TGO Growth Research Articles

Microstructural evolution and high-temperature oxidation behavior of plasma sprayed SiC-YSZ TBCs

In this work, the effect of SiC addition on the microstructure, porosity, phase composition, weight gain, and thickness of TGO of the coatings after repeated oxidation at 1300 °C for various time of APS SiC-YSZ TBCs have been investigated. The phase of the as-sprayed coatings is consisted of t’-ZrO2, SiC and very little SiO2. With the increase of SiC addition, the diffraction peak intensity of SiC and SiO2 are both increased. SiC and SiO2 can fill into the pores in YSZ effectively. However, the porosity in the coatings has little changed with the increased of SiC addition. The addition of SiC during repeated oxidation at 1300 °C can limit the development of the m-ZrO2 phase while causing no ZrO2 lattice deformation. Increasing the SiC addition can impede the growth of TGO between the bonding layer and the ceramic layer, lowering the thermal weight gain rate of the SiC-YSZ coating. The lowest thermal weight gain rate occurs when the SiC composite concentration is 15 wt%. When the composite content of SiC is increased by 10 wt%, the percentage of O element at the interface between the bonding layer and the ceramic layer decreases by approximately 6.56 wt%. Increasing the SiC concentration will improve the compactness of the interface between the bonding and ceramic layers, and limit grain formation in the bonding layer. However, the SiC composite composition must match the ceramic coating's porosity. When the SiC percentage is too low, the ceramic layer's porosity increases and TGO development accelerates. When the SiC concentration is too high, it agglomerates and connects within the coating, causing the coating to fail off during high temperature service.

Enhanced structural stability of yttria‐stabilized zirconia–LaMgAl11O19 dual ceramic coating by an α‐Al2O3 layer

AbstractTo prolong the service life and structural stability of CoCrNiAlY–yttria stabilized zirconia–LaMgAl11O19 (LaMA) dual ceramic coating during high temperature exposure, an AlY plating layer was deposited on top LaMA. Results revealed that this coating without AlY plating layer showed a rapid mass increase from 6.62 mg/cm2 at 20 h to 10.63 mg/cm2 at 80 h, but also suffered from a fast TGO growth and severe elemental diffusion, resulting in an exacerbating interface fracture of the coating. However, AlY plating layer was first oxidized, and Y2O3 induced the formation of a dense α‐Al2O3 reaction layer on the top LaMA layer, which greatly prevented the internal penetration of O2. This coating with AlY plating layer showed a relatively stable structure, a very slow TGO growth, and a slight mass increase trend from 8.19 mg/cm2 at 20 h to 8.73 mg/cm2 at 80 h. Potential oxidation damage mechanisms of the coatings with or without AlY plating layer were comprehensively clarified.

TGO Growth Behavior in Environmental Barrier Coatings with Modified Silicon Bond Coat

TGO Growth Behavior in Environmental Barrier Coatings with Modified Silicon Bond Coat

Constrained sintering and thermal ageing behaviour of electrophoretically deposited Yb2Si2O7 environmental barrier coating

The oxidation of SiC and the formation of a thermally grown oxide layer (TGO) limit the lifetime of environmental barrier coatings. Thus, this paper focuses on the deposition of denser Yb2Si2O7 coatings using electrophoretic deposition to reduce the TGO growth rate. The findings show densification for Yb2Si2O7 can be achieved with an optimized sintering profile (heating/cooling rate, temperature, and time). However, the addition of 1.5 wt. % of Al2O3 to Yb2Si2O7 promoted densification and lowered the required sintering temperature, at 1380 °C using 2 °C/min heating/cooling rate for 10 hours provided efficient coating density. Moreover, adding Al2O3 reduced the TGO growth rate by more than 70% compared to the Al2O3-free coatings, without cracking in TGO after 150 hours of thermal ageing at 1350 °C. Results within this study suggest electrophoretic deposition with Al2O3 addition produces promising Yb2Si2O7 environmental barrier coatings on SiC substrate with low oxidation rates and increased lifetime.

Hot corrosion behavior of CoCrNiAlY–YSZ–RSZ dual-ceramic thermal barrier coating with Al plating in molten sulfate mixture

A novel Al plating layer was deposited on the CoCrNiAlY–YSZ–RSZ dual - ceramic thermal barrier coating. The hot corrosion behaviors of these coatings in molten sulfate mixture were studied by the detail characterizations. This coating without Al plating possessed a fast mass gain trend, severe elemental diffusion and rapid TGO growth during the hot sulfate exposure, resulting in a significant increase in mass gain from 6.12 mg/cm2 at 20 h to 13.67 mg/cm2 at 100 h and aggravated coating fracture. In comparison, a continuously dense Al2O3 barrier layer formed on the coating's surface, preventing or delaying the infiltration of the molten sulfate and oxygen, and eventually led to an enhanced corrosion resistance. This coating with Al plating showed a slight mass gain from 8.20 mg/cm2 at 20 h to 9.31 mg/cm2 at 100 h, but also exhibited very slow TGO growth and stable structure. The potential molten sulfate corrosion mechanisms of these coatings with and without Al plating were systematically discussed.

