Thermal Barrier Coatings Material Research Articles

Fabrication of samarium strontium aluminate ceramic and deposition of thermal barrier coatings by air plasma spray process

Thermal barrier coatings (TBC) with the metallic NiCrAlY bond coat are often used in many aircraft engines to protect superalloy components from high-temperature corrosion thereby to improve the life of gas turbine components. The search for new TBC material has been intensified in recent years due to lack of thermo-physical properties of conventionally used Yttria stabilized Zirconia (YSZ) TBCs. Recently, the rare earth containing Samarium Strontium Aluminate (SSA) based ceramic was proposed as a new TBC material due to its matching thermo-physical properties with the substrate. The present work focused on the synthesis of SSA ceramics for TBCs application and its coatings development on Ni-based superalloy Inconel 718 substrate by air plasma spray process. The X-ray photoelectron spectroscopy (XPS) result confirmed the formation of single phase SSA ceramic after synthesis. The surface morphology of SSA TBCs is mainly composed of melted splats, semi and un-melted particles. The cross-sectional SEM micrographs did not show any spallation at the interface which indicated good mechanical interlocking between the bond coat and ceramic top coat. The Young’s modulus and hardness of SSA TBCs were found to be 80 and 6.1 GPa, respectively. The load-depth curve of SSA TBC showed good elastic recovery about 47 %.

Fabrication of samarium strontium aluminate ceramic and deposition of thermal barrier coatings by air plasma spray process

Thermal barrier coatings (TBC) with the metallic NiCrAlY bond coat are often used in many aircraft engines to protect superalloy components from high-temperature corrosion thereby to improve the life of gas turbine components. The search for new TBC material has been intensified in recent years due to lack of thermo-physical properties of conventionally used Yttria stabilized Zirconia (YSZ) TBCs. Recently, the rare earth containing Samarium Strontium Aluminate (SSA) based ceramic was proposed as a new TBC material due to its matching thermo-physical properties with the substrate. The present work focused on the synthesis of SSA ceramics for TBCs application and its coatings development on Ni-based superalloy Inconel 718 substrate by air plasma spray process. The X-ray photoelectron spectroscopy (XPS) result confirmed the formation of single phase SSA ceramic after synthesis. The surface morphology of SSA TBCs is mainly composed of melted splats, semi and un-melted particles. The cross-sectional SEM micrographs did not show any spallation at the interface which indicated good mechanical interlocking between the bond coat and ceramic top coat. The Young’s modulus and hardness of SSA TBCs were found to be 80 and 6.1 GPa, respectively. The load-depth curve of SSA TBC showed good elastic recovery about 47 %.

Thermal Conductivity and Expansion Coefficient of (Sm1−x Yb x )2Ce2O7 Ceramics for Thermal Barrier Coatings

In the current paper, the (Sm1−x Yb x )2Ce2O7 ceramics were prepared via sol–gel and high-temperature solid reaction methods. The phase composition, microstructure, thermal conductivity, and expansion coefficient were investigated. Results indicate that pure (Sm1−x Yb x )2Ce2O7 ceramics with single defect-fluorite structure are synthesized successfully. Owing to the phonon scattering caused by Yb addition, the thermal conductivity of (Sm1−x Yb x )2Ce2O7 ceramics decreases with increasing Yb2O3 content at identical temperatures, which is lower than that of YSZ. Due to the relatively low ionic radius of Yb3+ ions, the addition of Yb2O3 decreases the thermal expansion coefficient of (Sm1−x Yb x )2Ce2O7 ceramics, which is higher than that of 8YSZ. The synthesized (Sm1−x Yb x )2Ce2O7 ceramics can be explored as candidate materials for thermal barrier coatings.

Hot corrosion behavior of YSZ, Gd2Zr2O7 and YSZ/Gd2Zr2O7 thermal barrier coatings exposed to molten sulfate and vanadate salt

