Failure Of Thermal Barrier Coatings Research Articles

Numerical investigation of thermal stresses induced interface delamination in plasma-sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are designed to be able to withstand a severe environment with coupled conditions including high temperature and thermal stresses. However, the complexity of their microstructure and the severity of these conditions give rise to premature TBCs failure and consequently reduce their service lifetime. This work aims to investigate the mechanisms of failure in plasma-sprayed TBCs and one of the most important failure mechanisms is related to thermal residual stresses developed in TBCs which leads to coating delamination at the interface contact between layers. A discrete element-based model has been developed to investigate thermal stresses generated at the interface on either side of the thermally grown oxide (TGO) layer due to the mismatch of coefficients of thermal expansion (CTE) between layers, and their material properties are assumed to vary with temperature. Thermal stresses induced in local areas due to the TGO interface irregularities are considered as well in the model by using the image analysis, which consists of a segmentation processing on actual microstructure images of TBCs, thereof allow to account for the undulation of the TGO interface which has been found to be associated with the failure of TBCs.

Molten-Volcanic-Ash-Phobic Thermal Barrier Coating based on Biomimetic Structure.

Volcanic ash is a major threat to aviation safety. The softening/melting temperatures of volcanic ash lie far below typical aero-engine operating temperatures. Thus, molten ash can accelerate the failure of thermal barrier coatings (TBCs). Here, inspired by natural superhydrophobic surfaces (e.g., the lotus leaf), a molten-volcanic-ash-phobic TBC, which provides a large possibility to eliminate molten ash issues of TBCs, is developed. A hierarchically structured surface is first prepared on a (Gd0.9 Yb0.1 )2 Zr2 O7 (GYbZ) pellet by ultrafast laser direct writing technology, aiming to confirm the feasibility of the biomimetic microstructure to repel molten volcanic ash wetting. Then biomimetic-structured GYbZ TBCs are successfully fabricated using plasma spray physical vapor deposition, which reveals "silicate" phobicity at high temperatures. The exciting molten-volcanic-ash-phobic attribute of the designed surfaces is attributed to the lotus-leaf-like dual-scale microstructure, emulating in particular the existence of nanoparticles. These findings may be an important step toward the development of next-generation aviation engines with greatly reduced vulnerability to environmental siliceous debris.

Effect of Thermal Growth Oxide Composition and Morphology on Local Stresses in Thermal Barrier Coatings

The failure of thermal barrier coatings (TBCs) during operation depends mainly on the thermal mismatch between the ceramic top coat (TC) and the metal bond coat (BC). The thermal mismatch at the interface is influenced by the dynamic changes in the composition and morphology of the thermally grown oxide (TGO) between TC and BC during thermal cycling. This work focuses on the establishment of a TGO dynamic growth model, which considers the changes in TGO composition and morphology for investigating the effect of dynamic growth of TGO on local mismatch stresses during thermal cycling. The results show that the sharp locations at the TGO/BC interface are more prone to high tensile stresses during thermal cycling due to the uneven growth behavior of TGO, leading to crack initiation. The valley region of the interface is in a state of compressive stress σxx during the early stages of thermal exposure. The peak region preferentially forms a concentration of tensile stress σyy. Once large-scale "layer" (Ni, Co)Al2O4-based spinel-like mixed oxides(MO) growth occurs in TGO, the stress σxx changes from compressive stress to tensile stress in the valley region, eventually forming high tensile stress (Max: +158 MPa). The maximum tensile stress σyy in the peak region is increased to 256 MPa, which is more than two times larger than the early period of thermal exposure. As a result, the dramatic changes in local stresses seriously affect the time and location of microcracks.

Experimental Study of the Thermally Grown Oxide and Interface of Thermal Barrier Coatings Using TEM In-Situ Heating.

Thermal barrier coating (TBC) materials play important roles in gas turbine engines to protect the Ni-based superalloys from high-temperature airflow damage. In this work, the nano-mechanism of TBC failure is analyzed. A scanning transmission electron microscopy-energy dispersive spectrometer (STEM-EDS)-based analysis method was used to study the influence of element migration on the deformation behavior of the bond-coat (BC) layer during heating. The content of elements in the same region varied greatly at different temperatures, which could prove the contribution of element migration to the deformation of the BC layer. TEM in-situ heating experiments were designed and carried out to study the deformation behavior near the ceramic topcoat (TC)/thermally grown oxide (TGO) and the TGO/BC interface. The TC/TGO interface was deformed violently during heating, and obvious deformation occurred at 100 °C, while the TGO/BC interface was relatively stable. A subset geometric phase analysis method was used for full field-strain measurement. The strain value near the TGO/BC interface was relatively small and did not change significantly at lower temperatures. The TC/TGO interface is more unstable and easier to deform than the TGO/BC interface. The stress and strain evolution in the internal region of TGO at high temperatures was quantitatively analyzed. The TGO layer has a tensile stress of GPa magnitude along the interface direction at the peak position, and the shear stress is small.

