Failure Of Thermal Barrier Coatings Research Articles

THz-TDS characterization of stress evolution of EB-PVD TBCs under thermal cycling

The failure of thermal barrier coatings (TBCs) during service is a critical issue affecting aeroengine efficiency. In this study, electron beam physical vapor deposition (EB-PVD) TBCs were subjected to thermal cycling at 1100 °C. Terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the relationship between the evolution of characteristic frequency and the number of thermal cycles, along with the development of stress in the ceramic layer. In situ tensile test revealed a linear correlation between the characteristic frequency and micro strain derived from THz-TDS data, with a proportional coefficient of −9.94 between the characteristic frequency and the slope of the refractive index curve. The results show that the stress evolution can be effectively monitored by recording the trend of characteristic frequency. During the non-failure stage, the characteristic frequency followed a parabolic trend as thermal cycles increased, influenced by the growth of thermally grown oxide (TGO), ceramic layer sintering, and thermal mismatch. Approaching the failure stage, stress evolution was dominated by the expansion of vertical cracks in the ceramic layer and the widening of transverse cracks at the interface, which corresponded to a sudden change in the characteristic frequency.

Thick Columnar-Structured Thermal Barrier Coatings Using the Suspension Plasma Spray Process

Higher operating temperatures for gas turbine engines require highly durable thermal barrier coatings (TBCs) with improved insulation properties. A suspension plasma spray process (SPS) had been developed for the deposition of columnar-structured TBCs. SPS columnar TBCs are normally achieved at a short standoff distance (50.0 mm–75.0 mm), which is not practical when coating complex-shaped engine hardware since the plasma torch may collide with the components being sprayed. Therefore, it is critical to develop SPS columnar TBCs at longer standoff distances. In this work, a commercially available pressure-based suspension delivery system was used to deliver the suspension to the plasma jet, and a high-enthalpy TriplexPro-210 plasma torch was used for the SPS coating deposition. Suspension injection pressure was optimized to maximize the number of droplets injected into the hot plasma core and achieving the best particle-melting states and deposition efficiency. The highest deposition efficiency of 51% was achieved at 0.34 MPa injection pressure with a suspension flow rate of 31.0 g/min. With the optimized process parameters, 1000 μm thick columnar-structured SPS 8 wt% Y2O3-stabilized ZrO2 (8YSZ) TBCs were successfully developed at a standoff distance of 100.0 mm. The SPS TBCs have a columnar width between 100 μm and 300 μm with a porosity of ~22%. Furnace cycling tests at 1125 °C showed the SPS columnar TBCs had an average life of 1012 cycles, which is ~2.5 times that of reference air-plasma-sprayed dense vertically cracked TBCs with the same coating thickness. The superior durability of the SPS columnar TBCs can be attributed to the high-strain-tolerant microstructure. SEM cross-section characterization indicated the failure of the SPS TBCs occurred at the ceramic top coat and thermally grown oxide (TGO) interface.

Modeling 3D TGO dynamic growth and stresses and the resulting modes of buckling failure in thermal barrier coatings

AbstractBased on experimental data, this study presents a model of the dynamic growth of a thermally grown oxide (TGO) in thermal barrier coatings (TBCs) during thermal cycling. The model was solved using the finite element method. The thermal stress results of the model were compared with experimental results to verify the accuracy of the model. The stress analysis revealed that the combined effect of radial tensile stress and circumferential compressive stress can lead to the delamination of TBCs in a buckling form, which can induce the formation of cracks in TBCs along the axial direction. This observation was consistent with the conclusions drawn from the experimental results. Crack initiation and propagation result from the action of the maximum shear and radial stresses. In addition, the relationship between buckling failure and microscopic cracks was investigated. Thermal cycling induces radial and circumferential cracks, leading to buckling, delamination, and ultimate coating failure as the radial and circumferential cracks interact at the interface.