Thermal barrier coatings (TBCs) are mostly used in critical components of aircraft gas turbine engines. Hot corrosion is among the main deteriorating factors in TBCs which results from the effect of molten salt on the coating–gas interface. This type of corrosion is observed as a result of contamination accumulated during combustion processes. Fuels used in aviation industry generally contain impurities such as vanadium oxide (V2O5) and sodium sulfate (Na2SO4). These impurities damage turbines’ inlet at elevated temperatures because of chemical reaction. Yttria stabilized zirconia (YSZ) is a conventional top coating material for TBCs while Gd2Zr2O7 is a new promising top coating material for TBCs. In this study, CoNiCrAlY metallic bond coat was deposited on Inconel 718 nickel based superalloy substrate material with a thickness about 100μm using cold gas dynamic spray (CGDS) method. Production of TBCs were done with deposition of YSZ, Gd2Zr2O7, YSZ/Gd2Zr2O7 ceramic top coating materials using EB-PVD method, having a total thickness of 300μm. Hot corrosion behavior of YSZ, Gd2Zr2O7, YSZ/Gd2Zr2O7 TBC systems were exposed to 45wt.% Na2SO4 and 55wt.% V2O5 molten salt mixtures at 1000°C temperature. TBC samples were investigated and compared using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis and X-ray diffractometer (XRD). The hot corrosion failure mechanisms of YSZ, Gd2Zr2O7 and YSZ/Gd2Zr2O7 TBCs in the molten salts were evaluated.

Numerical Investigation on the Effects of Different Thermal Insulation Strategies for a Passenger Car Diesel Engine

AbstractOne of the key technologies for the improvement of the diesel engine thermal efficiency is the reduction of the engine heat transfer through the thermal insulation of the combustion chamber. This paper presents a numerical investigation on the effects of the combustion chamber insulation on the heat transfer, thermal efficiency and exhaust temperatures of a 1.6 l passenger car, turbo-charged diesel engine. First, the complete insulation of the engine components, like pistons, liner, firedeck and valves, has been simulated. This analysis has showed that the piston is the component with the greatest potential for the in-cylinder heat transfer reduction and for Brake Specific Fuel Consumption (BSFC) reduction, followed by firedeck, liner and valves. Afterwards, the study has been focused on the impact of different piston Thermal Barrier Coatings (TBCs) on heat transfer, performance and wall temperatures. This analysis has been performed using a 1-D engine simulation code coupled with a lumped mass thermal model, representing the engine structure. A time-periodic wall conduction model has been used to calculate the wall temperature swings along the combustion chamber surface and within the engine cycle. Two different TBC materials, Yttria-Partially Stabilized Zirconia (Y-PSZ) and anodized aluminum, and different layer thicknesses have been simulated.

CMAS behavior of yttrium aluminum garnet (YAG) and yttria-stabilized zirconia (YSZ) thermal barrier coatings

Calcium magnesium aluminosilicate (CMAS) that is formed from the ingested deposits in gas turbines degrades thermal barrier coatings (TBCs), especially for the most widely used material; yttria-stabilized zirconia (YSZ). In the present work, we examine the behavior of yttrium aluminum garnet (YAG) as an alternative material for TBCs. CMAS interaction studies were conducted by making composite pellets of YAG-CMAS and YSZ-CMAS powders. These pellets, after being subjected to heat treatment between 1100°C and 1500°C were characterized by X-ray Diffraction (XRD), scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS), which showed YAG to be almost inert to CMAS whereas YSZ exhibited significant phase changes. To test the behavior of TBCs with YAG and 8YSZ as the topcoat material in a CMAS environment, cyclic furnace tests were conducted in which a controlled amount of CMAS was applied and then the samples were cycled to failure. In addition, to simulate the continuous accumulation of CMAS expected in service, a cyclic furnace test was devised in which a small dose of aqueous solution of CMAS was applied on TBC specimens at the start of every cycle until the samples were cycled to failure. In all these tests YAG TBCs outperformed YSZ in terms of durability. The mechanisms of CMAS attack are described and the relative resistance of YAG and YSZ is shown to be consistent with the Optical Basicity (OB) theory.

Stability of rare‐earth‐doped spherical yttria‐stabilized zirconia synthesized by ultrasonic spray pyrolysis

AbstractPhase stability is one of the crucial requirements for any material that can be used at elevated temperature applications such as thermal barrier coating (TBC). The most traditional TBC material, partially stabilized zirconia, limits the operating temperature due to its phase transformation. Conversely, the low thermal conductivity of fully stabilized zirconia (YSZ) may enable effective reduction in the surface temperature on the coated component, while avoiding deleterious phase transitions, although still being subjected to sintering and grain growth. It has been reported that addition of rare‐earths as dopants to YSZ can significantly increase resistance to grain growth at high temperature. Keeping this under consideration, this work investigates the role of rare‐earths (lanthanum and gadolinium) in increasing thermal stability of YSZ microspheres, the building blocks for high‐temperature photonics for reflective TBC. The spheres were produced by ultrasonic spray pyrolysis, and the doping led to significant improvement of stability by significant inhibition of grain growth. While the individual dopants showed significant growth and sintering inhibition up to 900°C, co‐doping with 4% (in mol) of each (Gd and La) led to coarsening resistance up to 1000°C for 3 hours, when particles retained reasonable spherical features with nanometric crystallite sizes.

