Degradation of a TBC with HVOF-CoNiCrAlY Bond Coat

Thermal barrier coatings (TBCs) provide both thermal insulation and oxidation and corrosion protection to the substrate metal, and their durability is influenced by delamination near the interface between the ceramic topcoat and the metallic bond coat, where a layer of thermally grown oxide (TGO) forms during service exposure. In the present work, the degradation process of a TBC with an air-plasma-spray ZrO₂-8 wt.%Y₂O₃ topcoat and a high-velocity oxy-fuel CoNiCrAlY bond coat was studied, in terms of TGO growth kinetics and aluminum depletion in the bond coat, as well as cracking behavior. The results show that the TGO growth kinetics can be described by a transient oxidation stage with δ³ = k ₁ t followed by a steady-state oxidation stage with δ² = c + k ₂ t. Significant aluminum depletion was observed in the bond coat after extended thermal exposure; however, chemical failure of the bond coat did not occur even after the aluminum content near the TGO/CoNiCrAlY interface decreased to 4.5 at.%. A power-law relationship between the maximum crack length in the TBC and the TGO thickness was observed, which may serve as the basis for TBC life prediction.     

Investigation of TBCs on turbine blades by photoluminescence piezospectroscopy

Thermal barrier coating (TBC) blade specimens with Pt diffusion bond coats were subjected to thermal cycling with periodic measurements of the residual stress in the thermally grown oxide (TGO) using photoluminescence piezospectroscopy. Two distinct stress levels were generally found to coexist in the probed volume, i.e. a high stress (～4 GPa) and a low stress (～500 MPa) level. Both the high and low stress levels were independent of the curvature of the blade surface, in agreement with numerical modelling based on a composite cylinder stress model. The relative contributions of the two stress levels appear to be correlated with the θ-Al₂O₃ content of the TGO, which was dependent on the position on the blade. The TBCs tended to fail along the TGO/bond coat interface in thermal cycling. This was modified by the presence of mixed transition metal oxides in the TGO. The results are interpreted in terms of a likely failure mechanism.     

Characterization of a cyclic displacement instability for a thermally grown oxide in a thermal barrier system

The mechanism responsible for the performance of a commercial thermal barrier system upon thermal cycling has been investigated. It comprises an electron beam physical vapor deposited (EB–PVD) yttria-stabilized zirconia thermal barrier coating (TBC) on a (Ni,Pt)Al bond coat. At periodic interfacial sites, the thermally grown oxide (TGO) that forms between the TBC and the bond coat at high temperature displaces into the bond coat with each thermal cycle. These displacements induce strains in the superposed TBC that cause it to crack. The cracks extend laterally as the TGO displaces, until those from neighboring sites coalesce. Once this happens, the system fails by large scale buckling. The displacements are accommodated by visco-plastic flow of the bond coat and “vectored” by a lateral component of the growth strain in the TGO. They depend upon the initial morphology of the metal/oxide interface. The observed responses are compared with the predictions of a ratcheting model.     

The Protectiveness of Thermally Grown Oxides on Cold Sprayed CoNiCrAlY Bond Coat in Thermal Barrier Coating

This paper presents the results of an oxidation behavior study for a thermal barrier coating (TBC) with air plasma sprayed yttria-stabilized zirconia top coat and CoNiCrAlY bond coat deposited using low pressure plasma spray (LPPS) and cold spray (CS). The TBC is subjected to isothermal oxidation and creep tests at 900?°C and evaluated using scanning electron microscopy, energy dispersive x-ray spectrometry transmission electron microscopy and electron backscatter diffraction. The thermally grown oxide (TGO) developed in the TBC with the LPPS bond coat was composed of only ??-Al₂O₃ and the TGO developed in the TBC with a CS bond coat is composed of ??-Al₂O₃ and ??-Al₂O₃. Despite the presence of this metastable ?? phase, the TGO in the CS specimens exhibits a dense microstructure and lower amounts of mixed oxides. The correlation between ??-Al₂O₃ and the formation of mixed oxides was investigated through the measurement of ??-Al₂O₃ thickness ratio and mixed oxides coverage ratio. It was found that the mixed oxides coverage ratio is inversely proportional to the ??-Al₂O₃ thickness ratio.     

