Durability Of Thermal Barrier Coatings Research Articles

Unveiling thermal stresses in RETaO4 (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho and Er) by first-principles calculations and finite element simulations

Thermal stress (σ) plays a critical role in regulating the stability and durability of thermal barrier coatings (TBCs) during service. However, its measurements are limited due to technical challenges. Here, thermal stresses in RETaO4 (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) TBCs have been evaluated by a cross-scale model, integrating first-principles calculations and finite element simulations (FEM). The isobaric heat capacity, thermal expansion coefficients, density, Young's modulus, and Poisson's ratio of RETaO4 as a function of temperature have been determined by the quasi-harmonic approach (QHA) and the quasi-static approach (QSA). Taking these properties as the input data of FEM, the σ of TBCs with RETaO4 as ceramic coatings is estimated. It indicates that the maximum tensile stress exists at the ceramic and thermal-grown oxide coating interface. Notably, TBCs_NdTaO4 exhibits the highest tensile stress, reaching up to 2093 MPa, while TBCs_GdTaO4 has the lowest value at 1880 MPa. Furthermore, the key factors affecting σ are estimated using interpretable machine learning based on the decision tree algorithm. The RETaO4 with small Poisson's ratio, strong electronegativity, small heat capacity, and thermal expansion coefficients is helpful to achieving low σ in TBCs. GdTaO4, YTaO4 and (Ho0.5Er0.5)TaO4 emerge as more favorable options as ceramic layer due to their lower σ in TBCs. This present cross-scale model provides a successful tool for predicting σ and the reverse design of TBCs materials based on low σ in TBCs.

High temperature hardness and thermal analysis of CoNiCrAlY alloys used as bond coats for thermal barrier coatings

CoNiCrAlY alloys, commonly used as the bond coat of thermal barrier coatings (TBCs), are often exposed to high temperatures of about 1000 °C, the evolution of mechanical and thermal properties is critical to the reliability and durability of TBCs. In this paper, the hardness and thermal properties (weight change and heat flow) of CoNiCrAlY alloys at temperatures within 1000 °C were evaluated by indentation and TGA-DSC thermal analysis. It was found that hardness and thermal properties change gradually within 600 °C but change sharply above 600 °C, exhibiting an approximately synchronized evolution. An illustrative explanation was given for the correlation in terms of the Boltzmann energy distribution theory of microscopic particles. Based on these findings, an idea of inter-calibration between the two evolutions was proposed.

The Oxidation Behaviors of NiCoCrAlY Coatings After Pre-Oxidation Treatment During High-Temperature Oxidation at 800 ℃ and 900 ℃

Thermal barrier coatings (TBCs) are essential for protecting the high-temperature components in gas turbines. However, the durability of TBCs is limited, and new technologies to improve their lifetime are necessary. This study focuses on the pre-oxidation treatment of the NiCoCrAlY bond coat in TBCs to reduce the growth rate of the thermally grown oxide (TGO) and improve the TBC lifetime. The mechanism of the TGO growth rate reduction by the pre-oxidation treatment of NiCoCrAlY was investigated. Oxidation behaviors of the surface of NiCoCrAlY coating samples with or without pre-oxidation treatment were evaluated in air at 800 °C and 900 °C. In-situ X-ray diffraction (XRD) measurements and isothermal oxidation tests were performed. The obtained results revealed that the growth rate of TGO is significantly suppressed by α-Al2O3 layer formed by the pre-oxidation treatment. The generation of γ-Al2O3 and θ-Al2O3, which are typically formed below 900 °C, was significantly reduced by pre-oxidation. It is suggested that this reduction suppresses of the TGO growth.

