Cracks In Thermal Barrier Coatings Research Articles

Numerical Simulation of the Residual Stress at the Interface between Thermal Barrier Coating and Nickel-Based Single-Crystal Superalloy Based on Crystal Plasticity Theory

Residual stress plays an important role in the formation and growth of cracks in thermal barrier coatings and single-crystal superalloy substrates. In this study, a finite element model for a planar double-layer thermal barrier coating and a crystal plasticity finite element model based on dislocation slip-induced plastic deformation of single-crystal materials were established to analyze the residual stress in the coatings and the substrate, considering the creep and crystal plasticity of the substrate materials. The simulation results show that the thermal barrier coatings bear most of the stress generated by high temperatures, and the residual stress of the substrate is small. By comparing the two material properties to calculate the interface stress when the amplitude of the interface between the substrate and the coating is 30 μm and the thickness of the thermal grown oxide layer is 5 µm, the interfacial stress of the substrate at the macro scale was found to be similar to the interfacial stress at the micro slip system scale. Based on the cumulative shear strain, it was determined that the [001]-, [011]-, and [111]-oriented alloys activated the 12, 8, and 4 groups, respectively, under the combined action of thermal stress and centrifugal force of the coating. Comparing the activation of different initial orientation slip systems and the magnitude of the yield stress provides a theoretical foundation to study the structural integrity of single-crystal alloys.

Thermal stress analysis of optimized functionally graded coatings during crack propagation based on finite element simulation

Thermal barrier coatings (TBCs) are widely used in aircraft engines to protect their superalloy turbine blades in high inlet-temperature environments. However, the mismatch of the thermal expansion between the ceramic top layer and the bonding layer causes thermal stresses during thermal cycling, which can lead to cracks and failure of the TBCs system. In this paper, three different structures of TBCs were analyzed, which were single ceramic layer coatings (SCLC), double ceramic layer coatings (DCLC), and optimized functionally graded coatings (OFGC). In this paper, the propagation of horizontal and longitudinal cracks in TBCs of different structures during thermal cycling was analyzed by finite element simulations. The axial and radial maximum stress values and minimum stress values at the crack tip during thermal cycling of TBCs were analyzed. The results indicated that the OFGC structure could effectively reduce the thermal stress at the crack tip of the coating system during thermal cycling. The introduction of a gradient structure could improve the lifetime and thermal shock resistance of the coating.

Coupling effect of delamination cracks and vertical cracks on local phase transition of ceramic topcoat in thermal barrier coatings

The phase transition of atmospheric plasma-sprayed yttria-stabilized zirconia (APS YSZ) coating over delamination cracks in thermal barrier coatings (TBCs) was addressed by gradient thermal cycling test at 1150 °C to illustrate the coupling effect of delamination cracks and vertical cracks in TBCs on the phase transition. Phase structure of 15 dot-matrix micro-regions across YSZ thickness was examined by Raman spectroscopy. Three two-dimensional models were built to elaborate the local temperature evolution in the YSZ coating. The phase transition of YSZ had a markedly time-space characteristic with application duration, depending on the temperature increment. The phase-transition quantity decreased radioactively from the delamination crack center to its tip and gradually decreased from the YSZ free surface to the YSZ/bond coat interface. Compared with YSZ coating only containing delamination cracks or only vertical cracks, the temperature in the YSZ coating was significantly elevated by the combination of vertical cracks and delamination cracks. The increment was in 75 °C–295 °C when the delamination crack propagated from 0.5 mm to 4.0 mm. Consequently, due to the coupling effect of delamination cracks and vertical cracks, serious sintering and phase transition of the YSZ coating would occur once the delamination crack length was beyond 0.5 mm.

Investigating the interface cracking mechanism of CMAS-corroded thermal barrier coatings based on the cohesive zone model

This study enhances the cohesive zone model to describe the propagation of interfacial cracks in thermal barrier coatings (TBCs) corroded by calcium-magnesium-alumino-silicates (CMAS) at high temperatures. The finite element method is used to build a multi-layer 2D plate model that simultaneously simulates stress distributions, infiltration of CMAS, corrosion of ceramic coat and interface crack growth in TBCs. Furthermore, this study provides a detailed elaboration of the growth mechanism of interface cracks and the performed corrosion experiment. The theoretical model presented here validates the spalling phenomena of TBCs caused by CMAS corrosion at high temperatures.

