Thermal Barrier Research Articles

Degradation mechanism of dual-phase thermal barrier coatings Gd2Zr2O7 + 8YSZ under hot corrosion in pure Na2SO4

AbstractThe research presented in the article concerns the identification and characterisation of degradation mechanisms of two-phase coating thermal barriers based on the 8YSZ/Gd2Zr2O7 (8YSZ/GZO) system, and analysed under the conditions of exposure to pure sulphate salts of the Na2SO4 type. The scope of the presented research included the characterisation of microstructural phenomena in conditions of heating at a temperature of 920 °C for 240 h, followed by heating at a temperature of 970 °C for another 96 h until visually significant degradation effects of the ceramic coating were achieved. Detailed studies of the phase and chemical composition using XRD and SEM/EDS methods were carried out on the surface of the tested coatings and their cross sections to describe microstructural phenomena. The analysis of the phase composition of the ceramic layer showed the course of degradation processes based on the disintegration of the pyrochlore phase into intermediate forms of non-stoichiometric fluorite with the simultaneous enrichment of tetragonal zirconium oxide into a cubic form with a high content of the stabilising phase (Gd2O3). Tests on model powder systems confirmed the degradation mechanism of two-phase TBC coatings.

Optimization Design and Performance Verification of the CeYSZ/Al2O3 Double Ceramic Layer Thermal Barrier Coatings Structure Parameters

Optimization Design and Performance Verification of the CeYSZ/Al2O3 Double Ceramic Layer Thermal Barrier Coatings Structure Parameters

Extending interfacial toughness and thermal cycling lifetime of thermal barrier coatings via interfacial 3D pattern

AbstractWeak interface bonding is a critical factor that compromises the long‐term durability of the thermal barrier coatings. To mitigate this, we introduce an innovative interface‐strengthening approach by integrating four types of 3D patterns at the top coat/bond coat interface using the laser cladding technique. Tensile tests demonstrate a significant enhancement in interfacial bonding strength, increasing by over 139.2% from 21.6 ± 1.7 MPa for conventional thermal barrier coatings without patterns to 51.67 MPa for the patterned coatings. Moreover, the thermal cycling lifetime has been extended by 41%, from 1026 ± 25 cycles for the conventional coating to 1936 ± 164 cycles for the patterned coating. This enhancement in interfacial toughness and thermal cycling lifetime is primarily because of the blocking and deflection effects of 3D patterns on interfacial crack propagation. Besides, the geometric parameters of the patterns play a crucial role in determining the failure behavior of the thermal barrier coatings. This strategy presents a practical solution for the development of durable thermal barrier coatings and provides the valuable insights for the optimization of other heterogeneous interfaces.

A Trajectory planning technique for grinding and polishing aerospace turbine blades’ thermal barrier coatings based on PCL point cloud processing

A Trajectory planning technique for grinding and polishing aerospace turbine blades’ thermal barrier coatings based on PCL point cloud processing

Anti-Sintering Behavior of GYYSZ, Thermophysical Properties, and Thermal Shock Behavior of Thermal Barrier Coating with YSZ/Composite/GYYSZ System by Atmospheric Plasma Spraying

Anti-Sintering Behavior of GYYSZ, Thermophysical Properties, and Thermal Shock Behavior of Thermal Barrier Coating with YSZ/Composite/GYYSZ System by Atmospheric Plasma Spraying

Synthesis and characterization of the RE2A2O7 oxides with an ultrahigh-entropy sublattice occupied by rare-earth elements

The possibility of fabricating novel high-entropy oxides (HEOs) with one ultrahigh-entropy sublattice consisting of 13 cations in RE2A2O7 (RE = [La, Sm, Nd, Gd, Dy, Yb, Y, Er, Eu, Tb, Ho, Lu, Tm], A = Zr, Hf, Ti, Sn, Ce or Pr) is demonstrated. All the samples exhibited a pure pyrochlore-type or defective fluorite crystalline structure with various degrees of cation disordering. Particularly, RE2Ti2O7 and RE2Sn2O7 exhibited a pure pyrochlore-type structure with high degree of ordering, while RE2Ce2O7 and RE2Pr2O7 demonstrated a single fluorite type HEO structure. The results pave the way to fabricate RE2A2O7 HEOs with an ultrahigh-entropy sublattice and potential application as the ceramic layer in thermal barrier coatings.

