Yttria-stabilized Zirconia Coatings Research Articles

EBSD analysis of microstructure changes in YSZ coatings during thermal cycling

Failure of atmospheric plasma sprayed (APS) yttria-stabilized zirconia (YSZ) coatings is complicated. Therefore, determining the microstructural changes that occur in the top coat during thermal cycling is beneficial to figure out the dominant failure mechanism in thermal barrier coatings. In this study, thermal cycling tests were conducted on YSZ coatings sprayed by APS. For comparison, coatings with the same spray parameters were held at 1100 °C for 25 h. Areas of interest in the cross-sections of the coatings were chosen prior to thermal cycling, in which ex-situ observation and analysis were performed via scanning electron microscope and electron backscatter diffraction. Residual stress values of coatings with different thermal cycling times were measured by X-ray diffraction. The growth of quasi-columnar crystals, which were related to a depletion in Al and an increase in stress, was observed. During thermal loading, the vertical cracks propagate comparatively easier leading to the spallation. The residual stress varies continually during the thermal cycling test and the changes are divided into several stages. The monoclinic phase transformation tends to occurred inside grains, creating compressive stress inside the coating, which improve the longevity of the coating.

Thermal Insulation of YSZ and Erbia-Doped Yttria-Stabilised Zirconia EB-PVD Thermal Barrier Coating Systems after CMAS Attack.

The impact of small deposits of calcium–magnesium–aluminium silicates (CMAS) on the top of thermal barrier coatings (TBCs) made of yttria-stabilised zirconia (YSZ) produced via electron-beam physical vapour deposition (EB-PVD) is shown to play a role in the microstructural and chemical stability of the coatings; hence, it also affects the thermal insulation potential of TBCs. Therefore, the present work investigates the degradation potential of minor CMAS deposits (from 0.25 to 5 mg·cm−2) annealed at 1250 °C for 1 h on a novel Er2O3-Y2O3 co-stabilised ZrO2 (ErYSZ) EB-PVD TBC, which is compared to the standard YSZ coating. Due to the higher reactivity of ErYSZ coatings with CMAS, its penetration is limited in comparison with the standard YSZ coatings, hence resulting in a better thermal insulation of the former after ageing.

Thermal Stability of PS-PVD YSZ Coatings with Typical Dense Layered and Columnar Structures

Yttria-stabilized zirconia (YSZ) coatings with typical pyramid columnar and dense layered structure were prepared by plasma spray-physical vapor deposition (PS-PVD). The evolution behavior of microstructure and crystallography of the coatings before and after thermal aging treatment were observed by electron backscatter diffraction (EBSD) and field emission scanning electron microscopy (FE-SEM). Results showed that the as-deposited coatings exhibited many types of structures and were mainly composed of a nonequilibrium tetragonal (t’-ZrO2) phase. With the prolonging of thermal exposure time, the initial nonequilibrium tetragonal phase of YSZ coatings gradually transformed into a monoclinic (m-ZrO2) phase. During the process of stationary deposition, at a proper spraying distance, each column exhibited a certain preferred orientation, but the ceramic topcoat did not exhibit distinct preferred orientation statistically.

Development of thermal barrier coatings using reactive pulsed dc magnetron sputtering for thermal protection of titanium alloys

There is a need to thermally protect titanium (Ti) alloys so that they can be used in high temperature environments, such as during hypersonic flight. In this study, a thermal barrier coating (TBC) system was developed using reactive magnetron sputtering. The coating system consists of a nickel chromium aluminum yttrium (NiCrAlY) bond coat and an yttria-stabilized zirconia (YSZ) top coat. The YSZ layer was deposited by reactive pulsed direct current (DC) magnetron sputtering a Zr-Y target in a gas mixture comprised of argon (Ar) and oxygen (O2). The pulsed DC magnetron sputtering process used closed-loop control with the cathode voltage as the feedback signal. The YSZ coatings were deposited at different cathode voltage feedback signals. The phase structure, elemental composition, microstructure, and thickness of the YSZ coatings were examined by different means. It was found that up to 69% of the metallic deposition rate can be achieved for obtaining near stoichiometric YSZ coatings. The YSZ coatings exhibit single cubic phase structure with an Y2O3% in the range from 6.5 mol% to 8 mol%. The developed TBC consists of a 2 μm Ti adhesion layer, a 30 μm NiCrAlY bond coat, and a 40 μm YSZ top coat deposited on Ti-6Al-4 V substrate. The NiCrAlY layer exhibited a dense structure and the YSZ layer exhibited a porous columnar grain structure. The TBC showed excellent adhesion on Ti-6Al-4 V. The coated Ti-6Al-4 V was annealed from 800 °C to 1000 °C in air to evaluate the thermal protection capability of the coating. No degradation of the TBC was observed and the Ti-6Al-4 V alloy preserved fine gains and desired mechanical properties up to 900 °C. However, the coating showed horizontal cracks and degradation at the interface between the Ti-6Al-4 V substrate and the NiCrAlY bond coat at 1000 °C. It is assumed that the outward diffusion of Al from the Ti alloy led to the coating failure at the interface. This indicates that new diffusion barrier layers are needed as part of the TBC system.

