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.     

Microstructural development of a ZrO2 Naβ″-Al2O3 composite

Microstructural development in Na-Al₂O₃ containing 15 vol% ZrO₂ particles is described. Intergranular ZrO₂ particles inhibit abnormal grain growth of the Na-Al₂O₃. The growth of the Na-Al₂O₃ grains and the intergranular ZrO₂ followed a cubic time law. Direct particle coalescence consisting of encounter and spheroidizing processes was found to be the basic growth mechanism for the intergranular ZrO₂     

Effects of heteroflocculation of powders on mechanical properties of zirconia alumina composites

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (>50 vol% solids) had ZrO₂ content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabilized, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the non-transformable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to 99% theoretical density, the elastic properties, flexure strength and fracture toughness were examined at room temperature. Dynamic moduli of elasticity of fully deagglomerated compositions did not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made from submicron powders and with low content of dispersed phase (less than 10 to 20 vol %), the strength increased with increasing ZrO₂ content to a maximum of 1 GPa, irrespective of the degree of stabilization of t-ZrO₂. With increasing content of the dispersed phase (> 20 vol%), heteroflocculation of powder mixtures during wet-processing led to the formation of ZrO₂ grain clusters of increasing size. Residual tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO₂ phase transformation in final composites with transformable t-ZrO₂, but also led to lateral microcracking of ZrO₂/Al₂O₃ interfaces. This enhanced fracture toughness, but at larger ZrO₂ contents the flexure strength always decreased due to intensive microcracking, both radial and lateral. The important microstructural aspects of strengthening and toughening mechanisms in ZTA composites are related in discussion to the effects of heteroflocculation of powder mixtures during wet-processing.     

Low temperature phase equilibria and ordering in the ZrO2-rich region of the system ZrO2-CaO

From co-precipitated powder samples, the solid state reactions occurring between room temperature and 1500° C in the ZrO₂-CaO system have been studied. At low temperatures, compositions containing < 25 mol% CaO show a complex picture of phase transformation and ordering in the system. From the obtained results the following singular reactions have been established. (i) Tetragonal zirconia solid solution decomposes eutectoidally at 7 mol% CaO and 1048 ± 4° C into monoclinic zirconia solid solution and calcium zirconate (CZ). (ii) Cubic zirconia solid solution undergoes a eutectoidal decomposition at 17.5 mol% Cao and 1080 ±20° C into tetragonal solid solution + calcium zirconate. (iii) The monoclinic ordered phase, CaZr₄O₉ (₁), ), undergoes an order-disorder transformation into cubic zirconia solid solution at 1232 ± 5° C. (iv) Cubic zirconia solid solution undergoes a eutectoidal decomposition into two ordered phases, ₁ + ₂ at 21 mol% CaO and 1200 ± 10°C. (v) Hexagonal ordered phase Ca₆Zr₁₉O₄₄ (₂) decomposes peritectoidally into cubic zirconia solid solution + calcium zirconate at 1360 ± 10° C. The two ordered phases ₁ and ₂ seem to be unstable below 1100° C. By using DTA, X-ray diffraction and SEM techniques, the extent of the tetragonal and cubic zirconia solid solution fields have been established. From the above experimental results a new tentative phase diagram is given for the ZrO₂-rich region of the system, ZrO₂-CaO.     

Mechanical properties of (Y-TZP) — alumina-silicon carbide nanocomposites and the phase stability of Y-TZP particles in it

Al₂O₃-SiC-ZrO₂ composites were investigated to obtain a better understanding of the effect of SiC particles and the stress-induced transformation of Y-TZP on its mechanical properties. The Al₂O₃-SiC-ZrO₂ composites were fabricated by hot pressing using -Al₂O₃, SiC and ZrO₂ mixtures. Fracture toughness and strength of Al₂O₃ were greatly improved by incorporating SiC and ZrO₂ particles which were located mainly inside and between Al₂O₃ grains, respectively. The toughening and strengthening mechanism of these composites and the phase stability of the tetragonal ZrO₂ in the composites before and after high-temperature annealing were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. It was observed that there is a critical volume fraction of zirconia, above which the phase stability of the tetragonal zirconia increases, despite the grain growth of the zirconia. It is considered that another phenomenon, the residual stresses, affect the phase stability of the tetragonal zirconia. To remove the residual stresses the composites were annealed at 1100 °C. After slow cooling, the tetragonal zirconia became very unstable, especially in samples with the highest fabrication temperature and increasing zirconia content. Even quenching from 1100 °C caused an increase in the monoclinic phase of these samples.     

