TGO Growth Research Articles

Behavior and mechanism of the stress buffer effect of the inside ceramic layer to the top ceramic layer in a double-ceramic-layer thermal barrier coating

The Double-Ceramic-Layer Thermal Barrier Coating (DCL-TBC) consists of a top ceramic layer (TC1), an inside ceramic layer (TC2), bond coat (BC) and alloy substrate. The top ceramic layer is made by new ceramic materials which has the lower thermal conductivity, such as LZ, LZ7C3, LMA etc. Although these materials have good high temperature performance and thermal insulation properties, their thermal expansion coefficients are very low which cause higher degree of mismatch with material properties of alloy substrate.The behavior of the stress buffer effect of the layer of TC2 is investigated aiming at the DCL-TBC with the two ceramic layers as LZ7C3(TC1) and 8YSZ(TC2). A FE model of the DCL-TBC is established and the stress of the TC1 during the thermal cycles is calculated. Results show that the layer of TC2 has the behaviors of stress buffer effect on reducing the stress of TC1 and reducing the influence of the TGO growth upon the stress of TC1. The inner mechanism of the stress buffer effect is investigated based on the influence of the thermal expansion coefficient ρ of the TC2 layer. Results show that the TC1 layer varies with the TC2 layer, having different thermal expansion coefficients; and the stress buffer effect is dominant, affected by the thermal expansion coefficients of the TC2 layer. The thermal expansion coefficient should be neither too large nor too low; if its values are too low or too large, the stress levels of TC1 are all high. The reasonable range of the thermal expansion coefficient is investigated. Aiming at the DCL-TBC with the two ceramic layer thicknesses as 150μm, the reasonable range of the thermal expansion coefficient of TC2 is from 10.35°C−1 (×10−6) to 10.887°C−1 (×10−6).

Microstructure degradation of EB-PVD TBCs on Pd–Pt-modified aluminide coatings under cyclic oxidation conditions

The study concerns Pd/Pt-modified aluminide bond coatings for EB-PVD TBCs on N5 superalloy in terms of microstructural evolution during cyclic oxidation tests.The coating was deposited by Pd+Pt electroplating, followed by high activity vapor phase aluminizing at 1050°C and pre-oxidation heat treatment in order to form an α-alumina TGO. The 7wt.% yttria stabilized zirconia TBC was deposited using electron beam physical vapor deposition. Cyclic oxidation tests of the coating were performed at 1100°C in 1hcycles in laboratory air. The microstructure of the coatings in the as-deposited state as well as after cyclic oxidation test was studied using SEM. The evolution and phenomena occurring at the interface between the bond coating, thermally grown oxide and TBC during high temperature exposure were studied in detail using high resolution FEG-S/TEM. The samples for S/TEM analysis were prepared using FIB (Focused Ion Beam). The study is mostly focused on the growth of TGO during high temperature exposure and the overall performance of the Pd/Pt-modified aluminide coatings as a cost effective alternative to Pt-modified aluminide coatings commonly used as EB-PVD bond coats.

Controlling the Chemical Composition of the Interface Thermally Grown Oxides to Prolong the Lifespan of Thermal Barrier Coatings Based on Interfering Aluminum Diffusion Mechanism

This study places great emphasis on the relationship between the TGO chemical composition and the TGO growth rate and the mechanical property of thermal barrier coatings upon isothermal oxidation. The results indicate that the control of the θ-Al2O3α-Al2O3 phase transformation can have significant effects on the subsequent isothermal oxidation during the earlier stage of isothermal oxidation. The formation of (Al, Cr)2O3 oxide sub-layer originated from the textured and larger grain of α-Al2O3 can influence the potential TBCs lifetime. Additionally, the potential longer oxidation lifetime of TBCs that subjected to the appropriate vacuum heat pre-treatment can be attributed to the larger absorptive energy of the TGO area prior to its fully microcrack imperfection propagation. Most importantly, TBCs delamination was determined by the steady-state energy release rate in ceramic topcoat and TGO, the TGO and the interface, as well as the formation and development of the microstructure and composition of imperfections in the vicinity of the interface.

