Hot Section Components Research Articles

Influence of substrate temperature on the microstructure of YSZ films and their application as the insulating layer of thin film sensors for harsh temperature environments

Thin film sensors are employed to monitor the health of hot-section components of aeroengine intelligence (for instance, blades), and electrical insulating layers are needed between the metal components and thin film sensors. For this purpose, the electrical insulation characteristics of an yttria-stabilized zirconia (YSZ)/Al2O3 multilayer insulating structure were investigated. First, YSZ thin films were deposited by DC reactive sputtering at various substrate temperatures, and the microstructural features were investigated by scanning electron microscopy and X-ray diffraction. The results indicate that the micromorphology of the YSZ thin film gradually became denser with increasing substrate temperature, and no new phases appeared. The compact and uniform topography of the YSZ thin film improved the insulation properties of the multilayer insulating structure and enhanced the adhesion of the thin film sensors. In addition, the electrical insulation properties of the YSZ/Al2O3 multilayer insulating structure were evaluated via insulation resistance tests from 25 to 800 °C, in which the YSZ thin film was deposited at 550 °C. The results show that the insulation resistance of the multilayer structure increased by an order of magnitude compared with that of the conventional Al2O3 insulating layer, reaching 135 kΩ (5.1 × 10−6 S/m) at 800 °C. Notably, the insulation resistance was still greater than 75 kΩ after annealing at 800 °C for 5 h. Finally, the shunt effect of the YSZ/Al2O3 multilayer insulating structure was estimated using a PdCr thin film strain gauge. The relative resistance error was 0.24%, which demonstrates that the YSZ/Al2O3 multilayer insulating structure is suitable for thin film sensors.

Silicate ash-resistant novel thermal barrier coatings in gas turbines

Gas turbines, used to propel aircraft or generate electricity, are vulnerable to attack by environmental silicate debris. When silicate ash is ingested into gas turbines, it melts and may adhere to the hot-section components of gas turbines (typically operating at > 1250 °C), infiltrating into their thermal barrier coatings (TBCs). These ash-related TBC issues pose a critical hazard for aircraft aviation. Here, we have developed here a novel NdYbZr2O7 coating substrate which addresses these challenges. We have quantitatively analyzed the in-situ wetting process of three representative silicate ash compositions—CMAS, volcanic ash and fly ash, and compare their infiltration behavior in the newly-developed NdYbZr2O7 coating substrate. The widely differing chemical compositions of the three ash samples yield differences in their high temperature viscosities which in turn impact their respective propensities for wetting and infiltration. CMAS exhibits the greatest dynamic wettability due to its lower viscosity over the entire range of temperature. Volcanic ash exhibits the largest infiltration depth, probably related to the relatively high solubility of rare-earth oxides (RE2O3) in volcanic ash melt. Nevertheless, compared with a traditional YSZ coating, the NdYbZr2O7 coating exhibits a better corrosion resistance in the presence of all three silicate ash samples due to the rapid precipitation of (Nd, Yb)-apatite and c-ZrO2 phases, which build a dense reaction layer, inhibiting further melt infiltration.

Relative Stress Analysis of Gas Turbine Blade for Various Alloying Materials

Gas turbines provide a reliable and efficient production of power in both pilot power plants and aircraft propulsion. The operating cost of the modern gas turbines is greatly influenced by the durability of hot section components. To cope up with the increasing temperature, there has been an evolution of new generation blade materials. At elevated temperature conditions, thermal stress and resulting deformations can affect the power developed and efficiency. In this paper a finite element simulation has been made on a fixed blade profile to explore various factors affecting the turbine blade. Variety of existing and new generation materials have been considered a under a boundary condition of constant high pressure and high Operating temperature was varied between 1000°C-1400°C. The development of stress and deformation along with the heat flux have been studied for finding the most effective manufacturing alloy for gas turbines.

