Comprehensive understanding of horizontal and vertical crack effects on failure mechanism of lamellar structured thermal barrier coatings

The local spalling induced by the propagation and coalescence of cracks in the ceramic layer is the fundamental reason for the thermal barrier coatings (TBCs) failure. To clarify the effects of horizontal and vertical cracks on the coating failure, an integrated model combining dynamic TGO growth and ceramic sintering is developed. The effects of cracks on the normal and shear stress characteristics are analyzed. The driving force and propagation ability of cracks under different configurations are evaluated. The interaction between horizontal and vertical cracks is explored by analyzing the variation of the crack driving force. The results show that TGO growth causes the ratcheting increase of σ₂₂ tensile stress above the valley, and the σ₁₂ shear stress is on both sides of the peak. Ceramic sintering mainly contributes to the ratcheting increase of σ₁₁ tensile stress. There is minimum strain energy when the horizontal crack extends to the peak. The vertical cracks on the surface of the ceramic layer are easier to propagate through the coating than that of other locations. When the horizontal and vertical cracks simultaneously appear near the valley, they can promote the propagation of each other. The present results can offer theoretical support for the design of an advanced TBC system in the future.     

The sintering behavior of plasma-sprayed YSZ coating over the delamination crack in low temperature environment

The sintering behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) coating over the delamination crack and its influence on YSZ cracking were investigated via gradient thermal cycling test and finite element model (FEM). The gradient thermal cycling test was performed at a peak surface temperature of 1150 °C with a duration of 240 s for each cycle. A three-dimensional model including delamination cracks with different lengths was employed to elaborate the temperature evolution characteristics in YSZ coating over the delamination cracks. The temperature over the delamination crack increases linearly with the crack propagation, which continuously promotes the sintering of YSZ coating in the region. As a result, the YSZ coating over the delamination crack sinters dramatically despite of the low temperature exposure. Meanwhile, the temperature distribution difference in YSZ coating induces an nonuniform sintering along both free surface and thickness of YSZ coating. Correspondingly, the maximum vertical crack driving force locates at the YSZ free surface over the delamination crack center, which makes the vertical cracks generate in this region and propagate to the interface of YSZ /bond coat with YSZ further sintering. The vertical crack promotes the delamination crack propagation via accelerating the oxidation velocity of the bond coat. The influence of temperature rise on delamination crack propagation can be divided into two stages: the little contribution stage and the promotion stage. For the actual engine exposure to low temperature, the study of phase transformation of YSZ over the delamination crack is indeed needed because of an extended remarkable temperature rise period.     

Simulation of thermal barrier coating spallation induced by the initiation/growth/coalescence of internal crack and interfacial crack based on a real image model

In the furnace cycle test, the growth of oxide film leads to the propagation and coalescence of multiple cracks near the interface, which should be responsible for the spallation of thermal barrier coatings (TBCs). A TBC model with real interface morphology is created, and the near-interface large pore is retained. The purpose of this work is to clarify the mechanism of TBC spallation caused by successive initiation, propagation, and linkage of cracks near the interface during thermal cycle. The dynamic growth of thermally grown oxide (TGO) is carried out by applying a stress-free strain. The crack nucleation and arbitrary path propagation in YSZ and TGO are simulated by the extended finite element method (XFEM). The debonding along the YSZ/TGO/BC interface is evaluated using a surface-based cohesive behavior. The large-scale pore in YSZ near the interface can initiate a new crack. The ceramic crack can propagate to the YSZ/TGO interface, which will accelerate the interfacial damage and debonding. For the TGO/BC interface, the normal compressive stress and small shear stress at the valley hinder the further crack propagation. The growth of YSZ crack and the formation of through-TGO crack are the main causes of TBC delamination. The accelerated BC oxidation increases the lateral growth strain of TGO, which will promote crack propagation and coalescence. The optimization design proposed in this work can provide another option for developing TBC with high durability.     