Surface Modification of Thermal Barrier Coatings by Single-Shot Defocused Laser Treatments

Thermal barrier coatings (TBC) consisting of atmospheric plasma-sprayed ZrO₂-8 wt.% Y₂O₃ and a high velocity oxygen fuel-sprayed metallic bond coat were subjected to CO₂ continuous wave laser treatments. The effects of laser power on TBCs were investigated as was the thermally grown oxide (TGO) layer development of all as-sprayed and laser-treated coatings after thermal oxidation tests in air environment for 50, 100, and 200 h at 1100 °C. The effects of laser power on TBCs were investigated. TGO layer development was examined on all as-sprayed and laser-treated coatings after thermal oxidation tests in air environment for 50, 100, and 200 h at 1100 °C. Melted and heat-affected zone regions were observed in all the laser-treated samples. Oxidation tests showed a stable alumina layer and mixed spinel oxides in the TGO layers of the as-sprayed and laser-treated TBCs.     

真实TGO界面形貌对热障涂层界面应力的影响

采用MSC.MARC有限元分析软件,以真实服役的某重型燃气轮机透平第一级动叶片表面热障涂层为研究对象,研究真实TGO界面形貌对热障涂层界面应力的影响。结果表明:在TC/TGO界面的TC层中,法向应力σ₂₂分布中的拉应力位于波谷区域,压应力位于波峰区域,而在BC/TGO界面的BC层中,σ₂₂应力分布与TC层相反;TC层与BC层的剪切应力σ₁₂分布规律相同,均是波谷左侧的应力方向为负,波谷右侧的应力方向为正。TGO界面的波峰和波谷处的法向应力σ₂₂值随TGO厚度的增大而增加;当TGO厚度不变时,BC/TGO界面振幅增大,TGO内和BC内的法向应力σ₂₂值也随之增大。     

Adherence and Failure of an EBPVD 7YSZ Coating on a β/γ-NiCrAl Substrate: A Pilot Study

Oxidation, lifetime and crack path at failure upon furnace oxidation testing at 1100 °C were evaluated for an undoped alumina-forming NiCrAl alloy/7YSZ (7 wt% yttria stabilized zirconia) coating system. The coating was deposited by electron beam physical vapor deposition (EBPVD). The thermally grown oxide (TGO) was investigated by energy dispersive spectroscopy. The outer zone of the TGO comprised a particular interlayer-denominated mixed zone (MZ), which revealed accumulation of Cr together with Y and Zr. The contents of Y and Zr in the MZ of the undoped sample were in good agreement with compositional data previously evaluated for thermal barrier coating (TBC) systems with EBPVD NiCoCrAlY bond coats versus relative lifetime. These data are considered characteristic measures of the stage of life for EBPVD TBC systems.     

TGO Growth and Crack Propagation in a Thermal Barrier Coating

In thermal barrier coating (TBC) systems, a continuous alumina layer developed at the ceramic topcoat/bond coat interface helps to protect the metallic bond coat from further oxidation and improve the durability of the TBC system under service conditions. However, other oxides such as spinel and nickel oxide, formed in the oxidizing environment, are believed to be detrimental to TBC durability during service at high temperatures. It was shown that in an air-plasma-sprayed (APS) TBC system, postspraying heat treatments in low-pressure oxygen environments could suppress the formation of the detrimental oxides by promoting the formation of an alumina layer at the ceramic topcoat/bond coat interface, leading to an improved TBC durability. This work presents the influence of postspraying heat treatments in low-pressure oxygen environments on the oxidation behavior and durability of a thermally sprayed TBC system with high-velocity oxy-fuel (HVOF)-produced Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat. Oxidation behavior of the TBCs is evaluated by examining their microstructural evolution, growth kinetics of the thermally grown oxide (TGO) layers, and crack propagation during low-frequency thermal cycling at 1050 °C. The relationship between the TGO growth and crack propagation will also be discussed.     

