Research Progress on Sintering Resistance of Ceramic Thermal Protection Coatings

Abstract

Ceramic thermal protective coatings during long-term service in high-temperature environments are prone to micropore shrinkage, grain coarsening, and porous structure collapse, leading to severe densification. This consequently degrades the durability and reliability of the ceramic coatings. This paper elucidates the sintering densification mechanism of ceramic coatings, analyzes innovations in material systems and multidimensional structural design strategies, and summarizes the state-of-the-art research progress on anti-sintering densification of ceramic coatings. The limitations of conventional techniques that inhibit high-temperature sintering densification via “passive pore retention” are highlighted. A novel strategy based on phase transformation-induced pore formation for achieving “active in situ pore generation” is explored. On this basis, future research directions for enhancing the anti-sintering densification performance of ceramic thermal protective coatings are proposed.

1. Introduction

Ceramic thermal protective coatings are key enabling technologies for achieving a high thrust-to-weight ratio, high efficiency, and a long service life in advanced aero-engines. These coatings cover a broad class of high-temperature protective systems, including thermal barrier coatings (TBCs) [1,2], environmental barrier coatings (EBCs), and abradable environmental barrier coatings (AEBCs) [3,4,5,6]. All these systems fall into the broad category of ceramic thermal protective coatings and face the common challenge of performance degradation induced by high-temperature sintering.

Thermal-insulating ceramic coatings in TBC systems are mainly applied onto nickel-based or cobalt-based superalloy components, with their primary function being thermal insulation, which can reduce the substrate temperature by 100–300 °C. The most widely used material is yttria-stabilized zirconia (YSZ), owing to its low thermal conductivity and high toughness. Emerging candidate materials include rare earth zirconates with improved phase stability, alumina-based coatings with excellent oxidation resistance, and phosphate-based coatings with ultra-low thermal conductivity. The dominant fabrication processes are atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD). The columnar microstructure formed by EB-PVD provides excellent thermal cycling resistance, making it the preferred technology for engine turbine blades [7].

EBCs and AEBCs are specifically developed for ceramic matrix composites (CMCs, e.g., SiC/SiC) and represent the most advanced and multifunctional thermal protection systems. Their core functions include high-temperature water–oxygen corrosion resistance, calcia–magnesia–alumino–silicate (CMAS) corrosion resistance and abradable sealing. The mainstream materials are rare earth silicates, including monosilicates (e.g., ytterbium monosilicate, with superior corrosion resistance), disilicates (e.g., ytterbium disilicate, with excellent thermal expansion compatibility), and aluminosilicates with self-healing capability. The primary manufacturing process for EBCs and AEBCs is APS. The current development trend is to integrate thermal insulation with abradable and environmental protection functions to fabricate thermal/abradable environmental barrier coatings (T/AEBCs), which are applicable to next-generation ultra-high-temperature CMC components [8,9].

Through the combined effects of thermal insulation, gas-path sealing, corrosion resistance, and erosion resistance, ceramic thermal protective coatings enable reliable operation of hot-section components under extreme conditions far exceeding the service limits of substrate materials, acting as an indispensable protective shield for modern advanced engines. With the ongoing trend toward higher temperatures and higher efficiency, especially as CMCs become the primary materials for hot-section components, advanced ceramic thermal protective coatings with ultra-high temperature resistance (>1500 °C), high performance, multifunctionality, and long durability have become a major research focus.

During long-term service at high temperatures (>1000 °C) or ultra-high temperatures, ceramic thermal protective coatings inevitably undergo microstructural evolution, including internal pore shrinkage, grain coarsening, and the collapse of characteristic microstructures (e.g., columnar structures in EB-PVD coatings), which results in overall densification. This sintering-induced microstructural degradation leads to premature coating failure via three critical mechanisms: (i) Thermal insulation failure: Densification markedly increases thermal conductivity and impairs thermal insulation performance, thereby causing overheating, creep, accelerated oxidation, and even melting of metallic substrates and CMC substrates [10,11]. (ii) Excessive stiffening: The rise in elastic modulus reduces coating toughness and strain tolerance, seriously degrading its thermal cycling resistance [12,13]. (iii) Severe internal stress mismatch: Tensile stress originating from sintering shrinkage superimposes upon thermal cycling stress and exceeds the interfacial bonding strength. Microcracks are consequently initiated, further propagating into macroscopic delamination and eventually resulting in catastrophic spallation of the coating [14,15].