Solid Particle Erosion Behavior of La2Ce2O7/YSZ Double-Ceramic-Layer and Traditional YSZ Thermal Barrier Coatings at High Temperature

Thermal barrier coatings (TBC) used for turbine blades are indispensable for the most advanced aero-engines due to their excellent thermal insulation performance. Solid particle erosion (SPE) at high temperatures is one of the most critical factors in TBC failure. The high-temperature SPE failure behavior of TBC on circular sheets and turbine blades was investigated in this paper at erosion angles 60° and 90°. The high-temperature thermal shock behavior of TBC was also studied as the control group. The SPE failure mechanism of TBC is attributed to the spallation and thickness decrease of TBC. The formation of thermally grown oxide is the main reason for the TBC spallation, while the thickness decrease of TBC is due to the impaction of solid particles by near-surface cracking. The erosion angle is critical to the failure behavior of TBC, and TBC is more susceptible to SPE at an erosion angle of 60° than that at 90° because of the additional shear stress. Furthermore, a La2Ce2O7/YSZ double-ceramic-layer TBC was designed and deposited on turbine blades. The experimental results indicate that this type of double-layer TBC has more excellent performance under SPE than traditional YSZ TBC.

Microstructure evolution and thermal cyclic failure behavior of thermal barrier coatings with interface modification

Thermal barrier coatings (TBCs) interface between bond coat and ceramic top coat is critical since most of degradation presented in this region as TBCs failure analysis. Shot peening, as a valuable surface engineering, has been applied for TBCs interface modification. The interface modification by shot peening process to the morphology evolution and degradation optimization still need to be studied. In this work, three different shot peening parameters have been selected for interface modification of as-prepared TBCs. The divergent column microstructure has been reduced and surface roughness improved with the shot peening parameters changing. Thermal cyclic failure behavior has been summarized as two mode, thermally grown oxide (TGO) growth induced spalling and interface premature delamination. TGO growth induced spalling dominated failure at roughly interface condition, while interface delamination dominated failure at smooth interface condition. This failure mode has explained the thermal cyclic life tendency, climbing up and then declined, of TBCs.

Assessing the Structural Integrity of Plasma-sprayed Multilayer Thermal Barrier Coatings

Abstract The majority of lifetime models associates the failure of thermal barrier coatings to an oxidation of the bond coat. A thickening of the thermally grown oxide leads to a conversion of stresses at the undulated ceramic-metal interface, supporting the propagation of existing micro cracks. However, in plasma-sprayed multilayer thermal barrier coatings consisting of gadolinium zirconate (GZO) and yttria-stabilized zirconia (YSZ) a shift of the failure site from the ceramic-metal interface to the GZO-YSZ interface has been observed. Thus, an exclusively oxide-based formulation is not sufficient to describe the damage transition phenomena. Therefore, this paper outlines a mechanism-based approach for assessing the structural integrity, considering all relevant thermally activated processes as well as the interaction between thermal and elastic misfits. Oxidation of bond coat, creep of composite materials and sintering of ceramics are modeled in terms of temperature and exposure time. Finite element analysis of GZO-YSZ pairings with different microstructures reveal a strong influence of the initial porosities on the sintering behavior and thus on the resulting mechanical stresses and potential crack driving forces at the bi-material interfaces.

Shakedown Analysis and Experimental Study of Thermal Barrier Coatings.

Interfacial stress–strain fields become complicated in thermal barrier coatings (TBCs) under cyclic thermal loading, which affects the stability and spalling failure of TBCs directly. The convex and concave interfacial structures of TBCs were approximated as a multilayer cylinder model, and an analytical method of TBCs for shakedown analysis was established. A series of 8-YSZ TBC specimens were prepared by the plasma spraying process, followed by isothermal and thermal shock tests. The results showed that the stability limit is significantly greater than the elastic limit, the limit for the convex model was higher than that in the concave model, the first failure occurs in the concave area, and the main failure mode of a thermal barrier coating is the appearance of cracks at the interface layer during a thermal shock test. For the coating samples prepared in this study, the stability limits were between 950 °C and 1050 °C, and the validity of the stability limit analysis model of a multilayer structure was verified.

