Microstructure behaviour and influence on thermally grown oxide formation of double‐ceramic‐layer EB‐PVD thermal barrier coatings annealed at 1,300 °C under ambient isothermal conditions

To resist high thermal loads in turbines effectively, turbine blades are protected by thermal barrier coatings in combination with additional air cooling. State‐of‐the‐art yttria stabilised zirconia top coats do not operate at temperatures higher than 1,200 °C. Promising candidates for alternative top coats are pyrochlores, lanthanum zirconate and gadolinium zirconate. But lifetime of pyrochlores is short because of spallation. However, combinations of yttria stabilised zirconia and lanthanum zirconate or gadolinium zirconate as multilayer systems are promising top layers operating at higher temperatures than yttria stabilised zirconia. Such thermal barrier coatings top coats as double‐ceramic‐layer systems consisting of 7 wt.% yttria stabilised zirconia and lanthanum zirconate or gadolinium zirconate were deposited by Electron Beam‐Physical Vapour Deposition. The focus of the work was set on the influence of the coating design and the microstructure variation generated at different rotating speeds on the adhesion and thermally grown oxide behaviour after isothermal oxidation at 1,300 °C. Phase formation of the thermal barrier coatings top coats was obtained using X‐ray diffraction. After isothermal oxidation tests for 50 h at 1,300 °C, both, microstructure change and the formation of the thermally grown oxide were investigated. While the pyrochlore single‐ceramic‐layer are completely spalled off, microstructure of the double‐ceramic‐layer reveals only crack initiation. The thermally grown oxide thickness was determined by means of scanning electron microscopy. A high aluminum and oxygen content in the thermally grown oxide is found using X‐ray spectroscopy. Existence of α‐phase in Al₂O₃ was proved by X‐ray diffraction. After isothermal testing, no phase transformation can be detected regarding the double‐ceramic‐layer coatings.     

Evaluation of mixed oxide formation and sintering behavior in thermal barrier coatings on nickel‐based superalloy

Thermal barrier coatings are widely used in aircraft turbines to protect nickel‐based superalloys from the effect of high temperature oxidation and hot corrosion. In this study, both NiCrAlY bond coat and yttria‐stabilized zirconia top coat were deposited using atmospheric plasma spray technique. After coating production, specimens were exposed to oxidation in air atmosphere at 900 °C, 1000 °C and 1100 °C for different periods of time up to 50 h. Microstructural transformations in the ceramic top coat and growth behavior of the thermally grown oxide layer were examined using scanning electron microscopy, porosity calculation, elemental mapping and hardness measurement. Formation of different types of oxides in the thermally grown oxide layer shows that this process strongly depends on deposition technique as well as on oxidation time and temperature. Hardness values of the top coat increased with a decrease in the porosity of the top coat. Uniformity and homogeneity of the thermally grown oxide layer and densification of the top coat were evaluated in terms of the structural durability of thermal barrier coating systems.     

Tailoring protective coatings for all‐oxide ceramic matrix composites in high temperature‐/high heat flux environments and corrosive media

The application range of porous all‐oxide ceramic matrix composites (CMCs) can be significantly extended through deposition of protective coating systems. Typical applications include protection against erosion, wear and foreign object damage as well as a reduced permeability. Environmental barrier coatings (EBC) are mandatory in order to guarantee sufficient lifetime of the CMC components under high temperature‐, high heat flux conditions and corrosive attack (combustor liners, thermal protection systems for atmospheric reentry). Limited thermal stability of today’s oxide fibers requires additional thermal barrier functionality for EBCs in order to keep the effective CMC bulk temperatures below 1200 °C. Depending on the specific application DLR’s coating concept for all‐oxide CMCs is based on either a single reaction‐bonded aluminium oxide (RBAO) coating or a hybrid coating system consisting of a RBAO bond coat followed by an EB‐PVD YSZ/FSZ top coat and is highlighted for three case studies. Deposition techniques (magnetron sputtering, MOCVD) alternative to EB‐PVD as well as the suitability of fibrous and cellular materials for thick EBC/TBC layers are explored.     

