Characteristics Of Thermal Barrier Coatings Research Articles

Thermo-elastic Characteristics of Thermal Barrier Coating in a Gas Turbine Combustion Chamber with an Intermediate FGM Layer

This study is focused on the analysis of thermos-elastic characteristics of a gas turbine combustion chamber, where the inner surface experiences a higher temperature while the outer region of the wall should be thermally conductive to dissipate the heat of combustion. Using a single material is not suitable to ensure the above two requirements simultaneously. Instead of using a single material, a layered combustion chamber with high-temperature material at the inner region and thermally conductive material at the outer region can be a solution. This produces the problem of a sharp interface where disbonding occurs due to high thermal stresses. The problem of sharp interface can be eliminated by using a functionally graded material (FGM) layer in between the inner and outer layers of the combustion chamber. The present study considers a gas turbine combustion chamber consisting of three layers: the inner layer is used as a thermal barrier coating, the intermediate layer is a smooth transition from the inner layer to the outer layer, and the outer layer is a thermally conductive material. ANSYS simulation is used for the analysis of thermos-elastic characteristics of the combustion chamber with homogeneous and FGM intermediate layers. The effect of linear and nonlinear material distributions in the FGM layer on the thermos-elastic characteristics is studied. A comparison of results reveals that thermal stress smoothly changes from the inner layer to the outer layer in the case of the FGM intermediate layer compared to the homogeneous intermediate layer. This suggests that a combustion chamber with an FGM intermediate layer is more reliable than a homogeneous intermediate layer.

Effect of Pore Evolution on Thermal Diffusivity and Radiation Characteristics of Thermal Barrier Coatings after High-Temperature Exposures

Studying the impact of pores is crucial to enhancing the service performance of coatings, since they are a typical microstructure feature of thermal barrier coatings. In this paper, a coating prepared by the APS method was employed as the study object, and a scanning electron microscope and optical microscope were used to calculate the porosity after spraying or high-temperature exposures. Based on this, numerical calculations and simulations were used to evaluate the impacts of the pore structure and porosity on the heat conductivity and radiation characteristics of the coating. The results showed that, at high-temperature exposures, the horizontal pores inhibited thermal conductivity and radiation, but the column pores increased heat conductivity and radiation. The heat conductivity of the coating linearly decreased as the porosity increased, whereas the extinction coefficient increased, although at a slower and slower pace. When the porosity reached 15%, if the porosity was further increased, the thermal radiation energy did not change much, indicating that increasing the porosity would only block the heat radiation to a certain amount. This new and time-saving technique for materials research utilizing simulation and numerical computing may be utilized to optimize the microstructure of coatings to increase their service performance.

Influence of Additives on Characteristics of Thermal Barrier Coatings: A short Review

Influence of Additives on Characteristics of Thermal Barrier Coatings: A short Review

Degradation characteristics of thermal barrier coatings for hot corrosion and CaO–MgO–Al2O3–SiO2

Hot corrosion and CaO–MgO–Al 2 O 3 –SiO 2 (CMAS) resistance of thermal barrier coatings (TBCs) are required in an industrial gas turbine employed with impure fuels and high turbine inlet temperatures . Therefore, various TBC systems with different structural combinations were investigated in thermochemical aspects of hot corrosion and CMAS causing the degradation of TBCs. Four types of 8 wt% yttria-stabilized zirconia (8YSZ), namely dense microstructure (8YSZ), porous microstructure (C–8YSZ), no monoclinic phase (NoM–8YSZ), and low thermal conductivity (low-k), were employed to prepare topcoats using an atmospheric plasma spray on the two bondcoats coated by a vacuum plasma spray with feedstocks of NiCoCrAlY and NiCoCrAlY+Hf + Si. Hot corrosion components penetrated through the whole topcoat layer at 1000 °C however did not infiltrate to the bondcoat layers because of the thermally grown oxide layer formed between the topcoat and the bondcoat. In addition, the elements Hf and Si contained in the bondcoat composition suppressed diffusion of β phase aluminum. In the visual inspection after the hot corrosion at 1000 °C and CMAS test at 1250 °C, the TBC system with the NoM–YSZ topcoat layer showed a sound condition without failure unlike other TBCs, and no monoclinic phase after hot corrosion and the CMAS test, indicating that the phase stability plays a critical role in TBCs' lifetime in the hot corrosion and CMAS environments. • Superiority of Hf and Si elements in MCrAlY bondcoat was observed through hot corrosion test. • Conventional and advanced TBCs failed with a similar trend independent of chemicals and porosity. • low-k TBCs showed a negative dependency on both hot corrosion and CMAS • Phase stability of topcoat layer can also be proposed to have more life time under the environment of hot corrosion and CMAS.

