Thermally Grown Oxide Interface Research Articles

Failure behaviour of EB-PVD YSZ thermal barrier coatings under simulated aero-engine operating conditions

Thermal shock test simulating aero-engine operating conditions was carried out on electron beam physical vapor deposition (EB-PVD) yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs) using advanced multi factor coupling test equipment. Phase composition, thermally grown oxide (TGO) residual stress, surface morphology and cross-sectional microstructure evolution in coatings were systematically studied. Based on XRD results, coatings possessed tetragonal phase structure without any further transformation during testing. After 5000 cycles, the thickness of TGO gradually increased from 0.3 to 2 μm. TGO residual compressive stress also gradually increased and then drastically dropped when the coating spalled. Horizontal cracks first initiated from TGO/bond coat (BC) interface, then arose from top coat (TC)/TGO interface and finally coalesced through TGO to cause delamination. Repeated thermal shock and gas impact conditions promoted the emergence of vertical cracks in ceramic top coat. Once communicated with transverse delaminations, cracks eventually led to spalling failure of the coating system. The contribution to spallation failure was mainly from thermal stress as a result of thermal expansion mismatch between the coating and the substrate. In turn, elastic stress induced by high speed impact of gas and TGO growth stress promoted initiation and propagation of coating cracks.

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.

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.

Spallation mechanism of thermal barrier coatings with real interface morphology considering growth and thermal stresses based on fracture phase field

The interface of atmospheric plasma spray (APS) thermal barrier coatings (TBCs) is extremely rough and uneven, causing extremely complicated laws in thermally grown oxide (TGO) growth, stress distribution and evolution, as well as crack initiation and expansion. In this paper, a fracture phase field theory of TBCs interfaces with considerations of growth and thermal stresses is established. A two-dimensional geometric model of APS TBCs was constructed based on the real interface morphology. Next, the TGO growth laws and interface spallation mechanism during the service of TBCs were studied through numerical simulation and experiment. Results demonstrated that the TGO growth rate at the convex position on the interface is higher than those at other positions, especially at the local convex positions. TGO at the left and right sides of concave positions approach continuously with oxidisation of the interface, causing the TGO to wrinkle and the interface to become rougher. With an increase in TGO thickness, stresses in the TGO at peak areas perpendicular to the interface increase significantly and their maximum reaches to 1.58 GPa. Interface cracks initiated in peak areas around the TGO/BC interface at 877 h, and then formed in flat positions in the TGO at 912 h as well as valley areas around the TGO/TC interface at 928 h. Final, TBCs failed at 959 h due to the coalescence of these cracks. All four types of interface cracks could be found in the simulation results.

Combined effect of thermally grown oxide growth and calcium–magnesium–alumino–silicate infiltration in thermal barrier coating system

In recent years, thermal barrier coating systems (TBCs) have been commonly used in aerospace equipment to enhance the performance of substrates. However, premature fracture of TBCs occurs due to the infiltration of calcium–magnesium–alumino–silicate (CMAS) particulates and excessive thickness of thermally grown oxide (TGO). To comprehensively understand the performance of TBCs, the effects of CMAS infiltration and TGO growth on the fracture behavior of TBCs were investigated. Two thermo-mechanical models were constructed to investigate the residual stress evolution of TBCs under the combined effect of CMAS infiltration and TGO growth. CMAS infiltration, TGO growth, expansion due to separation of the grain boundary, and tetragonal-to-monoclinic phase transformation were implemented by the ABAQUS subroutine. The buckle height and TGO growth trends were consistent with the experimental and theoretical results, respectively. The results indicate that the crack has a high probability of initiating and propagating at the off-peak of the topcoat (TC)/TGO interface owing to the maximum interfacial stresses. It can be concluded that in the case without TGO growth, CMAS infiltration results in an increase in the interfacial stresses. However, in the case with TGO growth, CMAS infiltration suppresses the interfacial stresses.