Lifetime‐determining spalling mechanisms of NiCoCrAlRE / EB‐PVD zirconia TBC systems

The mechanisms that control the lifetime of thermal barrier coating (TBC) systems have been traced by two particular overlay bondcoats serving as model systems: superalloy pins (IN100, CMSX‐4) with two alternative NiCoCrAlRE (RE: Hf, Y) bond coat compositions (i) NiCoCrAlY without and (ii) with co‐dopants of silicon and hafnium. On top an electron‐beam physical‐vapor deposited (EB‐PVD) yttria partially stabilized zirconia (YPSZ) TBC commonly mixed with 2 wt.% hafnia, or, rarely with 10 wt.%, was applied. The test pins were thermo‐cycled at 1100 and 1150 °C until failure. Identical lifetimes in cyclic tests on YPSZ TBCs with 2 (relatively high sintering rate) and 10 wt.% hafnia (relatively low sintering rate) preclude an effect of diffusion mechanisms of the YPSZ TBC on lifetime. The fit of lifetimes and test temperatures to Arrhenius‐type relationships gives activation energies for failure. These energies agree with the activation energies for anion and cation diffusion in alumina for the respective bondcoat variant: (i) for the NiCoCrAlY/TBC system for O^2‐ diffusion in alumina, (ii) for the NiCoCrAlYSiHf/TBC system for Al³⁺ diffusion in alumina. SEM and EDS investigations of the thermally grown oxides (TGOs) confirm the mechanisms responsible for TBC failure as indicated by activation energies. Two categories of failure can be distinguished: (i) NiCoCrAlY coatings fail by an “adhesive mode of failure” along smooth bond coat/TGO interfaces driven by a critical TGO thickness. (ii) NiCoCrAlYSiHf coatings fail later and more reluctantly by a “cohesive” crack mode via de‐cohesion at the TGO/TBC interface. In the latter case a quasi‐integrity of the crack‐affected TGO is lengthily maintained up to failure by a crack‐pinning mechanism which runs via Al³⁺ supply from the bondcoat.     

On debonding of graded thermal barrier coatings

Thermal barrier coatings generally consist of a metallic substrate which is the primary structural component, a metallic bond coat which serves as oxygen diffusion barrier, a very thin layer of thermally grown oxide and a ceramic top coat that provides the main thermal shielding. Homogeneous ceramic coatings as top coats appear to have certain undesirable features such as high residual and thermal stresses, generally low toughness and relatively poor bonding strength. The new concept of compositional grading of the top coat may help to overcome some of these shortcomings by eliminating the material property discontinuities. A common mode of failure in thermal barrier coatings seems to be the debonding of the top coat. In this study the related interface crack problem for a graded ceramic/metal top coat is considered. It is assumed that the thermophysical properties of the top coat continuously vary between that of the bond coat at the top coat-bond coat interface and that of the ceramic at and near the free surface. The main objective of the study is to examine the influence of the material nonhomogeneity parameters and relative dimensions on the stress intensity factors and the crack opening displacements.     

Microstructure of MgAl5Ca3Sr alloy after creep deformation and high‐temperature heat treatment

The paper presents results of microstructural investigations of MgAl5Ca3Sr magnesium alloys in the as‐cast condition, after creep tests at 180 °C, and after heat treatment at 450 °C for 4.5 hours. The microstructure of MgAl5Ca3Sr alloy is composed of α‐Mg solid solution, irregular shaped (Mg,Al)₂Ca phase with C36 crystal structure, bulky (Mg,Al)₁₇(Sr,Ca)₂ phase, fine lamellar Mg₂Ca phase with C14 structure, needle‐shaped Al₂Ca precipitates with the C15 crystal structure. The precipitation of the needle‐shaped Al₂Ca phase in the α‐Mg grains and spheroidization of the C14 phase were found after heat treatment at 450 °C in argon atmosphere. The (Mg,Al)₂Ca (C36) and (Mg,Al)₁₇(Sr,Ca)₂ phases seems to be stable at 450 °C, however, the increasing of aluminum content in C36 compound was observed suggesting the initial stage of C36 → C15 transformation. After creep deformation at 180 °C precipitates of the Al₂Ca phase were found in α‐Mg phase. The intermetallic compounds are stable at 180 °C. The MgAl5Ca3Sr alloy exhibits good creep resistance up to 75 MPa. Tensile properties are comparable to those of Mg‐RE‐Zn–Zr alloys.     