Failure behaviors of EB-PVD thermal barrier coatings under isothermal oxidation and thermal cycling conditions

The thermal growth oxide (TGO) layer formed by oxidation of the bond coat (BC) layer is an important factor affecting the durability of thermal barrier coatings (TBCs). In this study, a series of isothermal oxidation and thermal cycling experiments were performed on samples containing TBCs at different temperatures. The initiation, propagation and connection process of cracks in the cross-section of the samples were studied, and the oxidation failure mechanism of the samples was analyzed. The results demonstrate that the diffusion of the Al element in NiCoCrAlYHf coating to the top coat (TC) layer promotes the formation and rapid thickening of TGO at TC/BC interface. With the consumption of Al, two sublayers of mixed oxide layer and Al2O3 layer are gradually formed in the TGO layer. The TC/TGO interface is the main initiation location of interface crack. In addition, the types and initiation locations of cracks caused by isothermal oxidation are different from those caused by thermal cycling. Under isothermal oxidation, the TC layer near the TC/TGO interface and the TC/TGO interface are prone to induce horizontal cracks, which finally lead to the delamination and spalling of TBCs. Under the thermal cycling, vertical cracks occur in the TC layer, extend towards the TC/TGO interface, and merge with the interface cracks, eventually resulting in the delamination failure of the TBCs.

SEM-Guided Finite Element Simulation of Thermal Stresses in Multilayered Suspension Plasma-Sprayed TBCs

This study presents novel insights into thermal stress development and crack propagation mechanisms in single- and multilayered suspension plasma-sprayed (SPS) coatings of gadolinium zirconate (GZ) and yttria-stabilized zirconia (YSZ), thermally treated at 1150 °C. By combining image processing with finite element simulation, we pinpointed sites of high-stress concentration in the coatings, leading to specific cracking patterns. Our findings reveal a dynamic shift in the location of stress concentration from intercolumnar gaps to pores near the top coat/thermally grown oxide (TGO) interface with TGO thickening at elevated temperatures, promoting horizontal crack development across the ceramic layers. Significantly, the interface between the ceramic layer and TGO was found to be a critical area, experiencing the highest levels of both normal and shear stresses. These stresses influence failure modes: in double-layer SPS structures, relatively higher shear stresses can result in mode II failure, while in single-layer systems, the predominant normal stresses tend to cause mode I failure. Understanding stress behavior and failure mechanisms is essential for enhancing the durability of thermal barrier coatings (TBCs) in high-temperature applications. Therefore, by controlling the interfaces’ roughness along with improving interfacial toughness, the initiation and propagation of cracks can be delayed along these interfaces. Moreover, efforts to optimize the level of microstructural discontinuities, such as intercolumnar gaps and pores, within the creaming layer and close to the TGO interface should be undertaken to reduce crack formation in the TBC system.

Integrating APS TBCs with a built-in Al2O3 protection network for superior CMAS resistance

Calcium–magnesium–alumina–silicate (CMAS) attack poses a great threat to the durability of thermal barrier coatings (TBCs). We introduce a novel Al2O3-modified TBC architecture to counter this threat. The architecture integrates an Al2O3 protection network into the microstructure of an atmospheric plasma sprayed (APS) TBC by drip coating method, which modifies the vulnerable infiltration pathway of CMAS melt and promotes the self-crystallization of CMAS at the infiltration front. Moreover, during sintering, the built-in Al2O3 can inhibit inter-splat crack healing. This innovative TBC design achieves robust CMAS resistance and strain tolerance, while also offering advantages during sintering.

CMAS corrosion resistance of YSZ thermal barrier coatings enhanced by Pt–Al films

Corrosion of calcium magnesium aluminum silicate (CMAS) severely affects the durability of thermal barrier coatings (TBCs). In this study, 8 wt% Y2O3 partially stabilized ZrO2 (8YSZ) coatings were prepared using atmospheric plasma spraying (APS) technique, and a Pt–Al film was magnetron-sputtered onto the surface of the YSZ coating (Pt–Al-YSZ) to enhance its resistance against CMAS. The results indicated that the Pt–Al-YSZ coating displayed superior non-wetting performance against CMAS, as well as stronger resistance to CMAS corrosion. The results evidenced by Raman spectra and X-ray diffraction showed that phase transformation was smaller and corrosion depth was lower after CMAS corrosion at 1250 °C for 2 h. The thermal cycling test was also performed. The result demonstrated that the Pt–Al coating did not have a negative impact on the thermal cycling life of the YSZ TBCs. However, platinum tended to soften at high temperatures and became easily penetrated by molten CMAS. Therefore, it is necessary to further improve the strength of platinum layer to fully utilize its protective effect against CMAS attack.

