Mass transfer in polycrystalline ytterbium monosilicate under oxygen potential gradients at high temperatures

Mass transfer in polycrystalline Yb₂SiO₅ wafers with precise composition control was evaluated and analyzed by oxygen permeation experiments at high temperatures using an oxygen tracer. Oxygen permeation proceeded due to mutual grain boundary diffusion of oxide ions and Yb ions without synergistic effects such as acceleration or suppression. The oxygen shielding properties of Yb₂SiO₅ were compared with those of the other line compounds such as Yb₂Si₂O₇ and Al₂O₃ based on the determined mass transfer parameters. It was found that the more preferentially an oxide ion diffuses in the grain boundary compared to the interior of the grain, the greater the effect of suppressing the movement of the oxide ion by applying an oxygen potential gradient becomes.     

Failure mechanisms in model thermal and environmental barrier coating systems

The durability of environmental barrier coating (EBC) systems in gas turbine engine environments depends upon their temperature dependent rates of degradation by processes such as steam volatilization and bond coat oxidation. While addition of a thermal barrier coating (TBC) reduces the temperature within the EBC system, it introduces new failure mechanisms. Deposition of a segmented HfO₂ TBC with a reduced in-plane Young’s modulus is essential to avoid bifurcated TBC channel cracking into a Yb₂Si₂O₇ EBC, and delamination, as a result of an approximately 50% difference in coefficients of thermal expansion (CTE) of the coating layers. During prolonged high temperature steam cycling, a thin fluorite phase reaction layer is observed to develop at the HfO₂-YbDS interface consistent with recent thermochemical assessments. The CTE of the fluorite phase is shown to be substantially higher than that of either of the layers to which it is bonded, resulting in tunnel cracking of the fluorite, and eventual coating delamination of the TBC at either the fluorite-HfO₂ or YbDS-fluorite interfaces upon cooling. The study highlights the importance of matching the CTEs of the TBC and EBC layers during coating system design, and those of the reaction products that may form between them.     

Structural characteristics and high-temperature oxidation behaviour of nano-HfO2-doped thermal barrier coatings

The uneven growth of thermally grown oxides (TGOs) in thermal barrier coating systems is an important cause of cracking failure at the coating interface in high-temperature environments. The doping of rare earth elements in the bonding layer can effectively inhibit the formation of spinel oxides in the TGO and improve the high-temperature oxidation resistance of the coating. However, a single rare earth element has a limited effect on inhibiting TGO failure. In this study, a NiCoCrAlYHf coating was prepared using a supersonic flame spraying (HVOF) technique. The effects of HfO₂ doping on the high-temperature oxidation behaviour of the coatings and diffusion behaviour of metallic elements in the coatings were investigated at 1100 °C. The results showed that the nano-sized HfO₂ filled the pores between the powder particles and improved the hardness of the coating. During the high-temperature oxidation process, the oxides formed by Hf and Y had a large size and low solubility, which effectively blocked the diffusion of Al. This slowed the generation of spinel oxides, effectively inhibited the growth of the TGO, it inhibits the initiation and propagation of cracks within the coating, reduces damage to the coating from tensile and compressive stresses at the interface, and improved the high-temperature oxidation resistance of the coating.     

Constrained sintering and thermal ageing behaviour of electrophoretically deposited Yb2Si2O7 environmental barrier coating

The oxidation of SiC and the formation of a thermally grown oxide layer (TGO) limit the lifetime of environmental barrier coatings. Thus, this paper focuses on the deposition of denser Yb₂Si₂O₇ coatings using electrophoretic deposition to reduce the TGO growth rate. The findings showed densification for Yb₂Si₂O₇ can be achieved with an optimized sintering profile (heating/cooling rate, temperature, and time). However, the addition of 1.5 wt% of Al₂O₃ to Yb₂Si₂O₇ promoted densification and lowered the required sintering temperature, 1380 °C using 2 °C/min heating/cooling rate for 10 h provided efficient coating density. Moreover, adding Al₂O₃ reduced the TGO growth rate by more than 70 % compared to the Al₂O₃-free coatings, without cracking in TGO after 150 h of thermal ageing at 1350 °C. Results within this study suggest electrophoretic deposition with Al₂O₃ addition produces promising Yb₂Si₂O₇ environmental barrier coatings on SiC substrate with low oxidation rates and increased lifetime.     

