Validierung der Beständigkeit von ZrO2‐Minikugeln gegenüber Heißdampfsterilisation

Validation of the Resistance of ZrO₂‐Mini Balls against Hot Vapour Sterilisation Mini‐balls are used in bearings of dental drills. As the bearings as well as the drills are hot vapour sterilised, the bearings are operated under non‐lubricated conditions. In the present study, Raman scattering experiments show that hot vapour sterilisations of Y₂O₃ stabilised ZrO₂‐Mini‐balls lead to a phase transition from tetragonal to monoclinic ZrO₂. This phase transition finally causes the failure of ZrO₂‐Mini‐balls under the tribological conditions typical for dental drills. If, however, the ZrO₂‐Mini‐balls are stabilised using CeO₂, such hot vapour sterilisation induced phase transitions could not be observed. Subsequent tribological experiments prove that CeO₂ stabilised ZrO₂‐Mini‐balls, produced in a Sol‐Gel process, are resistant to slip‐rolling conditions, if large pores can be avoided.     

Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia in hot water

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia caused by annealing in hot water was investigated in the temperature range 80 to 200° C using sintered bodies in zirconia containing 2, 3 and 4 mol % Y₂O₃. Three approaches, alloying ZrO₂(Y₂O₃) with 0 to 20wt% CeO₂, dispersing 0 to 40 wt % Al₂O₃ into ZrO₂(Y₂O₃) ceramics and decreasing the grain size of zirconia, were examined to inhibit the tetragonal-to-monoclinic phase transformation. The amount of monoclinic phase formed decreased with increasing concentrations of CeO₂ alloyed and Al₂O₃ dispersed, and with decreasing grain size of zirconia.     

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.     

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.     

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.     

Fabrication and sinterability in Y2O3-CeO2-ZrO2

CeO₂-stabilized tetragonal zirconia polycrystal (Ce-TZP) containing 1 to 6 mol % YO_1.5 have been fabricated as fine powders by a coprecipitation technique. The microstructure of the as-sintered surface and fracture surface were examined by electron microscopy. CeO₂ dopants reduced the phase transformation temperature from amorphous to tetragonal and stabilized the tetragonal phase at low temperature. The addition of Y₂O₃ to Ce-TZP inhibited the grain growth. The sintered density reached 99% theoretical for short sintering times at 1440 and 1540° C, but decreased slightly to 97 to 98% theoretical for longer sintering times. The decrease in density is attributed to the morphological development of agglomerates, which induce large pores during sintering. The average grain size decreased significantly as the yttrium content increased from 1 to 3 mol %. Specimens aged in water at low temperatures exhibited no phase transformation. This implies fairly good thermal stability in the Y₂O₃-doped Ce-TZP system.     

Phases in Plasma Sprayed Thermal Barrier Coatings from Yttria Partially Stabilized Zirconia

The application of thermal barrier coatings (TBC) is increasing in aeroengines. Surface temperatures up to 1450°C require the application of ceramic TBCs because the temperature capability of metallic substrate materials is not high enough. The service life of turbine components could be improved by the use of yttria partially stabilized zirconia top coatings. The most successfull TBCs are made from 7–9 wt‐% yttria partially stabilized zirconia. One of the most discussed reasons of damages of such TBCs is the transformation between monoclinic and tetragonal phase in zirconia in connection with a dramatic change in volume. Thus in this work resulting phases of plasma sprayed zirconia coatings were investigated. It was found that no monoclinic phase could be detected after heat treatments at 1300, 1400 and 1466°C with cooling rates > 2°/min. Only with cooling rates < 2°/min monoclinic phases occured. It can be concluded that the metastable tetragonal high temperature configuration of yttria partially stabilized zirconia is “very stable”. The conditions in aeroengines with cooling rates > 2°C prevent the formation of the monoclinic phase in zirconia.     

