Thermophysical behavior of thermal sprayed yttria stabilized zirconia based composite coatings

The effective thermal conductivity of a composite coating depends on intrinsic thermal conductivity of the constituent phases, its characteristics (size, shape) and volume fraction of porosities. The present study concerns studying the effect of CoNiCrAlY and Al₂O₃ content on the coefficient of thermal expansion and thermal conductivity of the YSZ (YSZ-CoNiCrAlY and YSZ-Al₂O₃) based composite coatings developed by thermal spray deposition technique. The coefficient of thermal expansion and thermal conductivity of the composite coatings were measured by push rod dilatometer and laser flash techniques, respectively, from room temperature to 1000 °C. Variation in density, porosity, coefficient of thermal expansion, and thermal conductivity was observed in the composite coatings with the addition of different volume fraction of CoNiCrAlY and Al₂O₃ powders in YSZ-CoNiCrAlY and YSZ-Al₂O₃ composites, respectively. Comparison between the theoretical and experimental thermal conductivities showed a mismatch varying from 4% to 58% for YSZ-CoNiCrAlY composite coatings and from 58% to 80% for YSZ-Al₂O₃ composite coatings. Model based analyses were used to understand the mechanism of thermal conductivity reduction in the composite coatings. It was concluded that the morphology of porosities varied with composition.     

Ultrasonic cavitation erosion of as-sprayed and laser-remelted yttria stabilized zirconia coatings

Cavitation erosion resistance of 8 wt.% yttria stabilized zirconia has been investigated in specimens prepared by atmospheric plasma spraying and laser remelting post treatment. The results indicate that as-sprayed coatings involve defects such as primary cavities and initial micro cracks inside a particle and among the interfaces of particles. When the specimens are subjected to cavitation erosion, the micro cracks initiate and coalesce along with chip removals. Laser remelting produces a dense glazed layer with some cracks though the coatings. With the increasing of erosion time, large pieces are delaminated from coating-substrate interface leading to a significant mass loss. However, the resistance of laser remelted coatings to cavitation erosion is significantly improved when they are impregnated with epoxy by vacuum castable mounting. The relationship between cracks formed inside the laser remelted YSZ coatings and their damage mechanism under cavitation is discussed.     

Microstructure and thermo-physical properties of yttria stabilized zirconia coatings with CMAS deposits

Yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) are used to protect hot-components in aero-engines from hot gases. In this paper, the microstructure and thermo-physical and mechanical properties of plasma sprayed YSZ coatings under the condition of calcium-magnesium-alumina-silicate (CMAS) deposits were investigated. Si and Ca in the CMAS rapidly penetrated the coating at 1250 °C and accelerated sintering of the coating. At the interface between the CMAS and YSZ coating, the YSZ coating was partially dissolved in the CMAS, inducing the phase transformation from tetragonal phase to monoclinic phase. Also, the porosity of the coating was reduced from ∼25% to 5%. As a result, the thermal diffusivity at 1200 °C increased from 0.3 mm²/s to 0.7 mm²/s, suggesting a significant degradation in the thermal barrier effect. Also, the coating showed a ∼40% increase in the microhardness. The degradation mechanism of TBC induced by CMAS was discussed.     

Isolation of high-purity zirconia from Australian zircon sands using an ion-exchange process

Zirconia produced by acid-leaching zircon sand can be significantly contaminated by other elements leached from the sand. A simple cationexchange procedure has been developed that allows separation of the impurity elements from the zirconium to give zirconia with up to 99.99% purity. The addition of formic acid to the acid-leach solution prior to ion exchange provides the removal of about three-quarters of the hafnium present in the product zirconia.     

The influence of laser treatment on hot corrosion behavior of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

In this study, the effect of laser glazing on the hot corrosion behavior of nanostructured thermal barrier coatings (TBCs) was investigated. To this end, the hot corrosion test of plasma-sprayed and laser-glazed thermal barrier coatings conducted against 45 wt.% Na₂SO₄ + 55 wt.% V₂O₅ molten salt at 910 °C for 30 h in open air atmosphere. The results obtained from hot corrosion test showed that the reaction between Y₂O₃ and the corrosive salt produced YVO₄, leached Y₂O₃ from YSZ and led to the progressive destabilization transformation of YSZ from tetragonal to the monoclinic phase. The lifetimes of the plasma-sprayed TBCs were enhanced approximately twofold by laser glazing. Reducing the reactive specific surface area of the dense glazed layer with the molten salts and improving the stress accommodation through network cracks produced by laser glazing were the main enhancement mechanisms accounting for TBC life extension.     

