Langzeitverhalten und Einsatzgrenzen von plasmagespritzten CeO2‐ und Y2O3‐ stabilisierten ZrO2‐Wärmedämmschichten

Long‐term behaviour and application limits of plasma‐sprayed CeO₂‐ and Y₂O₃‐stabilized ZrO₂ thermal barrier coatings Investigations of changes in phase composition and mechanical properties of CeO₂ – and Y₂O₃ – stabilized plasma‐sprayed ZrO₂‐based thermal barrier coatings (TBC) after long‐term heat treatments at typical service temperatures were performed. Experimental studies include X‐ray diffraction and mechanical testing. TBCs with a high amount of the tetragonal equilibrium phase t (8 mol‐% CeO₂) show strong degradation due to the high amount of transformation to the monoclinic phase and the related decrease in strength. TBCs with a high amount of tetragonal or cubic non‐equilibrium phases t′ or c′ are more suitable. Among these, TBCs with higher CeO₂‐stabilizer contents (19.5 mol‐% CeO₂/1.5 mol‐% Y₂O₃ and 35 mol‐% CeO₂) show less amounts of monoclinic phase with respect to Y₂O₃‐stabilized TBCs (4.5 mol‐% Y₂O₃), as commercially used. Therefore they seem to have a high potential for long‐term applications.     

Microstructure of Y-PSZ after ageing at high temperature

Partially stabilized zirconia (PSZ) materials containing 2.5 and 5.0 mol % Y₂O₃ were prepared by pressureless sintering and aged at 1200° C for 1000 IS, and their microstructures were analysed by transmission electron microscopy and electron diffraction methods. Tetragonal zirconia polycrystal (TZP) containing 2.5 mol % Y₂O₃ before ageing showed nearly 100% tetragonal microstructure and 0.5 m grain size, but after ageing the microstructure changed greatly, exhibiting no simple grain structure over wide areas. Repeated twin structures within the grains were observed. Y-PSZ material containing 5.0M01% Y₂O₃ before ageing showed a tetragonal (I structure within a cubic (c) stabilized ZrO₂ matrix, After ageing, structures of fine strip crystals crossed each other orthogonally within the cubic matrix and typical diffuse scattering in the diffraction pattern was observed. Repeated twins were found on the plane of (100)_m, and the orientational relationship between tetragonal (t) and monoclinic (m) crystal was determined to be (100)_m [(100)_t, [010]_m \tT [001b]_t.     

The role of porosity in the phase composition evolution and texture formation at the friction surface of partially stabilized tetragonal zirconia ceramics

The influence of porosity on the phase composition evolution and texture formation at the friction surface of yttria-partially-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics (ZrO₂ + 3 mol % Y₂O₃) in contact with steel was studied. The Y-TZP ceramic samples with a porosity of P = 0.33 and 3.3% were tested in a rod-on-disk configuration. The sample of higher porosity exhibited a much lower degree of wear. The friction wear was accompanied by irreversible phase transformations from the cubic to tetragonal and from the tetragonal to monoclinic structure. It is suggested that such irreversible phase transformations in porous ceramics favor the development of compressive stresses at the friction surface, which account for the observed behavior.     

Effect of gamma irradiation on the phase composition of ZrO<Subscript>2</Subscript>-based ceramic materials

We have studied the effect of gamma irradiation on the phase composition and crystallite size of ZrO₂ samples containing 0, 2, and 3 mol % Y₂O₃ as a stabilizer and heat-treated at 800°C. The results demonstrate that gamma irradiation leads to amorphization of the material and reduces the amount of the monoclinic phase. Irradiation had the strongest effect on the phase composition (increasing the percentage of the tetragonal phase and reducing that of the monoclinic phase) and caused the largest decrease in crystallite size in the case of the yttria-free samples.     

