Sintering and characterisation of nano-sized yttria-stabilised zirconia
Hasanuzzaman, Muhammad (2006) Sintering and characterisation of nano-sized yttria-stabilised zirconia. Master of Engineering thesis, Dublin City University.
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A study has been conducted on the pressure-less sintering of the ZrC>2- Y2O3 system that contains 3 mol% Y2O3, and the effect that the temperature / time relationship has on the properties of the final product. Experiments were performed on two types of commercially available, nano-sized, ZrC>2 - 3mol% Y2O3 (3Y-TZP) powders. An unstabilised zirconia powder was also investigated for comparison purposes. Particle size analysis of these powder yield particle sizes ranging from 1.29 |xm to 1.78 pm, suggesting that the particles are heavily prone to agglomeration in water. BET specific surface area analysis showed the powders to be nano-sized as determined using equivalent spherical diameter theory. The density of the powders was measured using helium gas pycnometry. DTA/TGA analysis indicates that binder burnout occurs on heating in the range 300-440°C. Shrinkage and densification rate characteristics of the powders during sintering was investigated using dilatometry. One of the powders demonstrated full shrinkage during isothermal sintering; the other did not. The point of maximum densification rate differed by approximately 100°C for these powders. A dramatic expansion associated with the tetragonal to monoclinic transformation on cooling from 1400°C is observed by dilatometry for unstabilised zirconia powder sample but not for stabilised powder samples. This indicates that the sintered samples retain in tetragonal phase on cooling to room temperature. Pressureless conventional sintering and two-step sintering were used to sinter pressed discs. The density of discs which were sintered across a temperature range of 1350°C to 1550°C varies between 5.72 g/cm3 and 6.03 g/cm3, corresponding to 97.40% and 99.10% of theoretical density respectively. Density as high as 98.85% of theoretical was recorded for twostep sintering applying holding time of 16 h. Vickers hardness values increased with increasing sintering temperature. Fracture toughness measurements were carried out. X-ray diffraction analysis confirmed that sintered discs were fully tetragonal. Microstructural analysis of sintered samples was conducted to assess microstructural changes with respect to sintering temperature.
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