Currently, the top layer of ASCs mostly consists of a matrix phase, a lubricating phase, and a certain amount of micropores [15]. Among these, the matrix phase material is usually a metal or ceramic phase possessing certain oxidation resistance, corrosion resistance, and erosion resistance. Common ceramic phases include yttria-stabilized zirconia (YSZ), ytterbium disilicate, etc. The lubricating phase is used to provide abradability, reduce the coefficient of friction, and disrupt the continuity of the matrix to ensure that the "spalled" fragments are sufficiently small, preventing blockage of cooling holes or causing downstream erosion. Commonly used high-temperature lubricating phase materials include hexagonal boron nitride (h-BN), calcium fluoride (CaF₂), and MAX phases [16]. Compared to wear-resistant coatings, abradable sealing coatings require high porosity and low hardness to ensure good abradability. Coating porosity can be increased, on one hand, by adjusting thermal spray process parameters; on the other hand, by introducing pore-forming phases such as polyhydroxybutyrate (PHB) into the feedstock powder. Subsequent heat treatment then forms pores in situ, thereby increasing the porosity of the coating[17].

ASCs are mostly manufactured by thermal spraying, among which flame spraying (FS) is often used for medium-to-low temperature ASCs, while plasma spraying is commonly used for high-temperature ASCs [12]. In the preparation of ceramic matrix ASCs, atmospheric plasma spraying (APS) technology is most frequently employed. The principle of this spraying technology [18] is: the plasma arc generated between the cathode and anode heats and ionizes the working gas, forming a plasma jet ejected from the nozzle; the injected spray powder is heated by the plasma flame and becomes molten or semi-molten; entrained by high-pressure gas, the heated and softened powder is propelled forward; when the high-temperature, high-velocity powder particles reach the pre-treated substrate surface, they create intense impact and collision, instantaneously deforming upon the substrate surface. The molten powder rapidly cools and contracts, finally flattening and spreading onto the substrate surface; as spraying time increases, powder particles continuously impact and adhere to the substrate surface, bonding to each other in an interlocked manner, ultimately forming a firmly bonded coating.

The microstructure of a plasma-sprayed coating is described as a lamellar stack containing microcracks and pores. When the number of voids and microcracks in the coating reaches a certain level, it reduces the overall thermal conductivity of the coating and improves abradability. Furthermore, APS technology features high coating preparation efficiency, fast deposition speed, and low cost, making it one of the most economical and effective methods for preparing porous ceramic coatings [19].

Before the emergence of ceramic matrix ASCs, turbine gas path seals were typically made from all-metal structures. As temperature requirements increased, metal-based coatings could no longer meet service demands. Ceramic matrix coatings with higher melting points thus came into the view of researchers. Thermal barrier coatings (TBCs) applied in similar service environments cannot be directly used as abradable coatings due to their high hardness. Soft phases must be added to TBCs or porosity deliberately created to make them easier to abrade. Furthermore, for higher service temperatures, superalloy substrates themselves can no longer meet the requirements. The demand for CMCs with better thermal stability and lighter weight is growing, making the development of ASCs suitable for CMCs particularly important. Therefore, multifunctional ASCs based on environmental barrier coatings (EBCs) have been developed and applied onto CMC substrates.

In exploring the wear mechanisms of ASCs, researchers have applied various models based on the weakest-link theory to describe the strength of brittle materials. Building on such studies, Seuba [49] conducted Weibull analysis and found that the strength of porous ceramics primarily depends on total porosity, while also being influenced by the spatial distribution and morphology of the pores. Irissou [50] designed CoNiCrAlY coatings with varying surface hardness and porosity, and evaluated their bonding strength, erosion resistance, and tribological performance. The results indicate that a higher porosity reduces surface hardness and facilitates wear behavior, but it also leads to a deterioration in erosion resistance and overall mechanical properties. Moreover, the pore structure exerts a significant influence on the coating's thermal shock resistance and corrosion resistance. These findings underscore the critical role of pore structure control in the design of ASCs. For ceramic coatings, pore structure can be adjusted through three main approaches: pore-forming agents, spray process parameter optimization, and feedstock powder structure design.

