Microcrack nucleation in thermal barrier coating systems

Crack nucleation in thermal-barrier coating (TBC) systems subjected to a monotonic cooling process is studied. The TBC system is modeled using the finite element method, where cracks are represented as discrete discontinuities across continuum elements using the partition-of-unity method. The numerical implementation used for crack nucleation is based on an algorithm where, at insertion of a discontinuity, the traction response is derived from a cohesive zone model that has been modified to (i) behave like an initially rigid cohesive model, and to (ii) ensure smoothness of the traction-separation law at zero crack opening. Accordingly, an adequate convergence behavior of the numerical formulation can be warranted in boundary value problems of systems with relatively complex geometries. In the present numerical study, a comparison is made between TBC systems composed of different constitutive models. The fracture patterns and evolutions of the overall crack growth resulting from the simulations clearly illustrate the importance of accounting for the effects of plasticity in the bond coating and anisotropy in the top coating. The computed fracture profile is in good correspondence with experimental observations reported in the literature.     

Correlation of processing technique and microstructure of platinum aluminide bond coats with performance of thermal barrier coatings deposited on nickel base superalloy

Abstract

We show that the performance of thermal barrier coating systems is critically dependent upon the processing technique and microstructure of platinum aluminides utilised as bond coats. It is demonstrated by thermal exposure tests at 1150°C in air with 24 h cycling period to room temperature that the average useful life of a coating system employing zirconia–7 wt-% yttria as top coat and alloy MAR M002DS as substrate is increased from 192 to 480 h by replacing a three-layer bond coat aluminised by conventional pack cementation with a two-layer bond coat aluminised by chemical vapour deposition. Before each aluminising process, the superalloy has been electroplated with a platinum layer about 7 μm in thickness. Microstructural characterisation using scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron-probe microanalysis, transmission electron microscopy and X-ray diffraction indicates that the superior performance provided by the two-layer bond coat is related to its higher thermal stability enhancing the adhesion of the thermally grown oxide. However, both coating systems are found to fail by the same mechanism involving loss of adhesion between the thermally grown oxide and bond coat.     

On the performance and failure mechanism of thermal barrier coating systems used in gas turbine blade applications: Influence of bond coat/superalloy combination

Surface engineering plays a major role in achieving the performance and design lives of gas turbine components such as the high pressure turbine aerofoils which operate under the most arduous conditions of temperature and stress leading to a wide range of thermal and mechanical loading during service. In this study, emphasis is placed upon the role of composite systems consisting of bond coat and superalloy substrate in determining the performance and useful life of thermal barrier coatings using yttria-stabilized zirconia as top coat processed by electron-beam physical vapor deposition. Three platinum-modified bond coats of the diffusion type and three nickel-based superalloys are included in the study. Thermal exposure tests at 1150 °C in air with a 24-hour cycling period to room temperature have been used to rank the performance of the coating systems. Various electron-optical techniques have been used to characterize the sequence of events leading to coating failure as marked by spallation of the top ceramic coat. It is shown that for a given superalloy substrate, the coating performance is dependent upon the type of bond coat. Conversely, for a given bond coat, the performance becomes a function of the superalloy composition used in the application. However, in both cases, coating failure is found to be predominated by loss of adhesion between the thermally grown oxide and bond coat indicating that the respective interface is the weakest link in the system. The results are interpreted in terms of the phase transformations which occur in the bond coats during exposure at elevated temperatures and the corresponding effects on their oxidation behavior.     

Effect of platinum on the oxide-to-metal adhesion in thermal barrier coating systems

An investigation was conducted to determine the role of Pt in a thermal barrier coating system deposited on a nickel-base superalloy. Three coating systems were included in the study using a layer of yttria-stabilized zirconia as a model top coat, and simple aluminide, Pt-aluminide, and Pt bond coats. Thermal exposure tests at 1,150 °C with a 24-h cycling period to room temperature were used to compare the coating performance. Additional exposure tests at 1,000, 1,050, and 1,100 °C were conducted to study the kinetics of interdiffusion. Microstructural features were characterized by scanning electron microscopy and transmission electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction. Wavelength dispersive spectroscopy was also used to qualitatively distinguish among various refractory transition metals. Particular emphasis was placed upon: (i) thermal stability of the bond coats, (ii) thickening rate of the thermally grown oxide, and (iii) failure mechanism of the coating. Experimental results indicated that Pt acts as a “cleanser” of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer Al₂O₃ scale of a slower growth rate increasing its effectiveness as “glue” holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Among the bond coats included in the study, a surface layer of Pt-rich γ′-phase (L1₂ superlattice) was found to provide longer coating life in comparison with a mixture of PtAl₂ and β-phase.     