2. Sintering Densification Mechanism

Sintering densification is one of the primary failure mechanisms in ceramic thermal barrier coatings during high-temperature service. The core drivers are significantly enhanced mass transport at elevated temperatures and spontaneous matter migration driven by minimizing the system surface energy. To meet the analytical requirement, surface diffusion, grain-boundary diffusion, lattice diffusion, stress-assisted sintering, pore coarsening, and true densification are clearly distinguished below.

2.1. Diffusion Mechanisms and Mass Transport

The rate of diffusive mass transfer follows the Arrhenius equation (Equation (1)) [16,17,18]. With every 100–200 °C temperature rise, the diffusion coefficient increases by 1–2 orders of magnitude. High temperatures promote atomic/ionic migration and activate four distinct diffusion pathways:

(1) Surface diffusion: Atoms migrate along pore and particle surfaces, causing pore rounding and pore coarsening without volume shrinkage; it does not contribute to true densification.

(2) Grain-boundary diffusion: Atoms migrate rapidly along grain boundaries with low activation energy, transporting matter to interparticle necks and driving true densification.

(3) Lattice (volume) diffusion: Atoms diffuse through the crystal interior, eliminating pores and increasing stiffness at ultra-high temperatures, which also promotes true densification.

(4) Stress-assisted sintering: Local thermal stress and constrained deformation reduce diffusion barriers and accelerate sintering kinetics, leading to premature densification and modulus stiffening.

2.2. Surface Energy-Driven Densification

The fundamental driving force throughout the entire sintering process of ceramic materials is the reduction in the total surface energy of the system (Equation (2)) [19,20,21]. Porous microstructures possess considerable solid–gas interfacial energy, and high-temperature conditions supply sufficient activation energy to overcome diffusion barriers. This drives the system to evolve spontaneously toward minimizing its surface area by eliminating internal pores and promoting grain growth, thereby achieving sintering densification through the aforementioned mass transport mechanisms.

2.3. Grain Growth and Late-Stage Densification

In the late sintering stage, grain boundary migration and grain growth follow the Beck equation (Equation (3)) [22,23], which further eliminates intergranular pores and increases density and stiffness. It is critical to distinguish between pore coarsening and true densification: pore coarsening only induces changes in pore size and morphology through surface diffusion without causing volume shrinkage or density improvement, whereas true densification leads to macroscopic shrinkage, obvious porosity reduction, and significant elastic modulus increase, which are mainly driven by grain-boundary diffusion and lattice diffusion.

$$D=D_0\exp \left(-{\displaystyle \frac{E_a}{R T}}\right)$$

(1)

In Equation (1), D is the diffusion coefficient. D₀ is the pre-exponential factor (related to lattice vibrational frequency). E_a is the diffusion activation energy (energy required to overcome the energy barrier, in J/mol). R is the gas constant (8.314 J/mol·K). T is the absolute temperature (in K).

$$\mathsf{\Delta }G=\gamma \cdot \mathsf{\Delta }A$$

(2)

In Equation (2), ΔG is the change in Gibbs free energy of the system. γ is the surface tension, A is the surface area.

$$G^n-G_0^n=K_{0}t\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\displaystyle \frac{Q}{RT}}\right)$$

(3)

In Equation (3), G is the grain size at time t. G₀ is the initial grain size. n is the grain growth exponent (dimensionless; reflects the dominant mechanism: n = 2 corresponds to ideal grain boundary migration, n = 3 to solute drag or pore pinning, and n = 4 to strong pinning by second-phase particles). t is the holding time. K₀ is the kinetic constant. Q is the activation energy for grain growth.