Analysis of performance and emission characteristics of TBC coated low heat rejection engine

ABSTRACT Present investigation is focused on the effect of thermal barrier coated piston on the performance and emission characteristics of diesel engine using diesel as a fuel. The piston crown top surface were fully coated with three different combinations of thermal barrier coatings (TBC) materials like 7% YSZ, 2% Gd2O3 + YSZ, 5% Gd2O3 + YSZ with thickness of 0.25 mm and in all combinations the bond coat NiCrAlY were used with a thickness of 0.05 mm which is deposited on the target surface using plasma spray technique. Experimental analysis is carried out under different loading conditions in a four-stroke single cylinder diesel engine with diesel as a fuel. Results of this study reveal that the brake thermal efficiency of the coated engine is increased by 12.1% with reduction of brake specific fuel consumption by up to 16.2% compared to baseline engine. Experimental results also revealed that in coated engine Hydro carbon (HC) and carbon monoxide (CO) emissions were reduced by 17.8% and 18.4% respectively and also a significant increment of Nitrogen oxide (NOx) emission by 14.87% compare to baseline engine.

서스펜션의 준비방법에 따른 서스펜션 플라즈마 용사를 이용한 La2O3-Gd2O3-ZrO2계 열차폐코팅의 제조와 특성

서스펜션의 준비방법에 따른 서스펜션 플라즈마 용사를 이용한 La₂O₃-Gd₂O₃-ZrO₂계 열차폐코팅의 제조와 특성

Evaluation of Degradation Characteristics of Thermal Barrier Coating on Gas Turbine Blades

In order to evaluate the lifespan of high-temperature parts with thermal barrier coating in gas turbines used for power generation, this study was performed on an 80 MW-class gas turbine exceeding 24 k equivalent operating hours. Degradation characteristics were evaluated by analyzing the YSZ (Yttria Stabilized Zirconia) top coat, which serves as the thermal barrier coating layer, the NiCrAlY bond coat, and interface layers. Microstructural analysis of the top, middle, and bottom sections showed that Thermal Growth Oxide (TGO) growth, Cr precipitate growth within the bond coat layer, and formation of diffusion layer occur actively in high-temperature sections. These microstructural changes were consistent with damaged areas of the thermal barrier coating layer observed at the surface of the used blade. The distribution of Cr precipitates within the bond coat layer, in addition to the thickness of TGO, is regarded as a key indicator in the evaluation of degradation characteristics.

Characterisation of the TiO2 coatings deposited by plasma spraying

Plasma spraying of materials such as ceramics and non-metals, which have high melting points, has become a well-established commercial process. Such coatings are increasingly used in aerospace, automobile, textile, medical, printing and electrical industries to impart proprieties such as corrosion resistance, thermal resistance, wear resistance, etc.One of the most important characteristics of thermal barrier coatings is the ability to undergo fast temperature changes without failing, the so called thermal shock resistance.The formation of residual stresses in plasma sprayed ceramic and metallic coatings is a very complex process. Several factors, such as substrate material, substrate thickness, physical properties of both the substrate and the coating material, deposition rate, relative velocity of the plasma torch, etc. determine the final residual stress state of the coating at room temperature.Our objective is to characterize the titanium oxide and aluminium oxide coatings deposited by plasma spraying in structural terms, the resistance to thermal shock and residual stresses.

Thermoelastic characteristics of thermal barrier coatings with layer thickness and edge conditions through mathematical analysis.

The thermoelastic behaviors of such as temperature distribution, displacements, and stresses in thermal barrier coatings (TBCs) are seriously influenced by top coat thickness and edge conditions, which were investigated based on the thermal and mechanical properties of plasma-sprayed TBCs. A couple of governing partial differential equations were derived based on the thermoelastic theory. Since the governing equations are too involved to solve analytically, a finite volume method was developed to obtain approximations. The thermoelastic characteristics of TBCs with the various thicknesses and microstructures were estimated through mathematical approaches with different edge conditions. The results demonstrated that the top coat thickness and the edge condition in theoretical analysis are crucial factors to be considered in controlling the thermoelastic characteristics of plasma-sprayed TBCs.