Evolution of the microstructure and mechanisms of performance degradation in EB-PVD YSZ thermal barrier coatings corroded by volcanic ash at 1150 °C

Volcanic ash (VA)/calcium-magnesium-alumina-silicate (CMAS) corrosion is a crucial issue causing the failure of thermal barrier coatings. This study focuses on the degradation process of electron-beam physical vapor deposition yttria-stabilized zirconia by natural VA at 1150 °C. The phase transformation and chemical reaction during VA attack were measured in real-time by in-situ high-temperature X-ray diffraction. The mechanical properties and the thermal conductivity of the as-deposited and VA corroded coatings were investigated. The correlation between these microstructural changes and the coatings’ properties are discussed. The degradation mechanisms are expected to guide the development of potential VA/CMAS mitigation methods.

Biaxial stress in La2Ce2O7/YSZ thermal barrier coatings for oxidation evolution

High temperature oxidation could cause the spalling and failure of thermal barrier coatings (TBCs). La2Ce2O7(LCO)/YSZ double-layer TBCs were investigated with high temperature oxidation at 1100 °C, 1150 °C and 1200 °C, respectively. Interfacial crack propagation and TGO growth behavior were comparatively investigated, and the failure of double-layer TBCs is the result of the joint action of interface transverse crack propagation and TGO growth. Due to the difference in thermal expansion coefficients, the YSZ layer is subjected to biaxial stress, which contains the TGO growth stress and the thermal mismatch stress during the oxidation process. The transverse crack propagation was developed at the LCO/YSZ interface and the YSZ/BC interface, eventually. The thermal barrier coatings TGO thickened abnormally at 1000 °C and 1150 °C, while delamination at the LCO/YSZ interface resulted in coating failure at 1200°C. The thermal mismatch stress at the LCO/YSZ interface increased mainly due to the thermal expansion coefficient between LCO and YSZ. Transverse microcracks at the LCO/YSZ interface gradually propagated, resulting in delamination failure to LCO/YSZ. Therefore, the failure mechanism of biaxial internal stress has important guiding significance for the application of double-layer TBCs.

Numerical Simulation of Initial Residual Stress in Thermal Barrier Coatings: Planar geometry model

Thermal barrier coatings (TBCs) are one of the key materials of turbines for propulsion and power generation. A typical TBCs system generally contains four layers, i.e. top ceramic coat (TC), thermally grown oxide (TGO), bond coat (BC) and substrate. The material parameters （such as Young’s modulus, Poisson’s ratio and thermal expansion coefficient） of each layer will result in the different cooling velocity of thermal barrier coatings (TBCs) system during the deposition process. Initial residual stresses in TBCs occur after cooling to the ambient, which play an important role for evaluating the failure and reliability of TBCs. In this paper, the initial residual stresses of a planar TBCs model were obtained by finite element method, the effects of cooling kinds and the thickness of TC and TGO on the fields of initial residual stresses were also considered. The results may be useful for the optimization of TBCs system design and preparation.

The thermal property and failure behaviors of SmErZrO thermal barrier coatings by EB-PVD

Sm2Zr2O7 material has attracted much attention due to the good thermal stability and high melting point over the past decades. However, the improvement of thermal durability and the study of failure still remain a challenge. In this work, we focuses on phase structure, thermal property and failure mechanism of the SmErZrO thermal barrier coatings (TBCs) by EB-PVD technology. The SZ, SE1Z, SE2Z and SE3Z TBCs both exhibit the pyrochlore structure after EB-PVD deposition. The as-deposited TBCs exhibit three coatings microstructure (top and middle ceramic coating with columnar structure, and NiCoCrAlYHf bottom coatings). The thermal conductivity of SZ ceramics decreases with the increase of Er content. The SE2Z TBCs exhibit long thermal durability (thermal shock life and thermal cycling life) at 1100°C. After thermal durability test, the horizontal cracks occur and develop in TGO and interface. The cracks and element evolution might be the key factor leading to the SE2Z/YSZ TBCs failure. The SE2Z/YSZ TBCs might be a potential TBCs in advanced aircraft engines.

