Microstructure Of Yttria-stabilized Zirconia Research Articles

Grain boundary effects on thermal shock responses of yttria-stabilized zirconia

The rapid temperature change or thermal shock is inevitable in many engineering applications of polycrystalline yttria-stabilized zirconia (YSZ). Nevertheless, the thermal shock response of YSZ especially its dependence on the grain boundary (GB) structure in YSZ remains almost unknown to date. In this study, the effect of the GB on thermal shock behaviors of YSZ ceramics was investigated by molecular dynamics (MD) simulations in conjunction with experimental characterizations. According to the distribution of misorientation angles of GBs experimentally extracted from our sintered YSZ samples, four most common GBs with a misorientation angle ranging from 35° to 63° were constructed in MD simulations through the annealing technique. The thermal shock simulations on the bi-crystalline YSZ with such GBs show a weakening effect of the GB on the thermo-elastic strain wave, which is found to be the most significant in the [100]/[111] GB and almost vanish in its [110]/[111] counterpart. Our simulations also reveal that the weakening effect of GB on the thermo-elastic strain wave is majorly attributed to the impedance mismatch between its two adjacent grains, although the grain-boundary thermal resistance observed also can weaken the temperature wave after crossing GBs. These findings provide valuable atomic insights into the thermomechanical behaviors of YSZ under thermal shock, which will guide the design of microstructures of YSZ for some special applications in solid oxide fuel cells and thermal barrier coatings.

Quantitative Determination of Porosity in Thermal Barrier Coatings Using Terahertz Reflectance Spectrum: Case Study of Atmospheric-Plasma-Sprayed YSZ Coatings

Microstructures of yttria-stabilized zirconia (YSZ) atmospheric-plasma-sprayed (APS) thermal barrier coatings (TBCs) and their terahertz dielectric properties were investigated to quantitatively determine porosities by effective medium theory (EMT). Various samples with different porosities ranged from 9.15% to 21.58% were prepared by different particle size powders and spray power. Both the theoretical and experimental results showed that the interferential oscillation period of reflectance spectra of APS YSZ TBCs samples depends on optical thickness of ceramic coatings, and optical thickness depends on permittivity for known geometric thickness. The real part of permittivity ϵ ′ decreased as the porosity increased, the depolarization factor ζ of APS YSZ TBCs pores, which was estimated by comparing the experimental value with the simulated value obtained by the Maxwell–Garnett EMT model and Bruggeman EMT model, approached to a fixed value, owing to the typical lamellar structure of APS-TBCs, and was 0.7 and 0.6 in respective models. Samples of 185–459 μ m thickness verified that the technique varied from scanning electron microscope statistics results by <15%. Finally, a novel, convenient, online, nondestructive, and noncontact porosity evaluation method is proposed that can potentially be utilized on the health monitoring of TBCs in gas turbine blades.

Characterization of thermal barrier coatings microstructural features using terahertz spectroscopy

A novel approach was presented to characterize microstructural features of thermal barrier coatings (TBCs) using terahertz spectroscopy based on machine learning algorithms. In this study, the microstructures of yttria-stabilized zirconia (YSZ) atmospheric-plasma-sprayed (APS) thermal barrier coatings were regulated by choosing different kinds of spray powders, distances and power during processing. A terahertz time-domain spectroscopy system configurated transmission mode with an incidence angle of 0° was employed to estimate terahertz properties of porous YSZ ceramic coatings, including refractive index, extinction coefficient and relative time-domain broadening ratio. The variation tendency of terahertz properties of YSZ ceramic coatings with different microstructure features (porosity, pore to crack ratio, pore size) were investigated. Principal component analysis (PCA) method was adopted to reduce the dimensions of refractive index and extinction coefficient spectra data at the range of 0.6–1.4 THz and to ensure that different terahertz properties could be treated as inputs with similar weights during modeling. Three models (multiple linear regression (MLR), back-propagation (BP) neural network and support vector machine (SVM)) were set up to conduct regression analysis. As a result, according to the contribution rates of eigenvectors, the top one principal component of refractive index spectra data and the top two principal components of extinction coefficient spectra data were selected as the model inputs. The correlation coefficient comparisons showed that the characterization accuracy of PCA-SVM reached by over 95% and outperformed the other models. Finally, this study proposed that THz nondestructive technology combined with machine learning technique is efficient and feasible for microstructural features characterization and has profound implications for the structure integrity of TBCs evaluation in gas turbine blades.

Effect of Microstructure Control of Thin Film Yttria Stabilized Zirconia Electrolyte for Solid Oxide Fuel Cells by Adjusting Oblique Angle and Target Substrate Distance of Sputtering Process

In the present study, thin film yttria stabilized zirconia (YSZ) electrolyte was fabricated for solid oxide fuel cells via radio frequency (RF) magnetron sputtering with variation in oblique angle and target substrate distance (TSD). Growth tendencies were differentiated through adjusted oblique angle and TSD during sputtering, thereby microstructure of YSZ was changed. Electrochemical behavior of microstructure-controlled thin film YSZ was investigated by means of applying YSZ to anodic alumina oxide (AAO) based solid oxide fuel cells (SOFC). On nanoporous AAO substrates, Pt anode, YSZ electrolyte, and Pt cathode were deposited serially all through sputtering. For depositing thin film YSZ electrolyte, oblique angle and TSD were differentiated into four cases each, while thickness was set to be equal. YSZ thin films were evaluated using FESEM, AFM, FIB-SEM, XRD, and XPS. Electrochemical performance was investigated through i-V plot and EIS.

