Phase transformation and thermal conductivity of the APS Y2O3 doped HfO2 coating with hybrid structure

Hafnia-based materials are very promising to serve as thermal protecting coatings at temperature above 1200 °C. In this work, two kinds of 8 mol% Y₂O₃ stabilized HfO₂ ceramic coatings (YSH-SN and YSH-MX) with conventional and hybrid structures were prepared by air plasma spray (APS) method. The microstructure, thermal conductivity and the mechanical properties of the coatings before and after thermal exposure at 1300 °C were compared in detail. Results show that the as-sprayed YSH-MX has a hybrid laminated structure of monoclinic HfO₂ and cubicY₂O₃ splats, and transforms to monoclinic HfO₂ and cubic YSH after thermal exposure, while the YSH-SN is composed of major tetragonal YSH phase and transforms to monoclinic HfO₂ and cubic YSH afterward. Thermal conductivities at ultra-high temperature (1600 °C) before and after thermal exposure for those two coatings are close, and the fracture toughness in the direction parallel to the interface exceeds 2.1 MPa m^0.5. The YSH-MX coating with a hybrid structure provides insights to conveniently prepare gradient coating or other coatings with complex structures.     

Effect of Y doping on microstructure and thermophysical properties of yttria stabilized hafnia ceramics

A series of Y₂O₃-doped HfO₂ ceramics (Hf_1-xY_xO_2-0.5×, x?=?0, 0.04, 0.08, 0.12, 0.16 and 0.2) were synthesized by solid-state reaction at 1600?°C. The microstructure, thermophysical properties and phase stability were investigated. Hf_1-xY_xO_2–0.5x ceramics were comprised of monoclinic (M) phase and cubic (C) phase when Y³⁺ ion concentration ranged from 0.04 to 0.16. The thermal conductivity of Hf_1-xY_xO_2–0.5x ceramic decreased as Y³⁺ ion concentration increased and Hf_0.8Y_0.2O_1.9 ceramic revealed the lowest thermal conductivity of ~?1.8?W/m*K at 1200?°C. The average thermal expansion coefficient (TEC) of Hf_1-xY_xO_2–0.5x between 200?°C and 1300?°C increased with the Y³⁺ ion concentration. Hf_0.8Y_0.2O_1.9 yielded the highest TEC of ~?10.4?×?10^?6 K^?1 while keeping good phase stability between room temperature and 1600?°C.     

Nitrate fusion synthesis and two-step sintering of nanocrystalline yttria stabilized hafnia powders

Bulk quantities of nanocrystalline yttria stabilized hafnia (YSH) powders with crystallite size ranging from 8 to 15 nm were successfully prepared for the first time through nitrate fusion synthesis at a temperature as low as 673 K. The yttrium content was varied from 6 to 30 mol%. The dependence of the properties of the final product on the quantity of the dopant was investigated. Microstructural investigations were carried out with scanning electron microscopy and transmission electron microscopy. A maximum relative sintered density of 98.2 ± 0.3% T.D (theoretical density) was obtained for YSH containing 10 mol% yttrium by using “two-step sintering” at a final temperature of 1773 K. Anisotropic shrinkage factor (0.70–0.95) was found to vary linearly with the compaction pressure. SEM investigations reaffirmed that the sintered pellets comprised uniform distribution of faceted grains and elemental mapping revealed that yttrium is distributed uniformly in these sintered YSH monoliths.     

New insights into the thermal aging mechanism of yttria stabilized zirconia: A phase field study

In this paper, a novel phase-field (PF) model is proposed to study the thermal aging mechanism of single crystalline t'-YSZ. The influences of the initial compositional content of yttria and the initial twin structure of the t' phase on the aging process are systematically discussed. The PF model can recover the modulated structure and nano/micro hybrid structure observed in experiments. The PF simulation results indicate that the initial compositional content of yttria is the most important influential factor of the thermal aging process. Besides that, the transformation strain, the initial twin structure and the anti-phase boundaries (APBs) of the t' phase can also have significant influences on the thermal aging kinetics. The typical spinodal region is more suitable to predict the thermal aging behavior of single domain YSZ. For multi-domain YSZ with initial twin structures and APBs, the spinodal region should be further divided into the kernel region and marginal region. In the kernel region, the thermal aging occurs by spinodal decomposition with the formation of a modulated structure, which is followed by merging and coarsening. In the marginal region and outside the spinodal region, the phase decomposition leads to a hybrid structure with coarse grained cubic phase and fine grained tetragonal phase, which exhibits the characteristics of nucleation and growth. The hybrid structure is consistent with previous experimental observations. It is revealed that the boundaries of the nano sized tetragonal grains evolve from the twin boundaries and APBs. The nucleation-growth mechanism should be properly understood when it is applied to illustrate the evolution process of the hybrid structure. The PF model and the new insights obtained in this study are helpful to understand the thermal aging mechanisms of t'-YSZ.     