Sintering-Induced Failure Mechanism of Thermal Barrier Coatings and Sintering-Resistant Design

Thermal barrier coatings (TBCs) have been developed to protect superalloys against high-temperature heat fluxes, which are required for the development of high-performance gas turbines. TBCs have porous structures, which are densified by sintering. The resulting stiffening is a major cause of TBC failure in service. Therefore, there is a need to reduce the negative sintering effect on the life span of TBCs. In this study, the sintering mechanism and the dominant factors causing changes in stiffening and mechanical properties were revealed experimentally. The experimental results show that the multiscale undulation of the originally smooth two-dimensional (2D) pore inner surface triggers multipoint contact between the upper and lower inner surfaces, resulting in pore healing during thermal exposure. The healing of 2D pores is the main structural characteristic change in TBCs after thermal exposure and the main reason for the stiffening and changes in mechanical properties. Then, the sintering effect on TBCs with vertically cracked structures was designed and simulated. We found that implanting vertical cracks in the topcoat can reduce the sintering effect and driving force for cracking by 87.9% and 79.9%, respectively. The degree of reduction depends on the space between vertical cracks. Finally, the mechanism responsible for the sintering-resistant TBCs was analyzed and discussed. Vertically cracked structures exhibited scale-sensitive stiffening, indicating that macroscopic stiffening is much lower than microscopic stiffening. In other words, the macroscopic sintering effect was lowered, and the TBCs remained highly resistant to global strain during thermal exposure. The resulting strain energy release rates are much lower than those of conventional TBCs. The results of this study contribute to the long-life thermal protection of superalloy-based components used in advanced gas turbines.

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.

Preparation and thermo-physical properties of La2AlTaO7 ceramic for thermal barrier coating application

In this work, La2AlTaO7 (LAT) ceramic was fabricated with the solid-state reaction method. The high temperature stability, mechanical properties and thermo-physical properties of LAT ceramic were studied. The thermal cycling lifetime of LAT thermal barrier coating (TBC) was also evaluated. Results showed that thermal conductivity of LAT ceramic was lower than that of 8% Y2O3-stabilized zirconia (YSZ) and decreased with the increase of the temperature, from 2.01 W/(m*K) at room temperature to 1.85 W/(m*K) at 1000 °C. The thermal expansion coefficient (TEC) of LATceramic was comparable to that of YSZ. The LAT ceramic also exhibited excellent thermal stability from room temperature to 1250 °C. The premature failure of the LAT TBCs during the thermal cycling test can be mainly attributed to the low fracture toughness, thermal expansion mismatch and high sintering rate.

Electron beam physical vapor deposited YbGYZ thermal barrier coatings: Phase stability, thermo-physical properties and thermal shock behavior

The rare earth oxide ceramics designed as 5.5 wt% Yb2O3-4.5 wt% Gd2O3-12 wt% Y2O3-78 wt% ZrO2 (YbGYZ) is a candidate material for thermal barrier coating (TBC) which will be suitable for application at higher temperatures. YbGYZ ceramic powders and bulks are fabricated by solid-state synthesis at temperatures above 1673 K, whose powders still have no phase transformation and exhibit excellent thermal stability despite long-term heat treatment at two different temperatures. In the temperature range from room temperature to 1573 K, the averaged coefficient of thermal expansion of YbGYZ ceramic bulk is 11.12 × 10−6 K−1, which meets the demand of novel ceramic materials of TBCs. Meanwhile, the averaged thermal diffusivity and thermal conductivity of YbGYZ ceramics are approximately 2.3% and 7.6% lower than those of the conventional YSZ bulk material respectively. The YbGYZ ceramic coatings are directly manufactured on the surface of (Ni, Pt)Al bond coat by means of electron beam physical vapor deposition (EB-PVD), whose phase structure consists primarily of cubic phase with co-existing of excess Y2O3 and ZrO2. A large number of regularly distributed “mud-like” microcracks appear on the surface of YbGYZ ceramic coating when it has experienced long-term thermal shock at 1373 K. The transverse microcracks originating in the ceramic coating have elongated to the interface of ceramic coat and thermally grown oxide (TGO) film that further cause the degeneration and separation of the interface. The spalling position of the YbGYZ coating where mainly occurs at the upper and lower adjacent interfaces of TGO layer. The serious rumpling, undulation, cross-linking, stress accumulation and rapid relaxation within TGO layer are the critical factors to accelerate interfacial delamination and spallation failure of YbGYZ TBCs.

Detection method and instrument of interfacial debonding inside a thermal barrier coating based on fluorescence imaging.