Structural changes during the thermal stabilization of modified and original polyacrylonitrile precursors

A polyacrylonitrile (PAN) precursor fiber of a special grade for preparing carbon fibers was modified by the impregnation of an aqueous KMnO₄ solution. The effects of the modification on the lateral and morphology structure, related to the crystalline properties of both the precursors and preoxidized fibers, such as the orientation index, crystal size, and crystallinity index, were measured by wide‐angle X‐ray diffraction. For both modified and original PAN fibers, a comparative study of the changes of the elemental content during the process of preoxidation, the relations between the thermal stress and heat‐treatment temperature, and the effect of the modification on the skin/core structure of a preoxidized fiber were also introduced by the use of elemental analysis, optical microscopy, and so on. The modification of KMnO₄ was demonstrated to increase the density, increase the crystallinity index, increase the preferred orientation index, and decrease the crystal size for a modified precursor fiber and for a preoxidized fiber developed from a modified precursor fiber after a different heat‐treatment temperature. KMnO₄ also showed a catalytic action, accelerating the rate of preoxidation and reducing the time of thermal stabilization; this improved the homogenization of the cross‐section structure and led to an improvement in the tensile strength of 15–20% and an improvement in the elongation of 20–30% in the resulting carbon fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 2047–2053, 2005     

Corrosion of RE2Si2O7 (RE=Y,Yb, and Lu) environmental barrier coating materials by molten calcium-magnesium-alumino-silicate glass at high temperatures

With the increased demand for high operating temperature of gas turbine engines, corrosion by molten calcium-magnesium-alumino-silicate (CMAS) exhibits a significant challenge to the development of durable environmental barrier coatings (EBCs). EBC candidates, γ-Y₂Si₂O₇, β-Yb₂Si₂O₇, and β-Lu₂Si₂O₇ were explored on their corrosion resistance to CMAS melts at 1300 °C and 1500 °C for 50 h. Interaction and degradation mechanisms were investigated and the corrosion behaviors showed different trends at high temperatures. At 1300 °C, RE₂Si₂O₇ dissolves into CMAS melts and apatite phases reprecipitate forming a thick recession layer. However, when the temperature increases to 1500 °C, CMAS melts vigorously penetrate through the grain boundary of RE₂Si₂O₇ and ‘blister’ cracks form throughout the samples. The reduced grain boundary stability at 1500 °C promotes the penetration of CMAS melts in RE₂Si₂O₇. Grain boundary engineering is critically demanded to optimize CMAS corrosion at high temperatures.     

Study on properties of Hf6Ta2O17/Ta2O5 system: A potential candidate for environmental barrier coating (EBC)

Ta₂O₅ doped Hf₆Ta₂O₁₇ system (Hf₆Ta₂O₁₇/Ta₂O₅) is considered to have potential application prospect in the field of aero-engine. We herein focus on the thermo-physical, mechanical properties and CMAS corrosion resistance of Hf₆Ta₂O₁₇/Ta₂O₅ to systematically evaluate the possibility for the application of environmental barrier coating (EBC). By changing the content of Ta₂O₅, the gradient adjustment of thermal expansion coefficient can be realized while maintaining low thermal conductivity (1.5–2.2 W/(m·K)). The introduction of Ta₂O₅ significantly reduces the modulus and improves the fracture toughness. Single-phase Hf₆Ta₂O₁₇ shows excellent corrosion resistance against molten calcium-magnesium-alumina-silicate (CMAS). The crystallization of CaTa₂O₆ and HfSiO₄ is the important factor to prevent further corrosion. The introduction of Ta₂O₅ weakens the ability to prevent Si penetration and greatly increases the thickness of the corrosion layer. The results highlight the merit of Hf₆Ta₂O₁₇/Ta₂O₅ system as potential candidate for multi-layer gradient coating on the surface of ceramic matrix composites.     