Mechanical properties and thermal conductivities of 3YSZ-toughened fully stabilized HfO2 ceramics

In this work, the 3 mol% yttria stabilized zirconia (3YSZ) composed of tetragonal phase has been introduced into the 10 mol% Er₂O₃ stabilized cubic hafnia (10ErSH) matrix to improve its fracture toughness. The effects of the addition of 3YSZ on the phase composition, microstructure, mechanical properties and thermal conductivities of the 10ErSH have been investigated. The results showed that all the 3YSZ-toughened 10ErSH samples were composed of cubic phase and a little (<10 mol%) monoclinic phase. The introduced tetragonal phase of 3YSZ fully disappeared even when the volume fraction of 3YSZ reached 50%, indicating that the phase transformation occurred during 1500 °C. The fracture toughness for the sample with 50% 3YSZ was improved by 60% compared with the pure 10ErSH ceramics owing to the sub-mico/micro hybrid structure, which changed the crack propagation mode and consumed part of the crack extension energy. Additionally, the thermal conductivity slightly decreased due to the mass and radius misfits induced by substitution atoms (Zr⁴⁺, Er³⁺ and Y³⁺). Considering the improved mechanical and thermal properties, the 3YSZ-toughened 10ErSH ceramics may be alternative TBC materials.     

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.     

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.     

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.     

Evolution of thermal insulation of plasma-sprayed thermal barrier coating systems with exposure to high temperature

The thermal insulation potential of plasma-sprayed yttria-stabilized zirconia thermal barrier coatings is generally assessed via the evaluation of the ceramic layer. However, ageing of the complete system leads to microstructural transformations that may also play a role in the heat transport properties. This study thus investigated the microstructure-heat insulation relationships of different TBC systems in their as-deposited state and when aged under various conditions, through the systematic analysis of both microstructure and thermal diffusivity. The latter was measured from room temperature up to 1100 °C using the laser-flash technique, while the porous microstructure was assessed using image analysis. The different coatings exhibited relatively similar thermal diffusivity values that were shown to be mostly influenced by the thin porosities in contrast to larger defects. The thermal insulation of the TBC systems after exposure to high temperature was shown to be stable despite the microstructural variations introduced by cracks, oxidation and chemical degradations.     

Microstructure and high-temperature sintering properties of APS coatings prepared from spherical thin-walled hollow-shell powders

High-performance thermal barrier coatings (TBCs) made of 4 mol.% Y₂O₃–stabilized ZrO₂ (4YSZ) powder with a spherical thin-walled hollow-shell (STHS) structure exhibited a special microstructure different from the conventional lamellar structure of air plasma-sprayed (APS) coatings. The as-sprayed STHS APS coatings had a completely tetragonal prime (t′) structure and non-lamellated closed-cell structure with high porosity, which resulted in relatively low thermal conductivity (～1.0 W m^?1 K^?1) and high Vicker's hardness (～6 GPa). The influences of high-temperature aging on the microstructure stability, phase stability, and sintering capability were investigated after long-time heat treatment at different temperatures. The characterization results indicated that the pore content was basically constant, and it was less than 0.5% for sintered linear shrinkage of the STHS coatings after heat treatment at 1500 °C for 100 h. Furthermore, no spalling appeared in the STHS APS coating with the t′ phase structure after 101 thermal cycles of the water-quenching method at 1050 °C, and no monoclinic ZrO₂ (m-ZrO₂) phase was present in all of the STHS coatings after aging at 1200 °C for 1–1100 h. The excellent anti-sintering properties and phase stability of the STHS coatings are attributed to the closed-pore microstructure and the highly pure t′ phase composition with uniform distribution of ions, respectively. The results suggested that the non-lamellated closed-cell microstructure is beneficial for improving the coating properties, and the results also provide guidelines for microstructure design of TBCs using a feedstock powder.     

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.     

