Preparation of nanocrystalline YSZ powders by the plasma technique

A plasma synthesis method has been devised to produce nanosize YSZ powders with various yttria contents. The powders are synthesized by introducing a mixture of coarse-grained zirconia and yttria into an r.f. inductively coupled plasma flame. The average particle size of the as-prepared powders is in the range 20–40 nm and the specific surface area is 18–50 m²g^–1. The phase and granulometric composition of the produced powders depend on the degree of evaporation of raw powders, reagent concentration in the gas flow and quenching rate, and on the content of Y₂O₃. Up to 5.5 mol% yttria, the major phase of nanosize powders is tetragonal ZrO₂, mostly as the non-transformable (t) form. For yttria contents higher than 6 mol%, the major phase is cubic ZrO₂.     

Polycrystalline t′-ZrO2(Ln2O3) formed by displacive transformations

ZrO₂ (3 mol% Y₂O₃) tetragonal and t-ceramics (displacively formed ceramics) were compared with ZrO₂ ceramics (3 mol% Ln₂O₃, where Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Er, or Yb) processed in an identical manner. Sintering at 1500 °C for 2 h produced mainly tetragonal polytypes for the dopants with smaller ionic radii than Dy(i.e., Er, Y and Yb) but when ZrO₂ was reaction sintered with dopants with larger ionic radii excessive monoclinic phase transformation and associated microcracking resulted. High-temperature annealing in the cubic stability regime and rapid cooling through the tetragonal stability regime was used to fabricate t-composites of ZrO₂ doped with Y, Yb, Er or Dy. Room-temperature fracture toughness and strength values are explained on the basis of a ferroelastic-cubic-to-tetragonal transformation. The domain structure was viewed by transmission optical microscopy (TOM) or transmission electron microscopy (TEM).     

Effect of NiO dissolution on the microstructural development of t′-ZrO2 with 5 mol% Y2O3

The ageing (1300 °C for 1–300 h) dependence of the microstructure of the metastable tetragonal (t-) ZrO₂ (plasma-sprayed Y-PSZ with 5 mol% Y₂O₃) in the sintered (1300 °C, 5 h) t-ZrO₂/NiO (10 mol%) composite was studied by transmission electron microscopy. Short-term (10 h) ageing caused incomplete decomposition of t-ZrO₂ into tweed assemblages containing t-ZrO₂ relics and tetragonal (t-) ZrO₂ precipitates in the cubic (c-) ZrO₂ matrix. The t-ZrO₂ relics retained anti-phase domain boundary (APB) and primary deformation twins, but no secondary deformation twins. Further ageing (e.g. up to 100 or 300 h) caused growth of the twin variants of t-ZrO₂ with {101} habit plane. The large t-ZrO₂ variants transformed directly into m-ZrO₂ twins, but not into t-ZrO₂ colonies. In addition to the relics in the aged samples, t-ZrO₂ with APB may also form locally from a solute-redistributed c-ZrO₂ by slow cooling.     

Formation of diffusionlessly transformed tetragonal phases in rapid quenching of ZrO2-Y2O3 melts

ZrO₂ polycrystals, partially stabilized by 2 to 7 mol% Y₂O₃, were arc-melted and rapidly quenched using an arc-imaging furnace with a hammer-anvil unit. Some of the specimens were further annealed at 1700° C for 3 h in air. The phases and the microstructures of these ZrO₂-Y₂O₃ polycrystals were examined through X-ray diffraction and transmission electron microscopy. Special emphasis was placed upon the examination of the microstructure of the metastable tetragonal phase (t phase) which was formed by a diffusionless transformation of the high-temperature cubic phase. It was found that the t phase exhibits a twinned and mosaic structure made of alternating layers of twin-related variants. A comparison of the present experimental results with other related works has also been made.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Effect of zirconia (3 mol% yttria) additive on mechanical properties and structure of alumina ceramics

The effects of ZrO₂-3 mol% Y₂O₃ additives containing 7.3, 15, 23.3 and 32 vol% of ZrO₂ on _f, K _IC, H _v and the microstructure of hot-pressed alumina-based ceramics were investigated. The presence of the m-, t- and t-ZrO₂ phases was discovered by using X-ray diffraction and transmission electron microscopy. An inhomogeneous distribution of Y₂O₃ in the ZrO₂ grains was observed. The variation of the mechanical properties of the ceramics is explained by the influence of different toughening mechanisms and by a change in the structure of the material.     