Since the 1990s, nanoceramic powders have been increasingly incorporated into thermal spray coatings. For ASCs, nanostructured composite coatings offer multiple advantageous properties. Compared to traditional lamellar thermal spray coatings, these coatings demonstrate not only improved wear resistance but also reduced hardness, which is beneficial for enhancing abradability. In addition, coatings with nanoscale features exhibit higher toughness and better resistance to crack propagation than their microscale counterparts, thereby improving thermal shock resistance during service. This improvement arises because conventional thermal spray coatings typically form a layered structure with weak interlamellar bonding, whereas in nanostructured coatings, semi-molten nanoparticles agglomerate to form dense nano-domains that enhance interfacial toughness and bonding strength [63]. The structure and performance of coatings prepared by different thermal spray techniques are compared in Table 5.

However, the use of ultrafine powder feedstock faces significant challenges: poor flowability, strong agglomeration tendency, and difficulty in retaining nanostructure at high temperatures. These limitations render conventional APS techniques unsuitable for producing nanostructured coatings [69]. To address this, Wang et al. [70] developed YSZ-based ASCs with nanostructured features by adding YSZ nanoparticles into a YSZ precursor solution and employing solution precursor plasma spraying (SPPS). The resulting coatings exhibited a bimodal pore structure comprising both macro- and micropores. By adjusting the concentration of nanoparticles, the coating structure could be effectively tailored. Sun et al. [67] further investigated the thermal shock behavior of 8YSZ-based nanostructured ASCs prepared via SPPS. They found that the presence of fine vertical microcracks in the hybrid structure increased the coating's strain tolerance and inhibited the propagation of larger horizontal cracks along the interlamellar interfaces, thereby enhancing thermal stability.

To further compare nanostructured and conventional coatings, Sun et al. [64] fabricated a conventional abradable sealing coating (C-YSZ) using APS and a nanostructured variant (M-YSZ) using SPPS, as shown in Fig. 7. They systematically evaluated both coatings in terms of microstructure, mechanical properties, and oxidation resistance. Compared with C-YSZ, the M-YSZ nanostructured coating exhibited finer and more uniformly distributed pores, lower hardness, and reduced bonding strength. Under high-temperature oxidation, large horizontal cracks formed in the C-YSZ coating, while only fine vertical cracks were observed in M-YSZ. This behavior was attributed to the homogeneous distribution of nanoscale pores, which helped reduce the risk of coating delamination.

Nevertheless, SPPS also presents inherent drawbacks. Due to the high solvent content in the precursor solution, significant thermal lag occurs during spraying as energy is consumed for solvent evaporation, reducing the thermal energy available for solute decomposition, which leads to low deposition efficiency. To address this limitation, Wang et al. [68] introduced the sol-precursor plasma spraying (Sol-PPS) technique for coating fabrication. Compared to SPPS, the Sol-PPS approach provided YSZ-based coatings with a more stable and loosely packed microstructure, while also lowering the friction coefficient, enhancing bonding strength, and preserving surface hardness. This approach is considered an effective strategy to improve both and overall performance of ASCs.

Ceramic coatings are typically characterized by high hardness and brittleness, which often result in high friction coefficients and poor wear behavior. To improve the tribological performance of ASCs, the incorporation of appropriate solid lubricants is essential. The lubricating phase in ASCs serves multiple key functions:(1) acting as a "soft phase" within the matrix to modulate coating strength; (2) disrupting matrix continuity to ensure sufficiently fine wear debris; (3) providing self-lubrication to reduce the friction coefficient; (4) increasing strain tolerance, thereby improving fracture resistance and thermal stability. Importantly, due to the high-temperature service environment and the thermal spray deposition process, the lubricating phase must exhibit excellent thermal stability. As shown in Table 6, commonly used solid lubricants for ASCs and their performance comparisons are presented.

Hexagonal boron nitride (h-BN), with its graphite-like layered structure and excellent corrosion resistance, is frequently used in high-temperature sealing coatings. Zhang et al. [23] incorporated h-BN into a YSZ matrix and reported a significant reduction in the coating's friction coefficient. Qin et al. [38] fabricated YbDS coatings containing various amounts of h-BN using APS, and found that h-BN effectively lowered hardness and created transition layers that reduced the friction coefficient and increased the volume wear rate-features beneficial to abradability. However, excessive h-BN content led to structural loosening and compromised coating stability. Despite its advantages, h-BN tends to oxidize above 900^∘C, limiting its use in ultra-high temperature applications. In contrast, calcium fluoride (CaF₂) is widely recognized as an ideal high-temperature lubricant due to its excellent thermal stability, low hardness, and low friction coefficient. Wang et al. [75] investigated the microstructure and properties of YbDS-CaF₂ coatings prepared by APS. They observed that increasing CaF₂ content led to higher porosity, reduced hardness, and lower friction coefficient. At room temperature, the dominant wear mechanism was abrasive wear, whereas at 900^∘C, adhesive wear became prevalent.