Hot corrosion behavior of double ceramic layered CaZrO3/Yttria‐stabilized zirconia coatings

A novel double ceramic layered (DCL) CaZrO₃/Yttria‐stabilized zirconia (YSZ) thermal barrier coatings (TBCs) was designed for improved service life against sulfate vanadate hot corrosion as compared with that of YSZ single layered coating. The hot corrosion behavior of DCL CaZrO₃/YSZ coatings was studied at 950°C after dry spreading 50%Na₂SO₄+50%V₂O₅ mixture onto a coated surface. The CaZrO₃ as the topmost layer in DCL CaZrO₃/YSZ coatings, served as a sacrificial layer during sulfate vanadate hot corrosion protecting the underneath YSZ coating. The corrosion reactions in this case were sluggish due to the initial formation of low melting point meta‐calcium vanadate (CaV₂O₆) that isothermally transformed to higher melting point calcium vanadates having higher calcia (CaO) content. The corrosion reaction products sealed the top surface, impeding the oxygen movement and eventually retarded the thermally grown oxide (TGO) growth. The sulfate vanadate hot corrosion life of the DCL CaZrO₃/YSZ coatings was observed to be more than double as compared with single ceramic layered YSZ coatings.     

Comparison of thermal shock resistances of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of the current study is evaluation and comparison of thermal shock behavior of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, the nanostructured and conventional YSZ coatings were deposited by atmospheric plasma spraying (APS) on NiCoCrAlY-coated Inconel 738LC substrates. The thermal shock test was administered by quenching the samples in cold water of temperature 20–25 °C from 950 °C. In order to characterize elastic modulus of plasma-sprayed coatings, the Knoop indentation method was employed. Microstructural evaluation, elemental analysis, and phase analysis were performed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffractometry (XRD) respectively. The results revealed that failures of both nanostructured and conventional TBCs were due to the spallation of ceramic top coat. Thermal stresses caused by mismatch of thermal expansion coefficients between the ceramic top coat and the underlying metallic components were recognized as the major factor of TBC failure. However, the nanostructured TBC, due to bimodal unique microstructure, presented an average thermal cycling lifetime that was approximately 1.5 times higher than that of the conventional TBC.     

Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition

Plasma spray physical vapor deposition (PS-PVD) was used to deposit yttria stabilized zirconia (YSZ) coatings with different columnar morphologies by varying the spray distance. Although similar quasi-columnar structures were formed at the spray distances of 600 mm and 1400 mm, the formation mechanisms of particles in the coatings were different. Besides, an electron beam physical vapor deposition (EB-PVD) like columnar coating out of pure vapor was deposited at a spray distance of 1000 mm and the columnar consisted of elongated nano-sized secondary columns. The hardness and Young׳s modulus of the coatings were investigated. Compared to the other two quasi-columnar structures, the EB-PVD like columnar coating exhibited higher hardness (~9.0 GPa ) and Young׳s modulus (~110.9 GPa), mainly due to its low porosity and defect.     

微波加热处理电熔ZrO2制备部分稳定ZrO2

以电熔法制备的CaO完全稳定ZrO2为原料,在微波高温管式加热装置内于1 350℃60 min下加热处理.利用XRD、SEM研究了微波加热对电熔ZrO2相组成和微观结构的影响.结果表明:电熔ZrO2具有良好的微波吸收能力,在微波场中能以400～450℃·min<'-1的速率实现快速升温;试样处理后出现单斜相ZrO2衍射峰,内部未出现裂纹,在低于传统推板窑处理电熔ZrO2原料的时间和温度条件下,得到了部分稳定ZrO2.微波加热在该领域具有显著的节能优势,为高效、低耗制备高品质部分稳定ZrO2提供了全新的途径.     