Effect of segmented cracks on TGO growth and life of thick thermal barrier coating under isothermal oxidation conditions

This paper is devoted to a comparative study on the isothermal oxidation of thick thermal barrier coating (TTBC) with and without segmented cracks produced by atmospheric plasma spray (APS) process. Accordingly, the growth of thermally grown oxide (TGO) and its effect on the degradation of the coating were investigated. Thick top coat in both segmented crack and conventional thick TBC reduced the double layered TGO growth rate slightly. The segmented crack thick TBC demonstrated longer isothermal oxidation life in comparison with that of the conventional thick TBC at 1100 °C. The dominant failure mechanism was spallation due to lateral cracking within the TGO and/or within TBC near the TGO layer, called mixed failure. Stress, and consequently strain, induced on the TTBC due to progressive TGO growth, seems to be primarily responsible for the crack initiation and propagation leading to the coating failure. Increment of elastic energy stored within the top coat due to the increasing of TGO thickness, finally causes thick thermal barrier coating failure in high temperature isothermal oxidation.     

An innovative model coupling TGO growth and crack propagation for the failure assessment of lamellar structured thermal barrier coatings

The failure of plasma-sprayed thermal barrier coating (TBC) is often caused by the coating spallation due to crack propagation. In this study, a new model with stacking lamellae is developed based on the cross-section micrograph to explore crack propagation behavior within the ceramic top coat (TC) during isothermal cycling. The dynamic growth process of thermally grown oxide (TGO) is simulated via material properties change step by step. The stress profiles in the lamellar model are first evaluated, and the pore and lamellar interface crack effects on the stress state are further explored. Then, the successive crack growth, linkage, and ultimate coating spallation process is simulated. The results show that the stress intensity in TC enhances with thermal cycling. Large stress concentration always occurs near the pore and lamellar interface crack, which can result in the incipient crack growth. Moreover, the lamellar interface crack also changes the stress distribution within the TC and at the TC/bond coat interface. The multiple crack propagation upon temperature cycling is explored, and the possible coalescence mechanism is proposed. The lamellar crack steadily propagates at the early stage. The crack length sharply increases before the occurrence of coating spallation. The simulated coat spalling path is in line with the experimental result. Therefore, the new lamellar model developed in this work is beneficial to further reveal coating failure mechanism and predict coating lifetime.     

Analysis of crack initiation and propagation in Thermal Barrier Coatings using SEM-Based geometrical model with extended finite element method

Thermal Barrier Coatings (TBC) are widely used to protect the metallic components that operate at harsh conditions of elevated temperatures and oxidizing environments. Thermally grown oxide (TGO) causes cracks formation in the top coat (TC) that may lead to spallation failure of TBC. This work investigates effect of pores and TGO thickness on crack initiation and propagation due to thermal mismatch between TBC layers. Image processing is used to convert an SEM image, including pores, into a finite element (FE) model. An FE model using XFEM implemented in ABAQUS was developed to investigate crack initiation and propagation for various TGO thicknesses considering the effect of plastic deformation of BC, TGO and substrate. Results show that presence of pores changes the critical sites for crack initiation from the TC/TGO interface to be around the pores within the TC. Crack initiation temperatures and crack lengths were found to be affected with both TGO thickness and pores.     

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.     

On crack evolution with texturization of bonding layer in thermal barrier coating

The interface morphology of the bonding layer has a considerable effect on the damage and failure of sandwich-structured thermal barrier coatings. This work investigated the comprehensive effects of a grooved texture produced using laser ablation on the local surface strain, interfacial stress and strain, and crack behavior of the bonding layer in a thermal barrier coating system. The distribution and evolution of the local surface strain was obtained using the digital image correlation method. The interfacial stress, and the strain between the ceramic and bonding layers, were determined through a simulation of the plane-strain model, and the morphology and propagation of cracks were observed in thermal barrier coatings under an external tensile load. The results indicated that the local surface strain of the thermal barrier coating increased with the texturization of the bonding layer, whereas the fluctuation decreased. There were two inflection points in the local surface strain–time curves, corresponding to the initiation of surface cracks and that of interfacial transverse cracks. The surface cracks were initiated earlier than those without the texturization of the bonding layer. However, the behavior of the interfacial cracks was more complicated. If the roughness of the texture, defined as Rc, was small, the surface cracks propagated vertically to the interface between the ceramic and bonding layers, and turned into transverse cracks, leading to a separation of the ceramic layer. If Rc was greater than 22 μm, the surface cracks went further down to the interface between the bonding layer and substrate, and propagated horizontally, resulting in the separation of both the ceramic and bonding layers. Meanwhile, interfacial cracking and separation were deferred. A large roughness resulted in good cohesion between the ceramic and bonding layers, and a high stiffness for the coating, which improved the damage resistance and extended the life of the coating.     