Ali et al. [76] studied the tribological properties and wear mechanisms of YSZ-based ASCs by introducing a small amount of MAXphase Ti₃AlC₂ as a lubricating phase. During plasma spraying, partial decomposition and oxidation of Ti₃AlC₂ produced hard secondary phases that filled voids and enhanced overall hardness. As the Ti₃AlC₂ content increased, the coating's volume wear rate decreased, resulting in improved wear resistance. An optimal balance between abradability and wear resistance was achieved at 10wt%Ti₃AlC₂. Similarly, Hajian Foroushani et al. [79] incorporated LaPO₄ into YSZ as a high-temperature lubricating phase, which not only improved frictional performance but also enhanced ablation resistance and increased porosity.

To improve compatibility with ceramic matrices and reduce combustion or oxidation losses during plasma spraying, coating the lubricating phase within the feedstock powder has proven to be an effective strategy, as illustrated in Fig. 8. As reported in our earlier study [71], ZrO₂-coated h-BN powders were prepared and subsequently blended with YSZ, followed by deposition using APS. The coating approach significantly reduced h-BN degradation during spraying and improved compatibility with the ceramic matrix. Compared to uncoated systems, the modified coatings exhibited enhanced abradability and thermal stability. Zeng et al. [73] applied a similar strategy by coating CaF₂ particles with ZrO₂ and reported comparable improvements in performance.

Research on plasma-sprayed ceramic-based ASCs faces two major challenges: (1) the extremely low deposition efficiency of ceramic coatings, typically only 10-15%, which is significantly lower than the average 30-50 % efficiency of general thermal spray coatings; and (2) premature failure of the ceramic topcoat, including cracking and delamination during thermal cycling. These issues largely arise from the complex multilayer architecture of ASCs and the harsh service environments in which they operate. Introducing low-melting-point binder phases has proven to be an effective strategy to address both problems.

Nickel (Ni) is commonly used as a binder phase due to its low melting point and good bonding with both the substrate and the transition layer. Kang et al. [59] coated hollow spherical YSZ particles with a thin Ni film. During plasma spraying, the Ni melted and acted as a binder, forming a continuous spherical pore structure, which significantly improved the coating's bonding strength and thermal stability. Similarly, Zhang [23] introduced a small amount of Ni powder into an APS-deposited YSZ-based abradable coating and achieved enhanced bonding strength and thermal shock resistance, along with a deposition efficiency exceeding 70%.Cheng et al. [80] selected low-melting-point ceramic phases such as alumina (Al₂O₃), yttrium aluminum garnet (Y₃Al₅O₁₂,YAG), and magnesium aluminate spinel (MgAl₂O₄) as high-temperature binders and mixed them into YSZ powder. As shown in Table 7, Plasma-sprayed coatings incorporating these phases showed improved deposition efficiency, with the YAGcontaining coatings achieving the highest improvement. Compared to pure YSZ coatings, the YAG-containing composite exhibited a 43.68% increase in thermal cycling life. As ceramic coating delamination is typically driven by the brittle nature of ceramics and the accumulation of thermal stress-induced cracks during cycling, the dispersion strengthening effect of YAG helped suppress crack growth and extend service life. Notably, failure locations in these composite coatings shifted toward the inner YSZ layers adjacent to the substrate. To further improve coating toughness, Cheng et al. [81] incorporated SiC whiskers into the YSZ matrix. These whiskers retained their structural integrity and remained uniformly dispersed after plasma spraying, further enhancing the thermal cycling performance of the YSZ-based sealing coatings. The toughening mechanisms of these SiC-reinforced coatings include residual stress relaxation due to the porous structure and crack bridging and pull-out effects of the whiskers [82].

In corrosion resistance and failure mechanism studies, it has been observed that ASCs tend to fail through crack propagation parallel to the coating surface, mainly due to their continuous porous structure. To mitigate this issue, researchers have explored crack self-healing strategies to promote localized sintering and close large pores, especially horizontal ones, thereby enhancing coating toughness and extending service life [83]. Self-healing approaches in ceramics are typically classified into two types:(1) the addition of special phases that oxidize at high temperatures to form glassy products, which fill and seal cracks; (2) the use of sintering aids to promote grain growth and densification at elevated temperatures, thereby redistributing internal defects.

However, for highly porous coatings like ASCs, relying on a single mechanism may not achieve effective self-healing.