Sintering ofβ-sialon with 5mol% Y2O3-ZrO2 additives

The sintering behaviour ofβ-Sialon composition powders with 5 mol% Y₂O₃-ZrO₂ additives at 1750°C for 1.5 h in nitrogen or argon atmospheres was studied.β-Sialon composition powders could be pressureless-sintered to about 93% theoretical density by the addition of 5 wt% 5 mol% Y₂O₃-ZrO₂. By HIPing the pressureless-sintered bodies the density was increased to higher than 98% theoretical density, and uniform submicrometre ZrO₂ particles were homogeneously dispersed in theβ-Sialon matrix, resulting in an increase of fracture toughness,K _1c, from 5.1 to about 5.7 MN m^−1.5. Increasing the amount of tetragonal ZrO₂ transformable to monoclinic phase in theβ-Sialon matrix increasedK _1c.     

The microstructure and mechanical properties of yttria-stabilized zirconia prepared by arc-melting

The microstructure of ZrO₂-Y₂O₃ alloys prepared by arc-melting was examined mainly by electron microscopy. It was found that the microstructure changed markedly with yttria content between 0 and 8·7 mol%. Pure zirconia was a single monoclinic phase, while ZrO₂-8·7 mol% Y₂O₃ alloy was single cubic phase as expected from ZrO₂-Y₂O₃ phase diagram. Tetragonal phase was found in alloys with 1 to 6 mol% Y₂O₃ together with monoclinic or cubic phase. The tetragonal phase found in present alloys normally had a lenticular shape with a length 1 to 5m and a width 0.1 to 0.3m, which is much larger than that formed by annealing. The phase with a herring-bone appearance was found in alloys with Y₂O₃ between 2 and 3 mol%, which was recognized to be a metastable rhombohedral phase. The structure of the present alloys is likely to be formed by martensitic or bainitic transformation during fairly rapid cooling from the melt temperature. The change in hardness and toughness with yttria content of the alloys is discussed on the basis of microstructural observations.     

Nanocrystalline zirconia-yttria system–a Raman study

Nano sized zirconia (ZrO₂) powders doped with different amount of yttria (Y₂O₃) (3, 5 and 8 mol%) were prepared through coprecipitation method. The crystallite size estimated from the X-ray peak broadening is around 10 nm. Phase identification was carried out using XRD and Raman spectroscopy. Raman spectroscopic study of the synthesized materials show clear evidence of the presence of single phase cubic structure in the case of 8 mol% Y₂O₃ doped fully stabilized zirconia (8Y-FSZ); tetragonal phase in the case of zirconia doped with 3 mol% Y₂O₃ (3Y-TZP-tetragonal zirconia polycrystal) and a mixture of cubic and tetragonal phases for 5 mol% Y₂O₃ doped partially stabilized zirconia (5Y-PSZ). Raman technique is therefore an effective tool to distinguish the phases present in the calcined nano sized powders of zirconia.     

Preparation of monodispersed Y-doped ZrO2 powders

3 mol% Y-doped ZrO₂ powders prepared by the controlled hydrolysis of metal alkoxides were monodispersed and grown to 0.5m after 5 h ageing. The as-prepared powder was amorphous and hydrate but transformed into a tetragonal single phase by heating. Furthermore, the Y₂O₃ concentration of each particle was almost the same in all particles. The synthesis conditions such as ageing time, ageing temperature and water concentration greatly affected the particle morphology. The refluxing of the alkoxide solution was particularly necessary to prepare the monodispersed particles. On the basis of the variation of size distribution with ageing time, the mechanism of particle growth was discussed.     

Ageing behaviour of t′-phase in a hot-pressed ZrO2(4 mol% Y2O3) ceramic

The ageing behaviour of unequilibrium tetragonal (t) phase and its resultant effect on the mechanical properties of hot-pressed ZrO₂(4 mol% Y₂O₃) ceramic have been investigated by means of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Experimental results show that t-phase which is the product of diffusionless transformation from cubic (c) phase during rapid cooling after sintering is unstable when aged in a temperature range of 1400–1600 °C for up to 80 h in that it decomposes diffusionally into equilibrium tetragonal(t) phase and c-phase. Yttria contents of phases formed during decomposition are basically in agreement with those indicated by phase diagram. The stability of t-phase characterized by the existence of anti-phase domain microstructure under the microscopic dark field image is significantly associated with the tetragonality(c/a) measured by XRD and the larger the tetragonality, the more unstable the t-phase. Metastable precipitates of t-phase are triggered by applied stress to transform to monoclinic (m) phase during which the fracture toughness is enhanced and transformability of t-phase is critically dependent upon the solute content as well as size. It is found that when t- and m-phase coexist with adequate fractions of c- and t-phase, the fracture toughness of the aged specimen demonstrates a peak value that moves to shorter ageing times with increasing temperature while the Vickers hardness decreases monotonically with ageing time regardless of ageing temperature due to grain growth.     