Very low-pressure plasma-sprayed dense yttria-stabilized zirconia coatings using an axial bi-cathode plasma torch

In order to increase the plasma power and enhance the heating of sprayed powder, a bi-cathode plasma torch with axial powder injection was developed for the very low-pressure plasma spraying (VLPPS). By using this torch, uniform and dense yttria-stabilized zirconia (YSZ) coatings were prepared at different spraying distances. The plasma characteristics, microstructures and mechanical properties of YSZ coatings were investigated in detail. Results show that the new plasma torch had the ability to vaporize part of the YSZ powder. Meanwhile, the YSZ powder underwent a distinct phase change, at a maximum of 50% monoclinic zirconia phase transformed to the tetragonal-prime zirconia phase. The sprayed coatings had hybrid microstructures, including unmelted particles, molten droplets, and vapor clusters. With increasing spraying distance, the melting state of YSZ coating got better, while the microstructure became looser, leading to the porosity increased from 2.7% to 7.4%. The best mechanical properties were found in the densest coating, whose microhardness and Young's modulus was 11.7 ± 0.6 GPa, 167.8 ± 4.0 GPa, respectively, indicating that the mechanical performance of the YSZ coating was more dependent on the density, rather than the melting state. The dense VLPPS ceramic coatings may be applied for corrosion and heat resistance against oxidation, as well as the solid oxide fuel cells.

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

AbstractAbout 6‐8 wt% yttria‐stabilized zirconia (YSZ) is the industry standard material for thermal barrier coatings (TBC). However, it cannot meet the long‐term requirements for advanced engines due to the phase transformation and sintering issues above 1200°C. In this study, we have developed a magnetoplumbite‐type SrAl12O19 coating fabricated by atmospheric plasma spray, which shows potential capability to be operated above 1200°C. SrAl12O19 coating exhibits large concentrations of cracks and pores (~26% porosity) after 1000 hours heat treatment at 1300°C, while the total porosity of YSZ coatings progressively decreases from the initial value of ~18% to ~5%. Due to the contribution of porous microstructure, an ultralow thermal conductivity (~1.36 W m−1 K−1) can be maintained for SrAl12O19 coating even after 1000 hours aging at 1300°C, which is far lower than that of the YSZ coating (~1.98 W m−1 K−1). In thermal cyclic fatigue test, the SrAl12O19/YSZ double‐ceramic‐layer coating undertakes a thermal cycling lifetime of ~512 cycles, which is not only much longer than its single‐layer counterpart (~163 cycles), but also superior to that of YSZ coating (~392 cycles). These preliminary results suggest that SrAl12O19 might be a promising alternative TBC material to YSZ for applications above 1200°C.

Effects of the dissolution of thermal barrier coating materials on the viscosity of remelted volcanic ash

Abstract The chemical interaction between remelted volcanic ash and ceramic coatings of yttria-stabilized zirconia (YSZ) and/or gadolinium zirconate (GZO) is of special importance for the design of volcanic ash melt-resistant thermal barrier coatings (TBCs) for aviation turbine technologies. The spreading and infiltration potential of the melts is strongly influenced by the melt viscosity. Thus the interpretation of infiltration experiments and modeling of infiltration processes both rely on accurate viscosity data. Melt viscosity may be significantly altered by the dissolution of the YSZ or GZO thermal barrier coatings during the infiltration process. Here, we have determined the influence of YSZ and GZO additions to the viscosity of a series of volcanic ash melts using high-temperature concentric cylinder viscometry. All samples have been characterized fully after viscometry. At 6.5 wt% of YSZ or GZO, both dopants lead to a reduction of viscosity in a temperature range between 1297–1640 °C in air. The magnitude of the decrease in viscosity depends weakly on volcanic ash melt composition. The viscosity effect has been parameterized in the following form: Δ η = x ⋅ m dopant whereby x is a melt-composition specific coefficient of viscosity decrease, and mdopant represents the added amount of YSZ/GZO (wt%). This viscosity reduction should contribute to an acceleration of the physical infiltration of TBCs via remelted volcanic ash.