Free vibration analysis of a pre-twisted sandwich blade with thermal barrier coatings layers

A rotating cantilever sandwich-plate model with a pre-twisted and pre-set angle has been developed to investigate the vibrational behavior of an aero-engine turbine blade with thermal barrier coating (TBC) layers. The classic von Karman plate theory and the first-order shear deformation theory are applied to derive the energy equations of the rotating TBC blade, in which the geometric shapes, the work ambient temperature, and the TBC material properties are considered. The Chebyshev-Ritz method is used to obtain the nature frequency of the rotating TBC blade. For static frequency and modal analysis, the finite-element method (FEM) is also applied to compare and validate the results from the Chebyshev-Ritz method. A good agreement is found among these kinds of methods. For dynamic frequency, the results are analyzed in detail concerning the influence of system parameters such as the thickness of the TBC layer, the working temperature, and the pre-twisted and pre-set angle. Finally, the Campbell diagram is demonstrated to analyze the resonance property of the cantilever sandwich TBC blade model.

Suspension characterization and electrophoretic deposition of Yttria-stabilized Zirconia nanoparticles on an iron-nickel based superalloy

Yttria-Stabilized Zirconia (YSZ) is the most common material for thermal barrier coatings. Suspensions of 3mol% YSZ nanoparticles in acetone medium have been prepared in presence of different amounts of iodine as dispersant. Size distribution of particles in the suspensions and zeta potential were measured as a function of dispersant concentration. Adding 1.2g/l iodine was found to be effective for the dispersion of YSZ nanoparticles in acetone. The stability of YSZ suspension in acetone increased with iodine content increasing until reached 1.2g/l. Mean diameter of particles and zeta potential of the YSZ suspension in acetone were 912nm and 2.4mV respectively, and with addition of 1.2g/l iodine shifted to 111.6nm and 50.2mV respectively. Electrophoretic deposition (EPD) process has been carried out from this suspension at different applied voltages and deposition times. A uniform green coating was obtained at voltage of 6V and deposition time of 2min the thickness of the green coating is measured about 25µm.

Theoretical analysis of thermal shock resistance of ceramic foam coatings

Ceramic foams have a high resistance to corrosion and wear. They also have a good thermal insulation performance because of their high melting point and low thermal conductivity. The thermal shock resistance of a ceramic foam coating with an edge crack under a sudden temperature variation is investigated. The dynamic thermal stress fields in the ceramic foam coating are obtained. Using the superposition principle, the crack problem of the ceramic foam coating is reduced to the solution of a set of singular integral equations. Propagation of the edge crack is analyzed. Effects of the relative density and thermal properties of the ceramic foam and of crack length on the thermal shock resistance are identified. The results obtained can be useful in designing thermal protective ceramic materials for thermal barrier coatings.

Effect of CMAS Composition on Hot Corrosion Behavior of Gadolinium Zirconate Thermal Barrier Coating Materials

The resistance of synthesized pyrochlore-type Gd2Zr2O7 bulk specimens to four calcium-magnesium aluminosilicate (CMAS) compositions at different temperatures was investigated. The reaction products were identified by X-ray diffraction and penetration depths were examined using scanning electron microscopy. A dense reaction layer is comprised mainly of Ca2Gd8(SiO4)6O2 and a cubic fluorite phase formed during the CMAS attack, and some unreacted CMAS was found in a transition layer below the reaction layer. The overall infiltration depth changed slightly with temperature, however, the thickness of the reaction layer and the morphology of the transition layer varied distinctly with temperature. The sintered sample underwent the most severe degradation by the CaO-lean CMAS, whereas the effect of CaSO4 and CaCO3 was not significant. Moreover, the Gd content of the ZrO2-based cubic fluorite phase depends on the temperature and the molar ratio of Ca:Si in the CMAS.