Microstructure of IN738LC Fabricated Using Laser Powder Bed Fusion Additive Manufacturing

Abstract Nickel-base superalloys are extensively used in the production of gas turbine hot-section components as they offer exceptional creep strength and superior fatigue resistance at high temperatures. Such improved properties are due to the presence of precipitate-strengthening phases such as Ni3Ti or Ni3Al (γ′ phases) in the normally face-centered cubic (FCC) structure of the solidified nickel. Although this second phase is the main reason for the improvements in properties, the presence of such phases also results in increased processing difficulties as these alloys are prone to crack formation. In this work, specimens of IN738LC are fabricated on a Coherent Creator laser powder bed fusion (L-PBF) additive manufacturing (AM) equipment. Optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) are carried out to characterize the deposit region. Metallurgical continuity is achieved in the entire deposit region and the specimens do not show any warpage. However, the specimens show voids (e.g., pores and cracks) in the deposit region. The results show that the percentage void area decreases along the build height direction. The deposited IN738LC shows polycrystalline grains in the entire deposit region as confirmed by XRD and EBSD. The grain size also shows variations along the build direction. In summary, the results open opportunities for academic researchers and small-scale businesses in fabricating high-γ′ nickel-base superalloys on a desktop laser powder bed fusion AM equipment.

Sintering Modeling of Thermal Barrier Coatings at Elevated Temperatures: A Review of Recent Advances

To achieve a higher efficiency in gas turbine engine by increasing the inlet-temperature of burning gas is one of the primary goals in aviation industry. The development of thermal barrier coating system (TBCs) continuously raises the inlet-temperature of gas turbine engine in the past decades. Due to the complexity of TBCs and harsh operation environments, the degradation and failure mechanisms of hot section components have not been fully understood, and consequently limits the application of TBCs. It was identified that high-temperature sintering of the topcoat in a typical TBC could be one of the major sources of its failure since the microstructures of the constituent coating layers evolve dynamically during the service period, resulting in significant changes of mechanical and thermal physical properties of the coating system. This paper intends to review recent advances of analytical and numerical modeling of sintering of topcoat in TBCs including the modeling methodology and applications of the models, particularly the implementation of finite element combined with specific materials constitutive functions. Critical remarks on the future development and applications of these models are also discussed in the end.

Impact of Ceramic Matrix Composite Topology on Friction Factor and Heat Transfer

AbstractCeramic matrix composites (CMCs) are of interest for hot section components of gas turbine engines due to their low weight and favorable thermal properties. To implement this advanced composite in a gas turbine engine, characterizing the influence of CMC’s surface topology on heat transfer and cooling performance is critical. However, very few published studies have reported the flow and heat transfer effects caused by this unique surface topology. This study is an experimental and computational investigation to evaluate the effect of weave orientations, relevant to CMC surfaces, on the resulting pressure loss and convective heat transfer within an internal channel. The weave pattern was additively manufactured as the walls of a scaled-up coupon containing a single channel. For each of the three weave orientations, bulk pressure losses and convective heat transfer coefficients were measured over a range of Reynolds numbers. Scaling the pressure losses in terms of a friction factor and convective heat transfer coefficients in terms of a Nusselt number showed the importance of choosing the appropriate definition of the hydraulic diameter, which was particularly important for the friction factor. A coupon having one wall with the weave surface increased pressure loss and heat transfer compared to a smooth wall with the largest increases occurring when the CMC weave strands were perpendicular to the flow. Friction factor augmentations were much higher than heat transfer augmentations. When adding the weave to a second channel wall, pressure loss and heat transfer were further increased. Orienting the CMC strands perpendicular to the flow consistently showed the largest augmentations in heat transfer over a smooth channel, but at a much higher pressure loss penalty than that seen with the CMC strands parallel to the flow.

Influence of Minor Alloying Element Additions on the Crack Susceptibility of a Nickel Based Superalloy Manufactured by LPBF

Inconel 738LC (IN738LC) is a nickel-based superalloy specially used in the hot section components of turbine engines. One of its main drawbacks relies on the cracking susceptibility when it is manufactured by laser powder bed fusion (LPBF). This paper analyzes the influence of minor alloying element concentration on cracking tendency of IN738LC superalloy manufactured by LPBF. For that objective, samples were manufactured using two powders, which presented different minor alloying elements concentration (Si, Zr and B). It was shown that the samples crack tendency was very different depending on the powder used for their manufacturing. In fact, the measured crack density value was 2.73 mm/mm2 for the samples manufactured with the powder with higher minor alloying elements concentration, while 0.25 mm/mm2 for the others. Additionally, a special emphasis has been put on elemental composition characterization in cracked grain boundaries in order to quantify possible Si or Zr enrichment. It has been also studied the differences of solidification ranges and grain structures between both samples as a consequence of different minor alloying elements concentration in order to analyze their effect on crack susceptibility. In this sense, Scheil-Gulliver simulation results have shown that samples with higher Si and Zr contents presented higher solidification range temperature. This fact, as well as an increase of the presence of high angle grain boundaries (HAGB), leaded to an increment in the crack formation during solidification. Therefore, in this research work, an understanding of the factors affecting crack phenomenon in the LPBF manufactured IN738LC was accomplished.