Selective laser melting of yttria-stabilized zirconia

The mechanism of phase transformation and microstructure of yttria-stabilized zirconia (YSZ), prepared by selective laser melting (SLM) at a laser wavelength of 1 μm, has been investigated. Also, the effect of different working gases and heat treatment has been studied. Experimental results present that processing parameters can be optimized and determined by Liner Energy Density. X-ray diffraction analysis confirmed the absence of monoclinic phase under argon flow during SLM, with gray-color appearance due to the lack of oxygen, which impedes the transformation from tetragonal to monoclinic phase partially. Later, the color becomes white during heat treatment because samples are exposed to the air with sufficient amount of the oxygen and the diffraction peaks of monoclinic phase increased. When SLM and heat treatment was carried out in the air, the YSZ samples appeared in white-color, which indicates the presence of both monoclinic and tetragonal phase. The morphology and microstructure showed there occurred bonding phenomenons among particles, only surface partially melted between particles with weak bonding strength. The top surface morphology of air-prepared sample showed cracks, un-melting particles and cave-like pores because of uneven distribution of temperature variations. This paper also made a preliminary research and provided a feasibility method to melt zirconia ceramics completely, in which more energy input by multi-scanning in the air working gas.

INFLUENCE OF SINTERING TEMPERATURE ON TRANSLUCENCY OF YTTRIA-STABILIZED ZIRCONIA FOR DENTAL CROWN APPLICATIONS

This study aims to investigate the effect of sintering temperature on the translucency of yttria-stabilized zirconia (YSZ) for dental crown applications. YSZ suspension was treated by colloidal processing and 24 h of sedimentation to eliminate agglomerates and aggregates. The green bodies of YSZ were then shaped into pellets through slip casting. These bodies were sintered into a final shape at 1450 °C–1650 °C. The densities of the specimens were measured using Archimedes method. Light transmission of the YSZ specimen was also evaluated using a spectrophotometer with an integrating sphere. Morphological analysis was conducted with field-emission scanning electron microscopy. Results showed that sintering temperature significantly influenced the density, light transmission, and microstructure of YSZ. High sintering temperatures produced YSZ with a compact and homogeneous microstructure and a high density. Furthermore, the low light scattering effect on the porosity-free microstructure yielded light transmission as high as 37% in YSZ sintered at 1650 °C. The optimal sintering temperature was found to be 1600 °C, at which 34% light transmission was generated. In conclusion, high sintering temperatures improved the translucency of YSZ. This effect was attributed to effective densification of grains and elimination of pores at high temperatures, thereby alleviating the light scattering effect of the pores. At the optimal temperature, YSZ with high density and translucency and a compact microstructure was formed

Microstructures of YSZ and CGO Thin Films Deposited by Spray Pyrolysis: Influence of Processing Parameters on the Porosity

AbstractMicrostructures of yttria‐stabilized zirconia (YSZ) thin films deposited by spray pyrolysis at 370 °C on sapphire are investigated. The as‐deposited films are predominantly amorphous and crystallize upon heating at temperatures above 370 °C, developing grains in the range of 5 nm to several 100 nm. During post‐deposition heat treatment up to 800 °C, ∼ 50 vol% porosity develops in the center of the films with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin films consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 °C. In gadolinium‐doped‐ceria (CGO) thin films similar microstructures and porosity are found after low‐temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin films is observed in both the YSZ and in CGO thin films. Stagnating grain growth in the thin films is rather caused by reduced grain boundary mobility, here predominately due to a “secondary phase”, i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densification has taken place.

Evaluation of Microstructural Evolution of Nanostructured Yttria-stabilized Zirconia During Sintering Using Impedance Spectroscopy

A non-destructive evaluation (NDE) technique of impedance spectroscopy (IS) was employed for studying the sintering process of nanostructured yttria-stabilized zirconia (YSZ) after exposure at 1100 °C in air for different times. The variations within microstructure of YSZ were correlated with the EIS parameters. The results showed that the resistance and capacitance of YSZ grains (g) and grain boundaries (gb) varied with the sintering time. The resistance of YSZ g and gb increased significantly in 10 h, which may correspond to closure of pores. While during the stage from 10 to 200 h, the resistance of the g was basically consistent, and the gb decreased with the growth of g, which indicated that the gb resistance was more sensitive to grain size. The change of porosity and pore shape could be interpreted through impedance parameters. The porosity decreased and the shape of pores became smaller and rounder with the increasing sintering time.