Thermal cycling and isothermal oxidation behavior of quadruple EB‐PVD thermal barrier coatings

Increase of energy efficiency by increasing the turbine inlet temperature is the main driving force for further investigations regarding new thermal barrier coating materials. Today, thermal barrier coatings consisting of yttria stabilized zirconia are state of the art. In this study, thermal barrier coatings consisting of 7 weight percent yttria stabilized zirconia (7YSZ) and pyrochlore lanthanum zirconate (La₂Zr₂O₇) were deposited by electron beam physical vapor deposition. Regarding thermal cycling and isothermal oxidation behavior different layer architectures such as mono‐, double‐ and quadruple ceramic layers were investigated. The thermal shock behavior was examined by thermocycle tests at temperatures in the range between T = 50 °C ‐1,150 °C. Additionally, the isothermal oxidation behavior at a temperature of T = 1,150 °C with dwell times of t= 50 h and t = 100 h was studied in the present work. The conducted research concerning the behavior of various thermal barrier coating systems under thermal cycle and isothermal load highlights the potential of multilayer thermal barrier coatings for operating in high temperature areas.     

Comparison of CMAS corrosion and sintering induced microstructural characteristics of APS thermal barrier coatings

The microstructural features of high-temperature sintered and CaO-MgO-Al₂O₃-SiO₂ (CMAS) corroded air plasma sprayed Y₂O₃ stabilized ZrO₂ (YSZ) thermal barrier coatings (TBCs) under the thermal gradient condition were comparatively studied. As-sprayed YSZ has a lamellar structure and the lamellae are composed of closely aligned columnar crystals. The sintered and the CMAS corroded YSZ coatings maintain the t’-ZrO2 phase as the as-sprayed YSZ coating. The sintered YSZ remains the lamellar structure with reduced interlamellar gaps and grains coarsening. After the CMAS corrosion, the top layer of the YSZ coating keeps its lamellar structure consisting of some columnar grains with the CMAS infiltration into the intergrain gaps and the formation of striped Zr₂Y₂O₇. The typical lamellar structure transforms into more equiaxed grains in the middle and bottom layers of the ceramic coating along with significant infiltration of amorphous CMAS and anorthite formation in the bottom layer owing to the high contents of Ca and Al.     

Degradation of thermal barrier coatings due to thermal cycling up to 1150°C

The degradation of thermal barrier coatings (TBCs) due to thermal cycling up to 1150°C has been studied. During thermal cycling, the bond coat in the TBCs was oxidised to form an alumina and a mixed oxide layer between the top coat of yttria stabilised zirconia (YSZ) and the bond coat of MCrAlY alloy. The mixed oxide layer mainly consists of -Cr₂O₃ and (Ni,Co)(Cr,Al)₂O₄ spinel phases, which are formed above the -alumina layer. Interestingly, the alumina layer gradually disappeared during the oxidation while the content of chromium in the mixed oxide increased with increasing oxidation time. As the oxidation accelerated after the disappearance of the alumina layer, cracks initiated and propagated in the mixed oxide layer near the YSZ. Eventually, the crack propagation induced the spallation of some YSZ top coatings after the 2000 h oxidation.     

Thermal conductivity issues of EB‐PVD thermal barrier coatings

The thermal conductivity of electron‐beam physical vapor deposited (EB‐PVD) thermal barrier coatings (TBCs) was investigated by the Laser Flash technique. Sample type and methodology of data analyses as well as atmosphere during the measurement have some influence on the data. A large variation of the thermal conductivity was found by changes in TBC microstructure. Exposure at high temperature caused sintering of the porous microstructure that finally increased thermal conductivity up to 30 %. EB‐PVD TBCs show a distinct thickness dependence of the thermal conductivity due to the anisotropic microstructure in thickness direction. Thin TBCs had a 20 % lower thermal conductivity than thick coatings. New compositions of the ceramic top layer offer the largest potential to lower thermal conductivity. Values down to 0.8W/(mK) have been already demonstrated with virgin coatings of pyrochlore compositions.     