Interaction of Yb2Si2O7 environmental barrier coating material with Calcium-Ferrum-Alumina-Silicate (CFAS) at high temperature

Experimental investigations of Yb₂Si₂O₇ pellet exposed to Calcium-Ferrum-Alumina-Silicate (CFAS) at 1400 °C in ambient air were carried out to reveal corrosion reaction between molten silicate deposit and Yb₂Si₂O₇. Phase transformation, microstructure evolution and reaction mechanism were evaluated. Results indicated that the corrosion process was accompanied by the infiltration of CFAS melt, the dissolution of Yb₂Si₂O₇ and the reprecipitation of Yb₂Si₂O₇ and Ca₂Yb₈(SiO₄)₆O₂ apatite as reaction product. The formation of apatite decreased the concentration of Ca²⁺ in the melt. After CFAS exposure at 1400 °C for 30 h, the thickness of the apatite layer stopped increasing due to insufficient Ca²⁺ content, and remained at about 115.4 μm. However, the infiltration depth of CFAS melt increased with the extending corrosion duration and increasing deposit content. And the infiltration rate was preliminarily found to first decrease and then increase with time. Most of the residual CFAS were crystallized into garnet (Ca₃Fe₂(SiO₄)₃ and Yb₃Fe₅O₁₂) and mayerite (Ca₁₂Al₁₄O₃₃), while a small volume of amorphous glass was dispersed among the garnet and mayerite grains.     

Novel-structured plasma-sprayed thermal barrier coatings with low thermal conductivity,high sintering resistance and high durability

The microstructure of the ceramic topcoat has a great influence on the service performance of thermal barrier coatings (TBCs). In this study, conventional layered-structure TBCs, nanostructured TBCs, and novel-structured TBCs with a unique microstructure were fabricated by air plasma spraying. The relationship between the microstructure and properties of the three different TBCs was analysed. Their thermal insulation ability, sintering resistance, and durability were systematically evaluated. Additionally, their failure modes after being subjected to two kinds of thermal shock tests were analysed. The results revealed that the novel-structured TBCs had remarkably superior performances in all the examined aspects. The thermal conductivity of the novel-structured TBCs was significantly lower than those of the conventional and nanostructured TBCs both in the as-sprayed state and after thermal treatment for 500 h at 1100 °C. The macroscopic elastic modulus of the novel-structured TBCs after sintering at 1300 °C for 100 h was similar to those of the conventional and nanostructured TBCs in the as-sprayed state. During both a burner rig thermal shock test and a furnace cyclic oxidation test, the thermal shock lifetime of the novel-structured TBCs was much longer than those of the conventional and nanostructured TBCs. This study has demonstrated novel-structured plasma-sprayed TBCs with high thermal insulation ability and high durability.     

Effects of suspension properties on the fabrication of Yb2Si2O7 coatings using electrophoretic deposition

This study examines the electrophoretic deposition of Yb₂Si₂O₇ particles on SiC substrates to produce Environmental Barrier Coatings. To prepare crack-free and homogeneous green coatings, the effect of the solvent, dispersant concentration, and pH were investigated. Ethanol provided a well-dispersed suspension and crack-free coating which was shown by sedimentation tests and microstructure analysis. The effect of the dispersant concentration was investigated with zeta potential measurement and microstructure analysis with a concentration above 0.5 g/L resulting in higher ionic strength and producing cracked and uneven coatings. The ionic strength was also associated with the powder packing density with larger indentation impressions measured for loosely packed coatings. The deposition rate depended on the suspension properties influenced coating integrity with delamination evidenced by analysing the current density drop during deposition. Sintering of the green coatings having different densities and microstructure showed their importance in the preparation of uniform and dense sintered coatings.     

