EB-PVD Thermal Barrier Coatings Research Articles

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

Abstract 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.

Vapor-Phase CMAS-Induced Degradation of Adhesion of Thermal Barrier Coatings

An investigation of the interface degradation and failure of EB-PVD thermal barrier coatings (TBCs) during burner rig cyclic oxidation in the presence of calcium–magnesium–alumino-silicate (CMAS) deposits has been conducted. The formation of two compounds of major CMAS constituents, specifically MgAl2O4 (spinel) and CaAl2Si2O8 (anorthite), was observed at the interface between the alumina scale and the TBC. Spallation occurred as a result of cracking and delamination primarily through the spinel layer. The tests showed more than a tenfold decrease in TBC life relative to the oxidation test without CMAS at the same temperature. It is demonstrated that this particular form of coating degradation occurs below the melting temperature of this particular CMAS composition and hence does not involve TBC infiltration by molten silicates. The likely mechanism of interface degradation, which involves vapor-phase transport of CMAS constituents as hydroxides, is proposed and discussed.

Development of bending tests for near shear mode interfacial toughness measurement of EB-PVD thermal barrier coatings

Mechanisms leading to degradation of the mechanical adhesion of thermal barrier coatings (TBCs) used in aircraft turbines are numerous and complex. Direct measurement of interface toughness as a function of thermal cycling exposure must be a component of any practical approach to lifetime assessment based on a spallation criterion. This paper outlines several adhesion tests, where the multilayered system undergoes different mechanical loading. The notion of mode mixity angle is reminded and a method for its determination is described. Of course, this method to compute the mode mixity angle can be useful for any thin filmdelamination problem.Some results on the TBC adhesion energy are presented, relating to both the temperature and the time-of-aging for relevant mode mixity consistent with the spallation configurations observed from feedback on ex-service blades. The effect of the residual stresses induced during thermal cycling is considered here, in order to derive the intrinsic critical energy release rate. The whole set of data enables a description to be made of the intrinsic interfacial toughness decreasing under both thermal aging and mechanical loading. Finally, an energetic approach is used to compare this intrinsic interfacial toughness to the elastic stored energy during thermal cycling, in order to assess the lifetime corresponding to spontaneous spallation.

Water Vapor Effects on the CMAS Degradation of Thermal Barrier Coatings

A new damage mechanism of CMAS degradation of TBCs involving delamination through the thermally grown oxide (TGO) has been observed on burner rig tests of EB-PVD thermal barrier coatings. This mechanism operates in the absence of CMAS melting and is mediated by hydroxylation and volatilization of the constituent CMAS hydroxides, followed by vapor transport to the vicinity of the TGO through the TBC porosity. Analyses reveal the formation of spinel (MgAl2O4) and anorthite (CaAl2Si2O8) at the TGO/TBC interface, with concomitant evolution of porosity at the spinel/alumina interface. Tests were conducted in a controlled atmosphere tube furnace to validate the hypothesis and assess the factors that affect the evolution of the deleterious oxides. The microstructure of the reaction products formed in the lab test is compared to that formed in the burner rig.

Measurement of Young’s Modulus of Thermal Barrier Coatings by Suspended Coupled Flexural Resonance Method

Thermal barrier coatings(TBCs) has been extensively used on hot section components of gas turbine engine, especially turbine blades. Young’s modulus of TBCs is a significant mechanical parameter in life prediction research of turbine blades, but it is hard to measure the EB-PVD thermal barrier coatings by conventional tensile/compression method for its brittleness and porous microstructure. Therefore, Young’s modulus mathematical model of double-layer beam structure specimen was deduced on the basis of the first-order beam bending vibration equation and composite beam bending equation. An experimental platform with dynamic signal acquisition system was set up for measuring Young's modulus of thermal barrier coatings under high temperature. Young's modulus of ceramic coat was measured from ambient temperature to 1100°C to provide material data for subsequent research on life prediction of turbine blades with thermal barrier coatings.

Influence of Impact Damage on the Thermal Stress Distribution within the EB-PVD Thermal Barrier Coatings

Macroscopic holes often form on gas turbine blades surface by high velocity gas stream with some foreign impact particles during service. The influence of the impact-holes on the thermal stress distribution was investigated in this paper. The thermal stress distribution within the TBCs after impact-damages at high temperature was intensively studied by using finite element method. Analyze equivalent stress and thermal stress, it was found that the macroscopic holes on the surface of ceramic coatings could change the original temperature gradient and it transform the thermal stress distribution in the TBCs without impacting, resulting in the maximum tensile stress area expanding at the crest and promoting the generation of cracks and reducing coatings life. The impact-holes at the edge of the blades changed the former thermal stress distribution completely. The maximum thermal stress region existed in the alumina scale, severely decreases the life of thermal barrier coatings.

