Lifetime Of Thermal Barrier Coatings Research Articles

Impact of interfacial texture on failure behaviour of 8YSZ thermal barrier coatings under thermal cyclic loading

Thermal barrier coatings (TBCs) provide thermal protection for high-temperature components of aero-engines and steam turbines, but are susceptible to failure under thermal cyclic loads. The effective design of interfacial texture is one of the approaches to enhance the lifetime of ceramic TBCs. In this study, four types of interfacial textures were designed on the surface of substrates, and 8YSZ coatings of equal thickness were prepared using atmospheric plasma spraying, and thermal cycling tests were carried out. Meanwhile, the thermal cycle lifetime, microstructure evolution, residual stress, and energy release rate criterion were also studied. The optimal morphology and parameters of the texture are determined. It is found that the lifetime of the concave trapezoidal texture is the longest, with a deflection angle of 67°, diameter of 700 μm, depth of 500 μm, and spacing of 5 mm. This study can provide theoretical guidance for the interfacial texture design of ceramic TBCs.

Understanding the effect of bondcoat surface treatment on enhanced lifetime of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are extensively used in gas turbine engines in power generation and aerospace applications. Improvements in TBC performance and lifetime are continuously pursued by the gas turbine industry. Suspension plasma spraying (SPS) has shown promising results in recent years as it could produce a porous columnar TBC microstructure exhibiting low thermal conductivity and high durability. Further improvements in lifetime and fundamental understanding of failure mechanisms would lead to their wider application.Lifetime of a TBC system is dependent not only on the topcoat microstructure and chemistry, but also on topcoat-bondcoat interface and bondcoat characteristics as they influence the growth rate of thermally grown oxide (TGO) layer as well as the mismatch stresses generated in the topcoat during operation. Moreover, in SPS TBCs, the column density and porosity in the topcoat are also affected by the bondcoat surface roughness which in turn affects TBC lifetime. In previous work, it was shown that bondcoat surface treatment by shot peening followed by light grit blasting can significantly enhance the cyclic lifetime of SPS TBCs.The objective of this work was to gain further insight into the lifetime improvements due to surface treatment of bondcoat. Commercial NiCoCrAlY powder was deposited by high velocity air fuel (HVAF) and high velocity oxy fuel (HVOF) spraying while SPS was used to deposit yttria stabilised zirconia topcoat. Before spraying the topcoat, the bondcoat samples were subjected to vacuum heat treatment, shot peening, and/or grit blasting. Residual stress measurements were performed by X-ray diffraction on samples with different variants of bondcoat treatments and compared with Almen strip tests. The lifetime was examined by thermal cyclic fatigue testing. The relationships between changes in bondcoat surface roughness, residual stress state and microstructure due to surface treatments, and resultant topcoat microstructure and lifetime were studied and discussed.

The Oxidation Behaviors of NiCoCrAlY Coatings After Pre-Oxidation Treatment During High-Temperature Oxidation at 800 ℃ and 900 ℃

Thermal barrier coatings (TBCs) are essential for protecting the high-temperature components in gas turbines. However, the durability of TBCs is limited, and new technologies to improve their lifetime are necessary. This study focuses on the pre-oxidation treatment of the NiCoCrAlY bond coat in TBCs to reduce the growth rate of the thermally grown oxide (TGO) and improve the TBC lifetime. The mechanism of the TGO growth rate reduction by the pre-oxidation treatment of NiCoCrAlY was investigated. Oxidation behaviors of the surface of NiCoCrAlY coating samples with or without pre-oxidation treatment were evaluated in air at 800 °C and 900 °C. In-situ X-ray diffraction (XRD) measurements and isothermal oxidation tests were performed. The obtained results revealed that the growth rate of TGO is significantly suppressed by α-Al2O3 layer formed by the pre-oxidation treatment. The generation of γ-Al2O3 and θ-Al2O3, which are typically formed below 900 °C, was significantly reduced by pre-oxidation. It is suggested that this reduction suppresses of the TGO growth.