Combined pre-annealing and pre-oxidation treatment for the processing of thermal barrier coatings on NiCoCrAlY bond coatings

A combined pre-annealing and pre-oxidation treatment was developed for the processing of partially yttria stabilized (PYSZ) thermal barrier coatings (TBC) on top of NiCoCrAlY bond coatings (BC). To develop this pre-treatment, the influence of the oxygen potential during pre-annealing and pre-oxidation on the life span and failure mechanisms of the entire high temperature coating system upon thermal cycling was investigated. The results of this study showed that the service life of the coating system depended strongly on the composition and microstructure of the thermally grown oxide (TGO) after pre-oxidation. The longer life spans were obtained if the TGO thickened very slowly during thermal cycling due to a large α-Al₂O₃ grain size. Such a slow-growing TGO corresponded with a pre-treatment for which θ-Al₂O₃ was formed during pre-oxidation and for which the yttrium was located within a high density of pegs along the TGO/BC interface after pre-oxidation. If the yttrium was present on top of the TGO after pre-oxidation, a thick mixed alumina-zirconia layer formed upon thermal cycling. This mixed oxide layer contributed significantly to the total oxide layer thickness, resulting in short life spans. The formation of NiAl₂O₄ spinel in between the TBC and the α-Al₂O₃ should be avoided, since this can lead to premature failure along the spinel/α-Al₂O₃ interface.     

The Influence of La Doping on the Oxidation Mechanism and Stresses in the Thermally Grown Oxide on CMSX-4 with Pt-Aluminide Bond Coat

The thermally grown oxide (TGO) formed on CMSX-4 coated with Pt-aluminide bond coat was studied by isotopic oxidation and photo-luminescence piezo-spectroscopy (PLPS). It was found that La doping in the CMSX-4 substrate suppressed inward oxygen diffusion along TGO grain boundaries which was the dominant TGO growth mechanism without La doping. The different oxidation mechanisms led to distinct TGO stress distributions and different crystallographic orientations of the α-Al₂O₃ in the TGO. In the sample without La doping, the residual stress was uniform and the α-Al₂O₃ crystallites were found to be preferentially orientated with their c-axis in perpendicular to the substrate surface, whereas in the sample with La doping, the residual stress distribution was found to be bi-modal with the existence of low stress zones, and the α-Al₂O₃ crystallites were preferentially orientated with the c-axis in parallel to the substrate surface. The TGO growth stress was estimated to be ?0.6 GPa (compressive) for the sample without La doping and 0.2 GPa (tensile) for the La-doped sample. Low stress zones are speculated to correspond to local damage caused by the tensile growth stress at high temperature.     

Stress Analysis and Failure Mechanisms of Plasma-Sprayed Thermal Barrier Coatings

Yttria-stabilized zirconia coatings were deposited by plasma spraying and heat-treated at 1100 °C for 50, 100, 150, and 200 h in air, respectively. Mechanical properties including microhardness and Young’s modulus were evaluated using the nanoindentation test. Residual stresses in the ceramic topcoat and the thermally grown oxide (TGO) layer were measured using Raman spectroscopy and photoluminescence piezo-spectroscopy (PLPS) techniques, respectively. The results showed that both the modulus and hardness increased with the thermal exposure time up to 100 h and then gradually decreased. The accumulated tensile stress in the as-sprayed topcoat changed to compressive stress after thermal exposure, and the compressive stress in the topcoat increased with an increase of thermal exposure time up to 150 h. The average compressive stresses in the TGO layer were higher than that of the cross-sectional topcoat, and the measured in-plane compressive stress increased firstly and then gradually decreased with increasing exposure time. The local interface geometry strongly affect the nature and evolution of hydrostatic stresses in the TGO. Finally, the crack initiation and propagation at the topcoat/TGO/bondcoat interface has been discussed with respect to the residual stresses in the plasma-sprayed TBC system.     

Delamination resistance of thermal barrier coatings containing embedded ductile layers

Micromechanical models are developed to explore the effect of embedded metal layers upon thermal cycling delamination failure of thermal barrier coatings (TBCs) driven by thickening of a thermally grown oxide (TGO). The effects of reductions in the steady-state (i.e. maximum) energy release rate (ERR) controlling debonding from large interface flaws and decreases in the thickening kinetics of TGO are investigated. The models are used to quantify the dependence of the ERR and delamination lifetime upon the geometry and constitutive properties of metal/TBC/TGO multilayers. Combinations of multilayer properties are identified which maximize the increase in delamination lifetime. It is found that even in the absence of TGO growth rate effects, the delamination lifetime of TBC systems with weak TGO/bond coat interfaces can be more than doubled by replacing 10–20% of the ceramic TBC layer with a metal whose ambient temperature yield stress is in the ～100–200 MPa range.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.     