The Effect of High-Temperature Water Vapour on Degradation and Failure of Hot Section Components of Gas Turbine Engines

For the past decade, the aviation industry has been adopting sustainable aviation fuels (SAF) for use in aircraft to reduce the impact of aviation on climate change. Also, some nations look to SAF as an option for energy security for their military fleets. Understanding the critical impact of alternative fuel sources on hardware will provide the gas turbine industry with strategic options in sustainability and maintainability of the existing and new fleets. The alternative fuels with high hydrogen/carbon ratio (H/C) (such as synthetic paraffinic kerosenes (SPK)) could produce more water vapour content than the conventional jet fuels upon combustion, and this increased water vapour level could exert a significant impact over the long-term durability on hot section components such as the substrate blades, oxidation resistant coatings, thermal barrier coatings (TBCs), environmental barrier coatings (EBCs), resulting in an accelerated degradation of the turbine components. The possible detrimental effect of high-temperature water vapour on degradation and lifespan of hot section components was examined. Examples were specifically given on degradation and spallation of thermally grown oxides (TGO), formation of non-protective oxides and ceramics topcoats in TBCs. Results show that water vapour can lead to volatilization of TGO (Al2O3), and is responsible for the formation of non-protective oxides in both Pt-modified β-NiAl and MCrAlY coatings, leading to their early spallation. However, water vapour does not appear to directly affect the ceramic topcoat of the TBC. For EBCs coated on SiC-based substrates, the substrate recession via silica (TGO) volatilization was reviewed. These EBCs were observed undergoing degradation in highly hostile environments, e.g., constantly operating under high temperatures, pressures, and velocities condition in the presence of water vapour steam. The review intends to provide a perspective of high-temperature water vapour effect on the EBCs’ topcoat properties such as durability, degradation, crack nucleation and crack growth, and possible guidance for mitigating these degradation effects.

A modified damage accumulation model for life prediction of aero‐engine materials under combined high and low cycle fatigue loading

AbstractAero‐engine hot section components experience combined high and low cycle fatigue (CCF) loading under actual working conditions. This paper proposes a modified fatigue damage accumulation model to precisely predict lifetimes of aero‐engine materials according to the newly presented interaction factor, which is able to quantitatively map damage interactions introduced by high cycle fatigue (HCF) and low cycle fatigue (LCF) loads. With the model, the lifetimes of four typical engine materials are predicted and compared to their factual data from experiments. The validation of the model is illustrated by a comprehensive comparison of the results concluded by the proposed model and that of other classical methodologies, which indicates that the new model holds the best performance in lifetime predictions. Moreover, an actual engineering case, which is the lifetime prediction of third stage turbine blades of an aero‐engine, is completed to validate the feasibility of the proposed method.

Application of Machine Learning to Solid Particle Erosion of APS-TBC and EB-PVD TBC at Elevated Temperatures

Machine learning (ML) and deep learning (DL) for big data (BD) management are currently viable approaches that can significantly help in high-temperature materials design and development. ML-DL can accumulate knowledge by learning from existing data generated through multi-physics modelling (MPM) and experimental tests (ETs). DL mainly involves analyzing nonlinear correlations and high-dimensional datasets implemented through specifically designed numerical algorithms. DL also makes it possible to learn from new data and modify predictive models over time, identifying anomalies, signatures, and trends in machine performance, develop an understanding of patterns of behaviour, and estimate efficiencies in a machine. Machine learning was implemented to investigate the solid particle erosion of both APS (air plasma spray) and EB-PVD (electron beam physical vapour deposition) TBCs of hot section components. Several ML models and algorithms were used such as neural networks (NNs), gradient boosting regression (GBR), decision tree regression (DTR), and random forest regression (RFR). It was found that the test data are strongly associated with five key factors as identifiers. Following test data collection, the dataset is subjected to sorting, filtering, extracting, and exploratory analysis. The training and testing, and prediction results are analysed. The results suggest that neural networks using the BR model and GBR have better prediction capability.