3. Research Progress in Anti-Sintering Densification

To break through the high-temperature sintering bottleneck in ceramic coatings, research efforts by international scholars have been concentrated on two primary approaches: optimization of ceramic material systems and multidimensional structural design of coatings.

3.1. Optimization of Ceramic Material Systems

To address the structural instability, abnormal grain growth and uncontrolled densification of conventional YSZ ceramics caused by high-temperature phase transformation above 1175 °C, domestic and foreign scholars have developed four mainstream technical routes from the perspective of material composition and crystal structure design: rare earth oxide doping, second-phase introduction, high-entropy composition design, and novel ceramic matrix design.

3.1.1. Rare Earth Oxide Doping

Rare earth oxide doping introduces rare earth oxides such as La₂O₃, CeO₂, Sc₂O₃ and HfO₂ into the YSZ matrix. The radius and atomic mass differences between rare earth ions and matrix ions induce lattice distortion and regulate oxygen vacancy concentration, thereby inhibiting high-temperature grain boundary migration and grain coarsening at the atomic diffusion level and achieving the synergistic optimization of sintering resistance and low thermal conductivity. Its core feature is modifying the conventional YSZ matrix by adding rare earth dopants without changing the basic matrix system.

In 2009, the Japan Fine Ceramics Center [24] adopted La₂O₃ and HfO₂ as dopants to modify EB-PVD YSZ coatings. After annealing at 1400 °C for 50 h, the thermal conductivity of the co-doped coating remained as low as 1.0 W·m⁻¹·K⁻¹, while conventional 4YSZ exhibited a rapid increase to 2.0~2.5 W·m⁻¹·K⁻¹. Meanwhile, the feather-like sub-column structure of the doped coating was well retained, whereas that of 4YSZ disappeared completely due to severe sintering. In burner rig tests, the backside temperature of the co-doped double-layer coating was approximately 40 °C lower than that of conventional 4YSZ, and its thermal cycle life exceeded 200 cycles, twice that of 4YSZ (about 100 cycles). In 2021, researchers from Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences [25], prepared CeO₂ and Sc₂O₃ co-doped YSZ ceramics via a solid-state reaction. Compared with pure YSZ (2.00 W·m⁻¹·K⁻¹), the co-doped sample exhibited a thermal conductivity of only 1.53 W·m⁻¹·K⁻¹ at 1000 °C, corresponding to a reduction of 23%. After heat treatment at 1500 °C for 120 h, no detrimental monoclinic phase was detected in the co-doped ceramic, while obvious phase decomposition occurred in YSZ. In addition, the linear shrinkage of Ce-Sc co-doped YSZ after sintering was significantly lower than that of single-CeO₂-doped YSZ, demonstrating improved sintering resistance. Quantitative results from Refs. [24,25] consistently confirm that rare earth doping can effectively suppress sintering-induced densification and microstructure degradation. Compared with undoped YSZ, the thermal conductivity increase rate during high-temperature annealing is reduced, and porosity and microstructure stability are significantly improved.

Rare earth oxide doping features simple preparation procedures, good compatibility with existing YSZ coating manufacturing routes and controllable cost, making it the most mature matrix modification method for engineering applications. Nevertheless, it still has inherent limitations. A high doping content improves sintering resistance but inevitably deteriorates the fracture toughness of the ceramic matrix and causes thermal expansion mismatch, leading to cracking and spalling during thermal cycling. In addition, high-purity rare earth oxide raw materials are expensive, restricting large-scale industrial applications. The modification effect of single rare earth doping is limited, and rational composition matching of co-doping is relatively difficult.

3.1.2. Second-Phase Introduction

Second-phase introduction introduces additional secondary phase particles, such as multi-metal oxides, into the ceramic matrix. The grain boundary pinning effect hinders grain boundary migration and refines grain size, while oxygen vacancy regulation delays high-temperature densification, serving as a classic strategy to improve the high-temperature structural stability of YSZ. Its typical characteristic is forming a binary system of matrix plus external second phase, and the performance improvement is realized through the pinning effect of the second phase.

