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.     

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.     

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.     

An impedance spectroscopy study of high-temperature oxidation of thermal barrier coatings

Oxidation of air-plasma-sprayed (APS) thermal barrier coatings (TBCs) was carried out in air at 950 °C and was investigated using impedance spectroscopy coupled with scanning electron microscopy and X-ray diffraction. After oxidation for between 500 and 3000 h, a continuous alumina scale was formed at the bond coat/top coat interface in the TBCs, and was evaluated using impedance spectroscopy. The impedance spectra of the oxidised TBCs showed that there were four relaxation processes, which were attributed to the yttria-stabilised zirconia (YSZ) bulk of the TBCs, the thermally grown alumina scale, the YSZ grain boundary, and the metal electrode effect. Impedance analysis showed that the resistivity of the alumina scale increased with increasing oxidation time, demonstrating the thermally grown oxide (TGO) growth. The resistance of the YSZ grain boundaries also increased after oxidation for 2000 h, suggesting the composition change at grain boundaries after a long-term oxidation.     

Characterization of the degraded microstructures of a platinum aluminide coating

The degradation of a platinum modified aluminide (PtAl) coating and a CMSX-4 superalloy substrate were investigated for cyclic and quasi-isothermal heating to 1200 °C. To accelerate the oxidation of the specimens, the thermally grown oxide (TGO) was removed at 10 h intervals. For up to 80 h of exposure, comparisons of specimens with periodic oxide removal and those without oxide removal were made. Qualitatively, the major changes to the bond coat were associated with phase changes from β-(Ni,Pt)Al to γ′-(Ni₃Al) and precipitate coarsening. This evolution was quantified through backscatter scanning electron microcopy and image analysis. With instrumented indentation, the room temperature coating modulus was also measured at 10 h intervals. Additional results include observations of differences in waviness of the bond coat surface for cyclic, quasi-isothermal, and true-isothermal heating, and observations of rafting near the substrate/coating interface. The differences between cyclic and quasi-isothermal heating indicate that stresses associated with cooling and heating significantly alter microstructural evolution.     

Oxide layer development under thermal cycling and its role on damage evolution and spallation in TBC system

The nature and cause of failure of thermal barrier coatings (TBCs) consisting of physical vapor deposited (PVD) yttria stabilized zirconia (YSZ, 8 wt.% Y₂O₃) and a diffusion aluminide bond coat (Pt-Al) were investigated after oxidative thermal cycling and isothermal heat treatment at 1177 °C in air. Experiments were conducted for 45 and 10-minute hold times and for isothermal condition for disk specimens with and without TBC. It is found that microcracks starts in the oxide scales at the bond coat grain boundary protrusions. Total number of thermal cycles affect the density of microcracks within the TGO layer. Evidence is presented that higher density of microcracks in the 10-min hold-time experiments tend to separate the TBC from the TGO layer via extensive coating micro-decohesion and promotes 'complete' TBC separation as opposed to traditional 'partial' spallation of TBC from the substrate as in the 45-min hold-time and isothermal experiments.     

Structure and oxidation resistance of plasma sprayed Ni–Si coatings on carbon steel

Ni–Si coatings consisting of mainly NiSi₂ and NiSi were deposited on a carbon steel by air plasma spraying. Isothermal oxidation tests of the carbon steel substrates with the Ni–Si coatings at 500–800 °C have been carried out. The result indicated that a protective SiO₂-based oxide scale was formed on the surface of the coatings after oxidation. On the other hand, during oxidation, phase transformation occurred among the NiSi₂, NiSi and Ni₂Si phases constructing the Ni–Si coatings. This was caused by the extraction of silicon from the silicides and the reformation of silicides at the silcide/Si-blocks interface. Above 700 °C, the outward diffusion of iron and carbon became very fast and consequently decarburization happened at the coating/substrates interface, which induced the formation of pores in the substrates near the interface. In addition, grain boundary oxidation of Cr in the steel substrate was observed above 700 °C.     