The effect of ceramic YSZ powder morphology on coating performance for industrial TBCs

The increasing turbine inlet temperatures in modern gas turbines have raised concerns about the corrosion of ceramic thermal barrier coatings (TBCs) caused by molten silicate deposits, commonly referred to as “CMAS” due to their main constituents (CaO-MgO-Al2O3-SiO2). The objective of this study was to investigate the combined influence of powder morphology and chemical composition on the CMAS resistance and thermal cycling resistance of ceramic monolayer and bi-layer coatings created through Atmospheric Plasma Spraying (APS). Three powder morphologies were examined: porous Agglomerated and Sintered (A&S) granules, Hollow Spherical (HOSP) powders, and dense, irregular Fused and Crushed (F&C) particles. Monolayer 7-8YSZ coatings with both porous and dense vertically cracked (DVC) microstructures, and bi-layer coatings consisting of a bottom layer of porous standard 7-8YSZ and a top layer composed of a porous high‑yttrium ZrO2–55 wt% Y2O3 were obtained using all three powder types (A&S, HOSP, or F&C). Furthermore, the bi-layer systems were deposited with different ratios between the individual layer thicknesses and/or different total thickness. FEG-SEM, EDX, and micro-Raman analyses, were conducted to assess the coatings' performance. Nanoindentation high-speed mapping and pillar splitting test were performed to evaluate the mechanical behaviour. The study on 8YSZ monolayers shows that coatings from a F&C feedstock exhibit higher density, reducing the CMAS penetration. However, these coatings demonstrate poorer thermal cycling performance due to increased stiffness and thermal stresses. Coatings from HOSP and A&S powders allow CMAS penetration but offer stress relief pathways, enhancing the coating's ability to withstand thermal stresses. Bi-layer coatings with a 55YSZ top coat show superior CMAS resistance compared to 7-8YSZ monolayer coatings, with limited penetration causing top coat peeling. The thickness ratio between the layers also affects thermal cycling resistance, where a thinner 55YSZ layer extends the TBC lifetime.

Effect of Thermal Spraying Method, Time and Temperature on the Thickness of Thermally Grown Oxide and Lifetime of Thermal Barrier Coatings

Effect of Thermal Spraying Method, Time and Temperature on the Thickness of Thermally Grown Oxide and Lifetime of Thermal Barrier Coatings

Characteristics and performance of suspension plasma sprayed thermal barrier coatings on additively manufactured superalloy substrates

The complex-shaped hot-section parts of new-generation turbine engines demand unique design solutions. Additive Manufacturing (AM) is an emergent production method that can produce metallic parts with complex geometries and minimal material wastage. In this work, the characteristics and performance behavior of Thermal Barrier Coatings (TBCs) deposited on forged and AM-built HAYNES®282® superalloy substrates were studied and compared. The bond coats were produced by High-Velocity Air-Fuel (HVAF) spraying using NiCoCrAlY powder feedstock and TBC top-coats by Suspension Plasma Spraying (SPS) using water- and ethanol-based suspensions of Yttria-Stabilized Zirconia (YSZ). The microstructural features, adhesion, Thermal Cycling Fatigue (TCF) lifetime, and thermal shock lifetimes of the TBCs were comprehensively investigated. The results showed that the deposition of bond coats reduced the roughness and asperities of the AM-built substrates. Depending on the type of suspension used and the spray parameters employed, the TBCs exhibited vertically cracked and columnar microstructures. However, no significant differences in TCF and thermal shock lifetimes of TBCs on AM and forged substrates were observed. It is demonstrated that TBC systems can be produced on AM-built metallic substrates, and the resulting TBCs can have similar microstructures and properties as TBCs deposited on conventional substrates.