On the role of imperfections in the failure of a thermal barrier coating made by electron beam deposition

An impression test has been used to explore the remnant toughness and the delamination characteristics of thermal barrier coatings (TBCs) after extended thermal exposure. The delamination trajectory is found to change as the thermally grown oxide (TGO) thickens. At small thicknesses, delamination occurs predominantly within the TGO and TBC. With a thicker TGO, developed after 100 h exposure at 1100°C, the delamination extends predominantly along the TGO/bond coat interface, but with small oxide domains remaining embedded in the bond coat. The changes in the interface adhesion and the mechanics responsible for this transition are addressed, along with a discussion of the role of morphological imperfections in the TGO in failure nucleation. A method for determining the effective in-plane modulus of the TBC from the curvature of decohered TGO/TBC bilayers is also presented.     

Non-destructive microwave detection of layer thickness in degraded thermal barrier coatings using K- and W-band frequency range

Thermal barrier coatings have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. To monitor thermal barrier coating (TBC) integrity over its lifetime, the detection of top coat (TC) and thermally grown oxide (TGO) thicknesses was carried out using a microwave non-destructive technique. The results showed a good resolution of 1° change in phase for 15 μm TC thickness when a rectangular waveguide operating at relatively high frequency was applied. The phase of the reflection coefficient at the interface of TC and waveguide varies for different TGO and TC thicknesses. Therefore, measuring the phase of the reflection coefficient enables us to accurately calculate these thicknesses. Finally, a theoretical analysis was used to evaluate the reliability of the experimental results.     

Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions

Plasma sprayed thermal barrier coatings (TBCs) are applied to gas turbine components for providing thermal insulation and oxidation resistance. The TBC systems currently in use on superalloy substates typically consists of a metallic MCrAlY based bond coat and an insulating Y₂O₃ partially stabilized ZrO₂ as a ceramic top coat (ZrO₂ 7–8 wt.% Y₂O₃). The oxidation of bond coat underlying yttria stabilized zirconia (YSZ) is a significant factor in controlling the failure of TBCs. The oxidation of bond coat induces to the formation of a thermally grown oxide (TGO) layer at the bond coat/YSZ interface. The thickening of the TGO layer increases the stresses and leads to the spallation of TBCs. If the TGO were composed of a continuous scale of Al₂O₃, it would act as a diffusion barrier to suppress the formation of other detrimental mixed oxides during the extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability. The TBC layers are usually coated onto the superalloy substrate using the APS (Atmospheric plasma spray) process because of economic and practical considerations. As well as, HVOF (High velocity oxygen fuel) bond coat provides a good microstructure and better adhesion compared with the APS process. Therefore, there is a need to understand the cycling oxidation characteristic and failure mode in TBC systems having bond coat prepared using different processes. In the present investigation, the growth of TGO layers was studied to evaluate the cyclic oxidation behavior of YSZ/Al₂O₃ composite TBC systems with APS-NiCrAlY and HVOF-NiCrAlY bond coats. Interface morphology is significantly effective factor in occurrence of the oxide layer. Oxide layer thickening rate is slower in APS bond coated TBCs than HVOF bond coated systems under thermal cycle conditions at 1200 °C. The YSZ/Al₂O₃ particle composite systems with APS bond coat have a higher thermal cycle life time than with the HVOF bond coating.     