Direct observations of erosion-induced ferroelasticity in EB-PVD thermal barrier coatings

Imaging of the presence and location of erosion-induced ferroelastic toughening was completed on 8-weight percent yttria-stabilized zirconia EB-PVD thermal barrier coatings using confocal polarized Raman spectroscopy. A combination of measurements made using ultraviolet and visible laser radiation was conducted to determine the location and extent of ferroelastic twinning present at and below the eroded surfaces as well as along fracture surfaces of the coatings. Ferroelastic twinning events were identified at three major locations within and on the coating: the erosion surface, just below the surface, and in the bulk of the coating. The results shown here reveal that not all fracture events result in a ferroelastic response. This suggests there may be an opportunity to increase the toughness of thermal barrier coatings by increasing the possibility that a crack can produce a ferroelastic twin in the coating.

Infrared radiative properties of EB-PVD thermal barrier coatings

In the present paper the infrared radiative properties of electron beam physical deposition (EB-PVD) partially yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs) have been studied in combination with experimental measurement and theoretical model. The method of Finite-Difference-Time-Domain (FDTD) is employed to investigate the infrared spectral reflectance and transmittance of TBCs with various microstructural parameters and validated by a following experiment. The infrared radiation transfer mechanisms inside the coatings are explored and examined in detail. The results indicate that distinctive micro-nanostructures of EB-PVD coatings have important effects on its radiative performance, and the infrared spectral reflectance and transmittance of coatings are highly affected by the primary columnar morphology and pore architectures, such as intercolumnar gaps, feathery striation and closed globular pores. Further research suggests that scattering effects caused by different microstructures can greatly affect radiative transfer across the coatings. By optimizing the microstructure of coatings properly to enhance its backscattering effects, it can effectively increase the spectral reflectance and reduce the spectral transmittance, thereby reducing the radiation heat flows and providing a better thermal protection for the metallic substrate.

Lifetime Performance of EB-PVD Thermal Barrier Coatings with Coating Thickness in Cyclic Thermal Exposure

Lifetime Performance of EB-PVD Thermal Barrier Coatings with Coating Thickness in Cyclic Thermal Exposure

Compiled furnace cyclic lives of EB-PVD thermal barrier coatings

Furnace cycling has been widely used to study the failure of EB-PVD thermal barrier coatings. This contribution compiles TBC furnace cyclic lives over a broad literature base to highlight optimum systems and generalized trends not always apparent in one study. Systems included typical bond coats (Pt-modified aluminides, diffused Pt-only γ/γ′, and NiCoCrAlY (±Pt, Hf) overlays) and superalloy substrates (1st, 2nd, 3rd generation single crystals, directionally solidified, or conventionally cast). Pretreatments included controlled low p(O2) bond coat pre-oxidation and grit blasting (or none). The aggregate lives (~70) suggest a general trend with temperature, ~10-fold decrease for every 100°C increase. Measured alumina scale thicknesses (~30) were, on average, ~6.1±1.8μm at failure and independent of temperature for conventional systems. Most failures thus occurred in less time than that predicted to grow 7μm of alumina scale (as estimated from separate TGA studies of a Pt-modified aluminide coated 2nd generation single crystal superalloy). A tentative activation energy indicated from the broad distribution of failure times was ~280kJ/mol, while that from homogeneous TGA testing was ~380kJ/mol, with regression coefficients of r2=0.57 and 0.98, respectively.

Interaction of molten silicates with thermal barrier coatings under temperature gradients

This paper examines the effect of temperature gradients on the interaction between silicate deposits and thermal barrier coating (TBC) systems. A dedicated test facility, in which a CO2 laser is employed to impose a controllable thermal gradient through the coating and underlying substrate, is used to investigate the interaction between two silicate compositions with state-of-the-art 7YSZ EB-PVD TBCs. The experimental results are then used to guide the development of expressions that describe the nature of silicate infiltration in the TBCs, the evolution of coating elastic modulus, and the generation and release of stresses.

Finite element simulation of residual stress of EB-PVD thermal barrier coatings during thermal cycling

In this study, the residual stress of electron beam - physical vapour deposition thermal barrier coatings (TBCs) with various times of thermal cycling was calculated by the finite element (FE) meth...

