Effect of post-deposition heat treatment on electrodeposited NiCoCrAlY coatings

An electrolytic process was utilised to fabricate NiCoCrAlY coatings, involving deposition of a composite coating consisting of the (Ni,Co) matrix and CrAlY particles, followed by a diffusion heat treatment to convert the composite coating to the desired NiCoCrAlY microstructure. This study focused on the effects of the post-deposition heat-treating conditions (temperature and atmosphere) on the final coating microstructure and composition. The (Ni,Co)–CrAlY composite coatings were heat-treated for 2?h at 1000, 1100 and 1200°C in vacuum and Ar, respectively. Phase constituents and concentrations of the major elements in the coating after heat treatment were examined. Vacuum heat treatment at temperatures ≥1100°C resulted in a significant decrease in Cr content at the coating surface due to Cr evaporation. Heat treatment at 1200°C also caused outward diffusion of Ti from the substrate alloy to the coating layer.     

Estimation of metal temperature of MCrAlY coated IN738 components based on interdiffusion behaviour

Abstract

Interdiffusion at the interface between a Co–36·5Ni–17·5Cr–8Al–0·5Y, MCrAlY coating and the underlying IN738 superalloy was studied in a large matrix of specimens isothermally heat treated up to 12 000 h at temperatures 875, 925 or 950°C. Microstructural investigations and calculated phase fraction diagrams show that a precipitate free zone forms between the coating and superalloy and grows with time. The width of the growing zone was estimated on the basis of average intensity profiles obtained from experimental X-ray maps measured using energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM). A simple parabolic growth model was set up for estimating the metal temperature near the coating/substrate interface based on the growth kinetics of the precipitate free zone. Parameters for the model were extracted from measurements of the width of the growing precipitate free zone with time. The developed model was used to estimate metal temperatures for a service exposed, first stage gas turbine blade.     

Service-like TMF tests for the life assessment of an SX GT blade

Thermo-mechanical fatigue (TMF) is the main source of failure of gas turbine (GT) components, owing to thermal gradients and complex geometry. A GT-component life assessment procedure has to consider TMF and the effect of metallic coatings. A single crystal (SX) blade is analysed by FE simulation to identify the critical locations and their thermo-mechanical conditions during a ‘cold’ startup/shutdown transient. The defined TMF cycle is applied to SX superalloy specimens with two different coatings. The results highlight a significant life reduction for specimens with the coating having the highest Re content due to its microstructure. The experimental results are confirmed by field feedback from operated blades.

This paper is part of a thematic issue on the 9th International Charles Parsons Turbine and Generator Conference. All papers have been revised and extended before publication in Materials Science and Technology.     

Leading edge erosion of coated wind turbine blades: Review of coating life models

Erosion of the leading edge of wind turbine blades by droplet impingement wear, reduces blade aerodynamic efficiency and power output. Eventually, it compromises the integrity of blade surfaces. Elastomeric coatings are currently used for erosion resistance, yet the life of such coatings cannot be predicted accurately. This review paper gives an overview of experimentally validated erosion model blocks that can be used to predict the life of the leading edge of coated wind turbine blades. From the reviewed work it is concluded that surface fatigue, as nucleating wear mechanism for erosion damage, can explain erosive wear and failure of the coatings. An engineering approach to surface fatigue, using the Palmgren–Miner rule for cumulative damage, allows for the construction of a rain erosion incubation period equation. Coating life was described as a function of the rain intensity, the droplet diameter, the fatigue properties of the coating and the severity of the conditions. It is recommended to focus coating development on reduction of the impact pressure, e.g. by developing surfaces with a low modulus of elasticity; or on enlarging the safe area by: developing coatings with adjustable compressive stresses and hardness, or coatings without defects and impurities.     