For ASCs, h-BN is a common solid lubricant, but it tends to oxidize at around 1000 ^∘C, forming B₂O₃. In view of the dual requirements of abradability and mechanical stability in ASCs, our group previously developed an APS coating using 8YSZ as the ceramic matrix, polyester as a porogen, and h-BN as a self-lubricating phase, forming a cartilageinspired porous architecture [84]. As illustrated in Fig. 9, a smooth glaze-like surface layer resembling cartilage is formed at high temperatures due to the presence of h-BN and its oxidation product B₂O₃. Meanwhile, the porous structure generated by the burnout of the porogen mimics the underlying bone-like framework. In service environments, B₂O₃ also acts as a sintering aid to promote crack healing, effectively enhancing the high-temperature stability of the coating. The synergy between the lubricating "cartilage" layer and the supporting porous "bone" structure significantly improves the high-temperature abradability of the ASC.

In the early development of EBCs, Nguyen et al. [85,86] incorporated 10vol%SiC nanoparticles into sintered YbDS/YbMS ceramic composites. Upon crack formation at 1250^∘C, the SiC fillers reacted with ambient oxygen to produce viscous amorphous SiO₂, which could infiltrate and seal the cracks. Subsequently, the SiO₂ reacted with YbMS in the system to form YbDS, thereby achieving a crack-healing effect. Similar self-healing behavior of SiC was also demonstrated by Kim et al. [87] in the Sc₂Si₂O₇/SiC system. While these studies highlight the potential of SiC as a self-healing phase, the dense microstructure of sintered ceramics differs significantly from the porous structure of thermally sprayed coatings. Moreover, SiO₂ tends to react with steam at temperatures as low as 1200^∘C to form volatile Si(OH)₄, which is considerably below the expected service temperature of EBCs. If the kinetics of this volatilization reaction are faster than those of the SiO₂-YbMS reaction forming YbDS, the intended self-healing mechanism may be rendered ineffective under high-temperature service conditions.

Due to their unique layered crystal structure and bonding characteristics, MAX phases have attracted significant attention for their self-healing capabilities. During plasma spraying or high-temperature service, MAX phases can oxidize to form Al₂O₃ and rutile-type TiO₂ [77]. The volume expansion under compressive stress facilitates crack and pore closure. Additionally, TiO₂ can dissolve into Al₂O₃ to form numerous cation vacancies, promoting solid-state diffusion and accelerating densification. Lee et al. [78] incorporated Ti₂AlC MAX phase particles into mullite- YbMS composites and observed that the MAX phase acted as both a sintering aid and a self-healing agent. This significantly reduced the temperature and time required for crack healing and fully restored mechanical strength post-healing. Huang et al. [46,88] developed an abradable coating system using LaMgAl₁₁O₁₉ (LMA) as the structural matrix, polyester as the porogen, and Ti₃AlC₂ MAX phase as both lubricant and self-healing agent. After a high-temperature self-healing treatment at 1200^∘C, the coatings were subjected to CMAS corrosion testing at 1300^∘C for 2 h and 12 h. As shown in Fig. 10, increasing the content of Ti₃AlC₂ (TAC) led to a decrease in porosity, an increase in coating density, and the transformation of interconnected pores into isolated ones. Cracks gradually closed during the self-healing process. When the TAC content reached a threshold, the composite coating significantly reduced CMAS infiltration depth, demonstrating excellent corrosion resistance. Studies have demonstrated that the introduction of self-healing phases can effectively repair microcracks and significantly enhance the corrosion resistance and thermal shock resistance of coatings. However, most existing investigations have focused on ceramic sintered bodies or TBCs, leaving the question of how to implement this strategy in ASCs with abundant porous structures unresolved. Ensuring a uniform distribution of the self-healing phase and sufficient availability at the sites requiring repair will be crucial in determining its practical applicability in ASCs.

For ASCs, especially those applied to CMCs, multiple functional requirements must be satisfied simultaneously-including abradability, high-temperature oxidation resistance, and environmental corrosion resistance. These often cannot be achieved using a single-layer coating, thereby driving the development of multilayer coating architectures to achieve multifunctional integration. A typical high-temperature ASC system generally includes a porous abradable topcoat and a bond coat underneath. For coatings on CMC substrates, an additional EBC layer must be introduced between the bond coat and the topcoat to prevent hydrothermal corrosion.

