Multidisciplinary analysis of the operational temperature increase of turbine blades in combustion engines by application of the ceramic thermal barrier coatings (TBC)

The improvement of the temperature resistance of the aircraft engine elements can be obtained by application of a single ceramic thermal barrier coating (TBC) (e.g. Noda [1]) or several composite layers (e.g. Sadowski [2]). Engine elements protected by TBC can work safely in elevated temperature range above 1000 °C. Continuous endeavour to increase thermal resistance of engine the elements requires, apart from laboratory investigations, also numerical study of the different aero-engine parts. The most important are turbine blades, where high temperatures and stress concentrations during thermal shocks or thermal fatigue can be observed during engine exploitation. The high temperatures and stress concentrations can act as the local sources of damage initiation and defects propagation in the form of cracks.The present paper deals with the solution of the transient temperature transfer problem in bare and thermal barrier coated alloy Inconel 713 for the temperature range up to 1000 °C. The computational fluid dynamics (CFD) part of analysis was performed by application of ANSYS Fluent code receiving the temperature field of combustion gas, whereas computational structural mechanics (CMS) part concerning the temperature distribution inside the turbine blade was done by ABAQUS. Finally, the efficiency of the TBC layer (0.5 mm thickness) protecting and cooling channels was discussed in order to explore the operational temperature increase in the aero-engines.     

Numerical Simulation of Solidification Process on Single Crystal Ni-Based Superalloy Investment Castings

Bridgman directional solidification of investment castings is a key technology for the production of reliable and highly efficient gas turbine blades. In this paper, a mathematical model for three-dimensional （3D） simulation of solidification process of single crystal investment castings was developed based on basic heat transfer equations. Complex heat radiation among the multiple blade castings and the furnace wall was considered in the model. Temperature distribution and temperature gradient in superalloy investment castings of single blade and multiple ones were investigated, respectively. The calculated cooling curves were compared with the experimental results and agreed well with the latter. It is indicated that the unsymmetrical temperature distribution and curved liquid-solid interface caused by the circle distribution of multiple turbine blades are probably main reasons why the stray grain and other casting defects occur in the turbine blade.     

Design and modeling of multiple layered TBC system with high reflectance

Ceramic thermal barrier coatings (TBCs) are playing an increasingly critical role in advanced gas turbine engines due to their ability to sustain further increases in operating temperatures. However, these increases in temperature could raise considerable issues associated with increased radiative heat transfer into the TBC systems. This study was conducted to design a ceramic based multiple layered TBC system with high reflectance to radiation. Mathematical modeling was used to calculate the potential temperature reduction on the substrate surface when the multiple layered TBC is applied. The result of the simulation shows that a temperature reduction up to 90 °C is possible when utilizing the designed multiple layered TBC coatings.     

Design and Properties of Thermal Barrier Coatings for advanced turbine engines

The efficiency and performance of advanced aircraft turbines can be markedly increased if higher gas temperatures are used. Although the highly loaded blades and vanes in the high pressure turbine are heavily cooled, today's substrate materials are unable to provide sufficient strength in the temperature range up to 1500°C and above. If thermal barrier coatings (TBCs) are applied on superalloy turbine blades a substantial temperature drop of the parts can be achieved. The resulting increase in efficiency comes from reduced cooling and/or increased gas turbine inlet temperatures of up to 150°C. TBCs are either processed by plasma spraying (PS) or electron beam physical vapour deposition (EB-PVD). While PS is lower in cost EB-PVD leads to superior strain and thermoshock tolerant coatings. Furthermore, cooling hole closure of turbine blades and vanes is prevented and aerodynamic design maintained. Finally, future research and development needs in TBC technology are stressed.     