Effects of heat treatment on adhesion strength of thermal barrier coating systems

Thermal barrier coatings (TBCs) are widely used as protective and insulative coatings on hot section components of gas turbines and their applications, like blades and combustion chambers. The quality and performance properties of TBCs are of great importance in terms of their resistance to service conditions. In a TBC system, there is a close relationship between the adhesion properties of coating layers. The adhesion strength of TBCs varies depending on the coating technique used and the surface treatments. In this study, CoNiCrAlY and YSZ (ZrO₂ + Y₂O₃) powders were deposited on stainless steel substrate. High Velocity Oxy-Fuel (HVOF) and Atmospheric Plasma Spraying (APS) techniques were used to produce the bond coats. The ceramic top layers on CoNiCrAlY bond coats were produced by the APS technique. The TBC specimens were subjected to heat-treatment tests. Adhesion strength for top coat/bond coat interface of as-sprayed and heat-treated samples was investigated. The results showed that the heat treatment of the coatings in different temperatures led to an increase in the adhesion strength of TBCs.     

Microstructural features of mechanical failure in thermal barrier coating systems under static loadings

Abstract

In order to clarify qualitatively and quantitatively the failure mechanism of plasma-sprayed thermal barrier coating (TBC) systems from the microstructural viewpoint, in situ observation of the mechanical failure behavior was conducted for TBC systems under the static loadings at ambient temperature; as the most fundamental aspect, by means of an optical microscopy. Several kinds of TBC systems were prepared by using different sorts of ceramic coating materials. Mechanical tensile loading or compressive loading was gradually applied to the plate shape of TBC specimen using a four-point bending test methodology. It was found that the tensile failure behavior of TBC systems depends strongly on the top-coat microstructures as well as heat treatment after the plasma spraying. The compressive failures were also found rather incidental and depended on the strength of top-coat at the interfacial region. Among different TBC systems, those with the finely segmented top-coat exhibited a good spalling resistance.     

Effect of superalloy substrate composition on the performance of a thermal barrier coating system

An investigation was carried out to determine the performance of a thermal barrier coating system consisting of (ZrO₂-8% Y₂O₃)/(Pt) on two single-crystal Ni-base superalloys. Coating/alloy behavior was studied with reference to: (i) initial microstructural features, (ii) oxidation properties, (iii) thermal stability characteristics, and (iv) failure mechanism. All thermal exposure tests were carried out at 1150°C in still air with a 24-h cycling period to room temperature. Failure of the coating system was indicated by macroscopic spallation of the ceramic top coat. Scanning electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction were used to characterize the microstructure.Decohesion between the thermally grown oxide and bond coat was found to be the mode of failure of the coating system for both alloys. This was correlated with the formation of Ti-rich and/or Ti+Ta-rich oxide particles near the oxide-bond coat interface degrading the adherence of the thermally grown oxide. However, the thickening rate of the oxide had very little or no effect on the relative coating performance. It was concluded that the coating performance is critically dependent on alloy substrate composition particularly the concentration of elements, which could have adverse effects on oxidation resistance such as Ti.     

The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract

Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.     

EB-PVD process and thermal properties of hafnia-based thermal barrier coating

Thermal barrier coatings (TBCs) are being developed for the key technology of gas turbine and diesel engine applications. In general, 8 mass% Y₂O₃–ZrO₂ (8YSZ) coating materials are used as the top coating of TBCs. The development of hafnia-based TBC was started in order to realize the high reliability and durability in comparison with 8YSZ, and the 7.5 mass% Y₂O₃–HfO₂ (7.5YSH) was selected for coating material. By the investigation of electron-beam physical vapor deposition (EB-PVD) process using 7.5YSH ceramic ingot, 7.5YSH top coating with about 200 µm thickness could be formed. The microstructure of the 7.5YSH coated at coating temperature of 850 °C showed columnars of laminated thin crystals. On the other hand, the structure of the 7.5YSH coated at coating temperature of 950 °C showed solid columnars. From the result of sintering behavior obtained by heating test of 7.5YSH coating, it was recognized that the thermal durability of 7.5YSH coating was improved up to about 100 °C in comparison with 8YSZ coating. This tendency was confirmed by the experimental result of the thermal expansion characteristics of sintered 7.5YSH and 8YSZ.

©2003 Elsevier Science Ltd. All rights reserved.     

Bond coat oxidation and its significance for life expectancy of thermal barrier coating systems

Abstract

The time and temperature dependent evolution of the microstructure of thermal barrier coating systems under isothermal conditions between 950 and 1100 ° C up to 5000 h are investigated for two APS - TBC systems and two EB-PVD systems. Kinetics for the thermally grown oxide thickness values, the β phase depletion underneath the oxide scale and the physical defects in and around the thermally grown oxide are determined by extensive SEM and subsequent interactive image analysis. In the case of physical defects, the size of pores, interacting pore populations and maximum crack lengths are measured. Additionally, the latter are classified with respect to their local orientation in the thermally grown oxide or its vicinity. Finally, the results are discussed with regard to their significance in lifetime modelling of gas turbine components.     