A DFT exploration of the stabilities, physical properties, and tensile strength of new synthesized Nb2AC (A=Ni and Co) MAX phases

Based on first-principles calculations, this study explored the structural stability, elastic anisotropy, tensile strength, and the mechanical, electronic, and thermodynamic properties of the newly synthesized MAX phases Nb2NiC and Nb2CoC. It has been found that these compounds are thermodynamically and mechanically stable, exhibit metallic conductivity, and possess ductile nature. The ultimate tensile strengths of Nb2NiC and Nb2CoC depend on their crystallographic directions, specifically [0001] and [112̄0]. In the [0001] direction, Nb2CoC has a tensile strength of about 36.63 GPa at a strain of 26%, compared to Nb2NiC, which has a tensile strength of 31.70 GPa at a strain of 24%. In the [112̄0] direction, Nb2CoC exhibits a tensile strength of around 23.29 GPa at a strain of 12%, while Nb2NiC has a tensile strength of approximately 18.51 GPa at an strain of 8%. Both Nb2CoC and Nb2NiC demonstrate significant elastic deformation before reaching their ultimate tensile strengths, indicating good ductility. It is noteworthy that Nb2NiC is less elastic than Nb2CoC in both the [0001] and [112̄0] directions, as the elastic constants of Nb2CoC are comparatively higher than those of Nb2NiC. Furthermore, the estimated thermal parameters show that these compounds exhibit a relatively low Debye temperature, a high melting point, low minimum thermal conductivity, and thermal expansion coefficient values that are similar to those of well-established thermal barrier coating (TBC) materials such as Al2O3, LaPO4, and TiO2. Consequently, the newly synthesized MAX phases Nb2NiC and Nb2CoC are promising candidates for TBC applications.

Assessment of dwell-fatigue properties of nickel-based superalloy coated with a multi-layered thermal and environmental barrier coating

A three-layered experimental thermal and environmental barrier coating (TEBC) was deposited using air plasma spraying technology on cylindrical specimens of nickel-based superalloy MAR-M247. TEBC consists of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) upper layer, Y2O3 stabilised ZrO2 interlayer and CoNiCrAlY bond coat deposited on the grit-blasted surface of MAR-M247. The cyclic plastic response and damage mechanisms in uncoated and TEBC-coated MAR-M247 have been studied in isothermal dwell-fatigue tests conducted under strain control with constant strain amplitude at 900 °C. In each cycle, 5-minute dwells were introduced in both tensile and compression peaks of the hysteresis loop. Fatigue hardening/softening curves, stress relaxation curves, cyclic stress-strain curves and fatigue life curves are reported. Ceaseless mild softening has been found in both uncoated and TEBC-coated MAR-M247. TEBC-coated MAR-M247 showed an improved lifetime in the whole range of tested strain amplitudes. Data obtained from stress relaxation curves were used to assess the fraction of creep damage. The generalised damage accumulation rule was used to evaluate damage due to fatigue-creep-environment interaction. A study of the surface relief and internal microstructure using SEM and TEM helped to interpret the specifics of fatigue behaviour of uncoated and TEBC-coated material. The effectiveness of this newly developed TEBC, together with the dwell sensitivity of MAR-M247, were discussed from the perspective relevant to dwell-fatigue cyclic straining.

Investigation of the CMAS thermal corrosion resistance of novel Al2O3/YTaO4 thermal barrier coatings

Investigation of the CMAS thermal corrosion resistance of novel Al2O3/YTaO4 thermal barrier coatings

Thermal life assessment of laser powder-directed energy deposited NiCrAlY/CuCrZr bimetallic clad for rocket nozzle applications