Effect of Plasma Pretreatment on Thermal Durability of Thermal Barrier Coatings in Cyclic Thermal Exposure

Plasma pretreatment on the top and bond coats was performed and its influence on the thermal durability of thermal barrier coating (TBC) system was investigated through cyclic thermal exposure. Two types of bond coat were prepared by different methods, namely, air plasma spray (APS) and high-velocity oxy-fuel (HVOF), and two kinds of feedstock powder were employed for preparing the top coat in APS process. The better thermal durability was achieved in the vertically cracked TBC with the surface modified bond coat or with the bond coat prepared by APS process. The hardness and fracture toughness values of TBCs increased because of densification of the top coat during cyclic thermal exposure, and the bond coat prepared by HVOF process showed higher values than that by APS process. The TBCs with the surface modified bond coat were more efficient in improving adhesive strength than those without plasma pretreatment on the bond coat. The relationship between microstructure evolution and thermomechanical characteristics of TBCs with plasma pretreatment was discussed in cyclic thermal exposure.

Thermal and Mechanical Characteristics of Thermal Barrier Coatings in Cyclic Thermal Fatigue Systems

In this study, the relationship between microstructural evolution and mechanical properties of thermal barrier coatings (TBCs) has been investigated through different thermal fatigue systems, electric thermal fatigue (ETF) and flame thermal fatigue (FTF), including the thermal stability through the interface between the bond and top coats. The TBC system with the thicknesses of 300 µm in both the top and bond coats was prepared with METCO 204 NS and AMDRY 962, respectively, with the air plasma spray (APS) system using 9MB gun. To observe the oxidation resistance and thermal stability of TBC, the thermal exposure tests were performed with both thermal fatigue tests at a surface temperature of 850 °C with a temperature difference of 200 °C between the surface and bottom of sample, for 12,000 EOH in designed apparatuses. The hardness values are slightly increased due to the densification of top coat with increasing the thermal exposure time in both thermal fatigue tests. The influence of thermal fatigue condition on the microstructural evolution and interfacial stability of TBC is discussed.

Mathematical Analysis of Thermoelastic Characteristics in Plasma-Sprayed Thermal Barrier Coatings

The thermoelastic characteristics of plasma-sprayed thermal barrier coatings (TBCs) have been analyzed using mathematical modeling. Two types of TBC model, cylinder and circular disk which are commercial plasma-sprayed TBCs, subjecting to symmetric temperature distribution to the radial and longitudinal directions, respectively, were taken into consideration. Based on the thermoelastic theories, a second order ordinary differential equation was derived for the cylinder model and a pair of partial differential equations were set up for the circular disk model. The analytic solution was obtained from the ordinary differential equation, while a finite volume method was developed for numerical solutions to the pair of partial differential equations due to the complexity of governing equations. The thermoelastic characteristics of TBC models, such as temperature distributions, displacements, and stresses, were displayed according to the obtained solutions. The rate of heat conduction in the section of the top coat is relatively slow in comparison with the substrate, and no profound difference appears in the temperature distribution between two TBC models. The highest longitudinal tensile stress is expressed at the bond coat of both models, and the substrate is under the compressive stresses to the circumferential direction. While the cylinder expands to the positive longitudinal direction only, the expansion in the circular disk occurs to both the positive and negative longitudinal directions. Relatively large displacement and stresses exhibit in the cylinder as compared with the circular disk. In the circular disk, the stresses to the radial direction undulate at each section, and the displacement profile displays that the width of the circular disk is slightly narrowed. The results demonstrate that the mechanical and thermal properties of the top and bond coats are the crucial factors to be considered in controlling the thermoelastic characteristics of plasma-sprayed TBCs.

Analysis of thermoelastic characteristics for vertical-cracked thermal barrier coatings through mathematical approaches

Thermoelastic characteristics of thermal barrier coatings (TBCs) with vertical cracks were analyzed through mathematical approaches to investigate the thermoelastic behaviors of TBCs in a service temperature. TriplexPro™-200 system was applied to prepare the relatively dense TBC using METECO 204NS powder. The microstructure of top coat in the TBC was just controlled to create vertical type cracks by reheating without powder feeding in same equipment and rapid cooling process. A couple of governing partial differential equations were derived based on the thermoelastic theory, and a finite volume model was developed to the governing equations to evaluate the thermoelastic characteristics, such as temperature distribution profile, displacement, and stress, inducing a thermal fatigue. For the specimen with two or more vertical type cracks, smaller displacement appears to longitudinal direction and larger displacement to radial direction as the number of crack increases. In the longitudinal stress distribution profiles to z-direction, the tensile stress at the interface between the bond coat and the substrate converts into the compressive stress when the specimen has vertical cracks more than two, while larger magnitude undulation develops for the specimen with smaller number of crack in the radial stress distribution profiles. The results obtained demonstrate that multiple vertical cracks enhance the thermal durability and extend the lifetime of TBCs.

Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work

A review is presented of how heat transfer takes place in plasma-sprayed (zirconia-based) thermal barrier coatings (TBCs) during operation of gas turbines. These characteristics of TBCs are naturally of central importance to their function. Current state-of-the-art TBCs have relatively high levels of porosity (~15%) and the pore architecture (i.e., its morphology, connectivity, and scale) has a strong influence on the heat flow. Contributions from convective, conductive and radiative heat transfer are considered, under a range of operating conditions, and the characteristics are illustrated with experimental data and modeling predictions. In fact, convective heat flow within TBCs usually makes a negligible contribution to the overall heat transfer through the coating, although what might be described as convection can be important if there are gross through-thickness defects such as segmentation cracks. Radiative heat transfer, on the other hand, can be significant within TBCs, depending on temperature and radiation scattering lengths, which in turn are sensitive to the grain structure and the pore architecture. Under most conditions of current interest, conductive heat transfer is largely predominant. However, it is not only conduction through solid ceramic that is important. Depending on the pore architecture, conduction through gas in the pores can play a significant role, particularly at the high gas pressures typically acting in gas turbines (although rarely applied in laboratory measurements of conductivity). The durability of the pore structure under service conditions is also of importance, and this review covers some recent work on how the pore architecture, and hence the conductivity, is affected by sintering phenomena. Some information is presented concerning the areas in which research and development work needs to be focussed if improvements in coating performance are to be achieved.

Evaluation of residual stress distribution and deformation characteristics of thermal barrier coatings using hard synchrotron X-rays

Thermal barrier coatings (TBCs) were made on cylindrical substrates with radii of 5, 10 and 15 mm. In addition to the cylindrical substrates, the TBC were also made on a plate substrate. First, NiCoCrAlY was pressureless plasma sprayed as a bond coating, and then zirconia with 8 mol% yittria was air plasma sprayed as the top coating. The TBC specimens were exposed in an atmosphere of air at 1373 K for 0, 200 and 500 h. The radial strain was measured with the strain scanning method using hard synchrotron X-rays. The hoop and axial stresses of the cylindrical specimens were measured using laboratory X-rays with successive removal of the surface by polishing. Using these measured stresses and strains, the distributions of the 3-dimensional components of the residual stress in the top coating were determined. The spalling stress became a maximum at about 50 μm from the interface. This position corresponded to the location of initiation of the spalling cracks. The value of the measured maximum spalling stress ...

Residual Stress and Deformation Characteristics of Thermal Barrier Coatings on Curved Substrate

Thermal barrier coatings (TBCs) were made on the round bar substrates of which radii were 5, 10 and 15mm. In addition to the round bar substrates, the TBC was made on a plate substrate. First, NiCoCrAlY was pressureless plasma-sprayed as bond coating, and then zirconia with 8 mol% yittria was air plasma-sprayed as top coating. The TBC specimens were exposed in air atmosphere at 1373K for 0h, 200h or 500h. The radial strain was first measured with the strain scanning method using high energy synchrotron X-rays. The hoop and axial stresses of the round bar specimens were measured using laboratory X-rays with successive removal of the surface by polishing. Using these measured stresses and strain, the distributions of the 3-dimentional components of the residual stress in the top coating were determined. The spalling stress became a maximum at about 50μm apart from the interface. This position corresponded to the location of initiation of the spalling cracks. The maximum of the measured spalling stress increased with the decrese in the radius of curvature. The difference of the measured distribution of the residual stress from the calculated distribution of the thermal residual stress was explained on the basis of the characteristic of the top coatings, such as the stress relaxation due to sliding between lamelli, a binding effect by the interface roughness and the spalling stress induced by the interface roughness.

