Y2O3 Coatings Research Articles

Interaction phenomena between ceramic coatings and liquid uranium alloys

Owing to the high chemical reactivity of molten uranium alloys, the use of traditional graphite crucibles for casting fuel slugs for a sodium-cooled fast reactor (SFR) is problematic. Moreover, rare earth (RE) elements retained in the fuel slugs for an SFR, which are extracted from the spent fuel by pyro-processing, are more reactive than uranium melt. Therefore, in this study, Y2O3 single-layer coatings with thicknesses of approximately 50, 70, and 120 μm and double-layer coatings of TaC/Y2O3 and Y2O3/TaC were plasma-sprayed onto niobium substrates and tested for thermal shock resistance and compatibility against U–10 wt% Zr and U–10 wt% Zr–5 wt% RE melt. The Y2O3 single-layer coating, regardless of coating thickness, and the TaC/Y2O3 double-layer coating showed good contact at the interface between the coating and the niobium substrate, with no deterioration after the thermal cycling test. In the interaction studies, the single- and double-layer coatings showed good compatibility with the U–Zr melt. However, the Y2O3 coatings with thicknesses of approximately 50 and 70 μm showed severe penetration of the U–Zr–RE melt and reacted with the niobium substrate. The single-layer Y2O3 coating with a thickness of 120 μm and the double-layer TaC/Y2O3 coating exhibited the most promising performance among the candidate coatings.

INTERACTION STUDIES OF CERAMIC VACUUM PLASMA SPRAYING FOR THE MELTING CRUCIBLE MATERIALS

Candidate coating materials for re-usable metallic nuclear fuel crucibles, TaC, TiC, ZrC, ZrO₂, and Y₂O₃, were plasmasprayed onto a niobium substrate. The microstructure of the plasma-sprayed coatings and thermal cycling behavior were characterized, and U-Zr melt interaction studies were carried out. The TaC and Y₂O₃ coating layers had a uniform thickness, and high density with only a few small closed pores showing good consolidation, while the ZrC, TiC, and ZrO₂ coatings were not well consolidated with a considerable amount of porosity. Thermal cycling tests showed that the adhesion of the TiC, ZrC, and ZrO₂ coating layers with niobium was relatively weak compared to the TaC and Y₂O₃ coatings. The TaC and Y2O₃ coatings had better cycling characteristics with no interconnected cracks. In the interaction studies, ZrC and ZrO₂ coated rods showed significant degradations after exposure to U-10 wt.% Zr melt at 1600℃ for 15 min., but TaC, TiC, and Y₂O₃ coatings showed good compatibility with U-Zr melt.

Microstructural characterization of plasma sprayed conventional and nanostructured coatings with nitrogen as primary plasma gas

Air plasma sprayed conventional and nanostructured Al2O3–13wt.%TiO2, WC–12–17wt.%Co, and ZrO2–7–8wt.%Y2O3 coatings were deposited using nitrogen as the primary plasma gas. In nanostructured coatings, as critical plasma spray parameter (CPSP) – which is defined as the ratio of gun or arc power to the primary gas flow rate – controls the volume fraction of unmelted/partially melted regions, which in turn control the properties of coatings, coatings for all systems considered in this work were obtained as a function of CPSP. Coating properties, e.g. porosity, microhardness and percentage of partially melted/unmelted regions, for nanostructured coatings, were seen to be monotonic functions of CPSP. However, the effect of CPSP on percentage of partially melted/unmelted regions was significantly smaller for nitrogen as compared to argon as the primary plasma gas, particularly for nanostructured Al2O3–13wt.%TiO2 coatings.