Comprehensive Understanding of TGO Morphology Effect on the Thermal Barrier Coatings Failure Under Free Edges

Abstract The growth stresses induced by the thermally grown oxide (TGO) will be amplified at the free-edge site, making the free-edge site a weak part of the thermal barrier coatings (TBCs). In this study, the TBCs failure behavior is investigated based on different TGO morphologies under free edges. The thermomechanical model is established by creating straight lines and simplified sinusoidal curves, respectively. Dynamic TGO growth is realized by the secondary development of the subroutine. The cohesive element is inserted at the TC/TGO interface to simulate the delamination. The stress evolution near different TGO morphologies under the influence of the free edge are examined. In addition, the interfacial cracking behavior near the free edge is also explored. The results show that the appearance of the free edge will deteriorate the stress condition in the nearby area, change the preferred cracking area, and induce the earlier failure behavior. The straight line morphology has the most “friendly” stress distribution. The sinusoidal curves have peaks and valleys, and different areas of the TGO shape are different under the influence of the free edge, but all of them have the effect of stress “convergence”. These results can provide significant guidance to develop the next-generation advanced TBCs.

Reliability analysis of thermal barrier coatings under CMAS deposition and penetration

To address the issue of thermal barrier coating (TBC) failure and spalling caused by the deposition and infiltration of environmental particulate matter, a thermal-mechanical coupling model for EB-PVD TBC considering CMAS deposition failure was developed. Based on this model, the effects of CMAS deposition thickness and infiltration depth on TBC spalling failure were numerically studied. First, the deposition characteristics of CMAS were obtained using a critical velocity model. Second, the corresponding TBC failure criteria were established. Then, the effects of CMAS deposition thickness and infiltration depth on TBC spalling failure were specifically studied under different CMAS particle sizes and service times. The results show that the CMAS deposition thickness is the greatest at the leading edge and the pressure surface near the trailing edge of the blade. The CMAS infiltration depth is influenced by the temperature field, with higher temperatures leading to greater infiltration depth. With the simultaneous increase of service time and particle size, the average damage value of the coating surface (D¯) initially increases slightly to 0.2, then rapidly to 0.9, and finally slowly to 0.97, resulting in almost complete failure and spalling of the coating.

Study of CMAS wettability, penetration, and degradation behaviors on EB-PVD 8YSZ coatings laser micro-glazed (LMGed) by an ultraviolet picosecond ultrashort pulsed laser

CaO-MgO-Al2O3-SiO2 (CMAS) is a primary contributor to the failure of thermal barrier coatings, particularly those with columnar structures fabricated via electron beam physical vapor deposition (EB-PVD). This paper presents the laser micro glazing (LMGing) of EB-PVD using an ultraviolet pulsed ultrashort pulsed laser. With the disappearance of inter-columnar gaps, a smooth LMGed layer with extremely narrow cracks (<0.9 μm in width) were formed on the surface. Employing the sessile-drop method, heat treatment demonstrated that the LMGed layer effectively suppressed the wettability, penetration, and degradation behaviors of CMAS. Compared to the original, the spreading area of CMAS on the LMGed layer was decreased by 48 % (37.8 mm2 → 19.5 mm2), while the contact angle was increased by 116 % (6.2° → 13.4°). Meanwhile, a small amount of CMAS was found in the inter-columnar gaps, while no CMAS aggregated at the bottom. The depth of the corrosion layer at the CMAS-coating interface decreased by 68 % (9.9 μm → 3.2 μm), ensuring the blocking effect of the LMGed layer.

