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.     

Measurement of high‐temperature strains in superalloy and carbon/carbon composites using chemical composition gratings

This paper describes an experimental and signal processing technique to perform high temperature tests on superalloy (INCONEL) and carbon/carbon structures using silica‐based chemical composition gratings (CCGs). The results obtained from applying this technique at 940°C in superalloys and 950°C for carbon/carbon (C/C) composites are benchmarked against data obtained from four different methods. The results show that the wavelength responses of the CCGs bonded on the superalloy and on the C/C plate increase nonlinearly with increasing temperatures. The temperature‐dependent strain transfer coefficients recorded during the superalloy tests show quite stable results below 600 °C and tend to slightly decrease thereafter. The values of the strain transfer coefficients below 1000 °C are significantly affected by the thermal expansion coefficient of the substrate material and the interface. We demonstrate that the strain transfer coefficient calculation method used in this paper is not suitable for low and/or negative expansion material. The results of the relative errors show that the CCGs‐F method based on the quadratic dependence of the wavelength shift versus the temperature appears to be the best to estimate the mechanical strains within the interval of temperatures considered and the measurement accuracy. The relative errors measured between 200 °C and 1000 °C are less than 5%.     

Study on behavior of NiAl coating with different Ni/Al ratios

β-NiAl coatings with different Ni/Al ratios were deposited on K403 superalloy substrates via magnetron sputtering. The phase transformation and diffusion phenomenon of the NiAl/Ni-based superalloy system after vacuum annealing at 900 and 1000 °C were analyzed using X-Ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and energy dispersive X-ray spectrometry (EDS). The effect of coating concentrations on the outward diffusion behavior of substrate elements was discussed. The high Cr concentrations in the Al-rich NiAl coatings were caused by the intense interdiffusion between Al and Cr. The Ti, W and Mo partitioned to γ′-Ni₃Al in the coatings. Several possible reasons for the formation of γ′-Ni₃Al at the surface of Ni-rich NiAl coating were identified, including: diffusion behavior of W and Mo in β-NiAl, destabilizing effect of substrate elements on β-NiAl, and diffusion rates of Ni and Al in β-NiAl. The volume change in β ⇛ γ′ transformation process shows Ni uphill diffused to the γ′-Ni₃Al islands at the surface of Ni-rich NiAl coatings. The IDZ (interdiffusion zone) thickness and precipitates in IDZ were related to the Al initial concentrations in the coatings.     

Survey and analysis of internal temperatures of Lebanese domestic refrigerators

In the light of power crisis that Lebanon is witnessing, temperatures were recorded, using data loggers, in 147 domestic refrigerators in three main cities every 5 min over 3 days. A questionnaire was administered to gather information on household characteristics, power supply and refrigerator. Temperatures were on average 8.0 °C with minimum −5.9 °C and maximum 37.0 °C. Income, number of household members, age, brand, load level and seal of the fridge, frequency of power cuts and availability of alternative power had no significant effect on the overall mean temperature, while the city and the distance to heat source had a significant effect. The number of high temperature readings (T > 6 °C) differed significantly between the cities, while the availability of an alternative power was borderline significant. The lowest average temperatures were recorded during the night and 70% of average temperatures were in the danger zone (above 6 °C). This study can provide input to food safety risk assessments.     

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.     

Effect of Ru on interdiffusion dynamics of β-NiAl/DD6 system: A combined experimental and first-principles studies

As a diffusion barrier between thermal barrier coating (TBC) and advanced single crystal (SC) superalloy DD6, RuNiAl coating has attracted increasing attention recently. In this work, different diffusion couples including NiAl/DD6, RuNiAl/DD6 and RuAl/DD6, were prepared and their interdiffusion behavior was investigated at 1100 °C, to understand the diffusion barrier mechanism of the RuNiAl coating. The addition of Ru to NiAl effectively reduced the interdiffusion coefficient of Al, thereby delaying the phase transformation from β to γ′ and suppressing the formation of topologically closed-packed (TCP) phases and primary interdiffusion zone (IDZ). It is verified by first-principles calculations that Ru restrained the formation of Ni and Al vacancies and Ni and Al antisite atoms. According to the calculations, the addition of Ru increases the defect formation energies, thus inhibiting the diffusion of Al in β-NiAl. The calculation results are consistent with the experimental data.     

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.     

A phenomenological model to predict the crack growth in single crystal superalloys at high temperature

This paper presents the formulation of a phenomenological model to predict the crack growth in single crystal superalloy at high temperature. The proposed model relies on an extensive experimental study performed on AM1 single crystal superalloy at temperatures ranging from 650 °C to 950 °C. Tests carried out in fatigue and creep–fatigue regimes investigate the effects of time on crack growth rates. The crack growth model follows the framework of classical linear elastic fracture mechanics. Time effects at high temperature are captured by creep–fatigue and oxidation–fatigue interactions. The specific model formulation for nonisothermal conditions is attractive for identifying parameters on a large temperature domain and for predicting complex Thermo-Mechanical Fatigue (TMF) tests. Model predictions are then compared with a large set of experimental results including TMF tests. The application of this model, which accounts for a better understanding and modeling of physical phenomena such as the environmental or creep effects on crack growth rate, should improve the prediction of crack growth regime in single crystal superalloys that are used to design critical components such as turbine blades.     