Hf and Ta co-doping MCrAlY alloy to improve the lifetime of coatings

Thermal barrier coating (TBC) is one of the key components of an aircraft turbine or stationary gas turbine. NiCoCrAlY is the most commonly used bond coat (BC) and its composition and impurities have an impact on the lifetime of the TBC. However, the performance of NiCoCrAlY is very unstable due to the influence of impurities. Reactive elements (RE) Hf and Ta are co-doped into the bond coat to improve the adhesion of thermally grown oxide (TGO) layers to extend the lifetime of TBC. Yttrium stabilized zirconia (YSZ) is sprayed on free-standing alloys of NiCoCrAlY and NiCoCrAlYHfTa, respectively. The samples are cyclically oxidized at 1050 °C until the peeling off of coatings. The lifetime of NiCoCrAlY coatings is only 240 h, while the lifetime of NiCoCrAlYHfTa coatings reaches 2664 h. The morphology and structure of the oxide scale are characterized to study the mechanisms of lifetime extension.

The relationship between the film hole distribution and thermal cycle performance of TBCs by APS and HVOF

Thermal barrier coatings (TBCs) play an important role in protecting hot-end components of aero-engines from high-temperature oxidation and corrosion. The peeling of the TBCs around film holes has emerged as a major source of failure in thermal cycle oxidation. This study examines the failure behavior of TBCs on floating tiles in the combustion chamber of an aero-engine. Ninety thermal cycles were performed on two types of floating tiles with different film hole distribution. Results showed that the surface TBCs of sample No.1 peeled off by approximately 40 % of the total area, while 90 % of the surface TBCs of sample No.2 floating tile remained intact. A thermal cycling experiment of the TBCs on the surface of the floating tiles was performed to determine the stress distribution of the film holes by finite element (FE) simulation. The results indicated that there was a significant stress gradient on the TBCs surface around the film holes after thermal cycles, with 86.09 MPa and 62.84 MPa higher stress gradient around the film holes of sample No.1 floating tile compared to the film hole of sample No.2 floating tile on the surface and back side, respectively. This difference in the stress gradient and the film hole location on the TBCs led to spalling of the TBCs on the surface of both floating tiles. This study elucidates the failure mechanism of TBCs around film holes under thermal cycling through experiments and FE simulations, offering insights to improve the lifetime of TBCs.

Effects of thermal and mechanical properties of Hf and Ta-doped NiCoCrAlY bonding layer alloys on the high-temperature oxidation resistance of thermal barrier coatings

The MCrAlY bonding layer alloy was prepared by vacuum melting, and the alloy composition was NiCoCrAlY and NiCoCrAlYHfTa. The high-temperature cyclic oxidation experiment was carried out in air at 1050∘C until it failed. The results show that Hf and Ta have a significant influence on the oxidation resistance of the alloy. The doped specimens have longer oxidation lifetimes compared to the undoped samples. The study also found that the Hf would diffuse out and combine with O to form HfO2 in the thermally grown oxide (TGO) layer, which would improve the adhesion and reduce the wrinkles of the oxide layer. Ta in the doped specimens hinders the diffusion of Al and reduces the oxidation rate, while improving the thermal mismatch. Therefore, the specimens doped with both Hf and Ta have the highest oxidation lifetimes, though the oxidation lifetime was 10 times longer than that of the undoped specimens. It is indicated that the doping of Hf and Ta is an important method to improve the oxidation lifetime of thermal barrier coatings (TBCs).

High temperature oxidation resistance of laser shock modified NiAlHf metallic bond layer

The growth state of the thermally grown oxide (TGO) in thermal barrier coatings (TBC) has a crucial influence on the lifetime of TBC. Laser shock processing (LSP) is a new method for modifying the metallic bond layer. In this paper, NiAlHf coatings were modified through LSP. The effect of LSP on TGO formation in the early oxidation phase, the growth state of TGO at the long oxidation stage and the distribution of Hf element within the NiAlHf coatings after LSP were investigated. The results show that LSP successfully induce two effects on NiAlHf coatings. (1) The original oxide particles on the coatings surface are smaller and distribute more densely after LSP. In the oxidation process, these smaller and denser particles act as nucleation points, which make it easier for them to grow rapidly into a continuous oxide film. (2) LSP introduces high density dislocation lines, dislocation cells, sub-grain boundaries and other crystal defects, which provides more short diffusion channels for the selective oxidation of Al element and promotes the growth of oxide films. Both of these effects promote the rapid formation of a dense, continuous and homogeneous oxide film during the early oxidation stages. In addition, Hf element in the No-LSP samples mainly precipitate along the NiAl grain boundaries during the long-time oxidation process. This tends to cause the accumulation of Hf-rich oxides and accelerates the cracking of the oxide film. In contrast, LSP-induced crystal defects allow Hf-rich oxides uniformly distribute within the coatings. The uniform distribution of Hf-rich oxides is beneficial for improving the oxidation resistance of TGO films.