A Comprehensive Understanding of Thermal Barrier Coatings (TBCs): Applications, Materials, Coating Design and Failure Mechanisms

This review offers a comprehensive analysis of thermal barrier coatings (TBCs) applied to metallic materials. By reviewing the recent literature, this paper reports on a collection of technical information, involving the structure and role of TBCs, various materials and coating processes, as well as the mechanisms involved in the durability and failure of TBCs. Although TBCs have been successfully utilized in advanced applications for nearly five decades, they continue to be a subject of keen interest and ongoing study in the world of materials science, with overviews of the field’s evolution remaining ever relevant. Thus, this paper outlines the current requirements of the main application areas of TBCs (aerospace, power generation and the automotive and naval industries) and the properties and resistance to thermal, mechanical and chemical stress of the different types of materials used, such as zirconates, niobates, tantalates or mullite. Additionally, recent approaches in the literature, such as high-entropy coatings and multilayer coatings, are presented and discussed. By analyzing the failure processes of TBCs, issues related to delamination, spallation, erosion and oxidation are revealed. Integrating TBCs with the latest generations of superalloys, as well as examining heat transfer mechanisms, could represent key areas for in-depth study.

Thermal oxidation description methodology of thermal barrier coatings on gas turbine blades considering service characteristics

Predicting the thickness of Thermally Grown Oxide (TGO) and the residual stresses induced by TGO growth in the Thermal Barrier Coatings (TBCs) is significant work in analyzing the failure of TBCs. In practical applications, the operating temperature, oxygen partial pressure, aluminium content in TBCs, and porosity of the topcoat affect the oxidation rate. Therefore, a novel oxidation rate model is proposed to describe the effect of such factors. The parameters in the proposed model were fitted with isothermal degradation test data from prior literature. Then, a method for calculating the residual stress in a disk-shaped specimen was developed based on the oxidized growth strain model and the oxidized model. Finally, a variable-temperature oxidation experiment was performed to validate the new oxidation model. The average error between the predicted and experimental thicknesses is approximately 10%, and the average error between the residual stresses is 30%. Under the variable-temperature condition, the error between the predicted thickness and the average experimental thickness is 3.9%.

Comprehensive Understanding of the Effect of TGO Growth Modes on Thermal Barrier Coating Failure Based on a Simulation.

The growth stress induced by thermally grown oxide (TGO) is one of the main reasons for the failure of thermal barrier coatings (TBCs). In this study, the failure behavior of TBCs was examined based on different growth modes of TGO. A TBC thermo-mechanical model with a simplified sinusoidal interface morphology was established by the secondary development of a numerical simulation. The plasticity and creep behavior of materials were considered. Based on the subroutine development, the non-uniform growth of the TGO layer was realized. Cohesive elements were also applied to the TC/TGO interface. The stress distribution and evolution at the TC/TGO interface were investigated. Then, the cracking behavior near the interface was studied. The results show that lateral growth causes the off-valley site to replace the previous off-peak site as a vulnerable site. The non-uniform growth accelerates damage in the off-valley site, which leads to a change in the failure behavior. These results will provide significant guidance for understanding the TBC failure and the development of advanced TBCs.

Corrosion behavior of Fe-containing silicate over ytterbium hafnate ceramics with different HfO2 content

High-temperature molten silicate deposit corrosion has become a critical factor for the failure of thermal barrier coatings. In this work, promising ytterbium hafnate ceramics with different HfO2 content were prepared by solid-phase reaction, and the effect of HfO2 content on Fe-containing silicate corrosion behavior of ytterbium hafnate ceramics was investigated. The evolution of HfO2 content showed negligible influences on reaction products and corrosion mechanism. The attack of Fe-containing silicate melts on the ceramic led to the formation of a reaction layer consisting of apatite, fluorite, and garnet. This layer plays a critical role in preventing further penetration of the melts. However, ytterbium hafnate ceramics with lower HfO2 content showed better resistance to melt attack.

