Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Phase Relations in the System ZrO2-Y2O3 at Low Y2O3 Contents

Subsolidus phase relations in the low-Y₂O₃ portion of the system ZrO_2-Y₂O₃ were studied using DTA with fired samples and X-ray phase identification and lattice parameter techniques with quenched samples. Approximately 1.5% Y₂O₃ is soluble in monoclinic ZrO₂, a two-phase monoclinic solid solution plus cubic solid solution region exists to ∼7.5% Y₂O₃ below ∼500°C, and a two-phase tetragonal solid solution plus cubic solid solution exists from ∼1.5 to 7.5% Y₂O₃ from ∼500° to ∼1600°C. At higher Y₂O₃ compositions, cubic ZrO₂ solid solution occurs.     

Phase Transformation in Y2O3-Partially-Stabilized ZrO2 Polycrystals of Various Grain Sizes during Low-Temperature Aging in Water

ZrO₂-2 mol% Y₂O₃ crystals with average grain sizes from 0.51 to 0.96 µm were prepared by sintering in air at 1400°C for 2 to 100 h. The tetragonal-to-monoclinic phase transformation associated with the low-temperature degradation was investigated to clarify how the presence of water directly affects the influence of grain size on transformation. The specimens were exposed to water at 80–120°C, a temperature range in which transformation by thermal activation is difficult in the absence of water. Contrary to expectations, this type of low-temperature transformation did not accelerate monotonously with increasing grain size. Instead, the amount of phase transformation first decreased, reaching a constant value, and then increased with increasing grain size. Such interesting results can be explained satisfactorily by the combined influences of grain size on the nucleation process, because of preferential dissolution of yttrium at the grain boundaries, and the intrinsic transformability of Y₂O₃-doped tetragonal ZrO₂ grains.     

Phase Equilibration in ZrO2-Y2O3 Alloys by Liquid-Film Migration

The tetragonal ( t ) and cubic ( c ) ZrO₂ solid solutions in two-phase ZrO₂-8 wt% Y₂O₃ ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y₂O₃ content. The enrichment in Y₂O₃ content of the c -ZrO₂ grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO₂ increases by the nucleation and growth of cap-shaped t -ZrO₂ lenses. The interfaces between the c -ZrO₂ matrix and the growing t -ZrO₂ lenses are semicoherent.     

Grain-Size Distribution in Al2O3─ZrO2 Generated by High-Temperature Annealing

Grain-size distribution in various Al₂O₃─ZrO₂ (2.5 mol% Y₂O₃) ceramics during high-temperature annealing was examined. In alumina-rich alloys, the grain size of major and minor phases was very different, while grain size was almost uniform in zirconia-rich alloys. This difference in grainsize distribution was related to the difference in grain growth rate of the major phase and to the effectiveness of grain-boundary pinning by minor-phase grains.     

Degradation During Aging of Transformation-Toughened ZrO2-Y2O3 Materials at 250°C

The detrimental aging phenomenon observed in ZrO₂-Y₂O₃ materials, which causes tetragonal ZrO₂ to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y₂O₃ and SiO₂ content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO₂ to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)₃. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y₂O₃) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.     

Sintering, Microstructure, Hardness, and Fracture Toughness Behavior of Y2O3-CeO2-ZrO2

A wet-chemical approach is applied to derive fine powders with compositions 11 mol% CeO₂-ZrO₂, 1 mol% YO_1.5-10 mol% CeO₂-ZrO₂, 12 mol% CeO₂-ZrO₂, and 2 mol% YO_1.5-10 mol% CeO₂-ZrO₂ by the coprecipitation method. The characteristics of the as-derived powders are evaluated through thermal analysis and electron microscopy. The sintering behavior of the calcined powders is carried out at 1400° and 1500°C for 1 to 10 h. Sintered density higher than 98% of theoretical is achieved for sintering at 1400°C for several hours. The as-sintered density dependence on the sintering condition is related to the extent of tetragonal-to-monoclinic phase transformation as well as the associated microcracks. Partial substitution by Y₂O₃ in CeO₂-ZrO₂ results in reduced grain size and tends to stabilize the tetragonal structure. Y₂O₃ is more effective than CeO₂ with respect to the grain size refinement and tetragonal stability. In addition, Y₂O₃ substitution in CeO₂-ZrO₂ increases the hardness, while it decreases the fracture toughness.     

Load Dependence of Hardness in Sintered Submicrometer Al2O3 and ZrO2

Monolithic alumina bodies with a grain size of 0.5 μm and submicrometer tetragonal ZrO₂ (3 mol% Y₂O₃) polycrystals were produced and investigated. With decreasing indentation load, the hardness increases but may again decrease below about 5 N; relevant measurements require the application of fairly high loads. Decreasing grain sizes increase the hardness of alumina and zirconia ceramics even for very small grain sizes in the submicrometer range.     