Comprehensive dynamic failure mechanism of thermal barrier coatings based on a novel crack propagation and TGO growth coupling model

Comprehensive understanding of failure mechanism of thermal barrier coatings (TBCs) is essential to develop the next generation advanced TBCs with longer lifetime. In this study, a novel numerical model coupling crack propagation and thermally grown oxide (TGO) growth is developed. The residual stresses induced in the top coat (TC) and in the TGO are calculated during thermal cycling. The stresses in the TC are used to calculate strain energy release rates (SERRs) for in-plane cracking above the valley of undulation. The overall dynamic failure process, including successive crack propagation, coalescence and spalling, is examined using extended finite element method (XFEM). The results show that the tensile stress in the TC increases continuously with an increase in an undulation amplitude. The SERRs for TC cracks accumulate with cycling, resulting in the propagation of crack toward the TC/TGO interface. The TGO cracks nucleate at the peak of the TGO/bond coat (BC) interface and propagate toward the flank region of the TC/TGO interface. Both TC cracks and TGO cracks successively propagate and finally linkup leading to coating spallation. The propagation and coalescence behavior of cracks predicted by this model are in accordance with the experiment observations. Therefore, this study proposed coating optimization methods towards advanced TBCs with prolonged thermal cyclic lifetime.     

Evolution of internal cracks and residual stress during deposition of TBC

Thermal barrier coating (TBCs) are ceramic coatings that are deposited on metallic substrates to provide high thermal resistance. Residual stress is among the critical factors that affect the performance of TBCs. It evolves during the process of coating deposition and in-service loading. High residual stresses result in significant cracking and premature delamination of the TBC layer. In the present study, a hybrid computational approach is used to predict the evolution of internal cracks and residual stress in TBC. Smooth particle hydrodynamics (SPH) is first used to model the deposition of yttria-stabilized zirconia (YSZ) layer that contains various interfaces and micropores on a steel substrate. Then, three-dimensional (3D) finite element analysis is utilized to predict the evolution of internal cracks and residual stress in the ceramic coating layer. It is found that multiple cracks emerge during the solidification of the coating layer due to the development of high tensile (quenching) stresses. The cracking density is higher at regions near the coating interface. It is also found that compressive (residual) stresses are developed when the deposited coating is cooled to room temperature. The residual stress state is equibiaxial and nonlinear across the thickness/width of the TBC layer. The residual stress profile predicted compares well with that of hole drilling experiments.     

Thermal cycling behavior of plasma-sprayed yttria-stabilized zirconia thermal barrier coating with La0.8Ba0.2TiO3−δ top layer

In this study, a La_0.8Ba_0.2TiO_3?δ (LBT) upper layer was deposited on an yttria-stabilized zirconia (YSZ) thermal barrier coating (TBC) through atmospheric plasma spraying. The thermal cycling behaviors of the YSZ single-ceramic-layer and LBT–YSZ double-ceramic-layer coatings at 1000 °C were investigated through a water quenching method. Moreover, phases, microstructural evolution, and elemental distributions were studied through by X-ray diffraction and scanning electron microscopy–energy-dispersive X-ray spectroscopy. The results showed that the thermal cycling lifetime of the LBT–YSZ coating was 27% higher than that of conventional YSZ coating. The conventional YSZ coating failed after 251 cycles because of the joining of the continuous horizontal and vertical cracks caused by the formation of thermal growth oxides and the bending effect of the single-ceramic-layer structure. The thermal cycling behavior of the LBT–YSZ coating was different from that of the YSZ coating at the edge and center. Although the former was similar to the failure behavior of the YSZ coating, the cracks in the vertical direction were deflected as a result of the bending effect of the double-ceramic-layer structure during quenching. This deflection led to the formation of slope cracks with longer propagation paths and slope spallation zones. The latter showed small-debris spallation on top of the LBT upper layer due to the lower fracture toughness of the LBT, which protected the central coating from the structural damage of the ceramic coating. These two behaviors would either release the thermal stress or increase the crack-propagation energy requirement in the ceramic coating, consequently improving the thermal cycling lifetime of the LBT–YSZ coating. In summary, depositing an LBT upper layer could potentially improve the thermal cycling lifetimes of TBCs.     