Modelling and evaluating thermal conductivity of porous thermal barrier coatings at elevated temperatures

Thermal conductivity of various porous thermal barrier coatings (TBCs) used at elevated temperatures for gas turbines has been evaluated using the proposed six-phase model. These TBCs rely on microstructural properties and yield different types of porosities. This paper studies the thermal conductivity of TBCs based on microstructural features to evaluate the effect of different types of porosities on thermal conductivity. The first part of this paper investigates the microstructural characterization of various TBCs using image analysis (IA) technique. The second part of this paper evaluates the thermal conductivity using the image analysis. The volumetric fraction of porosities along with their orientation, shape and morphology, shows a considerable impact on the overall thermal conductivity of TBCs. The proposed six-phase model can predict thermal conductivity of porous TBCs with a good agreement with the measured values. The model results can help to better understand the effect of microstructural changes on thermal conductivity and can provide useful guide to fabricate TBCs with low thermal conductivity.     

Interaction of CMAS on thermal sprayed ytterbium disilicate environmental barrier coatings: A story of porosity

Molten calcium magnesium alumina-silicates (CMAS) represent a challenge for the current generation of rare earth silicates environmental barrier coatings (EBCs). Their interaction with ytterbium disilicate (Yb₂Si₂O₇) free-standing coatings deposited using thermal spraying technique has been studied to further understand the reaction mechanisms. Three coatings, deposited with different porosity levels and thickness, representing traditional EBCs (<3% porosity and ～350 μm thickness) and abradable coatings (～20% porosity and 500–1000 μm thickness) were exposed to CMAS at 1350 °C. The results show that higher porosity levels facilitates CMAS infiltration in the first hour of exposure, in combination with infiltration through the inter-splat boundaries. Preferential dissolution of ytterbium monosilicate (Yb₂SiO₅) takes place, forming a 10–15 μm Ca₂Yb₈(SiO₄)₆O₂ apatite layer as the reaction product, producing a network of fine porosity (<10 μm) as the inter-splat boundary material is consumed. After exposure for 48 h, CMAS has completely infiltrated all three coatings, with apatite crystals present across the coatings, up to a depth of ～550 μm. Despite the extensive CMAS infiltration and apatite formation, no damage could be observed in any of the coatings, providing a promising first step for environmental barrier abradable coatings.     

储能硝酸熔盐的热稳定性分析

研究了商业化储能二元混合硝酸熔盐Solar salt (60% NaNO3?40% KNO3)的热稳定性和恒温热稳定性,采用热重法分析了其热分解温度,通过恒温实验考察了处于开放空气气氛中的NaNO3, KNO3和Solar salt在不同温度下的热稳定性,研究了NaNO3和KNO3在Solar salt高温不稳定性中的作用. 结果表明,大于500℃时温度越高,熔盐越不稳定,相同温度下不稳定程度NaNO3>Solar salt>KNO3. 温度升高,分解产物NO2?平衡含量增大而达到平衡的时间缩短,O2?含量很低. Solar salt中硝酸盐热分解和挥发的质量损失比接近1:1,NaNO3是造成体系热分解的主要原因,而KNO3起到抗挥发的作用.     

Interactions of hafnia/hafnon composites with molten silicate deposits

The article examines the chemical interactions between HfO₂/HfSiO₄ composites and melts that originate from siliceous debris ingested into gas turbine engines. Pellets with hafnon volume fractions of 50%, 70% and 100% were synthesized from powders of the pure components and exposed to two types of quinary siliceous deposits (one acidic and one basic) at 1400 °C for times ranging from 10 min to 4 h. Scanning and transmission electron microscopy examinations of reacted pellets show extensive melt penetration without evidence of an effective mitigating mechanism. Acidic melts preferentially react with hafnia to form hafnon while basic melts dissolve hafnon to form hafnia; in both cases, however, the melts penetrate extensively along grain and interphase boundaries. These processes are accompanied by swelling of the reaction layer followed by blistering and exfoliation of the affected coating material. The thermodynamics of the reactions, mechanisms of melt penetration, and implications for coating applications are discussed.     