Due to mismatches in the CTE between the porous ceramic abradable layer and the CMC substrate, a bond coat is essential to provide both CTE compatibility and enhanced interfacial adhesion. However, after long-term service, reactions with atmospheric oxygen lead to the formation of a thermally grown oxide (TGO) layer between the bond coat and the topcoat. The TGO layer is a known weak link in multilayer coating systems. Once it reaches a critical thickness, cracking and coating failure may occur. Therefore, suppressing the formation and growth of TGO is crucial for enhancing coating durability, and the bond coat plays a pivotal role as the primary oxidation barrier [89]. On metallic substrates such as Ni-based superalloys, mature MCrAlY (M = Ni,Co, or Ni+Co) bond coats are widely used [20,73]. In contrast, for CMCs, Si-based bond coats are more common due to their CTE compatibility.

The incorporation of a Si bond coat significantly enhances the interfacial adhesion strength of EBCs and serves as a sacrificial layer during high-temperature service by forming a protective SiO₂-based TGO layer. However, under thermal cycling conditions, the reversible α-β quartz phase transformation within the TGO layer can lead to approximately 5% volume shrinkage, potentially inducing cracking within the oxide layer [90]. In recent years, NASA [91] proposed a composite bond coat consisting of HfO₂ and Si. In oxidative environments, Si preferentially oxidizes to form SiO₂, which subsequently reacts with HfO₂ to generate the HfSiO₄ phase.

Table 8 summarizes the coefficients of thermal expansion (CTEs) of the individual phases in the HfO₂-Si system, demonstrating that this reaction pathway avoids CTE mismatch while mitigating the volumetric instability associated with the SiO₂ phase transformation [92]. Deijkers et al. [93] investigated the formation mechanism of HfSiO₄ and found that its formation significantly reduces the thickness of the SiO₂ layer, thereby improving thermal cycling resistance. Zhu [94] further reported that HfO₂-Si bond coats exhibit high strength and fracture toughness, with a service temperature capability up to 1400^∘C.

Anton et al. [99] fabricated Si-HfO₂ bond coats with varying HfO₂ doping levels via magnetron sputtering, and observed excellent thermal cycling performance without delamination caused by phase transformations. However, it has been noted that HfO₂ doping in Si bond coats does not effectively suppress the growth rate of the TGO layer. The continuous HfO₂ network provides rapid oxygen diffusion pathways, and excessive HfO₂ content can accelerate the oxidation of silicon. Li et al. [92], using atmospheric plasma spraying (APS), prepared HfO₂-Si bond coats with different compositions, and confirmed that those with lower HfO₂ content exhibit superior thermal shock resistance at 1300 ^∘C. Due to the relatively high Si content, oxidation of HfO₂-Si bond coats is unavoidable in high-temperature air or steam environments. In steam at 1371 ^∘C for 31 h, complete oxidation of elemental Si was observed, with TGO growth rates exceeding those in dry air.

Harder [98] employed plasma spray-physical vapor deposition (PSPVD) to prepare an EBC system with a HfO₂/Si bond coat and a YbDS top coat. The coating exhibited excellent oxidation and steam corrosion resistance, and the presence of HfO₂ effectively reduced TGO thickness and improved the bond coat's oxidation resistance. To further inhibit TGO growth and prolong coating lifespan, Lee et al. [99] modified APSdeposited Si/YbDS EBCs by incorporating various oxide additives. Their results indicated that low concentrations of TiO₂,Al₂O₃, or Al₂O₃-containing compounds (e.g., mullite and YAG) effectively suppressed TGO growth, reducing its thickness by more than 80% under steam cycling conditions.

For ASC systems applied to CMC substrates, multiple performance requirements must be simultaneously met. Abradability demands a loose and porous microstructure; resistance to water vapor and CMAS corrosion requires a relatively dense structure that can prevent corrosive species infiltration; meanwhile, adequate interfacial bonding strength is also essential. These conflicting demands pose significant challenges to coating design. A multilayer coating architecture offers a straightforward yet effective solution. By stacking a bond coat at the bottom, an EBC in the middle, and an abradable top coat, such trilayer structures can simultaneously achieve water vapor resistance and abradability.

In studies of EBC systems, Fan [100] employed APS to fabricate a trilayer thermal environmental barrier coating (T-EBC) system composed of Si/YbDS/LMA, demonstrating effective protection of CMC substrates under high-temperature water vapor environments. Building upon this, Huang et al. [30,101] further deposited a porous YSZ top coat on the existing Si/YbDS/LMA system, and a thick porous LMA layer on a Si/YbDS/YbMS system, respectively. These multilayer coatings exhibited both favorable abradability and excellent corrosion resistance. Notably, the CTEs of Si,YbDS,YbMS, and LMA increase progressively (4.1, 5.5,7.5, and 9.2×10^-6 K^-1, respectively), bridging the thermal expansion mismatch between the CMC substrate and the YSZ top layer. This gradation effectively mitigates interfacial thermal stresses and reduces the risk of cracking.