Residual stress evolution of thermally grown oxide in thermal barrier coatings deposited onto nickel-base superalloy and iron-base alloy with thermal exposure ageing

Abstract The objective of this work is to understand the influences of different chemical compositions of iron-based and nickel-based substrates on the TGO growth rate, the distribution of residual stress in TGO, the Quantity of the Spinel Cap Presence (QSCP) at the convex of undulated TGO, as well as the θ-Al 2 O 3 to α-Al 2 O 3 phase transformation. A thermal barrier coatings (TBCs) system consisting of the NiCrAlY bond coat and the 8YSZ topcoat (500 μm thickness) were successfully produced onto the substrates of nickel-based superalloy and iron-based alloy by atmospheric plasma spraying (APS), respectively. The results suggest that, compared with the iron-based substrate, both the TGO growth rate and the aluminum depletion in the bond coat with the nickel-based substrate develop significantly slower as the thermal exposure proceeds. It is proposed that the Cr 3+ and Al 3+ with small radii may considerably influence the mechanisms of phase transformation in the TGO, and these small ions can promote the θ-Al 2 O 3 to α-Al 2 O 3 phase transformation. Furthermore, the TBCs system with iron-based substrate was prone to have much shorter stage of θ-Al 2 O 3 to α-Al 2 O 3 phase transformation and a dramatic reduction of θ-Al 2 O 3 content in the TGO, which had indirect effect on the thermo-dynamic behavior of TGO during the subsequent stage of isothermal oxidation. Furthermore, the apex of the convex of undulated TGO usually has the largest compressive residual stress. The spinel oxides can be inclined to form at the apex region of the convex of the undulated TGO with a lower value of b/a geometric parameter. However, only few spinel oxides appeared at the flank region of the convex. Nevertheless, the ‘spinel cap’ feature usually formed at the area of the apex and the flank of the convex of undulated TGO with a higher value of b/a geometric parameter. Compared with the nickel-based substrate, the more presence of ‘spinel cap’ at the area of the convex of undulated TGO with a iron-based substrate can be found at the convex of undulated TGO, and its earlier end of the θ-Al 2 O 3 to α-Al 2 O 3 phase transformation may result in the lower value of compressive residual stress in average.

Progress of Research on the Failure Mechanism of Thermal Barrier Coatings

Thermal barrier coatings(TBCs) is one of the important materials of turbines for propulsion and power generation with excellent thermal insulation properties.At high temperature,the failure of TBCs will lead to serious problems,so the failure mechanisms is the hotspot of TBCs research.The microcracks caused by thermal stress and the TGO formation and growth will cause the failure of TBCs.The whole failure progress,such as the oxidation of the BC,the TGO growth process,initiation and propagation of the microcracks are reviewed in this paper.The factors led to the final failure are discussed,and the recent research progress on improving the lifetime of TBCs is summarized.In addition,the trend of the failure mechanism of thermal barrier coatings is described.

TBCs for Gas Turbines under Thermomechanical Loadings: Failure Behaviour and Life Prediction

The present contribution gives an overview about recent research on a thermal barrier coating (TBC) system consisted of (i) an intermetallic MCrAlY-alloy Bondcoat (BC) applied by vacuum plasma spraying (VPS) and (ii) an Yttria Stabilised Zirconia (YSZ) top coat air plasma sprayed (APS) at Forschungszentrum Juelich, Institute of Energy and Climate Research (IEK-1). The influence of high temperature dwell time, maximum and minimum temperature on crack growth kinetics during thermal cycling of such plasma sprayed TBCs is investigated using infrared pulse thermography (IT), acoustic emission (AE) analysis and scanning electron microscopy. Thermocyclic life in terms of accumulated time at maximum temperature decreases with increasing high temperature dwell time and increases with increasing minimum temperature. AE analysis proves that crack growth mainly occurs during cooling at temperatures below the ductile-to-brittle transition temperature of the BC. Superimposed mechanical load cycles accelerate delamination crack growth and, in case of sufficiently high mechanical loadings, result in premature fatigue failure of the substrate. A life prediction model based on TGO growth kinetics and a fracture mechanics approach has been developed which accounts for the influence of maximum and minimum temperature as well as of high temperature dwell time with good accuracy in an extremely wide parameter range.

Oxidation Study of an EB-PVD MCrAlY Thermal Barrier Coating System

In this paper the oxidation kinetics data on a TBC system where the NiCoCrAlY bond coat and the YSZ topcoat are both applied by the EB-PVD process are presented. The oxidation testing was performed at 950, 1050 and 1150 °C with times up to 1000 h. SEM analysis and modelling of the TGO growth rate show that the TGO is formed in two stages and that the initial oxide formed during the manufacture of the topcoat is non-protective. A global expression is derived for the oxidation kinetics at the three temperatures.