PVD thermal barrier coating systems for Mo–Si–B alloys

Abstract

Oxidation protective layers with chemical compositions of Mo–70Al, Mo–46Si–24B, Mo–37Si–15B and Mo–47Si–24Al (at.-%) were deposited on Mo–9Si–8B specimens by magnetron sputtering. After pre-oxidation of the coated samples, ceramic topcoats of yttria partially stabilized zirconia (YSZ) and gadolinium zirconate (GZO) were applied using electron-beam physical vapour deposition. Both as-deposited YSZ and GZO topcoats exhibited good adhesion to the pre-oxidised bond coats. The different thermal barrier coating (TBC) systems were exposed to air at 1000 °C for periods between 20 and 100 h. The YSZ topcoat was tightly-adherent to the borosilicate scale grown on the Mo–46Si–24B bond coat after 20 h of exposure. Similar results were obtained for GZO topcoats deposited on Mo–46Si–24B and Mo–37Si–15B bond coats. The TBC system consisting of GZO topcoat and Mo–47Si–24Al bond coat, which formed a mixed scale of silica and mullite-like oxides, survived 100 h at 1000 °C. However, after this exposure time, the bond coats were approaching their lifetime due to the low layer thickness (5–10 μm). Oxidation of the Mo–Si–B substrate at unprotected areas around the suspension hole of the samples caused severe deterioration of the Mo–70Al bond coat and substantial degradation of the outer region of the GZO topcoat due to chemical reactions with MoO₃.     

Entwicklung von Oxid – Keramik zur Anwendung als Wärmedämmschichten

Development of Oxide Ceramics for an Application as TBC The standard thermal barrier coating material yttria stabilised zirconia (YSZ) is limited in long term operation to a maximum temperature of about 1200°C. As a result further increase of the gas inlet temperature and hence the efficiency of gas turbines are hardly to achieve with YSZ coatings. In a screening procedure especially perowskite (ABO₃, A = Sr,Ba, B = Zr) and pyrochlore (A₂B₂O₇, e.g. A = La and other rare earth elements, B = e.g. Zr) materials have been identified as possible candidates for thermal barrier coatings. Basic physical properties (e.g. thermal expansion coefficient, thermal diffusivity and conductivity) of several candidates have been determined using sintered, dense samples. The possibility of optimization of the properties by using specific compositions will be discussed. From promising materials powders which are suitable for plasma‐spraying have been produced by spray‐drying. New TBC systems consisting of new materials (BaZrO₃, La₂Zr₂O₇) deposited by atmospheric plasma spraying and vacuum plasma sprayed MCrAlY bondcoats were tested in a gas burner facility. Especially La₂Zr₂O₇ coatings gave promising results. A further improvements could be achieved by the use of layered or graded coatings with a YSZ coating at the bondcoat interface and on top a layer of the new TBC material. First results of thermal cycling tests with 1250 and 1350°C surface temperature will be presented.     

Bending fatigue failure of atmospheric-plasma-sprayed CoNiCrAlY + YSZ thermal barrier coatings

Bending fatigue failure of conventional atmospheric-plasma-sprayed CoNiCrAlY + ZrO₂–8 wt.% Y₂O₃ thermal barrier coatings with/without the thermally grown oxide layer generated between the bond coat and the top coat was experimentally studied at room temperature. Microscopical and profilometrical characterization of as-received and fractured specimens and a simplified finite element study of cooling thermal stresses show that the same fatigue strength of both the as-coated and the oxidized specimens (i.e. its insensitivity to the presence of the thermally grown oxide) is most likely caused by a preferential through-the-thickness cracking of the thermally grown oxide layer. Moreover, the bond-coat/substrate interface is identified as the weakest part of the studied thermal barrier system under both low and high crack growth rates.     