Micromechanical modeling for the in-plane mechanical behavior of orthogonal three-dimensional woven ceramic matrix composites with transverse and matrix cracking

Ceramic matrix composites (CMCs) are currently being considered for applications in the hot-section components of aviation gas turbines owing to their favorable characteristics. Herein, a micromechanical modeling is presented for orthogonal 3 D woven CMCs under in-plane loading. The three-dimensional effective compliance of the 3 D woven composite was derived using three-dimensional laminate theory and continuum damage mechanics. The damage variables were used to describe the stiffness reduction due to the transverse and matrix cracking in each fiber bundle. The calculation method for the transverse and matrix cracking evolutions under in-plane loading was established by introducing mixed-mode damage criteria. The stress redistribution among the fiber bundles of 3 D woven CMCs due to the fiber/matrix interfacial debonding around matrix cracking was considered to capture the interaction between the matrix and transverse crack evolutions. Additionally, a mesomechanical model comprising finite element analysis and damage mechanics was established to evaluate the stress perturbation due to the geometry of the woven structure. The edge face of the 3 D woven CMC was experimentally observed to measure the transverse and matrix cracks that occurred in each fiber bundle. The transverse and matrix crack densities predicted by the micromechanical and mesomechanical models reasonably agreed with the experimental results up to crack saturation. Furthermore, the micromechanical model reproduced the nonlinear stress–strain response under tensile and shear loading using mixed-mode damage criteria.

Aircraft engine hot-section virtual sensor creation and gas path performance monitoring

Various Kalman filter approaches have been presented for performance estimation of aircraft engines provided that sensor measurement numbers are sufficient and all of them are available. However, it is difficult to collect all physical parameters along the gas path since the complex structure limits sensor installation, especially around the high-pressure turbine. The contribution of this article is to create a virtual sensor in the hot-section based on the component physical characteristics and aerothermodynamic theory and develop a dual hypothesis test strategy combined with a state estimator to track abrupt degradation of the engine component. A novel couple algorithm of the unscented Kalman filter using a virtual sensor is developed that has three primary advantages: (i) The performance degradation of four rotatory components can be estimated without sensor P43. (ii) The accuracy of state estimation using the virtual sensor is equivalent to that of sensor P43, and it adapts to the abrupt change of engine operating condition in the whole flight envelope. (iii) If the sensor can be installed in the future, it can be engaged to its analytical redundancy. Simulations are carried out on typical abrupt degradation datasets of aircraft engines, which demonstrate the superiority of anomaly detection of gas path component performance using virtual sensor measurements, especially hot-section components.

TGO growth and kinetic study of single and double layered TBC systems

Thermal barrier coating (TBC) systems are widely used to prolong the lifetime of hot section components in gas turbines. Oxidation degradation at high temperature is inevitable despite the use of TBC systems. In the current study, Yttria stabilized zirconia (YSZ)/CoNiCrAlY/Inconel 718 and La2Zr2O7 (LZO)/YSZ/CoNiCrAlY/Inconel 718 TBC systems were produced and exposed to high temperature oxidation tests. After the tests, a thermally grown oxide (TGO) layer formed at the interface between the bond and top coating due to the oxidation of bond coating. This layer has a critical importance for the lifetime of TBCs. The oxidation kinetics, rate constants, and activation energy values of both TBC systems were calculated using TGO thickness values. At the end of the oxidation tests, the double-layered LZO/YSZ TBC system exhibited better performance thanks to the lower oxygen permeability of LZO considering oxidation kinetics results and microstructural investigations. However, the formed TGO layer consisting of alumina and mixed oxides (MOs) led to crack formation at the interface. Besides, higher temperatures and increasing oxidation periods also degraded the integrity of top coatings due to the sintering effect. Chemical incompatibility did not affect the interface durability of the LZO/YSZ TBC system during the oxidation tests.