Cohesive zone modelling of thermal barrier coatings interfacial properties based on three-dimensional observations and mechanical testing

This study aims to identify interfacial mechanical properties of a typical EB-PVD TBC system. High temperature influence on the evolution of the phase transformation in the bond-coat layer was quantitatively measured using synchrotron laminography 3D imaging of the thermal barrier coating (TBC) microstructure. Mechanical compressive tests have enabled strain at spallation to be characterised as a decreasing function of time spent at high temperature. A higher level of compression has also enabled the growth of interfacial crack after the first spallation event. On this basis the parameters of the Needleman's cohesive zone model were identified. The influence of the morphology of the ceramic/metal interface on damage evolution was numerically tested and was seen to be consistent with known delamination process in TBCs.

Influence of Surface Finish on the Cyclic Oxidation Lifetime of an EB-PVD TBC, Deposited on PtAl and Pt-diffused Bondcoats

Thermal barrier coatings (TBCs) are well established as protective systems for gas turbine hot path components, due to their ability, with substrate cooling, to reduce the maximum surface temperature experienced by the metal component. However, when subject to high temperature oxidation, cyclic heating and cooling during service, TBCs degrade in both thermal protection capability and mechanical stability as a result of a combined thickening of the alumina-thermally grown oxide and sintering of the ceramic top coat. Eventually the ceramic top coat spalls from the metallic substrates. The detailed failure mechanisms for the TBC often are complicated, reflecting a balance between defects introduced into the TBC during manufacture and service and the stored energy generated in the TBC as a result of cyclic thermal exposure. It has been shown that the surface finish influences the residual stress in the thermally grown oxide and thus the stored energy. In this study, the influence of substrate surface finish, prior to bondcoat manufacture, on the cyclic oxidation lifetime is examined. Two EB-PVD TBC systems, a zirconia 8 wt% yttria topcoat on a platinum aluminide bondcoat and a zirconia 8 wt% yttria topcoat on a platinum diffused γ+γ′ bondcoat have been studied. For these two systems, various substrate surface finishes have been investigated, including ground, grit blasted and polished and grit blasted surfaces. The lifetime data for these cyclic oxidation tests of EB-PVD TBCs on these two diffusion bondcoats, platinum aluminide and platinum diffused, on CMSX4, have been analysed statistically for the various surface finishes. It is shown that the variability in measured lifetime can be modelled using Weibull statistics. The role of surface finish on the Weibull model parameters, characteristic life (η) and Weibull modulus (β), are discussed in this paper and hence the role surface finish plays on the likelihood of early, short life, TBC failure. Based on this analysis a more optimised surface finish is recommended to extend TBC lifetimes with diffusion based bondcoats. Further, the platinum diffusion bondcoat is shown to outperform the platinum aluminide system once the substrate surface finish has been optimised.

Computational modelling of constrained sintering in EB-PVD thermal barrier coatings

A micromechanical model is developed to simulate the evolution of microstructure during in-service sintering and eventual inter-columnar cracking in coatings made using electron beam vapour deposition (EB-PVD) route. The coating is idealized with a discrete distribution of axisymmetric asperities across interfaces between columnar grains. The model assumes that inter-columnar sintering is driven by changes in interface free energy of columns and the potential energy of the applied stress. Much faster diffusion that occurs over the free surfaces of the asperities is neglected. It is further assumed that the rate of sintering of the contacting asperities is determined by diffusion along the interface between the contacting asperities. Time evolution of contact modulus of the coating is accounted for as a function of sintering strain. The developed macroscopic constitutive model is employed to evaluate the sensitivity of the sintering response to imperfections and examine the conditions under which inter-columnar cracks can develop within the coating.

Degradation of La2Zr2O7 and other novel EB-PVD thermal barrier coatings by CMAS (CaO–MgO–Al2O3–SiO2) and volcanic ash deposits

Corrosive attack of CMAS deposits is studied for a series of advanced EB-PVD thermal barrier coatings including 14YSZ, HfSZ, 29DySZ, CeSZ, La2Zr2O7, and Gd2Zr2O7 upon a 2hour anneal at 1260°C in air. Furthermore, the effects of volcanic ash deposits on La2Zr2O7, and Gd2Zr2O7 are studied. A 7YSZ standard coating is employed for benchmarking the resulting microstructures and the coating performances in terms of infiltration depth and chemical reactivity. While all investigated zirconia-based coatings become fully infiltrated by the deposits, the zirconates form crystalline reaction products that prevent complete infiltration. The La2Zr2O7 system is treated as a case study with detailed insight on the newly formed phases via analytical TEM.