To enhance the thermal life of rocket exhaust nozzles, the hot side of copper liners is coated with thermal barrier coatings (TBCs) to provide thermal insulation and oxidation resistance. However, interface failures often occur between M-CrAlY bond coats and nozzle liners due to significant differences in their thermal expansion coefficients (CTE). This study explores the use of Laser Powder-Directed Energy Deposition (LP-DED) to clad NiCrAlY onto a CuCrZr substrate, as the process offers localized heating which can offer better bond strength. Optimization trials were conducted using single and multi-track studies to identify optimal parameters. Due to the low energy absorption of the CuCrZr substrate to 1070 nm laser sources, cladding was performed at a high energy density of 135 J/mm2 with a 1.2 g/min feed rate to achieve defect-free clads with sufficient diffusion. The bulk of the NiCrAlY clads showed γ′-Ni3Al, β-NiAl, and γ-Ni phases, while Y4Al2O9 and Y2O3 oxides formed on the top surface due to aluminum and yttrium depletion at high temperatures. The clads exhibited cellular dendritic microstructures at the bulk region, and planar microstructures were observed at the dilution zone. EBSD-KAM maps showed higher dislocation density near the interface due to CTE mismatch across substrate and clad. Scratch tests confirmed strong adhesion with no interface cracks, though crack propagation was observed from the edges after 50 isothermal cycles, driven by copper erosion. With Cu diffusion, interface region exhibited a graded microstructure which could enhance CTE, improving compatibility compared to standard NiCrAlY alloys.

Sustainable synthesis of pyrochlore gadolinium zirconate through the molten salt method: A green approach

Gadolinium zirconate (GZO) with pyrochlore phase exhibits exceptional promise for thermal barrier coatings due to its notable low thermal conductivity. Current synthesis methods for GZO, however, are frequently complex, energy-intensive, and environmentally hazardous. A more sustainable and eco-friendly synthesis route for GZO is thus imperative. This research introduces a green synthesis process for pyrochlore GZO utilizing the molten salt method.Experimental synthesis at varying temperatures revealed changes in the phase composition of GZO via the molten salt method. Fluorite GZO formed at 1200 °C, while pyrochlore GZO appeared at 1400 °C. XRD refinement, TEM, and Raman spectroscopy confirmed these results. Density Functional Theory (DFT) calculations established the minimum theoretical synthesis temperature for pyrochlore GZO at 1370 °C. Limiting factors for reactions at different temperatures during the molten salt synthesis process were identified. DFT calculations and experiments verified that the fluorite structure GZO transitions to pyrochlore GZO at temperatures exceeding 1370 °C. Property tests on pyrochlore GZO samples synthesized through two pathways showed minimal differences. This study thus lays a theoretical foundation for the eco-friendly synthesis of pyrochlore GZO material using the molten salt method and offers insights for future process development.

Microstructural evolution and high temperature hot corrosion behaviour of AlCoCrFeNiTi high-entropy alloy coatings

The fuels used contain undesirable impurities such as Na, V, K, S, and Mg in gas turbine engines. These impurities react with hot components under service conditions, leading to the deterioration of the thermal barrier coatings (TBCs) structure and the formation of hot corrosion damage. In this study, metallic powders containing CoNiCrAlY were deposited as bond coat on Inconel 718 superalloy material using high velocity oxygen fuel (HVOF) coating technique. High entropy alloy (HEA) powder containing AlCoCrFeNiTi was synthesized as coating material using mechanical alloying (MA) method. AlCoCrFeNiTi powders were produced using atmospheric plasma spray (APS) coating technique. The produced coating system was subjected to hot corrosion tests at 700 °C and 800 °C temperatures for different time periods in corrosive environment conditions containing Na2SO4 (sodium sulfate) and V2O5 (vanadium pentoxide). Hot corrosion behavior of the coating system was determined, damage formations and degradation processes were investigated in detail. As a result of the determination and synthesis of HEA powder materials, production of coatings, experimental studies and hot corrosion test studies under service conditions, it was seen that the Ti-containing AlCoCrFeNi HEA coating system has usable qualities without any damage such as cracking, buckling or spalling.