Phase transformation kinetics of 3 mol% yttria partially stabilized zirconia (3Y-PSZ) nanopowders prepared by a non-isothermal process

A crystalline nanopowder of 3 mol% yttria-partially stabilized zirconia (3Y-PSZ) has been synthesized using ZrOCl₂ and Y(NO₃)₃ as raw materials throughout a co-precipitation process in an alcohol-water solution. The phase transformation kinetics of the 3Y-PSZ freeze dried precursor powders have been investigated by nonisothermal methods. Differential thermal and thermogravimetric analyses (DTA/TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been utilized to characterize the 3Y-PSZ nanocrystallites. When the 3Y-PSZ freeze dried powders are calcined in the range of 703-1073 K for 2 h, the crystal structure is composed of tetragonal and monoclinic ZrO₂. The BET specific surface area of the 3Y-PSZ freeze dried precursor powders calcined at 703 K for 2 h is 118.42 m²/g, which is equivalent to a crystallite size of 8.14 nm. The activation energy from tetragonal ZrO₂ converted to monoclinic ZrO₂ in the 3Y-PSZ freeze dried precursor powders was determined as 401.89 kJ/mol. The tetragonal (T) and monoclinic (M) ZrO₂ phases coexist with a spherical morphology, and based on TEM examination have a size distribution between 10 and 20 nm. When sintering green compacts of the 3Y-PSZ, a significant linear shrinkage of 8% is observed at about 1283 K. On sintering the densification cycle is complete at approximately 1623 K when a total shrinkage of 32% is observed and a final density above 99% of theoretical was achieved.     

Effects of manganese oxide addition and reductive atmosphere annealing on the phase stability and microstructure of yttria stabilized zirconia

The effects of Mn₃O₄ addition and reductive atmosphere (N₂:H₂ = 97:3) annealing on the microstructure and phase stability of yttria stabilized zirconia (YSZ) ceramics during sintering at 1500 °C for 3 h in air and subsequent annealing in a reductive atmosphere were investigated. Mn₃O₄ added 6 mol% YSZ (6YSZ) and 10 mol% YSZ (10YSZ) ceramics were prepared via the conventional solid-state reaction processes. The X-ray diffraction results showed that a single cubic phase of ZrO₂ was obtained in 1 mol% Mn₃O₄ added 6YSZ ceramic at a sintering temperature of 1500 °C for 3 h. A trace amount of monoclinic ZrO₂ phases were observed for 1 mol% Mn₃O₄ added 6YSZ ceramics after annealing at 1300 °C for 60 cycles in a reductive atmosphere by transmission electron microscopy. Furthermore, a single cubic ZrO₂ phase existed stably as Mn₃O₄ added 10YSZ ceramics was annealed at 1300 °C for 60 cycles in reductive atmosphere.     

Interplay of the phase and the chemical composition of the powder feedstock on the properties of porous 8YSZ thermal barrier coatings

Some chemical impurities enhance sintering kinetics of ceramic Thermal Barrier Coatings (TBCs) which can cause their premature failure during operation in gas turbine engine by causing reduction in coating’s strain compliance as well as faster bond-coat oxidation due to increased thermal conductivity. Certain chemical impurities are also believed to suppress resistance to tetragonal to monoclinic phase transformation in 8YSZ, which can also be an important factor regarding TBC’s performance. Most of the impurities and some of the monoclinic phase present in the powder feedstocks can survive into the as-sprayed coating. Therefore, there is a general trend towards OEMs requiring the lowest amounts of chemical impurities and the lowest amounts of monoclinic phase in the powder feedstocks. This paper presents a comprehensive investigation aimed at understanding the role and the relative importance of the chemical and phase purities of the powder feedstock for the properties and performance of thick 8YSZ TBCs.     

Effect of high-content Yttria on the thermal expansion behaviour and ionic conductivity of a stabilised cubic Hafnia

The needs for space propulsion thruster induce the development of new designs and material compositions able to withstand 3000 K of flame combustion temperature. Cubic-stabilised hafnia appears as one of the most promising candidates to protect refractory materials in such conditions. Here, the influence of dopant content on the thermal expansion (473−1823 K) and ionic conductivity (600−1150 K) in highly doped-hafnia (12−33 mol% Y₂O₃) with disordered cubic systems is reported. The composition and the homogeneity of the samples were carefully checked using crystallographic, chemical and spectroscopy analyses. Finally, the study of thermal and oxygen conductivity properties highlighted their dependence on the amount of dopant. The average thermal expansion coefficient was lowered from 11.3 to 10.9 10⁻⁶/K and the ionic conductivity decreased by two decades with 33 mol% of Y₂O₃ by using the optimised substitution ratio. Interactions and local ordering of oxygen vacancies can explain this behaviour.     

