Simultaneous synthesis by spark plasma sintering of a thermal barrier coating system with a NiCrAlY bond coat

As-fabricated thermal barrier coating (TBC) systems generally consist of a superalloy substrate, a MCrAlY bond coat (M = Ni, Co, Fe), and a ceramic (usually partially stabilized zirconia) top coat. The conventional methods for producing the two coating layers generally derive from thermal spray and physical vapor deposition techniques. Thermal exposure leads to the formation of an additional layer: the thermally grown oxide (TGO) between the bond coat and top coat. In the present work, a TBC system is synthesized through the application of spark plasma sintering (SPS), which provides not only the opportunity to synthesize all three layers at once, but the process is quite rapid and can produce dense layers. More specifically, this paper describes the application of this method to an yttria-stabilized ZrO₂ (YSZ) top coat and a NiCrAlY bond coat on a Ni-base Hastelloy X substrate. A one-micron thick Al₂O₃ TGO layer is also created from the reaction between an Al foil layer inserted in the stack prior to sintering and the ZrO₂ in the top coat. The effects of select process conditions are considered. The resulting multi-layer system is characterized with optical microscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD). Differential thermal analysis (DTA) is used to investigate the reaction between the Al foil and the YSZ top coat.     

Elastic Modulus Evolution and Behavior of Si/Mullite/BSAS-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapor Environment

Si-based ceramics (e.g., SiC and Si₃N₄) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the surface recession that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H₂O vapor), appropriate refractory oxides are engineered to serve as environmental barrier coatings (EBCs). The current state-of-the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al₂O₃·2SiO₂) interlayer and (1 ? x)BaO·xSrO·Al₂O₃·2SiO₂, 0 ?? x ?? 1 (BSAS) top coat. In this article, the role of high-temperature exposure (1300 °C) performed in H₂O vapor environment (for time intervals up to 500 h) on the elastic moduli of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates was investigated via depth-sensing indentation. Laser-ultrasonics was employed to evaluate the E values of as-sprayed BSAS coatings as an attempt to validate the indentation results. Fully crystalline, crack-free, and near-crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The absence of phase transformation and stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers after harsh environmental exposure provides a plausible explanation for the almost crack-free coatings observed. The relationships between the measured elastic moduli of the EBCs and their microstructural behavior during the high-temperature exposure are discussed.     

An Al+Y Coating Process for Improvement of the High-Temperature Oxidation Resistance of a TiAl Alloy

An Al+Y codeposition by a single EB–PVD process has been developed to improve the high-temperature oxidation resistance of a TiAl alloy. The Al+Y codeposited TiAl alloy with various ratios of Al and Y evaporation sources was evaluated by isothermal and cyclic-oxidation tests. The coating layer has a composition gradient because of the difference in vapor pressure between Y and Al. The oxidation resistance can be extensively improved by the formation of an Al+Y codeposition layer and it depends on the ratio of the source material of Al and Y. The best oxidation resistance was obtained from the Al+Y codeposited TiAl alloy with a ratio of Al:2Y. With the proper ratio of Al:Y, the Al+Y codeposition coating forms two layers of the oxides during high-temperature oxidation+the outer (Y, Al)O layer and the inner Al₂O₃ layer, containing small amounts of Y oxide. These oxide layers, which have a fine grain size, act as a diffusion barrier, thus suppressing the rate of inward diffusion of O and may readily relieve thermal stresses.     

Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions

Plasma sprayed thermal barrier coatings (TBCs) are applied to gas turbine components for providing thermal insulation and oxidation resistance. The TBC systems currently in use on superalloy substates typically consists of a metallic MCrAlY based bond coat and an insulating Y₂O₃ partially stabilized ZrO₂ as a ceramic top coat (ZrO₂ 7–8 wt.% Y₂O₃). The oxidation of bond coat underlying yttria stabilized zirconia (YSZ) is a significant factor in controlling the failure of TBCs. The oxidation of bond coat induces to the formation of a thermally grown oxide (TGO) layer at the bond coat/YSZ interface. The thickening of the TGO layer increases the stresses and leads to the spallation of TBCs. If the TGO were composed of a continuous scale of Al₂O₃, it would act as a diffusion barrier to suppress the formation of other detrimental mixed oxides during the extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability. The TBC layers are usually coated onto the superalloy substrate using the APS (Atmospheric plasma spray) process because of economic and practical considerations. As well as, HVOF (High velocity oxygen fuel) bond coat provides a good microstructure and better adhesion compared with the APS process. Therefore, there is a need to understand the cycling oxidation characteristic and failure mode in TBC systems having bond coat prepared using different processes. In the present investigation, the growth of TGO layers was studied to evaluate the cyclic oxidation behavior of YSZ/Al₂O₃ composite TBC systems with APS-NiCrAlY and HVOF-NiCrAlY bond coats. Interface morphology is significantly effective factor in occurrence of the oxide layer. Oxide layer thickening rate is slower in APS bond coated TBCs than HVOF bond coated systems under thermal cycle conditions at 1200 °C. The YSZ/Al₂O₃ particle composite systems with APS bond coat have a higher thermal cycle life time than with the HVOF bond coating.     