Erosion–corrosion behaviour of Ni–20Cr plasma coating in actual boiler environment

Abstract

Erosion–corrosion is encountered in a large variety of engineering industries. In such environments, protective coatings are used. In this investigation, erosion–corrosion of the Ni–20Cr coating on nickel and iron based superalloys has been investigated by subjecting them to the boiler of coal fired thermal power plant at the temperature zone of 540°C for 1000 h duration. The erosion–corrosion kinetics of the plasma sprayed Ni–20Cr coating on different superalloys has been investigated. XRD, SEM with EDS and EPMA have been used to analyse the eroded–corroded products along the surface and cross-section. Main phases identified in all the Ni–20Cr coated superalloys after exposure are NiO, Cr₂O₃, Al₂O₃ and SiO₂. Aluminium has penetrated from the bond coat to the top coat along the splat boundaries. Oxides of chromium, nickel and aluminium are recognized as protective oxides for boiler environment. The probable mechanism of attack for Ni–20Cr coating in the given boiler environment is discussed.     

Evaluation,degradation and life assessment of coatings for land based combustion turbines

Abstract

Use of metallic and thermal barrier coatings to protect hot section blades and vanes of combustion turbines for power generation has been common practice for the past three and one decades respectively. Because these coatings must be optimised with respect to both different forms of corrosion and modes of operation (base load versus peak load), their performance may be machine specific. Industrial end users generally do not have detailed knowledge of the failure mechanisms of the coatings and the basis for selecting coatings to suit specific requirements, topics the present review seeks to address. The evolution of protective coatings, coating failure mechanisms and a methodology for selecting machine specific coatings are described. The methodology, which can be used to rank and optimise coating systems and to predict the remaining life of coatings, forms the basis of a computer code known as COATLIFE. The ingredients of this methodology, i.e. degradation modelling and thermomechanical fatigue life prediction, are reviewed.     

Development of EQ coating for new TBC system in Ni based superalloys

Abstract

Ni based single crystal superalloys containing high concentrations of refractory elements are prone to generation of a diffusion layer called secondary reaction zone (SRZ) beneath their bond coating during exposure at high temperatures. The SRZ causes a reduction of the load bearing cross-section and is detrimental to the creep properties of thin wall turbine airfoils. In the present study, a new bond coat system, 'EQ coating' which is stable and suppresses formation of the SRZ is proposed. The characteristic of EQ system is that the coating stays in equilibrium state and never reacts with the substrate. Diffusion couples of coating materials and substrate alloys were made and were heat treated at 1100°C for 300 and 1000 h. The concentration profiles of alloying elements in these diffusion couples were analysed by electron probe microanalyser to investigate the existence of the diffusion zone. Cyclic oxidation examinations were carried out at 1100°C in air and the oxidation properties of EQ coating materials were discussed.     

Finite element modelling of development of stresses in thermal barrier coatings

Abstract

Thermal barrier coatings (TBCs) are used to allow higher gas temperatures (and hence greater efficiencies) in power generation gas turbines and/or to lengthen blade lifetimes, by reducing the heat transfer from the combustion gases to the blade substrate materials. However, the lives of TBC coated components tend to be limited by the growth of an oxide layer between the thermally insulating top coat and the MCrAlY coated superalloy substrate; this results in stresses which can lead to spallation (flaking-off) of the top coat. The present paper gives an overview of a recent programme of modelling work undertaken to understand the development of stresses due to the growth of the oxide layer. Typical examples of the rough interface between top coat and bond coat are characterised in terms of their aspect ratios. Representative geometries are then studied using a series of 2D finite element models of the interface layer. Initial models assumed a simple parabolic growth law for the oxide layer; the models were then developed to consider the evolving properties of the substrate and bond coat, and a more rigorous model of the oxidation process was implemented. The resulting model takes as its input the results of a microstructure evolution model developed at Loughborough University, which provides phase proportions. These in turn are used in conjunction with a constitutive model based upon an analytical homogenisation (based on Eshelby approach) that allows the substrate and bond coat creep and elastic behaviour to be predicted as the microstructure evolves. The formation of the thermally grown oxide (TGO) is modelled by considering the volume change due to oxidation. In turn, the model predicts the evolution of stresses at positions within the TGO layer. The influences of interface roughness, temperature and bond coat formulations are all explored by running the coupled model with different input parameters.     