High-pressure sintered yttria stabilized zirconia ceramics

Fast densification of 8YSZ ceramics under a high pressure of 4.5 GPa was carried out at different temperatures (800, 1000, 1450 °C), by which a high relative density above 92% could be obtained. FT-Raman spectra indicate that the 8YSZ underwent a phase transition from partially tetragonal to partially cubic phase as temperatures increase from 1000 to 1450 °C when sintering under high pressure. The electrical properties of the samples under different high-pressure sintering conditions were measured by complex impedance method. The total conductivity of 0.92 × 10⁻² S cm⁻¹ at 800 °C has been obtained for 8YSZ under high pressure at 1450 °C, which is about 200 °C lower than that of the samples prepared by conventional pressureless sintering.     

Highly refractory lightweights from zirconia and zircon

Summary Using a simple technology and the method of foaming, it is possible to make zirconia and zircon lightweight refractories possessing excellent heat-insulating properties. Their high refractoriness means they can be used in high-temperature furnaces; this applies especially to zirconia lightweight.     

Phase stability,microstructure and mechanical properties of nanostructured yttria stabilized zirconia coatings subjected to moisture degradation

The nanostructured (8 wt%) yttria stabilized zirconia coatings (n-YSZ) were deposited by atmospheric plasma spraying (APS) to study the effect of moisture degradation on the properties of n-YSZ coatings. Variations in phase composition, microstructure and mechanical properties were comprehensively characterized. The single-edge notched beam method used for fracture toughness testing is sufficiently reliable for evaluating the integral properties of the coatings in this study. Results indicated that the microstructure of the n-YSZ coatings was significantly affected by hydrothermal degradation. Hydrothermal degradation resulted in substantial defects, such as pores and cracks, which severely decreased the mechanical properties of the n-YSZ coatings. In addition, the ceramic coat was in a state of compressive stress, and the stress initially increased and then decreased with increasing degradation time. The variations in the stress of the n-YSZ coatings are closely related to the transformation of tetragonal to monoclinic phase, which is induced by hydrothermal degradation. Additionally, several major mechanical properties of the n-YSZ coatings decreased significantly with the hydrothermal degradation, including fracture toughness from 1.28 ± 0.05 to 0.08 ± 0.01 MPa m^1/2, flexural strength from 60.51 ± 2.98 to 4.54 ± 0.14 MPa, and Young's modulus from 21.98 ± 0.96 to 1.46 ± 0.33 GPa.     

Comparison of microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia thermal barrier coatings

The main goal of this paper was to evaluate and compare the microstructure and mechanical properties of plasma-sprayed nanostructured and conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). To this end, NiCrAlY bond coat, nanostructured, and conventional YSZ coatings were deposited on Inconel 738LC substrate by atmospheric plasma spraying (APS). The mechanical properties of the coating were evaluated using nanoindentation and bonding strength tests. The microstructure and phase composition of the coating were characterized by field emission scanning electron microscopy (FESEM) and X-ray diffractometry (XRD). The nanostructured YSZ coating contained both nanosized particles retained from the powder and microcolumnar grains formed through the resolidification of the molten part of the powder, whereas the microstructure of the conventional YSZ coating consisted of columnar grain splats only. The phase composition of the as-sprayed nanostructured coating consisted of the non-transformable tetragonal phase, while the conventional coating showed the presence of both the monoclinic and non-transformable tetragonal phases. The results of nanoindentation and bonding strength tests indicated that the mechanical properties of the nanostructured coating were better than those of the conventional coating.     

钇稳定纳米氧化锆的制备工艺研究

用溶胶-凝胶法制备了钇稳定纳米氧化锆(YSZ),系统研究了沉淀剂滴加方式、锆离子浓度、溶剂、干燥方式和煅烧温度对产物性能的影响,并采用FT-IR、TG-DSC、XRD、SEM等对前驱体干凝胶粉末和产物进行了表征。结果表明：以水作溶剂,采用正加方式,选择锆离子浓度为0.5 mol/L,真空干燥,550 ℃煅烧2 h,可得到粒径为39 nm左右、组分为单一四方相的YSZ粉体。     