Evaluation of TGO growth in thermal barrier coatings using impedance spectroscopy

Impedance spectroscopy was employed to evaluation of TGO growth on the interface of YSZ/bond coat in plasma sprayed thermal barrier coatings during isothermal oxidation and exposed to Na2SO4 at 950 °C. Various observed electrical responses relating to the thickness, continuous nature and purity of the TGO layer were explained using an equivalent circuit model. The results show the thickness of TGO increases with exposure time. The TGO after isothermal oxidation mainly contains Al2O3. However, the TGO after hot corrosion changes gradually from Al2O3 to a mixture of NiO, Al2O3 and Ni(Cr, Al)2O4 spinel. The thickness of the continuous alumina layer formed in TBCs can be evaluated by electrical modulus spectra.

The Oxidation Behavior of TBC with Cold Spray CoNiCrAlY Bond Coat

Cold gas dynamic spray (CGDS) has been considered a potential technique to produce the metallic bond coat for TBC applications, because of its fast deposition rate and low deposition temperature. This article presents the influence of spray processes for bond coat, including air plasma spray, high velocity oxy-fuel, and in particular CGDS, on the oxidation performance of TBCs with a Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat and an air plasma sprayed topcoat. Oxidation behavior of the TBCs was evaluated by examining the coating microstructural evolution, TGO growth behavior, and crack propagation during thermal exposure at 1050 °C. The relationship between the TGO growth and crack propagation will be discussed.

Effect of Chemical Compositions and Surface Morphologies of MCrAlY Coating on Its Isothermal Oxidation Behavior

Chemical composition and surface morphology of MCrAlY coatings are factors which influence the oxidation behavior and the thermal durability of thermal barrier coatings. In this study, Cold-sprayed Ni20Cr10AlY and Ni23Co20Cr8.5Al4.0Ta0.6Y coatings with polished surfaces were employed to study the effect of composition on the oxidation behavior. The cold-sprayed MCrAlY coatings at the as-sprayed and shot-peened surface conditions, along with the low pressure plasma-sprayed MCrAlY coating with sputters adhered weakly on the surface, were employed to investigate the effects of surface morphologies of MCrAlY coatings on their oxidation behavior. Cold-sprayed Ni20Cr10AlY coating exhibited a two-stage oxidation behavior and a higher TGO growth rate than that of the cold-sprayed Ni23Co20Cr8.5Al4.0Ta0.6Y coating at the rapid growth stage. After 10-h oxidation, the TGO on the as-cold-sprayed coating surface was mainly constituted by Al2O3, while the TGO on the coating surface attached with sputters was composed of Al2O3 and Cr/Ni-oxides. After 500-h oxidation, Cr2O3 and porous spinel appeared in the TGO on the surface of the as-cold-sprayed coatings with different compositions. The growth of Cr/Ni-oxides was attributed to the Al depletion. The content of spinel decreased on the cold-sprayed NiCrAlY with a shot-peened surface compared with the as-sprayed coating.

M-13 Determination of Wear Behaviour of Titanium Alloys Implant Materials Blasted With Mixture of Zirconia/Silica/Hydroxyapatite Powders

In this research, the effects of thin Al2O3 diffusion barrier layer (including Y-containing small oxide precipitates) and nanostructured YSZ top coat on the high temperature oxidation behavior of HVOF NiCoCrAlTaY/APS YSZ coatings were examined by using isothermal oxidation tests at 1100 °C. Nanostructured YSZ top coat could slightly reduce the double layered TGO growth rate and weight gain of thermal barrier coating systems. However, Al2O3 diffusion barrier layer (consisting of Y-containing small oxide precipitates) could substantially decline the oxidation rate and Ta movement (Ta-enriched oxides formation) in the NiCoCrAlTaY/Al2O3/nanostructured YSZ coating at 1100 °C.