Toolbox for optimizing anti‐erosion protective coatings of wind turbine blades: Overview of mechanisms and technical solutions

Factors and structural parameters of coatings influencing the efficiency of protective coating systems against rain erosion of wind turbine blades are reviewed. The possibilities to enhance the attenuation of wave energy from droplet impact, increase damping and varying stiffness, creating interfaces for wave reflection, adding reinforcement for wave scattering, and creating additional layers or skeleton‐like reinforcing or wave absorbing structures, are discussed in the paper. Formulas for quantitative estimation of these effects as well as qualitative relationships between the structural parameters of coatings and their performance are presented. Recommendations and promising directions for the improvement of the protective coating systems for rain erosion protection of wind turbine blades are presented.     

大功率汽轮机末级长叶片三维动态应力及服役寿命的研究

提出了一种预测汽轮机叶片服役寿命的综合数值模型.首先,开发了分析汽轮机叶片激振力、动频和动应力的计算模型,基于动应力的分析结果,提出了考虑制造和腐蚀环境等多种因素的汽轮机叶片服役寿命模型.给出了叶片材料的疲劳特性试验结果,为叶片寿命分析提供了必要参数据.此外,还对汽轮机末级680 mm叶片进行了分析.结果表明:所建立的叶片激振力、动频和动应力分析模型具有良好的工程精度,开发的叶片服役寿命分析模型可以定量考虑影响叶片寿命的各个因素,从而为在设计阶段及运行中保证叶片的可靠性提供了有力的依据.     

Non-destructive and contactless defect detection inside leading edge coatings for wind turbine blades using mid-infrared optical coherence tomography

Leading edge erosion of wind turbine blades is one of the most critical issues in wind energy production, resulting in lower efficiency, as well as increased maintenance costs and downtime. Erosion is initiated by impacts from rain droplets and other atmospheric particles, so to protect the blades, special protective coatings are applied to increase their lifetime without adding significantly to the weight or friction of the blade. These coatings should ideally absorb and distribute the force away from the point of impact; however, microscopic defects, such as bubbles, reduce the mechanical performance of the coating, leading to cracks and eventually erosion. In this work, mid-infrared (MIR) Optical Coherence Tomography (OCT) is investigated for non-destructive, contactless inspection of coated glass-fiber composite samples to identify subsurface coating defects. The samples were tested using rubber projectiles to simulate rain droplet and particle impacts. The samples were subsequently imaged using OCT, optical microscopy, and X-ray tomography. OCT scanning revealed both bubbles and cracks below the surface, which would not have been detected using ultrasonic or similar non-destructive methods. In this way, OCT can complement the existing quality control in turbine blade manufacturing, help improve the blade lifetime, and reduce the environmental impact from erosion.     

Leading edge erosion of wind turbine blades: Multiaxial critical plane fatigue model of coating degradation under random liquid impacts

A computational model of rain erosion of wind turbine blades is presented. The model is based on the transient fluid–solid coupled finite element (FE) analysis of rain droplet/coating interaction and fatigue degradation analysis. The fatigue analysis of the surface degradation is based on multiaxial fatigue model and critical plane theory. The random rain fields are constructed computationally, and the estimated droplet sizes are included in FE model to acquire a library of load histories. Subsequently, the resulted nonproportional multiaxial high cycle fatigue problem is solved to assess the damage and lifetimes of the coatings. The approach can be used to design new coating systems withstanding longer service times.     

Experimental Study of Heat Transfer in Gas Turbine Blades Using a Transient Inverse Technique

To enhance specific power output and thermal efficiency of gas turbine engines, industry searches for ways to increase the turbine inlet temperatures. Therefore, temperatures of turbine blades increase as well and necessitate active cooling of these components. Experimental design work on such internal cooling schemes is carried out to find acceptable compromises between heat transfer and pressure losses. It is often carried out by using transient thermochromic liquid crystal techniques in combination with Plexiglas models. However, for real turbine blades this experimental technique is inappropriate due to the lack of optical access. Therefore, to study actual turbine blades there is need for development of noninvasive, nondestructive methodologies. This article describes a measurement technique that allows determination of internal heat transfer coefficients of real turbine blades experimentally. Thus, a test rig with a rapidly responding heater was designed to fulfill the requirement of a sudden increase in the air temperature within the cooling passages. The outer surface temperatures were measured using infrared thermography. To estimate the spatial distribution of internal heat transfer coefficients from transient surface temperatures the inverse heat transfer problem was solved. As optimization algorithm the Levenberg–Marquardt method was chosen. Outer surface temperature data was measured for a rectangular reference model with rib turbulators and compared with simultaneously acquired data using the thermochromic liquid crystal technique. It is concluded that the new experimental measurement technique could be used to quantitatively determine internal heat transfer coefficients.     