Deuterium permeation behavior through yttria-stabilized zirconia coating fabricated by magnetron sputtering

Abstract For the realization of a sufficient fuel cycle in a fusion reactor, tritium permeation barriers have been studied mainly using ceramics coatings to suppress tritium leakage through structural materials. In this study, the first trial of fabrication and characterization of yttria-stabilized zirconia (YSZ) coatings has been carried out. The YSZ coatings containing 4 mol% Y2O3 were deposited on reduced activation ferritic/martensitic steels by radio-frequency magnetron sputtering devices in two different deposition modes. The coating fabricated in the metallic mode showed a lower density with many cracks, and deteriorated after annealing and deuterium permeation tests at 600 °C. On the other hand, the coating fabricated in the poison mode showed relatively higher density, and did not deteriorate from the result of the deuterium permeation measurements at up to 600 °C. These results suggest that the YSZ coating deposited in the poison mode would be applied as a tritium permeation barrier.

The Research of Yttria Stabilized Zirconia Coatings formed by a low Pressure Thermal Spraying Plasma Torch

To enhance deposit efficiency and improve quality of the coatings of the Yttria stabilized zirconia (YSZ) formed by the thermal plasma spraying method. A new kind plasma touch with diverging-converging nozzle and an internal powder feedstock were designed. The optical emission spectroscopy method was used for evaluating powder molten and vaporized degrees. The results indicated that the plasma jet produced by the two kinds of plasma gases under the 700 a condition made the feedstock powder of YSZ vaporized partially, but different melting degrees. Denser column microstructure YSZ coatings were first successfully deposited at an arc current of 700 A by using Ar-He-H2 plasma gas. In the Ar-H2 plasma jet, the temperature is higher at the same axial distance. The micro-hardness of the coatings deposited by Ar-He-H2 plasma gases is higher.

Crystallinity, Stresses, and Cracks of YSZ Coatings Characterized by SEM–EBSD–Raman Spectroscopy

Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were prepared by atmospheric plasma spraying (APS). SEM–EBSD–Raman spectroscopy evaluated the relationship between the crystallinity, stresses, and cracks in YSZ coatings. Cracks were more likely to form in locations having poor crystallinity, due to the tensile stresses. Analysis between the formation of cracks and stresses shows that transgranular cracks are more likely created in areas with poor crystallinity. Moreover, intergranular cracks are more likely to occur around columnar grains, because the tensile stresses of columnar grains are greater than those of equiaxed grains. Resultantly, the formation and propagation of cracks can be controlled by controlling the cooling rate and grain shape.

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.

MICROSTRUCTURAL, THERMAL AND WEAR BEHAVIOR OF YSZ/Al2O3 THERMAL BARRIER COATINGS FOR GUN BARREL APPLICATIONS

The present study aims to investigate the performance of Yttria Stabilized Zirconia and Alumina Thermal Barrier Coatings deposited on EN steel substrates used in gun barrel through Atmospheric Plasma Spray Process. Chemical compositions of the EN steels (EN8, EN19, EN24 and EN36C) as well as the particles were examined prior to the coating process. Atmospheric Plasma Spray Process was carried out using carrier gas with two different flow rates of 3 and 4scfh. A bond coat was made for 50 µm by depositing NiCrAlY and the top coats were made for 200 µm distinctly using Yttria Stabilized Zirconia and Alumina. X-ray Diffraction and Field Emission Scanning Electron Microscope examination were performed on the coated specimens to observe the phase and morphology respectively. Thermal testing and Infrared thermal imaging were performed to measure the insulation performance and spallation performance under different exposure time (upto 20 minutes) for the input temperature of 200˚C. Wear and scratch test were conducted on the specimens to determine the coating integrity as well as bonding of top and bond coat to substrate. YSZ coated EN36C steel produced under carrier gas flow rate of 3scfh possessed greater insulation performance compared to all other steels. Also, YSZ coated EN36C steel has better wear and scratch resistance indicating lesser deformation compared to other materials.

