Air Plasma-Sprayed Yttria and Yttria-Stabilized Zirconia Thermal Barrier Coatings Subjected to Calcium-Magnesium-Alumino-Silicate (CMAS)

Yttria (Y₂O₃) and zirconia (ZrO₂) stabilized by 8 and 20 wt.%Y₂O₃ thermal barrier coatings (TBCs) subjected to calcium-magnesium-alumino-silicate (CMAS) have been investigated. Free-standing Y₂O₃, 8 and 20 wt.%YSZ coatings covered with synthetic CMAS slurry were heated at 1300 °C in air for 24 h in order to assess the effect of Y₂O₃ on the corrosion resistance of the coatings subjected to CMAS. The microstructures and phase compositions of the coatings were characterized by SEM, EDS, XRD, RS, and TEM. TBCs with higher Y₂O₃ content exhibited better CMAS corrosion resistance. Phase transformation of ZrO₂ from tetragonal (t) to monoclinic (m) occurred during the interaction of 8YSZ TBCs and CMAS, due to the depletion of Y₂O₃ in the coating. Some amounts of original c-ZrO₂ still survived in 20YSZ TBCs along with a small amount of m-ZrO₂ that appeared after reaction with CMAS. Furthermore, Y₂O₃ coating was found to be particularly highly effective in resisting the penetration of molten CMAS glass at high temperature (1300 °C). This may be ascribed to the formation of sealing layers composed of Y-apatite phase [based on Ca₄Y₆ (SiO₄)₆O and Y_4.67(SiO₄)₃O] by the high-temperature chemical interactions of Y₂O₃ coating and CMAS glass.     

Wear and corrosion properties of plasma sprayed AI2O3 and Cr2O3 coatings sealed by aluminum phosphates

Plasma-sprayed aluminum oxide (AI₂O₃) and chromium oxide (Cr₂O₃) coatings were sealed by aluminum phosphates. Phosphates were formed throughout the coating, down to the substrate, and were verified by scanning electron microscopy and hardness measurements. The sealing increased the hardness of the coatings by 200 to 300 Vickers hardness (HV) units. Abrasion and erosion wear resistances were increased by the sealing treatment. Sealing also substantially closed the open porosity, as shown in electro-chemical corrosion tests. The sealed structures had good resistance against corrosion during 30 days of immersion in both acidic and alkaline solutions with pH values from 0 to 10. No decrease in abrasion wear resistance was observed after immersion.     

Einfluß betriebsnaher thermischer Belastung auf die Eigenschaften von Y2O3-teilstabilisierten ZrO2-Wärmedämmschichten

Influence of close to practice thermal loads on the properties of thermal barrier layers of ZrO₂ partially stabilized with Y₂ O₃ The use of plasma sprayed thermal barrier coatings offers the possibility to protect thermally stressed components in engines against overstressing. A suitable material for that is Y₂O₃-partially stabilized zirconium oxide. Considering the high temperatures as they occur in use the thermal barrier coatings were examined for their resistance concerning phase composition, porosity, hardness and structure. It turned out that changes in structure lead to a considerable increase of porosity and thus to a decline of hardness. Concerning the phase composition there was only a slight change after heat treatment in favour of monoclinic and cubic modification.     

Effect of Microstructure on the Thermal Conductivity of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-YSZ Coatings

The microstructures of three atmospheric plasma-sprayed (APS) Al₂O₃-ZrO₂ coatings were investigated using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The differences in the microstructures of the three Al₂O₃-ZrO₂ coatings, including their phase compositions, cracks, pores, grain sizes, and solid solutions, were analyzed in detail. A close relationship was observed between the thermal conductivities of the coatings and the microstructures, and the Al₂O₃-YSZ coatings with more spherical pores, fewer vertical cracks, and finer grains exhibited the lowest thermal conductivity of 0.91 W/m·K. Compared with YSZ coatings, Al₂O₃-YSZ coatings can exhibit lower thermal conductivity, which may be attributed to the formation of an amorphous phase, smaller grains, and Al₂O₃-YSZ solid solution.     