Characterization and densification of lanthana-zirconia powders prepared by high temperature hydrolysis

Zirconia-lanthana powders containing 4.5, 7, 10, 15 and 20 mol % La₂O₃ were prepared by hydrolysis. The hydrolysis process was carried out in a laboratory stainless steel autoclave for their equivalent hydroxides for 2 h at 200‡ C. The powders were investigated using X-ray diffraction, infrared spectrometry, and transmission electron microscopy techniques. No other phases except the cubic phase zirconia of fluorite-type structure were detected. The prepared materials were examined for their thermal stability and phase constitution, by X-ray and infrared analyses, on heating up to 1400‡ C. The cubic phase zirconia remains stable up to 1000‡ C at which it starts to decompose yielding monoclinic zirconia and lanthanum zirconate. At 1200‡ C, the cubic phase nearly disappears in the sample containing 4.5 mol % La₂O₃. Increasing La₂O₃ content up to 20 mol % retards its destabilization, reduces the yielded monoclinic phase, and in the same time increases the formed lanthanum zirconate phase. At 1500‡ C only monoclinic ZrO₂ and La₂Zr₂O₇ are present. The La₂Zr₂O₇/monoclinic ZrO₂ ratio increases with increasing La₂O₃ content. Pressed specimens of the prepared materials were fired for 2 h at 800 to 1400‡ C. The sintering activity of the prepared powders resulted in a 92% theoretical density body for the 4.5 mol % ZrO₂ material. The densification properties in relation to changes in the phase constitution are discussed.     

Einfluß der Korngröße auf Reibung und Gleitverschleiß von TZP?ZrO2-Keramik bei Festkörper- und Mischreibung

Effect of grain size on friction and wear of TZP? ZrO₂ ceramic in unlubricated and boundary lubricated sliding contact . Microstructures with systematically between 0.30 and 1.62 μm varying average grain sizes were produced by cold isostatically pressing and sintering of commercial 3 mol.% Y₂O₃ stabilized ZrO₂ powder. Hardness and fracture toughness of the different structures as well as the amount of tetragonal, cubic and monoclinic phases were measured. Wear tests were carried out on the different self-mated microstructures in unlubricated and oil or water lubricated unidirectional and unlubricated reciprocating sliding contact, respectively. Worn surfaces were systematically analysed using scanning electron microscopy. The experimental results showed a significant influence of the average grain size as well on mechanical properties such as hardness and fracture toughness as on tribological properties. Friction and wear are discussed in relation to grain size and phase stability of the microstructures and also as a function of the different tribosystems and lubricants used.     

Preparation and properties of Y2O3-doped ZrO2 thin films by the sol–gel process

Y₂O₃-doped ZrO₂ (YZ) thin films were prepared on alumina substrates by the dip-coating process. The dip-coating solution consisted of a homogeneous sol, and was prepared by using the respective chlorides as raw materials, with ethylene glycol, 2-butanol and distilled water as solvents. The thin films containing 0–20 mol % Y₂O₃ were successfully produced by thermal treatment above 600 °C. The characterization for the film preparation was performed by means of thermogravimetric–differential thermal analysis for the thermal analysis, and scanning electron microscopy for the morphological analysis and thickness measurements. The properties of the films were characterized in terms of a study of the crystalline phase, the crystallite size, the microstructure and the electrical conductivity by using X-ray diffraction, scanning electron microscopy and the complex impedance techniques. In all YZ thin films, the tetragonal phase was stable at low temperatures as a result of the crystallite size effect. However, at higher temperatures, the tetragonal phase was transformed into either the monoclinic phase or the cubic phase, depending on the doping concentration. The YZ thin film of 8 mol% Y₂O₃ content was stabilized to almost cubic phase at 1000 °C. Resonable conductivity behaviour at YZ was observed for the YZ thin films. The electrical conductivity of YZ thin films was similar to the values of the sintered body. This revised version was published online in November 2006 with corrections to the Cover Date.     

Sintering of 3 mol % Y2O3-TZP and its fracture after ageing treatment

TZP ceramic of 99.7% theoretical relative final density was obtained by pressureless sintering a commercial co-precipitated 3 mol % Y₂O₃-ZrO₂ powder at 1400° C for 10 h. Fracture surfaces of the aged material revealed that the fracture of TZP ceramic was typified by an intergranular mode in areas where the phase was mainly tetragonal, whereas the transgranular mode was found predominantly in the area containing more monoclinic phase. Microcracks induced by the (t) (m) transformation provided short paths for water to accelerate the property degradation of TZP upon low-temperature ageing in a humid atmosphere.     

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.     