Friction delamination mechanism of EB-PVD thermal barrier coatings in high-temperature and high-speed rotating service environment

High-speed rotation is an indispensable working state in the service process of aero-engines, therefore, the centrifugal load cannot be ignored in the failure analysis of thermal barrier coatings. However, due to the lack of service environment simulators that can realize high-temperature as well as high-speed rotation, the failure mechanism of high-speed rotation thermal barrier coatings is still unclear. Here, the effects of rotational speed variation on the service life and failure mode of thermal barrier coatings at high temperatures are studied by experiments and finite element method (FEM). The results show that the service life of high-speed rotating thermal barrier coatings decreases with the increase of rotational speed. The failure is mainly governed by the thinning and spalling of the columnar crystal region of the ceramic layer and the delamination and exfoliation of the equiaxed crystal region, rather than the abnormal growth of TGO. Further in-depth analysis shows that the failure of high-speed rotating thermal barrier coatings is mainly due to the joint driving of centrifugal force and wall shear stress, as well as the contribution of thermal fatigue at high temperatures. This work adds to the understanding of the failure mechanism of thermal barrier coatings under extreme working conditions, and also provides guidance for the safe and reliable service of thermal barrier coatings on working blades.

Properties evaluation and failure behaviour of plasma sprayed Y4Hf3O12 for thermal barrier coating applications

In this study, thermal barrier coating (TBC) of Y4Hf3O12 compound was produced by the plasma spray method. Phase composition, microstructure, thermophysical properties, high temperature stability, as well as CaO-MgO-Al2O3-SiO2 (CMAS) corrosion behaviour were investigated for Y4Hf3O12 coating. Results reveal Y4Hf3O12 coating of defective fluorite structure, as a result of high concentration of Y cation, to exhibit low thermal conductivity, outstanding structure stability up to 1300 °C and super good protectiveness against CMAS, but with relative low TEC. Preliminary findings support that Y4Hf3O12 ceramic can be top-layer materials on YSZ to form a double-ceramic-layer (DCL) TBC system. Thus, by adding a YSZ buffer layer, the Y4Hf3O12/YSZ DCL TBC was further developed on Ni-based superalloy and its thermal cycling durability was evaluated. It indicates that this DCL TBC system could undertake a cyclic lifetime longest to ~459 cycles. The failure behaviour in thermal shock test is related to the crack evolution induced by a combination of TEC mismatch, TGO growth and sintering of top-layer Y4Hf3O12.

LaYZrO/YSZ double ceramic layer thermal barrier coatings by EB-PVD: Thermal performance, morphology and failure behavior

Thermal barrier coatings have been widely investigated over the last decade. Lanthanum zirconate is known for its high melting point and good thermal stability in aircraft engine environment. Owing to the distinctive thermophysical properties, Y doping in pyrochlore La2Zr2O7 might be a promising TBCs material as the top ceramic layer. It remains a challenge to completely understand the LaYZrO/YSZ double ceramic layer TBCs properties and microstructure evolution during thermal cyclic. In this paper, LaYZrO/YSZ double ceramic layer TBCs have been prepared by EB-PVD. The phase structure, thermal performance, microstructure and thermal cyclic lifetime have been researched. Thermal conductivity has been decreased compared with YSZ and La2Zr2O7, simultaneously, coefficient of thermal expansion has been increased compared with La2Zr2O7. The failure behavior of LaYZrO/YSZ double ceramic layer TBCs have been elucidated as typical TGO growth mode and specific sintering crack evolution mode. This study of properties and failure behaviors might provide the valuable investigation and thought for other advanced TBCs.