Nano/Micro-Laminated (ZrO2–Y2O3)/(Al2O3–Y2O3) Composite Coatings and Their Oxidation Resistance

Nano/micro-laminated (ZrO₂–Y₂O₃)/(A1₂O₃–Y₂O₃) composite coatings were deposited onto an Fe–25Cr–7Ni–N alloy substrate by using alternate electrochemical and sintering processes. The thickness of each layer was in the range of 80–500 nm. Experimental results indicated that the multi-laminated coatings were more effective in providing oxidation resistance than monolithic ZrO₂–Y₂O₃ or A1₂O₃–Y₂O₃ coatings, with the oxidation resistance of the former increasing with increasing number of laminated layers. The microstructural studies suggest that the laminated coatings possess the advantages of these two types of coatings and avoid the weakness of single ZrO₂–Y₂O₃ or A1₂O₃–Y₂O₃ coatings. Reactive elements Y and Zr also played a role in this nano-layered setting in improving the oxidation resistance of the coatings.     

Structure and corrosion resistance of ZrO2 ceramic coatings on AZ91D Mg alloys by plasma electrolytic oxidation

The aim of this work is to study the structure and the corrosion resistance of the plasma electrolytic oxidation ZrO₂ ceramic coatings on Mg alloys. The ceramic coatings were prepared on AZ91D Mg alloy in Na₅P₃O₁₀ and K₂ZrF₆ solution by pulsed single-polar plasma electrolytic oxidation (PEO). The phase composition, morphology and element distribution in the coating were investigated by X-ray diffractometry, scanning electron microscopy and energy distribution spectroscopy, respectively. The results show that the coating thickness and surface roughness were increased with the increase of the reaction time. The ceramic coatings were of double-layer structure with the loose and porous outer layer and the compact inner layer. And the coating was composed of P, Zr, Mg and K, of which P and Zr were the main elements in the coating. P in the coating existed in the form of amorphous state, while Zr crystallized in the form of t-ZrO₂ and a little c-ZrO₂ in the coating. Electrochemical impedance spectra (EIS) and the polarizing curve tests of the coatings were measured through CHI604 electrochemical analyzer in 3.5% NaCl solution to evaluate the corrosion resistance. The polarization resistance obtained from the equivalent circuit of the EIS was consistent with the results of the polarizing curves tests.     

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.     

Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings

Al₂O₃-ZrO₂ coatings were deposited by the suspension plasma spray (SPS) molecularly mixed amorphous powder and the conventional air plasma spray (APS) Al₂O₃-ZrO₂ crystalline powder. The amorphous powder was produced by heat treatment of molecularly mixed chemical solution precursors below their crystallization temperatures. Phase composition and microstructure of the as-synthesized and heat-treated SPS and APS coatings were characterized by XRD and SEM. XRD analysis shows that the as-sprayed SPS coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases, while the as-sprayed APS coating consists of tetragonal ZrO₂, α-Al₂O₃, and γ-Al₂O₃ phases. Microstructure characterization revealed that the Al₂O₃ and ZrO₂ phase distribution in SPS coatings is much more homogeneous than that of APS coatings.     

Effect of Sealing Treatment on Corrosion Resistance of Plasma-Sprayed NiCrAl/Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>-8 wt.%TiO<Subscript>2</Subscript> Coating

Plasma-sprayed ceramic coatings inherently contain pores and micro-cracks which is deleterious when performed in aggressive environment. Various methods were applied to the as-sprayed coatings in order to improve the corrosion resistance. In the investigation of this study, plasma-sprayed NiCrAl/Cr₂O₃-8 wt.%TiO₂ coatings were sealed by epoxy resin and silicone resin, respectively. Coatings were characterized by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), optical microscopy (OM) and x-ray diffraction (XRD). The possible corrosion mechanism was discussed. The results of salt spray test and electrochemical measurements indicated that after the sealing treatment, the porosity of coatings decreased obviously and a compact layer was formed to protect the coating from corrosion. The silicone resin proved to be more effective than epoxy resin in enhancing the corrosion resistance of the coatings used in this research.     