An analytical model for predicting residual stress in TBC-film cooling system considering non-uniform temperature field

Residual stress is an important parameter to evaluate and predict the interfacial peeling and failure of a thermal barrier coating (TBC) system in the thermal cycle. Considering a non-uniform temperature field, an analytical model for predicting residual stress of the TBC-film cooling system was established. Based on the double-layer TBC-film cooling system, the analytical solutions were validated by comparing with the previous results. Then, the four-layer system, including the superalloy substrate (SUB), the metallic bond coat (BC), the thermally grown oxide (TGO), and the ceramic top coat (TC), was analyzed. The distribution of residual stress was discontinuous at the interface among the layers. The maximum peeling moment occurred at the BC/SUB interface, and the maximum shear stress occurred at the TGO/BC interface. Therefore, the probabilities, of which opening edge cracks (mode I) appeared at the BC/SUB interface and shearing edge cracks (mode II) appeared at the TGO/BC interface, were higher than that at other interfaces. The deflection and bending direction of the system were affected by the temperature difference and coefficient of thermal expansions. The large thickness of the coating could effectively improve the thermal insulation capacity of the system, but the peeling moment and shear stress also increased. Therefore, preventing the thermal growth of the TGO layer and reducing the thickness of the TC layer can improve the stability of the system and extend service lifetime of the system while ensuring that the cooling requirements for hot section components are met.

Strain response of fiber-reinforced ceramic-matrix composites subjected to stress-rupture with stochastic load at intermediate temperature

To ensure the reliability and safety of ceramic-matrix composites (CMCs) hot-section components used in the aero engines, it is necessary to perform the strain response of CMCs at intermediate temperatures (600 to 1000 °C) under stress-rupture with stochastic loading. In this paper, the strain response of SiC/SiC composite under stress-rupture with stochastic load at intermediate temperatures is investigated. Multiple damage mechanisms of matrix cracking, interface debonding and oxidation, and fiber’s oxidation and fracture are considered. Matrix crack spacing, interface oxidation and debonding length, fiber’s broken probability, and intact fiber’s stress are determined using micromechanical damage models. Experimental strain response and internal damage evolution of SiC/SiC composite under constant and stochastic stress are predicted. Effects of stochastic stress level and stochastic time, material properties, damage state and environment temperature on composite’s strain response and stress-rupture life are discussed. When stochastic stress level and environment temperature increase, the composite’s strain at the stochastic stress increases, the time for the interface complete debonding and oxidation decreases, and the stress-rupture life decreases.

Shaping quality, microstructure, and mechanical properties of melt-grown mullite ceramics by directed laser deposition

Directed laser deposition (DLD) has demonstrated the potential for rapid preparing high-performance melt-grown ceramics applied for the aero-engine hot section components. Mullite has excellent potential for applications in harsh environments and extreme conditions. However, there are defects (pores and cracks) in melt-grown mullite ceramics (MGMC), limiting its application, related to unclear processes, microstructure, and properties. In this work, we investigated the impact of z-increment on shaping quality, microstructure, and mechanical properties. The results reveal that oxygen vacancy concentration is responsible for the variety of sample colors. Low z-increment is beneficial to decreasing porosity, while high z-increment is beneficial to crack suppression. The formation and propagation mechanism of internal cracks at low z-increment are discussed in detail. Mullite crystals in the sample center are "tabular cellular", which are 2:1 high-alumina mullite. While mullite crystals at the sample edge are 3:2 mullite, undergo four stages as z-increment increase; "tabular cellular", "facet dendrite", "rod cellular", and mixed crystal. This evolution of crystal morphology is attributed to the decrease in temperature gradient and the increase of supersaturation. Besides, the flexural strength and microhardness increase first and then decrease with the rise of z-increment, which is the function of defects, compactness, and microstructure.