Yttria-stabilized hafnia-zirconia thermal barrier coatings: The influence of hafnia addition on TBC structure and high-temperature behaviour

The search for more reliable and durable thermal barrier systems is a key factor for future aircraft turbine engines success. Hafnia is therefore an attractive ceramic component due to its similarity to zirconia and its elevated structural transformation temperatures. We report here structural and thermomechanical investigations of various plasma-sprayed coatings composed of ZrO₂+x mol% HfO₂ (x=0, 25, 50 and 100), partially stabilized by 4.53 mol% yttria. X-ray diffraction studies show that, a metastable, non-transformable, high yttrium content, tetragonal solid solution is the only phase observed on the as-sprayed samples. This phase is crystallographically equivalent to the t phase described for classical yttrium-partially stabilized zirconia (Y-PSZ) thermal barrier coatings (TBCs). Upon high-temperature annealing in air (T=1200C), however, the return of this t phase to equilibrium differs from the classical tt+c reaction. According to literature data, reactions of the type tt+c+m should prevail at the highest hafnia contents (x50). Indeed, important quantities of monoclinic phase are accordingly being observed upon cooling. Thermal cycling of TBC samples in air has been performed at 1100C. The Young's modulus of the ceramic coating, which progressively increases when hafnia is substituted for zirconia, has a strong influence on TBC thermomechanical resistance.     

Sintering and compensation effect of donor- and acceptor-codoped 3mol% Y2O3-ZrO2

Addition of 0.15–0.5 mol% acceptor oxide, Al₂O₃, to 3 mol% Y₂O₃-ZrO₂ results in enhanced densification at 1350 °C. The enhancement is accounted for by a liquid phase sintering mechanism. The addition of donor oxide, Ta₂O₅, of 0.15–2.5 mol % at 1300–1600 °C results in the destabilization of tetragonal (t-) phase and the decrease of final density in 3 mol% Y₂O₃-TZP (tetragonal ZrO₂ polycrystals). X-ray diffractometry (XRD) reveals that the Ta₂O₅-added 3 mol% Y₂O₃-ZrO₂ contains monoclinic (m-) ZrO₂ and a second phase of Ta₂Zr₆O₁₇. The decreasing in final density is attributed to the increase of m-ZrO₂ content. Complete destabilization of t-ZrO₂ to m-ZrO₂ in samples added with 2.5 mol% Ta₂O₅ is interpreted by the compensation effect based on donor- and acceptor-codoping defect chemistry.     

Preparation of tetragonal ZrO2-Gd2O3 powders

ZrO₂-Gd₂O₃ alloys containing 2,3,5 and 8 mol.% Gd₂O₃ have been prepared by mixed oxide (MO), hybrid sol-gel (SG), and co-precipitation (CP) routes. No tetragonal (t) phase is retained in the MO method, while 100% t phase is obtained in the calcined CP samples; the SG method leads to only partial stabilization of the t phase. Washing of the CP powders with propan-2-ol leads to unagglomerated powders with increased specific surface area (145 versus 89 m²g^–1) and sintered density (98% versus 79%). Cubic and t phase also appear on sintering the samples with >2 mol.% Gd₂O₃.     

The ZrO2-rich region of the ZrO2-MgO system

The solubility limits of MgO in tetragonal zirconia were studied by combining the differential thermal analysis data and X-ray disappearing phase method. From these experiments a eutectoid reaction, tetragonal ZrO₂ solid solution monoclinic ZrO₂ solid solution + MgO, at 1120±10 °C and 1.6±0.2 mol% MgO was established. The solubility of MgO in tetragonal ZrO₂ diminished as the temperature increased, and at 1700 °C the solubility was less than 0.5 mol% MgO. The extent of the cubic zirconia solid solution single field was determined by using precise lattice parameter measurements and SEM observations. In this way an invariant eutectoid point, cubic ZrO₂ solid solution tetragonal ZrO₂ solid solution + MgO, was located at 1420±10 °C and 14.8±0.5 mol% MgO.     