Peridynamics–FEM Coupling for Interfacial Delamination Effected by Vertical Crack Density in Thermal Barrier Coatings

A coupling model of peridynamics and finite element method is proposed to study the interfacial delamination influenced by vertical crack density in thermal barrier coating (TBC) systems. Specifically, the progressive failure progress under static and fatigue loads in TBCs, including vertical cracks propagation, the evolution of vertical cracks to interfacial cracks, and interfacial delamination, is simulated by the proposed model. The difference between static failure mechanism and fatigue failure mechanism of TBCs is numerically elucidated. The simulated fracture morphology is in good agreement with the experiential observation. In both static and fatigue loads, a higher vertical crack density is found to correspond to a shorter delamination length, and there is no interfacial delamination when the vertical crack density is high enough. The results provide important insight of vertical crack density on interfacial delamination, and the durability of TBCs can be enhanced by ensuring an appropriately high vertical crack density.

Corrosion behaviour of La2Hf2O7 thermal barrier coatings against molten CMAS melts at 1300 and 1500 °C

Thermal barrier coatings (TBCs) are susceptible to accelerated degradation caused by molten calcium-magnesium-alumino-silicate (CMAS), which exhibits a significant challenge to design and develop durable TBCs. Rare-earth hafnates are regarded as promising TBCs candidates. In this study, the corrosion behaviour of plasma-sprayed La2Hf2O7 coating at 1300 and 1500 °C was investigated and compared with the traditional YSZ coating. The study indicated that the La2Hf2O7 exhibited better corrosion resistance than the YSZ. During the corrosion process, La2Hf2O7 coating dissolved into CMAS melts and reprecipitated m-HfO2 and apatite which effectively mitigate the corrosion, while the YSZ coating only produced ZrO2 with a transition from tetragonal to monoclinic phase. The recession depth of the coatings increases noticeably when the reaction temperature raised from 1300 to 1500 °C. The YSZ coating failed at 1500 °C, while the La2Hf2O7 coating still remained intact. This study may provide a theoretical basis for selecting and design of rare-earth hafnates TBCs with excellent corrosion resistance.

Recent Development in Advance Ceramic Materials and Understanding the Mechanisms of Thermal Barrier Coatings Degradation

AbstractMetallic alloys' behavior at high temperatures, especially their response to corrosion and formation of protective surface layers, has long been a focus of scientific inquiry. Although certain alloy compositions require an initiation period before hot corrosion advances to the propagation stage, no combination of alloys can be considered impervious to hot corrosion indefinitely. The capacity of nickel-based materials to tolerate extreme circumstances such high temperatures, acidity, corrosion, and scratching is highly valued. However, they are unable to satisfy the strict demands of today's high-temperature applications. The durability of thermal barrier coatings (TBCs), which are prone to oxidation, rust, and degradation from sulphates and foreign object damage, has been the subject of recent study. For sophisticated ceramic materials exposed to high temperatures, hot rust degradation poses a considerable challenge. The main objective of this study is to investigate the effects of severe degradation on several advanced ceramic material types and their level of advancement. The purpose of the inquiry is to comprehend the deteriorating processes at the long term working condition, including the function of oxidation and liquid salts. Additionally, we investigate the effects of temperature, environment, and contact duration on the heated weathering behavior of earthenware. Finally, we discuss strategies for mitigating hot corrosion degradation in ceramics, such as protective coatings like new design of TBCs, doping, and composition optimization. This paper aims to offer a thorough understanding of the hot corrosion behavior of ceramics, which is crucial for developing durable materials suitable for high-temperature applications. Additionally, it explores the fabrication of protective coatings and addresses the challenges faced in this regard. The insights gained from this research can contribute to the advancement of resilient ceramic fabrics and the development of effective protective coatings.