Assessment of stress characteristic and interfacial delamination in thermal barrier coatings considering the connected crack network

The coalescence induced by the propagation of inter-lamella and intra-lamellar cracks in thermal barrier coatings (TBCs) by atmospheric plasma spraying (APS) is the root cause of coating spallation. To clarify the influence of defects such as cracks on the coating failure, a real TBC model including the connected crack networks, spherical pores, and unmelted particles is developed. A defect-free equivalent model is also established for comparison. The through-thickness thermal conductivity and elastic modulus of the ceramic layer are determined by experiments and simulations. The stress state and damage evolution of the TC/TGO interface under these two models are evaluated. The results show that the thermal insulation of APS TBC is mainly dominated by the inter-lamella crack. The defects in the coating cause a great increase of stresses in the ceramic layer and at the TC/TGO interface. The TBC lifetime by an equivalent model will be higher than the experimental value. The TGO creep can delay the damage of the TC/TGO interface. The results of this study can provide significant theory guidance for designing advanced TBCs toward high insulation and long life.

Mechanical Properties Durability of Sc2O3-Y2O3 Co-Stabilized ZrO2 Thermal Barrier Materials for High Temperature Application

The poor phase stability of conventional 6–8 wt.% yttria-partially stabilized zirconia (YSZ) results in the formation of detrimental cracks in thermal barrier coatings (TBCs) as well as the decrease of fracture toughness, limiting its long-term application above 1200 °C. Here, we try to prove that a higher t′-phase stability is the prerequisite for the mechanical properties durability of ZrO2-based TBC materials. Compared with YSZ ceramic, 7.0 mol.% Sc2O3 and 0.5 mol.% Y2O3 co-stabilized ZrO2 (ScYSZ) shows a superior t′ phase stability at 1300 °C. In addition, the fracture toughness of ScYSZ is nearly stable after being annealed at 1300 °C for 0–40 h, while the value of YSZ gradually declines during the aging process. The stable fracture toughness of ScYSZ is closely related to its prominent t′ phase stability. The elastic modulus of ScYSZ is nearly unchanged and lower than that of YSZ. The excellent stability of the phase structure and mechanical properties makes ScYSZ a great potential material for next generation high-temperature TBCs.

A coupled theory for deformation and phase transformation due to CMAS infiltration and corrosion of thermal barrier coatings

This work proposes a mechanism-based phase-field approach for tetragonal-to-monoclinic transformation in thermal barrier coatings (TBCs) corroded by calcium-magnesium-alumino-silicates (CMAS). A 2D plate model is established and verified by experiments results of the distribution of CMAS mass density and the extension of the monoclinic phase after different heating times. In addition, the simulation results show that the tensile stress accumulates in the interface between two phases, which can be seen as the motivation for horizontal cracks in TBCs. The theoretical model presented here shows excellent capacities to capture the evolution of the monoclinic phase in TBCs corroded by CMAS.

Propagation feature of cracks in plasma-sprayed YSZ coatings under gradient thermal cycling

The cracking behavior of plasma-sprayed yttria-stabilized zirconia (YSZ) during thermal gradient cycling test was investigated. The relationship between the size of the bright spot observed on YSZ coating surface and the size of the crack in thermal barrier coatings (TBCs) was examined by introducing pre-cracks in TBCs assisted with finite element analysis (FEA). The cracking behavior during thermal cycling, and the relationship between the residual lifetime of TBCs and the crack size were examined. Results show that the bright spot observed on YSZ surface corresponds to the crack in the coating and thus the size of bright spot increases with crack size. It was found that the crack size increases linearly with thermal cycle number when the relative thermal cycle number is less than 85% of thermal cyclic lifetime. The crack propagation acceleration at the later stage of lifetime is attributed to coalescence of numerous cracks. The obvious coalescence of cracks in TBCs begins to take place when the residual lifetime becomes approximately 15%, which leads to a rapid decrease of the residual lifetime of YSZ coating with the increase of thermal cycle number.