Evaluation of AlCoCrFeNiTi-high entropy alloy (HEA) as top coat material in thermal barrier coating (TBC) system and investigation of its high temperature oxidation behavior

Evaluation of AlCoCrFeNiTi-high entropy alloy (HEA) as top coat material in thermal barrier coating (TBC) system and investigation of its high temperature oxidation behavior

Effect of laser cladding layers on microstructure and mechanical properties of NiCoCrAlY bond coat

The NiCoCrAlY alloy is usually used as a bonding layer material in thermal barrier coatings (TBCs) system. Owing its excellent high-temperature oxidation resistance and corrosion resistance, it plays a crucial role in maintaining the structural integrity and functionality of TBCs. Laser cladding technology is a flexible and efficient processing technique for the manufacturing of NiCoCrAlY bond coat, with the characteristics of precisely control the layers of the laser cladding and further modulate the quality of the bond coat. Therefore, the investigation on the influence of cladding layers on the microstructure and mechanical properties of the bond coat is of great significance for improving the service performance and lifetime of aircraft engines. To this end, the scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used in this study to explore the influence of cladding layers on the microstructure and mechanical properties of the NiCrAlCoY bond coat. The results indicate that the microstructure of the cladded alloy can be divided into substrate, heat-affected zone, and cladded zone. The hardness of both the heat-affected zone and the cladded zone increases as the number of cladding layers increases, while the hardness of the substrate maintains constant. In addition, as the number of laser cladding layers increases, so does the residual stress of the cladded alloy.

Achieving durable double-layered thermal barrier coatings by tailoring multi-scale structures

Achieving durable double-layered thermal barrier coatings by tailoring multi-scale structures

Failure behavior study of EB-PVD TBCs under CMAS corrosion and thermal shock cycles

Abstract Calcia-magnesia-alumino-silicate (CMAS) erosion has become a major obstacle, limiting the operating temperature and service life of Thermal barrier coatings (TBCs) in aircraft engines. Constructing simulation environments that replicate TBCs’ working conditions and exploring online, non-destructive detection techniques are reliable approaches to studying coatings’ failure, representing both a global research hotspot and a challenge in this field. The paper presents an initial endeavor to establish a simulation experiment for TBCs in aviation-engine within a CMAS environment. Experimental results show that electron beam physical vapor deposition (EB-PVD) Y2O3-stabilized ZrO2 (YSZ), one of the mainstream TBCs technologies, produced 20% surface spallation after 50 thermal-shock cycles under simulated CMAS corrosion conditions. Testing and analysis of the macroscopic and microscopic structures of the failed samples, combined with SEM, EDS, and XRD findings, revealed significant physical and chemical interactions between the ceramic layer and CMAS deposits, as well as phase transformation within the coatings, leading to substantial alterations in mechanical properties and ultimately causing the failure of EB-PVD YSZ.

Comparison of microstructural evolution and high temperature oxidation behavior of AlCoCrFeNiTi and AlCoCrFeNiZr high-entropy alloy coatings

In this study, the isothermal oxidation behaviors of HEA coating systems with AlCoCrFeNiTi and AlCoCrFeNiZr contents were evaluated by comparing microstructurally and investigated in terms of potential applications of thermal barrier coatings (TBCs). For this reason, their oxidation resistance under isothermal conditions, as well as diffusion phenomena occurring at the bond and top coating interface, were investigated. CoNiCrAlY bond coating was applied to the surface of a Nickel-based superalloy substrate by high velocity oxygen-fuel (HVOF) spraying. HEA coating materials were produced by using the mechanical alloying (MA) process. The mechanical activated synthesized HEAs were sprayed on a CoNiCrAlY bond coating by Atmospheric Plasma Spraying (APS). The microstructural changes at the coating interface of CoNiCrAlY, AlCoCrFeNiTi and AlCoCrFeNiZr (HEAs) coatings resulting from oxidation processes at 1200 °C for 5, 25, 50 and 100 h, and the formation and growth behaviors of the thermally grown oxide (TGO) layer were investigated. The present results demonstrate a potential of HEAs in TBC applications. After oxidation tests, crystal lattice transformations took place in both coating systems. In the AlCoCrFeNiZr-HEA TBC system, an increase in diffusion rate, oxygen permeability, and consequently TGO layer thickness was observed by the transformation from the cubic lattice structure to a monoclinic lattice structure with a larger volume. In the AlCoCrFeNiTi-HEA TBC system, the transformation of a narrow rhombohedral lattice with tight planes was maintained. Therefore, the TBC system with AlCoCrFeNiTi-HEA has a lower TGO layer thickness compared to the TBC system with AlCoCrFeNiZr-HEA and has a longer service life against oxidation failure at high operating temperatures. As a result of the production, experimental studies and high temperature oxidation test studies under service conditions, it was seen that AlCoCrFeNiTi and AlCoCrFeNiZr-HEA coatings can be used in the structure of TBC systems.

