Long-term stability and performance of nanostructured thermal barrier coatings deposited by magnetron sputtering on superalloys: A review

Advantages of nanostructured magnetron sputtered TBCs

Magnetron sputtered nanostructured thermal barrier coatings have shown critical advantages for aviation gas turbine blade applications that improve engine performance and efficiency. The unique characteristics of nanostructured materials and the control exhibited by sputtering are combined to afford these benefits. Magnetron sputtering deposited nanostructured TBCs are a promising technological means of improving the performance and efficiency of aviation gas turbine blades. This technology provides lower thermal conductivity, improved adhesion strength, and precise control over coating characteristics that increase engine efficiency, increase operating temperatures, and increase component lifetimes. Because nanostructured TBCs deposited by magnetron sputtering will continue to be a key part of developing more efficient and durable gas turbines, the demand for them will only grow as the demand for aviation propulsion systems increases.

Enhanced Thermal Stability

Thermal barrier coatings (TBCs) for high-temperature applications

TBCs are significant in gas turbines and aerospace engines as they help resist severe thermal cycling and oxidation stress. Oxidation mechanisms of TBC and how they impact durability have been studied by several. A comprehensive review of oxidation mechanisms in TBCs was published by Roy et al., ³³ where they illustrated how TGO formation and bond coat adhesion influence coating longevity. Microstructure effects on oxidation resistance in MCrAlY coatings, a common bond coat material, were studied by Zhao et al., ³⁴ who showed that deposition methods could significantly affect performance.

Phase stability and sintering of TBCs have been investigated through modifications of the material composition. A Pt-Al protective film on YSZ TBCs designed to resist CMAS (calcium magnesia alumina silicate) corrosion, the primary source of degradation, was proposed by Wang et al. ³⁵ According to their findings, Pt-Al films can massively improve CMAS resistance without deteriorating thermal cycling life, although the softening of platinum at high temperatures is an issue. Researchers have studied the improvement of composite TBC thermal cycling durability. Functionally graded YSZ-Al₂O₃ coatings were analyzed by Ramesh et al. ³⁶ as a scenario in which a gradual compositional transition reduces thermal stress (better thermal cycling resistance) between the layers. Their work points out the advantages of multi-material coatings in enhancing the TBC performance under high-temperature conditions.

It has also been the effect of coating thickness and bonding strength on TBC performance. As stated by Wang et al., ³⁷ TBC thickness, bonding strength, and thermal cycling life have a relationship in which thicker coatings provide better thermal insulation. However, inferior quality bonding strength leads to decreased lifespan. Therefore, to ease this problem, they fabricated a gradient NiCoCrAlY/YSZ coating using a synchronous dual powder feeding method, providing better bonding and oxidation resistance. To further improve corrosion resistance, it was shown by Ali Raza et al. ³⁸ also examined how the porosity and the corrosion resistance of HVOF coating deposition can depend on oxygen flow rate and spray distance. They show that to tune the TBC properties for better durability, it is important to control process parameters. Meng et al. ³⁹ also investigated the effect of various superalloy substrates (K38, N5) on the oxidation behavior of nanocrystalline coatings. They confirmed that substrate selection is an important influencing factor for coating performance.

Moreover, composite coatings have been introduced to improve the TBC properties further. PS-PVD 7YSZ TBCs were modified with Al₂O₃ by Zhang et al., ⁴⁰ showing that additions of Al₂O₃ help enhance oxidation resistance and coating reliability in the harsh operating environments of turbine components.

Diffusion barrier coatings for nuclear applications

Diffusion barrier coatings are essential in nuclear reactors because they improve mechanical stability and prevent oxidation; TBCs, on the other hand, focus on thermal insulation. Chromium coatings on zirconium alloy fuel claddings, an important component of nuclear reactors, have been investigated by Li et al. ⁴¹ Using radio frequency magnetron sputtering, they show that a 6 μm Cr coating greatly enhances oxidation resistance in steam at 1200°C. A chromium oxide stable layer blocks oxidation, improving mechanical strength and corrosion resistance.

Li et al. ⁴² further researched (Cr/CrN) multilayer coatings deposited on Zr—4 alloy, proving their excellent oxidation resistance under extremely harsh conditions (1200°C steam exposure). Based on their results, the authors learned more about the oxidation mechanisms and how layered structures can improve durability in nuclear fuel claddings.

Peng et al. ⁴³ have also studied the role of oxygen addition to NiCrAlY coatings and found that oxygen-containing coatings contain nano-crystalline Y₂O₃ structure and better wear and hardness resistance at high temperatures. They find that microstructural engineering can improve diffusion barrier performance in nuclear environments.

Advances in coating deposition techniques

Coating deposition techniques have a high influence on performance and durability of TBCs and diffusion barrier coatings. Substrate temperature, deposition method and layering strategy to investigate how they affect the mechanical properties have been investigated by researchers. In addition, it is observed by Verma et al. ⁴⁴ that coating hardness and Young's modulus are affected significantly by substrate temperature, with the higher values of mechanical properties reaching their maxima at 500°C as a result of a dense microstructure.

The hardness of Al₂O₃-Ta₂O₅ multilayer coatings was investigated by Zhang et al., ⁴⁵ and hardness variation was shown with layer structure; however, the elastic modulus was constant (145–155 GPa). Kadam et al. ⁴⁶ further investigated how spray angle variation in air plasma sprayed 8YSZ coating directly impacts porosity, hardness, and roughness, thus emphasizing the significance of precise process control in TBC fabrication. Simulation techniques have also been employed to optimize coating design. Finite Element Modeling (FEM) was used by Price et al. ⁴⁷ to assess hardness variation within multilayer coatings. It showed that FEM may be utilized as a predictive tool in optimizing coating performance. For example, Solovyev et al. ⁴⁸ investigated how residual stress is formed in YSZ electrolyte films. They found that with optimized deposition modes, stress is reduced, allowing for a more stable coating.