Heterogeneous layered structure in thermal barrier coatings by plasma spray-physical vapor deposition

The unique columnar structure endows thermal barrier coatings (TBCs) prepared by plasma spray-physical vapor deposition (PS-PVD) with high thermal insulation and long lifetime. However, the coating delamination failure resulting from an intra-column fracture (within a column rather than between columns) is a bottleneck in the solid dust particle impact environment for aero-engine. To clarify the intra-column fracture mechanism, a basic layer deposition model is developed to explore a heterogeneous weak-to-strong layered structure formed by a local transient <i>in-situ</i> deposit temperature. During the PS-PVD, an <i>in-situ</i> deposit surface is continuously updated due to constantly being covered by vapor condensation, showing a transient temperature, which means that the <i>in-situ</i> deposit surface temperature rises sharply in short period of 0.2 s of depositing a thin layer during a single pass. Meanwhile, the increasing temperature of the <i>in-situ</i> deposit surface results in an experimentally observed heterogeneous weak-to-strong structure, showing a continuous transition from a porous weak structure at the bottom region to a dense strong structure at the top region. This structure easily makes the intra-column fracture at the porous weak region. The results shed light on improving TBC lifetime by restraining the intra-column fracture.

Structure and durability evaluation of blast furnace slag coatings and thermal barrier coatings (TBCs) under high temperature conditions

A high temperature aggressive environment is a great challenge for the long lifetime of thermal barrier coatings (TBCs). In this study, the hot corrosion and isothermal oxidation behavior of blast furnace slag (BFS) and yttria stabilized zirconia (YSZ) coated TBC systems were systematically investigated under high temperature conditions. This study aims to analyze the evaluation of microstructural properties of thermally sprayed CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings before and after high temperature oxidation and hot corrosion tests. For this purpose, the coatings were isothermally oxidized at 900 °C for 8, 24, 50, and 75 h. Hot corrosion behavior of CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings deposited on Inconel 718 superalloy substrates were studied at 900 °C in a Na2SO4 and V2O5 molten salt environment. CoNiCrAlY/BFS and CoNiCrAlY/YSZ TBC systems exposed to Na2SO4+ V2O5 molten salt at 900 °C investigate the hot corrosion behavior. The microstructure of coatings, formation of thermally grown oxide (TGO) layer, and degradation of coatings during isothermal oxidation and hot corrosion conditions were investigated. The oxidation and hot corrosion mechanisms were discussed. BFS, could be a potential alternative for the YSZ ceramic top coating in TBCs. The TBC system with BFS can be preferred for oxidation damage due to its low cost. YSZ and BFS TBCs are highly resistant to Na2SO4 and V2O5 molten salt environment.

High temperature oxidation resistance of TNM alloy coated with/without 8YSZ/NiCoCrAlY thermal barrier coatings

The oxidation resistance of the TNM alloy (Ti-44Al-4Nb-1.5Mo (B, Y) (at. %)) coated with/without 8YSZ/NiCoCrAlY thermal barrier coating (TBC) was conducted through cyclic oxidation at 950 ℃. The formation mechanism of oxide scale, interface interdiffusion mechanism, and evolution law of thermal growth oxide (TGO) were systematically investigated. Results show that the weight gains of the TNM alloy coated with/without TBC are 2.45 mg/cm2 and 6.98 mg/cm2, respectively. The Ti-Al-N layer and the (Nb, Mo)-rich layer near the surface play a role in blocking the internal diffusion of air and the external diffusion of metal ions. After a comprehensive analysis of EPMA and SEM results, it is concluded that the diffusion rate of elements in NiCoCrAlY is Ni > Co > Cr. At the same time, the relationship between the interface diffusion layer thickness and oxidation time is built based on Fick’s first law. In addition, as the oxidation time increases, TGO evolves from a dense Al2O3 layer to CS (chromia, spinel oxide) layer, and CSN (chromia, spinel, and nickel oxide) cluster oxides. The mixed oxides induce crack nucleation and growth, thereby reducing TBC lifetime.