Electron microscopy and diffraction studies of suspension-plasma-sprayed ZrO2 + 8 wt.% Y2O3 coatings

The microstructure of ZrO2+8wt.% Y2O3 coatings has been studied by means of transmission electron microscopy (TEM) including selected-area electron diffraction (SAED) and electron backscatter diffraction (EBSD). The coatings were suspension-plasma-sprayed onto stainless steel substrates. The parameters spray distance and torch scan speed had been varied. Both diffraction methods have confirmed the tetragonal lattice structure of the suspension-plasma-sprayed zirconia. The combination of TEM and EBSD allowed a detailed analysis of the size, shape and orientation of grains. The microstructure of the coatings is mainly lamellar, consisting of columnar grains. Additional microstructural features indicate that powder particles have been subjected to different modifications during spraying. The influence of the variation of spraying parameters on the microstructure is marginal.

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al2O3–ZrO2–Y2O3 composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al2O3 particles and Al2O3–Y2O3 sol. Their microstructure showed that YSZ particles were covered with spatial-network Al2O3–Y2O3 blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al2O3–Y2O3 or ZrO2–Y2O3 coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al2O3–Y2O3, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.

Thermal cycling behaviour of plasma sprayed lanthanum zirconate based coatings under concurrent infiltration by a molten glass concoction

Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operating temperatures, resulting in enhanced efficiencies and performance. However, during aero engine operation, environmentally ingested airborne particles, which includes mineral debris, sand dust and volcanic ashes get ingested by the turbine with the intake air. As engine temperatures increase, the finer debris tends to adhere to the coating surface and form calcium magnesium alumino-silicate (CMAS) melts that penetrate the open void spaces in the coating. Upon cooling at the end of an operation cycle, the melt freezes and the infiltrated volume of the coating becomes rigid and starts to spall by losing its ability to accommodate strains arising from the thermal expansion mismatch with the underlying metal. The state-of-the-art ZrO2-7-weight% Y2O3 (YSZ) coatings are susceptible to the aforementioned degradation. Rare-earth zirconates have generated substantial interest as novel thermal barrier coatings (TBC) based primarily on their intrinsically lower thermal conductivity and higher resistance to sintering than YSZ. In addition, the pyrochlore zirconates are stable as single phases at up to their melting point. La2Zr2O7 (LZ) is one among such candidates. Hence, the present study focusses on the comparison of cyclic molten CMAS infiltration behaviour of the base metal Inconel 738 (BM), the bond coat NiCrAlY (BC), the duplex YSZ, the LZ coating and a five layered coated specimen with LZ as top layer. Among those coatings mentioned above, the five layer coated specimen showed excellent CMAS infiltration resistance under thermal cycling conditions.

Modeling of elastic modulus and hardness determination by indentation of porous yttria stabilized zirconia coatings

The mechanical properties of materials can be determined by means of the instrumented indentation experiment. However, when the indentation test leads to a regular load–depth curve for homogeneous massive materials, the presence of defects such as roughness, porosity or cracks present in the region close to the indented zone can greatly modify the shape of this curve. To avoid influence of such defects on the indentation measurement, a cautious polishing is generally performed to obtain a smooth surface. However, for the study of coated materials when polishing is not possible, the defects can interfere with the results of the mechanical property determination and they must be definitely taken into consideration for the indentation data analysis. In this work, the indentation test is employed to characterize suspension plasma sprayed porous ZrO2 + 8 wt.% Y2O3 (8YSZ) coatings. By comparing different models for hardness analysis, we selected the most appropriate one allowing the calculation of the macrohardness taking into account the influence of roughness, porosity and cracks. Afterwards, we showed how the roughness interferes with the depth measurement at the beginning of the indentation test where a fast depth increase is observed. We also showed how the presence of confine defects leads to an abnormal effect during the loading, such as a horizontal plate or a jump in depth. Finally, we propose a methodology to avoid the influence of the roughness and of the porosity on the hardness determination and to model a selected part of the loading curve for the determination of the macrohardness and a parameter representative of the indentation size effect.