In their works, Kurian et al. ⁴⁹ compare the different coating processes, i.e., atmospheric plasma spraying and laser cladding, to be suitable for industrial applications. Their study highlighted how the coating method affects possible microstructure, porosity, and adhesion strength of TBC and diffusion barrier.

The fabrication of nanocrystalline coating by magnetron sputtering has been widely explored. Uniform thickness control in the sputtering deposited nanocrystalline coating has been emphasized by Wang et al. ⁵⁰ as a method to achieve favorable coating properties. For aerospace applications, Lin et al. ⁵¹ used titanium alloys to apply thermal barrier coatings while using reactive pulsed DC magnetron sputtering as a scalable technique.

Reactive pulsed DC magnetron sputtering recently proved to be a promising way of depositing high-performance TBCs. Its high deposition rates and scalability make it suitable for aerospace applications, as mentioned by Lin et al. ⁵² Various control parameters of the stoichiometry, the microstructure, and the adhesion properties are being improved to optimize this technique for next-generation coatings.

Table 1 shows how the studies summarized advanced TBC properties and performance over various substrates and deposition methods for magnetron-sputtered thermal barrier coatings (TBCs). Important findings demonstrate the enhanced oxidation resistance, thermal stability, and protection against corrosion due to dense ceramic layers and bond coats. For example, Al modifications were shown to improve CMAS corrosion resistance and super-hydrophobicity by Zhuo et al. (2024), and Dai et al. (2024) achieved more excellent durability with aluminized NiCrAlY coatings. Zhang et al. (2023) and Liu et al. (2022) demonstrated thermal insulation and electrical resistivity improvements through dense layers and insulating coatings. While these advancements are promising, little information currently exists regarding the coating's long-term stability and longevity under actual service conditions on a global scale. The findings in Table 1 identify the key influential factors on TBC performance and provide directions for future research to optimize TBC applications in extreme environments.

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Table 1.

Summary of reports on properties and deposition methods of thermal barrier coatings (TBCs).

