INTRODUCTION
In the field of structural ceramics, yttria-tetragonally stabilized zirconia (Y-TZP) is of great interest because of its high strength and toughness and good wear resistance.1-1-3-1 Previous studies have demonstrated that wet processing routes provide the best possible mechanism for obtaining ”bulk” nanocrystalline ceramics because wet processing does not call for extremely high pressures for consolidation as are needed with conventional dry forming techniques.1-4-6-1 The chief problem with wet processing is aqueous attack on Y-TZP.[7-21] Aqueous degradation can lead to processing dilemmas as the dissolution of yttrium ions from the surface of the powder changes the surface chemistry of the powder while leading to a lack of control over suspension pH values. Furthermore, dissolution of the yttrium-stabilizing agent can lead to catastrophic degradation of the ceramic in service as a result of the tetragonal to monoclinic phase change which leads to spallation of the ceramic because of stresses produced by the 5-6 vol.% expansion associated with the phase change. It is the aim of this review to summarize the current scientific understanding about the aqueous chemistry of Y-TZP and the implications toward synthesis, dispersion, and processing of nanosized Y-TZP. After reviewing the wealth of knowledge in the literature, elucidation of the techniques that will allow for the aqueous synthesis and processing of nanosized Y-TZP will be apparent. Improvement and utilization of these techniques will lead to the accomplishment of a universal goal in structural ceramics which is the production of bulk ceramics from nanosized ceramic powders.
AQUEOUS CHEMISTRY OF YTTRIA-TETRAGONALLY STABILIZED ZIRCONIA
Several research groups have investigated aqueous attack of Y-TZP ceramics.[7-21] There are two major theories for the reaction of Y-TZP in water. The first theory comes from work performed by Lange et al.[7] They performed a study on sintered, polycrystalline Y-TZP with yttrium content ranging from 2 to 6 mol%. The aging experimentsin this study were performed at 250°C in a tube furnace in a humid environment. From this study, Lange et al. concluded that water attacks a tetragonal grain on the surface of the ceramic. Lange et al. hypothesized that during aqueous attack, water leaches a sufficient amount of yttrium such that the surface tetragonal grain transforms into the monoclinic phase. Further depletion of yttrium causes the monoclinic structure to grow until a critical size is reached. At this critical size, the monoclinic structure will grow spontaneously until the entire grain is transformed to the monoclinic phase. If this new mono-clinic grain is greater than a critical size, then microcracks will form exposing more virgin surface from which yttrium can be leached. Lange et al. further showed that transformation of tetragonal phase to monoclinic phase decreases with increasing yttrium content. Conclusions by Lange et al. were reached through the use of high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD), but no solution analysis or kinetic studies were performed. Therefore no direct measurement of yttrium in solution was made nor was the rate at which the proposed leaching mechanism took place measured. Yoshimura et al.[8] have also performed studies on the aqueous attack of sintered, polycrystalline Y-TZP. The aging experiments in this study were performed over a range of temperatures from 250°C to 800°C under hydrothermal conditions. It was concluded by Yoshimura et al. that water chemically adsorbs to the surface of the ceramic. Then hydroxide groups migrate through the oxygen vacancies in the Y-TZP lattice leading to the formation of zirconium and yttrium hydroxides. The increase in the size of the hydroxide groups with respect to the oxygen vacancy gives rise to stressed sites at the oxygen vacancies within the lattice. The stressed site defect, in turn, acts as a site for the nucleation of the monoclinic phase. The transformation of tetragonal to monoclinic then leads to microcracking and macrocrack-ing. The major difference between the mechanisms proposed by Lange et al. and Yoshimura et al. is that in the Yoshimura et al. model, hydroxide transport into the material, not yttrium leaching, contributes to the failure of the ceramic seen in aqueous environments. Studies on oxygen diffusion in Y-TZP and the effect of oxygen on the tetragonal to monoclinic transformation pose problems for the Yoshimura et al. theory in the temperature range at which Yoshimura et al. conducted aging experiments.1-22’23-1 Research has demonstrated that as the yttrium content increases, oxygen diffusion rates increase.1-22-1 Thus one might expect that OH- diffusion rates would also increase which would lead to the presence of many stressed sites in the ceramic according to the degradation model proposed by Yoshimura et al. These stressed sites, which would lead to degradation of properties in the ceramic, have not been observed in other studies. Research has also been shown that simply replacing 16O in Y-TZP with 18O leads to a decrease in the amount of tetragonal to monoclinic transformation.1-23-1 If such a small mass change leads to such a significant change in transformation, it is reasonable to expect that replacing 16O with OH- would have a similar effect, but such has not been observed in the literature.