Additionally, Guo [33] designed a multilayer structure comprising Si/80 % mullite + 20% BSAS/dense BSAS/porous BSAS (Fig. 11a). The system showed good abradability at 1100 ^∘C, offering effective protection to both the CMC substrate and the rubbing blade tip. However, this study lacked in-depth evaluation of failure mechanisms and longterm thermal stability under extended high-temperature service. While multilayer architectures enable functional integration, they also introduce issues such as thermal expansion mismatch across interfaces and cracking during thermal cycling, which must be addressed in future work.

Gradient coating design is an effective strategy to alleviate the CTE mismatch in multilayer structures and prevent cracking during thermal cycling. As shown in Fig. 11b, Jing et al. [48] developed a graded abradable top layer composed of Sc₂O₃-Y₂O₃-ZrO₂ on the EBC interlayer of a CMC substrate, which effectively relieved the CTE mismatch between the intermediate and surface layers. Compared with the nongraded structure, the graded coating exhibited enhanced interfacial bonding strength and improved thermal shock resistance.

YbMS possesses superior corrosion resistance, while YbDS exhibits a CTE that more closely matches that of CMC substrates. Their combination has been explored as a way to simultaneously achieve corrosion resistance and thermal compatibility in EBC systems [43]. However, in practice, it is difficult to precisely control the phase ratio of the two silicates during deposition. Although YbMS offers low thermal conductivity, excellent high-temperature phase stability, and strong resistance to steam and CMAS attack, its CTE mismatch with the substrate limits its application as a standalone EBC layer.

To address this issue, our team [102] proposed a compositional gradient EBC composed of YbDS-YbMS, with a bottom layer rich in YbDS to match the substrate's CTE, an intermediate layer with a balanced phase composition to transition CTE across the interface, and a top layer rich in YbMS to ensure excellent corrosion resistance. As schematically illustrated in Fig. 12, the water vapor corrosion mechanism of the graded coating is highly dependent on phase composition and distribution. For example, after heat treatment, the 75YbD-S-25YbMS composition primarily contains two phases: YbDS and YbMS, whereas the 25YbDS-75YbMS composition contains a ternary phase mixture of Yb₂O₃,YbDS, and YbMS.

As shown in Fig. 12a, the 75YbDS-25YbMS coating exhibited higher corrosion rates due to the inferior steam resistance of YbDS, leading to the rapid formation of a porous corrosion layer. This layer facilitated the ingress of steam and oxygen, accelerating the growth of the TGO layer at the interface between the bond coat and the top coat. Although a certain thickness of TGO is beneficial for substrate protection, excessive growth and the accompanying phase transformation shrinkage of SiO₂ may result in crack formation and eventual coating spallation.

In contrast, as illustrated in Fig. 12b, the single-layer 25YbD-S-75YbMS coating demonstrated excellent corrosion resistance due to its high YbMS content; however, its significant CTE mismatch led to thermal stress accumulation and top coat cracking during service. The graded coating shown in Fig. 12c combined the advantages of both systems. The YbDS-rich bottom layer effectively relieved thermal stress during thermal cycling, while the YbMS-rich top layer enhanced corrosion resistance by suppressing oxygen and steam diffusion into the coating, resulting in a much thinner TGO. After 100 h of exposure, the graded EBC showed no signs of cracking or spallation, indicating excellent structural integrity.

Interestingly, although pores generated by steam corrosion in the top layer may be detrimental to long-term corrosion resistance, they may conversely benefit the abradability of the coating, which is favorable for ASC applications.

