Effect of Yttria Content on the Zirconia Unit Cell Parameters (original) (raw)
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Phase Stability of t′-Zirconia-Based Thermal Barrier Coatings: Mechanistic Insights
Journal of the American Ceramic Society, 2011
The temperature capability of yttria-stabilized zirconia thermal barrier coatings (TBCs) is ultimately tied to the rate of evolution of the ''nontransformable'' t 0 phase into a depleted tetragonal form predisposed to the monoclinic transformation on cooling. The t 0 phase, however, has been shown to decompose in a small fraction of the time necessary to form the monoclinic phase. Instead, a modulated microstructure consisting of a coherent array of Y-rich and Y-lean lamellar phases develops early in the process, with mechanistic features suggestive of spinodal decomposition. Coarsening of this microstructure leads to loss of coherency and ultimately transformation into the monoclinic form, making the kinetics of this process, and not the initial decomposition, the critical factor in determining the phase stability of TBCs. Transmission electron microscopy is shown to be essential not only for characterizing the microstructure but also for proper interpretation of X-ray diffraction analysis.
Advances in Materials Science and Engineering : An International Journal (MSEJ), 2016
Thermal Barrier Coatings (TBCs), routinely prepared from Ceramic based compositions (typically 8%Y2O3-ZrO2or 8YSZ) are being engineered to protect the metallic components from degradation in applications like gas turbines, jet and automotive engines. With a goal of finding improved TBC materials a wide variety of ceramics are being researched worldwide. Before physically preparing the TBCs of uncommon compositions in the laboratory, their suitability to perform can be predicted. Limited accessibility to detailed and realistic information on the influence of newer compositions (other than 8YSZ) on TBCs warrants methods to obtain this information.
Journal of the American Ceramic Society, 2009
A multicomponent microstructure model is applied in ultrasmallangle X-ray scattering studies of two groups of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings (TBCs). One group was sprayed from a single powder feedstock using controlled processing conditions. The other group included three different feedstock morphologies (obtained from different manufacturing methods), each with a similar particle size distribution and sprayed under the same average controlled processing conditions. The microstructure is quantitatively related to the feedstock morphology and processing conditions. Relationships are explored among these microstructures and the coating properties (e.g., thermal conductivity, elastic modulus). The degree of microstructural anisotropy is demonstrated to be pore-size dependent, being more pronounced for larger pores, and more sensitive to feedstock morphology (powder processing) than to spray processing. The microstructure analysis indicates two broad distributions of interlamellar pores, which combined, account for 70%-80% of the pore volume. The total porosity is found to increase with decreasing particle temperature or velocity. For all coatings, a negative linear relationship exists between thermal conductivity and total porosity. Comparison of the new analysis is made with earlier small-angle neutron scattering results, and implications are considered for a more general application of this metrology in TBC microstructure design.
Surface and Coatings Technology, 2006
This paper deals with the development of new synthesis techniques for functional materials such as Yttria Stabilized Zirconia (YSZ) used in the field of thermal barriers coatings. Currently, Thermal Barrier Coatings (TBCs) are manufactured by dry route technologies (EB-PVD or plasma spray) but such methods are directional and often require costly investments and complicated operations. We have carried out significant work aimed at developing sol-gel routes, which are nondirectional methods, to prepare, by suitable chemical modifications, nanocrystalline materials with a controlled morphology. The main advantage of this method is to decrease the crystallization temperature, much lower than the conventional processes, allowing the synthesis of reactive powders with nanometric particles size. In this paper, the formulation of an alkoxide sol has been optimized in order to obtain homogeneous YSZ films. Nature and quantity of binders have been studied. Superalloys have been then immersed in the sol and withdrawn at several controlled rates before being annealed at different temperatures. The films microstructures have been investigated using scanning electron microscopy. It appears that the combination of a slower withdrawal speed (17 cm/min) with a 3 wt.% content of DBP allows to obtain the most homogeneous and the thicker coatings. Moreover, SEM-FEG observations have shown that the deposit is present all over the rough surface of the substrate and is composed of two morphologies: a YSZ thin covering film and a thicker discontinuous layer duplicating the substrate topography.
Stabilize Consequences of Y2O3 in Zirconia Thermal Barrier Coatings (TBC)
The accumulation of cubic stabilizing oxides is a pre-requisite for the use of Zirconia as a main voter in thermal barrier coating. These can be added in ample amounts to form a partially stabilized Zirconia or to form a fully stabilized Zirconia. Zirconia yttria (ZrO2.Y2O3) coating deposited by plasma spray method is widely used in industry as a thermal barrier coating (TBC). Development of narrative and innovative ceramic materials, which have brought about significant hi-tech change, predominately by Y 2 O 3 for the accumulation of varying amounts of cubic stabilizing oxide. This paper is a study on the effect of stabilizer on performance of ZrO 2 thermal barrier coatings. For experimentation, Y2O3 complete characterization was done with ZrO2.Y2O3 coated samples to optimize the micro structural, mechanical properties and characterization, using the latest techniques. It was observed that porosity content in all the coated samples was not very high. However the porosity content was uniformly distributed and pore size was small. It has been revealed that, in case of small porosity content, hardness values of thermal barrier coating with stabilizing effect of yttria were not very high.