Evaluation of mixed oxide formation and sintering behavior in thermal barrier coatings on nickel‐based superalloy

Thermal barrier coatings are widely used in aircraft turbines to protect nickel‐based superalloys from the effect of high temperature oxidation and hot corrosion. In this study, both NiCrAlY bond coat and yttria‐stabilized zirconia top coat were deposited using atmospheric plasma spray technique. After coating production, specimens were exposed to oxidation in air atmosphere at 900 °C, 1000 °C and 1100 °C for different periods of time up to 50 h. Microstructural transformations in the ceramic top coat and growth behavior of the thermally grown oxide layer were examined using scanning electron microscopy, porosity calculation, elemental mapping and hardness measurement. Formation of different types of oxides in the thermally grown oxide layer shows that this process strongly depends on deposition technique as well as on oxidation time and temperature. Hardness values of the top coat increased with a decrease in the porosity of the top coat. Uniformity and homogeneity of the thermally grown oxide layer and densification of the top coat were evaluated in terms of the structural durability of thermal barrier coating systems.     

Crack propagation studies and bond coat properties in thermal barrier coatings under bending

Ceramic based thermal barrier coatings (TBC) are currently considered as a candidate material for advanced stationary gas turbine components. Crack propagation studies under bending are described that were performed on plasma sprayed ZrO₂, bonded by MCrAlY layer to Ni base superalloy. The crack propagation behaviour of the coatings at room temperature in as received and oxidized conditions revealed a linear growth of the cracks on the coating till the yield point of the super alloy was reached. High threshold load at the interface between the ceramic layer and the bond coat was required to propagate the crack further into the bond coat. Once the threshold load was surpassed the crack propagated into the brittle bond coat without an appreciable increase in the load. At temperatures of 800°C the crack propagated only in the TBC (ceramic layer), as the ductile bond coat offered an attractive sink for the stress relaxation. Effects of bond coat oxidation on crack propagation in the interface region have been examined and are discussed.     

Effect of microstructure and low cycle fatigue deformation on tensile properties of P91 steel

Isothermal furnace heat treatments were carried out to simulate the microstructures of inter-critical, fine grain and coarse grain heat-affected zones of P91 steel weld joint at different soaking temperatures ranging from just above AC₁ (837 °C) to well above AC₃ (903 °C). Interrupted low cycle fatigue tests were performed on the specimens of P91 steel up to 5 %, 10 %, 30 %, and 50 % of the total fatigue life at the strain amplitude of ±0.6 %, strain rate of 0.003 s⁻¹ and temperatures of 550 °C and 600 °C. Subsequently, tensile tests were conducted on the interrupt tested specimens at the same strain rate and temperatures. Soaking at the inter-critical temperature region reduces / deteriorates the tensile and yield strengths of base metal compared to fine grain and coarse grain regions. The inter-critical heat-affected zone accounted higher damage contribution towards the overall tensile behavior of the actual P91 steel weld joint. Substructural coarsening during strain cycling at elevated temperatures attributes to the rapid reduction in the initial yield strength up to 10 % of fatigue life of P91 steel. A higher amount of plastic strain accumulation during low cycle fatigue deformation resulted in a decrease in fatigue life of the inter-critical heat-affected zone of P91 steel.     

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.     

Experimental and numerical analysis of an air-cooled double-lift NH3–H2O absorption refrigeration system

The prototype of an air-cooled double-lift NH₃–H₂O absorption chiller driven by hot water at low temperature is presented. The main objective of the study is to illustrate the experimental performances of the prototype under different operating conditions. A mathematical model of the cycle is developed, along with a procedure for the identification of otherwise difficult to measure data, with the purpose of providing the complete picture of the internal thermodynamic cycle. The combined experimental and numerical data allowed assessing the effects on the thermodynamic cycle with varying operating conditions. The unit operated steadily with chilled water inlet 12 °C, outlet 7 °C, air temperature between 22 °C and 38 °C, and hot water driving temperatures between 80 °C and 90 °C. The reference cooling capacity at air temperature of 30 °C is 2.5 kW, with thermal COP about 0.3 and electrical COP about 10.     

Thermal effects on GFRP rebars: experimental study and analytical analysis

The bond mechanism between Glass Fiber Reinforced Polymer (GFRP) bars and concrete is investigated through experimental testing and analytical modeling. This bond depends on several parameters such as temperature. The present paper studies the thermal effect, under high temperature up to 80°C, on bond behaviour at the interface GFRP bars/concrete through pullout-testing. These tests are conducted on specimens after 24 h of exposure at various temperatures. The thermal effect on an average short-term bond strengths and the pullout-load versus end-slip behaviours are compared to untreated specimens (20°C). Some pullout-tests on steel bars/concrete are also performed for the comparison. Experimental results show no significant change in the average bond strength for specimens subjected to temperatures up to +60°C. On the other hand at 80°C, there is a decrease of bond strength of about 22 and 28% for the 8 mm and the 16 mm diameter rods, respectively. An analytical model of the bond stress-slip response of a GFRP/concrete bar has been proposed. The results show good accuracy between the model and the experimental results.     