Measurement Drift in 3-Hole Yaw Pressure Probes From 5µm Sand Fouling at 1050 °C

AbstractThe present paper focuses on the resilience of 3-hole pressure probes to hot sand fouling in turbomachinery environments. These probes are utilized inside jet engine hot sections for diagnostics and flow characterization. Ingestion of sand and other particulates pose a significant risk to hot section components and measurement devices in gas turbine engines. In this study, wedge, cylindrical, and trapezoidal probes were exposed to hot section turbine aerothermal conditions of 1050 °C and 65–70 m/s flow velocity and fouled with 0–5 µm Arizona Road Dust (ARD). Sand accumulated more rapidly on the surface of the trapezoidal and cylindrical probe geometries than on the surface of the wedge probe geometry. Probe calibrations following sand fouling were performed in an ambient temperature, open air, calibration jet at Mach 0.3 and 0.5. Calibration curves using nondimensional coefficients were used to assess probe error in yaw angle due to sand fouling. Probe error was based on each probe’s ability to accurately measure flow direction over a flow angle range of [−10 deg, 10 deg]. On average, the probes displayed greater error at Mach 0.5 than Mach 0.3. The wedge probe performed the best after sand fouling and displayed a maximum error of less than ±2 deg in yaw angle. In contrast, the cylindrical probe performed the worst after sand fouling and displayed maximum errors of more than ±8 deg in yaw angle. Transient response did not change notably with sand fouling.

Quantitative mapping of service process-microstructural degradation-property deterioration for a Ni-based superalloy based on chord length distribution imaging process

Directionally solidified (DS) and single crystal (SC) Ni-based superalloys inevitably underwent microstructural degradation induced by the harsh operating environment. For the safety service and economic overhaul, constructing a quantitative mapping chain from service process to microstructural degradation and to property deterioration is critically essential. The present work started with stress-free and stress-assisted pre-service treatments of a DS Ni-based superalloy to obtain microstructures with different degraded states. An imaging process based on two-phase rotary chord length distributions was established to extract the high dimensional statistical information for identifying the morphology and size features of microstructures. To reduce the dimension of the statistical information and quantitatively characterize the microstructural states in fewer parameters, principal component analysis was employed to capture the core microstructural indicators, which was utilized to establish the response surface between the deterioration of fatigue resistance and the microstructural degradation. Finally, a multi-output support vector regression (SVR) model was constructed to map between service process and microstructural degradation. The results showed acceptable accuracy to estimate the microstructural degradation of pre-serviced alloys. Meanwhile, the framework provides a technical chain for the waste determination and microstructural degradation estimation of the hot section components made by DS and SC Ni-based superalloys.

Failure analysis of a gas turbine blade: A review

Blade is an essential component of a gas turbine that functions to convert thermal gas energy at a high temperature and pressure into mechanical energy. Gas in the turbine blade works at a high temperature and pressure. High temperatures and pressures cause turbine blades to be one part of turbine components that often fails. Failure of the blade results in performance and turbine efficiency. The cause of failure on the blade is influenced by several factors, including fatigue, creep, oxidation, degradation of the coating on the turbine blade, corrosion, erosion, and surface degradation due to working at high temperatures. The hot temperature on the blades work on turbines around 1927 F to 3500 F and is one of the main factors in the failure rate. Individual blade failure rates differ due to operating temperature, rotation speed, operating mode, total service time, and differences in fabrication. This article discusses several case studies of failure in turbine hot section components such as blades to support the failure investigation is the key to improving turbine efficiency.

Evolution behavior of liquid film in the heat-affected zone of laser cladding non-weldable nickel-based superalloy

Laser cladding with a paraxial powder-feeding device was used to repair as-cast K447A nickel-based superalloy, a kind of non-weldable alloy with a high Hf content and working as gas-turbine hot-section components. In the present study, the metallurgical evolution behavior of liquid film and liquation cracking mechanisms are studied. The results showed that a kind of uncommon Ni21Hf8 phase was found in the incipient melting regions (IMR) of the intergranular region. In the thermal cycle, γ/Ni21Hf8 eutectic liquefaction, M5B3 dissolved, and γ/γ’ eutectic liquefaction successively occurred in the heating process in IMR, and liquid → γ + γ/γ’ + M5B3 + γ/Ni21Hf8 → γ + γ’(3) + γ/γ’ + γ/Ni21Hf8 + M5B3 occurred in the cooling process. The inhomogeneity of the intergranular structure leads to the liquid film with the characteristics of discontinuous distribution and uneven width. The cracking criterion of the liquid film was established based on pressure drop. When the closed liquid film meets the condition of 2γsl<1/K*(ρs/ρl−1*R*dt+∆hε), the liquid film with a thickness that is greater than critical thickness will be induced into a liquation cracking.