Effects of TGO growth on the stress distribution and evolution of three-dimensional cylindrical thermal barrier coatings based on finite element simulations

Based on the ultrasonic C-scan results of 8YSZ coatings after thermal cycles, three-dimensional cylindrical numerical simulations of the physical geometry model of the thermal barrier coating (TBC) sinusoidal surfaces were conducted with finite elements to estimate the stress distribution and evolution law of the top coat (TC)/bond coat (BC) interface, including the centre and edge of the specimen affected by the dynamic growth of the thermally grown oxide (TGO). The results show that when a layer of TGO is grown on the TC/BC interface, compressive stress is uniformly distributed on the TGO interface, and the stress value decreases as a function of the TGO layer thickness. When the thickness of the TGO exceeds a certain value, the compressive stress of all parts of the interface gradually changes to tensile stress; meanwhile, the edges of the model affected by the crest and trough effects of the wave are reflected in the radial and circumferential directions, especially along the axial direction, with alternating concentrated tensile and compressive stresses. TGO growth imposes a minor influence on the magnitude and distributions of the radial and circumferential stresses at the BC interface. The linear elasticity, creep, fatigue, and stress accumulation effects of each layer of TBCs in each thermal cycle were fully considered in this model. The model not only interprets the crest and trough effects of the TC/BC surface interface during the growth of TGO, but also interprets the effects of the core and edge of the cylindrical model, further revealing the reason for which the core and edge of the TBC will most likely form cracks.

Delamination mechanism of thermal barrier coatings induced by thermal cycling and growth stresses

The growth of thermally grown oxide (TGO) is often regarded as a key factor causing failures of thermal barrier coatings (TBCs) because it leads to thermal cycling and growth stresses. However, the delamination mechanism of TBCs induced by the growth process of TGO is still unclear. Here, a finite element method (FEM) considering both thermal cycling and growth stresses is developed. The accumulation of damage and crack nucleation, propagation, and coalescence are obtained using a phase field damage model. The results indicate that there is a critical TGO thickness of 8–10 μm, which may result in delamination of the TBCs. Both thermal cycling and growth stresses induce the premature failure of TBCs. The delamination mechanism induced by thermal cycling and growth stresses is obtained. When the interface roughness is A/L ≥ 1/4, the cracks pass through the TGO/TC interface and propagate in the TC layer. When the interface roughness is A/L ≤ 1/8, the cracks propagate along the vicinity of the TGO and will not pass through the TC/TGO interface. When the interface roughness is 1/8 < A/L less than 1/4, the cracks bifurcate at the vicinity of TGO interface, and one branch propagates in the TC and the other propagates along the vicinity of the TGO.

Thermal barrier coatings failure mechanism during the interfacial oxidation process under the interaction between interface by cohesive zone model and brittle fracture by phase-field

Failure of Thermal barrier coatings (TBCs) caused by interface oxidation directly determines the ultimate durability of TBCs. The interfacial delamination and brittle fracture will co-occur during the interface oxidation of the TBCs. To study the interaction between the two failure modes, we develop a thermodynamically-consistent coupling framework combining the phase-field model of the brittle fracture in the bulk material and the cohesive zone model of the pre-existing adhesive interface for TBCs. We divide the critical energy release into two parts to distinguish interface and bulk material failure. According to the variational principle, the phase-field crack evolution equation is obtained. Moreover, the other variables’ coupled constitutive and evolving equations are obtained through the dissipation inequality and balance equations. Models and experiments show interface oxidation of APS (air plasma spraying)-TBCs is tending to cracks in the valley of the TC (top coating)/TGO (thermally grown oxide) interface and the peak of the TGO/BC (bond coating) interface. The newly generated TGO will turn the crack at the TGO/BC interface peak into TGO inner crack and eventually lead to equidistant cracks in the TGO peak. Besides, by studying the interaction between cracks at two locations, we find that the TGO/BC interface cracks promote the TC/TGO interface crack initiation and inhibit its propagation. The TC/TGO interface crack very weakly promoted the TGO/BC interface cracks initiation and propagation. The framework present here provides excellent potential for modeling the oxidation failure process in TBCs.