Einfluss einer PVD‐Al‐Zwischenschicht auf die Eigenschaften eines thermisch gespritzten Wärmedämmschichtsystems nach Temperaturwechselbelastung

Thermal barrier coatings (TBC) generally consist of a metallic bond coat (BC) and a ceramic top coat (TC). Co–Ni–Cr–Al–Y metallic super alloys and Yttria stabilised zirconia (YSZ) have been widely used as bond coat and top coat for thermal barrier coatings systems, respectively. As a result of long‐term exposure of thermal barrier coatings systems to oxygen‐containing atmospheres at high temperatures, a diffusion of oxygen through the porous ceramic layer occurs and consequently an oxidation zone is formed in the interface between ceramic top coat and metallic bond coat. Alloying components of the BC layer create a so‐called thermally grown oxides layer (TGO). One included oxide type is α‐Al₂O₃. α‐Al₂O₃ lowers oxygen diffusion and thus slows down the oxidation process of the bond coat and consequently affects the service life of the coating system positively. The distribution of the alloying elements in the bond coat layer, however, generally causes the formation of mixed oxide phases. The different oxide phases have different growth rates, which cause local stresses, micro‐cracking and, finally, delamination and failure of the ceramic top coat layer. In the present study, a thin Al inter‐layer was deposited by DC‐Magnetron Sputtering on top of the Co–Ni–Cr–Al–Y metallic bond coat, followed by thermal spraying of yttria‐stabilised zirconia (YSZ) as a top coat layer. The deposited Al inter‐layer is meant to transform under operating conditions into a closed layer with high share of α‐Al₂O₃ that slows down the growth rate of the resulting thermally grown oxides layer. Surface morphology and microstructure characteristics as well as thermal cycling behaviour were investigated to study the effect of the intermediate Al layer on the oxidation of the bond coat compared to standard system. The system with Al inter‐layer shows a smaller thermally grown oxides layer thickness compared to standard system after thermal cycling under same conditions.     

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.     

A study on the oxidation behavior of HVOF sprayed NiCrAlY–0.4 wt.% CeO2 coatings on superalloys at elevated temperature

The high temperature oxidation behavior of HVOF sprayed ceria added NiCrAlY coatings on superalloys has been studied. Oxidation kinetics of the bare and coated superalloys in air at 900 °C under cyclic conditions was investigated. X-ray diffraction, field emission scanning electron microscope with energy dispersive spectroscopy and X-ray mapping were used to analyse the scales formed on the surface of the oxidised samples. The NiCrAlY–0.4 wt.% CeO₂ coated specimen showed negligible microspalling of the scales. The incorporation of ceria in NiCrAlY powder has contributed to the development of adherent oxide scale, in the coating, at elevated temperature and provided the better oxidation resistance.     

In situ chemical vapour co-deposition of Al and Si to form diffusion coatings on TZM

Multilayer alumino-silicide and silicide coatings were formed by in situ chemical vapour co-deposition of Al and Si on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy for improving its high-temperature oxidation resistance. MoSi₂ and Mo (Si, Al)₂ layers were formed in the inner and the outer layers, respectively in the case of alumino-silicide coating. Whereas silicide coating consisted of Mo₅Si₃ and MoSi₂ phases in the inner and the outer layers, respectively. 24–100-μm thick coatings were formed by optimizing the pack mixture of Al and or Si, NH₄F and Al₂O₃ powders and conducting the experiments at 1000 °C for 8–36 h. MoSi₂ layer showed a faster growth rate and presence of columnar grains. A small weight gain at the initial stages was observed during the oxidation tests of the coated samples under continuous or cyclic heating at 1300 °C in air. Neither cracks nor peeling of the coating layers were noticed after oxidation tests.     

Development of graded Ni-YSZ composite coating on Alloy 690 by Pulsed Laser Deposition technique to reduce hazardous metallic nuclear waste inventory

Alloy 690 based ‘nuclear waste vitrification furnace’ components degrade prematurely due to molten glass-alloy interactions at high temperatures and thereby increase the volume of metallic nuclear waste. In order to reduce the waste inventory, compositionally graded Ni-YSZ (Y₂O₃ stabilized ZrO₂) composite coating has been developed on Alloy 690 using Pulsed Laser Deposition technique. Five different thin-films starting with Ni80YSZ20 (Ni 80 wt% + YSZ 20 wt%), through Ni60YSZ40 (Ni 60 wt% + YSZ 40 wt%), Ni40YSZ60 (Ni 40 wt% + YSZ 60 wt%), Ni20YSZ80 (Ni 20 wt% + YSZ 80 wt%) and Ni0YSZ100 (Ni 0 wt% + YSZ 100 wt%), were deposited successively on Alloy 690 coupons. Detailed analyses of the thin-films identify them as homogeneous, uniform, pore free and crystalline in nature. A comparative study of coated and uncoated Alloy 690 coupons, exposed to sodium borosilicate melt at 1000 °C for 1-6 h suggests that the graded composite coating could substantially reduced the chemical interactions between Alloy 690 and borosilicate melt.     