Microstructural changes to metal bond coatings on gas turbine alloys with time at high temperature

Complex coating systems are required to protect nickel-based super alloys from high temperature oxidation and corrosion. Industrial gas turbine blades and heat shields are generally plasma sprayed with a metal bond coating containing nickel, chromium, cobalt, aluminium and yttrium, and then an external thermal barrier coating of yttria-stabilised zirconia is applied. In this study, samples of an IN939 alloy heat shield with both a metal bond coat and a ceramic thermal barrier coating have been heated in air at high temperature for up to 2000 hours to assess the long term stability of the metal bond coat. Polished sections of the heat treated samples were examined by SEM and EDX to determine microstructural changes. The Ni-Cr-Co-Al-Y coating was found to be a very effective barrier against oxidation; the only apparent oxidation being the growth of an alumina layer between the bond coat and ceramic thermal barrier coating. With time, the growth of the Ni₃Al phase in the metallic bond coat was observed, with extensive diffusion of other elements to and from the bond coat.     

Modeling of thermal barrier coating temperature due to transmissive radiative heating

Thermal barrier coatings are generally designed to possess very low thermal conductivity to reduce the conduction heat transfer from the coating surface to the metal turbine blade beneath the coating. In high-temperature power generation systems, however, a considerable amount of radiative heat is produced during the combustion of fuels. This radiative heat can propagate through the coating and heat up the metal blade, and thereby reduce the effectiveness of the coating in lowering the thermal load on the blade. Therefore, radiative properties are essential parameters to design radiative barrier coatings. This article presents a combined radiation and conduction heat transfer model for the steady-state temperature distribution in semitransparent yttria-stabilized zirconia (YSZ) coatings. The results of the model show a temperature reduction up to 45 K for YSZ of high reflectance (80%) compared to the YSZ of low reflectance (20%). The reflectivities of YSZ and metal blade affect the temperature distribution significantly. Additionally, the absorption and scattering coefficients of YSZ, the thickness of the coating, and the thermal conductivities of YSZ and metal blade affect the temperature distribution.     

Effect of protective coatings on mechanical properties of ZhS32-VI heat-resistant alloy

We discuss the effect of diffusion heat-resistant and condensation gradient thermal barrier coatings used for protection of gas turbine disk operating blades on mechanical characteristics (long- and short-term durability and plasticity at temperature of 1000°C) of specimens from ZhS32-VI heat-resistant alloy. We study the effect of isothermal annealing in air at temperature 1100°C during 240 h on mechanical characteristics of uncoated and coated specimens. We note the expediency of application of gradient thermal barrier coating consisting from external heat-insulating ceramic layer and thermostable heat-resistant metal layer, which provide temperature reduction of metal walls of cooled blades made from ZhS32-VI heat-resistant alloy with the purpose of achieving the required residual life.     

TUBA 800 — die Realisierung eines Inline-Coaters für Thermo-Barriere-Beschichtungen mittels EB-PVD

TUBA 800 — an In-line Plant for Thermal Barrier Coating by EB-PVD TUBA 800 represents a highly productive multichamber plant for thermal barrier coating (TBC) of gas turbine blades, furnished with VON ARDENNE high power EB guns for PVD. Basis of the plant design is a new type of carrier (trolley) which is handling the blades during travelling from air to air through the plant. Each trolley is equipped with a docking unit for connecting it to cooling and electric supplies and for transmission of measured data to external PC. The realized concept is compared with well established sting and sting-trolley plants.     

Property gradation for modification of response of rotating MMC discs

Abstract

Aerospace and industrial gas turbine engine components such as discs, blades, and propeller shafts for which long fibre reinforced metal matrix composites are being made, operate under complex mechanical and thermal loading conditions. A major aim of functionally graded materials application is to optimise component response through appropriately tailored microstructures. This paper explores the influences of property gradation, centrifugal body force loading, and thermal loading on stresses in rotating discs. The discs were modelled as non-homogeneous orthotropic materials such as those obtained through non-uniform reinforcement of a metal matrix by long fibres. The results show how different temperature change distribution patterns and property gradation types correlate with hoop stresses developed in the disc.     