Cyclic-Oxidation Behavior of Thermal-barrier Coatings Exposed to NaCl Vapor

A Ni–24Cr–6Al–0.7Y (NiCrAlY) coating was deposited on a nickel-base superalloy by low-pressure plasma spraying, and the top coating, ZrO₂ partially stabilized with Y₂O₃ (7.5 wt%), was deposited on the NiCrAlY coating by air-plasma spraying. The cyclic-oxidation behavior of the NiCrAlY + YSZ coating exposed to NaCl vapor was investigated under atmospheric pressure at 1,050 °C, 1,100 °C and 1,150 °C. The cyclic-oxidation life of the NiCrAlY + YSZ coating in the presence of NaCl vapor was shortened compared with that in air. The higher the temperature is, the shorter the cyclic oxidation life. The oxide scale formed at the interface between the bond coat and the ceramic layer after exposure to NaCl vapor consisted of voluminous and non-protective NiO, Al₂O₃ and NiCr₂O₄ spinel. The failure of the TBC exposed to NaCl vapor occurs within the top coat and close to the YSZ/thermal growth oxide interface. The failure mechanism has been discussed based on the experimental results and thermodynamics.     

High-temperature oxidation and hot corrosion behavior of two kinds of thermal barrier coating systems for advanced gas turbines

High-temperature oxidation and hot corrosion tests were conducted at 800 to 1100 °C under isothermal and thermal-cycle conditions for two kinds of thermal barrier coating (TBC) systems with different compositions of ceramic top coat: Y₂O₃-stabiIized zirconia (YSZ) and CaO-SiO₂-ZrO₂ (C₂S-CZ). Qualitative and quantitative failure analyses were carried out to clarify the failure mechanisms of TBC systems. In high-temperature oxidation up to 1100 °C, the YSZ-TBC system was subjected more easily to spalling of the ceramic top coat. This is attributed to the localized oxidation along the ceramic top coat/metallic (NiCrAlY) bond coat interface, as compared with the case of the C₂S-CZ-TBC system. Thus, the most significant oxidation damage resulted in the YSZ system under the thermal-cycle condition. On the other hand, for hot corrosion by Na₂SO₄-NaCI molten salt up to 1000 °C, the C₂S-CZ system was more reactive with the molten salt to form a new phase layer composed of both the metallic bond coat constituents, such as aluminum and chromium, and corrosive species such as oxygen at the inner region of the ceramic top coat. Furthermore, effects of both the heat treatment, in particular the atmosphere after plasma spraying, and the chromium content of the bond coat were investigated for each coating system.     

Mechanical Behavior of Air Plasma-Sprayed YSZ Functionally Graded Mullite Coatings Investigated via Instrumented Indentation

Yttria-stabilized zirconia (YSZ)-mullite multilayer architectures with compositional grading between the bond coat and YSZ top coat are envisioned as solutions to ease their coefficient of thermal expansion mismatch induced stress. In this work, two different types of mullite powder (spray-dried and freeze-granulated) and a mullite-YSZ 75/25 vol.% mixture spray-dried powder were employed. Using instrumented indentation with loads between 10 and 500 mN, the role of the powder characteristics on the mechanical behavior of air plasma-sprayed mullite bond coats deposited on SiC substrates was investigated. Hardness (H) and elastic modulus (E) were measured for the as-sprayed coatings and for coatings heat-treated at 1300 °C, in water vapor environment, for periods up to 500 h. Both H and E values of the coatings are found to be highly dependent on the size distribution of the starting powders. It is aimed the fabrication of an efficient and cost-effective EBC prototype based on YSZ compositionally graded mullite.     

