Mechanical properties of nanocrystalline materials

This review describes the mechanisms responsible for low-temperature degradation (LTD) of zirconia ceramics and its detrimental consequences for biomedical devices. Special emphasis is given to the critical issue of zirconia degradation actually observed for hip prostheses. Experimental methods to accurately measure and predict LTD in a given zirconia ceramic are presented. Different solutions to inhibit LTD or at least reduce its kinetics are reviewed, with the objective of highlighting alternative options for the generation of new zirconia-based biomedical ceramic devices.

Low-Temperature Degradation of Zirconia and Implications for Biomedical Implants

Of late, nanotechnology seems to be rapidly thrusting its applications in all aspects of life including engineering and medicine. Materials science and engineering has experienced a tremendous growth in the field of nanocomposite development with enhanced chemical, mechanical, and physical properties. A wide array of research has been conducted in the processing of nanocomposites. Consolidation of these systems from loose particles to bulk free form entities has always been a challenge. To name a few, traditional consolidation techniques such as cold pressing and sintering at high temperatures, hot pressing, and hot isostatic pressing have strong limitations of not being able to retain the nanoscale grain size due to the excessive grain growth during processing. This article reviews in detail the results from numerous studies on various methods for manufacturing nanocomposites with improved properties and retained nanostructures. Both challenges and recent advances are discussed in detail in this review.

Challenges and advances in nanocomposite processing techniques

Solute segregation at grain boundaries has been correlated with grain-boundary conductivity in high-purity 15-mol%-CaO-stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels 103 at 500°C. At the lowest levels of segregation achieved, <0.1 monolayer, σspgb remains ∼102 less, and possibly represents an “intrinsic” limiting value for the grain boundary. Comparison with Y2O3-doped ZrO2 suggests similar behavior in this system. The control of grain-boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain-boundary impedance of polycrystalline ionic conductors.

Solute Segregation and Grain-Boundary Impedance in High-Purity Stabilized Zirconia

The recent interest in the properties of ceramics with grain sizes less than 100 nm has created a need for processing routes with which to manufacture such ceramics. This article briefly reviews the variety of techniques currently used to manufacture ultrafine starting powders, compact the powders, and sinter them into bulk nanocrystalline ceramics. The unique challenges associated with processing such fine structures are discussed together with each technique. Major obstacles have included the difficulty (now largely overcome) of producing sufficient quantities of ultrafine powders; the strong tendency of nanocrystalline powders to agglomerate; the difficulty of manufacturing homogeneous, stress free compacts from ultrafine powders; and the ever present obstacle of unwanted grain growth during sintering. Despite numerous technical difficulties, researchers have managed to develop techniques for processing ultrafine powders into nanocrystalline ceramics which are both fully dense (or near fully de...

Processing of nanocrystalline ceramics from ultrafine particles

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.

https://ris.utwente.nl/ws/files/6646412/Boutz94yttria.pdf

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

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.

/pdf/characterization-of-grain-boundaries-in-superplastically-buw61tdgus.pdf

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

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.

/pdf/the-effect-of-ceria-co-doping-on-chemical-stability-and-32vuio90jo.pdf

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

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.

/pdf/effect-of-additives-on-densification-and-deformation-of-17wonrcigr.pdf

Effect of additives on densification and deformation of tetragonal zirconia

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.

https://ris.utwente.nl/ws/files/6646464/Boutz94low.pdf

M.M.R. Boutz

Papers

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

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

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

Effect of additives on densification and deformation of tetragonal zirconia

Low temperature superplastic flow of yttria stabilized tetragonal zirconia polycrystals