Influence of long time post annealing on thermal stability and thermophysical properties of plasma sprayed La2Zr2O7 coatings

Thermal barrier coatings (TBCs) were applied to gas turbines to function at higher gas temperatures. Extensive effort has been invested in identifying new materials with even better performance compared to yttria-stabilized zirconia (YSZ) being the industry standard up-to-date. La2Zr2O7 (LZ) with the pyrochlore structure has received attention and been identified as a new candidate material for TBCs. In this study, two different coatings produced with commercial and high-energy ball milled LZ powders were investigated. Coatings were annealed for 50 h, 100 h, 200 h and 500 h at 1150 °C. Microstructures of the coatings were examined with Scanning Electron Microscopy and thermal conductivities were investigated with laser flash measurements. Mercury porosimetry was used as well to identify pore size changes that might occur with annealing process. X-Ray diffraction analysis was used to determine any possible phase changes. The results showed a sintering effect leading to an increase in thermal conductivity, and after 500 h heat treatment time, phase decomposition occurred on LZ based coatings.

Thermal properties of La3TaO7 and La2AlTaO7 oxides

The La3TaO7 and La2AlTaO7 oxides were prepared by solid state reaction method. The phase composition, microstructure and thermal properties were examined. Results show that pure La3TaO7 and La2AlTaO7 oxides are prepared successfully. From 20°C to 1200°C, the average thermal conductivities of La3TaO7 and La2AlTaO7 ceramics are 1.12W/mK and 1.71W/mK, respectively, which are much lower than that of YSZ. Compared to La2AlTaO7, La3TaO7 has lower thermal conductivity due to its higher atom-mass difference between the substituting and the substituted atoms. The thermal expansion coefficient of La3TaO7 is higher than that of YSZ, and the thermal expansion coefficient of La2AlTaO7 is close to that of YSZ. As compared with La2AlTaO7, the higher thermal expansion coefficient of La3TaO7 can be ascribed to its lower electro-negativity difference between ions at sites A and B. The La3TaO7 and La2AlTaO7 oxides can be explored as potential ceramic materials for thermal barrier coatings.

Parameter Optimization of Thermal Barrier Coatings used in Two Stoke Externally Scavenged S.I. Engine using Non-Traditional Optimization Algorithms

Various researchers have studied and introduced, although limited, varieties of thermal barrier coatings (TBC) materials. Each of these TBC materials has their own respective properties. Considering all these properties which one will be the effective choice among the available lot is very difficult to estimate. The optimisation is carried out using non-traditional optimisation techniques namely simple additive weighting method (SAW), weighted product method (WPM), technique for order preference by similarity to ideal solution (TOPSIS) & preference ranking organization method for enrichment evaluations (PROMETHEE) are used to find out the best optimal choice for the specified engine. The results of the above mentioned algorithms are compared and presented in this paper to decide which tbc material will perform comparatively better & give accordingly the good results.

On the resistance of rare earth oxide-doped YSZ to high temperature volcanic ash attack

Abstract Thermal barrier coatings (TBCs), such as 7–8 wt.% yttria stabilized zirconia (YSZ), are widely used to reduce the temperature of coated materials. However, when a turbine operates in a harsh environment, for example a volcanic ash attack, sand and ash particles ingested by the engine could be deposited on the TBC surfaces as molten calcium-magnesium-alumino-silicate (CMAS). CMAS melts and penetrates into TBCs at high temperatures, which causes a loss of strain tolerance and results in the premature failure of the top coat. It is recognized that the formation of CMAS is inevitable due to exposing the turbine to sand and ash; therefore, CMAS mitigation solutions are among the top challenges for materials scientists. To some extent, CMAS is the biggest weakness of the traditional 7-8YSZ TBC material. The thermochemical interactions between YSZ and real volcanic ash are investigated in this paper as a means to alleviate the volcanic ash attack by understanding the underlying mechanisms. 7YSZ, 0.5Gd-8YSZ and 3Er-7YSZ powders and free-standing plates were prepared, respectively. The effect of the volcanic ash on the three different samples was investigated using an X-ray diffraction, scanning electron microscopy, a scanning transmission electron microscopy, and a differential scanning calorimetry. The phase transformations and reaction of TBCs materials subjected to a volcanic ash attack were examined and the volcanic ash degradation and mitigation mechanisms were discussed. It was found that rare earth-doped YSZ samples suffered less damage from the volcanic ash attack than the standard 7YSZ. The results demonstrate that rare earth-doped YSZ may be effectively utilized to mitigate a volcanic ash attack.