Method for Predicting Thermal Fatigue Life of Thermal Barrier Coatings Using TGO Interface Stress

Thermal barrier coatings (TBCs) applied to high-temperature components of gas turbines consist of a ceramic top coat, a metallic bond coat, and thermally grown oxide (TGO) generated between the top coat and bond coat. Because TBCs are subjected to repeated thermal stress at the coating interface under thermal fatigue conditions and eventually breakage, it is crucial to evaluate the thermal fatigue durability of TBCs according to the stress. In this study, coin-type TBC specimens were prepared by depositing commercial coating powders on Ni-based super alloys via the air plasma spray method, and the thermal fatigue life of the TBCs was experimentally evaluated. According to the test results and references, a finite-element analysis was conducted. The maximum stress of the TGO interface was evaluated according to the thickness and equivalent elastic modulus, and simulating the microstructure including the pores of the top coat. Using these relationships, a thermal fatigue life prediction equation considering the coating thickness (t), equivalent elastic modulus (E), and operating temperature (T) was derived, and subsequently verified.

Influence of ceramic top coat and thermally grown oxide microstructures of air plasma sprayed Sm2SrAl2O7 thermal barrier coatings on the electrochemical impedance behavior

Electrochemical impedance spectroscopy (EIS) technique is used to examine the top coat and thermally grown oxide (TGO) microstructures of Samarium Strontium Aluminate (SSA) thermal barrier coatings (TBCs) after exposed to pre-oxidation and oxidation at 1050 and 1100 °C, respectively. EIS spectra showed that the three relaxations frequencies in Bode plot corresponded to SSA top coat, TGO, and TGO-bond coat interface. A significant reduction in polarization resistance of SSA top coat and increase in capacitance for different pre-oxidation times of 10, 20 and 30 h are being found due to increasing the defects (pores and cracks) of about 13, 29 and 45%, respectively at 1050 °C. The reduction in bi-axial residual stresses was calculated to be about 57% as a consequence of decrement in top coat resistance. The growth of SSA TGO was found from 10 to 20 h of pre-oxidation treatment caused to decrease the capacitance which indicates the presence of highly enriched α-Al2O3 at the bond coat-top coat interface. The highest charge transfer resistance and lowest capacitance were found to be about 0.48 × 106 Ω cm2 and 1.1 nF cm−2 respectively for 20 h of pre-oxidation which could reflect the overall impedance kinetics of TBC system (SSA top coat and TGO) at the TGO-bond coat interface.The impedance responses of SSA top coat, TGO, and TGO-bond coat interface were reduced drastically after oxidation at 1100 °C for 10 h pre-oxidized specimen as compared to 20 and 30 h due to the compositional change of pure α-Al2O3 based TGO into more conductive NiCr2O4. The lowest diffusion coefficient, DCr3+ in the NiO lattice which reduced the formation of metal ion vacancies at the TGO-top coat interface caused to exhibit higher TGO resistance for 20 h pre-oxidized specimens over 10 and 30 h after oxidation at 1100 °C.

Residual stress distribution along interfaces in thermal barrier coating system under thermal cycles

The residual interfacial stress plays an important role in crack initiating and propagating along the interface, which could result in delamination failure of the thermal barrier coatings (TBCs). In this study, the finite element model of air plasma spraying(APS) TBCs was established to assess the level and distribution of residual stress along top coat(TC)/thermally grown oxide (TGO) and bond coat (BC)/TGO interfaces under thermal cycles. Instead of using vertical stress S22 in global coordinate system, the normal and tangential components in the local system along the interfaces, transformed from stress components S11, S22, and S12 in the global one, were used to evaluate the way the cracks initiate and propagate along the interfaces. Firstly, the effect of the number of thermal cycles on residual stress was investigated. It was found that, for the TBCs model without TGO growth and crack, the impact of the number of thermal cycles on the stress is very insignificant and could be ignored. So the present study only chose to focus on the first thermal cycle. Then the influence of the TGO thickness and the interface amplitude on the normal and tangential residual stresses for both homogeneous and inhomogeneous temperature fields was explored. The results show that the TGO thickness, interface amplitude and temperature field affect the residual stress level and distribution, leading to different fracture mechanisms along TC/TGO and TGO/BC interfaces. Finally, the difference between the vertical stress in the global coordinate system and the normal stress in the local coordinate system was studied. Compared with vertical stress S22, the stress components normal and tangential to the TC/TGO and TGO/BC interfaces are more appropriate to describing the stress distribution along the interfaces and predicting the propensity of crack initiating and propagating along the interfaces.