S.no	References	Deposition Method	Bond / Topcoat	Substrate	Findings
1	Xueshi Zhuo et al. (2024) 53	DC Magnetron Sputtering + PS-PVD + Laser Texturing	Bond Coat: NiCrAlY, Top Coat: 7YSZ	DZ40M superalloy	A 5 μm Al film improved super-hydrophobicity (contact angle: 12.3° to 168.8°) and CMAS corrosion resistance at 1230°C for 1800s.
2	Shengkai Dai et al. (2024) 54	Magnetron Sputtering + Arc Ion Plating	NiCrAlY coating with Al layer	GH4169 superalloy	Aluminizing improves oxidation resistance, delays Al depletion rate, and prolongs durability.
3	Shafaq Ashraf Lone et al. (2024) 55	Magnetron Sputtering	Nanostructured Co-Al coating (with and without Cr)	Superalloy Superni-718	Cr enhanced cyclic oxidation resistance. Oxidation resistance was enhanced at higher substrate temperatures.
4	Bo Meng et al. (2023) 56	Magnetron Sputtering	NiCrAlY (Bond Coat) with YSZ (Topcoat)	single-crystal superalloy N5	Because of thermal expansion mismatches, cracks occurred at the TGO/BC interface or YSZ/TGO interface of NiCrAlY-TBC.
5	Li Ai et al. (2023) 57	Magnetron Sputtering	Dense zirconium oxide layer	nickel-based superalloy substrate	Corrosion resistance under CMAS (Calcium–Magnesium–Aluminum Silicon oxide systems) (1300°C) and NV (NaCl-Na2SO4-V2O5) conditions (900°C) was significantly enhanced by dense layers.
6	Jun-Gui Zhang et al. (2023) 29	Magnetron Sputtering for Al film	ZrO₂-7 wt% Y2O3 (7YSZ), Al modification via magnetron sputtering	A DZ125 nickel-based superalloy	The thermal insulation properties of 7YSZ TBCs at 1000°C have been significantly modified by Al modification using magnetron sputtering, with a maximum improvement of 100°C for PS-PVD TBCs.
7	Yang Liu et al. (2022) 58	Magnetron Sputtering	NiCrAlY (Bond Coat), Al2O3 insulating layer, thermally grown alumina (TGA)	Nickel-based superalloy (Turbine Blade)	The electrical insulation was >2 × 109 Ω·cm at 1000°C, and the coatings demonstrated outstanding thermal stability.
8	Ali, I et al. (2021) 59	Direct Current Magnetron Sputtering for Al interlayer + Atmospheric Plasma Spraying (APS) for bond/topcoat	CoNiCrAlY bond coat Yttria-stabilized zirconia (YSZ) top coat.	Inconel 600 substrate.	Meanwhile, the oxidation resistance of the Al-TBCs was enhanced, and YSZ cracks were reduced. Mixed failure was observed after 600 h at 1150°C.
9	Shiyu Cuiet al. (2021) 60	Magnetron Sputtering	Sealed layers (Zr, Al, and Al-Y) on TBCs	Nickel-based superalloy	Sealed layers reduced CMAS penetration. Al-Y reacted with CMAS and formed Al2Y4O9, but when in reaction with Al-Y, CMAS formed Ca3Y2(SiO4)6O₂, capable of forming fast crystallization and permanent protection.
10	Budinovskii, S. Aet al (2019) 61	Magnetron sputtering.	NiCrAlY (Re, Ta, Hf) + AlNiY (Hf) Y- and Gd-containing zirconium alloy targets for ceramic layer.	nickel alloys	The alloy materials suffered the least damage and the lowest mass losses in the coatings.
11	Panpan Zhang et al (2019) 62	Magnetron Sputtering + Plasma Spraying + Laser Remelting	Topcoat: Yttria-Stabilized Zirconia (YSZ) with α-Al2O3 overlay	nickel-based superalloy K417G	It was found that hot corrosion resistance was improved by laser remelting, which decreases surface roughness and seal defects, and Al deposition by magnetron sputtering, which increased resistance by the formation of α-Al₂O₃ and decreased molten salt reactions. In a 55 wt.% V2O5 and 45 wt.% Na2SO4 mixture at 1000°C for 30 h, the coatings exhibited resistance to hot corrosion.
12	C. Zhu et al. (2012) 63	DC Magnetron Sputtering	Topcoat: Al film (∼1 μm) on YSZ	Ni-based	Thermal treatment at 1000°C in static air was utilized to deposit Al film (∼1 μm) on the YSZ topcoat surface and oxidize to form Al2O3 in the YSZ voids. Meanwhile, Al2O3/YSZ TBCs were marked for these samples.
13	Seraffon et al. (2011) 64	Magnetron sputtering.	NiCoCrAlY with a ceramic layer	Nickel-based superalloys.	Co-sputtering of different targets produced coatings with minimal damage.
14	Xin Wang et al. (2010) 65	DC Magnetron Sputtering for bond coat + EB-PVD for top coat	Bond Coat: Pt-diffused γ–γ′ layer (10 μm) Topcoat: YSZ (200 μm)	CMSX-4 (Ni-based superalloy)	It was found that coarse (Ra = 1.165 μm) and fine (Ra = 0.682 μm) grooves can be produced using a process approach such as directional grinding. After sputtering and heat treatment, the former Pt bond coat retained grooves. EB-PVD deposited YSZ.
15	Li, Z et al. (2010) 66	DC Magnetron sputtering.	NiCoCrAlY.	GH3128 Nickel-based superalloy.	The bond-coated specimen (NiCoCrAlY) gained 48.16% of the weight of the superalloy substrate sample after the isothermal-oxidation test. The high-temperature oxidation resistance of the GH3128 superalloy has been significantly enhanced.
16	Rahman, A et al. (2009) 67	Magnetron sputtered.	Ni-Al coatings	Superni-718	The formation of nonporous, adhesive oxide scales with superior oxidation resistance was found in Ni-Al coatings.
17	Lee, J. H et al. (2009) 68	HVOF for bond coat + Magnetron Sputtering for Pt film + Pack Aluminizing	Bond Coat: CoNiCrAlY (with and without Pt), TopCoat: YSZ	Hastelloy-X superalloy	Pt-aluminized coatings retained oxide scales after 150 h at 1100°C, outperforming aluminized and HVOF coatings.
18	P. Kuppusamiet al. (2005) 69	DC Magnetron Cosputtering	Bondcoat: Ir–24Ta (at %)	Ni-based single-crystal superalloy (TMS-75)	Ir–Ta coatings were stable up to 1073 K, and interdiffusion occurred at 1273 K. Hardness peaked at 16.2−23.9 at—% Ta.
19	Ning et al. (2004) 70	Direct Current Magnetron Sputtering	NiAl–0.1 at % Hf	Glass, Si (1 1 1), CMSX-4® (Ni-based superalloy)	B2 NiAl coatings exhibited dense zone T microstructures. Texture depended on power and substrate: (1 1 1) surface orientation is formed on Si, (2 1 1) on glass, and random on CMSX-4®. Heat treatment reduced hardness. Textured coatings showed higher modulus and lower hardness.
20	Kuppusami, P et al. (2004) 71	DC Magnetron Sputtering	Ir–Ta coatings (16.2–65.1 at % Ta)	Nickel-base single-crystal superalloy TMS-75	Nanocrystalline structures were formed in Ir–Ta coatings. The highest hardness and elastic modulus limit were achieved at 16.2–23.9% Ta; further increment of Ta increased hardness and modulus.

This review highlights a critical gap in magnetron-sputtered nanostructured thermal barrier coatings (TBCs) because of the absence of real-world test samples to test the long-term stability under actual service conditions. However, such coatings exhibit good wear resistance and excellent properties in a laboratory environment. However, today, test methods neglect the complex and aggressive conditions in applications such as gas turbines or jet engines. However, this gap between laboratory simulations and actual applications limits the development of durable and efficient TBCs and points toward the need for testing methods that represent realistic operating conditions.

