The effect of bond coat oxidation on the microstructure and endurance of a thermal barrier coating system

Abstract

Isothermal oxidation tests have been carried out on a thermal barrier coating (TBC) system consisting of a nickel-based superalloy, CoNiCrAlY bond coat applied by HVOF and yttria-stabilised zirconia (YSZ) top coat applied by EB-PVD. Bond coat microstructure, coating cracking and failure were characterised using high resolution scanning electron microscopy complemented with compositional analyses using energy dispersive X-ray spectrometry. A protective alumina layer formed during the deposition of the YSZ top coat and this grew with sub-parabolic kinetics during subsequent isothermal oxidation at temperatures in the range 950 to 1150°C. After short exposures at 1050°C and final cooling, small sub-critical cracks were found to exist within the YSZ but adjacent to bond coat protuberances. Their formation is related to the development of local tensile strains associated with the growth of an alumina layer (TGO) on the non-planar bond coat surface. However, for the specimens examined, these cracks did not propagate, in contrast to other TBC systems, and final spallation was always found to have occurred at the bond coat/TGO interface. This shows that the strain energy within the TGO layer made a significant contribution to the delamination process.     

Local stress around cap-like portions of anisotropically and nonuniformly grown oxide layer in thermal barrier coating system

The intrinsic deformation accompanying the growth of thermally grown oxide (TGO) can induce significant local stress potentially causing interfacial delamination and coatings fracture in a thermal barrier coating system (TBCs). Multiple mechanisms can be involved in a TGO growth process which is sensitive to the reactive elements contained in the coatings, and as a result anisotropic and nonuniform growth deformation can be produced in the TGO layer. The objective of this study is to analytically and numerically investigate the oxide-growth-induced local stress around the cap-like portions of a TGO layer having grown to a certain thickness and furthermore demonstrate the associated micro-crack patterns. A sphere model is proposed to analytically derive the elastic and elastic–plastic solutions of the stress field, which takes into account the anisotropy and nonuniformity of growth strain as well as the yielding of coating materials. On the other hand, finite element analysis is carried out to consider more realistic undulation morphology of the TGO layer and to verify the analytical prediction. It is seen that the through-thickness and lateral components of the anisotropic growth strain compete in the stress generation and there exist critical conditions for the dominance of different growth strains. The effect of growth strain gradient is examined to disclose the consequence of TGO dominant growth at the TGO/BC (bond coat) interface. The effects of the roughness and thickness of TGO and the plastic behaviour of different coating layers are also analysed. Finally, the possible micro-crack patterns due to TGO growth in typical TBC systems are illustrated with suggestions about how to reduce the driving force for the related structural failure.     

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.     

Preparation of thermally reduced graphene oxide and the influence of its reduction temperature on the thermal,mechanical, flame retardant performances of PS nanocomposites

In this article, we investigated the influence of thermally reduced graphene oxides (TGOs) at different reduction temperatures on the thermal, mechanical and flame retardant performances of polystyrene (PS). The results indicated that disordered expanded layer structure can be obtained as the reduction temperature increases from 200 to 500 and 800 °C (the resulted composites are named as PS/TGO2, PS/TGO5 and PS/GTO8, respectively), which could lead to better dispersion of TGO sheets in PS matrix. Dynamic mechanical thermal analysis showed that both the storage modulus and T_g of PS/TGO5 and PS/TGO8 nanocomposites are significantly improved compared with that of neat PS. Noticeable improvement in flame retardant performance were achieved with the addition of TGO5 and TGO8, particularly TGO8, due to the removal of the functional oxygen groups from GO and the barrier effect of intumescent and loosely structure of char layers.     

Microstructure and thermal behaviour of plasma sprayed zirconia/alumina composite coating

In thermal barrier coatings (TBC), failure occurs near or at the interface between the metallic bondcoat and topcoat. On high temperature conditions, an oxide scale which is named thermally grown oxide (TGO) occurs along the bond/topcoat interface. For diminishing the creation of TGO, a dense coating with low residual stress and thermal stress buffer layer was preferable. High hardness ceramic coatings could be obtained by gas tunnel type plasma spraying, and the deposited coating had superior property in comparison with those deposited by conventional type plasma spray method. In this study, the gas tunnel type plasma spraying system was utilized to produce a zirconia/alumina functionally graded thermal barrier coating and discussed its physical and mechanical properties, thermal behavior and high temperature oxidation resistance of the coating are discussed. Consequently, the proposed system exhibited superior mechanical properties and oxidation resistance at the expenses of a slightly lower thermal insulating effect. This interlayer is preferred in order to minimize the detrimental effect of the phase transformation of gamma-Al2O3 to alpha-Al2O3.     