Finite Element Analysis of the Effects of Thermally Grown Oxide Thickness and Interface Asperity on the Cracking Behavior Between the Thermally Grown Oxide and the Bond Coat

Finite element simulations based on an interface cohesive zone model (CZM) have been developed to mimic the interfacial cracking behavior between the α−Al2O3 thermally grown oxide (TGO) and the aluminum-rich Pt–Al metallic bond coat (BC) during cooling from high temperature to ambient temperature. A two-dimensional half-periodic sinusoidal geometry corresponding to interface undulation is modeled. The effects of TGO thickness and interface asperity on the stress distribution and the cracking behavior are examined by parametric studies. The simulation results show that cracking behavior due to residual stress and interface asperity during cooling process leads to stress redistribution around the rough interface. The TGO thickness has strong influence on the maximum tensile stress of TGO and the interfacial crack development. For the sinusoidal asperities, there exists a critical amplitude above which the interfacial cracking is energetically favored. For any specific TGO thickness, crack initiation is dominated by the amplitude while crack propagation is restricted to the combine actions of the wavelength and the amplitude of the sinusoidal asperity.

Microstructural examination of TGO formed during pre‐oxidation on Pt‐aluminized Ni‐based superalloy

The study concerns electron beam physical deposition (EB‐PVD) of yttria stabilized zirconia (YSZ) deposited on pre‐oxidized Pt‐modified aluminide bond coating. The coating was prepared by the means of Pt galvanizing followed by vapor phase aluminizing at 1050 °C. During annealing at 1100 °C in air atmosphere for 4 h a 1 µm thick layer of thermally grown oxide (TGO) consisting of equiaxed α‐alumina grains was formed. Focused ion beam and scanning/transmission electron microscopy analysis revealed the presence of nanoscale Cr‐rich precipitates distributed in the TGO as well as Cr precipitation in the grain boundaries of the β phase at the interface between the TGO and bond coat. Deposition of the YSZ on a pre‐oxidized substrate provided mechanical keying with the TGO. Void formation was observed at the TGO and bondcoat interface presumably due to initial transient alumina formation.

Predicting failure within TBC system: Finite element simulation of stress within TBC system as affected by sintering of APS TBC, geometry of substrate and creep of TGO

Demand for economically efficient and environmental friendly gas turbine engines leads to the usage of a thermal barrier coating (TBC) system, which is usually sprayed on the top of a superalloy substrate. The system includes a ceramic TBC, a bond coat (BC) and a thermally grown oxide (TGO) layer. Thermo-mechanical mismatch stresses created within the coating at the end of a thermal cycle lead to spallation of the ceramic coating and a rapid increase in the temperature of the substrate. The thickness of the oxide layer and the amount of aluminium depleted during high temperature operation also affect the lifetime of the TBC. As a first step to the prediction of the failure mechanisms and the lifetimes of TBCs, a preliminary study of how the stress distribution within the TBC system is affected by different factors is required. This paper investigates the effects of the sintering of the ceramic layer, of the geometry of the substrate and of the creep of the TGO, on the stresses built up in the TBC system. Three different TBC system geometries were modelled using plane strain FE models with three different sets of TGO creep properties. An Arrhenius equation was fitted to the temperature dependent modulus of the sintered TBC using results published in the open literature. The equation was later implemented within the FE model. It is concluded that the TBC on the top of flatter regions of substrate produces smaller tensile residual stresses compared to sharp corners of the substrate. It was also found that the initiation and propagation of cracks within a TBC, during steady state operation depends on the choice of the creep parameters of the TGO. At the cooling stage, increase in the modulus of the TBC, due to sintering, has been shown to produce stresses within the TBC near the TGO interface that are as large as twice the value that is predicted using a model without sintering.