Grain boundaries

Nanostructured coatings distinguish themselves from conventional coatings through their significantly reduced grain sizes, typically falling below 100 nanometers. This shift to the nanoscale realm profoundly alters material behavior, primarily due to the increased dominance of grain boundaries. Often overlooked in larger-grained materials, these interfaces take center stage at the nanoscale, dictating a wide array of properties. This transition to nanostructured configurations gives rise to many unusual and often desirable characteristics, enhancing performance across various metrics compared to their micron-sized grain counterparts. ⁸⁴ (Badea et al., 2023) elaborate on the benefits of nanostructured YSZ, highlighting the role of finer grain size in reducing thermal conductivity and enhancing resistance to sintering. The increased density of grain boundaries in these materials acts as scattering centers for phonons, hindering heat transfer. ⁸⁵

Enhanced grain boundary density

Nanostructured coatings have a high density of grain boundaries because of the reduced grain size. An increased grain boundary is a plus, as it not only impedes heat transfer but also helps to improve thermal insulation properties. ⁸⁶ Nanostructured TBCs often have controlled porosity. The porous, defective, layered microstructure reduces intrinsic thermal conductivity. Porosity contributes to mechanical compliance and thermomechanical compatibility during thermal expansion. TBCs can exhibit various pore morphologies, including interlamellar pores, microcracks, and nanopores. (Badea et al., 2023) investigates the role of interlamellar porosities in the bond coat, suggesting that controlled porosity can enhance TGO distribution without compromising overall performance.^87,88 Nanostructured coatings are characterized by their remarkably small grain sizes, which directly translates to a significantly higher density of grain boundaries than their conventional, larger-grained counterparts. This abundance of grain boundaries creates a unique internal structure that profoundly impacts the coating's properties. A high density of grain boundaries is highly desirable for achieving exceptional thermal insulation. i.e., in the form of vibrations called phonons, heat travels through materials like a wave. When these phonons encounter a grain boundary, they are scattered, and their movement is disrupted. With a higher density of grain boundaries acting as scattering points, heat flow through the nanostructured coating is significantly impeded, resulting in enhanced thermal insulation. Nevertheless, the influence of grain boundaries goes beyond purely due to their thermal properties. It also affects the optical behavior of these coatings. As a result, optical scattering occurs due to the increased density of interfaces. When the coating is in contact with the exiting light, it becomes a maze with grain boundaries in which the light scatters out into many directions. Therefore, the high density of grain boundaries within nanostructured coatings has multiple roles in influencing their thermal and optical behavior. This unique characteristic creates exciting possibilities for preparing coatings with tailored properties for different applications. ⁸⁸

Thermal insulation benefits

TBCs generally provide improved thermal insulation, strain compliance, and lower thermal conductivity than conventional TBCs. To reconcile conflicting data, further research is needed to improve predictive modeling. ⁸³ For the past decade, zirconia nanostructured TBCs, long a focus of research interest, have drawn much interest for their superior performance versus the conventional TBC. This interest results from the excellent improvements observed in other properties. Nanostructured zirconia TBCs have great wear and tear resistance and perform better for longer under harsh operational conditions. TBCs must have the ability to withstand rapid temperature changes without degradation. In this area, nanostructured zirconia has proved to be particularly beneficial. TBCs’ primary function is thermal insulation, and nanostructured zirconia enhances performance by providing lower thermal conductivity, thus improving efficiency. The strong TBC and underlying material bond are essential for longevity. Nanostructured zirconia exhibits high bonding strength, ensuring reliable performance. By reducing heat transfer, nanostructured zirconia TBCs contribute to lower operating temperatures and increased component lifespan. The defining characteristic of a successful Thermal Barrier Coating lies in its ability to impede heat flow effectively. This is precisely where nanostructured zirconia TBCs excel, boasting significantly lower thermal conductivity than their conventional counterparts. This exceptional characteristic stems from the unique arrangement of atoms within their structure. These advancements highlight the potential of nanostructured zirconia TBCs to revolutionize high-temperature applications, paving the way for more efficient and durable systems.^89,90

Enhanced adhesion strength

The performance and longevity of thermal barrier coatings are critically dependent on their ability to adhere firmly to the underlying substrate. This adhesion demands high-temperature applications where the coatings are subjected to significant thermal and mechanical stresses. A key factor influencing adhesion strength is the microstructure of the TBC, particularly at the interface between the coating and the substrate. ⁸³ (Kim et al., 2014) highlights the importance of Finely granulated coatings, characterized by small grain sizes, which offer a larger interfacial bonding area than coarse-grained coatings. This increased interfacial area allows for more bonds to form between the coating and substrate, resulting in enhanced adhesion strength. In addition, well-controlled porosity in the coating can improve the coating further. Mechanical interlocking between the coating and substrate can be augmented by a certain degree of porosity, enhancing the delamination resistance. The importance of bond coat structure in TBC performance is well-documented. Tailoring the microstructure of the bond coat, particularly at the nanoscale, can significantly influence adhesion strength. For example, creating a nanostructured bond coat with a high density of grain boundaries can promote adhesion by providing sites for mechanical interlocking and increasing the diffusion path for detrimental species that can weaken the interface. Hence, optimizing the microstructure of TBCs, including grain size, porosity, and bond coat structure, is crucial for achieving robust adhesion and ensuring the long-term reliability of these coatings in high-temperature applications. ⁹¹

Increased thermal conductivity

The thermal conductivity of YSZ nanocoatings is highly dependent upon the material's grain size. The thermal conductivity increases with increasing grain size. The role of grain boundaries in phonon scattering is responsible for this phenomenon. The significant carriers of heat energy in crystalline material such as YSZ are phonons or quanta of lattice vibrations. An interface, grain boundaries, exists between regions of different crystallographic orientations. Such boundaries scatter phonons and break the smooth heat flow through the material. Grains with smaller sizes have more grain boundaries in a coating. It reduces phonon scattering and is thus detrimental to increasing efficient heat transfer and low thermal conductivity. On the contrary, fewer grain boundaries are made possible by larger grains. The increased thermal transport through the coating results from phonons moving more easily with fewer interfaces to stop them.