Microstructural study of aluminum phosphate-sealed, plasma-sprayed chromium oxide coating

Microstructural characterization of aluminum phosphate-sealed, plasma-sprayed chromium oxide coating was carried out in order to study the strengthening mechanisms of the aluminum phosphate sealant in the coating. Characterization was performed using x-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The structure of the sealed coating was lamellar with columnar α-Cr₂O₃ grains extending through the lamella thickness. Amorphous aluminum phosphate sealant had penetrated into the structural defects of the coating such as cracks, gaps, and pores between the lamellae. The relative composition was 25 at.% aluminum and 75 at.% phosphorus for the sealant in the coating, giving the molar ratio P/Al of 3, which corresponds to that of metaphosphates Al(PO₃)₃. There is no indication of reaction products from the chemical reactions between the sealant and the coating. Thus, the aluminum phosphate sealing in the chromium oxide coatings can be explained mainly by adhesive binding resulting from the formation of the condensed phosphates with the appropriate adhesive properties to the coating, and not by chemical bonding resulting from the chemical reactions between the sealant and the coating.     

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.     

Effect of Y2O3 content in the pack on microstructure and hot corrosion resistance of Y–Co-modified aluminide coating

Y–Co-modified aluminide coatings on nickel base superalloys were prepared by pack cementation method. Effect of Y₂O₃ content in the pack mixture on microstructure and hot corrosion resistance of the coatings was investigated. The results show that with the increase in Y₂O₃ content, the content of Co in the coatings increases. The mass gain of the coatings with Y₂O₃ addition of 1, 2 and 3 wt.% is 0.6, 0.55 and 0.42 mg/cm² after hot corrosion at 1173 K for 100 h, respectively. Y₂O₃ addition accelerates the diffusion of Co and thus increases the hot corrosion resistance of the coating.     

Al2O3-ZrO2 mixed oxide composite incorporated aluminium rich zinc coatings for high wear resistance

Corrosion resistance and wear resistance are the two important parameters for high performance of zinc galvanic coating. In the present work, the improvement of these two characteristics was achieved by the incorporation of Al₂O₃-ZrO₂ mixed oxide composite in the coating. Al₂O₃-ZrO₂ mixed oxide composite was synthesized from ZrOCl₂·8H₂O. Aluminium rich zinc coatings with high sliding wear resistance was developed from a galvanic bath containing the mixed oxide. Based on the performance of the coating during physicochemical and electrochemical characterization, the concentration of mixed oxide composite in the bath was optimized as 0.50 wt% Al₂O₃-0.50 wt% ZrO₂. While rich in Al-metal content in the coating caused high corrosion resistance, the incorporation of the mixed oxide improved structural characteristics of the coating resulting in high wear resistance also. The coating was nonporous in nature and even the interior layers had high stability. The coatings have potential scope for high industrial utility.     

Thermal barrier coating application of zircon sand

Naturally occurring zircon sand was plasma spray coated on steel substrates previously coated with NiCrAlY bond coat. The coatings were characterized for their microstructure, chemical composition, thermal shock resistance, and the nature of structural phases present. The as-sprayed coatings consisted of t-ZrO₂ (major phase), m-ZrO₂, ZrSiO₄ (minor phases), and amorphous SiO₂. These coatings, when annealed at 1200 °C/1.44 × 10⁴s yielded a ZrSiO₄ phase as a result of the reaction between ZrO₂ and SiO₂. Dramatic changes occurred in the characteristics of the coatings when a mixture of zircon sand and Y₂O₃ was plasma spray coated and annealed at 1400 °C/1.44 × 10⁴s. The t-ZrO₂ phase was completely stabilized, and these coatings were found to have considerable potential for thermal barrier applications.     

Corrosion and protection characteristics of phosphate coatings: effect of yttrium oxide as additive

Abstract

The corrosion and protection characteristics of phosphate coatings formed in a phosphating solution containing mainly ZnO, H₃PO₄ and NaF, using Y₂O₃ as an additive, were investigated through SEM, polarisation curves and EIS diagrams. The results show that the corrosion protection of phosphate coatings has been improved when Y₂O₃ is added to the phosphating solution, making the free corrosion potential shift to the positive direction and causing the corrosion current to decrease. The protection ability of phosphate coatings depends mainly on their barrier performance. The phosphate coatings formed in the phosphating solution with 10 and 20 mg L^–1 Y₂O₃ have finer crystal structures and smaller porosity; therefore, they exhibit better corrosion resistance and adhesion properties than those without Y₂O₃ and with 40 mg L^–1 Y₂O₃.     