Evolution of stress and morphology in thermal barrier coatings

Residual stress in the thermally grown oxide (TGO) in thermal barrier coatings (TBCs) was measured by photoluminescence piezospectroscopy (PLPS) and stress maps created to track local stress changes as a function of thermal cycling. The local stress images were observed to be correlated with morphological features on the metal surface that were purposely introduced during specimen preparation. Local stress relaxation and morphological evolution with thermal cycling were studied using the stress maps combined by post-mortem SEM examination. It was found that the morphology in the specimen having an initial polished surface was quite stable, while that in the specimen with a rough surface was unstable. The average residual stress in the specimen with the unstable morphology decreased with thermal cycling and it eventually failed along TGO/YSZ interface. The specimen with stable morphology maintained a high TGO stress throughout the thermal cycling process and failed along TGO/bond coat interface. The rough surface was also found to give rise to the formation of transition alumina (θ-Al₂O₃) in the TGO which was correlated with a reduced TGO stress.     

Simultaneous synthesis by spark plasma sintering of a thermal barrier coating system with a NiCrAlY bond coat

As-fabricated thermal barrier coating (TBC) systems generally consist of a superalloy substrate, a MCrAlY bond coat (M = Ni, Co, Fe), and a ceramic (usually partially stabilized zirconia) top coat. The conventional methods for producing the two coating layers generally derive from thermal spray and physical vapor deposition techniques. Thermal exposure leads to the formation of an additional layer: the thermally grown oxide (TGO) between the bond coat and top coat. In the present work, a TBC system is synthesized through the application of spark plasma sintering (SPS), which provides not only the opportunity to synthesize all three layers at once, but the process is quite rapid and can produce dense layers. More specifically, this paper describes the application of this method to an yttria-stabilized ZrO₂ (YSZ) top coat and a NiCrAlY bond coat on a Ni-base Hastelloy X substrate. A one-micron thick Al₂O₃ TGO layer is also created from the reaction between an Al foil layer inserted in the stack prior to sintering and the ZrO₂ in the top coat. The effects of select process conditions are considered. The resulting multi-layer system is characterized with optical microscopy, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray analysis (EDAX) and X-ray diffraction (XRD). Differential thermal analysis (DTA) is used to investigate the reaction between the Al foil and the YSZ top coat.     

Stiffness of free-standing thermal barrier coating top coats measured by bending tests

Free-standing miniature beam specimens of thermal barrier coating (TBC) top coats were prepared by metal dissolution from high pressure turbine blades coated with TBC by electron beam physical vapour deposition (EBPVD) and thermally cycled to 1150 °C for various times. The beams comprised the yttria stabilized zirconia (YSZ) TBC and the thermally grown oxide (TGO) and their effective elastic modulus was measured using a miniaturized three-point bending test. The measured effective modulus was typically in the range of 10–22 GPa, with large specimen-to-specimen variations. The modulus increased with thermal exposure of the coated blades up to 85 cycles, but decreased for a larger number of cycles. The Young’s modulus of the YSZ layer alone was derived from the effective modulus of the composite beams (YSZ and TGO) by taking into account the contribution of the TGO. The derived Young’s modulus of the YSZ was in the range 5–10 GPa, and was verified independently by TGO residual stress measurement. Significant inelastic deformation was found to occur during the bending test when a relatively high load was applied and is speculated to be due to micro-fractures between columns in the YSZ. Specimens prepared from the concave part of the turbine blade were found to be approximately four times stiffer than those taken from a flat part of the blade, indicating that the modulus of the TBC is strongly dependent on the microstructure of the YSZ coating.     

Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

The microstructure and durability of a thermal barrier coating (TBC) produced by the thermal spray method have been characterized. Upon exposure, the bond coat chemistry and microstructure change by inter-diffusion with the substrate and upon thickening of the thermally grown oxide (TGO). A wedge impression test, in conjunction with observations by scanning electron microscopy, has been used to probe the failure mechanisms. At short exposure times, when the TGO thickness is less than about 5 μm, the growth of the TGO does not affect the crack patterns in the TBC and delaminations induced by wedge impression propagate within the TBC about 30 μm from the interface. An amorphous phase at the splat interfaces promotes this failure mode. As the thickness of TGO increases during exposure, cracks form in the TBC around imperfections at the interface. Moreover, induced delaminations develop a trajectory close to the interface, propagating not only through the TBC but also within the TGO and along the interfaces. A scaling result based on the misfit around imperfections caused by TGO growth has been used to rationalize the critical TGO thickness when the TBC fails.