Thermal expansion of EB-PVD yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) layers, suited for thermal barrier coatings (TBCs) in turbine engines, were produced by electron-beam physical vapour deposition (EB-PVD) on metallic (Ni-based alloys) ceramic (Al₂O₃) substrates (typically at 1000 °C) using ingot compositions of about 4 mol% Y₂O₃–ZrO₂. Employing powdered samples, removed from the surface of the as-deposited YSZ layers, precision X-ray diffraction data sets were recorded between 60 and 900 °C and the lattice parameters of the metastable, tetragonal (t′) and the cubic (c) phases were refined using Rietveld's method. It was unambiguously verified that phase (c) was present in all investigated samples as a minority phase already in the as-deposited state.The thermal expansion coefficients of the tetragonal phase (t′) (α₁₁ = 9.3(2) × 10^?6 K^?1, α₃₃ = 10.8(1) × 10^?6 K^?1) and the cubic phase (c) (α₁₁ = 8.5(2) × 10^?6 K^?1) were evaluated from the refined temperature dependent lattice parameter values of this study, and turned out to be close to each other, but significantly different from the coefficient of the monoclinic phase (m) (α₁₁ = 9.0 × 10^?6 K^?1, α₂₂ = 1.2 × 10^?6 K^?1, α₃₃ = 11.9 × 10^?6 K^?1, α₁₃ = 0.0 × 10^?6 K^?1), which was derived from temperature dependent lattice parameter measurements published by Touloukian et al.⁵ The expansion coefficient of the monoclinic phase (m) exhibited a very pronounced anisotropy in contrast to the tetragonal phases (t′) and (t). Our values for (t′) and (c) were in excellent agreement with that of other tetragonal and cubic phases from the literature with quite different Y₂O₃ contents.     

Flash grain welding in yttria stabilized zirconia

Self-standing samples made of isostatically pressed powders of ZrO₂:8 mol% Y₂O₃ were submitted to AC signals of 60 and 1000 Hz. At temperatures of around 900 °C, a current density exceeding approximately 100 mA/cm² starts an avalanche process with a fast increasing current under constant voltage. It lasts about 60 s and it is preceded by an induction period of the order of 30 s. After that, the material is sintered as confirmed by impedance diagrams. Relative densities of 94% could be obtained.     

High-concentrated zirconia suspensions stabilized by cellulose nanocrystals

In this work, cellulose nano crystals (CNC) as a biodegradable and non-toxic dispersant were used in water-based suspensions of zirconia to examine their stabilization in ceramic processing. The zirconia suspensions for solid content, pH and CNC concentration were studied and the optimum pH value and minimum CNC concentration to produce stable highly concentrated water-based suspensions were determined. The achieved stability was found to be due to the adsorption of CNC nanofibers around the zirconia particles revealed by scanning electron microscopy (SEM). Suspensions with the lowest viscosity and highest stability over 24 h achieved at pH 4 i.e., 30 wt% zirconia particles stabilized with the addition of 1 wt% CNC.     

High temperature mechanical properties of zirconia metastable t'-Phase degraded yttria stabilized zirconia

Air plasma sprayed (APS) 8 wt%-yttria stabilized zirconia (8YSZ) with metastable tetragonal prime phase (t′) has been widely applied as thermal barrier coatings (TBCs) for gas turbine blades because of its outstanding mechanical properties at high temperatures. In the present research, a carefully designed process was used to prepare 8YSZ samples with different phase composition (t′, t and c) simulating the phase degradation of the material during operation conditions. High temperature (1000–1200 °C) bending strength, elastic modulus, and thermal expansion coefficient were measured, which exhibit strong dependence on the phase degradation during heat treatment. Effect of the phase composition on high temperature thermo-mechanical properties and the enhancement of the bending strength have been discussed, providing a new perspective for further improvements.     

Influence of porosity on mechanical properties of tetragonal stabilized zirconia

3YSZ specimens with variable open porosity (1–57%) were fabricated, and the stiffness, strength and fracture properties (fracture toughness and R-curve) were measured to investigate their potential use as support structures for solid oxide fuel or electrolysis cells. The ball-on-ring test was used to characterize Young's modulus and Weibull strength. The variation of fracture toughness with porosity was investigated and modelled using the results from fracture mechanical testing. A distinct R-curve behaviour was observed in dense 3YSZ specimens, in samples with a porosity around 15% and in some of the highly porous samples (porosities ～45%) reflecting a transformation toughening in the material. For the most porous samples, the “R-curve behaviour” disappeared and subcritical crack growth was observed. The studies indicate that even highly porous 3YSZ structures (porosities exceeding 40%) are feasible supports for SOFC/SOECs from a mechanical point of view.