Interfacial reactions between Ti-6Al-4V alloy and zirconia mold during casting

The interface of Ti-6Al-4V casting and ZrO₂ mold with silica binder was investigated by using electron probe microanalyses (EPMA), X-ray diffraction (XRD), and analytical transmission electron microscope (TEM). The interfacial reactions were proceeded by the penetration of liquid titanium through open pores near the mold surface. The metal side consisted of an -phase layer on the top of the typical + two-phase substrate. In the ceramic side, zirconia was reduced by titanium to form oxygen-deficient zirconia ZrO_2–x and evolved a gaseous phase (presumably oxygen). The SiO₂ binder, dissolved in the ZrO₂ mold, could react with titanium to form Ti₅Si₃ in the metal side. Meanwhile, titanium could transform to titanium suboxides Ti_yO (y 2) and the lower phase boundary of cubic ZrO_2–x was shifted to ZrO_1.76. Some amount of the stabilizer CaO, dissolved in Ti along with ZrO₂, could react with Ti(O) to form Ca₃Ti₂O₇ and CaAl₄O₇ in the reaction zone.     

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.     

Fast firing of reaction-bonded aluminium oxide RBAO composites

Al₂O₃/ZrO₂ composites have been prepared by fast firing of oxidized Al/Al₂O₃/ZrO₂ precursors produced by the reaction-bonded aluminium oxide (RBAO) technique. This fabrication route results in high-strength ceramics at relatively low densities. For example, after fast firing for 20 min at 1550 °C, RBAO containing 20 vol% ZrO₂ shows four-point bending strengths of > 600 MPa at a density of 95% theoretical which is comparable to conventionally sintered RBAO.     

Phase Composition of Films Deposited by ZrB2 RF Magnetron Sputtering

X-ray diffraction and electron-microscopic studies demonstrate that the conditions of ZrB₂ rf magnetron sputtering (primarily, the argon pressure) have a significant effect on the thickness, phase composition, and structure of the growing films. The films deposited on silicon substrates at low argon pressures (0.15–0.18 Pa) consist of zirconium diboride and a very thin zirconia layer. At higher argon pressures, from 0.21 to 0.42 Pa, the film thickness is larger, and the film is composed of four layers: ZrB₂/ZrB/ZrO₂/B₂O₃. Increasing the argon pressure to 0.47 Pa, reduces the deposition rate and the thickness of the zirconia layer. The resultant films contain neither ZrB nor B₂O₃. At still higher argon pressures, up to 0.65 Pa, film thickness continues to decrease because of the reduction in the thickness of the ZrB₂ layer. The substrate temperature influences the structural perfection of the growing films. Other sputtering parameters influence the argon pressure and, accordingly, the phase composition of the films. At deposition temperatures of 60–70°C, there are certain orientational relationships between the films and single-crystal substrates. On glass-ceramic substrates, the deposition rate is substantially faster, and the deposits consist of randomly oriented crystallites.     

Duplex spinel-ZrO2 ceramics

Duplex spinel-ZrO₂ ceramic composites were produced by an emulsion-hot kerosene drying technique. The sintered duplex spinel-ZrO₂ ceramics which had the composition of 55 wt% Al₂O₃-20 wt% ZrO₂-25 wt% MgO, consisted of a spinel matrix, whose grain size was in the range of 1.5 to 2.0 m, and uniformly dispersed zirconia agglomerates having grain sizes ranging from 1.0 to 2.0 m. Zirconia agglomerates began to appear at a temperature of 1500 °C and the duplex spinel-ZrO₂ structure was formed with the weight ratio of Al₂O₃/MgO being within 1.67 to 2.20 and the amount of ZrO₂ addition being within 5 to 25 wt %. The relative density, fracture toughness, flexural strength, and critical temperature difference of the spinel-ZrO₂ composite were 97.8%, 1.98 MPam^0.5, 390 MPa, and 275 °C, respectively.     