Nonlinear design-point performance adaptation approaches and their comparisons for gas turbine applications

Accurate performance simulation and understanding of gas turbine engines is very useful for gas turbine manufacturers and users alike and such a simulation normally starts from its design point. When some of the engine component parameters for an existing engine are not available, they must be estimated in order that the performance analysis can be started. Therefore, the simulated design point performance of an engine may be slightly different from its actual performance. In this paper, two nonlinear gas turbine design-point performance adaptation approaches have been presented to best estimate the unknown component parameters and match available design point engine performance, one using a nonlinear matrix inverse adaptation method and the other using a Genetic Algorithm-based adaptation approach. The advantages and disadvantages of the two adaptation methods have been compared with each other. In the approaches, the component parameters may be compressor pressure ratios and efficiencies, turbine entry temperature, turbine efficiencies, engine mass flow rate, cooling flows, and bypass ratio, etc. The engine performance parameters may be thrust and SFC for aero engines, shaft power, and thermal efficiency for industrial engines, gas path pressures, temperatures, etc. To select the most appropriate to-be-adapted component parameters, a sensitivity bar chart is used to analyze the sensitivity of all potential component parameters against the engine performance parameters. The two adaptation approaches have been applied to a model gas turbine engine. The application shows that the sensitivity bar chart is very useful in the selection of the to-be-adapted component parameters, and both adaptation approaches are able to produce good quality engine models at design point. The comparison of the two adaptation methods shows that the nonlinear matrix inverse method is faster and more accurate, while the genetic algorithm-based adaptation method is more robust but slower. Theoretically, both adaptation methods can be extended to other gas turbine engine performance modelling applications.     

Nonlinear design-point performance adaptation approaches and their comparisons for gas turbine applications

Accurate performance simulation and understanding of gas turbine engines is very useful for gas turbine manufacturers and users alike and such a simulation normally starts from its design point. When some of the engine component parameters for an existing engine are not available, they must be estimated in order that the performance analysis can be started. Therefore, the simulated design point performance of an engine may be slightly different from its actual performance. In this paper, two nonlinear gas turbine design-point performance adaptation approaches have been presented to best estimate the unknown component parameters and match available design point engine performance, one using a nonlinear matrix inverse adaptation method and the other using a Genetic Algorithm-based adaptation approach. The advantages and disadvantages of the two adaptation methods have been compared with each other. In the approaches, the component parameters may be compressor pressure ratios and efficiencies, turbine entry temperature, turbine efficiencies, engine mass flow rate, cooling flows, and by-pass ratio, etc. The engine performance parameters may be thrust and SFC for aero engines, shaft power, and thermal efficiency for industrial engines, gas path pressures, temperatures, etc. To select the most appropriate to-be-adapted component parameters, a sensitivity bar chart is used to analyze the sensitivity of all potential component parameters against the engine performance parameters. The two adaptation approaches have been applied to a model gas turbine engine. The application shows that the sensitivity bar chart is very useful in the selection of the to-be-adapted component parameters, and both adaptation approaches are able to produce good quality engine models at design point. The comparison of the two adaptation methods shows that the nonlinear matrix inverse method is faster and more accurate, while the genetic algorithm-based adaptation method is more robust but slower. Theoretically, both adaptation methods can be extended to other gas turbine engine performance modelling applications.     