Relation of microstructural and compositional features to the electrical properties in degraded thermal barrier coating systems

Degraded thermal barrier coating samples cut from different after-service gas turbine components are examined by both electron microscopy and impedance spectroscopy. There is a relationship between the microstructural and compositional features of the thermally grown oxide (TGO) and its electrical properties. The resistance of the TGO decreases with the TGO evolution from alumina to porous mixed oxides composed probably of NiO, spinel, Cr₂O₃, and Al₂O₃, while the relaxation frequency corresponding to the TGO increases. For seriously degraded TBCs, there is an additional semicircle in the impedance spectra in the extremely low frequency range, possibly arising from cracking in the vicinity of YSZ-TGO interface regions.     

Thermal cycling behaviour of thermal barrier coating systems based on first- and fourthgeneration Ni-based superalloys

Abstract

This study deals with the cyclic oxidation behaviour of thermal barrier coating systems. The systems consist of an yttria-stabilised zircona ceramic top coat deposited by EB-PVD, a β-(Ni,Pt)Al bond coat and a Ni-based superalloy. Two different superalloys are studied: a first-generation one and a fourthgeneration one containing Re, Ru and Hf. The aim of this work is to characterise the microstructural evolution of those systems and to correlate it to their resistance to spallation. Thermal cycling is carried out at 1100°C in laboratory air, with the number of cycles ranging between 10 and 1000. Each cycle consists of a 1 h dwell followed by forced-air cooling for 15 min down to room temperature. Among the main results of this work, it is shown that the MCNG-based system is significantly more resistant to spallation than the AM1-based one. Up to 50 cycles, both systems exhibit similar oxidation rate and phase transformations but major differences are observed after long-term ageing. In particular, a Ru-rich β-phase is formed in the bond coat of the MCNG-based system while the AM1- based one undergoes strong rumpling of the TGO/bond coat interface due to the loss of the thermal barrier coating.     

Mixed-mode interfacial fracture toughness for thermal barrier coating

A new interfacial fracture test method was developed for measuring the mixed-mode interfacial fracture toughness of thermal barrier coated material over a wide range of loading phase angles. The principle of this developed method is based on peeling the coating from the substrate due to compressive loading to the coating edge, as forming a shear loading to the interface, and slinging loading such as beam bending, as normal loading to the interface. The complete closed form of the energy release rate and associated complex stress intensity factor for our testing method is shown. An yttria stabilized zirconia (YSZ) coating, which was sprayed thermally on Ni-based superalloy, was tested using the testing device developed here.The results showed that the energy release rate for the coating-interfacial crack increased with loading phase angle, which is defined by tan⁻¹ for a ratio of stress intensity factor K₂ to K₁. It was noticed that the interfacial energy release rate increasing with mode II loading could be mainly associated with the contact shielding effect due to crack surface roughness rubbing together.     

表面粗糙度对PS-PVD热障涂层陶瓷层沉积的影响

研究基于等离子喷涂-物理气相沉积(PS-PVD)工艺的沉积表面的粗糙度对YSZ陶瓷层结构的影响,初步阐明了表面粗糙度对陶瓷层气相沉积过程的影响和涂层结构的形成规律。采用PS-PVD工艺在预制有NiCoCrAlYTa黏结层的K417G高温合金上制备YSZ陶瓷层;采用SEM、粗糙度检测仪、3D表面形貌仪等方法分析PS-PVD YSZ陶瓷涂层的形貌和结构特征。基体表面粗糙度对PS-PVD涂层结构有很大影响。结果表明:当基体表面粗糙度分别为 R a≤2μm, 2μm< R a<6μm, R a≥6μm时,涂层粗糙度分别在3.5~5,6~10,10~15μm区间;特征表面形貌"菜花头"的直径随着基体表面粗糙度的增加而逐渐增大, d P=38.5μm, d 280S =25.5μm, d 60S =38.7μm, d 24S =102μm, d S=137μm。表面粗糙度主要通过PS-PVD气相沉积过程中的阴影效应来影响涂层生长和形成差异性结构,随着基体表面粗糙度的增加,YSZ陶瓷层受阴影效应影响增大,表面形貌"菜花头"尺寸和柱状结构间间隙增大,形成更加疏松的结构。     

EB-PVD在热障涂层中的研究及应用

EB-PVD是以高能电子束为热源的一种蒸发镀膜技术.在真空的环境下,高能离子束轰击靶材(金属,陶瓷等),使其融化、升华、蒸发,最后沉积在基片上.由于EB-PVD技术具有蒸发和沉积速率高,涂层致密,化学成分易于精确控制,可得到柱状晶组织,无污染,热效率高,基片与薄膜之间有较强的结合力等诸多优点,已被广泛应用于国防和民用领域.本文介绍了EB-PVD技术在制备热障涂层时优势、不足与改进措施.     