Conversion of copper and manganese metallic films to spinel coating

Oxidation of copper + manganese metallic thin films on UNS 430 stainless steel was studied at temperatures of 600–950 °C. The Cu–Mn metallic films were converted to Cu–Mn spinel coating in the temperature range of 600–950 °C as confirmed by X-ray diffraction. Conversion mechanisms of metals to the spinel were investigated. Examination by SEM showed that negligible oxidation of the substrate alloy occurred in the coated samples at temperatures up to 750 °C; a transition layer formed between the substrate and spinel coating at higher temperatures. Cu–Mn spinel coating can provide protection to the metallic substrate at temperatures up to 850 °C. Formation of the spinel coating was due to reactions of CuO, which formed through outward diffusion of copper, with manganese oxides, which formed through inward diffusion of oxygen.     

Determining a critical strain for APS thermal barrier coatings under service relevant loading conditions

Despite the huge progress made in recent years in analysing the degradation behavior and the reliability of thermal barrier coating systems, there is still some deficit in the capability to predict damage evolution in terms of crack initiation and crack growth, which ultimately leads to macroscopic delamination and spallation of the coating system. In order to obtain this prediction capability, a fundamental understanding of the damage evolution processes under isothermal, thermo-cyclic and under thermo-mechanical loading conditions has to be developed.The aim of the presented work is to determine the critical strain, i.e. the strain at which cracking initiates, and to analyse the evolution of a network of cracks for widely used atmospheric plasma sprayed (APS) thermal barrier coating (TBC) systems. The TBC system has been exposed in our study to service relevant loading conditions, namely to thermal gradient mechanical fatigue (TGMF). TGMF tests for in-phase as well as out-of-phase loading cycles were performed on hollow cylindrical specimens made of the single crystal super alloy CMSX-4, loaded mechanically in 〈0 0 1〉 orientation, and being coated with a duplex system comprised of a CoNiCrAlY bond coat and a 8 wt.% Yttria partially stabilized Zirconia (YSZ) TBC. The CoNiCrAlY bond coat was deposited by Low Pressure Plasma Spraying (LPPS), while the ceramic top coat was deposited using the APS process. The loading cycles were chosen to represent an industrial gas turbine engine. Critical strains measured for delamination (within the ceramic coating or at the CoNiCrAlY – TBC interface) and through cracking, i.e. segmentation of the ceramic top coat was determined using a special compression test equipped with in situ acoustic emission technique. The mechanical testing was performed at room temperature after TGMF exposure. In order to study the impact of thermally grown oxide (TGO), specimens have been TGMF tested in the “as received” conditions as well as after isothermal aging (up to 3000 h at 1000 °C). To correlate the signal obtained by acoustic emission (AE) with the evolution of (micro-) cracks, the specimens have been carefully sectioned and investigated by standard metallographic means.The measured critical strains are used as a data basis for a strain-based lifetime model developed for isothermal and cyclic oxidation as well as thermo-mechanical loading. The lifetime model considers two failure modes, namely delamination and (vertical) through cracking.Metallographically obtained crack patterns within the TBC system have been incorporated into finite element models to quantify stress–relaxation as a consequence of damage evolution in the TBC system.The observations show that thermal gradient fatigue loading under in-phase loading leads to a shorter lifetime compared to out-of-phase loading.For the delamination mode, the critical strain values of the model are in good agreement with the experimental data of the TGMF experiments. The modeled critical strain for through cracking, on the other hand, is consistently lower than the experimentally determined failure strains, implying that the model describes the failure situation in a conservative manner.     