Characterization of the thermal conductivity of EB-PVD ZrO2–Y2O3 coatings with a pulsed thermal imaging method

ZrO2-(2–8mol%)Y2O3 coatings were deposited on zirconia substrates by EB-PVD. The coated layer had a columnar microstructure with intercolumnar gaps between columnar grains. Nanosized pores could be observed around feather-like grains as well as inside the columnar grains. The main phase of all coated specimens was the tetragonal phase. However, the monoclinic and cubic phase of the second phase could be detected for ZrO2-2mol%Y2O3 coatings and ZrO2-(6–8mol%)Y2O3 coatings, respectively. The thermal conductivity of coatings was successfully evaluated with the pulsed thermal imaging method using a combined sample of coatings and a substrate. Both the thermal conductivity and heat capacity of Y2O3-doped ZrO2 coatings tended to decrease with increasing amounts of Y2O3.

Wettability of Y₂O₃: A Relative Analysis of Thermally Oxidized, Reactively Sputtered and Template Assisted Nanostructured Coatings.

The wettability of reactively sputtered Y2O3, thermally oxidized Y-Y2O3 and Cd-CdO template assisted Y2O3 coatings has been studied. The wettability of as-deposited Y2O3 coatings was determined by contact angle measurements. The water contact angles for reactively sputtered, thermally oxidized and template assisted Y2O3 nanostructured coatings were 99°, 117° and 155°, respectively. The average surface roughness values of reactively sputtered, thermally oxidized and template assisted Y2O3 coatings were determined by using atomic force microscopy and the corresponding values were 3, 11 and 180 nm, respectively. The low contact angle of the sputter deposited Y2O3 and thermally oxidized Y-Y2O3 coatings is attributed to a densely packed nano-grain like microstructure without any void space, leading to low surface roughness. A water droplet on such surfaces is mostly in contact with a solid surface relative to a void space, leading to a hydrophobic surface (low contact angle). Surface roughness is a crucial factor for the fabrication of a superhydrophobic surface. For Y2O3 coatings, the surface roughness was improved by depositing a thin film of Y2O3 on the Cd-CdO template (average roughness = 178 nm), which resulted in a contact angle greater than 150°. The work of adhesion of water was very high for the reactively sputtered Y2O3 (54 mJ/m2) and thermally oxidized Y-Y2O3 coatings (43 mJ/m2) compared to the Cd-CdO template assisted Y2O3 coating (7 mJ/m2).

Improving the Bonding Strength of Y<sub>2</sub>O<sub>3</sub> Coatings on Steel Substrates through a YO<sub>X</sub> Adhesion Layer

Y2O3/YOx composite coatings with different YOx adhesion layers were fabricated on 316L stainless steel substrates through reactive magnetron sputtering. The YOx adhesion layers were deposited by radio frequency reactive magnetron sputtering with different oxygen partial pressures at a total pressure of 1.2 Pa. The Y2O3 coatings were deposited by bipolar pulse reactive magnetron sputtering. The relationships between the microstructures, phase compositions, interfacial adhesion of the composite coatings and the oxygen partial pressure used for deposition of the YOx adhesion layers were investigated. The microstructures and phase compositions of the coatings were characterized by scanning electronic microscopy and X-ray diffraction. The bonding strength of the coatings was measured by scratch tester. The results showed that the bonding strength of the Y2O3/YOx composite coatings reached the maximum value when the oxygen partial pressure was 0.18 Pa.

Preparation and Characterization of Yttrium Oxide Coatings by Plasma Spraying

Y2O3 coatings were prepared on aluminum substrates using plasma spraying with argon as primary gas. Structure and morphology were characterized by using X-ray diffraction, scanning electron microscopy and metallographic analysis. Results show that Y2O3 coating’s porosity is 5.6%. Coatings prepared with helium as secondary gas are purely white. However, there are black spots when hydrogen is used as secondary gas. A phase transformation from cubic to monoclinic tends to take place during the spraying operation under argon and hydrogen atmosphere. However this prejudicial transformation can be suppressed by using helium instead of hydrogen.