Evolution of Lamellar Interface Cracks During Isothermal Cyclic Test of Plasma-Sprayed 8YSZ Coating with a Columnar-Structured YSZ Interlayer

The failure of plasma-sprayed thermal barrier coatings (TBC) usually occurs through spalling of ceramic coating. The crack evolution during thermal cycling of TBC is directly associated with its spalling. In this paper, the cracks in TBC along the direction of the interface between ceramic coating and bond coat were examined from cross-section of TBC experienced different numbers of thermal cycle, and crack number and the total length of cracks were measured to aim at understanding the failure mechanism. TBC consists of cold-sprayed NiCoCrAlTaY bond coat on IN738 superalloy and double layered plasma-sprayed 8YSZ with a columnar grain structured YSZ interlayer of about 20 μm thick and about 230 μm lamellar YSZ. With each isothermal cyclic test, the TBC samples were kept at 1150 °C for 26 min hold and then cooled down to a temperature less than 80 °C in 4 min by air forced cooling. Results showed that cracks propagated primarily within lamellar-structured YSZ over the columnar YSZ along lamellar interface. The measurement from the cross-section revealed that crack number and total crack length apparently increased with the increase of the number of thermal cycle. It was found that cracks with a length less than a typical size of 200 μm accounted for the majority of cracks despite the number of thermal cycle during the test. A crack initiation and propagation model for plasma-sprayed TBC is proposed with a uniform distribution of circular cracks. The propagatable cracks form homogeneously within plasma-sprayed porous YSZ coating at the early stage of thermal cycling and propagate at an identical rate during thermal cycling. Only a few of large cracks are formed before most cracks reach to the critical size for multi-cracks linking-up. The propagation of most cracks to the critical size will leads to the rapid crack bridging and subsequent spalling of top ceramic TBC.

Crack Engineering in Thick Coatings Prepared by Spray Pyrolysis Deposition

Cracks should normally be avoided in the deposition of coatings, but vertical cracks in thermal barrier coatings are engineered to absorb thermo-mechanical stress. Thick lanthanum zirconate coatings were deposited by spray pyrolysis deposition from aqueous nitrate-based precursor solutions, and cracks were formed during decomposition of the nitrate species due to the associated volume change. The crack spacing and crack opening in the deposited coatings were analyzed in terms of thickness, pH of the precursor solution, and deposition and decomposition temperatures and kinetics by thermogravimetry, scanning calorimetry, mass spectroscopy, and electron microscopy. The thickness of the coatings demonstrated the most important effect on the crack pattern. The crack opening and the crack spacing varied linearly with increasing thickness, leading to small delamination at the interface. The cracks were stable after the crystallization of the films by further heat treatment. Knowing the influence of the different parameters, coatings with a designed crack pattern can be deposited.

Finite element simulation of interface cracks in thermal barrier coatings

Thermal barrier coatings, frequently used in turbine blades, comprise a corrosion-protecting bond coat, a ceramic top coat and a growing oxide layer in between. Failure mechanisms of these coating systems are still not satisfactorily understood. In this paper, crack initiation and propagation in thermal barrier coatings is studied with two-dimensional finite element simulations. Cracks are propagated at the interface between the TGO and the TBC either at the peak or at the valley position of asperities for different values of the creep strength of the materials. It is shown that, usually, cracks starting in the valley position have a larger energy release rate than those starting at peaks. Cracks in the peak position propagate in mode II even with rather thin TGO and thus do not relax stresses as effectively as cracks in the valley position. Whether cracks are arrested or not strongly depends on the creep properties. Additional effects that may have to be taken into account to model crack arrest more realistically are also discussed.

Boundary element prediction of TBC fracture resistance

ABSTRACTThe present study investigates the transient behaviour of interfacial cracks in thermal barrier coatings. It is assumed that a TBC withstands a thermal shock in the presence of external mechanical load acting on its surface. Crack closure takes place while the thermal contact resistance as well the friction between the crack faces is considered. The dependence of the thermal resistance on the contact pressure invokes coupling between the temperature and displacement fields. An appropriate boundary element procedure based on two‐dimensional time‐dependent thermoelasticity, is utilized for the numerical solution. A series of parametric analyses examines the impact of the coefficient of friction, thermal contact resistance and coefficient of heat convection on the crack severity.