TGO对热障涂层失效的作用分析

热生长氧化物(TGO)的形成与长大是热障涂层失效的根本原因。先在IC10高温合金基体上超音速火焰喷涂(HVOF)NiCoCrAlTaY粘结层(BC层),再等离子喷涂二元稀土氧化物稳定氧化锆Sc2O3-Y2O3-ZrO2,喷涂样在1 100℃恒温氧化,利用扫描电镜(SEM)、能谱仪对断面形貌、成分进行分析,讨论了TGO的形成机理及其与热障涂层失效的关系。结果表明:随着恒温氧化时间增加,TGO层底部的Al含量下降,上部、中间弥散颗粒及底部的Ni含量均增加,上部、中间弥散颗粒中Cr含量均减少;喷涂样氧化140 h后,TGO层由靠近陶瓷层的富(Cr,Al)2O3层、弥散其间的富Ni颗粒和靠近BC层的Al2O3层组成;TGO的生长速度先由Al与O2化学反应速度决定,接着受BC层金属元素扩散速度影响,最后由化学反应速度和扩散速度共同控制;减少TGO中的有害氧化物含量以降低涂层内的应力,可有效提高涂层的使用寿命。     

Analytical electron microscopy of the mixed zone in NiCoCrAlY-based EB-PVD thermal barrier coatings: as-coated condition versus late stages of TBC lifetime

Abstract

The microstructural evolution of the alumina-zirconia mixed zone in a NiCoCrAlY-based electron beam physical vapor deposited (EB-PVD) yttria partially stabilized zirconia (Y-PSZ) thermal barrier coating (TBC) system from the as-coated condition into the advanced stages of TBC lifetime is monitored by analytical transmission electron microscopy (TEM). In the as-coated condition yttria-rich islands at the thermally-grown oxide (TGO)/TBC interface locally impede zirconia uptake of the scale during TBC deposition and give rise to the formation of an “off-plane” alumina-zirconia mixed zone textured perpendicular to the TGO/TBC interface. During prolonged isothermal/cyclic oxidation an increased chromium diffusion through the TGO scale turns the mixed zone into a reaction zone introducing a morphological instability of the mixed zone/TBC interface due to solutioning of the bottom TBC layer.

This microstructural pattern is corroborated by a triple-stage growth model for the mixed zone during three successive stages in TBC lifetime: (i) during TBC deposition, the thickness of the mixed zone increases due to predominant outward aluminum diffusion and uptake of zirconia. No columnar alumina zone (CAZ) has formed at this stage, (ii) upon completion of the transition alumina-to-corundum phase transformation the thickness of the mixed zone remains constant while the change in diffusion mechanism for an inward oxygen diffusion process now initiates parabolic growth of the columnar alumina sublayer of the TGO scale, (iii) in the late stage of TBC lifetime an marked outward chromium diffusion from the bond coat causes the mixed zone to resume growth due to TBC destabilization and the formation of a (Al, Cr)₂O₃ mixed oxide matrix phase.

A transient YCrO₃ phase is proposed for driving the destabilization of yttria-rich sections of the bottom TBC layer.     

Adhesion of thermal barrier coatings – the development of metastable alumina

Abstract

The adhesion of thermal barrier coatings (TBCs) is dependent upon the characteristics of the thermally grown oxide (TGO) that forms between the TBC and the corrosion resistant bond coat. Work has been carried out to investigate the properties of the TGO as a function of ageing treatments using piezospectroscopy. Residual stress maps were generated for an electron beam physical vapour deposited (EB-PVD) TBC which showed a large variation in residual stress over the surface of a coated sample. The two peaks generally associated with a alumina (R1 and R2) frequently appear as doublets with a high and low stress component. In addition, the presence of a metastable θ alumina was detected in aged samples. It is believed that these observations can be related to incipient spallation of the TBC. The development of residual stress and the metastable oxide have been studied and correlated with the spallation behaviour of the TBC.     