Finite element analysis of stress distribution in thermal barrier coatings

A numerical simulation of crack development within APS TBC systems is presented. The TGO thickening and creep deformation of all system constituents is modelled. Two dimensional periodic unit cell is used to examine the effect of interfacial asperity on stress distribution and subsequent delamination of APS TBC. A study of cyclic loading and of creep of the base material on the stress distribution close to the asperity at the TGO/BC interface is made, revealing a small influence influence of both on the stress state in the thermal barrier coating system subjected to temperature loading. Cohesive zone elements at the oxide/ceramic interface model the development of the interfacial micro-crack. The finite element analysis shows that the development of the interfacial crack allows for a micro-crack formation within APS TBC. Subsequent TGO growth results in a tensional zone within the oxide layer. Linking of the micro-cracks at the interface and within TBC through TGO could lead to a coating delamination in the unit cell.

In-situ studies of the TGO growth stresses and the martensitic transformation in the B2 phase in commercial Pt-modified NiAl and NiCoCrAlY bond coat alloys

Abstract Oxide growth stresses were measured in situ at 1100 °C on commercial Pt-modified NiAl and NiCoCrAlY bond coat alloys using synchrotron X-rays. Measurements were taken on samples that had no preoxidation, as well as on samples that had experienced 24 one-hour thermal exposures at 1150 °C, a condition known to induce rumpling in the Pt-modified NiAl alloy, but not in the NiCoCrAlY alloy. The NiCoCrAlY alloy showed continuous stress relaxation under all conditions, whereas the Pt-modified NiAl alloys would typically stabilize at a fixed (often non-zero) stress suggesting a higher creep strength in the “Thermally Grown Oxide” on the latter alloy, though the precise behavior was dependent on initial surface preparation. The formation of martensite in the Pt-modified NiAl alloys was also observed upon cooling and occurred at temperatures below 200 °C for all of the samples observed. Based on existing models, this M s temperature is too low to account for the rumpling observed in these alloys.

Pulsed electron beam treatment of MCrAlY bondcoats for EB PVD TBC systems part 2 of 2: Cyclic oxidation of the coatings

This paper discusses the effect of pulsed electron beam (PEB) treatment of thermal sprayed MCrAlY bondcoats on the cyclic life of thermal barrier coatings (TBCs). Standard MCrAlY bondcoats were produced via HVOF, VPS and LPPS thermal spray methods. Some of the HVOF and VPS coatings were then given a PEB treatment before all the samples were coated with an EB PVD 7-8 wt.% yttria partially stabilized zirconia topcoat. The samples were all tested under cyclic oxidation conditions at 1150 °C with 1 h at temperature and 15 min cooling, samples were removed after 20% coating spallation and prepared for cross sectional analyses. Cyclic testing revealed that although the PEB treatment had no measurable effect on the VPS sprayed samples, the HVOF coatings showed a significant increase in the cyclic life after the PEB treatment. The effect of the PEB treatment on the various samples is discussed as well as its effect on TGO growth morphology. PEB treatment of the HVOF bondcoat was found to reduce the rate of alumina growth and to suppress the formation of oxide pegs resulting in a smoother bondcoat interface.

Evaluation of interfacial mechanical properties under shear loading in EB-PVD TBCs by the pushout method

A new simple pushout technique for evaluation of interfacial shear mechanical properties in thermal barrier coatings has been developed. The technique is similar to the pushout test of fiber-reinforced ceramics, except for the specimen shape and support method. The technique has been applied to evaluation of interfacial delamination toughness, Γ i , of the electron beam physical vapor deposition (EB-PVD) ZrO 2 thermal barrier coating (TBC) system. The change of Γ i in the EB-PVD system with thermal exposure is measured and discussed in terms of microstructural change and delamination crack path. The measured delamination toughness varied from Γ i = 10 to 115 J/m 2. The delamination path and TGO growth were found to be closely related. The delamination toughness significantly decreases due to the formation and growth of a spinel phase in the TGO layer. The relation between delamination toughness and delamination behavior is discussed.

Development of Intermixed Zones of Alumina/Zirconia in Thermal Barrier Coating Systems

The mechanisms whereby intermixed zones of alumina and zirconia are formed at the interface between the metallic bond coat and the ceramic top coat (yttria-stabilized zirconia) in thermal barrier coating systems have been investigated. The results lead to the following mechanism for the formation of the zones. The predominant mechanism for intermixed zone formation involves formation of a metastable alumina polymorph (θ or γ) during TBC deposition, with a significant amount of zirconia dissolved in it. The outward growth also begins to incorporate zirconia particles, which initiates the formation of the intermixed zone. Upon thermal exposure, the metastable TGO continues to grow outward, extending the intermixed zone, and eventually transforms to the equilibrium α-Al2O3. The transformation to α-Al2O3 results in an increase in the volume fraction of zirconia in the intermixed zone as it is rejected from solution. Once the α-Al2O3 appears, subsequent TGO growth produces a columnar zone of the TGO without a second phase. When α-alumina was preformed on the bond coat, prior to TBC deposition, no intermixed zone was formed for Pt-modified aluminide bond coats.