The effect of high temperature aging on fluorescence properties of Eu3+-doped La2(Zr0.7Ce0.3)2O7

Introduction of luminescent rare earth ions into thermal barrier coatings (TBCs) for the study of TBCs failure mechanism is a very effective non-destructive method. In this paper, Eu3+ ions were doped into La2(Zr0.7Ce0.3)2O7 (LZ7C3) to form LZ7C3:Eu fluorescent material using a high-temperature solid-phase synthesis method. By analyzing the physical structure, fluorescence properties and thermophysical properties of LZ7C3:Eu powders synthesized with different Eu3+ dopant concentrations at different temperatures, we determined that a synthesis temperature of 1400 °C, a dopant concentration of 1 mol% are the optimal synthesis conditions for LZ7C3:Eu. Independent LZ7C3:Eu coating was prepared by atmospheric plasma spraying technology, and high temperature aging test was carried out at 1400 °C. The influence of high temperature aging on the 5D0→7F2 energy level transition peak of Eu3+ in LZ7C3 powder and its coating was examined by microscopic emission spectroscopy. The results show that the peak of the 5D0→7F2 energy level transition is red-shifted, and the peak intensity and the full width at half maxima (FWHM) of the powder decrease gradually with the increase of high temperature aging time. The peak of the 5D0→7F2 energy level transition of the coating exhibits a constantly fluctuating tendency of red-shifting and then blue-shifting, and the peak intensity and the FWHM also continue to decrease. High temperature aging has a negative effect on the fluorescence properties of LZ7C3:Eu materials.

A matching design co-enhancing thermal barrier and thermal cyclic performances of sintering-resistant coatings

Sintering-induced healing of intrinsic micropores is the main cause of thermal insulation degradation in plasma-sprayed thermal barrier coatings (TBCs). Although large pores can effectively retain the thermal-insulation performance, they can lower the mechanical performance during thermal cyclic tests. Therefore, balancing the tradeoff between the thermal and mechanical performances is a challenging task. Herein, sintering-resistant coatings were achieved through a matching design that co-enhances the thermal insulation and thermal cycling span. TBCs with different contents of sintering-resistant pores were prepared. The thermal-insulation performance was found to be positively correlated with the contents of sintering-resistant pores. However, in thermal cyclic tests, the lifespan increased as the pore content increased from 0 vol% to 20 vol% but drastically decreased as the pore content increased further above 20 vol%. Next, the competing effects of the newly formed pores on the crack driving force and fracture toughness, which might hasten TBC failure, are discussed. Based on the critical pore content, the matching design enhances the thermal insulation and lifespan of the resultant TBCs by 50% and 40%, respectively, from those of conventional TBCs. Therefore, the sintering-resistant structure is expected to considerably contribute to next-generation applications of TBCs.

Enhancing the performances of EB-PVD TBCs via overlayer Al-modification

Thermal barrier coatings (TBCs) guarantee the service life of high-temperature alloy substrates under harsh service environments. Premature failure of TBCs due to coating fracture, high-temperature oxidation, and calcium oxide–magnesium oxide–aluminum oxide–silicon oxide (CMAS) corrosion, which significantly reduce their service lifespan. In this study, Al film was applied on the surface of EB-PVD TBCs using arc ion plating, and then converted it to α-Al2O3 by an in situ reaction between Al and ZrO2. The α-Al2O3 have toughening effect resulting in a higher bending resistance of EB-PVD TBCs. Because α-Al2O3 could seal the columnar inter-crystal gaps on the surface of EB-PVD TBCs, it reduced oxygen penetration, which enhanced the high-temperature oxidation resistance. CMAS reacted with α-Al2O3 to generate spinel and calcium‑aluminum yellow feldspar compounds at 1350 °C, which prevented following CMAS penetration and corrosion. Furthermore, Al-modification reduced the surface roughness, which made Al-modified TBCs have better wetting resistance.

Life Analysis of TBC on an Aero Engine Combustor Based On In-Service Failures Data

TBC (Thermal Barrier Coatings) mostly find application in the combustion sections of aircraft turbine engines. With the demand for fuel economy and increased power, combustion temperatures are approaching the design limits of the metal alloys from which hot end components are made. Modern TBCs are required to not only limit heat transfer through the coating but to also protect engine components from oxidation and hot corrosion. When a TBC fails, it exposes the underlying substrate to very high gas temperatures and the life of the component gets reduce drastically. With a failed TBC, a component can even fail within its prescribed service life. This is an airworthiness issue. So it is essential to estimate the TBC life accurate possibly to ensure airworthiness of the aircraft at any given time. Premature failure are observed in the TBC applied over combustion section of a turboprop engine. The engine is 800 shp class and it powers a commuter aircraft. The objective of this paper is to find out mean life of the coating by life data analysis. The data has been fitted to Weibull distribution. The Weibull parameters have been discussed which highlights the dominating cause for TBC failure and hence suggestion for life improvement of TBC system has also been made. The need for TBC system redesign has been emphasized. In addition to this, it also presents a comparison between different users/ operators.