Strength improvement in beta″ alumina by incorporation of zirconia

Beta alumina ceramic electrolytes for use in Na/S batteries are inherently weaker than most engineering ceramics due to the presence of weakly-bonded conduction planes in the crystal structure and to difficulties in controlling grain growth during firing. Substantial improvement in microstructural control is obtained by incorporation of monoclinic zirconia (m-ZrO₂) or partially stabilized zirconia (PSZ) resulting in increases in strength and fracture toughness to around 350 MPa and 4 MPam^1/2, respectively. PSZ may adversely influence the electrical resistivity of the ceramic owing to the presence of impurities. With most zirconia powders a high level of retention of tetragonal zirconia (t-ZrO₂) is obtained at levels of addition up to 15% by weight ZrO₂. At these levels ZrO₂/-Al₂O₃ ceramics show low resistivity and stable resistance in Na/S cells.     

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO₂-5.2 mol % Y₂O₃ alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 111 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.Graduate Student, Tohoku Univerisy, Sendai, Japan.     

Hydroxylapatite-zirconia composites: Thermal stability of phases and sinterability as related to the CaO-ZrO<Subscript>2</Subscript>phase diagram

Composites of hydroxylapatite (HA) with pure zirconia, and 3 and 8% Y₂O₃ in zirconia, were pressure-less sintered at temperatures from 900 to 1300C, and hot-pressed at 1200C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000,C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200,C, CaZrO₃ was formed. These reactions and transformations were interpreted in terms of the ZrO₂-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ sintered at 1300,C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon air-sintering.     

Subcritical crack growth in partially stabilized zirconia(PSZ)

Studies on the subcritical crack growth behaviour of partially stabilized zirconia (ZrO₂-I, 5 to 10 vol% tetragonal phase; ZrO₂-II, 35%) were carried out using the double-torsion technique and data from the dynamic fatigue of unnotched bend specimens. The results of subcritical crack growth support the model of stress induced transformation from the tetragonal to monoclinic modification. Differences in the crack growth parameter n (as-received condition) using the double-torsion technique or bend specimens may be explained by the special nature of subcritical crack extension at stressed surfaces for these different specimen types. The log v-log K _i plot of ZrO₂-I using the double torsion technique shows a plateau of constant velocity, which has to be attributed to a tetragonal-monoclinic transformation. After annealing (1500° C, 5 h) the plateau has vanished and the n value is comparable to bend test in an as-received condition.     

Scanning electron microscope observations of polymorphic transitions occurring on a 12CaO · 7Al2O3 surface under electron beam action

The compound 12CaO · 7Al₂O₃ has been widely studied for a long time. There are many controversies concerning its polymorphism and other properties in the literature. The variable conditions of synthesis, i.e. Temperature, atmosphere, as well as the different cooling rate conditions allowed us to obtain an optically anisotropic polymorphic modification of the 12CaO · 7Al₂O₃ in vacuum. X-ray investigation of that anisotropic phase indicated that the d-values (in nm) of the diffraction maxima did not correspond to that of the isotropic cubic phase presented in the literature. The polymorphic transition temperature was determined and reported earlier. SEM observations shed new light on the 12CaO · 7Al₂O₃ polymorphism problem. A sample of CaO/Al₂O₃ of weight ratio 0.94, which has been synthesized and cooled in air, was examined under the scanning electron microscope. The sample was exposed to electron beam action for 20 min. In the dark spots formed during this operation, bulges appeared which then enlarged and cracked. On the surface a transverse crack was visible. This phenomenon could also be observed in other areas of the sample surface during exposure. This phenomenon is connected presumably with the polymorphic transition of the 12CaO · 7Al₂O₃ phase.     