Investigation of a new type of composite ceramics for thermal barrier coatings

In order to explore a novel material for thermal barrier coatings (TBCs), the composite ceramic materials of lanthanum zirconate (La2Zr2O7) and lanthanum phosphate (LaPO4) were prepared by calcining. The phases and micro-structures of La2Zr2O7/LaPO4 composite ceramic materials were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The properties of La2Zr2O7/LaPO4 material, such as thermal conductivity, coefficient of thermal expansion (CTE) and mechanical properties of La2Zr2O7/LaPO4 were investigated using laser flash method, high-temperature dilatometer and micro-hardness test. Based on XRD patterns, the pyrochlore and monazite phases were obtained in La2Zr2O7/LaPO4 composite ceramic material without any chemical reaction. According to SEM morphology, the La2Zr2O7 was closely near to LaPO4 along with a lot of pores, it might cause the decrease of thermal conductivity. The thermal conductivities of composite ceramics were similar to that of La2Zr2O7. The CTE of them were about 10×10−6K−1, which was close to the value of LaPO4. Because of doping LaPO4, hardness and Young's modulus of samples were lower than that of La2Zr2O7. The studies revealed that the La2Zr2O7/LaPO4 composite ceramics had good mechanical and thermal physical properties and could be applied as new candidate materials for TBCs in the future.

Ce1−x Sm x O2−x/2—A novel type of ceramic material for thermal barrier coatings

Abstract In this study, Ce1−x Sm x O2−x/2 ceramics were synthesized by sol–gel route and solid state sintering method. The phase structure was analyzed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy. The morphologies of the synthesized powders and the corresponding bulk samples were observed using scanning electron microscopy (SEM). Their thermal diffusivities and thermal expansion coefficients were measured by the laser-flash method and the pushing-rod method, respectively. Results show that pure Ce1−x Sm x O2−x/2 powders with single fluorite structure are synthesized successfully, and their microstructures of the corresponding bulk samples are very dense. With the increase of Sm2O3 content, their thermal expansion coefficients decrease due to the higher electro-negativity of Sm3+ ions as compared with that of Ce4+ ions. Their thermal conductivities at 1000 °C lie in the range of 1.62–2.02 W/(m·K) due to the phonon scattering caused by the substituted atoms and oxygen vacancies. The Ce1−x Sm x O2−x/2 ceramics can be used as ceramic candidates for novel thermal barrier coatings (TBCs).

Defect engineering in development of low thermal conductivity materials: A review

Low thermal conductivity is the key property dominating the heat insulation ability of thermal barrier coatings (TBC). Reducing the intrinsic thermal conductivity is the major topic for developing advanced TBCs. Defect engineering has attracted much attention in seeking better TBC materials since lattice defects play a crucial role in phonon scattering and thermal conductivity reduction. Oxygen vacancies and substitutions are proven to be the most effective, while the accompanying lattice distortion is also of great importance. In this paper, recent advances of reducing the thermal conductivity of potential thermal barrier coating materials by defect engineering are comprehensively reviewed. Effects of the mass and size mismatch between the defects and the host lattice are quantitatively estimated and unconventional thermal conductivity reduction caused by the lattice distortions is also discussed. Finally, challenges and potential opportunities are briefly assessed to further minimize the thermal conductivity of TBC materials in the future.

Nondestructive evaluation of the interface between ceramic coating and stainless steel by electromagnetic method

Protecting coatings as thermal barrier coating (TBC) are used for yield improvement of equipment working at high temperature. Zirconia doped with yttria ceramics are considered a good TBC material due of its low thermal conductivity, refractory, chemical inertness and compatible thermal expansion coefficient with metallic support. The paper proposes the use of an electromagnetic method for evaluation of coatings on stainless steel using a sensor with metamaterial lens and comparison of the results with those obtained by complementary methods.

Synthesis and Characterization of LaMgAl<sub>11</sub>O<sub>19</sub> as Thermal Barrier Coatings Material

LaMgAl11O19 was synthesized at 1550 °C using La2O3, MgO and Al2O3 as raw materials. The samples were characterization by XRD and SEM. The tablet shaped crystals free of impurity phase formed under this condition. The thermal diffusivities were measured by laser flash method and the determined intrinsic thermal conductivities decreased as temperature increases from 25 °C to 1000 °C. As comparison, intrinsic thermal conductivities of LaMgAl11O19 are lower than that of 7YSZ. The synthesized LaMgAl11O19 was heat treated at higher temperature from 1600 °C to 1700 °C and no change in the phase indicates that the LaMgAl11O19 phase is stable under 1700 °C, which is very important for thermal barrier coatings (TBCs) serving at elevated temperature.