Microstructure and phase composition of ZrO2-CeO2 thermal barrier coatings

The microstructure and phase composition of zirconia plasma-sprayed thermal barrier coatings containing 12 to 25 wt% ceria addition have been investigated. Coatings containing less than 20 wt% CeO₂ are composed of a monoclinic and retained metastable transformable tetragonal phase due to the constraint developed by the small crystal size. This phase transforms readily under moderate thermal stresses. Compositions greater than 20 wt% CeO₂ are composed only of a metastable non-transformable tetragonal, tz′ structure, resistant to transformation under thermal or mechanical stresses. The microstructure of this phase shows microstructural similarities to the high-yttria t′-phase in the ZrO₂-Y₂O₃ system, such as transformation twins and anti-phase boundaries. This suggests that the phase observed in the ZrO₂-CeO₂ system forms by a similar mechanism.     

Phase composition and thermal conductivity of zirconia-based thermal barrier coatings

Atmospheric plasma spraying of powder materials has been used to produce thermal barrier coatings (TBCs) based on ZrO₂ stabilized with 7 wt % Y₂O₃, including coatings doped with neodymium and samarium oxides, for state-of-the-art and next-generation high-temperature gas turbine engines. Doping with neodymium and samarium oxides has been shown to reduce the thermal conductivity of the TBCs by 10–20%. At the same time, changes in the phase composition, crystal structure parameters, and microstructure of the TBCs during heat treatment at the service temperature lead to an increase in the thermal conductivity of all the coatings by 50–70%.     

Mikrostruktur und Eigenschaften HVOF‐gespritzter Schichten im System TiO2 – Cr2O3

Microstructure and properties of HVOF‐sprayed coatings of the TiO₂ – Cr₂O₃ system Thermally sprayed titanium oxide coatings are known for their good tribological properties and their electrical conductivity. The latter is due to oxygen deficiency from the stoichiometric composition TiO₂. These lattice defects can be ordered and are called crystallograhic shear planes. These structures are known as Magnéli phases. At high temperature in oxygen‐containing atmospheres the material forms isolating TiO₂, therefore the application under such conditions is restricted. At the titania‐rich side of the system TiO₂‐Cr₂O₃ also compounds with the structure of Magnéli‐phases are formed. According to information from the literature, these phases are stable in oxygen‐containing atmospheres and are therefore promising for corresponding coating applications at elevated temperatures. In this paper first results of systematic studies of microstructure and properties of HVOF‐sprayed coatings are presented.     

Y2O3 stabilized ZrO2 thin films deposited by electron-beam evaporation: Optical properties, structure and residual stresses

This paper describes the preparation and the characterization of Y₂O₃ stabilized ZrO₂ thin films produced by electric-beam evaporation method. The optical properties, microstructure, surface morphology and the residual stress of the deposited films were investigated by optical spectroscopy, X-ray diffraction (XRD), scanning probe microscope and optical interferometer. It is shown that the optical transmission spectra of all the YSZ thin films are similar with those of ZrO₂ thin film, possessing high transparency in the visible and near-infrared regions. The refractive index of the samples decreases with increasing of Y₂O₃ content. The crystalline structure of pure ZrO₂ films is a mixture of tetragonal phase and monoclinic phase, however, Y₂O₃ stabilized ZrO₂ thin films only exhibit the cubic phase independently of how much the added Y₂O₃ content is. The surface morphology spectrum indicates that all thin films present a crystalline columnar texture with columnar grains perpendicular to the substrate and with a predominantly open microporosity. The residual stress of films transforms tensile from compressive with the increasing of Y₂O₃ molar content, which corresponds to the evolutions of the structure and packing densities.     

Grenzflächenphänomene und Haftung bei thermisch gespritzten Verbundwerkstoffen

In this paper. results from investigations on the bonding mechanism between thermally sprayed metal and ceramic coatings and different substrates are presented. The materials were sprayed by atmospheric arc (Ni, Mo und Cu) and plasma spraying techniques (NiCr-Al, Al₂O₃, ZrO₂-7Y₂O₃) on iron (Armco-Fe) and steel (St37) substrates. The morphology, microstructure and impact behaviour of sprayed particles on the polished substrates have been analysed as a function of particle properties (temperature, heat content, size and velocity, surface tension, viscosity) and substrate properties (composition, temperature, surface structure). The results of the influence of investigated properties on the adherence mechanism of sprayed particles can be also directly correlated to the experimentally measured bond strength of are sprayed nickel coatings.     