Thermal conductivity and phase stability of plasma sprayed ZrO2–Y2O3–La2O3 coatings

Thermal conductivity and phase stability of La₂O₃,Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. La₂O₃ was selected as a stabilizer because it had a significant effect on reducing densification of Y₂O₃ stabilized ZrO₂ (YSZ). The developed coating showed low thermal conductivity even after high temperature exposure due to its high resistance to sintering. However, phase stability and thermal cycle life of the coating decreased as the amount of La₂O₃ was increased. It was concluded that optimum amount of La₂O₃ addition was about 1 mol% to suppress sintering with little degradation of other properties.     

A study on preparation and corrosion behavior of nano rare earth oxide-modified chromized coatings

The corrosion resistance of carbon steel in a harsh environment was improved by preparing a chromized coating on the surface through the method of pack cementation. Nano rare earth-oxides (NREOs), Y₂O₃, CeO₂, and La₂O₃ were added to the pack and to further enhance the performance of the chromized coatings. Morphological results showed that NREO refined the microstructure of chromized coatings by retarding the growth of grains. The Mott–Schottky plots demonstrated that coating Cr–La₂O₃ displayed fewer defects than the other two composite coatings, which was beneficial to reduce the corrosion tendency. In addition, the potentiodynamic polarization and impedance spectroscopy analysis showed that the order of the corrosion rates of the achieved coatings was pure Cr > Cr–Y₂O₃ > Cr–CeO₂ > Cr–La₂O₃, which revealed that the corrosion resistance increased accordingly.     

Thermal cycling resistance of modified thick thermal barrier coatings

The thermal cycling properties of several modified thick thermal barrier coatings (TTBC) were studied in three test series in which the maximum coating temperature was fixed to 1000, 1150 and 1300 °C. The modified coating structures were all segmentation-cracked coatings and some of these coatings were surface-sealed. The segmentation-cracked coatings were produced by laser glazing or by using appropriate plasma spray parameters. The sealing treatments were made by using aluminium phosphate or sol–gel-based sealant. In this paper, it was demonstrated that regardless of whether the segmentation-cracked TTBCs were made by using specific plasma spray parameters or by laser glazing, the strain tolerance of the coating improved significantly. Instead, both sealing treatments reduced the thermal cycling resistance of the TTBCs to some degree, especially in the case of aluminium phosphate sealing. Coating microstructures, their mechanical and elastic properties and residual stresses were taken into consideration when estimating the thermal cycling properties and failure modes of the coatings.     

Microstructure and Thermal Properties of Nanostructured 8 wt.% CeO2 Doped YSZ Coatings Prepared by Atmospheric Plasma Spraying

Nanostructured 8 wt.% CeO₂-5.4 wt.% Y₂O₃-ZrO₂ (CeYSZ) coatings were prepared by atmospheric plasma spraying technology. The microstructure, thermal diffusivity, and thermal cycle behavior of CeYSZ were investigated. The results show that the as-sprayed nano-CeYSZ coatings consist of tetragonal ZrO₂ and Ce element is in solid solution with ZrO₂. The CeYSZ coatings are characterized by nano-zones (unmelted nanoparticles), melted dense areas, splats, and pores. The thermal diffusivity of nano-CeYSZ coatings is 0.548 × 10^?6 m²/s at room temperature. The addition of CeO₂ decreases the thermal diffusivity of nano-YSZ coatings, which is mainly caused by the point defect scattering and grain-boundary scattering. The thermal cycle life of nano-CeYSZ coatings is about 860 cycles at 1050 °C. The spallation of the coatings occurs at the interface of CeYSZ/TGO.     

Effect of Submicron and Nano SiC Particles on Erosion Wear and Scratch Behavior of Plasma-Sprayed Al2O3/8YSZ Coatings

This paper reports studies into the effect of submicron and nano SiC particles on microstructure, phase composition, hardness, erosion wear, and scratch behavior of Al₂O₃-20wt.%8YSZ (ZrO₂ + 8 wt.% Y₂O₃) coatings fabricated by atmospheric plasma spraying. The failure mode of erosion wear and scratch for coatings was established and analyzed. The hardness, density, erosion wear, and anti-scratch resistance of coatings fabricated from plasma treating feedstocks were higher than that of coatings made from sintering feedstocks. The erosion wear rate of coatings with SiC was evidently decreased, and there was some small debris on worn surface with characteristic of translamellar fracture. The spallation, fracture, plough, and cracking were main failure mechanism for coatings. In the scratch process, the critical load of coating with SiC was increased. The crack growth resistance of coatings was analyzed from crack length at end of scratch test.