| Zirconia (ZrO2) and its toughened multiphase ceramics, with the advantages of high strength under room temperature, good wear resistance and high fracture toughness, which have been widely used in many fields, such as military equipments, electronics, machinery and biomedical sciences. Still lots of severe weaknesses was existing in the production and practical applications, such as poor reliability, reproducibility, which brought obstacles to engineering applications.In this work, ZrO2based composite nanopowders were synthesized via the technique of cross spraying-coprecipitation method combined with hydrothermal method under normal pressure, which effectively reduced the agglomeration of precursors and the particle size of the final products. The as-prepared ZrO2nanopowders have good dispersibility, uniform particle size, smooth surface, and good flowability, which can be calcined into the dense ceramic, with uniform microstructure and enhanced toughness.. Furthermore, low cost, simple equipments and convenient process technology were needed in this cross spray-atmospheric hydrothermal method, which was a cost-efficient and energy saving synthetic method, suitable for large-scale synthesis of ZrO2nanomaterials.The main research work is as follows:(1)3Y-ZrO2nanopowders were prepared by homogeneous precipitation method with cross spraying and hydrothermal method under normal pressure in liquid phase, using ZrOCl2·8H2O and YCl3·6H2O as the raw materials, and ammonia as precipitator. The influences on particle size had been comparatively studied between the cross spray-atmospheric hydrothermal method and traditional titration method. The influences of the former process, including ZrOCl2concentration, pH, reaction temperature and time, on the crystal structure and particle size of the ZrO2, were systematically studied. The as-obtained samples were characterized by XRD, TEM, and etc. The optimum condition of preparing3Y-ZrO2nanopowders was as follows: in the stage of cross spray,2mol/L ZrOCl2, NH3-H2O(pH=13) and1%PEG reacted under stirring; then the as-made slurry had been keeping on hydrothermal reacting at95℃and pH=13for12h. Finally, the3Y-ZrO2nanopowders was obtained after calcination at500℃, which had even particle size and good dispersibility, with an average particle size of about20nm.(2) Dense3Y-ZrO2ceramics were obtained using the above3Y-ZrO2nanopowders as the raw materials, via the process of ultrafine ball milling, granulating, molding and calcination. The density and linear shrinkage rate of the sintered ceramics were tested, and their microstructures were observed using SEM. The effects of the sintering process on the densification and crystal sizes of the3Y-ZrO2ceramics were investigated. The best process of3Y-ZrO2nano-ceramics was explored as follows:zirconium balls of2mm was added to mixture to mill at the speed of35r/min for10h, with the mass ratio of zirconium ball, water and mixed powders of6:1:1, then granulated with1.5%PVA, isostatic pressed. It takes two step to sinter:one is the process for pre-sintering to remove the organic substrate from r.t to500℃with a heating rate of2℃/min; the other is the process for microwave sintering using a five-stages heating strategy (r.tâ†'450℃â†'990℃â†'1200℃â†'1450℃), in which the total time was80min and the duration at1450℃was20min., and the natural cooling time of the samples was90min. It indicated that the3Y-ZrO2ceramics obtained by this process have a relative density of99.2%, bending strength of1098MPa, and a fracture toughness of7.8MPa·m1/2.(3) Al2O3/ZrO2composite nanopowders were prepared by the process of homogeneous precipitation method combined with cross spraying and hydrothermal method under normal pressure in liquid phase, using ZrOCl2·8H2O, YCl3·6H2O and as main raw materials, and ammonia as precipitator. The dense composite ceramics with relative density of99.6%were prepared in a similar microwave process to that of the3Y-ZrO2ceramics. It indicated that Al2O3/ZrO2composite ceramics being more uniform particle-size distribution and smaller crystal particle size had improved bending strength and fracture toughness. Meanwhile, the effects of the various nanopowders synthesis methods and the amounts of Al2O3on the mechanical properties of the Al2O3/ZrO2composite ceramics were also investigated. It showed that the materias, added with0.5%mol Al2O3, sintered at1430℃using the microwave process, have the best over-all properties of bending strength of1150MPa and a fracture toughness of8.2MPa·m1/2, higher than those of the3Y-ZrO2ceramics without Al2O3. |
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