According to Bai et al. (2014), this effect is also affected by unmelted nanoparticles embedded in the coating. Since these nanoparticles are much smaller than the surrounding nano/micro-grained domains, they also lead to a higher density of interfaces. Enhanced phonon scattering can balance the rise in thermal conductivity due to increased grain growth. However, unmelted nanoparticles’ grain size and distribution in the coating process must be controlled. Control over these thermal properties is, therefore, key to tailoring nanocoatings of YSZ for particular applications.^91,109 Schulz et al. address the thermal conductivity problems of electron beam-physical vapor deposition TBCs. This focuses on microstructure, grain size, and porosity as key specifiers of thermal conductivity. By these microstructural features having control during the deposition, the authors suggest it is important. microstructure, including grain size and porosity, in dictating thermal conductivity. The authors suggested that controlling these microstructural features during deposition is crucial for achieving the desired thermal properties. ¹¹⁰

Sintering's impact on hardness and strength

Sintering is a fundamental step in material consolidation but can also result in a trade-off in desirable properties. Particularly, the process often decreases hardness and strength, affecting the material's mechanical performance. Sintering occurs; materials are heated to high temperatures, which aids in densification and grain growth. Grains will pass through a series of stages, ending when particles fuse and grains increase in size, decreasing the total area of grain boundaries. However, these boundaries are critical in their way, too. They serve as barriers to the movement of the dislocations; defects that let materials deform under stress. Dislocation movement with fewer boundaries becomes easier, resulting in a reduction of the hardness and strength.

In addition, sintering tends to remove pores and increase density, but it also deleteriously degrades strength. Sometimes, the cracks are hindered by pores, which act as obstacles. Though useful for density and possibly other properties, removing them leaves a more homogeneous, possibly more susceptible to crack propagation and fracture, microstructure. Thus, careful control in the sintering process has been necessary for the desired properties, such as density and mechanical strength. For a desired microstructure that, in turn, gives a desired balance of properties for the particular use, parameters such as temperature and time must be optimized.^111,112 (Notably, Karaoğlanlı et al., 2016) examines how high-temperature oxidation affects ceramic top coat properties Figure 4, particularly hardness. The study reveals a clear trend: After oxidation, hardness values increase, and the effect increases with time and temperature. The phenomenon is attributed to the sintering effect during the oxidation process. The constituent particles diffuse and coalesce to form a densified coating as the coating is subjected to high temperatures. The densification process through sintering increases the material's hardness, resistance to indentation, and deformation. The sintering occurs under the high-temperature oxidation environment, further strengthening the ceramic top coat and decreasing sensitivity to mechanical stress and wear. ¹¹² (Zhou et al.2008) focus on the thermal stability of nanograined metals. It highlights that nanostructuring can enhance properties like strength at lower temperatures, making these materials more susceptible to grain growth at elevated temperatures. This grain growth can reduce strength and hardness, particularly in high-temperature applications like TBCs. ¹¹³

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Figure 4.

Oxidation time dependence of hardness evolution of the ceramic top coating on the TBC system. ¹¹⁴

Increased susceptibilities to cracking

The relationship between grain size and a material's mechanical properties is well-established in materials science. Generally, smaller grains contribute to increased strength and toughness, while larger grains correlate with lower strength and increased susceptibility to cracking. This principle also holds for thermal barrier coatings, where the grain size of the ceramic top coat, often composed of yttria-stabilized zirconia, significantly influences its resistance to thermal stress cracking. An increase in grain size within the YSZ top coat implies that individual grains are larger, reducing the total grain boundary area. Grain boundaries, the interfaces between adjacent grains, act as barriers to crack propagation. Cracks can be deflected or arrested, and energy can be dissipated well before catastrophic failure. But as the grains get larger, the spacing between these crack-arresting boundaries gets larger. For this reason, crack initiation in the larger-grained material occurs with less resistance, and the coating becomes susceptible to thermal stress cracking.

Additionally, larger grains are in accommodating strain. Given thermal cycling, the coating layers have a severe mismatch of thermal expansion coefficients; this generates much stress. These stresses can be more accommodated with smaller grains, shorter diffusion distances, and more remarkable plasticity. Conversely, larger grains can crack more easily under the same thermal stresses. Ultimately, this increased susceptibility to cracking in larger-grained YSZ coatings becomes delamination and spallation. Under cyclic thermal loading, once initiated, cracks can propagate, separating the top coat from the underlying bond coat, a process called delamination. When cracks coalesce and the delaminated area grows, delaminated areas can detach completely from the substrate, a failure mode known as spallation.^85,115

Initial grain size and sintering behavior

The sintering behavior of a material depends strongly on the initial grain size of the material. The basis for this dependence is that the relation between grain size and surface energy is a fundamental material property dictating material stability. Surface energy is the energy present at the material's surface compared to the bulk. Their higher surface area to volume ratio inherently means that smaller grains have higher surface energy. The system is driven by this excess energy to minimize its overall energy state, which is the same thing that happens in sintering the elevated temperatures during sintering-induced diffusion and mass transport of the materials. The high surface energy of smaller grains catalyzes these processes. One way to think of what this surplus energy is doing is that atoms at the grain boundaries attempt to rearrange themselves to take on more stable configurations; this type of grain migration results in densification of particles packing closer together and grain growth. Small grains join to make large ones.