Service life assessment of yttria stabilized zirconia (YSZ) based thermal barrier coating through wear behaviour

Countless research has suggested Yttria-stabilized Zirconia (YSZ) to be a top candidate for being implemented as thermal barrier coatings (TBC). However, when exposed to prolonged service, temperature and stress variations succeed in initiating a catastrophic phase transformation from tetragonal to monoclinic structure in Zirconia. Hence, the estimation of endurance for YSZ-based TBC is necessary to minimize failure in such situations. The main purpose of this research was to determine the relationship between tribological investigations and the estimated lifespan of YSZ coatings accurately. The study used various methods such as wear resistance testing, optical profilometry, specific wear rate, and coefficient of friction to estimate the maximum durability of TBCs. The research also provided insights into the composition and microstructure of the TBC system and found the optimized concentration of Yttrium doping to be 3.5 wt %. The study discovered that erosion was the main cause of roughness depreciation from SN to S1000. The estimation of the service life was primarily made based on optical profilometry, specific wear rate (SWR), coefficient of friction (COF) and wear resistance values which were further supported by the results of chemical characterization of the samples through electron dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS) and X-Ray Diffraction (XRD) analysis. The results were reliable and accurate and suggested future areas of investigation, such as 3D profilometry for surface roughness and thermal conductivity evaluation using laser-assisted infrared thermometers.

Novel Thermal Barrier Coatings with Phase Composite Structures for Extreme Environment Applications: Concept, Process, Evaluation and Performance

In this paper, a novel concept in the field of phase composite ceramics has been proposed and applied for creating the topcoats of durable thermal barrier coatings (TBCs), which is one of the most critical technologies for advanced high-efficiency gas turbine engines in extreme environments. The phase composite ceramic TBCs were designed to demonstrate superior and comprehensive performance-related merits, benefits, and advantages over conventional single-phase TBCs with a topcoat of 8YSZ or Gd2Zr2O7, including thermal phase stability, thermal shock durability, low thermal conductivity, and solid particle erosion resistance. In this paper, we review and summarize the development work conducted so far related to the phase composite ceramic concept, coatings processing, and experimental investigation into TBC behaviors at elevated temperatures (typically, ≥1250 °C) using different evaluation and characterization methods, including isothermal sintering, a burner rig test, a solid particle-impinging erosion test, and a CMAS corrosion test. Two-phase (t’+c) zirconia-based TBCs demonstrated improved thermal shock and erosion resistance in comparison to conventional single-phase (t’), 8YSZ TBC, and Gd2Zr2O7 TBC, when used separately. Additionally, a triple-phase (t’+c+YAG) TBC sample demonstrated superior CMAS resistance. The TBC’s damage modes and failure mechanisms for thermal phase stability, thermal cycling resistance, solid particle erosion behavior, and CMAS infiltration are also characterized and discussed in detail, in terms of microstructural characterization and performance evaluation.

Study of toughening behavior of SiC whiskers on 8YSZ thermal barrier coatings

The embrittlement of thermal barrier coatings (TBCs) in a high-temperature service environment inhibits the ability of the top-coat to resist crack initiation and propagation, in which the cracks are generated by erosion or wear of particles, limiting the service durability of TBCs. This study was based on the preparation of SiC whiskers (SiCw) / Y2O3 partially stabilized ZrO2 (YSZ) integrated composite powder, and the SiCw/YSZ composite TBCs was prepared through the atmospheric plasma spraying (APS) to improve the toughness of the top-coat. The effects of SiCw on the fracture toughness, tribological properties, erosion resistance and bending strength of the coatings were studied. The results showed that the fracture toughness of the composite coating with a SiCw length-to-diameter ratio (L/D ratio) of 250 was 24.6 % higher than the YSZ coating. Moreover, the composite coating presented excellent performance in wear, erosion, and resistance to crack propagation and spallation. The low amplitude of the Weibull modulus m of hardness distribution of the doped coating proved the stability effect of SiCw on the reinforcement and toughening of the coating.