Beyond these two aforementioned studies, a recent publication has shown that at room temperature, a diffusion-controlled leaching model is supported by data from kinetic studies of the aqueous degradation of 3 mole percent yttria tetragonally stabilized zirconia (3Y-TZP).[21] Experiments performed by Kimel and Adair allowed for leaching kinetics of yttrium to be measured and the effects of the aqueous degradation to be established. Interestingly, it was shown that the leaching of yttrium from 3Y-TZP did not have an adverse effect on the crystal structure of the 3Y-TZP powder within the sensitivity of XRD. However, it was also shown that yttrium leaching has significant effects on the surface chemistry of 3Y-TZP powder which can have a deleterious effect on the stability of 3Y-TZP aqueous suspensions.
In addition to characterizing the mechanism behind the aqueous degradation of 3Y-TZP, the work by Kimel and Adair showed that the addition of a surface passivation agent could trivialize the effects of aqueous degradation. It was determined that the addition of oxalic acid to the aqueous suspension of 3Y-TZP led to the formation of a passivation layer, suspected to be yttrium oxalate, that reduced yttrium ions in solution beyond detection. Furthermore, the formation of the passivation layer led to the development of significant surface charge that could be used to facilitate dispersion of the 3Y-TZP powder in aqueous suspension.
SYNTHESIS METHODS FOR YTTRIA-TETRAGONALLY STABILIZED ZIRCONIA
There are a variety of ways to produce nanosized Y-TZP powder including, but not limited to, polymerization/sol-gel, coprecipitation, and hydrothermal synthesis.[24-42] Synthesis by polymerization or sol-gel methods typicallytakes on a procedure similar to that of Pechini.[24- In the Pechini process, a complexing agent, such as citric acid, chelates cationic precursors. This suspension is then reacted with a glycol to form organic ester monomers, which polymerize with the addition of heat to form a sol of homogeneously distributed metal ions in an organic matrix. This sol is further heated to produce a resin, which is further heated to remove organic residuals producing the desired stoichiometric oxide powder. Coprecipitation involves the precipitation of individual amorphous metal hydroxides from solutions of metal salts in a solvent. Precipitation is typically achieved by changing the suspension pH or by the addition of heat to evaporate solvent which leads to a supersaturated environment that facilitates precipitation. The metal hydroxides are then calcined to form the desired stoichiometric metal oxide. Hydrothermal synthesis is precipitation with the addition of heat in excess of the boiling point of the solvent in a pressure vessel.
In some instances, precipitation occurs out of a homogeneous solution.[31'34'43'44] The homogeneous solution is produced by keeping the participating metal ions in solution by some complexation chemistry. Precipitation takes place when the complex is broken and large amounts of nuclei are dumped into solution. The complex is usually broken by the addition of a reagent, a change in the solution pH, or heat is added to the homogeneous solution causing a physical breaking of the complex as opposed to the chemical breaking that occurs with the addition of a reagent. The method of precipitation by the addition of heat takes advantage of nucleation/growth ideas to produce ultrafine particles. With the addition of a complexing agent, the critical limit of supersaturation is increased. When the complex is broken, the time available for self-nucleation is increased. After the complex is broken at temperature, the temperature is decreased. When the suspension temperature reaches the point at which the complex is stable again, any metal ions present in solution are recomplexed and thus not available for growth after self-nucleation. As a consequence, no growth occurs because of the decrease in the time available for growth and the resulting particles are ultrafine in size.