In 2010, Chromalloy Gas Turbine LLC (Limited Liability Company), Palm Beach Gardens, FL, USA [26], developed a novel Nd_xZr_1−xO_γ composite oxide system and introduced multi-metal oxide second phases to regulate the microstructure. Nd_xZr_1−xO_γ and conventional 7YSZ coatings were fabricated by EB-PVD and APS, respectively. After cyclic sintering at 1107 °C for 706 h, obvious sintering bridges between columns appeared in EB-PVD 7YSZ, while such bridging was effectively inhibited in the second-phase-modified coating. After isothermal sintering at 1371 °C for 500 h, the intra-columnar pores in 7YSZ were completely closed, and the structure was fully densified. In contrast, the intra-columnar pores and column gaps in Nd_xZr_11−xO_γ were well retained. For APS coatings, after sintering at 1200 °C for 50 h, the splat boundaries in 7YSZ began to merge and intra-splat cracks gradually healed. After extending to 100 h, 7YSZ showed severe densification. In comparison, APS Nd_xZr_1−xO_γ still maintained clear inter-splat pores and intra-splat cracks under the same conditions. The quantitative and microstructural results confirmed that the Nd_xZr_1−xO_γ coatings exhibited significantly higher sintering resistance than their YSZ counterparts prepared by the same processing route.

Second-phase introduction possesses a clear modification mechanism and flexible performance adjustability, which can simultaneously optimize microstructure and high-temperature mechanical properties; it is compatible with mainstream preparation technologies, including EB-PVD and APS. However, obvious drawbacks remain. Secondary phase particles are prone to agglomeration and uneven distribution, resulting in local stress concentration and scattered performance. It is difficult to precisely match the thermal expansion coefficient between the second phase and the matrix, easily inducing interfacial debonding and delamination under long-term thermal cycling. Moreover, most high-performance second phases rely on rare earth raw materials, which further increase material cost and may cause gradual degradation of phase stability during long-term high-temperature service.

3.1.3. High-Entropy Composition Design

High-entropy composition design adopts multi-principal rare earth ions with equimolar or near-equimolar random occupation. Relying on the sluggish diffusion effect, the atomic migration rate at high temperature is greatly reduced, fundamentally inhibiting grain growth and sintering densification, which represents a cutting-edge material design strategy for ultra-high-temperature service conditions.

In 2023, researchers from the Engineering Research Center of Nano-Geomaterials of the Ministry of Education (MOE), China University of Geosciences [27], synthesized a novel high-entropy rare earth zirconate (Dy_0.2Nd_0.2Sm_0.2Eu_0.2Yb_0.2)₂Zr₂O₇ with a pyrochlore structure via reverse co-precipitation. After annealing at 1600 °C for 1–50 h, the average grain size only increased from 0.73 μm to 2.22 μm, corresponding to an ultra-low grain growth rate of ~0.026 μm·h⁻¹, which is significantly lower than that of conventional single-component La₂Zr₂O₇ ceramics. Meanwhile, this high-entropy ceramic achieved a thermal conductivity of 1.14 ± 0.09 W·m⁻¹·K⁻¹ at 1500 °C (34% lower than La₂Zr₂O₇), a thermal expansion coefficient of 10.59 × 10⁻⁶ K⁻¹ (19% higher than La₂Zr₂O₇), and a fracture toughness of 2.07 MPa·m^1/2 (50% higher than La₂Zr₂O₇). In 2024, researchers at Harbin Engineering University [28] designed a series of multi-component (LaGdYSm)_x2Yb_x1Zr₂O₇ high-entropy ceramics by regulating ionic size disorder and electronegativity difference. After heat treatment at 1500 °C for 50 h, all compositions maintained excellent phase stability. With the increase in ionic size disorder δ_r, the structure transformed from a pyrochlore/fluorite dual phase to a single pyrochlore phase. The sample with the maximum size disorder (Yb_0.6) exhibited the lowest grain growth rate and the best thermal insulation performance, with a thermal insulation temperature difference up to 403 °C at 1200 °C. In the dual-phase region, grain growth was further suppressed by the synergistic effect of lattice distortion and sluggish diffusion. In addition, the thermal expansion coefficient of the optimal sample reached 11.49–11.58 × 10⁻⁶ K⁻¹ at 1000–1100 °C, even higher than that of conventional YSZ. Quantitative results from Refs. [27,28] confirm that high-entropy design can simultaneously achieve ultra-low grain growth rate, low thermal conductivity, high phase stability, and improved fracture toughness. The synergistic effect of severe lattice distortion and sluggish diffusion effectively inhibits sintering densification and grain coarsening, making high-entropy ceramics superior to traditional doping and second-phase introduction strategies in ultra-high-temperature environments.