Failure mechanisms of thermal barrier coatings on MCrAlY-type bondcoats associated with the formation of the thermally grown oxide

The effect of the thermally grown oxide (TGO) formation on the lifetime of the thermal barrier coatings (TBC) with MCrAlY-bondcoats (BC) is reviewed. A number of factors affecting the TGO-formation and TBC-failure are discussed including the coating microstructure, geometrical (coating roughness and thickness) and processing parameters. Under given testing conditions for a specific EB-PVD-TBC-system forming a flat, uniform alumina TGO a critical TGO-thickness for TBC-failure can be defined. This TGO-morphology is, however, not necessarily optimum for obtaining long TBC-lifetime, which can be extended by formation of TGO’s with an uneven TGO/BC interface. In contrast, APS-TBC-systems are prone to formation of intrinsically inhomogeneous TGO-morphologies. This is attributed to non-uniform depletion of Y and Al underneath rough MCrAlY-surfaces as well as due to the commonly observed repeated-cracking/re-growth of the TGO during temperature cycling. The latter phenomenon depends on the exposure temperature and the mechanical properties of the APS-TBC. In both types of TBC-systems the TGO-formation and TBC-lifetime appear to be very sensitive to the manufacturing parameters, such as vacuum quality during bondcoat spraying and temperature regime of the bondcoat vacuum heat-treatment.     

Relation of microstructural and compositional features to the electrical properties in degraded thermal barrier coating systems

Degraded thermal barrier coating samples cut from different after-service gas turbine components are examined by both electron microscopy and impedance spectroscopy. There is a relationship between the microstructural and compositional features of the thermally grown oxide (TGO) and its electrical properties. The resistance of the TGO decreases with the TGO evolution from alumina to porous mixed oxides composed probably of NiO, spinel, Cr₂O₃, and Al₂O₃, while the relaxation frequency corresponding to the TGO increases. For seriously degraded TBCs, there is an additional semicircle in the impedance spectra in the extremely low frequency range, possibly arising from cracking in the vicinity of YSZ-TGO interface regions.     

Role of Y4Al2O9 in High Temperature Oxidation Resistance of NiCoCrAlY-ZrO2.Y2O3 Coatings

NiCoCrAlY-ZrO2·Y2O3 coatings were deposited on the substrates by using a technology of combining electron,atom and ion beams （three beams）. Isothermal oxidation for these samples was performed at 1100℃ for 100-300 h. The results show that a thermally grown oxide （TGO） layer was formed between NiCoCrAlY layer and oxidation. The TGO contains α-Al2O3 and Y4Al2O9 etc. oxides. The intensity ratio of α-Al2O3/Y4Al2O9 was monotonously decreased with increasing oxidation time based on XRD （X-ray diffraction） analysis. The Y4Al2O9 phase plays the most important role in high temperature oxidation resistance at 1100℃. The related mechanism was also discussed.     

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.     

X-ray determination of stresses in alumina scales on high temperature alloys

Abstract

The resistance of alumina scales to cracking and spalling under the influence of growth and thermal stresses is a critical aspect of the environmental resistance of high temperature structural alloys, oxidation resistant coatings, and bond coats for thermal barrier coatings. However, the relative magnitudes of the stresses and their distribution are often not known. In this study several X-ray diffraction techniques are being used to measure the strains in alumina scales on a variety of high temperature alloys both during oxidation and after cooling to room temperature. The corresponding stresses are being calculated using appropriate elastic constants. The results include the observations that: (1) Growth stresses are higher in alumina formed on FeCrAl alloys as compared to that formed on nickel-base alloys, such as NiAl or single crystal superalloys (studies have not yet been performed on NiCrAl or CoCrAl alloys). (2) Yttrium additions to do not result in lower growth stresses in alumina scales on FeCrAl alloys even though the additions decrease the amount of lateral scale growth. (3) Growth stresses can be relaxed by plastic deformation of both the alloy and oxide. The implications of these results with regard to alumina adhesion are discussed.     