The choice of complexing agent is made based on several considerations. First, the complexing agent needs to form a stable complex with the metal ions over a wide range of pH values. Second, a complexing agent is chosen such that it will impart some sort of surface passivation to the precipitated particles to prevent aggregation of the as-synthesized particles. The surface passivation or colloid protection of the particles by the complexing agent can be accomplished by electrostatic repulsion, steric repulsion, or both occurring simultaneously. Beyond those two considerations, a complexing agent with simple chemistry (i.e., requiring least amount of reagents, time forformation of homogeneous solution, lowest cost, etc.) is desirable for ease in preparing solutions and for future processing steps. Complexing agents have been studied in detail for many years.[45-49] Intorre and Martell[45-47] had a series of papers in which they studied complexes for zirconium in aqueous solution. The complexing agents were based on structures having multidentate ligands.
Recent research has been performed on the aqueous synthesis of sub-10-nm Y-TZP using a metal ligand approach.[42] In this study by Kimel, sub-10-nm 1.7 mole percent yttria tetragonally stabilized zirconia (1.7Y-TZP) powder was produced hydrothermally by precipitation from homogeneous solution. Bicine was used as the metal ligand complexing agent. The powder was precipitated after hydrothermal treatment of the homogeneous solution at 200°C for 8 hr. This treatment yielded 2-wt.% solids with a nominal particle size of 7-8 nm. The as-precipitated powder was crystalline and did not require further calcinations. In addition, it was found that the bicine provided a form of surface protection that would allow for dispersion of the as-precipitated powder.
WET PROCESSING OF NANOSIZED PARTICLES
To pursue wet processing techniques for the fabrication of bulk ceramics from nanoparticles, obtaining well-dispersed nanoparticles suspensions is essential. However, obtaining well-dispersed nanoparticle suspensions proves to be a mountainous challenge. This challenge is primarily because of the fact that as particle size decreases, surface area increases. In nature, a system will evolve to the lowest energy state, so too it is with nanoparticles. With an increase in surface area generally comes an increase in surface energy, thus nature will try to lower the energy of a nanoparticles system in a way that will be detrimental to developing a well-dispersed suspension for wet processing.
The detrimental result of lowering the energy of a nanoparticle system will be agglomeration, which effectively increases the particle size and lowers the surface area. Generally, agglomeration will occur by the collision of two particles in suspension. Particles collide via random motion caused by thermal energy present in the system. Upon collision, two scenarios can occur—the particles can stick together or the particles can repel each other. Which scenario occurs depends on the interaction energy between the two particles. The theory of the interaction energy between two-particle double layers is described by Derjaguin, Landau, Verwey, and Overbeek (DLVO).[50,51] The DLVO theory predicts the interaction of two-particle double layers by accounting for the repulsive forces present. The presence of repulsive forcesleads to colloidal stability as these forces prevent particles from sticking together.
In terms of colloidal stability, there are generally three ways in which a colloid can be stabilized.[52,53] The three general types of colloid stability are known as steric, electrostatic, and electrosteric. The third type of stability, electrosteric, is a combination of both electrostatic and steric stabilization. In the realm of polymeric stabilization of colloids, electrosteric stabilization is also associated with the term protective or association colloid. The coinage of the term of ”protective colloid” is given to Zsigmondy, who conducted experiments with gold colloids and gelatin around the turn of the 20th century.[54] The idea of protective or association colloid has been discussed thoroughly in the literature.[54-63] Some of the benefits of this type of protection are insensitivity to electrolyte concentration, applicability in both aqueous and nonaqueous systems, and effectiveness in a large range of solid loading. In a research by Kimel,[64] it was found that complexing agent, bicine, formed a protective colloid around the as-precipitated 1.7Y-TZP powder. The 1.7Y-TZP powder was dispersed, recovered, and concentrated to a 50-wt.% (14 vol.%) aqueous suspension by centrifugation and ultracentrifugation. The bicine was found to flocculate the powder but prevents particle to particle contact. The flocculation was reversed by displacing the bicine on the surface of the 1.7Y-TZP powder with the oxalate anion by washing the suspension in an oxalic acid solution using centrifugation. Centrifugation then allowed for concentration of the aqueous suspension by forming a pellet, followed by redispersion in a smaller volume of an oxalic acid solution.