Anisotropic Thermal Diffusivities of Plasma-Sprayed Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are used to shield the blades of gas turbines from heat and wear. There is a pressing need to evaluate the thermal conductivity of TBCs in the thermal design of advanced gas turbines with high energy efficiency. These TBCs consist of a ceramic-based top coat and a bond coat on a superalloy substrate. Usually, the focus is on the thermal conductivity in the thickness direction of the TBC because heat tends to diffuse from the surface of the top coat to the substrate. However, the in-plane thermal conductivity is also important in the thermal design of gas turbines because the temperature distribution within the turbine cannot be ignored. Accordingly, a method is developed in this study for measuring the in-plane thermal diffusivity of the top coat. Yttria-stabilized zirconia top coats are prepared by thermal spraying under different conditions. The in-plane and cross-plane thermal diffusivities of the top coats are measured by the flash method to investigate the anisotropy of thermal conduction in a TBC. It is found that the in-plane thermal diffusivity is higher than the cross-plane one for each top coat and that the top coats have significantly anisotropic thermal diffusivity. The cross-sectional and in-plane microstructures of the top coats are observed, from which their porosities are evaluated. The thermal diffusivity and its anisotropy are discussed in detail in relation to microstructure and porosity.     

热流温度场下功能梯度板的热问题研究

功能梯度材料因其内部组分沿着空间位置连续变化,能有效缓解热应力集中等现象,在高超音速飞行器的热防护系统设计中具有良好的应用前景。以金属-陶瓷功能梯度板为研究对象,探讨在不同热环境下功能梯度板热传导、热变形和热应力的变化规律。首先,基于功能梯度材料的幂律分布模型,分析了线性温度场、正弦温度场、热流温度场和非线性温度场四种热环境对功能梯度材料物理特性的影响。其次,基于有限元分层建模思想,通过Python编程建立了功能梯度板的有限元模型。仿真分析了在热流温度场下功能梯度板在不同板厚、陶瓷体积分数指数和热流密度等参数条件下热传导、热变形和热应力的变化规律。最后,探讨了基于参数调控的改善功能梯度板热防护性能的方法。结果表明：用功能梯度材料设计飞行器热防护板时,陶瓷体积分数指数应小于5.0;在热流温度场下,功能梯度板的厚度为7.2~9.0 mm、陶瓷体积分数指数为1.0~2.5时,可实现功能梯度板的轻质和高效隔热;热流密度一定时,调整飞行时间对确保飞行器安全至关重要;热流密度应限制在5~10 mW/mm²以控制功能梯度板的变形。研究结论对于热流温度场下金属-陶瓷功能梯度板的热防护设计具有参考价值。     

Functionally gradient ceramic/metallic coatings for gas turbine components by high-energy beams for high-temperature applications

Failure of turbine blades generally results from high-temperature oxidation, corrosion, erosion, or combinations of these procedures at the tip, and the leading and trailing edges of a turbine blade. To overcome these limitations, functionally gradient ceramic/metallic coatings have been produced by high-energy beams for high-temperature applications in the aerospace and turbine industries to increase the life of turbine components. Thermal spray processes have long been used to apply high-temperature thermal barrier coatings to improve the life of turbine components. However, these processes have not met the increased demand by the aerospace and turbine industries to obtain higher engine temperatures and increased life enhancement as a result of the inhomogeneous microstructure, unmelted particles, voids, and poor bonding with the substrate. High-energy beams, i.e. electron beam-physical vapour deposition (EB-PVD), laser glazing, laser surface alloying, and laser surface cladding, have been explored to enhance the life of turbine components and overcome the limitations of the thermal spray processes. EB-PVD has overcome some of the disadvantages of the thermal spray processes and has increased the life of turbine components by a factor of two as a result of the columnar microstructure in the thermal barrier coating (TBC). Laser glazing has been used to produce metastable phases, amorphous material, and a fine-grained microstructure, resulting in improved surface properties such as fatigue, wear, and corrosion resistance at elevated temperatures without changing the composition of the surface material. Laser surface alloying and laser surface cladding have shown promising results in improving the chemical, physical, and mechanical properties of the substrate's surface. Metal-matrix composite coatings have also been produced by a laser technique which resulted in increased wear and oxidation-resistant properties. The advantages and disadvantages of thermal spray processes, EB-PVD, laser glazing, laser surface alloying, and laser surface cladding will be discussed. Microstructural evolution of thermal barrier coatings, recent advancements in functionally gradient coatings, laser grooving, and multilayered textured coatings will also be discussed.     