Self-healing capacity of Mullite-Yb2SiO5 environmental barrier coating material with embedded Ti2AlC MAX phase particles

Repetitive heating and cooling cycles inevitably cause crack damage of hot gas components of gas turbine engines, such as blades and vanes. In this study the self-healing capacity is investigated of mullite + ytterbium monosilicate (Yb₂SiO₅) as EBC material with Ti₂AlC MAX phase particles embedded as a crack-healing agent. The effect of Ti₂AlC in the EBC was compared with the self-healing ability of the mullite + Yb₂SiO₅ material. After introducing cracks by Vickers indentation on the surface of each sample, crack healing was realized by controlling the temperature and time during the post-heat-treatment process. For the mullite + Yb₂SiO₅ composite with Ti₂AlC particles, crack healing occurred at 1000 °C, while in the case of the mullite + Yb₂SiO₅ composite without Ti₂AlC, a sustained temperature of 1300 °C or higher was required. Compared with the healing of the mullite + Yb₂SiO₅ composite by the formation of a eutectic phase, the addition of Ti₂AlC promoted healing via the oxidation of Ti and Al. Notably, the surface formation of a ternary oxide of Ti–Yb–O was confirmed, which completely covered the damage area. Consequently, the addition of a Ti₂AlC MAX phase to the EBC composite resulted in a complete strength recovery, while the mullite + Yb₂SiO₅ composite without Ti₂AlC showed a strength recovery of about 80%. Furthermore, by analyzing the indentation load–displacement curve to indicate the role of Ti₂AlC, the addition of Ti₂AlC improved both the hardness and stiffness of the composite.     

Physicochemical properties and structural evolutions of gas-phase carbonization chars at high temperatures

The gas-phase carbonization chars from hydrocarbons with low molecular weight (anthracene oil and petroleum ether) were prepared using a drop tube reactor at 1000-1200 °C, and their physicochemical properties and structural evolutions (elemental composition, carbon crystallite structure, surface morphology, pore structure and chemical composition of volatile matters) were mainly investigated. The chars obtained in the high temperature region, which appeared with high C/H atomic ratio and poor carbon crystallite structure far from natural graphite, could be used as high carbonaceous materials. The chars were composed of uniform spherical particles with a continuous pore size distribution. The average pore diameters of the chars were much smaller and in the rage of 5.0-8.7 nm. The increasing carbonization temperature led to an initial increasing and a sequent decreasing of specific surface areas from mico-meso-pores and an increasing of those from meso-macro-pores in the chars. The volatile matters in the chars were composed of an easily-extracted fraction (CS₂-soluble compounds with three to six aromatic rings) and a hard-extracted fraction (CS₂-insoluble compounds with higher aromaticity). The elevated carbonization temperature led to diminish the two volatile fractions. A liquid core formation mechanism was proposed to explain the gas-phase carbonization process of hydrocarbons.     

Interactions between Ca2Gd8(SiO4)6O2 apatite and calcium-magnesium-aluminosilicate (CMAS) at elevated temperatures

High temperature corrosion behavior of Ca₂Gd₈(SiO₄)₆O₂ (CGdS) apatite has been investigated in the presence of molten calcium-magnesium-aluminosilicate (CMAS) glass having the composition 21.9 CaO - 4.3 MgO - 5.4 Al₂O₃ - 63.0 SiO₂ - 4.3 Na₂O - 0.8K₂O - 0.1 Fe₂O₃ (weight %). CGdS apatite powder was prepared by solid state synthesis from constituent oxides. Pellets of CGdS apatite + CMAS mixed powder and CGdS-CMAS diffusion couples were annealed at 1200, 1300, 1400, and 1500 °C for 1 and 20 h in ambient atmosphere. Development of phases in heat treated specimens was characterized using various analytical techniques as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high angle annular dark field imaging, selected area electron diffraction and energy dispersive X-ray spectroscopy. In both pellets and diffusion couples, monoclinic cyclosilicate Ca₃Gd₂(Si₃O₉)₂ formed from reaction of apatite with CaO in the CMAS melt only in samples heat treated at 1200 °C for 1 and 20 h or at 1300 °C for 1 h. Triclinic CaSiO₃ and monoclinic diopside MgCaSi₂O₆ were also observed in samples annealed at 1200 and 1300 °C. At 1400 and 1500 °C, because of its low viscosity, CMAS infiltrated along the pores and grain boundaries of the apatite substrates in diffusion couples. Phase compositions predicted from thermochemical computation were in good agreement with those observed experimentally. Ca₂Gd₈(SiO₄)₆O₂ apatite has the potential for being an effective T/EBC in circumventing the penetration of molten CMAS up to about 1300 °C but not at higher temperatures.     