Al-rich precipitation in CoNiCrAlY bondcoat at high temperature

A thermal barrier coating (TBC) is applied on a surface of a gas turbine blade to provide a thermal barrier and oxidation resistant properties for the components. The ability to resist oxidation of the coating arises from the self-healing, protective Al₂O₃ scale on top of the bondcoat, which is formed during service. However, if Al depletion occurs within the bondcoat, the protective scale will lose its self-healing ability, and hence, its oxidation-resistant property. This paper investigated the depletion of Al within the bondcoat by studying the microstructure of the bondcoat on a gas turbine blade after it has been in 4000 h service at 1200 °C. The results showed that Al depletion had occurred at different levels throughout the turbine blade. In the area where Al depletion had not yet occurred, precipitation of an Al-rich phase was detected. Most of the Al was contained within this phase, leaving only small amount of Al in the surrounding matrix. A well-defined boundary was observed between the depleted and non-depleted regions.     

Proto-TGO formation in TBC systems fabricated by spark plasma sintering

Thermal barrier coatings (TBC) are commonly used in modern gas turbines for aeronautic and energy production applications. The conventional methods to fabricate such TBCs are EB-PVD or plasma spray deposition. Recently, the spark plasma sintering (SPS) technique was used to prepare new multilayered coatings. In this study, complete thermal barrier systems were fabricated on single crystal Ni-based superalloy (AM1®) substrate in a one-step SPS process. The lifetime of TBC systems is highly dependent on its ability to form during service a dense, continuous, slow-growing alumina layer (TGO) between an underlying bond coating and a ceramic top coat. In the present paper, we show that such kind of layer (called proto-TGO in the following) can be in situ formed during the SPS fabrication of TBC systems. This proto-TGO is continuous, dense and its nature has been determined using TEM-EDS-SAD and Raman spectroscopy. This amorphous oxide layer in the as-fabricated samples transforms to α-Al₂O₃ during thermal treatment under laboratory air at 1100 °C. Oxidation kinetics during annealing are in good agreement with the formation of a protective α-Al₂O₃ layer.     

Thermal shock testing of thermal barrier coating/bondcoat systems

Various methods of thermal shock testing are used by aircraft and industrial gas turbine engine (IGT) manufacturers to characterize new thermal barrier coating systems in the development stage as well as for quality control. The cyclic furnace oxidation test (FCT), widely used in aircraft applications, stresses the ceramic/bondcoat interface, predominantly through thermally grown oxide (TGO) growth stress. The jet engine thermal shock (JETS) test, derived from a burner rig test, creates a large thermal gradient across the thermal barrier coating (TBC), as well as thermomechanical stress at the interface. For IGT applications with long high-temperature exposure times, a combination of isothermal preoxidation and thermal shock testing in a fluidized bed reactor may better represent the actual engine conditions while both types of stress are present. A comparative evaluation of FCT, JETS, and a combined isothermal oxidation and fluidized bed thermal shock test has been conducted for selected ceramic/bondcoat systems. The results and the failure mechanisms as they relate to the TBC system are discussed. A recommendation on the test method of choice providing best discrimination between the thermal shock resistance of the ceramic layer, the ceramic/bondcoat interface, and even substrate related effects, is given. This paper was presented at the 2nd International Surface Engineering Congress sponsored by ASM International, on September 15–17, 2003, in Indianapolis, Indiana, and appeared on pp. 520–29.     

EB-PVD热障涂层热循环过程中粘结层的氧化和相结构

采用磁控溅射方法在镍基单晶高温合金基体上沉积Ni-30Cr-12Al-0.3Y（质量分数，％）粘结层，采用电子束物理气相沉积方法（EB－VPD）沉积7％Y2O3（质量分数）－ZrO2陶瓷顶层，结果表明，在热循环过程中，非平衡相t′-ZrO2中的Y2O3含量逐渐减少，t′－ZrO2相逐渐分解成平衡相t-ZrO2（冷却时变转变成斜相）和立方组ZrO2,1050℃循环200次，粘结层氧化物（Al2O3)厚度约为3μm，表明Ni-Cr-Al-Y达宜作粘结层，继续热循环，陶瓷层中出现单斜阳，粘结层中Al贫化，氧化层中出现NiO及尖晶石等，引起应力集中，导致涂层失效。     

Numerical modeling of short crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of microstructurally short inherent cracks within a preoxidized thermal barrier coating system upon thermal shock loading is considered. A thin alumina oxide layer holding residual stresses was induced at the ceramic/metal interface to simulate thermally grown oxide on the bond coat. Undulation of the oxidized bond coat was modeled as a sinusoidal surface. The variations of the stress-intensity factors of inherent centrally located cracks and of edge cracks were calculated during the thermal cycling. The instant crack shapes during the first thermal cycle and at steady state were investigated. It was found that oxide layer thickness, crack tip location, as well as interfacial undulation are factors influencing the risk of crack propagation. It was also found that an edge crack constitutes a greater threat to the coating durability than a central crack. The propagation of an edge crack, if it occurs, will take place during the first load cycle, whereas for a central crack, crack tip position decides the risk of crack propagation.     