Thus, smaller initially grain-sized materials have a higher driving force for sintering and are more prone to grain growth. This phenomenon is especially relevant for nanostructured materials, whose initial grain size is extremely small. Achieving high density is advantageous but at the risk of uncontrolled grain growth that can degrade desirable properties of the nanostructure. Therefore, understanding and controlling the initial grain size and sintering behavior is crucial to achieving the required material properties and application performance. ¹¹⁶ The study presented by (Hong Z. et al., 2007) is on nanostructured yttria stabilized zirconia coatings deposited by air plasma spraying. In the as-sprayed condition, the researchers report achieving a mean grain size of about 42 nm. They also emphasize that to minimize grain growth during deposition, grain growth would be controlled to some degree in such coatings at elevated temperatures. ¹¹⁷

Oxidation of the bond coat

The oxidation of the bond coat, a critical process in the performance and longevity of thermal barrier coatings, leads to the formation of a thermally grown oxide layer. This protective yet inherently brittle layer presents a paradoxical challenge in TBC design. While the TGO layer, primarily composed of alumina (Al₂O₃) in MCrAlY bond coats, serves as a vital barrier against further oxidation of the underlying bond coat and substrate, its brittle nature makes it susceptible to cracking under the significant thermal stresses induced by the inevitable mismatch in thermal expansion coefficients between the ceramic top coat, bond coat, and substrate. This susceptibility to cracking can ultimately lead to two primary failure modes: spallation, the detachment and loss of the ceramic top coat, exposing the underlying layers to the harsh operating environment, and delamination, the separation of the ceramic top coat from the bond coat, compromising the thermal insulation properties of the TBC and reducing its effectiveness in protecting the underlying component. The growth rate, morphology, and mechanical properties of the TGO layer, all of which significantly influence the TBC's lifespan, are influenced by factors such as bond coat composition, operating temperature, and thermal cycling, making the understanding and control of these factors crucial for developing more robust and longer-lasting TBCs.^91,118 Zhou et al. This research investigates the role of internal oxidation in the failure of air plasma-sprayed TBCs with a double-layered bond coat. The study demonstrates that while a certain degree of internal oxidation can benefit adhesion, excessive internal oxidation can lead to a thick, non-uniform TGO layer, increasing the susceptibility to cracking and delamination. ¹¹⁹

Corrosion of the ceramic top coat

The degradation under hot corrosion conditions is also an issue with the ceramic top coats in thermal barrier coatings (TBCs), e.g., the YSZ and CSZ layers. It has been shown in previous studies that hot corrosion can cause the formation of cracks around the bond coat; nanostructured Al₂O₃/CSZ composite coatings were demonstrated to spall after prolonged exposure to hot corrosive environments, suggesting that such degradation mechanisms could potentially impact the coating's lifetime. While this work is focused on thermal cycling behavior, the learning from the hot corrosion studies can offer more insight into the overall challenges for TBCs in extremely high-temperature environments.^16,32 Studies have demonstrated superior hot corrosion resistance of ceria-stabilized zirconia coatings compared to yttria-stabilized zirconia coatings. Furthermore, the incorporation of nano-alumina has been shown to enhance the hot corrosion resistance of CSZ at elevated temperatures (e.g., 1050°C) in the presence of corrosive salts like V₂O₅ and Na₂SO₄. This improvement is attributed to forming a dense nano-alumina layer that effectively acts as a barrier, hindering the infiltration of molten salts into the CSZ layer. Consequently, Al₂O₃/CSZ composite coatings have exhibited the highest resistance to hot corrosion. The addition of nano-alumina also appears to influence the mechanical response of the coating to hot corrosion. The more minor increase in elastic modulus observed in nanostructured Al₂O₃/CSZ composite coatings, compared to unmodified CSZ, suggests that the presence of nano-alumina may limit the formation of detrimental phases like monoclinic zirconia, YVO₄, and CeVO₄, which can contribute to coating degradation.^32,91

Controlled grain size and morphology

Microstructures with refined grains, as a rule, possess superior mechanical characteristics, including enhanced hardness and wear resistance. This improvement stems from the increased grain boundary area inherent in fine-grained structures. Grain boundaries are barriers to dislocation motion, the primary mechanism of plastic deformation in materials. With a higher density of grain boundaries, dislocation movement is hindered, leading to increased strength and hardness. Furthermore, refined grain structures exhibit enhanced resistance to high-temperature sintering. Sintering, the process of material densification at elevated temperatures, is often accompanied by grain growth. This grain growth can harm mechanical properties, as larger grains offer less resistance to dislocation motion. Fine-grained microstructures, with their inherently smaller grain size, effectively impede grain growth during sintering, preserving their desirable mechanical properties at high temperatures. Controlling grain size and morphology manipulation during coating fabrication is paramount to achieving optimal performance. Techniques like controlled cooling rates, severe plastic deformation, and adding grain refiners during deposition can be employed to tailor the microstructure. By carefully controlling these parameters, it is possible to engineer coatings with superior mechanical properties and enhanced resistance to high-temperature degradation, ultimately leading to improved performance and extended service life in demanding applications. ¹²³

As revealed by SEM, the morphology of CeO₂ coatings on Superni-718 exhibited a dependence on the concentration of cerium acetate in the deposition bath. The coating displayed uniform, small spherical grains at 0.1 Min (Figure 7(a)). Increasing the concentration to 0.2 M (Figure 7(b)) resulted in a mixture of small spherical and rod-type grains. The coating shown in (Figure 7c), needle-like grains at the highest concentration of 0.3 M. Cross-sectional SEM analysis (Figure 7d) of the 0.3 M sample indicated a coating thickness of approximately 4 μm. EDS analysis confirmed the presence of cerium within the coatings. ¹²⁴ Lóh, N. Jet al. It focuses on two-step sintering, a technique used to achieve fine-grained ceramics. The authors describe how this method allows for densification while limiting grain growth, resulting in improved mechanical properties. This technique has implications for fabricating TBCs with tailored microstructures. ¹²⁵

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Figure 7.