Methods for characterizing the interfacial adhesion in thermal barrier coating systems

Thermal barrier coatings (TBC) are widely used to protect the hot section structural components, such as aircraft gas turbine, against hot corrosion and large thermal gradients during the service life. The resistance to interfacial debonding (adhesion) is one of the key factors for predicting the TBC durability during operation. The goal of the study is analysis of the existing approaches to quantification of the interfacial adhesion for multi-layered thermal barrier coating systems and thermally-grown oxide on Ni-based superalloys and discussion of the problem regarding the reasons for a large spread of the adhesion characteristics. Theoretical prerequisites for determining the adhesion characteristics are considered to provide a theoretical background for quantification of the interfacial adhesion. The absence of national standard tests applicable to TBC is marked along with a significant scatter of the values obtained using international test-regulating documents and proprietary technologies. The values of the intensity of the strain energy release are found to vary from 0.3 to 230 J/m2 depending on the method for measuring adhesion, experimental details and type of the system considered. The factors that determine a large spread of the interfacial adhesion values were divided into two groups: 1 — intrinsic, attributed to the features of TBC layer formation and temperature affecting the value and distribution of residual strains and stresses and direction of the failure initiation; 2 — extrinsic, which are related to test conditions and technique of experimental data processing. A complex approach involving both intrinsic and extrinsic parameters is required to address the problem of interfacial adhesion quantification.

Critical scale grain size for optimal lifetime of TBCs

• The critical scale grain size d c of ∼2.5 μm is identified for optimal TBC lifetime. • When d < d c , the rapid growth of the scale causes delamination failure of TBCs. • When d > d c , the growth stress causes vertical cracking and early failure of TBCs. Thermal barrier coatings (TBCs) suffer from spalling failure resulting from the rapid growth of thermally grown oxides (TGO) on the metallic bond coat. In this study, to achieve the optimal lifetime of TBCs, a lifetime phase diagram was constructed for the entire service of TBCs. Herein, both the thermal mismatch stress during thermal cycling and the TGO growth stress during thermal exposure were considered. First, thickening behavior of the Al 2 O 3 scale related to grain size was investigated. Second, transverse delamination failure due to the thermal mismatch stress was analyzed. Third, vertical cracking failure related to early scale growth was explored. Finally, a critical scale grain size of ∼2.5 μm was identified for achieving the optimal lifetime of TBCs. It was experimentally proved that vertical cracking of the Al 2 O 3 scale occurred during early oxidation as the scale grain size was larger than ∼4 μm after pre-oxidation. The proposed lifetime phase diagram and critical scale grain size effect serve as novel criteria for the design of next-generation durable TBCs.

Influence of non-uniform feature of thermally grown oxide thickness on the local stress state and cracking behavior in TBC

The growth of thermally grown oxide (TGO) at high temperatures is the main reason for inducing the cracking near the YSZ/bond coat interface in the thermal barrier coating (TBC). The non-uniform thickening of TGO can change the local stress state and crack evolution. In this work, a TBC model including non-uniform TGO is established to explore the effect of non-uniform feature of TGO on the local cracking behavior. The continuous growth of TGO is simulated by applying a stress-free strain. The arbitrary path propagation of cracks in YSZ and TGO is realized by the XFEM method. The influences of TGO non-uniform degree and lateral growth rate on the stress variation and crack evolution are discussed. The results show that the non-uniform growth of TGO increases the local oxide film thickness and decreases the amplitude of TGO/bond coat interface, which leads to a greater crack driving force than TGO uniform growth. The increase of TGO non-uniform degree will induce longer TGO crack. The coating life predicted by the uniform TGO model is overestimated. The increase of TGO lateral growth strain leads to the improvement of YSZ crack driving force and TGO crack length, which is independent of TGO non-uniform degree. The results in this study indicate that obtaining uniform TGO with a low growth rate by process optimization will be an important option to improve the durability of TBC.

Fracture-based transient thermo-mechanical analysis of reciprocating engine thermal barrier coatings

The durability of thermal barrier coatings for reciprocating internal combustion engine applications was investigated using a coupled, unsteady fracture-based thermomechanics model. The intra-cycle engine heat flux varies substantially in magnitude and has frequency similar to that of the coating thermal timescale, giving rise to relatively large spatial temperature nonuniformity within the coating that varies temporally. A finite difference scheme was used to calculate the temperature field. An analytical model that assumes equi-biaxial stress followed by plane strain delamination was used to evaluate an energy release rate that represents the driving force for cracking. Because the location and time of the peak energy release rate cannot be established a priori, they were computed for all times in the cycle. The peak energy release rate was found to occur later in the cycle than the peak surface temperature, and was often located within the coating as compared to at the coating–substrate interface where there was a mismatch in coefficient of thermal expansion. Coating thickness was found to affect both the magnitude of the peak energy release rate and its location. The residual stress, which arises from the coating deposition process, was found to have a significant effect on the peak energy release rate, and may be a process parameter that can be used to promote coating life. The computational framework demonstrated insight regarding the trade-off between engine thermodynamic performance and coating durability.