Carbon nanotube reinforced pyrocarbon matrix composites with high coefficient of thermal expansion for self-adapting ultra-high-temperature ceramic coatings

The mismatch in the coefficients of thermal expansion (CTE) of the carbon fiber reinforced pyrocarbon (Cf/C) composites and their thermal barrier coatings (TBCs) has significantly restricted the service life of Cf/C composites in high-temperature environments. Owing to the high CTE of TBCs, it is vital to find a material with similar mechanical properties and higher CTE than Cf/C composites. In this work, carbon nanotube reinforced pyrocarbon (Ct/C) nanocomposites with high CTEs were prepared to self-adapt to the TBCs. Different CTEs (～4.0–6.5 × 10^?6/°C) were obtained by varying the carbon nanotube (CNT) content of the Ct/C composites. Owing to the decreased mismatch in the CTEs, no cracks were formed in the TBCs (SiC and HfB₂-SiC-HfC coatings) deposited on the Ct/C composites. After heat treatment at 2100 °C, several wide cracks were found in the TBCs on the Cf/C composite, whereas the TBCs on the Ct/C composites were intact without cracks. We found that the CTE-tunable Ct/C composites can self-adapt to different TBCs, protecting the composites from oxidation at high temperatures.     

Thermal properties of plasma-sprayed thermal barrier coating with bimodal structure

Nanostructured zirconia coatings have been prepared by atmospherical plasma spraying (APS) on NiCrAlY-coated superalloy substrates. The isothermal oxidation test results indicate that the oxidation kinetics of nanostructured TBC follows a parabolic law and the oxidation resistance of the nanostructured TBC is comparable to that of the conventional TBC. The nanostructured thermal barrier coatings exhibit excellent thermal cyclic resistance and low thermal diffusivity. The failure of the nanostructured TBC occurs within the top coat and close to the YSZ/thermal growth oxide interface. The thermal diffusivity of the coating is 90% of that of conventional thermal barrier coatings, and it increases after heat treatment at 1050 °C for 34 h. The increase in the thermal diffusivity of the coating is ascribed to grain growth, the crack healing as well as sintering neck formation.     

Reactions of molten silicate deposits with yttrium monosilicate

The article addresses effects of silicate deposit composition on reactions with yttrium monosilicate (YMS), a candidate environmental barrier coating for aero-engine components. Computed phase equilibria are used to predict the nature and relative proportions of reaction products and the extent of YMS consumption upon reaction with twelve deposits of varying composition at 1300°C. These predictions are compared with results of a corresponding experimental study on three exemplary deposits. Although the nature and sequence of reaction products formed (typically apatite and yttrium disilicate) depend on the Ca:Si ratio of the deposit, the degree of consumption of YMS at equilibrium is relatively insensitive to deposit composition and is predicted to proceed to a greater extent than that in yttrium disilicate. However, sluggish reaction kinetics associated with the formation of a thin apatite layer above the YMS prevents reactions from reaching their terminal equilibrium states within the experimental times investigated (250 hours). For deposit loadings of 18 mg/cm² (corresponding to a thickness of about 100 µm), the degree of consumption following 250 hours exposures is only about 10%-40% of the predicted terminal values, depending on deposit composition.