Phase stability, thermophysical properties, thermal shock behavior and CMAS resistance of Sc2O3-CeO2 co-stabilized ZrO2 TBCs

In this study, 16 mol% CeO2–2 mol% Sc2O3 co-stabilized ZrO2 (4S16CZ) coating was prepared by atmospheric plasma spraying (APS) technology. Parameters related to the application of 4S16CZ as a thermal barrier coatings (TBCs) were studied, including high-temperature stability, thermal insulation performance, coefficient of thermal expansion (CTE), thermal shock behavior and CaO-MgO-Al2O3-SiO2 (CMAS) resistance. After heat treatment at 1400 °C for 100 h, the 4S16CZ coating maintained a single non-transformable tetragonal phase (t'), exhibiting excellent high-temperature stability. Additionally, at the test temperatures (25 °C–1000 °C), the thermal conductivity of the 4S16CZ coating ranged from 0.79 W/(m·K) to 1.02 W/(m·K), lower than that of the ZrO2 coating stabilized by 7.6 wt% Y2O3 (YSZ, 0.88–1.13 W/(m·K)), The CTE of the 4S16CZ coating was from 8.3 × 10−6 K−1 to 11.2 × 10−6 K−1, which was comparable to the YSZ. Moreover, the thermal shock behaviors of 4S16CZ TBCs and 4S16CZ/YSZ TBCs were evaluated at 1050 °C. The thermal durability of the 4S16CZ/YSZ TBCs was higher than that of 4S16CZ TBCs due to the buffering effect of the YSZ layer. The main reasons for the failure of both TBCs were the oxidation of the bond coat, thermal mismatch stress, low fracture toughness and sintering effects. In CMAS isothermal corrosion tests, CMAS exhibited low wettability on the 4S16CZ coating. Compared with the YSZ coating, the 4S16CZ coating showed better resistance to CMAS attacks.

Stress evaluation of TBCs with inclined film-cooling hole considering CMAS penetration-induced change of thermo-mechanical properties

CaO-MgO-Al2O3-SiO2 (CMAS) penetration is an essential factor leading to the failure of thermal barrier coatings (TBCs), especially around the film-cooling hole structure of high-temperature gas turbine components. The complex temperature distribution and the free-edge effect around the cooling holes aggravate the influence of CMAS penetration on the failure of the TBCs. In this study, a three-dimensional (3D) model of a TBC-film cooling system considering CMAS attack is presented based on the observation of a retired turbine blade. CMAS penetration-induced change of thermo-mechanical properties of the ceramic top coat (TC) was considered. The thermal gradient around a cooling hole and the uneven growth of the thermally grown oxide (TGO) on the stress distribution in TBCs were investigated. The results showed that the thermal insulation performance of TBCs declined considerably under CMAS penetration, and the induced maximum increase in the temperature at the TC/bond coat (BC)interface reached 60 K. CMAS penetration also led to a considerable stress increase in the TBC-film system. We therefore conclude that CMAS penetration accelerates the crack initiation at the TC/TGO and TGO/BC interfaces.

Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

AbstractThe stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce3+ ions (YAG:Ce3+)/yttrium oxide partially stabilized zirconia double‐ceramic‐layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce3+ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce3+ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.