UV Raman spectroscopic study on the surface phase of ZrO2 modified with Nd2O3

The Nd₂O₃ modified ZrO₂ was synthesized using two methods of co-precipitation (Nd-ZrO₂) and wet impregnation (Nd/ZrO₂). The surface and bulk crystalline phases of Nd₂O₃ modified ZrO₂ were investigated by using UV Raman spectroscopy, visible Raman spectroscopy, and X-ray diffraction (XRD). It is observed that the tetragonal phase in the surface region of Nd-ZrO₂ was not effectively stabilized by Nd₂O₃, as Nd₂O₃ is mainly present in the bulk of Nd-ZrO₂. However, in Nd/ZrO₂, it is found that with the impregnation of 0.5 mol% Nd₂O₃ on ZrO₂, the surface tetragonal phase of Nd/ZrO₂ can be stabilized even after calcination at 700 °C. The UV Raman results indicate that a disordered structure, or intermediate structure, which is involved in the transition from the tetragonal to the cubic phase, is formed at the surface region of Nd/ZrO₂. The formation of the aforementioned intermediate structure inhibits the phase transition from tetragonal to monoclinic in the surface region of Nd/ZrO₂. Furthermore, it is observed that the mixed tetragonal and monoclinic phases in the surface region of ZrO₂ which has been impregnated with Nd₂O₃ can also be stabilized after calcination at 700 °C. This work provides a simple method for controlling the surface phase of ZrO₂ at high temperatures.     

Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings

The microstructural features of high-temperature sintered and CaO-MgO-Al₂O₃-SiO₂ (CMAS) corroded air plasma sprayed Y₂O₃ stabilized ZrO₂ (YSZ) thermal barrier coatings (TBCs) under the thermal gradient condition were comparatively studied. As-sprayed YSZ has a lamellar structure and the lamellae are composed of closely aligned columnar crystals. The sintered and the CMAS corroded YSZ coatings maintain the t’-ZrO2 phase as the as-sprayed YSZ coating. The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grains coarsening. After the CMAS corrosion, the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of striped Zr₂Y₂O₇. The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating along with significant infiltration of amorphous CMAS and anorthite formation in the bottom layer owing to the high contents of Ca and Al.     

Affect of grinding and polishing on near-surface phase transformations in zirconia

The transformation of near-surface material on grinding and polishing has been investigated in sintered zirconia of 1 μm grain size and 99 % density containing 4.5 and 7.0 mol % Y₂O₃. Rough wet and dry grinding transformed ca 20 % cubic phase into 18 % tetragonal and 2 % monoclinic in material initially 47 % cubic and 53 % tetragonal (4.5 mol % Y₂O₃) but no change of phase in material that was fully cubic (7.0 mol % Y₂O₃). Annealing and polishing reduced lattice strain but only polishing reduced the concentration of monoclinic and tetragonal phases. Microhardness studies indicated that lattice strain and the phase transformations increased the penetration hardness to a depth of ca 4 μm.     

Synthesis and sintering behaviour in CeO2-ZrO2 ceramics

The co-precipitation method has been employed to prepare CeO₂-ZrO₂ ceramics. The application of a wet chemical method is expected to yield highly sinterable material at lower sintering temperatures. The characteristics of the synthesized powders are evaluated with respect to the particle size distribution, calcination step, and the degree of agglomeration. The sintering behaviour of the prepared powder is studied at various temperatures to obtain different phase distributions and grain sizes. The amount of the monoclinic phase in the as-sintered specimen is decreased with increasing CeO₂ contents in CeO₂-ZrO₂. 13.7 mol% CeO₂ is sufficient to achieve a tetragonal phase in the CeO₂-ZrO₂ system. In addition, Y₂O₃ and MgO dopants in CeO₂-ZrO₃ reduce the grain size and result in a fully tetragonal phase for the 10 mol% CeO₂ matrix.     

Thermisch gespritzte Schichten im System Al2O3–Cr2O3–TiO2 – ein Update

With exception of ZrO₂, the individual oxides and binary compositions in the system Al₂O₃–Cr₂O₃–TiO₂ are the most important oxide materials for the preparation of thermally sprayed coatings. In this contribution selected results of recent own research activities are summarized. This includes the comparison of microstructures, phase compositions, and properties of coatings, deposited by atmospheric plasma spraying (APS) and high velocity oxy‐fuel (HVOF) spraying. The possibilities arriving from the use of suspensions as feedstock are reviewed. Special attention is paid to the advantage of use of binary compositions in this system. Tribological, electrical and corrosion properties of the coatings are discussed.     