Influence of Water Vapor on the Isothermal Oxidation Behavior of Thermal Barrier Coatings

The oxidation of specimens with thermal barrier coating (TBC) consisted of nickel-base superalloy, low-pressure plasma sprayed Ni-28Cr-6AI-0.4Y (wt pct) bond coating and electron beam physical vapor deposited 7.5 wt pct yttria stabilized zirconia (YSZ) top coating was studied at 1050℃ respectively in flows of 02, and mixture of O2 and 5%H2O under atmospheric pressure. The thermal barrier coating has relatively low oxidation rate at 1050℃ in pure O2. Oxidation rate of thermal barrier coating in the atmosphere of O% and 5%H2O is increased The oxidation kinetics obeys almost linear law after long exposure time in the presence of 5% water vapor. Oxide formed along the interface between bond coat and top coat after oxidation at 1050℃ in pure O2 consisted of Al2O3, whereas interfacial scales formed after oxidation at 1050℃ in a mixture of O2 and 5%H2O were mainly composed of Ni(AI,Cr)2O4,NiO and AI2O3. It is suggested that the effect of water vapor on the oxidation of the NiCrAlY coating may be attributed     

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.     

Effects of heat treatment on adhesion strength of thermal barrier coating systems

Thermal barrier coatings (TBCs) are widely used as protective and insulative coatings on hot section components of gas turbines and their applications, like blades and combustion chambers. The quality and performance properties of TBCs are of great importance in terms of their resistance to service conditions. In a TBC system, there is a close relationship between the adhesion properties of coating layers. The adhesion strength of TBCs varies depending on the coating technique used and the surface treatments. In this study, CoNiCrAlY and YSZ (ZrO₂ + Y₂O₃) powders were deposited on stainless steel substrate. High Velocity Oxy-Fuel (HVOF) and Atmospheric Plasma Spraying (APS) techniques were used to produce the bond coats. The ceramic top layers on CoNiCrAlY bond coats were produced by the APS technique. The TBC specimens were subjected to heat-treatment tests. Adhesion strength for top coat/bond coat interface of as-sprayed and heat-treated samples was investigated. The results showed that the heat treatment of the coatings in different temperatures led to an increase in the adhesion strength of TBCs.     

Effects of oxide thickness, Al2O3 interlayer and interface asperity on residual stresses in thermal barrier coatings

During high temperature operation, the thermally grown oxide (TGO) usually forms along the bondcoat/topcoat interface in thermal barrier coating (TBC) and was characterized as a driving force for the failure of the coating system. The effects of TGO thickness and Al₂O₃ interlayer applied as an oxygen barrier layer between the bondcoat and topcoat on the magnitude of residual stresses in TBC during cooling process were interpreted using concentric-circle model. The results were coupled with finite element method. The influences of interface asperity and interface topography on the distribution of residual stresses normal to interfaces in TBC were also discussed.     

Thermal cycling behaviour of thermal barrier coating systems based on first- and fourthgeneration Ni-based superalloys

Abstract

This study deals with the cyclic oxidation behaviour of thermal barrier coating systems. The systems consist of an yttria-stabilised zircona ceramic top coat deposited by EB-PVD, a β-(Ni,Pt)Al bond coat and a Ni-based superalloy. Two different superalloys are studied: a first-generation one and a fourthgeneration one containing Re, Ru and Hf. The aim of this work is to characterise the microstructural evolution of those systems and to correlate it to their resistance to spallation. Thermal cycling is carried out at 1100°C in laboratory air, with the number of cycles ranging between 10 and 1000. Each cycle consists of a 1 h dwell followed by forced-air cooling for 15 min down to room temperature. Among the main results of this work, it is shown that the MCNG-based system is significantly more resistant to spallation than the AM1-based one. Up to 50 cycles, both systems exhibit similar oxidation rate and phase transformations but major differences are observed after long-term ageing. In particular, a Ru-rich β-phase is formed in the bond coat of the MCNG-based system while the AM1- based one undergoes strong rumpling of the TGO/bond coat interface due to the loss of the thermal barrier coating.     

Environment and time dependent effects on the fatigue response of an advanced nickel based superalloy

The fatigue behaviour of the nickel based superalloy RR1000 is characterised using double edge notch specimens incorporating shot peening. Evaluations were conducted at two test temperatures, 300 °C and 650 °C, employing baseline and dwell waveforms. The effects of air and vacuum environments plus prior exposure at 650 °C were also assessed. It is demonstrated that surface oxidation does not control performance at the test conditions of interest. Rather, the modification to stabilized peak and mean stresses resulting from either thermal relaxation of peened stresses or a time dependent shake down of stress under mechanical loading governs ultimate behaviour.