Phase Equilibria and Ordering in the System ZrO2-Y2O3

The phase diagram for the system ZrO₂-Y₂O₃ was redetermined. The extent of the fluorite-type ZrO₂-Y_zO₃ solid solution field was determined with a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. Long range ordering occurred at 40 mol% Y₂O₃ and the corresponding ordered phase was Zr₃Y₄OL₁₂. The compound has rhombohedra1 symmetry (space group R 3), is isostructural with UY₆O_l2 and decomposes above 1250±50°C. The results indicate that the eutectoid may occur at a temperature <400°C at a composition between 20 and 30 mol% Y₂O₃ Determination of the liquidus line indicated a eutectic at 83± 1 mol% Y₂O₃ and a peritectic at 76 ± 1 mol% Y₂O₃.     

Rhombohedral Phase in Y2O3-Partially-Stabilized ZrO2

Tetragonal-to-rhombohedral stress-induced phase transformation was studied by X-ray diffraction on the ground surfaces of tetragonal zirconia polycrystals and partially stabilized zirconia containing 2.0 to 5.0 mol% Y₂O₃ prepared by hot isostatic pressing. The rhombohedral phase increased with Y₂O₃ content and also with hot isostatic pressing temperature. The stability of the rhombohedral phase was studied with regard to surface finish and thermal annealing. The subsequent heat treatment of the specimens was found to cause the reverse rhombohedral-to-tetragonal transformation.     

Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3

Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO₂–3 mol% Y₂O₃ powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO₂–3 mol% Y₂O₃ produced mixed results. In the case of submicrometer ZrO₂–3 mol% Y₂O₃, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO₂–3 mol% Y₂O₃, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO₂–3 mol% Y₂O₃ specimens.     

Microstructural Evolution in a ZrO2 Wt% Y2O3 Ceramic

Several unusual microstructural features, i.e., 90° tetragonal ZrO₂ twins containing antiphase domain boundaries, tetragonal ZrO₂ precipitates in a colony morphology, and precipitate-free zones at the perimeter of cubic ZrO₂ grains containing fine tetragonal ZrO₂ precipitates, were observed in a single ZrO₂-12 wt% Y₂O₂ ceramic annealed at 1550°, 1400°, and 1250°C, respectively. The type of phase transformation responsible for each microstructural feature is described.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Formation and Transformation of Cubic ZrO2 Solid Solutions in the System ZrO2—Al2O3

In the system ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al₂O₃ content. At higher temperatures, the solid solutions transform into tetragonal ZrO₂ and α-Al₂O₃. Pure ZrO₂ crystallizes in the tetragonal form at 415° to 440°C.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

Indentation Studies on Y2O3-Stabilized ZrO2: II, Toughness Determination from Stable Growth of Indentation-Induced Cracks

Stable indentation cracks were grown in four-point bend tests to study the fracture toughness of two Y₂O₃-stabilized ZrO₂ ceramics containing 3 and 4 mol% Y₂O₃. By combining microscopic in situ stable crack growth observations at discrete stresses with crack profile measurements, the dependence of toughness on crack extension was determined from crack extension plots, which graphically separate the crack driving residual stress intensity and applied stress intensity factors. Both materials exhibit steeply rising R -curves, with a plateau toughness of 4.5 and 3.1 Mpa·m^1/2 for the 3- and 4-mol% materials, respectively. The magnitude of the plateau toughness reflects the fraction of tetragonal grains contributing to transformation toughening.     

Effect of Powder Coating on Stabilizer Distribution in CeO2-Stabilized ZrO2 Ceramics

The phase and microstructure relationship of 12 mol% CeO₂-stabilized ZrO₂ ceramics prepared from coated powder was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersed X-ray spectroscopy (EDS). As compared with the sample prepared with co-precipitated method, which exhibited a similar grain size distribution, the EDS analysis revealed that the powder coating induced a wide distribution of CeO₂ solubility, which decreases monotonically with the increase of grain size. This variation of stabilizer content from grain to grain rendered many large grains in the monoclinic phase. Stronger cerium segregation to grain boundaries was observed between large grains, which often form thin amorphous films there. The inhomogeneous CeO₂ distribution keeps more tetragonal ZrO₂ grains close to the phase boundary to facilitate the transforming toughness. Addition of an Al₂O₃ precursor in coated powders effectively raises the overall CeO₂ stabilizer content in the grains and preserves more transformable tetragonal phase in the microstructure, which further enhanced the fracture toughness. The dependence of CeO₂ solubility on grain size may be explained in a simple coating-controlled diffusion and growth process that deserves further investigation.     

Phase Equilibria in the Pseudoternary System ZrO2−Y2O3−Cr2O3 at 1600°C in Air

The pseudoternary system ZrO₂-Y₂O₃-Cr₂O₃ was studied at 1600°C in air by the quenching method. Only one intermediate compound, YCrO₃, was observed on the Y₂O₃−Cr₂O₃ join. ZrO₂ and Y₂O₃ formed solid solutions with solubility limits of 47 and 38 mol%, respectively. The apex of the compatibility triangle for the cubic ZrO₂, Cr₂O₃, and YCrO₃ three-phase region was located at =17 mol% Y₂O₃ (83 mol% ZrO₂). Below 17 mol% Y₂O₃, ZrO₂ solid solution coexisted with Cr₂O₃. Cr₂O₃ appears to be slightly soluble in ZrO₂(ss).