SEM surface morphology of CeO₂ coatings deposited on a superni-718 substrate with varying cerium acetate concentrations: (a) 0.1 M, (b) 0.2 M, and (c) 0.3 M. (d) Cross-sectional SEM micrograph of the CeO₂ coating deposited with a 0.3 M cerium acetate concentration on a Superni-718 substrate. (e) EDS spectra were obtained from surface and cross-sectional analysis of the CeO₂ coating on the Superni-718 substrate to confirm cerium's presence. ¹²⁴

Optimized pore size and distribution

The size and distribution of pores within a coating significantly influence its mechanical and thermal properties. Smaller, uniformly distributed pores are generally desirable for enhancing both the hardness and insulation efficiency of the coating. A dense microstructure with minimal porosity contributes to increased hardness. Tiny, homogenous pores obstruct crack propagation, forcing cracks to follow a more tortuous path. This increased crack path length requires more energy to propagate the crack, effectively increasing the material's resistance to fracture and improving its hardness. ¹²⁶

Phase composition, stability, and alloying

Ceramic coatings’ phase composition and stability are crucial in determining their high-temperature performance. Alloying, specifically the incorporation of stabilizing agents like rare earth oxides, presents an effective strategy for enhancing these characteristics. When introduced into the ceramic matrix, rare earth oxides act as stabilizers by preferentially segregating to grain boundaries and inhibiting grain growth. This stabilization effect arises from the larger ionic radii of rare earth elements compared to the host cations in the ceramic lattice. These larger ions at grain boundaries increase the energy required for grain boundary migration, hindering grain growth and maintaining a finer, more stable microstructure. The reduced grain growth from adding rare earth oxides translates into improved resistance to sintering at high temperatures. Specifically, high-temperature sintering is generated by mass transport, inducing desirable properties in the material that are undesired in the microstructure.

Multi-phase composites: Better performances are obtained, and better adhesion and durability of the ceramic coating are achieved by introducing phases of the ceramics with various characteristics of hardness, chemical stability, thermal shock resistance, fracture toughness, and other parameters. Padture, N. Pet al. Investigated the phase stability of ceramic thermal barrier coatings, which is crucial for their performance in high-temperature applications. Alloying with stabilizing agents, particularly rare earth oxides, has significantly enhanced this stability. These stabilizing agents effectively inhibit sintering and mitigate detrimental phase transformations at elevated temperatures when incorporated into the ceramic matrix. This stabilization mechanism helps maintain the TBC's structural integrity, thermal insulation properties, and oxidation resistance, ultimately prolonging its operational life.^127,128

Multilayer designs

Multilayer coatings, featuring alternating layers with distinct characteristics, offer several advantages over conventional single-layer coatings. The interfaces between these layers act as barriers to crack propagation, deflecting or arresting cracks and preventing them from propagating through the entire coating thickness. This enhanced crack resistance translates into improved coating toughness and durability. Moreover, the strategic arrangement of layers with different thermal expansion coefficients can effectively mitigate thermal stresses generated during thermal cycling. By tailoring the thermal expansion mismatch between layers, thermal stresses can be distributed more evenly, reducing the likelihood of coating delamination or cracking. Multilayer designs also offer enhanced oxidation resistance. Incorporating layers with high oxidation resistance, such as alumina or zirconia, can protect the underlying substrate from high-temperature oxidation. These layers act as diffusion barriers, hindering the inward diffusion of oxygen and the outward diffusion of metal ions, thereby slowing the oxidation process.^129,130

Microstructurally graded structures

Microstructurally graded coatings, characterized by controlled variations in grain size, porosity, or phase composition across the coating thickness, create desirable property gradients. This approach allows for tailoring specific properties in different coating regions to meet specific functional requirements. For instance, a coating could be designed with a fine-grained, dense surface layer for enhanced hardness and wear resistance, gradually transitioning to a coarser-grained, more porous layer underneath for improved thermal insulation. This graded microstructure allows the coating to withstand both mechanical wear and high temperatures effectively. The ability to tailor properties spatially controlled makes microstructurally graded coatings particularly attractive for applications involving complex geometries and severe operating conditions. ¹³¹

Oxidation-Resistant coatings

Utilizing thin, successive layers composed of alumina (Al₂O₃) or silica presents a highly effective strategy for significantly enhancing thermal barrier coatings’ oxidation resistance at elevated temperatures. These materials, renowned for their exceptional thermodynamic stability and inherently low oxygen permeability, function as robust diffusion barriers, effectively mitigating oxygen ingress to the underlying substrate. In contrast to single-layer depositions, this layered approach provides a more tortuous diffusion path for oxygen, further impeding its penetration and enhancing the coating's protective capabilities. The inherent redundancy of multiple layers ensures that even in the event of a crack in a layer, subsequent layers continue to provide a robust defense against oxidation, preventing catastrophic failure. This strategic material design effectively extends the lifespan of components operating in demanding, high-temperature oxidizing environments, contributing to enhanced reliability, prolonged service life, and improved overall system performance. ¹³¹ These protective coatings work to good advantage here in excluding contaminants such as oxygen and other reactive species, increasing the thermal barrier coating system's overall life and functional efficiency. ¹³²

Environmental barrier coatings

It is intended to shield the TBC from particular aggressive environments, including exposure to molten salts or other compounds containing sulfur that contribute to deterioration and early failure of the TBC coating. These are selected to withstand the corrosive species in the operation environment to ensure the TBC remains continuously functional.^130,133

Novel TBC materials

TBC materials exhibit superior high-temperature behavior, enabling elevated operating temperatures and extended service life. Rare-earth zirconates and hafnates have a combination of properties that address the limitations of conventional TBC materials. These materials, characterized by their high melting points, low thermal conductivity, and excellent chemical stability, demonstrate significant potential for next-generation TBC applications. Their crystal structures, often incorporating rare-earth elements like yttrium and lanthanum, contribute to their remarkable thermal properties. Rare-earth elements can also enhance phase stability, mitigating detrimental phase transformations at high temperatures and improving performance and durability.