Development of ZrO2 and YSZ coatings deposited by PE-CVD below 800 °C for the protection of Ni alloys

One possibility to increase the safety of structural Ni alloys is the use of yttria stabilized zirconia coatings in order to avoid deleterious thermal effects on their mechanical properties and corrosion resistance above 800 °C. However, the deposition of YSZ with a fully cubic structure below 800 °C cannot be achieved by traditional processing routes. In this work, we show the deposition of ZrO2 and Y2O3-stabilized ZrO2 coatings by Plasma Enhanced Chemical Vapor Deposition (PE-CVD). To this purpose, the effect of deposition temperature (500–800 °C), plasma power (50–250 W) and precursor concentration (0.5–2 g) on the microstructure, composition and crystalline phase of ZrO2 and YSZ coatings was studied. Contrary to previous reports, we show that it is feasible to use zirconium (IV) acetylacetonate [Zr (acac)4] to produce nanostructured zirconia coatings with low carbon content (1.8 at.%) and metastable tetragonal phase. A fully cubic YSZ structure was achieved with the use of Y-acetylacetonate with concentrations above 12 mol% at 700 °C and obtain a columnar microstructure.

Thermal Properties and Microstructures Analysis of YSZ and YSZ-Al2O3 Thermal Barrier Coatings

Yttria-stabilized zirconia (YSZ), 15 wt.%Al2O3-doped YSZ (YSZ-15 wt.%Al2O3), and 35 wt.%Al2O3-doped YSZ (YSZ-35 wt.%Al2O3) coatings were prepared by atmospheric plasma spraying. Composition and microstructure of the coatings were characterized. Moreover, the effect of composition and microstructure on the thermal conductivities and thermal expansion coefficients (TECs) of the coatings was investigated. The YSZ-15 wt.%Al2O3 coatings possess much lower thermal conductivity compared to YSZ coating which is attributed to the smaller grains, more oxygen vacancies and the large atomic weight difference between Al and Zr. The mentioned factors improve the phonon scattering thereby reducing the thermal conductivity. The lower TECs of YSZ-15 wt.%Al2O3 coatings are attributed to the smaller lattice parameters due to the substitutional Al3+ doping at Zr4+ sites. The decrease in lattice parameter values increases the crystal lattice energy. And the TEC is inversely proportional to the lattice energy. Moreover, the TEC of the YSZ-35 wt.%Al2O3 coating is much lower than the YSZ coating, which is due to the precipitation of Al2O3 and the formation of amorphous phases. Compared to ZrO2 phase, the Al2O3 phase possesses lower TECs values. The amorphous phase can account for larger expansion tolerance compared to the corresponding crystalline phase due to its loose atomic arrangement.

Synthesis of Yttria-Stabilized Zirconia Nano Powders for Plasma Sprayed Nano Coatings

Abstract Plasma sprayed Yttria Stabilized Zirconia (YSZ) coatings, with few microns sized microstructure/grain morphology has been well researched, reported and established as an industrial Thermal Barrier Coatings (TBC) material/system. However, nano structured YSZ coatings possess improved characteristics when compared with their micron sized counterparts. However, due to their nano sizes, light weight, and low density, plasma spray coating process of nano powders suffers from flowability issues due to lack of nano powder inertia/momentum, leading to poor deposition/ uneven coating thickness. In this research work, nano structured YSZ coatings were synthesized by using an Atmospheric Spray Coating (APS) facility. Nano powders of YSZ were used as the starting materials to prepare micron sized plasma sprayable powders. 80μm thick NiCrAlY bond coat (commercial) and 200μm thick YSZ top coat with nano microstructure (lab synthesized) were built on steel substrates. The starting nano crystalline (YSZ) powders, measuring 30-70 nanometers (nm) were synthesized in the laboratory via chemical method (sol-gel) by employing zirconium oxy chloride hexa-hydrate and yttrium nitrate as precursors, citric acid as chelating agent and ethylene glycol for the diversification reaction followed by calcination @ 1000°C. They were then re-constituted into micron sized (53-106 μm) plasma sprayable powders by agglomerating with polyvinyl alcohol (PVA) binders. The nano crystallite morphology of powders and coatings were analyzed by Scanning Electron Microscope (SEM), chemical composition by Energy Dispersive spectroscopy (EDS) and crystal structural phase by X-ray diffraction (XRD). The influence of calcination temperature of 1150°C on nano crystallite morphology was also studied.

Thermal Properties of Plasma-Sprayed Multilayer Al2O3/Yttria-Stabilized Zirconia Coating

Yttria-stabilized zirconia (YSZ) and Al₂O₃ multilayer coatings were fabricated by plasma spraying. Thermal conductivity and thermal shock test were investigated to find out the thermal properties of the coating layer and the surface of the crack. Thermal conductivity was investigated using laser flash method and thermal shock were measured by water quenching method. Furthermore, the factors influencing thermal properties of these coatings were analyzed in detail. Multilayer coatings have imperfect interfaces. At an imperfect interface, the heat transfer coefficient was close to zero, indicating a low thermal conductivity. Multilayer coatings increase thermal shock resistance. This is because ZrO₂ phases transformation from tetragonal to monoclinic occurring to the cooling process, resulting in microcracks due to volume expansion. The crack generated at this time dispersed and absorbed the thermal shock, so that it exhibited excellent thermal shock resistance.