High-entropy design delivers the best intrinsic sintering resistance among the four categories, with a much lower grain growth rate than doping and second-phase-modified materials. It can simultaneously achieve excellent ultra-high-temperature phase stability and low thermal conductivity, suitable for harsh service environments above 1600 °C. Nevertheless, considerable challenges remain. The composition ratio, synthesis process and heat treatment system of multi-component high-entropy ceramics are extremely demanding with complicated fabrication procedures. Imbalanced multi-element diffusion kinetics easily induce composition segregation and phase separation. In addition, high-entropy rare earth ceramics generally suffer from low fracture toughness. Balancing sintering resistance, mechanical reliability and manufacturing cost remains difficult, and current research is still limited to laboratory-scale exploration, far from large-scale engineering applications.

3.1.4. Novel Ceramic Matrix Design

This strategy does not rely on rare earth oxide doping, external second-phase introduction or high-entropy multi-component design. Instead, it develops brand-new single-phase ceramic matrices with typical crystal structures such as pyrochlore, fluorite and hexaaluminate. The low sintering rate, low thermal diffusivity and excellent high-temperature structural stability are derived from the intrinsic crystal structure, bonding characteristics and morphological features, serving as a promising alternative route to replace conventional YSZ.

In 2016, researchers from Technische Universität Darmstadt and Forschungszentrum Jülich, Germany [29], investigated the sintering behavior of Gd₂Zr₂O₇ (GZO) pyrochlore coatings. Quantitative measurements showed that after annealing at 1300 °C for 100 h, the relative stiffness increase in GZO was only 2.2–2.4, much lower than 3.1–3.5 for conventional YSZ. The unique microstructure of GZO effectively suppressed microcrack healing and sintering of unmolten particles, providing significantly higher sintering stability above 1300 °C. In 2018, researchers from the Institute of Materials Engineering, Silesian University of Technology, Poland [30], synthesized Eu₂Zr₂O₇, Eu₂Ce₂O₇ and Eu₂Hf₂O₇ with pyrochlore and fluorite structures. Laser flash analysis confirmed that Eu₂Ce₂O₇ exhibited the lowest thermal diffusivity above 1000 °C, far superior to Eu₂Zr₂O₇, Eu₂Hf₂O₇ and YSZ. All three compositions showed lower sintering densification rates than YSZ due to strong phonon scattering from lattice defects. In 2020, researchers from Ondokuz Mayis University, Turkey [31], compared long-term isothermal oxidation of YSZ and GZO coatings. After 100 h at 1000 °C, the porosity of YSZ decreased by approximately 40%, while that of GZO only decreased by 25%. Meanwhile, the hardness of YSZ increased from 3.9 GPa to 8.6 GPa, whereas that of GZO only increased from 2.98 GPa to 4.8 GPa. The lower hardness increment and porosity attenuation originated from the intrinsically high thermal stability and low elastic modulus of GZO. In 2025, researchers from Gas Turbine Institute, Shanghai Electric Gas Turbine Co., Ltd., Shanghai, China [32], prepared LaMgAl₁₁O₁₉ (LMA) hexaaluminate coatings via atmospheric plasma spraying. After testing at 1100 °C and 1300 °C, the porosity of LMA increased from 7.79% to approximately 10%–11% with prolonged sintering, while that of YSZ decreased from 16.21% to 12.25%. After 1300 °C for 1000 h, the t′ phase in YSZ decreased from 82.67% to 27.69%, while the monoclinic phase increased from 0.19% to 15.39%, causing 3%–5% volume expansion and coating spallation. In contrast, LMA formed in situ needle-like and strip-like grains to build a self-supporting framework, maintaining outstanding microstructural stability.