Structure, microstructure and electrical conductivity of 8YSZ containing NiO

The effects of NiO addition on the structure and microstructure of yttria-stabilized zirconia were investigated to clarify the role of the additive in the microstructure-related electrical conductivity of the solid electrolyte. Specimens of 8 mol% yttria-stabilized zirconia with NiO contents up to 5.0 mol% were prepared using nickel oxide and trihydroxi nickel carbonate as precursors. The specimens were sintered at 1350 °C for several holding times. The evolution of the lattice parameter with NiO content was evaluated by X-ray diffraction and the microstructural features by scanning electron microscopy. Electrical conductivity was evaluated by impedance spectroscopy measurements. The solubility limit of NiO at 1350 °C was found to be 1.5 mol% by X-ray diffraction. Energy dispersive spectroscopy results revealed Ni segregation for large holding times at 1350 °C. The grain boundary conductivity was found to be influenced by Ni segregation and to decrease with increasing holding times at high temperature.

Indentation on YSZ thermal barrier coating layers deposited by electron beam PVD

We have deposited an yttria-stabilized zirconia (YSZ) layer consisting of microsized grains on an alumina substrate using electron beam physical vapour deposition (EBPVD). The effects of Y2O3 concentration and the coating microstructures of YSZ on indentation damage are investigated. Different coating microstructures are prepared by varying the substrate temperature during deposition. Hertzian and nanoindentation are used to characterize the damage responses of the YSZ thermal barrier-coated alumina layered systems. The damage and failure behaviours have been investigated in relation to the effects of microstructures formed at various substrate temperatures as well as indentation loads from indentation tests. Elastic modulus, hardness, and stress–strain behaviours are characterized by results of the indentation tests. With increasing substrate temperature during the EBPVD process, the overall grain sizes became coarser with a smaller fraction of weak interfaces between the columns, and became more faceted. This change in microstructure ultimately influences the indentation behaviour. The results according to the use of smaller and larger scaled indenters during Hertzian/nanoindentations were also analyzed.

Influence of Hydrogen on the Microstructure of Plasma-Sprayed Yttria-Stabilized Zirconia Coatings

The influence of secondary hydrogen and current on the deposition efficiency (DE) and microstructure of yttria-stabilized zirconia (YSZ) coatings was evaluated. To better understand the influence of the spray process on coating consistency, a YSZ powder, −125 +44 µm, was sprayed with nitrogen/hydrogen parameters and a 9 MB plasma gun from Sulzer Metco. DE and coating porosity, which were produced using two different spray gun conditions yielding the same input power, were compared. Amperage was allowed to vary between 500 and 560 A, and hydrogen was adjusted to maintain constant power, while nitrogen flow was kept at a fixed level. Several power conditions, ranging from 32 to 39 kW, were tested. Different injection geometries (i.e., radial with and without a backward component) were also compared. The latter was found to produce higher in-flight temperatures due to a longer residence time of the powder particles in the hotter portion of the plasma. Porosity was based on cross-sectional micrographs. In-flight particle temperature and velocity measurements were also carried out with a special sensor for each condition. Test results showed that DE and coating density could vary significantly when a different hydrogen flow rate was used to maintain constant input power. On the other hand, DE was found to correlate very well with the temperature of the in-flight particles. Therefore, to obtain more consistent and reproducible DE and microstructures, it is preferable to maintain the in-flight particle temperature around a constant value instead of keeping a constant input power by adjusting the secondary hydrogen flow rate.

Photoluminescence properties of yttria-stabilized zirconia single crystal

The photoluminescence (PL) and PL excitation spectra of the single-crystal samples of yttria-stabilized zirconia (YSZ) were measured. We found that the PL spectra of two samples for the 280-nm excitation significantly differ from one another in intensity, although the two samples have the same macrostructure and the same chemical composition. These results indicate that PL spectroscopy is a useful method for studying the microstructure of YSZ. The result of the trace-impurity analysis suggests that the large difference in PL intensity between the two samples can be explained by considering the influence of Nb in the YSZ sample. It seems that Nb in YSZ caused an yttria-associated defect. The broad emission band around 550 nm would be attributed to this defect. These results suggest that information can be obtained from its PL measurement not only on the yttria-associated defect, but also on the trace impurity in the YSZ.

A Study of the Microstructure of Yttria-Stabilized Zirconia Deposited by Inductively Coupled Plasma Spraying

A Study of the Microstructure of Yttria-Stabilized Zirconia Deposited by Inductively Coupled Plasma Spraying

Biaxially aligned YBCO film tapes fabricated by all pulsed laser deposition

Abstract Biaxially aligned yttria-stabilized zirconia (YSZ) films on Ni-based alloy substrates were realized with high deposition rate of 0.5 μm min−1 by the inclined substrate deposition (ISD) technique without ion beam assistance. The microstructure of YSZ was examined to study the growth mechanism of biaxial alignment by ISD. Columnar structures toward the plasma plume suggested a self-shadowing effect in the ISD process. To raise Ic values, YBCO thickness was increased up to 5 μm. Thick YBCO films with high Jc values were realized on the ISD-grown YSZ. Long YBCO tapes with biaxial alignment were successfully fabricated using continuous pulsed laser deposition and a high Ic value of 37.0 A (77.3 K, 0 T) at a 75 cm voltage tap spacing was achieved.