Effect of platinum on the oxide-to-metal adhesion in thermal barrier coating systems

An investigation was conducted to determine the role of Pt in a thermal barrier coating system deposited on a nickel-base superalloy. Three coating systems were included in the study using a layer of yttria-stabilized zirconia as a model top coat, and simple aluminide, Pt-aluminide, and Pt bond coats. Thermal exposure tests at 1,150 °C with a 24-h cycling period to room temperature were used to compare the coating performance. Additional exposure tests at 1,000, 1,050, and 1,100 °C were conducted to study the kinetics of interdiffusion. Microstructural features were characterized by scanning electron microscopy and transmission electron microscopy combined with energy dispersive X-ray spectroscopy as well as X-ray diffraction. Wavelength dispersive spectroscopy was also used to qualitatively distinguish among various refractory transition metals. Particular emphasis was placed upon: (i) thermal stability of the bond coats, (ii) thickening rate of the thermally grown oxide, and (iii) failure mechanism of the coating. Experimental results indicated that Pt acts as a “cleanser” of the oxide-bond coat interface by decelerating the kinetics of interdiffusion between the bond coat and superalloy substrate. This was found to promote selective oxidation of Al resulting in a purer Al₂O₃ scale of a slower growth rate increasing its effectiveness as “glue” holding the ceramic top coat to the underlying metallic substrate. However, the exact effect of Pt was found to be a function of the state of its presence within the outermost coating layer. Among the bond coats included in the study, a surface layer of Pt-rich γ′-phase (L1₂ superlattice) was found to provide longer coating life in comparison with a mixture of PtAl₂ and β-phase.     

Mechanical properties and fracture behavior of interfacial alumina scales on plasma-sprayed thermal barrier coatings

Abstract

The mechanical properties and fracture behavior of Y-doped Al₂O₃ scales were investigated by furnace thermal cycling (to 1,150°C) of plasma-sprayed thermal barrier coatings (TBCs) with vacuum plasma-sprayed (VPS) or air plasma-sprayed (APS) Ni–22Cr–10Al–1Y bond coatings. No significant alterations in Al₂O₃ hardness or Young’s modulus (as measured by mechanical properties microprobe) were detected as a function of bond coat type, exposure time, or number of thermal cycles. The interfacial Al₂O₃ scales on VPS NiCrAlY exhibited progressive increases in localized fracture, buckling, and delamination during thermal cycling. The concentration of arrayed lenticular voids in the columnar Al₂O₃ grain boundaries significantly increased during cyclic oxidation (as compared to isothermal oxidation), but only in scales which formed on convex surfaces, suggesting internal void growth was stress-related. The amount and frequency of scale damage was higher on convex surfaces with a relatively large radius of curvature as compared to convex surfaces with a very small radius of curvature. Although the thermo-mechanical fracture resistance of Al₂O₃ scales on APS NiCrAlY was superior to scales on VPS NiCrAlY, TBC lifetimes on VPS NiCrAlY were greater by a factor of 2. Apparently, severe interfacial scale damage did not rapidly degrade the adherence of the ceramic top coatings.     

The effect of bond coat on mechanical properties of plasma-sprayed Al2O3 and Al2O3-13 wt% TiO2 coatings on AISI 316L stainless steel

In this study, Al₂O₃ and Al₂O₃-13 wt% TiO₂ were plasma sprayed onto AISI 316L stainless-steel substrate with and without Ni-5 wt% Al as bond coat layer. The coated specimens were characterized by optical microscopy, metallography and X-ray diffraction (XRD). Bonding strength of coatings were evaluated in accordance with the ASTM C-633 method. It was observed that the dominant phase was Al₂O₃ for both coatings. It was also found that the hardness of coating with bond coat was higher than that of coating without bond coat. Metallographic studies revealed that coating with bond coating has three different regions, which are the ceramic layer (Al₂O₃ or Al₂O₃-13 wt% TiO₂), the bond coating, and matrix, which is not affected by coating. The coating performed by plasma-spray process without bond coating has two zones, the gray one indicating the ceramic layer and the white one characterizing the matrix. No delamination or spalling was observed in coatings. However, there are some pinholes in coating layer, but they are very rare. The bonding strength of coatings with bond coat was higher than that of coating without bond coat. The strength of adhesion and cohesion was determined by means of a planemeter. It was seen that percentage of cohesion strength was higher than that of adhesion strength.     