Predicting failure within TBC system: Finite element simulation of stress within TBC system as affected by sintering of APS TBC,geometry of substrate and creep of TGO

Demand for economically efficient and environmental friendly gas turbine engines leads to the usage of a thermal barrier coating (TBC) system, which is usually sprayed on the top of a superalloy substrate. The system includes a ceramic TBC, a bond coat (BC) and a thermally grown oxide (TGO) layer. Thermo-mechanical mismatch stresses created within the coating at the end of a thermal cycle lead to spallation of the ceramic coating and a rapid increase in the temperature of the substrate. The thickness of the oxide layer and the amount of aluminium depleted during high temperature operation also affect the lifetime of the TBC. As a first step to the prediction of the failure mechanisms and the lifetimes of TBCs, a preliminary study of how the stress distribution within the TBC system is affected by different factors is required. This paper investigates the effects of the sintering of the ceramic layer, of the geometry of the substrate and of the creep of the TGO, on the stresses built up in the TBC system. Three different TBC system geometries were modelled using plane strain FE models with three different sets of TGO creep properties. An Arrhenius equation was fitted to the temperature dependent modulus of the sintered TBC using results published in the open literature. The equation was later implemented within the FE model. It is concluded that the TBC on the top of flatter regions of substrate produces smaller tensile residual stresses compared to sharp corners of the substrate. It was also found that the initiation and propagation of cracks within a TBC, during steady state operation depends on the choice of the creep parameters of the TGO. At the cooling stage, increase in the modulus of the TBC, due to sintering, has been shown to produce stresses within the TBC near the TGO interface that are as large as twice the value that is predicted using a model without sintering.     

Hot Isostatic Pressing of Plasma Sprayed Thermal Barrier Coating Systems

Thermal barrier coatings (TBC) are important to aerospace and high performance gas turbine engines because they help to keep the temperature experienced by the base metal low; thus, prolonging the life span of the material. Plasma spraying is a technique commonly used to deposit the ceramic-based TBC. An intermediate layer is applied to enhance the bond between the substrate and the ceramic top coat. However, the oxidation of the bond coat due to the infiltration of gas through the porous ceramic layer is a major problem encountered in TBC. This in turn leads to spalling and eventual destruction of the whole coating system. Hot isostatic pressing (HIP) was performed on a number of plasma sprayed thermal barrier coating systems to investigate the effects the process has on micro structure and other physical properties. Due to the fact that the majority of TBC is exposed to thermal cycling and thermal fatigue, it is hoped that the changes brought about by HIP in the porosity and microstructure will improve the life span and performance of TBC. HIP was performed in the temperature range 750-1300° C and pressures of 50-200 MPa. The bond coats that were studied include Ni-5% Al, Ni-20 percnt; Al, NiCrAl and NiCrAlY, while the ceramic coat was Zr0₂-5 wt percnt; CaO. Characterization of the coatings was carried out using scanning electron microscopy (SEM) and image analyser. The results showed the porosity of the coatings to be dramatically reduced to near zero levels. In addition, the other physical properties like hardness and Young's modulus increased over a wide temperature range.     

涡轮叶片材料及制造工艺的研究进展

针对航空发动机涡轮叶片的工作环境和使用要求,介绍了提高涡轮叶片耐温能力的两种途径,即加强叶片冷却和提高叶片材质自身的耐温性能。文中着重评述了用作叶片材料的合金及其制造工艺的研究进展与发展趋势。     

The effects of residual stresses and degradation on the response of viscoplastic functionally graded materials

Functionally graded materials (FGMs), having ceramic and metallic constituents, are commonly used for extreme temperature applications. Under extreme temperature changes, the mismatches in the thermo-mechanical properties of the ceramics and metallic constituents could cause pronounced thermal stresses and could lead to degradation in the properties of the constituents. High stresses in the metallic constituent lead to plastic deformations and high tensile stresses in the ceramic constituent induce cracking. An elastic–viscoplastic micromechanical model is formulated for analyzing residual stresses and strains and degradation in ceramic–metal FGMs undergoing temperature changes due to conduction of heat. A degradation parameter that depends on the temperature and strain is introduced in order to determine the level of material degradation in the ceramics and metallic constituents. The Perzyna viscoplastic model is considered for the metallic constituent while the ceramic constituent is assumed linear elastic. The material parameters in these constituents change with the degradation. The problem leads to time-dependent coupled thermal, deformation, and degradation behaviors. The micromechanical model is implemented in a displacement based finite element (FE), which is used to determine the performance of the viscoplastic functionally graded structures subject to external thermo-mechanical stimuli.     