Solid-state reaction,microstructure and phase relations in the ZrO2-rich region of the ZrO2-Yb2O3 system

Phase relations below 1700°C in the ZrO₂-rich region of the zirconia-ytterbia system have been established using thermal expansion, room-temperature X-ray diffraction, precision lattice parameter measurements and microscopic observations. The solubility limits of ytterbia in both monoclinic and tetragonal zirconia were determined. A eutectoid reaction, tetragonal zirconia solid solution monoclinic + cubic zirconia solid solutions at 400 ± 20°C and 2.4 mol % ytterbia was found. The left-hand boundary of the cubic zirconia solid-solution field was redetermined between room temperature and about 1700°C. Long-range ordering was present at 40 mol % ytterbia and the formation of an ordered phase, Zr₃Yb₄O₁₂, isostructural with M₇O₁₂-type compounds was found. Its thermal stability was established between room temperature and 1630 ± 10°C, in which it decomposes into cubic zirconia solid solution by an order-disorder reaction.     

Luminescence and crystallinity of flame-made Y2O3:Eu3+ nanoparticles

Cubic and/or monoclinic Y₂O₃:Eu³⁺ nanoparticles (10–50 nm) were made continuously without post-processing by single-step, flame spray pyrolysis (FSP). These particles were characterized by X-ray diffraction, nitrogen adsorption and transmission electron microscopy. Photoluminescence (PL) emission and time-resolved PL intensity decay were measured from these powders. The influence of particle size on PL was examined by annealing (at 700–1300°C for 10 h) as-prepared, initially monoclinic Y₂O₃:Eu³⁺ nanoparticles resulting in larger 0.025–1 μm, cubic Y₂O₃:Eu³⁺. The influence of europium (Eu³⁺) content (1–10 wt%) on sintering dynamics as well as optical properties of the resulting powders was investigated. Longer high-temperature particle residence time during FSP resulted in cubic nanoparticles with lower maximum PL intensity than measured by commercial micron-sized bulk Y₂O₃:Eu³⁺ phosphor powder. After annealing as-prepared 5 wt% Eu-doped Y₂O₃ particles at 900, 1100 and 1300°C for 10 h, the PL intensity increased as particle size increased and finally (at 1300°C) showed similar PL intensity as that of commercially available, bulk Y₂O₃:Eu³⁺ (5 μm particle size). Eu doping stabilized the monoclinic Y₂O₃ and shifted the monoclinic to cubic transition towards higher temperatures.     

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.     

Microstructure and phase composition of ZrO2-CeO2-Al2O3 materials modified with MgO and Y2O3

This paper presents our findings on phase formation processes during heat treatment of sol-gel synthesis products with the composition (mol %) 65(88ZrO₂ + 12CeO₂) + 35Al₂O₃ modified with 1 mol % MgO or Y₂O₃. The composites modified with 1 mol % MgO have been shown to differ significantly in phase composition from the parent nanopowders. Sintering is accompanied by partial decomposition of the tetragonal zirconia (T-ZrO₂) based solid solution and the formation of a monoclinic zirconia (M-ZrO₂) based solid solution and two aluminum-containing phases: corundum and the mixed oxide MgAl₁₁CeO₁₉. The addition of 1 mol % Y₂O₃ leads to successive formation of T-ZrO₂ and corundum and improvement of their structural perfection. The observed differences in phase formation during heat treatment result in different grain size compositions in the microstructure of the composites.     

Texture development in 3 mol% yttria-stabilized tetragonal zirconia

We have developed a method of forming textured tetragonal zirconia. A suspension containing 10 vol% solid loading of monoclinic ZrO₂ mixed with 3 mol% Y₂O₃ was prepared, and then a bead-milling process was performed using 50 μm diameter zirconia beads resulting in a well-dispersed suspension. The mixture suspension of monoclinic zirconia and yttria nanoparticles was slip cast under a magnetic field of 12 T to produce oriented monoclinic zirconia with yttria. The reaction sintering between yttria and the oriented monoclinic zirconia produces a final 3 mol% Y₂O₃ doped tetragonal zirconia that remains oriented.     