H_2O和Y(O)对NiCoCrAl热障涂层高温氧化的影响

分别利用真空等离子沉积和超音速火焰喷涂技术制备含有Y和含Y氧化物的NiCoCrAl涂层,用差热分析和光学及电子显微镜研究两种涂层在Ar-16.7%O_2,Ar-3.3%H_2O和Ar-0.2%H_2-0.9%H_2O气氛中1100℃时的氧化动力学和断面微观结构,通过第一性原理计算对比在不同气氛中含Y氧化物对涂层氧化的影响机理。结果表明:对于NiCoCrAl+Y涂层,Y倾向于向界面扩散并在界面富集导致Al_2O_3膜生成更多有利于内氧化的孔洞,水蒸气更会对内氧化产生促进作用。而对于NiCoCrAl+Y(O)涂层,由于Y在涂层制备过程中被氧钉扎,导致NiCoCrAl+Y(O)涂层在上述气氛中生成了平直而均匀的Al_2O_3层,不同气氛对其氧化行为影响较小。上述研究进一步揭示NiCoCrAl涂层中活性元素Y的存在状态和氧化气氛中的水蒸气对氧化铝组织结构和生长速率有重要影响。     

Mechanics-based scaling laws for the durability of thermal barrier coatings

The durability of thermal barrier systems is governed by a sequence of crack nucleation, propagation and coalescence events that accumulate prior to final failure by large scale buckling and spalling. This sequence is governed by the σ_zz stresses that develop normal to the substrate, around imperfections, as the thermally grown oxide (TGO) thickens. Their effect is manifest in the stress intensity factor, K, caused by the σ_zz stresses acting on cracks emanating from them. In turn, these events are governed by scaling laws, ascribed to non-dimensional groups governing σ_zz and K. In this article the basic scaling relations are identified and used to gain some understanding of the relative importance of the various mechanisms that arise for application scenarios with minimal thermal cycling. These mechanisms are based on stresses that develop because of TGO growth strains in combination with thermal expansion misfit. The results are used to identify a critical TGO thickness at failure and express it in terms of the governing material variables. The changes in behavior that arise upon extensive thermal cycling, in the presence of TGO ratcheting, are elaborated elsewhere.     

Tensile ductility behaviour of fine-grained alumina at elevated temperature

High-temperature tensile ductility behaviour of polycrystalline fine-grained alumina is shown to be classified into four regimes, depending on flow stress: (1) fast-crack growth regime, (2) single-crack growth regime, (3) microcracks growth regime, and (4) superplastic-crack growth regime, in the order of decreasing flow stress. The unique tensile ductility behaviour observed for each fracture regime is related to the type of damage accumulation. A fracture mechanics model is applied to interpret the tensile ductility of alumina in the superplastic-crack growth regime. The model correctly predicts the observed linear decrease in the true fracture strain with an increase in the logarithm of flow stress. In addition, the model is in quantitative agreement with the increase in the true fracture strain with decreasing grain size when compared at a given stress. The enhancement of tensile ductility in alumina by dilute MgO additions is attributed to an increase in the surface energy and/or decrease in the grain-boundary energy which resists the fracture process. On the other hand, the enhancement of tensile ductility in alumina by addition of a second phase of zirconia is attributed to an increase in the amount of alumina–zirconia grain boundaries which have a low grain-boundary energy. © 1998 Chapman & Hall     

Stress development and relaxation in Al2O3 during early stage oxidation of β-NiAl

Abstract

Using a glancing synchrotron X-ray beam (Advanced Photon Source, Beamline 12BM, Argonne National Laboratory), Debye-Scherrer diffraction patterns from thermally grown oxides on NiAl samples were recorded during oxidation at 1000 or 1100°C in air. The diffraction patterns were analyzed to determine strain and phase changes in the oxide scale as it developed and evolved. Strain was obtained from measurements of the elliptical distortion of the Debye-Scherrer rings, where data from several rings of a single phase were used. Results were obtained from α-Al₂O₃ as well as from the transition alumina, in this case θ-Al₂O₃, which formed during the early stage. Compressive stress was found in the first-formed transition alumina, but the initial stress in α-Al₂O₃ was tensile, with a magnitude high enough to cause Al₂O₃ fracture. New α-Al₂O₃ patches nucleated at the scale/alloy interface and spread laterally and upward. This transformation not only puts the alpha alumina in tension, but can also cause the transition alumina to be in tension. After a complete α-Al₂O₃ layer formed at the interface, the strain level in α-Al₂O₃ became compressive, reaching a steady state level around –75 MPa at 1100°C. To study a specimen’s response to stress perturbation, samples with different thickness, after several hours of oxidation at 1100°C, were quickly cooled to 950°C to impose a compressive thermal stress in the scale. The rate of stress relaxation was the same for 1 and 3.5 mm thick samples, having a strain rate of ～1×10^?8/s. This behavior indicates that oxide creep is the major stress relaxation mechanism.     