烟气轮机叶片失效分析

分析了烟气轮机叶片失效原因,对叶片裂纹,断口,组织及成分分析的结果表明,叶片失效属共振和冲刷穿孔导致的高周疲劳破裂;叶片涂层结构,组织缺陷分布以及烟气硬粒子是降低叶片使用寿命的重要因素。     

基于机器视觉的热障涂层内部裂纹在线检测方法及传热特性研究

燃气轮机服役过程中,热障涂层(以下简称涂层)内部裂纹萌生和扩展是导致涂层失效的主要原因。通过数值重构方法获得了含不同长度裂纹的热障涂层（TBCs）微结构,基于耦合双分布格子波尔兹曼方法（DDF-LBM）建立了热障涂层与冷却气膜流动传热模型,研究了热障涂层内部和表面温度分布特性。结果表明：出现裂纹会极大地改变涂层的温度分布情况,增加涂层温度不均匀性,造成局部烧结,进一步产生应力集中,极易导致涂层分层断裂,从而影响其耐久性。同时,基于耦合检测算法（GEMSS）通过大量机器学习训练,提出了热障涂层内裂纹定位和长度估算的在线检测评估方法。该方法能有效确定裂纹位置,高精度估算裂纹长度,为高温叶片在线健康度评估和寿命预测提供理论基础和技术支撑。     

Numerical simulation of steady state heat transfer in a ceramic-coated gas turbine blade

As gas turbine entry temperature (TET) increases, thermal loading on first stage blades increases and, therefore, a variety of cooling techniques and thermal barrier coatings (TBCs) are used. In the present work, steady state blade heat transfer mechanisms were studied via numerical simulations. Convection and radiation to the blade external surface were modeled for a super alloy blade with and without a TBC. The effects of surface emissivity changes, partial TBC coatings and uncertainties in external heat transfer coefficient were also simulated. The results show that at 1500 K TET, radiation heat transfer rate from gas to an uncoated blade is 8.4% of total heat transfer rate which decreases to 3.4% in the presence of a TBC. The TBC blocks radiation, suppresses metal temperatures and reduces heat loss to the coolant. These effects are more pronounced at higher TETs. With selective coating, substantial local temperature suppression occurs. In the presence of radiation and/or TBC, the uncertainties in convection heat transfer coefficient do not have a significant effect on metal temperatures.     

Phases of icing on wind turbine blades characterized by ice accumulation

Icing experiments on wind turbine blade profiles have been performed at the University of Manitoba Icing Tunnel Facility to facilitate a greater understanding of the mechanisms involved in the icing process for wind turbines exposed to cold climates. Blade icing results in the degradation of power performance and is a critical issue for the optimization of power performance and safe operation of wind turbines. Accumulation rate, the amount of ice that accumulates at the leading edge of the blade profile as a function of time, provides a characteristic measurement that can be used to classify the phases of icing in an icing event and further identify the severity of potential problems arising as a result of ice accumulation on wind turbine blades. To control this characteristic, the mitigation strategies that were employed involved coatings, heat treatments and the combination thereof, in both glaze and rime icing regimes. By understanding the icing process and its characteristic behavior to non-mitigated and mitigated scenarios, the phases of icing of both circumstances may be defined. This paper documents the data recorded from the experimental icing event and provides results of the comparative behavior of the icing mitigation strategies and extends this understanding to define the phases of icing on wind turbine blades.     

Influence of a wind turbine service life on the mechanical properties of the material and the blade

This paper presents results out of investigations of the DEBRA‐25 wind turbine blades. Almost unique in the history of modern wind energy, these blades were in operation for 18 years next to a weather station and were investigated afterward. Therefore, the loads experienced in the operational life could be post‐processed accurately with the measured data of the weather station and the turbine. The blades are made of materials that are similar with today's wind turbines. Furthermore, intensive laboratory tests and free field tests have been carried out, and all load assumptions and data and results are still available today. The results include experimental investigations on the moisture content of the load‐carrying material, static and fatigue behavior of the material, the relaxation of the coupling joints, the natural frequencies of the blade and a full scale static blade test. It is shown that the structural performance of the DEBRA‐25 service blades is comparable with modern wind turbine blades. Although some damage was found by visual inspection, the service blade of the DEBRA‐25 showed excellent mechanical behavior in the full scale blade test. Only small changes of the edgewise eigenfrequencies were detected. The pre‐tensioning forces of the IKEA bolts, where the two blade parts are connected, were measured and were still adequate. Copyright © 2012 John Wiley & Sons, Ltd.