Influence of Residual Blast-materials on Adhesion of Plasma-sprayed Yttria-stabilized Zirconia Coatings

Influence of Residual Blast-materials on Adhesion of Plasma-sprayed Yttria-stabilized Zirconia Coatings

Apparent Interfacial Toughness of Undoped and Photoluminescent Eu3+-Doped Yttria-Stabilized Zirconia Thermal Barrier Coatings

Most photoluminescence methods for the diagnostic of thermal barrier coatings (TBC) rely on the functionalization of yttria-stabilized zirconia (YSZ) with trivalent lanthanide ions. It consists in determining temperature and detecting preventively damages within the volume of the TBC prior to ceramic topcoat spallation. The latter depends on the interfacial toughness, which is an important factor to address thermal barrier coating’s performance and durability. In this paper, the influence of the addition of rare earth elements (Eu3+) on the interfacial toughness of TBC deposited by atmospheric plasma spray is investigated. Two types of coatings are deposited and investigated: (1) Type I: coating deposited using Eu3+-doped YSZ powder (2 mol.%), (2) Type II: coating deposited using undoped YSZ powder. Both types of coatings are heat-treated at 1100 °C under isothermal conditions using different oxidation exposure times: 100, 300 and 800 h. The morphology of the interface between the topcoat (TBC) and the bond coat is analyzed by scanning electron microscopy. The apparent interfacial toughness is investigated using indentation. It is shown that the interfacial apparent toughness decreases as the oxidation exposure time increases. Concomitantly, the thickness of the thermally grown oxide (TGO) layer between the bond coat and the topcoat increases. Results show as well that the partial substitution of Y3+ ions by a low amount of Eu3+ ions (2 mol.%) does not have influence on the microstructure and the interfacial toughness of the YSZ coatings. In addition, energy dispersive spectrometry reveals that there is no diffusion of Eu3+ into the TGO layer. It is therefore concluded that the use of Eu3+ for damage diagnostic based on photoluminescence methods will not induce any kind of degradation of the properties of TBCs.

The Thermal Conductivity of 8 Yttria Stabilized Zirconia and Mullite Thermal Barrier Coating on Medium Carbon Steel Substrate

Thermal conductivity is one of the main features of a thermal barrier coating (TBC) that is important in making sure that the TBC gives its best functionality to the system. A good TBC has low thermal conductivity, so that the temperature can drop across the coating which allows the system to operate in extremely high temperatures. There are several factors that can influence the thermal conductivity of the TBC such as the type of ceramic material used, the deposition method and the physical features of the TBC itself. For this research, air plasma spray (APS) is used to deposit 8 wt% yttria stabilized zirconia (8YSZ) and mullite on medium carbon steel substrates to study their respective thermal conductivities. The aim here is to develop a heat shield using TBC to protect the electric motor in an electrical turbocompounding system. The characteristics of the deposited TBC such as microstructure, element composition, phases and thermal conductivity are studied. The thermal conductivity is reduced when medium carbon steel substrate deposited with TBC. The thermal conductivity of 8YSZ, mullite and uncoated sample at minute 60 is 0.868 W/mK, 0.903 W/mK and 1.057 W/mK, respectively. Therefore, the deposition of 8YSZ TBC can lower the thermal conductivity of the medium carbon steel heat shield.

Inkjet printing yttria stabilized zirconia coatings on porous and nonporous substrates

This article describes the development of Yttria stabilized Zirconia coatings on porous and non-porous substrates through inkjet printing process and explores the role of substrate in defining coating characteristics through micro structural characterization and micro indentation. Zirconia based inks were prepared through high energy ball milling and printed on porous and non porous substrates. The low temperature sintering of coated substrates lead to stabilization of metastable t-phase of Zirconia, thus ensuring durability of coatings. However, the open pore sites in non porous substrate is incapable of retaining the fine particles in the ink on the substrate as they seep through the open pores. But, the combination of coarse and fine particles in ink resulted in controlling permeability of initial layers of coatings consequently forming thin coatings over porous substrates. Micro indentation tests on inkjet printed coatings on porous substrates and non porous substrate confirmed the capability of inkjet printing process to develop uniformly thick ceramic coatings irrespective of the type of substrate. Furthermore, it was revealed that multiple bi-layer coatings with coarse and fine PSD material powder based inks result in higher hardness in comparison to fine PSD material based inks.