Thermophysical performances of high-entropy (La0.2Nd0.2Yb0.2Y0.2Sm0.2)2Ce2O7 and (La0.2Nd0.2Yb0.2Y0.2Lu0.2)2Ce2O7 oxides

Two High-entropy ceramics, (La 0.2 Nd 0.2 Yb 0.2 Y 0.2 Sm 0.2 ) 2 Ce 2 O 7 (LNYSC) and (La 0.2 Nd 0.2 Yb 0.2 Y 0.2 Lu 0.2 ) 2 Ce 2 O 7 (LNYLC) were synthesized using a sol-gel technique and sintering at high temperatures. The lattice type, micro-morphology, and thermophysical and mechanical properties of these oxides were investigated. The results showed that the obtained high-entropy powders and bulk samples have single fluorite lattice, dense microstructure, and clear grain boundary. These oxides have less thermal conductivities than (7YSZ), and the thermal conductivity of LNYLC is lower than that of LNYSC. The thermal expansion coefficients of these high-entropy oxides are larger than that of 7YSZ. Owing to the relative less negative electrical difference in LNYLC that in LNYSC, the thermal expansion coefficient of LNYLC is larger than that of LNYSC. Further, these oxides exhibit outstanding lattice stability up to 1200 °C, which can help to improve the durability of thermal barrier coatings. However, these oxides have lower mechanical properties compared to 7YSZ.

Phase stability, thermal shock behavior and CMAS corrosion resistance of Yb2O3-Y2O3 co-stabilized zirconia thermal barrier coatings prepared by atmospheric plasma spraying

Rare-earth element doping has been extensively employed as one of the effective methods to enhance the durability of thermal barrier coatings (TBCs). In present study, ytterbia and yttria co-stabilized zirconia (YbYSZ) coatings and corresponding double-ceramic-layer (DCL) coatings comprised of a dense layer were produced by atmospheric plasma spraying. And the service capability including phase stability, thermal shock behavior and CMAS resistance were systemically evaluated. After heat treatment at 1500 °C for various durations, less monoclinic phase can be found in YbYSZ specimen, indicating excellent phase stability at high temperature. In the thermal shock test, the conventional YbYSZ coatings show a longest thermal shock lifetime, which increases by ~10% as compared with that of conventional yttria-stabilized ZrO 2 (YSZ) coatings. CMAS corrosion resistance of DCL coatings was studied using an isothermal corrosion test at 1300 °C. The results show that YbYSZ systems with a dense layer performed a high resistibility to CMAS corrosion, and less damage can be observed in coatings as well as smaller infiltration depth within the same time. • A double-ceramic-layer (DCL) coating comprised of dense layer was prepared. • The YbYSZ material exhibits higher resistance to high temperature. • The YbYSZ coatings have a longest thermal shock lifetime comparatively. • The DCL-YbYSZ coatings show enhanced CMAS resistance due to high chemical inertness.

Factors governing segmentation crack characteristics in air plasma sprayed ceramics

Air Plasma Spray (APS) is an industrially popular coating technique implemented in vast domains of applications. Among the inherent defects in an APS coating, segmentation cracks have been recognized to enhance the strain tolerance and durability of thermal barrier coatings. However, segmented coatings also exhibit higher thermal conductivity and are susceptible external debris infiltration. Hence, to optimize the coating properties, it is necessary to have processing control over the incorporation of such segmentation cracks.In this study, segmentation characteristics (thickness to cracking and crack spacing) of multiple Yttria Stabilized Zirconia (YSZ) and Gadolinium Zirconate (GZO) coatings were analyzed. The correlations between the processing parameters and the mechanical properties of the coatings were identified, and the cumulative effect of such variations on the segmentation characteristics were observed. In addition, the experimental observations were reconciled with fracture theories in thin films to provide a comprehensive phenomenological interpretation of segmentation in APS coatings.