Effect of oxygen content in NiCoCrAlY bondcoat on the lifetimes of EB-PVD and APS thermal barrier coatings

The effect of oxygen content in NiCoCrAlY bondcoat on the cyclic oxidation lifetimes of EB-PVD and APS thermal barrier coatings (TBC) has been studied. The EB-PVD TBC system with an oxygen content of 0.05 wt. % in NiCoCrAlY bondcoat shows five times longer lifetimes compared to a TBC system with 0.2wt% oxygen in the bondcoat. In the bondcoat with the high oxygen content the minor (0.3wt.%) yttrium addition was found to be tied up by oxygen into fine precipitates of yttrium aluminates. Thereby the beneficial effect of yttrium onto the adherence of the alumina scale was significantly reduced. The critical scale thickness at failure was by about a factor of two lower for the high oxygen bondcoat than for the low oxygen bondcoat. In contrast to EB-PVD TBC systems, no detrimental effect of increasing oxygen content on the lifetime of APS-TBC systems was observed. This can be explained by a different failure mechanism of APS-TBC systems, whereby the lifetime is mainly determined by the rate of crack propagation through the ceramic topcoat.

Effect of Atmosphere Composition on Lifetime and Oxidation Behavior of EB-PVD TBC with NiPtAl Bondcoats

NiPtAl coatings are widely used as bondcoats for thermal barrier coating (TBC) systems during high temperature exposure. Pt modified aluminide coatings on the CMSX-4 Ni-base alloy were oxidized at 1150°C in different atmospheres. Cross-section oxide layer morphologies on the NiPtAl coatings after TBC failure were similar in air with that in air+15%H2O. The surface morphologies of as-received low and high-Pt bondcoats showed great effect on the oxide layer growth and morphologies due to the different compositions within the two bondcoats. The irregular alumina surface on the low-Pt bondcoat was showed due to the slipping of the NiPtAl grains. Raman spectroscopy illustrated that the alumina mainly consisted θ and α-Al2O3 during the Ar+20%O2 exposure, however, only α-Al2O3 could be found in Ar+4%H2+2%H2O during short time exposure. Low oxygen partial pressure of Ar+4%H2+2%H2O perhaps is the reason that spinel and θ-Al2O3 can not formed, and directly promotes the α-Al2O3 formation.

Calcium–magnesium–alumino-silicate (CMAS) degradation of EB-PVD thermal barrier coatings: Characterization of CMAS damage on ex-service high pressure blade TBCs

The use of thermal barrier coatings (TBCs) on turbine blades and vanes located in the hot sections of gas turbine engines has allowed higher engine operating temperatures leading to temperatures of the order of 1200°C at the surface of the ceramic coating. At such temperatures, under service conditions, thermal barrier coatings are susceptible to corrosion by molten calcium–magnesium–alumino-silicates (CMAS) resulting from the ingestion of siliceous mineral debris (dust, sand, ash) by the engine.This study consists in a microstructural analysis of CMAS induced degradation of standard 8YPSZ EB-PVD thermal barriers observed on high pressure turbine blades of military engines removed from service. The CMAS/TBC interactions are mainly observed in the hottest zones of the blade pressure side. CMAS infiltration in the TBC porous microstructure (inter-columnar gaps, pores, cracks) down to the thermally grown oxide TGO interfacial layer is observed as well as dissolution of the 8YPSZ into the CMAS melt, TBC transformation from tetragonal 8YPSZ to monoclinic Y-depleted zirconia and formation of a Zr-bearing phase at the interface between CMAS and TBC. CMAS not only turns out to be mainly constituted of CaO, MgO, Al2O3, and SiO2 but also contains a large amount of iron oxide Fe2O3. Comparison with previous published data shows that CMAS composition depends on the flight conditions to a large extent. A part from the loss of column integrity and the modification of the porous morphology resulting from the CMAS chemical attack, large vertical separations between highly sintered columns are observed in the CMAS infiltrated TBC as well as delamination cracks in the upper part of the top coat possibly leading to progressive TBC spallation. These results are discussed in the light of similar studies on CMAS/TBC interaction and of existing sintering and delamination mechanisms.

Sintering and mud cracking in EB-PVD thermal barrier coatings

A multi-scale model is presented for the evolution of microstructure, sintering and deformation of columnar EB-PVD thermal barrier coatings. Computational studies are presented of the sintering of a coating constrained by a rigid substrate. It is demonstrated that the sintering process can become unstable when the in-plane stiffness exceeds a critical value. Imperfections in the coating can develop into cracks, which propagate through the thickness of the coating, creating a pattern of “mud cracks”. The simulations have motivated the development of simplified models of this process which describe the sintering response and the development of mud cracks in terms of the evolution of a small number of degrees of freedom. These simple models capture the major features of the more detailed computational simulations. They also allow the conditions under which imperfections grow into cracks to be readily identified and predict a density of mud cracks that is consistent with experimental observations.