The effect of a prefabricated crack on the crack growth in ceramics during quenching

The influence of a prefabricated crack on thermal-shock cracking during quenching is studied in real-time. The results show that after the thermal-shock crack extends to the prefabricated crack, the secondary crack may appear at the lower end of the prefabricated crack. The total vertical length of the crack and the probability of the secondary crack occurrence will gradually increase with the prefabricated crack angle. Besides, the influence of the prefabricated crack distance from the edge on thermal-shock crack growth is also considered. The simulation results of meso-damage mechanics are consistent with experimental observation. This article quantitatively investigates the effect of the prefabricated crack on the thermal-shock crack propagation in ceramics, expanding the research on the mechanism of thermal-shock failure.     

Study on mechanism of chip formation of grinding plasma-sprayed alumina ceramic coating

The mechanisms of ductile–brittle transition and surface/subsurface crack damage during the grinding of plasma–sprayed alumina ceramic coatings were investigated in an experiment and simulation on single diamond abrasive grain cutting. We observed that the brittle damage modes of alumina ceramic include boundary cracks, median cracks and lateral fractures. The normal force of the abrasive grain results in the initiation of median cracks, whereas the tangential force of the abrasive grain results in the propagation of median cracks in the direction of the abrasive grain cutting. Some cracks propagate downward to form machined surface cracks, whereas others propagate to the unmachined surface of the workpiece to produce brittle removal. Owing to the alternating tensile and compressive stresses, the material in contact with the top of the abrasive grain fractures continuously, forming the main morphology of the machined surface. The geometry and cutting depth of the abrasive grain have a significant influence on the ductile–brittle transition, whereas the cutting speed of the abrasive grain have no significant influence. On one hand, the stress concentration at the pore defects result in crack propagation to the deep layer; on the other hand, it reduces the local strength of the surface material, produces brittle fracturing, and interrupts crack propagation. The pores exposed on the machined surface and the broken morphology around them are important factors for reducing the surface roughness. Experimental observations show that the machined surface morphology of the alumina ceramic coating is composed of brittle fracturing, ductile cutting and plowing, cracks, original pores, and unmelted particles.     

Crack development behavior in thermally sprayed anti-oxidation coating under repeated thermal-oxygen coupling environment

The crack development behavior in thermally sprayed anti-oxidation coating was investigated after long-term and short-term oxidation with repeated thermal cycles from 1500 °C to room temperature. According to the distribution characteristics, the formed cracks can be divided into three types: type-A cracks with multi-directional features, type-B cracks originated from the inner interface bulges and type-C cracks initiating at surface oxide layer. Based on the analytical math models (blunt crack model and interface roughness model), the maximum stress at different positions was evaluated from the perspective of inner interface roughness, uneven oxide film, original microcracks and gathering micropores. The original vertical type-A cracks are most dangerous due to the highest crack tip stress. However, the micropore distribution or appropriate interface may promote transformation of vertical type-A cracks to less dangerous horizontal type-A cracks. This study on crack development behavior provides a fundamental insight and further avenues to optimize the composition and structure of thermally sprayed ceramic coating.     

Sand erosion and crack propagation mechanism of Cr/CrN/Cr/CrAlN multilayer coating

Herein, we have deposited Cr/CrN/Cr/CrAlN multilayer coatings with various modulation ratios on TC11 alloy substrate using cathodic arc system. The influence of various modulation ratios on microstructure and Al₂O₃ sand erosion behavior of coatings is systematically studied. Results reveal that the coatings are about 200 nm per cycle and total thickness is 8 μm. Five groups of coatings exhibit high hardness (>3000 HV_0.025). The coating with modulation ratio of 12 adhesion can reaches 55 N. The residual stress increases with the decrease of the modulation ratio, but the increase is generally low (less than ?2 GPa). In addition, according to sand erosion test, it is found that sand erosion resistance of multilayer coating is significantly around 5 times higher than TC11 alloy matrix. The erosion morphology shows that a large number of irregular cracks and layered spalling appear on the surface of the coating, indicating that the cracks are constantly initiated under the continuous impact of the sand and gravel，and finally gather together and then spalling. Moreover, dynamic response and stress field of the coating under the impact of single sand (Al₂O₃) are studied by numerical simulations. It is determined that coating cracking is caused by high tensile stress under CrAlN layer. In addition, according to crack propagation morphology and influence of different interfaces between multilayered structures on crack tips, propagation/termination mechanism of cracks is analyzed in detail. Cracks are easy to initiate in hard CrAlN layer and consume a lot of energy after propagating into soft Cr layer, thereby ending at next soft and hard interfaces. These results provide experimental and theoretical support for the study of high tenacity and anti-erosion coating.     