Microstructural characterization of the failures of thermal barrier coatings on Ni-base superalloys

Abstract

Typical thermal barrier coating (TBC) systems consist of a nickel-base superalloy substrate coated with a MCrAlY or diffusion aluminide bond coat, onto which is deposited a yttria-stabilized zirconia (YSZ) TBC. The bond coats are usually deposited via diffusion aluminizing processes or low pressure plasma spray processes (LPPS). The YSZ can be deposited by air plasma spraying (APS) or electron beam physical vapor deposition (EBPVD). A layer of thermally-grown oxide (TGO), which is usually alumina, forms between the bond coat and YSZ during TBC deposition and subsequent high-temperature exposure. The conventional wisdom is that APS coatings tend to fail in the YSZ and that EBPVD coatings tend to fail at the interface between the TGO and bond coat. However, current research has shown that the situation is much more complex and that the actual fracture path can be a function of the type of bond coat, the type of high-temperature exposure, and coating process parameters. This paper describes the results of a study of the failure of state-of-the-art EBPVD TBCs deposited on NiCoCrAlY and platinum-modified diffusion aluminide bond coats. The failure times and fracture morphology are described as a function of bond coat type. The failure times were found to be a strong function of temperature for both bond coats. The failure for NiCoCrAlY bond coats was found to initiate at defects in the coating, particularly at the TGO/YSZ interface, but the fracture propagated primarily along the TGO–bond coat interface. The failure times and morphologies for platinum-modified diffusion aluminide bond coats depended strongly on bond coat surface preparation. The mechanisms for failure of the two bond coats are described. Also, the effects of modifications to the bond coats and variations in processing parameters on these mechanisms are presented.     

Residual stress measurement by Piezospectroscopy of cross sections through thermal barrier coatings

Abstract

Piezo-spectroscopic measurements of the residual stress in the TGO have been demonstrated on cross sections through thermally cycled TBC systems with high spatial resolution (approximately 2 × 2 × 5 µm). The residual stress is perturbed by relaxation at the free surface, but this can be taken into account in an approximate way. This relaxation has a range approximately equal to the YSZ thickness indicating that the YSZ imposes significant mechanical constraint on the TGO despite its low modulus.

The measurements have shown that the non-planar morphology of the TGO induces large deviations from the thermo-elastic equi-biaxial stress expected for a planar TGO. The mean level of compressive residual stress is reduced by relaxation due to bending of the non-planar TGO, in agreement with elastic FEM analysis of sinusoidal TGO morphology. However, the real morphology is not sinusoidal and in some locations the local curvature is extremely high. In these regions the residual stress is observed to become tensile and as high as 1 GPa. The failure mechanism is by nucleation and growth of local damaged regions caused by these tensile stresses (which are evident as low stress regions on analysis through the YSZ) into larger regions that eventually become unstable to large-scale buckling and spalling.     

Al_2O_3短纤维/M124F复合材料的凝固组织

采用挤压铸造法制备了Al2O3短纤维增强M124F铝合金复合材料,并研究了其拉伸强度、基体凝固组织和界面。结果表明:用挤压铸造法制备的复合材料组织致密,纤维分布均匀,抗拉强度与M124F相比明显提高;基体组织的α-Al枝晶和Si相明显细化。分析表明,纤维的加入具有双重增强作用:高强度陶瓷纤维的介入增强了基体材料的力学性能;在凝固过程中,Al2O3短纤维阻碍了α-Al枝晶的生长,同时可作为Si相非自发形核的衬底,细化了基体组织,提高了复合材料的力学性能。纤维与基体间未发现界面生成物MgAl2O4。