Oxidation Control with Chromate Pretreatment of MCrAlY Unmelted Particle and Bond Coat in Thermal Barrier Systems

MCrAlY alloy bond coat is widely used in thermal barrier coating (TBC) systems to protect substrates from high-temperature oxidizing environments. However, failure of the ceramic topcoat can occur due to a thermally grown oxide (TGO) that grows at the interface between the bond coat and the topcoat. In this study, the effect of chromate treatment was investigated. Prior to topcoat deposition, a thin film of Cr₂O₃ was formed on the bond coat surface. High-temperature oxidation tests were carried out, and the oxidation rates were determined by inspection of cross sections. Similar oxidation tests were carried out using MCrAlY powder material assumed to be unmelted particles. As a result, the chromate-treated bond coat showed outstanding oxidation resistance. Calculations that take into account the oxidation of particles in the topcoat indicated the generation of internal stress to cause local fracture of the topcoat.     

Structural and Chemical Analyses of a Thermally Grown Oxide Scale in Thermal Barrier Coatings Containing a Platinum–Nickel–Aluminide Bondcoat

The structure and chemistry of the thermally grown oxide scale in a thermalbarrier coating having an electron beam-physical vapor depositedyttria-partially stabilized zirconia (YPSZ) topcoat and aplatinum–nickel–aluminide (Pt–Ni–Al) bondcoat werestudied using transmission electron microscopy. The scale consisted ofhexagonal aluminum oxide (-Al₂O₃) in both as-coated and thermallycycled specimens; no metastable Al₂O₃ polymorphs were observed. In as-coatedspecimens, the scale's grains had a fine, columnarmorphology. ZrO₂-rich dispersoids were observed both intra- andintergranularly throughout the scale. Thermally cycled specimens had aduplex scale structure: one band of grains adjacent to the YPSZ had anequiaxed morphology and contained ZrO₂-rich dispersoids; a second band ofdispersoid-free grains adjacent to the bondcoat had a coarse, columnarmorphology. Porosity and cracks were associated with the interface betweenthe bands. The formation of the banded morphology and the cracking areproposed to be associated with the presence of the ZrO₂-rich dispersoids.     

Microstructural analysis of YSZ and YSZ/Al2O3 plasma sprayed thermal barrier coatings after high temperature oxidation

Air plasma sprayed TBCs usually include lamellar structure with high interconnected porosities which transfer oxygen from YSZ layer towards bond coat and cause TGO growth and internal oxidation of bond coat.The growth of thermally grown oxide (TGO) at the interface of bond coat and ceramic layer and internal oxidation of bond coat are considered as the main destructive factors in thermal barrier coatings.Oxidation phenomena of two types of plasma sprayed TBC were evaluated: (a) usual YSZ (yttria stabilized zirconia), (b) layer composite of (YSZ/Al₂O₃) which Al₂O₃ is as a top coat over YSZ coating. Oxidation tests were carried out on these coatings at 1100°C for 22, 42 and 100h. Microstructure studies by SEM demonstrated the growth of TGO underneath usual YSZ coating is higher than for YSZ/Al₂O₃ coating. Also cracking was observed in usual YSZ coating at the YSZ/bond coat interface. In addition severe internal oxidation of the bond coat occurred for usual YSZ coating and micro-XRD analysis revealed the formation of the oxides such as NiCr₂O₄, NiCrO₃ and NiCrO₄ which are accompanied with rapid volume increase, but internal oxidation of the bond coat for YSZ/Al₂O₃ coating was lower and the mentioned oxides were not detected.     

Damage mechanisms and lifetime behavior of plasma-sprayed thermal barrier coating systems for gas turbines — Part II: Modeling

A phenomenological lifetime prediction tool for plasma-sprayed ZrO₂ based thermal barrier coating systems with MCrAlY bondcoat is presented. The analytical model uses a two step approach for calculating the development of delamination cracks: The initial crack growth is considered to be proportional to the thickness of the thermally grown oxide (TGO) scale on the bondcoat. After exceeding a critical TGO thickness, crack propagation is governed by linear elastic fracture mechanics taking into account stresses induced by thermal mismatch and by TGO growth. Validation using experimental data from thermal cycling tests with high temperature dwell times from few seconds up to 96 h gave evidence of the good predictive quality of the model.     