Measurements of coating density using ultrasonic reflection coefficient phase spectrum

A nondestructive method to determine the density of coating has been proposed in this paper based on the ultrasonic reflection coefficient phase spectrum (URCPS). A model was set up first to represent the ultrasonic waves reflected from a coating system at normal incident, and the relation between the extremum of URCPS and the coating density was established to provide the principle of determining the density. The ultrasonic method was validated on a series of ZrO2–7 wt.%Y2O3 (YSZ) coatings with various density. The specimens were prepared by electric beam physical vapor deposit (EB-PVD). After deposition, the specimens were irradiated using high-intensity pulsed ion beam (HIPIB) at different ion current density of 100 and 200 A/cm2 to change coating density. The coating densities of as-deposited and post irradiation by HIPIB were derived to be 4940–5030, 5200–5320 and 5390–5470 kg/m3, respectively. The relative error between the coating density measured by the ultrasonic method and Archimedean principle ranging from 2.53% to 6.11%, indicates that the proposed ultrasonic quantification method provides a reliable nondestructive way to determine coating density.

Structural, Mechanical and Erosion Properties of Yttrium Oxide Coatings by Axial Suspension Plasma Spraying for Electronics Applications

Yttrium oxide (Y2O3) coatings have been prepared by axial suspension plasma spraying with fine powders. It is clarified that the coatings have high hardness, low porosity, high erosion resistance against CF4 -containing plasma and retention of smooth eroded surface. This suggests that the axial suspension plasma spraying of Y2O3 is applicable to fabricating equipment for electronic devices, such as dry etching. Surface morphologies of the slurry coatings with splats are similar to conventional plasma-sprayed Y2O3 coatings, identified from microstructural analysis. Dense coating structures with no lamellar boundaries have been seen, which is apparently different from the conventional coatings. It has also been found that crystal structure of the suspension coatings mainly composed of metastable monoclinic phase, whereas the powders and the conventional plasma spray coatings have stable cubic phase. Mechanism of coating formation by plasma spraying with fine powder slurries is discussed based on the results.

Effect of Superficially Applied Y2O3 Coating on High-Temperature Corrosion Behavior of Ni-Base Superalloys

Inhibitors and oxide additives have been investigated with varying success to control high-temperature corrosion. Effect of Y2O3 on high-temperature corrosion of Superni 718 and Superni 601 superalloys was investigated in the Na2SO4-60 pct V2O5 environment at 1173 K (900 °C) for 50 cycles. Y2O3 was applied as a coating on the surfaces of the specimens. Superni 601 was found to have better corrosion resistance in comparison with Superni 718 in the Na2SO4-60 pct V2O5 environment. The Y2O3 superficial coating was successful in decreasing the reaction rate for both the superalloys. In the oxide scale of the alloy Superni 601, Y and V were observed to coexist, thereby indicating the formation of a protective YVO4 phase. There was a distinct presence of a protective Cr2O3-rich layer just above the substrate/scale interface in the alloy. Whereas Cr2O3 was present with Fe and Ni in the scale of Superni 718. Y2O3 seemed to be contributing to better adhesion of the scale, as comparatively lesser spalling was noticed in the presence of Y2O3.

Structure and Tribological Performance of Nanostructured ZrO2-3 mol% Y2O3 Coatings Deposited by Air Plasma Spraying

In this study, nanostructured ZrO2-3 mol% Y2O3 coatings were deposited by air plasma spray using reconstituted feedstock. The coating structures were characterized by x-ray diffractometer, micro-Raman spectrometer, field emission scanning electron microscope, and transmission electron microscope. It is revealed that the as-sprayed coating is mainly composed of columnar grains with diameters <100 nm and demonstrates the better toughness, higher microhardness, and lower porosity. It consists only of nontransformable tetragonal ZrO2 phase. The tribological performance of the coating was examined with a ball-on-disk apparatus under dry sliding conditions. The results show that the friction coefficient of as-sprayed coating was approximately one-fifth of the conventional zirconia coating and wear rate was lower one order of magnitude than the conventional zirconia coating. The dominant wear mechanism is abrasive wear. The improved wear resistance can be attributed to the increased mechanical properties of as-sprayed coating.