Novel ceramic matrix design fundamentally avoids the inherent high-temperature phase transformation and sintering instability drawbacks of conventional YSZ, acquiring excellent comprehensive high-temperature performance through intrinsic crystal structure advantages, which is an important development direction for future thermal barrier coating matrix replacement. However, the basic thermodynamic and kinetic mechanisms of most novel systems are still insufficiently understood. The phase evolution, element volatilization and corrosion behavior under long-term high-temperature service remain unclear. Some rare-earth-based novel matrices are limited by raw material reserves and high costs. In addition, the matching of new ceramics with existing preparation processes and substrate thermal expansion performance requires extensive parameter optimization, resulting in a long development cycle and difficulty in large-scale replacement of mature YSZ in the short term.

3.1.5. Quantitative Performance Comparison of Material Optimization Strategies

To intuitively compare and evaluate the anti-sintering effects of different material system optimization strategies, key quantitative indicators, including temperature, holding time, porosity change, thermal conductivity, elastic modulus, thermal cycling life, and failure mode, are summarized in

Table 1. Quantitative comparison of anti-sintering properties for ceramic coating material optimization strategies [24,25,26,27,28,29,30,31,32].

Strategy	Temperature	Time	Porosity Change	Thermal Conductivity (W·m−1·K−1)	Elastic Modulus/Stiffness	Thermal Cycling Life	Failure Mode
Conventional YSZ	1400 °C	50 h	Significant decrease	Rapid rise to 2.0–2.5	Significant increase	~100 cycles	Sintering densification, column bridging, spallation
Rare earth oxide doping (La2O3-HfO2/YSZ)	1400 °C	50 h	Well retained	Remain ~1.0	Moderate increase	>200 cycles	Retained feather-like structure, no obvious densification
Second-phase introduction (NdxZr1−xOγ)	1371 °C	500 h	Pores well retained	Low and stable	Slight increase	Significantly improved	No column bridging, inhibited densification
High-entropy rare earth zirconate	1600 °C	50 h	Negligible change	1.14 ± 0.09	Ultra-low grain growth rate	Excellent	No phase decomposition, ultra-low coarsening
Novel ceramic (Gd2Zr2O7, GZO)	1300 °C	100 h	Decrease by ~25%	Low thermal diffusivity	Relative stiffness increase: 2.2–2.4	Improved	Suppressed microcrack healing
Novel ceramic (LaMgAl11O19, LMA)	1300 °C	1000 h	Increased from 7.79% to 10%–11%	Low and stable	Slight increase	Excellent	No spallation, self-supporting framework retained

[24,25,26,27,28,29,30,31,32].

3.2. Multidimensional Structure Design of Coatings

Multimodal optimization of ceramic coating structures has become a critical approach complementary to material system innovation to address sintering issues. By regulating structural features, such as porosity, pore morphology and distribution, and crack networks, this strategy can reduce mass transport rates and sintering driving forces in high-temperature environments to varying degrees. Consequently, it delays densification processes while maintaining the low thermal conductivity and high strain tolerance of coatings.