Preparation and properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> partially stabilized ZrO<Subscript>2</Subscript> crystals

We have studied the effect of composition and growth conditions on the structure and properties of 2.5–5 mol % Y₂O₃ partially stabilized ZrO₂ crystals grown by directional solidification in a cold-wall crucible. The phase composition and density of the crystals have been determined. The crystals are shown to be uniform in composition, with local changes in Y₂O₃ content within ±0.5 mol %. The dimensions and quality of the single crystals are influenced by the growth conditions.     

New ZrO2Yb2O3 plasma-sprayed coatings for thermal barrier applications

New thermal barrier coatings with improved properties were identified. These ZrO₂Yb₂O₃ coatings were evaluated together with ZrO₂6.1Y₂O₃¹ and ZrO₂8.0Y₂O₃. All ceramic coatings were deposited onto Ni16.2Cr5.9A10.15Y and Ni18.3Cr6.4A10.22Yb bond coatings on MAR-M-200 + Hf substrates. Tests were run in a cyclic furnace at temperatures up to 1120°C. On the NiCrA1Y bond coating the ZrO₂12.4Yb₂O₃, ZrO₂14.7Yb₂O₃ and ZrO₂17.4Yb₂O₃ have respectively 60%, 30% and 15% longer lives than the ZrO₂6.1Y₂O₃ coating. They also have 80%, 50% and 25% longer lives than the ZrO₂8.0Y₂O₃ coating. On NiCrA1Yb bond coating the ZrO₂12.4Yb₂O₃ has about 40% longer life than the ZrO₂6.1Y₂O₃. The optimum Yb₂O₃ concentration in ZrO₂, at which maximum life is obtained, is believed to be between 12.4 and 14.7 wt.%. The ZrO₂Yb₂O₃ coatings fail in a similar fashion to the ZrO₂Y₂O₃ coatings.X-ray diffraction data from the limited study of the ZrO₂Yb₂O₃ system suggest that monoclonic, cubic and tetragonal phases, in addition to Zr₃Yb₄O₁₂ and Yb₂O₃, are present in zirconia-ytterbia powders containing between 2 and 90 wt.% Yb₂O₃. The ZrO₂Yb₂O₃ plasma-sprayed coatings were also found to contain monoclinic, cubic and tetragonal phases in addition to Zr₃Yb₄O₁₂, and they showed a few very weak X-ray diffraction peaks that belong to an unknown phase.     

Einfluß des ZrO2-Zusatzes auf mechanische Eigenschaften und den ungeschmierten Gleitverschleiß von TiB2-ZrO2-Mischkeramiken

Effect of ZrO₂ addition on mechanical and tribological properties of TiB₂-ZrO₂-composites TiB₂-ZrO₂-composites were produced by hot-pressing in argon. The mechanical and tribological properties of the composites were investigated in relation to the amount and the type of the ZrO₂ added. Addition of ZrO₂ reduced the sintering temperature of the TiB₂-ZrO₂-composites and led to growth inhibition of the TiB₂ phase. ZrO₂ also increased the bending strength and fracture toughness of the composites. Tribological tests were carried out using laboratory tribometers under unlubricated oscillating sliding contact against Al₂O₃-, Si₃N₄- and SiC-counterbodies. Abrasive wear of the ceramics against 80 mesh SiC grits was studied by using an abrasive wheel test. During sliding contact the composites showed high wear resistance but caused extensive wear of the counterbodies and high friction coefficients compaired to self-mated monolithic Al₂O₃- and ZrO₂-ceramics. Volumetric wear loss of the TiB₂-ZrO₂-composites due to abasive SiC grits was by a factor up to 30 lower than that of a commercial monolithic Al₂O₃. Wear resistance of the composites increased with decreasing amount of ZrO₂ and was higher for the composites with addition of partially stabilized tetragonal ZrO₂ than for those with addition of monoclinic ZrO₂.