The interfacial delamination or debonding induced by local stress concentration is one of the kernel factors leading to spalling failure of thermal barrier coatings (TBC). In this work, for the rapid and accurate characterization of the interface delamination and debonding in TBC, the detection method and instrument were investigated based on fluorescence imaging. First, based on the principle of reflection-enhanced fluorescence, an experimental method was presented to detect interfacial debondings in TBC. A coaxial fluorescence-imaging instrument was established to realize the method above. In addition, an alkaline soluble method (ASM) was presented in this work to prepare prefabricated debondings with different transverse sizes, shapes, and thicknesses, which are basically consistent with the interfacial debondings in an actual TBC structure. Through a series of experiments, this paper verified that, using the proposed method and optical instrument, the resolutions to detect internal debonding are as small as 6 µm in the thickness dimension and less than 0.3 mm in the transverse dimension.

Cracking behavior and delamination mechanism of lamellar structured TBC with localized mixed oxides

Mixed oxide (MO) with localized growth feature and high growth rate remarkably affects the lifetime of thermal barrier coatings (TBCs), which indicates that clarifying the ceramic cracking mechanism induced by MO is critical for developing new coatings with high durability. Two kinds of TBC models involving spherical and layered mixed oxides are created to explore the influence of MO growth on the local stress state and crack evolution during thermal cycle. The growth of α-Al2O3 is also included in the model. The undulating interface between ceramic coat and bond coat is approximated using a cosine curve. Dynamic ceramic cracking is realized by a surface-based cohesive interaction. The ceramic delamination by simulation agrees with the experimental observation. The effects of MO coverage ratio and growth rate on the TBC failure are also discussed. The results show that the MO growth causes the local ceramic coat to bear the normal tensile stress. The failure mode of coating is turned from α-Al2O3 thickness control to MO growth control. Once the mixed oxide appears, local ceramic cracking is easy to occur. When multiple cracks connect, ceramic delamination happens. Suppressing MO formation or decreasing MO growth can evidently improve the coating durability. These results in this work can provide important theoretical guidance for the development of anti-cracking TBCs.

LaNdZrO thermal barrier coatings by electron beam physical vapor deposition: Morphology, thermal property and failure mechanism

Thermal barrier coatings (TBCs) have been widely studied over the last decade. Lanthanum zirconate (LaZrO) is known for high melting point and good thermal stability in aircraft engine environment. The thermal property and failure mechanism of these advanced TBCs still remain a challenge. This study focuses on phase structure, thermal expansion coefficient, thermal conductivity, morphology, composition, thermal durability, and failure behaviors in the LaNdZrO/YSZ/NiCoCrAlYHf TBCs. The LaNdZrO and YSZ ceramic coatings have been deposited by EB-PVD, and NiCoCrAlYHf metallic coating has been deposited by AIP-PVD. The as-deposited TBCs exhibit good thermal cycling life and thermal shock life at 1100 °C due to the improvement of TEC and the formation of feathery microstructure. In the thermal durability test, the locally broken region of the YSZ/TGO interface, YSZ coating and further the LaNdZrO coating might facilitate propagation of the instability. The element diffusion and cracks evolution might be two critical factors for the failure mechanism of TBCs. This study would further explain the failure of other advanced TBCs.

Real-time detection of damage evolution and failure of EB-PVD thermal barrier coatings using an environmental simulator with high-temperature and high-speed rotation

Thermal barrier coatings (TBCs) are key to improving aero-engine performance. The lifetime assessment of TBCs in a simulation environment is an essential consideration for successful application. Environmental simulators for rotor blade TBCs are still scarce, largely owing to the difficulty in obtaining real-time TBC failure data in extreme environments. In this work, an environmental simulator with high-speed rotation and gas thermal shock characteristics was developed and tested. High-temperature resistant strain gauges and thermocouples were fixed onto the surface of turbine blades by a flame-sprayed Al2O3 layer. The damage evolution and failure mechanism of TBCs were analysed by the real-time detection of strain damage and temperature field. The results reveal only a little fluctuation in the temperature distribution of the TBC surface (122 °C) under the synergistic effect of high-speed rotation and thermal shock, and the strain at each measuring point is tensile. During thermal cycling tests of the TBCs, the strain at a specific point varied from 0.21% to 0.79%. The final thermal shock life was determined to be 156 cycles, thus proving that the coating enables good thermal shock resistance. The spalled area was mainly distributed from the suction surface to the leading edge, and TBC failure could be attributed to crack initiation and propagation caused by the impact of the high-temperature flame and accumulation of fatigue load. The present work paves the way for coating preparation process optimisation and real service performance evaluation.