Simultaneous determination of the coating thickness and its longitudinal velocity by ultrasonic nondestructive method

The ultrasonic reflection coefficient amplitude spectrum (URCAS) has been extended to obtain the coating thickness and its longitudinal velocity at the same time on thick substrates. A model was set up first to represent the ultrasonic waves reflected from a coating system at normal incident, and the expression of URCAS was derived in order to obtain the coating thickness and velocity. Then, an inverse algorithm based on the Gauss–Newton method was introduced to determine the thickness and velocity by comparing the theoretical and measured URCAS. Experimental validation was conducted on the homogeneous epoxy coatings on the aluminum substrates and inhomogeneous ZrO2−7wt%Y2O3 (YSZ) coatings on the superalloy substrates. The relative errors of the thickness and velocity measurement were in the ranges of 2.15–2.35% and 2.67–4.40% for epoxy coatings, and between 5.33–5.96% and 8.95–9.66% for YSZ coatings. It is concluded that the URCAS combined with inversion technique can be applied to obtain the thickness and longitudinal velocity of coatings simultaneously.

Microwave ECR plasma CVD of cubic Y2O3 coatings and their characterization

Abstract Yttrium oxide (Y2O3) thin films are deposited by microwave electron cyclotron resonance (ECR) plasma assisted metal organic chemical vapour deposition (MOCVD) process at a substrate temperature of 350 °C using indigenously developed metal organic precursors (2,2,6,6-tetra methyl-3,5-heptane dionate) yttrium, commonly known as Y(thd)3 synthesized by ultrasound method. The deposited coatings are characterized by X-ray photoelectron spectroscopy, glancing angle X-ray diffraction, scanning electron microscopy, EDS and infrared spectroscopy. The characterization results indicate that it is possible to deposit non-porous coatings with excellent uniformity of a single phase cubic Y2O3 on various substrates by this process at reasonably low substrate temperature that is desirable in various manufacturing processes.

Mullite and Mullite/ZrO2-7wt.%Y2O3 Powders for Thermal Spraying of Environmental Barrier Coatings

Mullite and mullite/ZrO2-7wt.%Y2O3 coatings could be thought among the main protective layers for environment barrier coatings (EBCs) to protect Si-based substrates in future gas turbine engines. Considering that feedstock of the compound powder is not commercially available, two powder processing routes Spray Drying (SD) and Flame Spheroidization (FS) were implemented for both types of powders. For each method the particle size, the morphology, and microstructure of the powder particles was determined. In addition, the effect of the heat treatment on the powder crystallinity and microstructure of FS powders was also investigated. To evaluate their suitability as feedstock materials, the powders were plasma sprayed and their in-flight particle characteristics monitored for coatings production. The powder morphology was correlated to the in-flight particle characteristics and splat morphology to gain insight about into the influence of powder characteristics on the coating formation.

Low Temperature Transformation of 3 mol% Yttria Stabilized Zirconia Coatings Deposited by Air Plasma Spraying

The low temperature transformation of ZrO2-3 mol% Y2O3 coatings were investigated at 40 ~100oC using Raman spectroscopy. The results show that the low temperature transformation of ZrO2-3 mol% Y2O3 coatings is considerably effected by water temperature and mainly occurred on the coating surface. However, the transformation can be retarded by addition of Al2O3. The reasons lie in that the addition of Al2O3 can result in improvement of deposition efficiency and Young’s modulus of ZrO2-3 mol% Y2O3 coatings containing Al2O3.

Effect of the porous structure of thermal-barrier coatings on their heat conductivity

A phenomenological model is proposed to show the dependence of the heat conductivity of a porous coating on the pore size using experimental data on ZrO2 + 7% Y2O3 coatings. To obtain plasmasprayed coatings with different structures, powders with different properties are used. The concept of critical pore size at a given porosity is proposed. It is the maximum pore size at which the heat conductivity depends not only on the total porosity but also on the pore size, which is especially important for materials with submicron and nanosized structural components. Since the free path of phonons scattered by pores depends on the distance between them, the concept of critical distance between pores is introduced. It is the maximum distance beyond which the heat conductivity depends on the integral porosity alone.