利用Rietveld方法Y2O3稳定ZrO2的定量相分析

利用多相混合物中某相的重量与它在Ｒｉｅｔｖｅｌｄ结构修正中求犁标度因子之间的关系，可以进行多相混合物的定量分析，本工作利用Ｒｉｅｔｖｅｌｄ修正和Ｘ射线步长扫描衍射数据对不同浓度Ｙ２Ｏ３掺杂的ＺｒＯ２试样地相含量的定量分析，给出了各相含量的定量数据，同时还证实了由研磨引起主相向斜相转变的规律。     

Preparation and thermal evolution of sol-gel derived transparent ZrO2 and MgO-ZrO2 gel monolith

Transparent gel monoliths of pure and MgO-doped zirconia having dopant concentrations in the range 0 to 15 mol % were prepared by chemical polymerization of zirconium n-propoxide and magnesium acetate tetrahydrate using 2-methoxy ethanol as solvent. The thermal evolution of amorphous gels was studied by differential thermal analysis, X-ray diffraction and transmission electron microscopy. The crystallization of pure and doped zirconia gels occurred in the temperature range 360 to 450° C. The first crystalline phase to appear is tetragonal for pure and 2 mol % doped zirconia, and cubic for 3 to 15 mol % doped samples. Both crystallization and decomposition temperatures are found to increase with increasing dopant concentration, approaching a saturation value for 10 mol % doped samples. It has been established that the transformation of the cubic to the monoclinic phase takes place via a metastable tetragonal phase. A linear relationship between the lattice parameter of cubic zirconia and MgO concentration has been established. X-ray diffraction studies have also revealed that the entire amount of MgO used in preparing doped zirconia gels remains in a single MgO-ZrO₂ crystalline phase formed initially by thermal treatment.[/p]     

Microwave processing: A potential technique for preparing NiO-YSZ composite and Ni-YSZ cermet

In the present study, microwave energy (2·45 GHz) has been used to prepare nickel oxide-yttria stabilized zirconia (NiO-YSZ) composites of composition, mNiO-(1 − m) Zr_0·9Y_0·1O_1·95 (m = 0·2, 0·3, 0·4, 0·5 and 0·6), from a precursor obtained by mixing NiO, Y₂O₃ and monoclinic ZrO₂ in their stoichiometric ratio. The composites have been prepared by conventional processing also to compare the products with those of microwave processed products. During comparison, it was observed that NiO-YSZ composites of each composition obtained by microwave processing had cubic phase of YSZ while in the conventionally prepared composites of compositions, m = 0·2 and 0·3, monoclinic, tetragonal and cubic phases of zirconia existed instead of its pure cubic phase. The composites were reduced to yield Ni-YSZ.     

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.     

Synthesis and characterization of vanadium-containing ZrO2 solid solutions pigmenting system from gels

A procedure is reported for the synthesis of vanadium-doped zirconia pigmenting system with different vanadium loadings which permitted their complete formation and further characterization. Monoclinic vanadium-zirconia solid solutions were prepared by gelling mixtures of zirconium n-propoxide and vanadyl acetylacetonate and studied over the range of temperature up to 1300 °C. Succesive steps of the reactions leading to the final monoclinic vanadium-zirconia solid solution phase were investigated by X-ray powder diffraction. It was found that the formation of the monoclinic solid solution took place by a phase transformation from a phase with the structure of tetragonal zirconia. The transformation temperature of metastable tetragonal to monoclinic phase was found to be governed by the nominal vanadium amount. Measurements of lattice parameters of monoclinic vanadium-zirconia solid solutions as a function of the nominal vanadium amount revealed that vanadium was dissolved in the zirconia lattice. Energy dispersive X-ray microanalysis and lattice parameters variation indicates that the maximum amount of vanadium into the monoclinic zirconia lattice was about 5 mol % of vanadium (3.7 wt % as V₂O₅). UV-Vis diffuse reflectance of monoclinic V-ZrO₂ solid solutions indicated that vanadium was dissolved as V⁺⁴ and that the color of vanadium-zirconia yellow pigments was produced by the dissolved vanadium.     

Phase structures in n-Y2O3

The phase structure of nanostructured Y₂O₃ (n-Y₂O₃) at low temperature was studied by x-ray diffraction profile refinement (XRDPR) method. All maximum X-ray diffraction peaks of Y₂O₃ were observed. The results show that the sample compacted at room temperature consists of two monoclinic phases B1 and B2 and a cubic phase. The samples sintered at 300°C and at 600°C for 5 hours, respectively, are also composed of monoclinic B1, B2 and cubic phases.The results show that with increasing temperature, grain growth, migration of grain boundary and phase transformation occur. This paper presents some quantitative information.