Research Progress of Self-Healing Thermal Barrier Coatings: A Review

Reliability and durability are two important performance indicators for thermal barrier coatings (TBCs). The reliability of TBCs usually includes high adhesive strength, low thermal conductivity and high thermal shock resistance. The high reliability of TBCs ensures basic usage requirements. Durability demands TBCs have a long service lifetime before their eventual failure. The lifetimes of TBCs under actual service conditions are strongly dependent on crack initiation and propagation. Controlling and delaying the dynamic process of crack initiation and propagation is a direct approach to prolonging the service lifetime of TBCs. Self-healing TBCs usually have the specific function of inhibiting crack propagation, and thus promote the self-healing process of TBCs. The research progress of self-healing TBCs was reviewed. Firstly, the concept of self-healing or self-healing materials is clarified. Secondly, the research progress about some self-healing ceramic materials as well as self-healing TBCs is reviewed. Based on the review, the micro-structure design, propagation patterns of the crack and self-healing mechanism are discussed systematically. Additionally, the future development trend of the self-healing TBCs is also overviewed in this paper.

Oxidation and interfacial cracking behaviors of TBCs with double-layered bond coat on different substrate materials

The oxidation and interfacial cracking behaviors of thermal barrier coatings (TBCs) with double-layered bond deposited on two typical substrate materials K452 and M247 was comparatively studied. Composition of the substrate can affect the element interdiffusion of the inner bond coat and the driving force of interfacial cracking. The capacity of the inner bond coat to supply Al for the upper bond coat is critical to the lifetime of TBCs with double-layered bond coat. Adopting a substrate with higher resistance to the inward-diffusion of Al can facilitate the formation of the protective Al2O3 scale and significantly extend the lifetime of TBCs.

Advanced MCrAlY alloys with doubled TBC lifetime

Thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades. However, the most commonly used MCrAlY bond coats which largely determine air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. APS TBC has been found predominate failure within top coat near interface/or at interface with thermally grown oxides (TGO). TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in the top coat or both. By focusing on the reduction of tensile stress in the top coats, three new bond coat alloys have been designed and developed. More than doubled TBC lifetime has been achieved by using the new alloys as bond coat. Extended thermal cycle lifetime is attributed to the unique properties of the new alloys, such as remarkably lower coefficient of thermal expansion (CTE), lower weight fraction of β phase, lower aluminum migration rate and the absence of mixed/spinel TGO oxides.

Critical scale grain size for optimal lifetime of TBCs

• The critical scale grain size d c of ∼2.5 μm is identified for optimal TBC lifetime. • When d < d c , the rapid growth of the scale causes delamination failure of TBCs. • When d > d c , the growth stress causes vertical cracking and early failure of TBCs. Thermal barrier coatings (TBCs) suffer from spalling failure resulting from the rapid growth of thermally grown oxides (TGO) on the metallic bond coat. In this study, to achieve the optimal lifetime of TBCs, a lifetime phase diagram was constructed for the entire service of TBCs. Herein, both the thermal mismatch stress during thermal cycling and the TGO growth stress during thermal exposure were considered. First, thickening behavior of the Al 2 O 3 scale related to grain size was investigated. Second, transverse delamination failure due to the thermal mismatch stress was analyzed. Third, vertical cracking failure related to early scale growth was explored. Finally, a critical scale grain size of ∼2.5 μm was identified for achieving the optimal lifetime of TBCs. It was experimentally proved that vertical cracking of the Al 2 O 3 scale occurred during early oxidation as the scale grain size was larger than ∼4 μm after pre-oxidation. The proposed lifetime phase diagram and critical scale grain size effect serve as novel criteria for the design of next-generation durable TBCs.