Investigation of the effect of BC layer surface roughness and TC layer porosity on stress values in plasma sprayed coatings based on SEM images

This study investigates the effect of BC surface roughness and TC porosity on the stress values in atmospheric plasma sprayed (APS) thermal barrier coatings using the finite element method based on SEM images. Thermal aging treatment was performed at 1100°C for 12 hours on TBC samples with different BC surface roughness. Grinding and grit blasting treatments were performed on the BC surface to achieve different roughnesses. It was observed that increasing the BC surface roughness increases the contact surface with oxygen and Al diffusion paths towards the BC/TC interface, increasing the TGO thickness. The finite element results show that in the initial state without the TGO layer, after the heating stage, the stress is tensile in the peak areas of the interface and compressive in the valley areas, and with the TGO growth, the stress in the peak becomes compressive and in the valley becomes tensile. By increasing the BC surface roughness from Ra=1.3μm to Ra=3.7μm, the maximum tensile stress in the valley region of the TGO/TC interface reached from 34.84 MPa to 91.70 MPa. As a result, increasing the surface roughness increases the stress values in TBCs. The TC porosity shifts the stress concentration sites to the boundary of pores and reduces the stress at the TGO/TC interface. For this reason, cracks have initiated from the discontinuities within the TC layer in the coating with the highest BC surface roughness. • Effect of BC surface roughness on the TGO thickness was experimentally investigated. • FE simulation was performed to calculate the stress distribution. • Effects of BC layer surface roughness and TC layer porosity on stress values were numerically investigated.

Biaxial stress in La2Ce2O7/YSZ thermal barrier coatings for oxidation evolution

High temperature oxidation could cause the spalling and failure of thermal barrier coatings (TBCs). La2Ce2O7(LCO)/YSZ double-layer TBCs were investigated with high temperature oxidation at 1100 °C, 1150 °C and 1200 °C, respectively. Interfacial crack propagation and TGO growth behavior were comparatively investigated, and the failure of double-layer TBCs is the result of the joint action of interface transverse crack propagation and TGO growth. Due to the difference in thermal expansion coefficients, the YSZ layer is subjected to biaxial stress, which contains the TGO growth stress and the thermal mismatch stress during the oxidation process. The transverse crack propagation was developed at the LCO/YSZ interface and the YSZ/BC interface, eventually. The thermal barrier coatings TGO thickened abnormally at 1000 °C and 1150 °C, while delamination at the LCO/YSZ interface resulted in coating failure at 1200°C. The thermal mismatch stress at the LCO/YSZ interface increased mainly due to the thermal expansion coefficient between LCO and YSZ. Transverse microcracks at the LCO/YSZ interface gradually propagated, resulting in delamination failure to LCO/YSZ. Therefore, the failure mechanism of biaxial internal stress has important guiding significance for the application of double-layer TBCs.

Effect of CeO2 doping on high temperature oxidation resistance of YSZ TBCs

In the present work, YSZ TBCs and 10 wt% CeO2-doped YSZ thermal barrier coatings (CeYSZ TBCs) were prepared via atmospheric plasma spraying(APS) respectively, whereupon high temperature oxidation experiment was carried out at 1100 °C to compare the high temperature oxidation behavior and mechanism of the two TBCs. The results showed that the doping of CeO2 reduced the porosity of YSZ TBCs by 23%, resulting in smaller oxidation weight gain and lower TGO growth rates for CeYSZ TBCs. Besides, the TGO generated in CeYSZ TBCs was obviously thinner and there were fewer defects inside it. For YSZ TBCs, as the oxidation process proceeded, Al, Cr, Co and Ni elements in the bonding coating were oxidized successively to form loose and porous spinel type oxides (CS), which was apt to cause the spalling failure of TBCs. While, the Al2O3 layer of the TGO generated in CeYSZ TBCs ruptured later than that in YSZ TBCs, which delayed the oxidation of Cr, Co, and Ni elements and the formation of CS accordingly. Therefore, CeO2 doping can effectively improve the high temperature oxidation resistance of YSZ TBCs.

Vertical crack distribution effect on the TBC delamination induced by crack growth from the ceramic surface and near the interface

The propagation of vertical crack on the surface of thermal barrier coatings (TBCs) may affect the interface cracking and local spallation. This research aims to establish a TBC model incorporating multiple cracks to comprehensively understand the effects of vertical crack distribution on the coating failure. The continuous TGO growth and ceramic sintering are together introduced in this model. The influence of the vertical crack spacing and non-uniform distribution on the stress state, crack driving force, and dynamic propagation is examined. Moreover, the influence of coating thickness on the crack growth driving is also explored. The results show that large spacing will lead to early crack propagation. The uniform distribution of vertical cracks can delay the spallation. When the spacing is less than 4 times ceramic coat thickness, the cracking driving force will come in a steady-state stage with the increase of vertical crack length. Prefabrication of vertical cracks with spacing less than 0.72 mm on the coating surface can greatly decrease the strain energy. The results in this study will contribute to the construction of an advanced TBC system with long lifetime.