Modeling and simulation of thermal fatigue crack in EB-PVD TBCs under non-uniform temperature

Demand for enhanced jet engine efficiencies has led to a significant increase in the combustion temperature. Thus protecting components against the combustion products is necessary and is possible by using thermal barrier coatings (TBCs). In this research, thermal fatigue and creep interaction are studied via analytical and numerical finite element methods. Thermal stress and crack propagation analyses in the ceramic top coat are carried out based on plane stress condition and under inhomogeneous temperature distribution across the layers. The crack is assumed as a penny-shaped crack in both vertical and inclined growth directions. The study proposed that the creep-plasticity results in thermal stress alleviation and the tensile stress transforms into a compressive stress of − 200 MPa in forwarding cycles that will induce crack closure. In addition, the results confirmed that vertical cracks grow much faster than oblique ones due to single mode crack propagation and about 0.14 MPa√m greater values in stress intensity factor. The modeling and simulation results match together, and the obtained crack behavior is in compliance with other researcher's output.     

Cracking behaviors of EB-PVD thermal barrier coating under temperature gradient

In this paper, we study the cracking behaviors of single-crystal nickel-based superalloy samples coated with electron-beam physical vapor deposited (EB-PVD) thermal barrier coatings (TBCs) under a thermal gradient experimentally and via the finite element method (FEM). Our results indicate that the stress distribution and failure mode of the TBC samples under the thermal gradient are different from those of samples under a uniform temperature field. The failure of the TBCs under uniform temperature is initiated by interfacial and horizontal cracks, which can result in the separation and buckling of the top coat (TC) layer. However, for the TBCs under a thermal gradient, failure is mainly caused by both vertical TC cracks and interfacial cracks because of the increased transversal stress in the TC layer. Moreover, the initiation and propagation of vertical and horizontal cracks change the failure mode to local spallation of the TC layer. We believe that our findings can contribute to further developments in TBC technology.     

Stress analysis of the thermal barrier coating system near a cooling hole considering the free-edge effect

Due to the thermal mismatch between layers and the free-edge effect, interfacial peeling and shear stresses are generated locally around the edges of cooling holes in a thermal barrier coating (TBC)–film cooling system. These interfacial peeling and shear stresses may lead to modes I and II edge delamination, resulting in TBC spallation around the cooling hole. In this study, analytical and numerical models were built to study the stress and interfacial cracking behaviors of TBCs near the cooling hole. Analytical solutions for interfacial peeling moment and shear force at each layer were obtained to analyze the free-edge effect on the stress distributions in TBCs, and they were verified by the finite element calculations. The results showed that interfacial peeling moment and shear force were functions of the hole radius and thicknesses of top coat and oxide layer. The increase of interfacial peeling moment and shear force raised the likelihood of edge cracking around the hole. Derived by the local stresses, the interfacial cracks in TBCs initiated and propagated from the hole edge upon cooling.     

Constrained sintering and cracking of air plasma sprayed thermal barrier coatings: Experimental observation and modeling

The development of vertical cracks in air plasma sprayed (APS) thermal barrier coatings (TBCs) during thermal cycling and constrained sintering under a temperature gradient is investigated. Microstructural analysis shows that the development of the vertical cracks is associated with multiple processes, including sintering during the hold period and cleavage during cooldown. Inspired by the experimental observations, an image-based sintering model is used to simulate the development of vertical cracks as the coating sinters while constrained by a substrate. The computational results show that microstructural imperfections can develop into vertical cracks, which then propagate toward the interface. A simple analytical model is presented for the threshold level of in-plane stress for the onset of propagation of a vertical crack during constrained sintering. By combining the results of these different modeling approaches, the cross-coupling of the material and geometric parameters, and how this determines the sintering response (microstructure evolution) and vertical crack formation is evaluated. In addition, the growth of vertical cracks by a cleavage mechanism during cooldown is examined and the coupling between sintering, cleavage crack growth, and TBC lifetime is explored.     

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.