On TGO creep and the initiation of a class of fatigue cracks in thermal barrier coatings

The initiation of a class of fatigue cracks observed in thermal barrier coatings (TBCs) subjected to thermal gradient mechanical fatigue testing is investigated. The coating system is based on a NiCoCrAlY bond coat and a partially yttria stabilized zirconia top coat. To explain the development of the cracks of interest, the thermo-mechanical response of the bond coat and the thermally grown oxide (TGO) is examined and quantified through finite element analyses. The models include non-linear and time-dependent behavior such as creep, TGO growth stress, and thermo-mechanical cyclic loading. The simulations suggest that stress-redistribution due to creep can lead to tensile stresses in the TGO during TGMF testing that are large enough to initiate the cracks investigated.

Effect of Hollow Spherical Powder Size Distribution on Porosity and Segmentation Cracks in Thermal Barrier Coatings

The effect of characteristics of hollow spherical (HOSP) powders on porosity and development of segmentation cracks in plasma‐sprayed thick thermal barrier coatings (TBCs) was investigated. Three powders with particle size ranges of 20–45, 53–75, and 90–120 μm were selected from a commercial HOSP powder feedstock for spraying the TBCs. The 20–45 μm powder has a higher deposition efficiency and a greater capability of producing segmented coatings than the other larger powders. Diagnostics of in‐flight particles show that the average surface temperature and velocity of the particles sprayed from the fine powder is higher by 250°C and 50 m/s compared with those sprayed from the 90 to 120 μm powder, respectively, due to its greater ratio of surface area to mass. The lower porosity of the coating sprayed from the fine powder is mainly attributed to the decreased volume of intersplat gaps and voids.

Formation of vertical cracks in solution-precursor plasma-sprayed thermal barrier coatings

When tailored to make durable thermal barrier coatings (TBCs), the Solution Precursor Plasma Spray (SPPS) process produces a microstructure containing uniformly vertical cracks. These cracks provide a high degree of strain tolerance to the ceramic top coat. In order to understand the formation of vertical crack in SPPS process, coatings of various thicknesses were deposited on a variety of substrates with vastly different thermal properties. These coatings were characterized in the as-sprayed state and after heat treatment. It has been determined that the tensile stress derived from the pyrolysis of precursor occurring during coating deposition or post heat-treatment is the major driving force for the formation of vertical cracks in SPPS TBCs.

Thick ceramic thermal barrier coatings with high durability deposited using solution-precursor plasma spray

The efficacy of ceramic thermal barrier coatings (TBCs) used to protect and insulate metal components in engines increases with the thickness of the TBCs. However, the durabilities of thick TBCs deposited using conventional ceramics-coating deposition methods have not been adequate. Here, we demonstrate the feasibility of depositing highly durable, 4 mm thick TBCs of ZrO 2–7 wt.% Y 2O 3 (7YSZ) on bond-coated superalloy substrates using the solution-precursor plasma spray (SPPS) method. It was found that the average thermal cycling life of the 4 mm thick SPPS TBCs is 820 cycles. While most of the conventional air plasma-sprayed (APS) coatings of the same composition and thickness deposited on identical bond-coated superalloy substrates were found to be detached partially from the substrates in the as-sprayed condition, the APS TBC that was intact failed after 40 thermal cycles. The dramatic improvement in the thermal cycling life in the SPPS TBCs can be attributed to: (i) the significantly higher in-plane indentation-fracture toughness (over five-fold) in the SPPS TBCs over APS TBCs and (ii) the presence of the vertical cracks in SPPS TBCs resulting in a high degree of strain tolerance. The large thickness of the SPPS coatings also allowed us to characterize the mechanical properties of the ceramic top-coat in some detail. To that end, we report here the results from indentation-toughness tests and uniaxial-compression tests on the SPPS TBCs and the reference APS TBCs.