3.2.1. Nano-Bimodal Structural Design

Spray granulation technology was employed to agglomerate nanoparticles into micrometer-sized particles. Through parameter adjustment in the APS process, these agglomerates achieved a gradient melting state: Particle surfaces heated above 2700 °C melted completely and spread into lamellar structures upon impact with the substrate. Ultra-rapid quenching formed dense columnar grains within the lamellar zones, accompanied by the generation of 2D pores and microcracks. Particle cores remained semimolten at 1600–2000 °C due to thermal lag. Unmelted nano-grains acted as “seeds” and recrystallized into equiaxed nanocrystals upon impact, forming porous nano-zones. The alternating deposition of these structures resulted in a bimodal topology integrating dense lamellar and porous nano-domains. This unique single-phase microstructure with dual micro/nanoscales contains abundant spherical pores, microcracks, and splats. Compared with conventional coatings, the bimodal-structured coatings exhibit superior sintering resistance.

In 2007, the National Research Council Canada [33] heat-treated nanostructured and conventional YSZ coatings at 1400 °C for 1, 5, and 20 h. After 1400 °C/20 h annealing, the thermal diffusivity of nanostructured YSZ was only 0.32 mm²/s, which was 59% lower than 0.58 mm²/s for conventional YSZ. Meanwhile, the in-plane elastic modulus of nanostructured YSZ remained at 40–60 GPa, much lower than 80–120 GPa for conventional YSZ. Quantitative porosity analysis further indicated that the coarse porosity of nanostructured YSZ increased from 8% to 14.4% after high-temperature sintering, while conventional YSZ exhibited a continuous porosity decrease from 8% to 4.1%. Such distinct behaviors originated from the differential sintering effect between nano-zones and the lamellar matrix, which formed coarse voids and counteracted fine pore healing, thus realizing a “self-compensation effect” against sintering-induced densification.

Subsequent research by scholars worldwide delved deeper into the microstructure and anti-sintering performance of bimodal-structured ceramic coatings. In 2010, Beihang University [34] fabricated nano-bimodal YSZ coatings via APS. The quantitative results confirmed that nanostructured coatings achieved a porosity of ~25%, significantly higher than ~15% for conventional coatings. The thermal conductivity was controlled at 0.8–1.1 W·m⁻¹·K⁻¹, approximately 40% lower than that of conventional YSZ (1.6–2.0 W·m⁻¹·K⁻¹). Thermal cycling tests demonstrated that bimodal coatings survived more than 500 cycles, whereas conventional coatings failed by spallation within 200 cycles. In 2015, Sapienza University of Rome [35] systematically compared thermomechanical properties of conventional and nanostructured APS-YSZ coatings. Porosity evolution tests revealed that conventional YSZ exhibited continuous porosity reduction with increasing temperature, while nanostructured YSZ showed an obvious porosity increase below 1000 °C and a slight decrease above 1000 °C. Quantitative mechanical tests indicated that the Young’s modulus of nanostructured YSZ remained at 18–25 GPa from room temperature to 1000 °C, about twice that of conventional YSZ, and maintained obvious plastic deformation at 1500 °C without fracture. In 2019, Xi’an Jiaotong University [36] investigated the degradation-resistant mechanism of nano-bimodal YSZ coatings. Quantitative statistics showed that 2D pore healing in lamellar zones dominated the initial rapid sintering stage (within 20 h), while differential densification between nano-zones and lamellar zones generated coarse voids, which counteracted 2D pore closure and retarded performance degradation. After 1150 °C/500 h thermal exposure, the elastic modulus increment of bimodal coatings was 30%–50% lower than conventional YSZ, and the thermal conductivity rise was suppressed by more than 30%. In 2022, Nanchang University [37] deposited YSZ/NiCrAlY bimodal TBCs on Ti-6Al-4V via spark plasma sintering (SPS). After 800 °C/100 h isothermal oxidation, the coating retained excellent interfacial adhesion with only 6% surface spallation under high-angle bending. Meanwhile, the mass gain of the coated Ti-6Al-4V substrate was only 0.51 times that of the bare substrate, indicating significantly improved oxidation resistance. Nevertheless, excessive porosity in nano-zones weakened inter-splat bonding and formed loose channels, accelerating TGO growth and deteriorating corrosion resistance.