Thermal Spray Yttria-Stabilized Zirconia Phase Changes during Annealing
Journal of Thermal Spray Technology, 2001
Phase stability of thermal barrier deposits made from yttria partially stabilized zirconia (Y-PSZ) is a requirement for extended service lifetime. The response of Y-PSZ plasma-sprayed deposits to annealing at 1000, 1200, and 1400 °C with times from 1 to 1000 h has been evaluated using the Rietveld analysis of neutron diffraction data. Results show that the yttria concentration of the as-sprayed tetragonal zirconia component generally decreased with increasing annealing temperature and time. As the yttria content in the tetragonal phase approached a limiting concentration, about 3.5 mol.% of YO 1.5
Characterization of Nanocrystalline Yttria-Stabilized Zirconia: An In Situ HTXRD Study
ISRN Nanotechnology, 2011
Nanocrystalline yttria-stabilized zirconia powders, synthesized by the citrate nitrate gel combustion route, with yttria concentration varying from 8 to 12 mol% were studied by in situ high temperature X-ray diffraction in the temperature range of 25-1000 • C. The sample obtained has a high specific surface area of 35 m 2 /g while calculated surface area was around 123 m 2 /g. The in situ high temperature X-ray diffraction study revealed that crystallite size remains in the range of 7-9 nm up to 800 • C and then rapidly grows up to 21-23 nm upto 1000 • C; only holding the material at 1000 • C for 30 minutes can promote grain growth in the range of 42-49 nm. Coefficient of thermal expansion ranges from 9.65 to 9.03 ppm/ • C for 8-12 mol% nanocrystalline yttria-stabilized zirconia.
Phase stability of scandia–yttria-stabilized zirconia TBCs
Surface & Coatings Technology, 1998
The evolution in phase composition resulting from 1400°C ageing of ZrO2 samples stabilized by ∼4–7 mol.% each of Y2O3 (YSZ), Sc2O3 (ScSZ), or a mixture of Sc2O3–Y2O3 (SYSZ) was studied by synchrotron radiation X-ray diffraction (SR XRD) using whole powder pattern fitting (WPPF), a recently proposed Rietveld-based method for the simultaneous refinement of phase composition and crystal microstructure. The results suggest that SYSZ has better high temperature tetragonal phase stability than the current state-of-art YSZ. For example, a plasma-sprayed coating of SYSZ (6.57 mol.%Sc2O3–1.00 mol.%Y2O3) remained 96.7% in its original tetragonal phase (presumably t′, c/a of 1.0058) after being aged 100 h at 1400°C and then 24 h at 1480°C, whereas a 4.5 mol.% YSZ coating was transformed into two new tetragonal phases, one a low-stabilizer phase with c/a of 1.0154 (44.8%) and the other a high-stabilizer zirconia phase with c/a of 1.003 (46.5%), with some monoclinic (8.7%) formed. The phase compositions found for the various powder and coating specimens, together with information regarding lattice parameters, crystalline domain size and microstrain, also obtained by WPPF, are discussed in terms of the high-temperature phase stability of the YSZ vs SYSZ systems.
Journal of the American Ceramic Society, 2005
Yttria-stabilized (8.6 mol% YO 1.5) zirconia thermal barrier coatings evolve at high temperatures from the ''non-transformable,'' metastable tetragonal-prime phase in their as-deposited condition to a mixture of the tetragonal and cubic phases. The kinetics of the transformation at 12001 and 14251C are reported based on X-ray diffraction measurements. Complementary Raman spectroscopy measurements indicate a sharpening of the tetragonal bands at 263 and 465 cm À1 that is attributed to a systematic decrease in disorder of the Y 31 and oxygen vacancies with annealing. No transformation to the monoclinic form of zirconia is observed immediately after high-temperature treatment. However, partial transformation to monoclinic occurs after a prolonged time (months) at room temperature in those samples treated at 14251C, indicating the development of isothermal martensite.
… of Materials for …
Increasing the turbine hot gas inlet temperature is a potential way to improve the efficiency of the land base gas turbines. Since the nickel and cobalt based superalloy materials can not face temperatures higher than 950 o C, thermal barrier coatings (TBC) with better insulation properties are needed. For this reason thicker TBCs (> 500µm) are needed to improve the thermal insulation. However, the increased thickness of TBCs may lead to a reduced coating lifetime. In order to overcome this drawback, two modifications of the thick 8Y 2 O 3-ZrO 2 TBC's structure were studied. Within these two modifications the TBC coating surface layer was sealed by using phosphate impregnation or laser glazing. These procedures are expected to improve coating hot corrosion and thermal cycling resistance, due to the denser coating surface or with the controlled vertical crack network. In this study thermal diffusivity and specific heat analysis together with microstructural characterization were carried out considering the sintering effect and possible phase transformations at elevated temperatures, up to 1250 o C. Qualitative explanation of experimental results was taken account by modelling the effect of porosity on the thermal properties of TBC. Broad variations in microstructural and thermophysical properties were observed within modified coatings.