Phase structure and thermal conductivities of Er2O3 stabilized ZrO2 toughened Gd2Zr2O7 ceramics for thermal barrier coatings

3.5 mol% Er₂O₃ stabilized ZrO₂ (ErSZ) and Gd₂Zr₂O₇ powders were produced by a chemical co-precipitation and calcination method, and ErSZ was used to toughen Gd₂Zr₂O₇. The phase structure, toughness and thermal conductivities of ErSZ toughened Gd₂Zr₂O₇ ceramics were investigated. When the ErSZ content was below 15 mol%, the compound consisted of pyrochlore phase, the ordering degree of which decreased with the increase of the ErSZ content. High ErSZ doping led to the formation of metastable tetragonal (t′) phase in the compound. The addition of ErSZ in Gd₂Zr₂O₇ benefited its toughness, mainly attributable to the presence of t′ phase in the compound. With the increase of the ErSZ content in the compound, the thermal conductivity first decreased and then showed an upward tendency, and 10 mol% ErSZ toughened Gd₂Zr₂O₇ exhibited the lowest thermal conductivity.     

High heat flux burner-rig testing of 8YSZ thermal barrier coatings: Influence of the powder feedstock

Burner-rig thermal cyclic testing of Thermal Barrier Coating (TBC) samples fabricated using different 8YSZ powders was conducted to investigate the influence of the chemical and phase compositions of the powder feedstocks. Four different powder feedstocks were selected. The chemical and phase compositions among the 8YSZ powders were systematically varied while the powder particle size and other physical characteristics were kept nominally the same. The coating process was also selected to achieve similar microstructure among the samples. The testing revealed that (1) higher impurity content (esp. silica) is detrimental to the cyclic life of the TBC; (2) coating porosity has a significant influence on the cyclic life of the TBC, the higher the porosity, the higher the cyclic life, for the range of porosity of the tested samples; (3) a low monoclinic content in the feedstock powder has not been shown to have a positive effect on the cyclic life of the TBC.     

Anti-CMAS corrosion mechanism of Al2O3 pore sealing and laser remelting on thermal barrier coatings

The sol-dip-coating method and surface laser remelting technology are applied to form an Al₂O₃ layer on a YSZ coating surface to effectively block the environmental sediment CMAS. The behaviour and mechanism for CMAS corrosion of the coating are investigated, and the interfacial reliability of the coating and matrix is calculated by using the Monte Carlo method. The bonding force between the Al₂O₃ sol and YSZ coating can be effectively improved by laser surface treatment. Samples subjected to a laser pretreatment and posttreatment (YL-AL) of the YSZ coating are found to show the best interfacial bonding strength between Al₂O₃ and YSZ. Furthermore, the YL-AL sample shows a higher CMAS resistance than the laser posttreatment (Y-AL) samples, which effectively combines the chemical resistance of Al₂O₃ to CMAS and the physical resistance of the laser re-melted densification layer against CMAS penetration.     

Phase composition,microstructure and thermophysical properties of the Srx(Zr0.9Y0.05Yb0.05)O1.95+x ceramics

Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9, 0.8, 0.7) ceramics were prepared by solid state reaction sintering. The sintered Sr_1.0(Zr_0.9Y_0.05Yb_0.05)O_2.95 is a single-phase solid solution while the sintered Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=0.9?0.7) are composites, and a significant grain growth inhibition is observed in the sintered Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9). Rare-earth elements distribution in the bulk materials indicates that Yb and Y preferentially substitute Zr-sites in SrZrO₃, and the highest solubility of RE₂O₃ in pure SrZrO₃ is ～0.8 mol%. The sintered Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x have high thermal expansion coefficients up to ～11.0×10^?6 K^-1 (1200°C). Sr_0.8(Zr_0.9Y_0.05Yb_0.05)O_2.75 has the lowest thermal conductivity of 1.38 W·m^-1·K^-1 at 800°C. Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9, 0.8) show no phase transition from 600 to 1400°C, whereas Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=0.9, 0.8) have excellent high-temperature phase stability over the whole investigated temperature range. Therefore, Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x (x=1.0, 0.9, 0.8) are considered as promising TBCs materials that might be operated at higher temperatures compared to YSZ.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.