Stress profile and crack evolution in a three-dimensional (3D) thermal barrier coatings during isothermal cyclic test

In a furnace cyclic test, the growth of TGO severely affects crack propagation near the interface and the coating failure. To fully understand the failure mechanism of thermal barrier coating (TBC), a three-dimensional (3D) TBC cracking model is developed. The surface morphology of bond coat (BC) is approximated using two 3D geometric surface (waviness and roughness). Dynamic growth behavior of crack during thermal cycling is evaluated by the extended finite element method (XFEM). The effects of location, size, and number of initial ceramic crack on the coating failure are also discussed. The results show that the interaction between thermal cycling and TGO growth intensifies the stress amplitude in the ceramic layer. In the TGO growth model involving all element swelling, the ceramic crack is more likely to extend in the early stage of oxidation. Compared with the waviness interface, the ratchet increase rate of stress in the roughness interface model is greater. Both large and small cracks above the valley can propagate to the interface. Only large-size cracks above the peak can grow under the shear fracture mode. The propagation of one crack can inhibit the further growth of nearby crack. The failure mechanism of 3D TBC induced by TGO growth under an isothermal cycling is revealed. The results in this work can provide important theoretical guidance for the development of TBC with higher cracking-resistant.

Effects of shot peening on microstructure, thermal performance, and failure mechanism of thermal barrier coatings

AbstractShot peening might be a potential technology to optimize the interface microstructure, plays a critical role on failure behaviors, of thermal barrier coatings (TBCs). It remains a significant challenge to understand the influence of shot peening on microstructure, oxidation resistance, and thermal shock life. In this work, the Y2O3‐stabilized ZrO2 TBCs have been deposited by EB‐PVD. The phase, microstructure, thermal performance, and failure mechanism of TBCs have been systemically investigated after shot peening. The shot peening process can improve the planeness of interface and reduce the formation of the cauliflower‐liked microstructure in TBCs. After shot peening, the TBC coatings exhibit relatively good isothermal oxidation resistance and high thermal shock life due to the optimization of TGO growth and the thermal stability. The phase transformation, TGO growth, and cracks extension might give rise to the failure of TBCs. This work might guide the investigation of the improvement of interface microstructure and failure behaviors.

Study on spalling mechanism of APS thermal barrier coatings considering surface vertical crack evolution affected by surrounding cracks

The propagation of vertical cracks on the surface of thermal barrier coating (TBC) may be affected by the surrounding cracks. In this study, a TBC model with multiple cracks is developed to study the effects of surrounding cracks on surface vertical crack evolution and coating spalling. Dynamic TGO growth, continuous ceramic sintering, and dynamic crack propagation are considered in the model. The general characteristics of stress state and crack evolution in the coating are first examined. Then, the effects of the surrounding horizontal and vertical cracks on the overall crack growth are investigated. The results show that the vertical crack expands to the interface and merges with the horizontal crack rapidly. The crack length variation by simulation is consistent with the reported results by experiment. The surrounding horizontal crack close to the coating surface can prevent the surface vertical crack from propagating to the interface, which is independent of the surrounding crack length. A certain density of vertical cracks with equal spacing and initial length is conducive to delaying the horizontal crack propagation near the interface. Some optimized design methods proposed in this work provide another option for developing TBC with high durability.

Understanding the Failure Mechanism of Thermal Barrier Coatings Considering the Local Bulge at the Interface between YSZ Ceramic and Bond Layer.

The TC/BC interface morphology in APS TBC is one of the important factors leading to crack propagation and coating failure. Long cracks are found near the bulge on the TC/BC interface. In this study, the TBC model with the bulge on the interface is developed to explore the influence of the bulge on the coating failure. Dynamic TGO growth and crack propagation are considered in the model. The effects of the bulge on the stress state and crack propagation in the ceramic layer are examined. Moreover, the effects of the distribution and number of bulges are also investigated. The results show that the bulge on the interface results in the redistribution of local stress. The early cracking of the ceramic layer occurs near the top of the bulge. One bulge near the peak or valley of the interface leads to a coating life reduction of about 75% compared with that without a bulge. The increase in the number of bulges further decreases the coating life, which is independent of the bulge location. The results in this work indicate that a smooth TC/BC interface obtained by some possible surface treatments may be an optional scenario for improving coating life.