Once a well-dispersed suspension is in place, a forming technique needs to be chosen. A majority of ceramic components made from powders containing a nominal particle size of 1 mm or greater are generally formed using dry-pressing techniques such as uniaxial or isostatic compression. However, use of these techniques with nanosized powders is inherently difficult because of consolidation and energy issues.[4,65-67] Nanosized powders are significantly affected by hydration forces which lead to aggregation which, in turn, can lead to the problems of poor powder flow and poor packing of the powder particles in the die. The high surface area associated with nanosized powders can lead to friction problems along the die and also lead to poor flow and powder packing. These problems also contribute to the need for forces larger than the capabilities of most conventional presses and also greatly inhibit the ability to make large bulk samples.
Wet processing techniques avoid the problems of dry forming techniques by keeping the powder in a suspension. Using a well-dispersed suspension circumvents the issues of powder flow and powder packing, leaving only the problem of consolidation to overcome. Consolidationduring wet processing can be addressed by drying techniques and/or the application of a load.
Drying without application of a load is a technique used in evaporation, sol-gel drying, and slip casting. Evaporation has problems as applied to nanoparticles because of agglomeration and phase separation while driving off solvent with the application of heat.[68,69] Evaporation without stringent control of heating can also lead to the development of drying stresses in the body which can result in internal defects and cracking. Sol-gel drying is similar to evaporation in terms of producing drying stresses because of the large amount of solvent that has to be removed for consolidation.1-70-1 As a consequence, sol-gel drying is typically only applied to bodies that are thin and have low dimensionality such as thin films and fibers.
Slip casting is performed by placing a suspension into a porous mold, where drying and consolidation then take place by water being removed from the suspension into the porous mold via capillarity forces resulting from the mold having a finer pore structure than the suspension body.[52,53] However, suspensions of well-dispersed nano-sized particles will have nanosized pore structures which will result in capillarity forces much greater than those found in a typical mold material. Thus there will be no driving force for drying of the nanosize particle body, and as a result, poor consolidation and low green densities will be obtained.
Wet processing techniques using the application of force for consolidation include centrifugation and filter pressing. Centrifugation uses centrifugal forces to pack powder particles by pulling them out of suspension. Submicron alumina powders have been consolidated in this manner as well as being used in this research for recovery of the well-dispersed as-synthesized Y-TZP powder.[70- However, a wide particle size distribution can lead to mass separation in the final compact and conventionally, centrifuge tubes are limited in size, thus limiting the size of the bulk body to be made.
Filter pressing consolidates the powder by pressing the suspension into the bottom of a die where a membrane is located. The membrane allows the water to pass through leaving behind a consolidated powder compact. Because the powder is in suspension, friction between the die walls and the powder is negated and as such, much lower pressures are needed for compaction as compared with dry pressing. Most studies on filter pressing in literature were performed on alumina powders.[65'68'69'71-74] While many of these studies resulted in well-packed green bodies, none of the studies was performed with nanosized powders on the order of 20 nm or less as is the case with this thesis work. Also, while the use of zirconia was discussed, there is a lack of literature on drying and sintering of such as-pressed pellets. Furthermore, cracking issues associatedwith drying filter-pressed pellets have not been satisfactorily addressed. Despite the lack of literature and the drying issues, filter pressing provides an excellent opportunity for forming bulk bodies from nanosized parti-cles.[75] Transparent pellets of 1.7Y-TZP with a nominal thickness of 2-3 mm were produced by filter pressing by Kimel.[76] The green pellets were found to have a 48% theoretical density and average grain size of 18 nm after drying to 120°C.
CONCLUSION
A goal of aqueous processing of nanosized Y-TZP powders is the production of bulk Y-TZP ceramics. To accomplish this goal, several issues will be addressed. First is determining the nature of aqueous degradation of Y-TZP and finding a method for the controlling degradation such that it becomes a nonissue. Surface passivation provides a mechanism by which such control can be found. However, as particle sizes decrease below 10 nm, conventional dispersants will need to be replaced by much smaller organic molecules. Once familiarity with the degradation issues is gained, a suitable synthesis technique to produce nanosized particles and allow for future aqueous processing of the nanosized particles must be developed. From the literature review, precipitation from homogeneous solution is a promising technique that provides for control over particle size as well as the surface chemistry of the particles. Upon the production of nanosized Y-TZP, recovery and dispersion of the powder must take place. From the literature review of processing techniques, the production of a well-dispersed suspension is imperative for the use of wet processing techniques to consolidate the nanosized Y-TZP powder into a bulk Y-TZP ceramic body.
