO5 ceramics is increased with the increase of solid solution content.Secondly, in order to improve the mechanical properties of Al2TiO5 ceramics, the Al2TiO5/Al2O3 composites are prepared using pressureless sintering. The sintering mechanism of Al2TiO5/Al2O3 composites is investigated and the phase compositions as well as the microstructures are characterized. The mechanical properties, thermal properties, thermal shock resistance and molten Al corrosion resistance are tested. The results suggest that the sintering mechanism of Al2TiO5/Al2O3 composites is solid state sintering. The bending strength and the elastic modulus of the Al2TiO5/Al2O3 composites increase gradually with the increase of Al2O3 content. When the Al2O3 content is exceeding 60 vol.%, thermal stress of Al2TiO5/Al2O3 composites is increased obviously with low strain, which is typical fracture of brittle ceramics. When the Al2O3 content is lower than 60 vol.%, more microcracks are formed in the Al2TiO5/Al2O3 composites, leading to a significant increase of strain with the increase of the stress. The thermal expansion coefficient of Al2TiO5/Al2O3 is 0.2～7.5×10-6/℃. The thermal expansion rate is gradually increased with the increase of Al2O3 content. When the Al2O3 content is 20 vol.%, the residual mechanical strength reaches the highest value, and is higher than the strength before thermal shock. The thermal shock property is obviously degraded with the increase of Al2O3 content. The thermal shock parameters R1 and R2 are decreased with the increase of Al2O3 content. When the Al2O3 content is 20 vol.%, the Al2TiO5/Al2O3 composites suggest the best thermal shock property. When the Al2O3 content is low, the Al2TiO5/Al2O3 composites show good molten Al resistance, while when the Al2O3 content is exceeding 90 vol.%, obvious Al penetration through the microcracks could be detected.On the basis of middle temperature stable Al2TiO5 matrix ceramics and Al2TiO5/Al2O3 composites, the thermal stress relaxation is applied in the preparation of graded materials. The finite element method is used to calculate the residual stress and the distribution P of the graded materials according to which Al2TiO5/Al2O3 graded materials are prepared. The relative density, phase composition and microstructure are characterized. The results show that P=1.2 is the optimal result. In the graded Al2TiO5/Al2O3 structure, the relative density is decreased layer by layer with the increase of Al2TiO5 content. The Ti and Al elements distribution shows good graded transition along the thickness direction. The thermal shock property of Al2TiO5/Al2O3 graded material at 700℃is higher than the Al2TiO5/Al2O3 composites.