Due to the functional complexity of multilayer coatings-particularly abradable sealing coatings applied on CMCs-the optimal approach is to deposit an abradable top layer on the EBC. However, such coating systems (EBC + abradable layer) often suffer from excessive thickness and poor stress tolerance, making them prone to delamination under harsh high-temperature service conditions, which poses a significant risk to engine safety [103]. Therefore, one of the greatest challenges in multilayer coating systems is enhancing the interfacial bonding strength. As a critical step in thermal spraying processes, surface pretreatment with optimized topography can significantly increase the contact area between the substrate and coating particles, thereby improving bonding quality. Grit blasting is a common pretreatment method used to roughen the substrate and enhance adhesion. Nevertheless, it may also introduce microcracks and entrapped grit particles, which could weaken the adhesion between the bond coat and the underlying layer [104]. Compared to the uncontrollable nature of grit blasting, laser surface texturing (LST) offers a programmable and precise approach for fabricating defined surface patterns on substrates while avoiding issues such as residual abrasive particles. LST is commonly classified into nanosecond (NS), picosecond (PS), and femtosecond (FS) laser systems, which differ significantly in their pulse durations and energy delivery. Among these, PS and FS lasers allow for highly controllable, accurate, and complex micro-texture fabrication due to their ultrashort pulse durations, and have been extensively applied to enhance tribological properties and coating adhesion in engineering surfaces [105].In contrast, NS lasers, due to their relatively long pulse durations, often lead to uncontrolled melting and re-solidification of the target material, resulting in recast layers that are undesirable for texture definition and may degrade interfacial properties. Therefore, NS lasers are generally not suitable for precise surface micro-structuring. On the other hand, PS and FS lasers can produce well-defined, high-resolution surface textures with minimal thermal damage, making them ideal for micro-texturing and fine surface modification [106]. LST technology enables the generation of periodic surface roughness on the substrate, which significantly enhances the mechanical interlocking and bonding strength between the substrate and thermally sprayed particles [105].

Typical laser-generated textures include dot arrays, grooves, square lattices, and hexagonal patterns [107,108]. Moreover, parameters such as texture size, depth, and spacing also play crucial roles in determining interfacial adhesion. To address coating damage during service, many researchers have conducted extensive studies on surface texturing. Lamraoui et al. [109] investigated the influence of geometric texture features on the adhesion strength of thermal-sprayed coatings on aluminum substrates and found that both the geometry and spatial distribution of textures significantly impact adhesion. Zheng et al. [110] used laser to etch micro-pits on alumina surfaces, resulting in twice the bond strength compared to chemically roughened samples. Tan et al. [111] studied the influence of pit diameter-to-spacing ratio, which was found to markedly affect adhesion strength. Reza et al. [112] reported that surface microstructures with optimized surface coverage not only enhanced interfacial bonding but also helped release residual stresses within the coating. Jing et al. [113] applied laser etching to create grooves on the substrate surface, followed by multilayer coating deposition. The results showed a 42.2% increase in adhesion strength, with tensile failures occurring primarily within the abradable layer itself. Heyl et al. [114] used nanosecond-pulsed lasers to fabricate square-grid textures on CoNiCrAlY bond coats, significantly improving thermal cycling life by approximately 180%. Matějíček et al.[115] conducted a systematic investigation into the effects of different laser surface textures, as illustrated in Fig. 13a-d, on interfacial and shear bonding strength. Their results demonstrated that LST significantly enhances coating adhesion. However, due to the fact that interfacial bonding strength exceeded the cohesive strength of the substrate, failure predominantly occurred within the substrate itself, thereby obscuring the direct influence of specific texture geometries on bonding performance. Zhan et al. [116] used atmospheric plasma spraying to deposit MoS₂ coatings on grit-blasted and biomimetic micro-textured substrates (as shown in Fig. 13f) and evaluated the effect of texture spacing. Compared with grit blasting, regular textures improved coating adhesion by over 21.1%, with optimal performance achieved when the texture occupied 70-80 % of the surface area. Our team [117] employed nanoscale laser engraving to fabricate a square microtexture with a checkerboard pattern (as shown in Fig. 13g) on the substrate. The bond strength between the CMC substrate and the Si- HfO₂ bond coat increased by more than 200% compared to conventional gritblasted samples. Cai et al. [118] employed hybrid laser manufacturing to design a multi-layered bioinspired surface structure inspired by monitor lizard skin, as illustrated in Fig. 13e. This architecture integrates sub-millimeter-scale skeletal frames with micron-scale anchoring features. The resulting multiscale hierarchy significantly enhanced the bonding strength at the coating-substrate interface. General guidelines for LST suggest that the texture volume should approximately match the average volume of spray particles, with texture angles exceeding 70^∘ and aperture sizes larger than the particle diameter. To further elucidate the bonding enhancement mechanism of LST, Qu et al. [119] conducted fracture surface analyses and attributed the improved adhesion to a combination of mechanical interlocking and metallurgical bonding induced by thermal spraying. Additionally, Kromer et al. [120] investigated the influence of laser texturing on the thermo-mechanical fatigue behavior of bondcoat-less TBCs. Their findings demonstrated that laser-induced textures can deflect crack propagation above the interface, thereby improving the long-term durability of the coating system.