Fracture characteristics of thermal barrier coatings after tensile and bending tests

Uniaxial tension and four-point bending tests were conducted on two-layer (non-functionally graded material) plasma-sprayed rectangular specimens, boasting a new, functionally graded material (FGM) layer. The top coat layer was composed of stabilized yttria-zirconia (YSZ), and the bond coat layer was NiCrAlY. The FGM was composed of five layers: 100% YSZ; 80% YSZ+20% NiCrAlY; 60% YSZ+40% NiCrAlY; 40% YSZ+60% NiCrAlY; and 20% YSZ+80% NiCrAlY. Fracture in the thermal barrier coating (TBC) system was examined by scanning electron microscopy (SEM), which showed that vertical multiple cracking first occurred in the top coat, followed by propagation of interface cracking between the top and bond coats. Spallation of non-FGM coatings occurred in tensile/bending experiments, but not for the FGM-coated specimens, with only localized delamination observed along the interface between FGM layers and the substrate. For specimens prepared with a gun–substrate distance of approximately 120 mm and gun operating power of approximately 32.5 kW, the interface fracture toughness, evaluated by a shear lag model for the uniaxial tensile test, was 0.94 and 0.67 MPa m 1/2 for type-A and -B coatings, respectively. Here, types A and B correspond to an average roughness of the bond coat surface of 12.8 μm and 6.8 μm, respectively. The interface fracture toughness evaluated by the Suo–Hutchinson model was 1.05–1.27 and 1.0–1.17 MPa m 1/2 for type-A and -B coatings, respectively. The results are very close to data obtained by the blister test method. For specimens prepared with a gun–substrate distance of 125–130 mm and gun operating power of approximately 38.5 kW, the interface fracture toughness, evaluated by the Suo–Hutchinson model for four-point bending, was 4.26–7.21 MPa m 1/2.

Fracture Characteristics of Thermal Barrier Coating in Tension and Bending Tests(Student Poster Session)

In the present paper, a method for determining the fracture characteristic of the ceramic coat layer/bond coat interface was developed. The uniaxial tension and four point bending tests were conducted on plasma-sprayed rectangular specimens of two-layer system and functionally gradient material (FGM) layer. The top coat layer was yttria-partially-stabilized zirconia (YSZ) and the bond coat layer was MCrAlY, both of which were deposited by plasma-spraying technique. FGM consisted of 5 layers: 100% YSZ, 80%YSZ+20%NiCrAlY, 60%YSZ+40%NiCrAlY, 40%YSZ+60%NiCrAlY, 20%YSZ+80%NiCrAlY. The detailed fracture process in the thermal barrier coating system was examined by using scanning electron microscopy (SEM). It was shown that vertical multiple cracking preceded in the top coat followed by the propagation of interface crack between top coat and bond coat. A shear lag model was used to evaluate the interface strength. The experimental results show that FGM specimens have higher interface strength than non-FGM specimens.

On the role of imperfections in the failure of a thermal barrier coating made by electron beam deposition

An impression test has been used to explore the remnant toughness and the delamination characteristics of thermal barrier coatings (TBCs) after extended thermal exposure. The delamination trajectory is found to change as the thermally grown oxide (TGO) thickens. At small thicknesses, delamination occurs predominantly within the TGO and TBC. With a thicker TGO, developed after 100 h exposure at 1100°C, the delamination extends predominantly along the TGO/bond coat interface, but with small oxide domains remaining embedded in the bond coat. The changes in the interface adhesion and the mechanics responsible for this transition are addressed, along with a discussion of the role of morphological imperfections in the TGO in failure nucleation. A method for determining the effective in-plane modulus of the TBC from the curvature of decohered TGO/TBC bilayers is also presented.

Quality Assurance Evaluation of Thermal Barrier Coatings by Electrochemical Impedance Spectroscopy

ABSTRACTThe technique of electrochemical impedance spectroscopy (EIS) was used to examine the behavior of intact thermal barrier coatings (TBC) at ambient temperature. Cross-sectional morphological examination of TBC was conducted by scanning electron microscope (SEM). By correlating the SEM visual examination with EIS data, TBC was characterized non-destructively. A model of EIS alternative current (AC) equivalent circuit was proposed to establish the relationship between the EIS elemental parameters in the circuit and the microstructural characteristics of TBC. A linear relationship was found to exist between the electrical impedance of TBC topcoat and the thickness of the topcoat. The porosity of TBC top coat showed a linear relationship with the capacitance of ceramic TBC, and the pore shape in the TBC topcoat was represented by the value of the electrical impedance of the pore. The result in the study has demonstrated that EIS can be used as a non-destructive evaluation (NDE) technique for quality assurance of TBC.