| Superplasticity is observed in many composite ceramics. It is an important way that superplasticity of nanocomposite ceramic at elevated temperatures is utilized to achieve near-net shape forming of ceramic components with complex shape. Al2O3-ZrO2 ceramics are important ceramic composites, which own excellent mechanical properties from ambient to elevated temperature and can be widely used to fabricate many parts of resistance to wear, to high temperature, et al. In the present research 3Y-TZP/Al2O3 nanocomposite ceramic with 20mol% ZrO2(3Y) content was prepared. Subsequently, the superplastic extrusion forming and compressive deformation were carried out. Moreover, the friction and lubrication during superplastic forming were evaluated and measured. The forming properties of composite ceramic, mechanical characters and microstructure before and after deformation were presented. The superplastic deformation mechanism of composite ceramic was studied.The 3Y-TZP/Al2O3 nanocomposite ceramics were prepared by hot pressed sintering at 1400℃, 1450℃, 1500℃and 1550℃. The addition of ZrO2 particles was very effective in improving the sintering character of materials and hindering the grain growth of Al2O3. The typical inter/intra granular mixed structure was achieved. The relative density of composite ceramic was more than 98% at above 1450℃, while he grain size increased with the increasing temperature. The maximum of bending strength, fracture toughness and Vickers hardness of composites were up to 591MPa, 7.9MPa·m1/2, 18.1GPa and 442.5GPa.The ring compression test was adopted to evaluate the friction and lubrication behavior of the hexagonal BN lubricant at the temperature range of 1400～1600℃and the strain rate of 1.1×10-4 s-1, 5.4×10-4 s-1 and 2.5×10-3 s-1, in order to measure the friction during the superplastic forming and seek for suitable lubricant. The friction factor on the BN lubricant was very stable, which viaration was analyzed with the increase of strain rate and temperature. The ring compression test was also used to evaluate the flow behavior of composite ceramic and measure the true flow stress.Superplastic extrusion on turbine disk model was attempted at temperature of at 1500℃, 1550℃, 1600℃and 1650℃. The material showed the best formability for extrusion at 1600℃, at which the maximum extrusion pressure was less than 25MPa and the maximum punch speed was up to 0.14mm·min-1. Due to the internal good forming property of composite ceramic and effective action of BN lubricant, the material showed good flow property during extrusion and no apparent strain hardening. Deformed samples owned good quality and there were almost no apparent flaws. Through the observation for the microstructure of SEM and TEM before and after deformation, the deformation mechanism was studied. The mechanical properties of deformed samples were measured, which were also compared with sintered samples.Superplastic compressive deformation was conducted at 1500℃and a constant pressure of 25MPa. According to the analysis of XRD pattern, orientation index and peak-intensity ratio, the orientation changes of crystal planes of ZrO2 and Al2O3 with the increasing strain were observed and compared. The results showed that the texture in deformed material was strong. Moreover, the measured and caculated {110}, {113} and {300} X-ray pole figures of Al2O3 matrix can distinctly illustrate the indensity of texture. The changes of fracture surface and surface characters in cross section parallel and perpendicular to compressive axis for deformed material, such as grain size, aspect ratio, grain shape factor and et.al, were analyzed by SEM. The microstructural evolution during deformation was observed by TEM, through which the main deformation mechanism and the forming and development of high texture were sdudied. The mechanical properties such as bending strength, fracture toughness and Vickers hardness showed the increasing tendency and then decreasing tendency with the increase of strain. The effect of strain, grain size, cavity and texture on the mechanical properties was studied. |
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