Currently, research on the influence of surface texture geometry on coating adhesion has made significant progress, particularly in optimizing laser processing parameters, material compatibility, and establishing correlations between texture dimensions and adhesion strength. However, studies on surface texturing for geometrically complex components, as well as the long-term stability of textured interfaces under thermal fatigue and oxidative environments, remain limited. Future work should integrate bioinspired structural design, finite element simulation, and high-precision fabrication methods to explore the mechanisms of multi-physics coupling at the textured interface, which will advance the engineering application of surface texturing in abradable sealing coatings.

During aircraft operation-including takeoff, acceleration, deceleration, and landing-the engine experiences rapid and repeated temperature fluctuations. These abrupt thermal changes can lead to coating spallation or failure. As a key indicator for assessing the durability of coatings under complex service conditions, thermal shock resistance is governed by intrinsic thermophysical properties of the material, fabrication process, and microstructural characteristics. To evaluate the thermal shock resistance of coatings, a common experimental method is employed: specimens are first heated to a target high temperature and held for a specified dwell time. Subsequently, rapid cooling is performed using water quenching or forced air cooling to bring the sample down to a designated low temperature. The coating is subjected to multiple heating-cooling cycles, and the number of cycles required for visible delamination is recorded as a quantitative measure of thermal shock resistance.

In recent years, numerical simulation techniques have also been applied to evaluate the thermal shock performance of coatings. For example, Svenia [27] conducted a detailed numerical analysis of the failure modes of YSZ ASCs under thermal loading, examining the stress distribution during thermal shocks. Tsui [140] established a three-dimensional model of the abradable coating and simulated the residual stress evolution under thermal gradients. Building on this, Johnston [141] explored the influence of coating material parameters on residual stress distribution during thermal shock, providing theoretical support for performance evaluation and optimization of coating systems.

Thermal gradients between the coating and the substrate induce thermal stresses within the material. When subjected to cyclic thermal loading, accumulated thermal stress may lead to crack initiation and propagation, thereby shortening the service life of the high-temperature coating. At present, two dominant theories explain thermal shock-induced damage mechanisms: thermoelastic theory and energy-based theory.

The thermoelastic theory suggests that during heating and cooling, constrained thermal expansion and contraction generate internal stress. When this thermal stress exceeds the inherent strength of the coating, cracks initiate and propagate [142]. However, this theory assumes a flawless, pore-free coating and does not consider the influence of preexisting microcracks, making it less applicable to ASCs, which inherently contain numerous pores and micro-defects. In contrast, the energy-based theory assumes the presence of intrinsic microcracks or pores in the material. During thermal cycling, differences in thermal expansion coefficients between the coating and substrate induce both axial and radial stresses at the interface. Axial stress cyclically opens and compresses existing microcracks and pores, while radial stresses cause localized stress concentrations at pore edges, promoting crack initiation and propagation [143]. This often results in the formation of horizontal cracks. As the number of thermal cycles increases, these horizontal cracks coalesce, ultimately leading to partial spallation of the coating.

For thermally sprayed porous ASCs, the presence of numerous horizontally oriented pores and cracks presents a significant challenge to thermal shock resistance and must be carefully considered during design and material selection.

Ceramic-based ASCs are currently undergoing rapid development but are also facing numerous challenges. Future development requires a synergistic approach integrating materials design, structural engineering, and simulation-assisted optimization to address emerging challenges under extreme service conditions. By selecting high-temperature-resistant materials and employing multiscale structural design strategies, the comprehensive performance of coatings can be significantly enhanced. Coupled with advanced computational simulations, in-depth investigations into the multi-physics failure mechanisms of coatings will pave the way for the successful implementation of ceramic-based ASCs in next-generation propulsion systems for aerospace applications.

Dechang Jia: Resources. Yu Zhou: Supervision. Leyao Wang: Writing - original draft, Formal analysis, Data curation. Shuqi Wang: Writing - review & editing, Project administration, Conceptualization. Guoliang Chen: Writing - review & editing, Conceptualization. Yongchun Zou: Project administration. Shuang Yu: Resources, Conceptualization. Enyu Xie: Data curation. Qingyuan Zhao: Formal analysis. Ye Zhiyun: Visualization. Jiahu Ouyang: Conceptualization. Yaming Wang: Writing - review & editing, Supervision, Resources, Project administration, Funding acquisition.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.