Modification of NiCoCrAlY with Pt: Part Ⅱ. Application in TBC with pure metastable tetragonal(t’) phase YSZ and thermal cycling behavior

A thermal barrier coating system comprising Pt-modified NiCoCrAlY bond coating and nanostructured 4mol.% yttria stabilized zirconia(4YSZ, hereafter) top coat was fabricated on a second generation Ni-base superalloy. Thermal cycling behavior of NiCoCrAlY-4 YSZ thermal barrier coatings(TBCs) with and without Pt modification was evaluated in ambient air at 1100?C up to 1000 cycles, aiming to investigate the effect of Pt on formation of thermally grown oxide(TGO) and oxidation resistance. Results indicated that a dual layered TGO, which consisted of top(Ni,Co)(Cr,Al)_2O_4 spinel and underlying α-Al_2O_3, was formed at the NiCoCrAlY/4 YSZ interface with thickness of 8.4μm, accompanying with visible cracks at the interface. In contrast, a single-layer and adherent α-Al_2O_3 scale with thickness of 5.6μm was formed at the interface of Pt-modified NiCoCrAlY and 4 YSZ top coating. The modification of Pt on NiCoCrAlY favored the exclusive formation of α-Al_2O_3 and the reduction of TGO growth rate, and thus could effectively improve overall oxidation performance and extend service life of TBCs. Oxidation and degradation mechanisms of the TBCs with/without Pt-modification were discussed.     

EB‐PVD Thermal Barrier Coatings for Aeroengines and Gas Turbines

Ceramic thermal barrier coatings (TBCs) offer the potential to significantly improve efficiencies of aero engines as well as stationary gas turbines for power generation. On internally cooled turbine parts temperature gradients of the order of 100 to 150 °C can be achieved. Today, state‐of‐the‐art TBCs, typically consisting of an yttria‐stabilised zirconia top coat and a metallic bond coat deposited onto a superalloy substrate, are mainly used to extend lifetime. Further efficiency improvements require TBCs being an integral part of the component which, in turn, requires reliable and predictable TBC performance. Presently, TBCs fabricated by electron beam physical vapor deposition are favoured for high performance applications. The paper highlights critical research and development needs for advanced TBC systems, such as reduced thermal conductivity, increased temperature capability, lifetime prediction modelling, process modelling, bond coat oxidation, and hot corrosion resistance as well as improved erosion behaviour.     

A study on gradient thermal barrier coatings by EB-PVD in a cyclic high-temperature hot-corrosion environment

Gradient thermal barrier coatings (GTBCs) have been produced by electron beam physical vapor deposition (EB-PVD). Their performance was evaluated by isothermal oxidation and cyclic high-temperature hot-corrosion tests. It is found that the GTBCs exhibited better resistance to high-temperature oxidation and cyclic high-temperature hot-corrosion (HTHC) than traditional two-layered TBCs. A dense Al₂O₃ layer on the bond coat of GTBCs can effectively prohibit inward diffusion of oxidants such as O and S and outward diffusion of Al and Cr. On the other hand, an inlaid interface, the formation of which resulted from the oxidation of Al diffusion into the gaps between the columns of bond coat during the fabrication of the GTBCs, contributes to reinforce the adherence of the Al₂O₃ layer to the bond coat. During fluxing of the Al₂O₃ layer, S and O diffused into the bond coat. Cracks developed in the surface layer of bond coat by the combined effect of sulfidation of the bond coat and thermal cycling, and finally led to failure of the GTBC.     

Improving long term oxidation protection for γ‐TiAl substrates

In previous work, a thermal spray multilayer system consisting of Zirconia (ZrO₂) and MCrAlY top coat showed promising results regarding the oxidation behavior of the Gamma Titanium Aluminides substrates tested, which encouraged further research activities. Diffusion of substrate material was successfully inhibited by a ceramic Zirconia coating. A building up of a dense and stable oxide layer could be achieved by additional application of an MCrAlY top coat, leading to improved oxidation resistance and thus showing feasibility. In this work the main focus for development was put on enhancing adhesion and lowering residual stresses of the coatings in order to allow long term and cyclic testing without delamination taking place. Being a very brittle material, Gamma Titanium Aluminides require special surface treatment to enable roughening which is crucial for a strong mechanical bond between substrate and coating. Alternatives to conventional grit blasting as a standard preparation method were investigated. These were micro‐abrasive blasting and blasting at elevated temperature (≈300–550°C) to allow a more ductile behavior. The paper will highlight the implications by means of these measures and will also show the present development status of the multilayer system.