Cracking behavior and delamination mechanism of lamellar structured TBC with localized mixed oxides

Mixed oxide (MO) with localized growth feature and high growth rate remarkably affects the lifetime of thermal barrier coatings (TBCs), which indicates that clarifying the ceramic cracking mechanism induced by MO is critical for developing new coatings with high durability. Two kinds of TBC models involving spherical and layered mixed oxides are created to explore the influence of MO growth on the local stress state and crack evolution during thermal cycle. The growth of α-Al2O3 is also included in the model. The undulating interface between ceramic coat and bond coat is approximated using a cosine curve. Dynamic ceramic cracking is realized by a surface-based cohesive interaction. The ceramic delamination by simulation agrees with the experimental observation. The effects of MO coverage ratio and growth rate on the TBC failure are also discussed. The results show that the MO growth causes the local ceramic coat to bear the normal tensile stress. The failure mode of coating is turned from α-Al2O3 thickness control to MO growth control. Once the mixed oxide appears, local ceramic cracking is easy to occur. When multiple cracks connect, ceramic delamination happens. Suppressing MO formation or decreasing MO growth can evidently improve the coating durability. These results in this work can provide important theoretical guidance for the development of anti-cracking TBCs.

Ultralow oxygen ion diffusivity in pyrochlore-type La2(Zr0.7Ce0.3)2O7

Thermally grown oxides (TGOs) at the ceramic top-coat/metallic bond-coat interface are a pressing challenge in advanced thermal barrier coating (TBC) systems as they can affect the performance and service lifetime of TBCs. Thus, developing novel TBC materials with ultralow oxygen ion diffusivity is very urgent. In this study, we reported the diffusive properties of oxygen ions in a novel pyrochlore-type La2(Zr0.7Ce0.3)2O7 (LZ7C3) material. The measured ionic conductivity and atomistic simulation revealed that the oxygen ion diffusivity in LZ7C3 grains is two orders of magnitude lower than that in conventional 8 wt.% yttria-stabilized zirconia (8YSZ) grains. This is due to the relatively high energy barrier for oxygen hopping in LZ7C3. In addition, it was found that enhancing the order distribution of cations is a strategy to reduce the intrinsic oxygen diffusion of pyrochlore-type oxides. On the other hand, we observed that La3+ cations segregate at the grain boundaries (GBs) of LZ7C3, which results in the electrostatic potential at GBs being comparable to that in the bulk. Furthermore, we found that the oxygen ion diffusion is facilitated at the GBs of LZ7C3 due to the stretched O–Zr/Ce bond and the low coordination at GBs. However, the segregations of Y3+ cations and the increase in the number of oxygen vacancies resulted in the formation of an electrostatic layer at the GBs of 8YSZ, which shielded the oxygen ion diffusion. Despite this, the oxygen ion diffusivity in LZ7C3 was still considerably less than that in conventional 8YSZ. This study offers a stepping stone toward utilizing pyrochlore-type LZ7C3 materials as advanced TBCs at high temperatures.

Factors affecting cyclic lifetime of EB-PVD thermal barrier coatings with various bond coats

Abstract Standard EB-PVD (electron beam physical vapor deposition) thermal barrier coatings (TBCs) of partially yttria-stabilized zirconia were applied on Ni-base substrate alloys and thermally cycled with T max = 1100 °C. Two different bond coats were investigated: NiCoCrAlY and NiPtAl. The longest lifetimes have been achieved on the Hf-containing Rene142 substrate. A rough interface between TGO (thermally grown oxide) and bond coat with hafnia pegs is characteristic for these alloys. A bond coat heat treatment in Ar–H significantly improved the lifetime of the TBC system with IN100 + PtAl bond coats, whereas reduced lifetimes were found on IN100 + NiCoCrAlY. Spallation of the TBCs was correlated to TGO thickness and microstructure. The TGO growth is faster for NiCoCrAlY than for NiPtAl bond coats, while TBC lifetime is longer for the NiCoCrAlY systems. Tests with reduced cycle length yielded an increased number of cycles to failure for IN100+NiCoCrAlY, but left the time to failure at temperature unchanged. TBC thickness had only a limited influence on cyclic lifetime.