M.M.R. Boutz

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.

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 μm. 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 μm. Reversible transformation and crack deflection may explain the observed toughness values.

Low temperature superplastic flow of yttria stabilized tetragonal zirconia polycrystals

Very fine-grained (0·2 um), dense Y-TZP ceramics have been produced by free sintering at 1150°C. The superplastic deformation of these materials is studied in compression at low temperatures (1100-1300°C). A significant enhancement in initial strain rates was observed compared to a coarser-grained (0·4 ?m) commercially available material (Tosoh, Japan). Doping with small amounts of Fe2O3 led to a further enhancement of strain rate. Deformation occurred via interface reaction controlled grain boundary sliding. Prior to deformation a continuous glassy silicate film was observed at the grain boundaries. The applicability of interface reaction controlled solution-precipitation creep models is discussed. The absence of a steady-state during deformation at low stresses is attributed to dewetting of the silicate film.

Effect of additives on densification and deformation of tetragonal zirconia

The effect of additives (Bi2O3, Fe2O3) on densification and creep rates of tetragonal ZrO2-Y2O3 has been investigated. In Bi2O3-doped Y-TZP, a reactive liquid forms at temperatures above 800–900‡C, which leads to a strong enhancement of densification for concentrations of 1–2 mol % Bi2O3. However, during cooling from the processing temperature a strong, undesirable transformation of the tetragonal to the monoclinic phase occurs. The addition of 0.6–1.2 mol % FeO3/2 promotes densification without destabilizing the tetragonal phase. A concentration of 1.2 mol %, however, induces discontinuous grain growth, while this is not the case for 0.6 mol %. Creep rates of Y-TZP were enhanced by a factor of 4–6 by adding 0.6 mol % FeO3/2.

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.

The electrical characterization of grain boundaries in ultra-fine grained Y-TZP

Starting from a sinter reactive powder prepared by a gel precipitation technique, dense, ultra-fine grained (100–200 nm) yttria stabilized tetragonal zirconia ceramics were obtained by sinter forging at a temperature of 1100 °C or by pressureless sintering at 1150 °C. The pressureless sintered compacts were subjected to further heat treatments at temperatures of 1250–1450 °C or compressive deformation at 1250 °C under uniaxial stresses of 20–100 MPa. The obtained samples were characterized mainly by impedance spectroscopy. After compressive deformation a decrease in grain boundary resistivity was found which increased with applied stress. This can be interpreted in terms of a decrease in impurity segregation and a partial removal by compressive deformation of a poorly conducting amorphous film around the grains. It was also found that the grain boundary resistivity of samples sintered at 1150 °C could be considerably reduced by further pressureless heat treatments at temperatures above 1250 °C. This effect is probably owing to dewetting of the grain boundary and dissolution of grain boundary impurities into the bulk of the grains.

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. 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.

Grain Growth During Sintering of Nanocrystalline Y and/or CE-Doped Tetragonal Zirconia

A non-isothermal analysis of grain growth in tetragonal zirconia ceramics doped with various amounts of yttria and/or ceria in the temperature range 700–1150°C is presented. In these nanocrystalline ceramics, prepared by the gel precipitation technique, two grain growth regimes are recognized. At temperatures up to 900–1000°C grain growth proceeds slowly at high values of porosity (55%→35%); probably by means of a surface diffusion mechanism. At higher temperatures grain growth is much faster and occurs in locally dense regions by a normal grain growth or an impurity drag mechanism. Apparent activation energies are given for five compositions in both regimes. The isothermal sintering behavior of 3 mol% Y 2 O 3 -containing TZP at 1050°C is analysed. A dense ceramic with a crystallite size in the nanometer regime (

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.