Phase transformation failure in YSZ TBCs induced by component-dependent CMAS corrosion

Generally, CMAS corrosion is a key factor leading to the spalling failure in thermal barrier coatings (TBCs). Figuring out the component-dependent failure mechanism is the premise to improve the service life of TBCs. Here, CMAS corrosion behaviors with different Ca/Si components have been investigated through analyzing microstructure evolution in 8-YSZ coatings by EB-PVD. Capillary force from the gap between columnar crystals could accelerate the infiltration speed of the molten CMAS, leading to a fully populated coating with lower strain tolerance in 4 h. Subsequently, CMAS infiltration in the coatings led to preferential dissolution of yttrium ions and precipitation of yttria lean YSZ, and secondary CMAS phases. The corrosiveness of CMAS could be further tailored by optical basicity and viscosity. TBC failure could be enhanced in high-Ca case, resulting in the morphology evolution from columnar crystals to coarsening grain structures. Raman spectra indicates CMAS corrosion with high-Ca content will facilitate the phase transformation of t-YSZ to harmful monoclinic phase. In addition, DFT calculation demonstrates Ca doping into 8-YSZ will make monoclinic phase more stable and generate a 4.1 % volume expansion from the view point of energy. Consequently, the combination of the above low strain tolerance, new phases and volume expansion will accelerate the harmful morphology evolution. This work provides direct insight into the failure mechanism of CMAS corrosion and paves the way to design of new TBCs.

A chemo-thermo-mechanical coupled phase field framework for failure in thermal barrier coatings

The failure process in a thermal barrier coating (TBC) system can be complicated due to complex microstructure, material property mismatch, high temperature change, chemical reactions during the failure process, etc. It brings considerable difficulties to the modeling technique. A complete failure process in TBC contains multiple failure types, e.g., bulk fracture, delamination of TC (top coating)/TGO (thermally grown oxide) interface and TGO/BC (bond coating) interface, and the interaction among them. However, existing approaches mainly focus on a single failure type and thus are faced with limits on exploring the complete failure mechanism. In this work, we propose a chemo-thermo-mechanical coupled phase field model based on thermodynamic laws, such that the complete TBC fracture process can be captured. The present contribution provides a step forward in comparison with the existing phase field models. Couplings among thermal conduction, material oxidation and damage are considered, and the fundamental differential equations are derived strictly follow the thermodynamic laws. The proposed model is first validated through several benchmarks, and then followed by several simulations of the failure process of a TBC system. The simulations and experiments show that the bulk fracture occurs when the TGO is thin at the cooling process, the growth effect of TGO layer causes damage initiation at the interface and finally transformed into the interfacial delamination, the interaction of interface cracking and bulk material cracking is captured. The proposed model is proven to be a powerful tool in solving the failure process of a TBC system and is promising to handle other similar problems.

Deformation Prediction Theory of Thermal Barrier Coatings near Cooling Holes under Thermal Cycling.

Thermal barrier coating (TBC) systems are widely adopted in gas turbine blades to improve the thermal efficiency of gas turbine engines. However, TBC failure will happen due to the thermal stress between the different layers of the TBC systems. The traditional two-layer theoretical model only considers TGO (thermally grown oxide) and a substrate in the inner cooling hole with the surface uncoated, which results in poor prediction of the deformations of the TBC systems. It should be mentioned that the effect of TBC is very important because the thickness of TBC is much larger than the TGO thickness. In this study, a new three-layer theoretical model was derived, which is composed of the cylindrical TGO and TBC mounted in the substrate with a circular hole, and the stress and strain of TGO near the cooling hole under the condition of the thermal cycles were calculated. The high temperature characteristics of TGO and the substrate including the high temperature strength and growth ratio were from the experiments. The results show that the strain of the developed three-layer model is irrelevant with increasing number of cycles, which indicates that TBC in the cooling hole significantly inhibits the deformation of TGO near the cooling hole. Therefore, aimed at confirming the feasibility of the three-layer theoretical model, the finite element analysis with coating in the cooling hole and on the surface was carried out with a three-layer axisymmetric model, which proves that the 3-layer theoretical model can predict the deformation trend near the cooling hole.