Dispersion of ZrO2 particles in aqueous suspensions by ammonium polyacrylate

The effects of poly anionic-electrolyte (ammonium polyacrylate, PAA) as a dispersant on two kinds of ZrO₂ (monoclinic and yttria-doped tetragonal zirconia) aqueous suspensions were examined by the measurements of-potential and viscosity, the sedimentation test and the determination of the wet point and flow point of the powders. Additions above 2.5 wt% PAA to zirconia gave a negative high-potential above –30 mV, and then –45 and –30 mV were obtained for monoclinic and tetragonal zirconia above 5 wt% PAA, respectively. A high negative-potential above –30 mV was retained with 5 wt% PAA for a change in pH over a wider range (pH 6 to 10 for monoclinic ZrO₂, 7 to 9 for tetragonal ZrO₂) in comparison to that of ZrO₂ without dispersant. The increase of the-potential resulted in a decrease in the viscosity. The evaluation of dispersion by the sedimentation test was correlated well with the value of-potential and the viscosity of the suspensions. The presence of native positive charge of monoclinic and tetragonal zirconia powders required an excess amount of PAA to attain dispersion of the suspension. There was a small difference in the least amount of PAA required to attain good dispersion between monoclinic and tetragonal ZrO₂. The difference was also indicated by changes of the flow point on PAA addition. Addition of 0.1% PAA to monoclinic ZrO₂ and 0.25 wt% to tetragonal ZrO₂ gave a maximum value of the flow point, whereas the positive-potential fell to zero. Measurement of the flow point was a simple and useful technique for rapid evaluation of a required amount of dispersant for ZrO₂ suspensions.     

Synthesis of monodisperse spherical nanometer ZrO2 (Y2O3) powders via the coupling route of w/o emulsion with urea homogenous precipitation

Using xylol as the oil phase, span-80 as the surfactant, and an aqueous solution containing zirconium (3 mol% Y₂O₃) and urea as the water phase, tetragonal phase ZrO₂ nano-powders have been prepared via the coupling route of w/o emulsion with urea homogenous precipitation. The effects of the zirconium concentration, the reaction temperature and the urea content on the average size of the products have been examined. The as-prepared ZrO₂ powders and the precursor powders were characterized by TGA–DTA, XRD, TEM and BET. Experimental results indicate that ZrO₂ powders prepared via the coupling route of w/o emulsion with urea homogenous precipitation possess some excellent characteristics, such as well-rounded spherical shape and excellent dispersing.     

Formation and sintering of 75 mol% alumina/25 mol% zirconia (2–3.5 mol% yttria) composite powder prepared by the hydrazine method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 25 mol% ZrO₂ have been prepared by the hydrazine method. As-prepared powders are the mixtures of AlO (OH) gel solid solutions and amorphous ZrO₂. The formation process leading to -Al₂O₃/t-ZrO₂ composite powders is investigated. Hot isostatic pressing has been performed for 1 h at 1500 °C under 196 MPa. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed.     

Inhibition of the formation of the B metastable phase in yttrium oxide plasma-spray coatings by the addition of zirconia

Yttrium oxide plasma-spray deposits show a metastable B phase together with the cubic stable form. This B form is prejudicial for thermal applications of these coatings (i.e. weakens mechanical properties, etc.). Attempts have been made to avoid the formation of this metastable phase during the plasma-spraying operation by use of an inhibitor, such as ZrO₂. A large solid solution with cubic C phase was found to form between ZrO₂ and Y₂O₃ (Y_1–xZr_xO_{1.5 + x/2 0.5 – /2}). A method is described for preparing powders suitable for plasma spraying. Several compositions were tested. B phase is no longer formed for ZrO₂ additions as low as x = 0.03. The influence of ZrO₂ addition on Y₂O₃ coatings properties is discussed.     

Phase relationships in the system Si3N4-Y2O3-SiO2

Examination of compositions in the system Si₃N₄-Y₂O₃-SiO₂ using sintered samples revealed the existence of two regions of melting and three silicon yttrium oxynitride phases. The regions of melting occur at 1600° C at high SiO₂ concentrations (13 mol% Si₃N₄ + 19 mol% Y₂O₃ + 68 mol% SiO₂) and at 1650° C at high Y₂O₃ concentrations (25 mol % Si₃N₄ + 75 mol % Y₂O₃). Two ternary phases 4Y₂O₃ ·SiO₂ ·Si₃N₄ and 10Y₂O₃ ·9SiO₂ ·Si₃N₄ and one binary phase Si₃N₄ ·Y₂O₃ were observed. The 4Y₂O₃ ·SiO₂ ·Si₃N₄ phase has a monoclinic structure (a= 11.038 Å, b=10.076 Å, c=7.552 Å, =108° 40) and appears to be isostructural with silicates of the wohlerite cuspidine series. The 10Y₂O₃ ·9SiO₂ ·Si₃N₄ phase has a hexagonal unit cell (a=7.598 Å c=4.908 Å). Features of the Si₃N₄-Y₂O₃-SiO₂ systems are discussed in terms of the role of Y₂O₃ in the hot-pressing of Si₃N₄, and it is suggested that Y₂O₃ promotes a liquid-phase sintering process which incorporates dissolution and precipitation of Si₃N₄ at the solid-liquid interface.Visiting Research Associate at Aerospace Research Laboratories, Wright-Patterson Air Force Base, Ohio 45433, under Contract No. F33615-73-C-4155 when this work was carried out.