Condensation in two phase and desuperheating zone for R1234ze(E), R134a and R32 in horizontal smooth tubes

Condensation is usually assumed to begin when the bulk enthalpy reaches the saturated vapor enthalpy, which leads to discontinuity of heat transfer coefficient calculation in modeling. This paper addresses the discontinuity by showing the presence of condensation in desuperheating region when the wall temperature decreases below the saturation temperature at any operating condition. The experiments have been conducted with R134a, R1234ze(E) and R32 for mass fluxes of 100–300 kgm⁻² s⁻¹, saturation temperatures of 30^°C–50 °C and from x = 0.05 to superheat of 50 °C in a horizontal smooth tube with 6.1 mm inner diameter. R134a is observed to have approximately 10% higher and 20% lower HTC compared to R1234ze(E) and R32 respectively. Cavallini correlation predicted the data within an accuracy of 12% while Kondo-Hrnjak correlation predicted HTC for condensation in de-superheating zone within accuracy of 23%.     

Hot deformation and processing maps of X-750 nickel-based superalloy

The deformation behavior of X-750 superalloy was investigated using the hot compression test in the temperature range of 850–1050 °C, and strain rate of 0.1–50 s⁻¹. The experimental results show that the flow stress of superalloy is significantly sensitive to the strain, the strain rate and the deformation temperature. Using dynamic materials model the processing maps of X-750 superalloy at strain of 0.1, 0.3 and 0.5 were established respectively. Microstructure observations reveal that the grain size as well as the volume fraction of the recrystallized grains increased at higher deformation temperature or lower strain rate. At strain of 0.5, the flow instability domain mainly located at lower temperature which is associated with shear band formation and flow localization. The optimum parameters for hot working of the alloy are deformation temperature of 1000–1050 °C and strain rate of 0.1–1 s⁻¹ according to the processing map and microstructure at true strain of 0.5.     

Characterizing the bond strength of geopolymers at ambient and elevated temperatures

This paper presents characterization of bond strength of geopolymers at ambient and elevated temperatures. The bond strength of 18 different formulations of metakaolin (MK)/fly ash (FA) based geopolymers is evaluated through double shear tests in 20–300 °C temperature range. The test parameters include fly ash content, SiO₂/K₂O ratio, solid-to-liquid ratio and Si/Al ratio. In addition the effect of additives, namely short carbon fibers, basalt fibers and styrene–acrylate emulsion in MK/FA precursor, on bond strength is studied. Data from the tests show that geopolymers exhibit slightly lower bond strength than that of epoxy resin at room temperature, however geopolymers retain much higher bond strength in 100–300 °C range. Addition of small quantity of short carbon fibers in MK/FA precursor does not significantly influence bond strength of geopolymers at ambient temperature, but greatly improve bond strength retention in 100–300 °C through crack control mechanism.     

Microstructural evolution of Pt-aluminide coating influenced by cycle oxidation service conditions

This work investigated the degradation of a platinum-modified aluminide diffusion coating on GTD 111 SC Ni-base superalloy turbine blades after ∼16,000 h of exposure to different thermal cycles (critical heating temperatures reported as ∼1000 °C and 1120 °C). The initial coating condition and the evolution of degradation were characterized with conventional microscopy and backscatter scanning electron microscopy. The initial microstructure consisted of a two-phase coating (intermetallic PtAl₂ dispersed in a matrix of β-(Ni,Pt)Al). The major microstructure degradation was associated with the formation of Kirkendall voids, the partial transformation of β-(Ni,Pt)Al to γ′-Ni₃Al, and the dissolution of the intermetallic PtAl₂, resulting in a more brittle, single-phase coating. This type of degradation facilitates the spallation of the coating and crack initiation, resulting in the loss of the coating and eventual blade failure.     

On the development of high temperature ammonia–water hybrid absorption–compression heat pumps

Ammonia–water hybrid absorption–compression heat pumps (HACHP) are a promising technology for development of efficient high temperature industrial heat pumps. Using 28 bar components HACHPs up to 100 °C are commercially available. Components developed for 50 bar and 140 bar show that these pressure limits may be possible to exceed if needed for actual applications. Feasible heat supply temperatures using these component limits are investigated. A feasible solution is defined as one that satisfies constraints on the COP, low and high pressure, compressor discharge temperature, vapour water content and volumetric heat capacity. The ammonia mass fraction and the liquid circulation ratio both influence these constraining parameters. The paper investigates feasible combinations of these parameters through the use of a numerical model. 28 bar components allow temperatures up to 111 °C, 50 bar up to 129 °C, and 140 bar up to 147 °C. If the compressor discharge temperature limit is increased to 250 °C and the vapour water content constraint is removed, this becomes: 182 °C, 193 °C and 223 °C.