Thermal Shock Resistance Of Laminated Porous Al₂O₃/ZrO₂ Ceramics Prepared By Gelcasting

Al₂O₃ and ZrO₂ ceramics with high hardness, strength, chemical stability and excellent wear resistance have been widely used as engineering materials. However, they may occur catastrophic fracture when it encounters thermal shock because of its inherent brittleness and lower thermal conductivity. Therefore, thermal shock resistance is a major issue in their structural application in elevated temperature. This thesis focus on the investigation of thermal shock resistance of laminated porous Al₂O₃/ZrO₂ ceramics. In addition, gelcasting process for preparing laminated porous Al₂O₃/ZrO₂ green bodies is studied. Main works include:Fabrication of polystyrene microsphere (pore-forming agent) and ZrO₂ nanoparticles was studied. Their surface property was investigated using layer-by-layer assembly technique. The results showed that by employing sodium poly (acrylic acid) and polyethylenimine to modify the zeta potential of the microsphere and ceramic particles, heterocoagulation takes place between them and the zirconia coating on the surface of polystyrene spheres was formed. The higher absolute value of zeta potential the ceramic particle has, the less aggregation can take place because of the electrostatic repulsion between them. Meanwhile, higher surface zeta potential is helpful to improve the fluidity of ceramic slurry.Al₂O₃/ZrO₂ slurry used for gelcasting was prepared and effects of solid loading, pH value and dispersant on the rheological behavior of the slurry were investigated. The results show that high solid loading of Al₂O₃/ZrO₂ suspension (50 vol%) with the lowest viscosity (less than 1 Pa·s) can be obtained at 1 wt% PAANa as the dispersant and pH value of 10.5. The ceramics green bodies made by gelcasting present excellent machinable capability. With WC cutting tools the green bodies can be machined at the cutting speed of 9.4 surface meters per minute (sm/min) and feed rate of 0.35 mm/revolution without damaging the sample and no cracks or bending occur during subsequently sinter.Porous Al₂O₃/ZrO₂ ceramics with porosity ranging from 6%-50% were fabricated by gelcasting, using polystyrene (PS) microsphere with diameter of 1μm as pore-forming agent. The effects of sintering temperature on porosity and strength as well as pore size were investigated. Flexural strengths of these porous ceramics at room temperature significantly decrease with the increases of porosity. However, thermal shock resistance of these ceramics was improved. Both the critical thermal-shock temperature (ΔT_c) and the residual strength of high porosity ceramics were higher than low porosity ceramics. These improvements are attributed to the porous structure in the specimen which relaxing the thermal shock stress and inhibiting the propagation of microcracks effectively rather than the phase transformation toughening effect of ZrO₂, which was confirmed by XRD analysis of specimens subjected to different thermal shock temperature.Multilayered porous Al₂O₃/ZrO₂ ceramics was prepared by gelcasting using layer-by-layer technique and thermal shock resistance of which was investigated. It was observed that the laminated ceramics have a high thermal shock resistance with the critical thermal-shock temperature (ΔT_c) about 500℃. The residual strength of about 30 MPa was obtained even after quenching at temperature of 1100℃. The specimens exhibited a gradual strength reduction rather than an abrupt strength decrease. This high thermal shock resistance was attributed to the crack deflection at the weak interfaces as well as the porous structure of the specimens, which relaxing the thermal stress and arrest the propagation of microcracks.