Comprehensive understanding of horizontal and vertical crack effects on failure mechanism of lamellar structured thermal barrier coatings

The local spalling induced by the propagation and coalescence of cracks in the ceramic layer is the fundamental reason for the thermal barrier coatings (TBCs) failure. To clarify the effects of horizontal and vertical cracks on the coating failure, an integrated model combining dynamic TGO growth and ceramic sintering is developed. The effects of cracks on the normal and shear stress characteristics are analyzed. The driving force and propagation ability of cracks under different configurations are evaluated. The interaction between horizontal and vertical cracks is explored by analyzing the variation of the crack driving force. The results show that TGO growth causes the ratcheting increase of σ22 tensile stress above the valley, and the σ12 shear stress is on both sides of the peak. Ceramic sintering mainly contributes to the ratcheting increase of σ11 tensile stress. There is minimum strain energy when the horizontal crack extends to the peak. The vertical cracks on the surface of the ceramic layer are easier to propagate through the coating than that of other locations. When the horizontal and vertical cracks simultaneously appear near the valley, they can promote the propagation of each other. The present results can offer theoretical support for the design of an advanced TBC system in the future.

Development of oxide-based High temperature environmental barrier coatings for ceramic matrix composites via the slurry process

A slurry process was employed to develop advanced environmental barrier coatings (EBCs) for SiC based composites. Yb2Si2O7 and mullite (3 Al2O3⋅2 SiO2) were chosen as the base materials for the bond coat to enable temperatures beyond the melting point of the current silicon bond coat (1414 °C). Various sintering aids, such as mullite, Yb2Si2O7, Al2O3, and silicon enabled sintering at 1500 °C – 1550 °C. Yb2Si2O7 was used as the top coat to provide water vapor recession resistance. A HfSiO4 intermediate layer prevented the 1500 °C eutectic reaction at the mullite/Yb2Si2O7 interface. Parabolic oxidation behavior and good adhesion as well as chemical and microstructural stability were observed after 500 h / 500 cycle steam oxidation at 1427 °C in 90 % H2O-10 % O2 steam. Kinetic analyses on Yb2Si2O7-coated SiC showed the independence of TGO growth rates on EBC thickness indicating that the oxidant permeation through the silica scale is the rate controlling factor.

Thermal cycling failure of the multilayer thermal barrier coatings based on LaMgAl11O19/YSZ

Thermal cycling failure of three multilayer TBCs based on LaMgAl11O19 (LaMA)/YSZ was comparatively investigated by using the burner-rig testing method in this work. Results indicate that through optimizing the weight ratio and thickness of the intermediate LaMA/YSZ composite layers, a five-layer TBC with much improved thermal cycling life of 11,749 cycles at 1372 °C surface and 1042 °C bond coat testing temperature has been realized. While, thermal cycling lifetimes of the tri- and six-layer TBCs were 7439 and 7804 cycles at surface/bond coat testing temperatures of 1378 °C/1065 °C and 1367 °C/1056 °C, respectively. Factors related to the 60 wt.% LaMA + 40 wt.% YSZ (60LaMA + 40YSZ) intermediate composite layer with the highest thermal expansion coefficient than other composite layers generating higher internal stress level to the tri- and six-layer TBCs, different bond coat temperature and TGO growth, as well as long-term stability of the LaMA coating during thermal cycling tests, were characterized and compared to understand the different thermal cycling lifetime and failure modes among such three multilayer TBCs.

Modeling and characterisation of depletion of aluminium in bond coat and growth of mixed oxides in thermal barrier coatings

Most existing thermal barrier coating (TBC) studies do not account for the depletion of Al in the BC and growth of mixed oxides (MOs). In this complementary study, we modify and extend our earlier (Lim and Meguid in Mater Des, 2019. https://doi.org/10.1016/j.matdes.2018.107543 ) coupled finite volume (FV)–finite element (FE) formulations to incorporate diffusion reactions of Al and Cr in the BC. The modified/expanded FV–FE formulations are supported by the introduction of appropriate diffusion–reaction equations and high temperature oxidation model. Three aspects of the work were accordingly examined. Firstly, the FV model is used to describe the diffusion and reaction of Al and O2 in the formation of α-Al2O3. The β-phase rate of depletion predicted by the model agrees well with experimental findings. Secondly, the diffusion of Cr through the TGO that leads to the formation of an external layer of MOs is simulated using the FV model. Our simulations reveal that MOs form in the early stage of thermal exposure, although there are sufficient Al in the BC to sustain the growth of α-Al2O3 phase. We reasoned this to the formation of internal oxides and diffusion cell in the BC. Lastly, we studied the effect of roughness on β-phase depletion and growth of TGO (α-Al2O3 + MOs). We show that β-phase in the peak of undulation depletes faster and the interface between the β + γ phase and γ phase in the BC straightens over time. Furthermore, our results reveal that MOs tend to form in the valley of undulation due to the shorter diffusion path.