2420 Development of High-Sensitive Non-Destructive Evaluation Technique of Degraded Thermal Barrier Coatings by Using Microwave

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. After service at high temperature, delamination or micro-cracks will initiate in TBC, leading to TBC separation from substrate and ultimately in unexpected gas turbine destruction. TBC failure is primarily attributed to the. growth of TGO and partly to TBC degradation. Currently, there are no reliable nondestructive techniques available to monitor TBC integrity over lifetime of the coating. Thus, to detect TBC and TGO thicknesses, a microwave nondestructive technique that utilizes a rectangular waveguide was developed. The phase of the reflection coefficient at the interface of TBC and waveguide varies for different TGO and TBC thicknesses. Therefore, measuring the phase of the reflection coefficient enables us to accurately calculate TGO and TBC thicknesses. Finally, a theoretical analysis was used to evaluate the reliability of the experimental results.

The determination of the delamination resistance in thermal barrier coating system by four-point bending tests

The delamination resistance in a plasma-sprayed thermal barrier coating (TBC) system was evaluated by means of a modified four-point bending test. In this work, in order to study the effect of the interface roughness between the bond coating (BC) and the top coating (TC), two kinds of the powder with the different particle size were used for spraying the BC material. In addition, the influence of the isothermal aging on the delamination resistance was also investigated. The experimental results indicate that the effect of the TC/BC interface roughness wasn't significant. The energy release rate G c, which was estimated by the modified four-point bending tests, increased with increasing aging time at 1000 °C, on the other hand, the scatter of it decreased with increasing aging time. The crack mainly propagated in the top coating for as-sprayed condition and at the top-coating/bond-coating interface after thermal aging for 2000 h. The energy release rate G c was correlated with the fracture strength of the weakest parts of the TBC specimen. The energy release rate G c increased with thermal aging until a critical time due to the sintering of the top-coating expired. It can be expected that G c will decrease with the thermal aging after the critical aging time because the top-coating/bond-coating interfacial strength decreases by the TGO growth. The critical aging time in this work is approximately 2000 h.

331 Microstructure Evolution in Thermal Spray Coatings and its relevance in High Temperature Behavior

This paper describes a detailed characterization study of the complex layered microstructures of thermal spray coatings. These are being characterized across length scales in order to look at various features such as porosity and phases in coating cross-sections, splat structure, splat interfaces, grain structure within splats, etc. This study incorporates the use of Optical microscopy, SEM & Image analysis for low magnification studies of ceramic and metallic systems and also the use of SEM and TEM for high magnification characterization. It explores and exhibits the necessity and applicability of each of these techniques for understanding the evolution of microstructure during formation of coatings. Porosity in YSZ topcoats is characterized and categorized using Image analysis and results are correlated with more advanced techniques of porosity measurement. The mechanisms of single splat formation and oxidation involved are studied and schematic models are presented to explain the deposit formation of Ni-5Al bond coats in case of different processes such as APS, Wire-arc, HVOF and Cold spray techniques. Having thus examined the microstructural intricacies in topcoat and bond coat systems, this study then explores the sintering and oxidation behavior of the TBC system as a whole. The effects of heat treatment temperature time and environment, and effect of feedstock and bond coat spraying system used, are studied for single splats of PSZ, sprayed onto CoNiCrAlY bond coats. Sintering effects are investigated in single splats rather than thick free-standing deposit to evaluate the role of splat-substrate interaction, thermal mismatch and TGO growth.

On the Understanding of TGO Growth and Spallation in Nickel Aluminides

This paper describes various parameters influencing oxidation kinetics, oxide’s morphology and spallation phenomena encountered while studying nickel aluminides in high temperature oxidation. Questions are raised about the possibility of making precise lifetime or failure predictions of coatings and TBCs systems. Changes in growth mechanism due to the presence of elements other than Ni and Al in nickel-aluminide base coatings, the effect of atmosphere, particularly water vapor, and the effect of surface preparation and of crystallographic orientation are the main parameters discussed in this work.