Improvement of thermally grown oxide layer in thermal barrier coating systems with nano alumina as third layer

A thermally grown oxide (TGO) layer is formed at the interface of bond coat/top coat. The TGO growth during thermal exposure in air plays an important role in the spallation of the ceramic layer from the bond coat. High temperature oxidation resistance of four types of atmospheric plasma sprayed TBCs was investigated. These coatings were oxidized at 1000 °C for 24, 48 and 120 h in a normal electric furnace under air atmosphere. Microstructural characterization showed that the growth of the TGO layer in nano NiCrAlY/YSZ/nano Al₂O₃ coating is much lower than in other coatings. Moreover, EDS and XRD analyses revealed the formation of Ni(Cr,Al)₂O₄ mixed oxides (as spinel) and NiO onto the Al₂O₃ (TGO) layer. The formation of detrimental mixed oxides (spinels) on the Al₂O₃(TGO) layer of nano NiCrAlY/YSZ/nano Al₂O₃ coating is much lower compared to that of other coatings after 120 h of high temperature oxidation at 1000 °C.     

Effects of alkali and alkaline-earth oxides on thermal, dielectrical, and optical properties of zinc borate glasses for transparent dielectric

The development of a transparent dielectric layer with a low dielectric constant is essential for improving the luminous efficacy of plasma display panels (PDPs). In this study, a ZnO-B₂O₃-SiO₂-Al₂O₃ glass containing alkaline-earth oxides and an alkali oxide was fabricated as a candidate Pb- and Bi-free transparent dielectric layer with a low dielectric constant. The thermal, electrical, and optical properties of these glasses were characterized. The changes in the properties in this glass system were investigated according to the alkalineearth oxides (RO)/alkali oxide (R₂O) ratio. The glass transition temperature (T_g), the coefficient of thermal expansion (CTE), and the dielectric constant of the glasses varied from 480°C to 540 °C, 7.9 to 10.6 × 10⁻⁶ /K, and 7.4–8.2, respectively. As the RO/R₂O ratio was increased, the T_g value increased, the CTE and transmittance decreased, and the dielectric constant increased slightly. The T_g or CTE can be calculated from other known values in these system glasses without requiring numerous experiments. The T_g and CTE are inversely proportional to each other and a constant was obtained by multiplying the two values.     

Thermodynamics of Zr/Hf-mixed silicates as a potential for environmental barrier coatings for Tyranno-hex materials

The synthesis and characterization of Zr/Hf-mixed silicates, which are promising as an environmental barrier coating (EBC) on Tyranno-hex materials, have been carried out. The coefficient of thermal expansion (CTE), the corrosion resistance against high- temperature water vapor, and the drops of strength and elastic modulus after exposure for high-temperature water vapor were investigated. Furthermore, the chemical stability of the Zr/Hf-mixed silicates under high-temperature water vapor and oxygen gas was thermodynamically investigated. The CTE value can change to (3.6-5.1) × 10^− 6 K^− 1 when the Zr/Hf ratio fluctuates. The weight losses of the mixed silicate samples after exposure for high-temperature water vapor were very small. The contents of the main component elements, such as Zr, Hf, and Si in the samples, were unchanged within the error limits by the exposure. The flexural strength and bending modulus of the samples decreased by 5-21 and 6-29% due to the exposure, respectively. The chemical stability of the Zr/Hf-mixed silicates under the experimental conditions was discussed.     

Thermal shock characteristics of plasma sprayed mullite coatings

Commercially available mullite (3Al₂O₃·2SiO₂) powders containing oxides of calcium and iron as impurities, have been made suitable for plasma spraying by using an organic binder. Stainless steel substrates covered with Ni-22Cr-10Al-1.0Y bond coat were spray coated with mullite. The 425 μm thick coatings were subjected to thermal shock cycling under burner rig conditions between 1000 and 1200 °C and less than 200 °C with holding times of 1, 5, and 30 min. While the coatings withstood as high as 1000 shock cycles without failure between 1000 and 200 °C, spallation occurred early at 120 cycles when shocked from 1200 °C. The coatings appeared to go through a process of self erosion at high temperatures resulting in loss of material. Also observed were changes attributable to melting of the silicate grains, which smooth down the surface. Oxidation of the bond coat did not appear to influence the failure. These observations were supported by detailed scanning electron microscopy and quantitative chemical composition analysis, differential thermal analysis, and surface roughness measurements.