Intelligent temperature measuring thermal spray multilayer thermal barrier coatings based on embedded thin film thermocouples

Thermal barrier coatings (TBCs) have garnered significant attention as crucial protective components for turbine blades. However, the current use of TBCs is limited by their singular functionality and the inability to accurately obtain the temperature gradient distribution within the coatings. Addressing the aforementioned issues, this paper proposes an intelligent thermal barrier coating embedded with thin-film thermocouples. This method not only provides effective thermal protection but also facilitates the precise measurement of the internal temperature gradient within the coating. To mitigate the thermal mismatch in TBCs under high-temperature environments, which can compromise their lifespan, this study employs multi-objective optimization of structural parameters to design an optimal coating thickness. This strategy ensures both superior thermal protection and extended service life. The intelligent temperature-sensing TBCs were fabricated using atmospheric plasma spraying and magnetron sputtering, followed by comprehensive characterization. To validate the performance of the intelligent temperature-sensing TBCs, static tests were conducted in a muffle furnace. The results demonstrated that the sensors exhibit excellent repeatability and high-temperature durability. Furthermore, a test platform replicating the thermal shock conditions of an engine environment was developed. This platform confirmed that the intelligent temperature-sensing TBCs are capable of accurately measuring the internal temperature gradient within the coating under engine-like conditions, offering a novel methodology for engine monitoring and diagnostics.

Data-driven AI for the automated classification of the isothermal heat-treated thermal barrier coatings using pulsed infrared thermography

Abstract Thermal barrier coating (TBC) is a multilayer coating applied to metallic structures exposed to high temperatures, such as aero-engine parts and gas turbine blades. These thermally insulating materials are used to extend the component life by minimizing the high-temperature exposure of structural components. As a result of the high-temperature oxidation tests, a thermally grown oxide (TGO) layer formed at the interface between the ceramic topcoat layer and the bond coat layer. The primary cause of TBC performance decline and structure failure is the growth of the oxide layer. Hence, it is crucial to regularly monitor the formation of the oxide layer and the degradation of coatings. This can be achieved by developing coating-life prediction models, which are vital for quality control, health monitoring of TBCs, maintenance decision-making, and the prevention of catastrophic failures. Existing methods for life estimation, which are dependent on empirical methods and microstructural analysis, often fail due to various material compositions and service conditions. This study proposes an automated classification of iso-thermal heat-treated TBC samples using temperature data, which helps in predicting the TBC life and monitoring the TBC degradation. TBC-coated samples are isothermal heat-treated at 1000 °C, and the initial growth of TGO is monitored using a non-destructive thermal imaging technique. Identifying distinctive features in each class corresponding to layer thickness measurement and service hours is challenging and time-consuming. So, we integrate data-driven AI models and feature extraction techniques to interpret complex thermal patterns. The performance of deep learning models and classical machine learning models demonstrates that our method can achieve maximum classification accuracy and F1-score of 89.2% and 89.0%, respectively. This automated classification framework promises a powerful tool for predictive maintenance and remaining useful lifetime estimation of hot section components reliant on TBCs.

Preparation of a polyurethane‐coated expandable graphite and its flame retardant application in rigid polyurethane foams

AbstractExpandable graphite (EG) is widely used in rigid polyurethane (PU) foam (RPUF) as flame retardant, however, the mechanical properties of RPUF would deteriorate with the increase of its content. To alleviate this problem, PU‐coated EG (PUEG) is prepared by coating PU on the surface of EG via interface polymerization. PUEG is characterized by FTIR, XPS, and SEM. The results show that PUEG is successfully prepared. Compared with the RPUF add with the pure EG, the compressive strength and flame retardancy of the RPUF prepared by adding 15% PUEG have been improved to varying degrees. A 15% PUEG and 20% PUEG modify RPUFs obtained both UL‐94 V‐0 rating with LOI values of 29.7% and 31.2%, respectively. The improvement in the flame retardancy and compressive strength of RPUF is due to the good interfacial compatibility between the RPUF and PU on the surface of the EG and the increased coking rate of the EG, which accelerates the formation of the thermal barrier carbon layer. The preparation process of PUEG is simple and suitable for large‐scale production.