The development and implementation of these advanced TBC materials represent a significant step towards enabling higher operating temperatures in gas turbines and other high-temperature applications. This advancement translates directly into improved fuel efficiency, reduced emissions, and enhanced overall performance. As research continues to unveil the full potential of rare-earth zirconates and hafnates, we can anticipate their increasing adoption in demanding thermal barrier coating applications. New materials have better thermal and environmental stability characteristics and improved capabilities against phase transformations and sintering at high temperatures.^134–136

Doping and alloying

It is possible to enhance the stability of the TBC material and raise the resistance to sintering or phase transformations and environmental impact by adding dopants or alloying elements. For instance, TBC material might be yttria-stabilized zirconia in which doping by yttria maintains the stability of the tetragonal phase of the material at high temperatures and hinders phase transformations that are disastrous to the coating properties. Clarke, D. R et al. (2005) prime example is yttria-stabilized zirconia, a widely used TBC material. The incorporation of yttria as a dopant effectively stabilizes the tetragonal phase of zirconia at high temperatures, preventing detrimental phase transformations that can lead to coating failure. This stabilization mechanism maintains the TBC's thermal insulation properties and longevity.^137,138

Bond coat optimization

The bond coat is also very important in TBC stability because it provides the top coat with the necessary bond to the substrate. The compatibility of bond coat materials, the right choice of composition, and the optimal microstructure will reduce interfacial reactions, increase adhesion, and increase the durability and service life of the surface coating. Thus, the proper design of the bond coat and its properties are crucial for maintaining the nanostructured TBCs’ service life and efficiency in severe high-temperature conditions. ¹³⁹

Thermal expansion mismatch and residual stresses

Challenge: Thermal expansion coefficients of TBC and superalloy substrate are generally different, and high residual stress could be developed during thermal cycling of the component, thus leading to coating crack formation, delamination, and early failure.

Mitigation: When designing TBCs, incorporate specific CTEs for the designed TBCs about the superalloy substrate, employ functionally graded layers to avoid high levels of stress concentration regions, and design deposition techniques that result in few levels of residual stresses.

Phase stability and sintering at high temperatures

Challenge: The TBCs nanostructured can grow grains and phase transformations. This sintering under high temperatures raises the thermal conductivity and stress tolerance and causes the loss of their valuable properties.

Mitigation: Restoring chemical composition changes, investigating the development of new materials for TBCs with higher service temperature and more stable phase, and regulating the deposition process specifically regarding grain size and shape.

Reproducibility and scalability of deposition processes

Challenge: Obtaining the desired and repeatable properties in nanostructured TBCs deposited by magnetron sputtering can be problematic because of the essential dependence of the process on such parameters as deposition pressure, target material, and substrate temperature.

Mitigation: Ensure tight control of deposition processes, strictly exercise quality control, and consider other methods, such as high-impulse magnetron sputtering, to improve deposition reproducibility and scalability.

Long-term oxidation and corrosion resistance

Investigating the evolution of the TGO layer: Long-term studies are needed to understand the growth kinetics, phase stability, and adhesion of the thermally grown oxide layer in nanostructured TBCs under realistic operating conditions. This includes examining the influence of coating architecture, residual stresses, and environmental factors on TGO behavior. Exploring novel bond coat compositions: Research into new bond coat alloys with improved oxidation and corrosion resistance at elevated temperatures is crucial. This includes investigating high-entropy alloys, diffusion barriers, and compositions tailored for specific operating environments.

Evaluating performance under thermal cycling

Systematic studies are needed to assess the impact of thermal cycling on the microstructure, adhesion, and overall performance of nanostructured TBCs. This includes understanding the role of thermal expansion mismatch stresses and developing strategies to mitigate their detrimental effects.

Microstructure control and stability

Tailoring nanostructure for enhanced properties: Further research is needed to optimize the deposition parameters of magnetron sputtering to achieve precise control over the nanostructure of TBCs. This includes tailoring grain size, porosity, and phase distribution to enhance thermal conductivity, mechanical strength, and strain tolerance properties.

Understanding and mitigating sintering effects

Investigating the sintering behavior of nanostructured TBCs at high temperatures is crucial to ensure long-term microstructural stability. This includes exploring strategies to inhibit grain growth and densification, such as doping with refractory elements or employing multilayered architectures.

Advanced characterization techniques

In-situ monitoring of coating degradation: Implementing advanced characterization techniques, such as in-situ X-ray diffraction and high-temperature microscopy, will provide valuable insights into the real-time degradation mechanisms of nanostructured TBCs under operating conditions. Developing non-destructive evaluation methods: Research into non-destructive evaluation techniques, such as acoustic emission and thermography, is essential for assessing the integrity and remaining life of TBCs in service.