A new tentative phase equilibrium diagram for the ZrO2-CeO2 system in air

The phase relationships over a wide range of temperature and compositions in the ZrO₂-CeO₂ system have been reinvestigated. From DTA results, thermal expansion measurements andK _IC determinations it was established that additions of CeO₂ to ZrO₂ decreases the monoclinic to tetragonal ZrO₂ transition temperature, from 990 ° C to 150 50 ° C, and an invariant eutectoid point at approximately 15 mol% CeO₂ exists. The extent of the different single- and two-phase fields were determined with precise lattice parameter measurements on quenched samples. Evidence for the existence of a binary compound Ce₂Zr₃O₁₀ (ø-phase) was obtained by X-ray diffraction. The ø-phase was stable below approximately 800 ° C, above which it decomposes into tetragonal zirconia + fluorite ceria solid solutions. Taking into account the polymorphic tetragonal-cubic transition and the narrowness of the two-phase tetragonal zirconia + fluorite ceria field above 2000 ° C, the existence of a new invariant eutectoid point was assumed, in which the metastable fluorite zirconia solid solution decomposes into tetragonal zirconia + fluorite ceria solid solutions. From the results obtained, the phase diagram also incorporates a eutectic point located at approximately 2300 ° C and 24 mol % CeO₂.     

Microstructural and chemical stability of Y-ZrO2 reinforced β″-alumina in molten sodium sulfide and sulfur

The stability of -alumina reinforced with 10 vol% of tetragonal partially stabilized 3 mol% Y₂O₃-ZrO₂ (3Y-ZrO₂) and with 10 vol% of cubic 8 mol% Y₂O₃-ZrO₂ (8Y-ZrO₂) in molten sulfur or molten Na₂S₄ has been examined using scanning electron microscopy (SEM) X-ray diffraction (XRD) and electron probe microanalysis (EPMA) both before and after immersion at 350 °C. Tetragonal partially stabilized 3 mol % Y₂O₃-ZrO₂ was destabilized when reinforced into -alumina and immersed in molten Na₂S₄. Destabilization without incorporation into -alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of -alumina enhanced zirconia destabilization in that -alumina can react with the molten corrodants to form corrosion products which are known corrosion agents for the leaching of Y₂O₃ from partially stabilized 3Y-ZrO₂. From XRD analyses, changing from partially stabilized 3Y-ZrO₂ to cubic 8Y-ZrO₂ in the composite increased resistance against phase destabilization. EPMA analyses revealed that the depletion was almost halted for cubic 8Y-ZrO₂ suggesting that the change in the zirconia phase used had reduced the chemical reactivity between Y₂O₃ and the corrodants. In order to avoid depletion destabilization of zirconia in -alumina, corrosion resistance can be increased by reducing chemical reactivity by using fully stabilizing zirconia. In addition, partially stabilized tetragonal zirconia may still be considered for use if a less reactive stabilizer such as CeO₂ is used.     

Microstructure and properties of a composite system for dental applications composed of glass-ceramics in the SiO2–Li2O–ZrO2–P2O5 system and ZrO2-ceramic (TZP)

The objective of the study was to develop a biocompatible composite system which was composed of TZP-ceramic (tetragonal zirconia polycrystals, ZrO₂ stabilized with 3 mol% Y₂O₃) and two glass-ceramics of the SiO₂–Li₂O–ZrO₂–P₂O₅ type. The metal-free composite system would satisfy the translucency, the biocompatibility and the strength requirements of dentistry. The two glass-ceramics of the SiO₂–Li₂O–ZrO₂–P₂O₅ type with a content of 15 and 20 wt% ZrO₂ respectively, were chemically and physically adapted to TZP-ceramic. The glass-ceramics were used as a dentin buildup material. The TZP-ceramic had the function of a root post. The shape of the post was cylindrical with a conical tip. The composite system was easy to process through viscous flow of the glass-ceramic at 900 and 1000°C, respectively. The microstructure and the mechanical properties of two glass-ceramics of the SiO₂–Li₂O–ZrO₂–P₂O₅ type were examined therefore.     

Autostresses induced by point defects in sintering phenomena: effect on mass transport and sintering stress

States of anelastic strain can be associated with excess concentrations of point defects, as generated by mass transport in a sintering compact. Correspondingly, states of mechanical long-range self-equilibrated stresses (autostresses) can be produced. The relationships between anelastic strain and autostresses have been derived for a two-particle model. A generalized relation between chemical potentials and autostresses, including surface stresses, is provided, which allows derivation of local thermodynamic driving forces for mass transport. The concept of equivalent external sintering stress, assumed to be the driving force for the global densification process, is shown to correspond, approximately, to the material- and history dependent normal autostress component acting on the neck cross-sections. Predictions made from the model provide a new interpretation of experimental observations of the effect of gaseous phases, such as H₂O and CO₂, on the sintering of MgO and CaO powders.