A hydrothermal route for production of dense, nanostructured Y-TZP

Y-TZP powders were prepared either by calcination in air or crystallization under hydrothermal conditions of a hydrous gel, obtained by coprecipitation. Differences in powder properties, green compact structure and sinterability were examined. Crystallization under hydrothermal conditions occurs at temperatures as low as 190°C in the presence of ammonia. The hydrothermally treated powders are composed of soft agglomerates, that collapse under very low pressures, resulting in green bodies with high densities and small pore radii. The sinterability is greatly improved by the hydrothermal treatment and allowed the production of dense, nanostructured Y-TZP by free sintering at 1050°C

Plasticity of nanocrystalline zirconia ceramics and composites

The deformation strain rate of nanocrystalline Y-TZP shows an increase by a factor 4 if the grain size decreases from 200 to 100 nm. Real superplastic deformation (strain rate > 10−4 s−1) is observed in these materials at relative low temperature (1100–1200 °C). Grain-boundary analysis indicates (partial) removal of an ultra-thin (1 nm), yttrium-rich grain boundary layer after deformation.\ud \ud Uniaxial pressure-assisted sintering techniques (=sinter-forging) provide the opportunity of large shear strains during densification. Sinter-forging experiments on zirconia-toughened alumina (15 wt% ZrO2/85 wt% Al2O3) resulted in a dense composite within 15 min at 1400 °C and 40 MPa, with effective shear strains up to 100%. Sinter-forging of Y-TZP and ZTA gives an increase in strength, reliability and fracture toughness. These improvements are caused by the large shear strains that result from the removal of processing flaws. Also, the number of microcraks at the grain boundaries and the interatomic spacing between the grains are reduced by the forging techniques, resulting in a strengthening of the grain boundaries if compared with pressureless sintering. K1C values of 10 MPa√m are obtained for Y-TZP, while no classical stress-induced phase transformation toughening is observed. Sinter-forged ZTA samples showed a better wear resistance than free sintered ones.\u

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress

The effect of ceria co-doping on chemical stability and fracture toughness of Y-TZP

The fracture toughness and ageing resistance of yttria, ceria-stabilized tetragonal zirconia polycrystals (Y, Ce-TZP) were evaluated as a function of grain size and ceria content. Very fine grained, fully dense materials could be produced by sinter forging at relatively low temperatures (1150–1200 °C). The ageing resistance in hot water (185 °C) of 2 mol% Y2O3-stabilized TZP is strongly enhanced by alloying with ceria. The ceria content necessary to avoid degradation completely, decreases with grain size. The toughness of fully dense Y, Ce-TZP is 7–9 MPa m1/2 for grain sizes down to 0.2 mgrm. No or very little transformation took place during fracturing and no clear variation with grain size was observed for the toughness at grain sizes up to 0.8 mgrm. Reversible transformation and crack deflection may explain the observed toughness values

Superplastic deep drawing of tetragonal zirconia ceramics at 1160°C

Superplastic forming under biaxial tension of tetragonal zirconia (Y-TZP) is investigated by pushing a hemispherical punch (radius 6 mm) on Y-TZP which was placed on a ring with an inner diameter of 16·7 mm. Dense Y-TZP samples with a grain diameter of 125 nm could be elongated to a dome height of at least 8 mm at a temperature as low as 1160°C. Such elongations could not be achieved at this very low temperature when the ceramic had a grain diameter of 250 nm. At a grain size of 250 nm differences in deformation behaviour were observed for different types of Y-TZP powders. This is explained by the fact that at higher cavity concentration in the sintered compact the sample deforms at lower forces but fractures at lower elongations

Yttria-Ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation

An analysis is presented of grain growth and densification of yttria-ceria stabilized tetragonal zirconia polycrystals (Y, Ce-TZPs) using both isothermal and non-isothermal techniques. The characteristics of Y, Ce-TZPs are compared to those of Y-TZP and Ce-TZP and the effect of increasing ceria concentration at constant yttria content is evaluated. During non-isothermal sintering two regimes are distinguished: below 900–1000°C the neck area increases strongly by surface diffusion accompanied by only very little densification and grain growth, in the temperature interval 900–1000°C to 1200°C the materials densify to 95% of the theoretical density via a grain boundary diffusion mechanism and grain growth accelerates. Dense materials with grain sizes of 0·15–0·20 μm can be prepared by isothermal sintering at 1100–1150°C.\ud \ud In Y, Ce-TZP it is yttrium that segregates to the grain boundaries at 1150–1400°C. The yttrium content of the grain boundaries in Y, Ce-TZP is independent of temperature and ceria-concentration under the investigated experimental conditions. Grain growth in dense TZP is controlled by a solute drag mechanism at elevated temperatures (>1200°C); this drag is highest for Y-TZP, absent for Ce-TZP and moderate for Y, Ce-TZP

Yttria-Ceria stabilized tetragonal zirconia polycrystals: Sintering, grain growth and grain boundary segregation

An analysis is presented of grain growth and densification of yttria-ceria stabilized tetragonal zirconia polycrystals (Y, Ce-TZPs) using both isothermal and non-isothermal techniques. The characteristics of Y, Ce-TZPs are compared to those of Y-TZP and Ce-TZP and the effect of increasing ceria concentration at constant yttria content is evaluated. During non-isothermal sintering two regimes are distinguished: below 900–1000°C the neck area increases strongly by surface diffusion accompanied by only very little densification and grain growth, in the temperature interval 900–1000°C to 1200°C the materials densify to 95% of the theoretical density via a grain boundary diffusion mechanism and grain growth accelerates. Dense materials with grain sizes of 0·15–0·20 μm can be prepared by isothermal sintering at 1100–1150°C. In Y, Ce-TZP it is yttrium that segregates to the grain boundaries at 1150–1400°C. The yttrium content of the grain boundaries in Y, Ce-TZP is independent of temperature and ceria-concentration under the investigated experimental conditions. Grain growth in dense TZP is controlled by a solute drag mechanism at elevated temperatures (>1200°C); this drag is highest for Y-TZP, absent for Ce-TZP and moderate for Y, Ce-TZP