Thermal fatigue failure induced by delamination in thermal barrier coating

The paper presents the experimental and theoretical investigation on the thermal fatigue failure induced by delamination in thermal barrier coating system. Laser heating method was used to simulate the operating state of TBC (thermal barrier coating) system. The non-destructive evaluation such as acoustic emission (AE) detect was used to study the evolution of TBC system damage. Micro-observation and AE detect both revealed that fatigue crack was in two forms: surface crack and interface delamination. It was found that interface delamination took place in the period of cooling or heating. Heating or cooling rate and temperature gradient had an important effect on interface delamination cracking propagation. A theoretical model on interface delamination cracking in TBC system at operating state is proposed. In the model, a membrane stress P and a bending moment M are designated the thermal loads of the thermal stress and temperature gradient in TBC system. In this case, the coupled effect of plastic deformation, creep of ceramic coating as well as thermal growth oxidation (TGO) and temperature gradient in TBC system was considered in the model. The thermal stress intensity factors (TSIFs) in non-FGM (functional gradient material) thermal barrier coating system is analytical obtained. The numerical results of TSIFs reveal some same results as obtained in experimental test. The model is based on fracture mechanics theory about heterogeneous materials and it gives a rigorous explanation of delaminations in TBC system loaded by thermal fatigue. Both theoretical analysis and experimental observation reveal an important fact: delaminations are fatigue cracks which grow during thermal shocks due to compressive stresses in the loading, this loads the delaminations cracks in mixed I and II mode.     

A dip process for thermal barrier coating on superalloys

A new cerium oxide thermal barrier coating for superalloys has been investigated. The coating is applied by hot dipping superalloy substrates into a molten bath of low melting point Ce-Ni alloy. Annealing and selective oxidation of cerium at low oxygen activities produce a duplex coating of a continuous outer layer of cerium oxide (CeO₂) and an inner composite CeO₂-substrate layer.The effects of coating composition and processing variables on the microstructure and integrity of the coating on nickel- and cobalt-base alloys were studied in detail. Isothermal and cyclic oxidation tests showed that coating spallation could occur and was associated with the oxidation of nickel and cobalt incorporated in the coating. A model for the formation of the CeO₂ barrier and the subscale region is proposed that explains the presence of unwanted nickel and cobalt.     

High-temperature thermal barrier coating formation by laser alloying of CP-Ti with pre-placed Ni-SiC coating

High-temperature thermal barrier coating was created on CP-Ti using a pre-placed Ni-SiC layer by laser alloying technique. The coating was developed using 80% Ni + 20% SiC, 50% Ni + 50% SiC and 60% Ni + 40% SiC, and the latter two compositions are found to be efficient in producing a uniform layer. The 100% SiC pre-placement was also used. A flaw-less coating of 0.4–0.6 mm thickness was produced at a lower power density of 1.3 to 1.9 × 10⁵ W cm^–2. Very high power density of 2.5–3.0 × 10⁵ W cm^–2 is inefficient to produce uniform coating. The laser alloyed coating consists of dendrites and intermetallic precipitates. The degree of dendrite population depends upon the coating composition and laser processing conditions. The coating hardness was 600–1200 HV, which is three to six times higher than the base titanium. Uniform hardness was obtained for the coatings produced at a laser power density of 1.3 × 10⁵ W cm^–2. The titanium silicide (TiNiSi, Ti₅Si₃, TiSi) and nickelide (NiTi₂) phases formed on the laser-alloyed coating surface was confirmed by X-ray analysis. These intermetallic phases can improve high-temperature properties of titanium and its alloys. The effect of laser power density and coating composition on the alloying depth alloying width, hardness and microstructure are discussed. The present work investigated the microstructure evolution, hardness and compound phases by means of optical and scanning electron microscopy, Vickers hardness testing, EDXRD and SIMS analysis. A 5 kW CW CO₂ laser was used for laser alloying experiments.     

Observations of the spallation modes in an overlay coating and the corresponding thermal barrier coating system

Abstract

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing and the sites of failure. Identification of the initiation sites and growth of cracks is important in understanding the spallation process. In this study, scanning electron microscopy is used to provide evidence of the processes involved in the two systems. A comparison of the two coating systems reveals the effect the outer ceramic layer has on the oxide scale growth, and the spallation processes crucial to the understanding of the failure mechanisms of these coating systems.