Thermal Shock Damage and Microstructure Evolution of Thermal Barrier Coatings on Mar-M247 Superalloy in a Combustion Gas Environment

The effect of preoxidation on the thermal shock of air plasma sprayed thermal barrier coatings (TBCs) was completely investigated in a combustion gas environment by burning jet fuel with high speed air. Results show that with increasing cycles, the as-oxidized TBCs lost more weight and enlarged larger spallation area than the as-sprayed ones. Thermally grown oxide (TGO) growth and thermal mismatch stress were proven to play critical roles on the as-oxidized TBC failure. Two types of significant cracks were identified: the type I crack was vertical to the TGO interface and the type II crack was parallel to the TGO interface. The former accelerated the TGO growth to develop the latter as long as the oxidizing gas continuously diffused inward and then oxidized the more bond coat (BC). The preoxidation treatment directly increased the TGO thickness, formed the parallel cracks earlier in the TGO during the thermal shocks, and eventually resulted in the worse thermal shock resistance.

Oxide growth and damage evolution in thermal barrier coatings

Cracking in thermal barrier coatings (TBC) is triggered by the development of a thermally-grown oxide (TGO) layer that develops during thermal cycling from the oxidation of aluminum present in the bond coat (BC). In the present communication a numerical model is presented that describes the interactive development of the TGO morphology and the fracture processes in TBC systems in a mesh-independent way. The evolution of the TGO–BC mixture zone is described by an oxygen diffusion–reaction model. The partition-of-unity method is employed for the simulation of discrete cracking, where cracks can nucleate and propagate across finite elements at arbitrary locations and orientations. The validity of the model is demonstrated through the analysis of a representative TBC system subjected to a specific thermal cycling process. A parametric analysis demonstrates the sensitivity of the response of the TBC system to the fracture strength of the top coat. The simulation results indicate that cracks appear primarily at the current location of the BC–TGO interface and may nucleate at early stages of thermal cycling. These results are in good agreement with recent experimental observations reported in the literature.

Evaluation of interface degradation during cyclic oxidation of EB-PVD thermal barrier coatings and correlation with TGO luminescence

The typical degradation mechanism of EB-PVD thermal barrier coatings (TBCs) that occurs during cyclic furnace testing of a commercial 7YSZ coating on a single-crystal nickel-base superalloy is analyzed. The mechanism includes local separation of the ceramic top coat from the thermally grown oxide (TGO), growth of the separated regions along the coating–substrate interface, and, upon reaching a critical size, large-scale TBC spallation. The extent of interface separation was evaluated by analyzing cross-section microstructures of the tested samples at different stages of TBC cyclic life. The morphological changes of the TGO as a result of roughening of the (Ni,Pt)Al bond coat surface were also quantified. Concurrently, the evolution of the TGO luminescence during cyclic oxidation and the development of TBC undulations, associated with large TBC–TGO interface separations, were monitored. It is demonstrated that there is a correlation between the extent of interface damage and the shift in TGO luminescence frequency, which can be evaluated nondestructively.

遮熱コーティング界面の酸化層成長に及ぼす負荷形態の影響(OS5-1 コーティングの酸化特性,OS5 コーティングプロセスとその皮膜特性評価)

The growths of thermally grown oxide (TGO) layer of thermal barrier coating (TBC) interface were investigated by thermal cycle test and heating test. A burner rig device was used for thermal cycle test of a plate with TBC system in air. A furnace device was also used for constant heating test of a circular plate with TBC system. The specimen was composed of two kinds of coatings (top coat and bond coat) that was sprayed on a substrate. The relationships between heating time and thickness of TGO layer were examined by those tests. The thickness of TGO layer was measured from the observation of the microstructures by SEM at TGO interface before and after those tests. As a results, it was found that the growth of TGO layer don't depend on heating loading type and can predict by a proposed growth law.