Capacitive sensor for active tip clearance control in a palm-sized gas turbine generator

The efficiency of a gas turbine has an inverse relationship to the clearance between the rotor blades and the casing. Recent efforts in miniaturization of micro gas turbine engines have created a new challenge in blade tip clearance measurement. This paper describes the development of a capacitive tip clearance measurement system, based on a synchronous detection of a phase-modulated signal, for a palm-sized gas turbine engine with an integral ceramic rotor piece. A surface modification of the ceramic compressor and rotor with conductive coating is utilized to create a novel configuration of a tip clearance probe. The probe capacitance varies by approximately 120 fF for a 100-/spl mu/m blade displacement. Periodic autocalibration is used to reduce the effects of temperature drift on the sensor output. The remaining measurement error drift of 1.5 fF//spl deg/C was caused by the temperature drift of the probe parasitic capacitor. The random uncertainty was between 1.9 and 6.9 /spl mu/m depending on the tip clearance gap.     

Microstructure and thermal behaviour of plasma sprayed zirconia/alumina composite coating

In thermal barrier coatings (TBC), failure occurs near or at the interface between the metallic bondcoat and topcoat. On high temperature conditions, an oxide scale which is named thermally grown oxide (TGO) occurs along the bond/topcoat interface. For diminishing the creation of TGO, a dense coating with low residual stress and thermal stress buffer layer was preferable. High hardness ceramic coatings could be obtained by gas tunnel type plasma spraying, and the deposited coating had superior property in comparison with those deposited by conventional type plasma spray method. In this study, the gas tunnel type plasma spraying system was utilized to produce a zirconia/alumina functionally graded thermal barrier coating and discussed its physical and mechanical properties, thermal behavior and high temperature oxidation resistance of the coating are discussed. Consequently, the proposed system exhibited superior mechanical properties and oxidation resistance at the expenses of a slightly lower thermal insulating effect. This interlayer is preferred in order to minimize the detrimental effect of the phase transformation of gamma-Al2O3 to alpha-Al2O3.     

重型燃机定向结晶空心叶片凝固过程的实验与模拟

采用ProCAST软件系统研究HRS(High Rate Solidification)与LMC(Liquid Metal Cooling)工艺下,不同工艺参数对重型燃机用大型定向结晶空心叶片凝固过程的影响。结果表明:与HRS工艺相比,LMC工艺下叶片的糊状区宽度更小,固/液界面形状更加平直。LMC工艺下叶片的纵向温度梯度约为HRS工艺下的3倍;利用LMC工艺制备该燃机叶片时冷却速率为0.3~2.00℃/s,远高于HRS工艺时的冷却速率(0.05~0.16℃/s);LMC工艺下,采用低的保温炉温度仍可保证叶片获得高的温度梯度和冷却速率;而为避免缘板处杂晶对原始晶粒的阻碍,HRS工艺应当采用高的保温炉温度与更低的抽拉速率。实验与模拟结果均表明:与HRS工艺相比,利用LMC工艺制备的燃机叶片,枝晶组织显著细化。     

Efficiency of a thermal screen behind rows of orifices on the leading edge of a gas turbine blade

Physical simulation of a thermal screen behin a row of standard orifices on the surface of the leading edge of a turbine blade is carried out by the heat and mass analogy method. A procedure is proposed for calculating the efficiency of thermal screens behind rows of orfices on the surface with a geometry typical of cooling systems on the leading edges of turbine blades.Institute of Engineering Thermophysics, Academy of Sciences of Ukraine, Kiev. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 65, No. 1, pp. 18–24, July, 1993.