Nano-bimodal structural design presents remarkable advantages in sintering resistance, thermal insulation stability and strain tolerance via differential sintering effects and multiscale pore regulation. This structural strategy is compatible with the commercial APS preparation route and has realized preliminary engineering verification. Nevertheless, the controllability of the bimodal microstructure remains insufficient, and excessive nano-zones tend to reduce inter-splat bonding strength and accelerate interfacial degradation. In addition, the long-term service stability of porous nano-domains above 1400 °C is still limited, and the contradiction between high porosity/low thermal conductivity and structural robustness needs to be further balanced.

3.2.2. Core–Shell Structural Design

Core–shell structured coatings are constructed by physically encapsulating or chemically synthesizing continuous, dense diffusion-barrier shells around core materials, which block sintering-induced mass transport pathways at the source and thereby suppress the densification of ceramic coatings.

In 2022, Xi’an Jiaotong University [38] developed a nanoporous composite doped with hollow/core–shell fibers featuring high thermal stability and low thermal conductivity. The hollow aerogel matrix significantly reduced solid-phase thermal conduction, while the core–shell structure and surface-doped Al₂O₃ or TiO₂ metal oxides synergistically suppressed radiative heat transfer and high-temperature sintering. The hollow-core ratio of the aerogel was precisely controlled in the range of 0–1, enabling tunable thermal conductivity and density for extreme thermal protection scenarios. In 2024, Xi’an Shiyou University [39] employed dual-feed atmospheric plasma spraying to prepare mechanically encapsulated “LZO(La₂Zr₂O₇) + YSZ + LZO” sandwich-structured powders by collisions between solid YSZ and molten La₂Zr₂O₇, forming heterogeneous interfaces that inhibited diffusion bridging. After sintering at 1200 °C for 200 h, the porosity of conventional YSZ decreased sharply from 31% to 13% with a linear shrinkage of 4.62%. In contrast, the porosity of core–shell structured coatings decreased moderately from 28% to 18% with a linear shrinkage of only 2.38%–2.56%, demonstrating significantly improved sintering resistance. Meanwhile, the hardness increment of core–shell coatings was controlled within 167%–183%, much lower than that of conventional YSZ. In 2024, Northwestern Polytechnical University [40] designed a nanoscale thermal conduction network using a ceramic@carbon core–shell structure to alleviate ablation heat accumulation and enhance the ablation resistance of ZrC–SiC/TaC coatings. Graphene-encapsulated SiC/TaC nanoparticles were introduced into ZrC coatings via supersonic atmospheric plasma spraying. The thermal conduction network formed by graphene shells improved in-plane heat diffusion, reducing the ablation surface temperature by approximately 200 °C (from 2224 °C to 2032 °C). This design suppressed the volatilization of low-melting-point phases and effectively delayed the sintering and coarsening of ZrO₂ particles. The average grain size of ZrO₂ was refined from 1.88 μm to 0.58 μm, and the linear ablation rate was reduced by 93.7%. In 2025, Central South University [41] synthesized (Ti, Zr, Hf, Ta)CN/SiCN nanocomposites via a single-source precursor method. EDS elemental mapping confirmed uniform elemental distribution, and the in situ-formed carbon shell, with a thickness of 3–5 nm, effectively inhibited grain growth at elevated temperatures. The average grain size of (Ti, Zr, Hf, Ta)CN nanoparticles was controlled within 16.05–21.5 nm, and the composite exhibited a hardness of 35–37 GPa and flexural strength up to 603 MPa. At 2200 °C, the linear ablation rate reached 0.033 μm/s, two orders of magnitude lower than that of conventional ultra-high-temperature ceramics.

Core–shell structural design blocks atomic diffusion and grain bridging at the source by constructing dense shell barriers, thus significantly suppressing sintering-induced densification, porosity attenuation and performance degradation. This strategy shows unique superiority in structural stability and thermal insulation durability under ultra-high temperatures. However, the uniform coating and dispersion of core–shell structured powders are technically difficult, and mismatched thermal expansion between core and shell phases easily induces interfacial thermal stress. Meanwhile, interfacial diffusion and shell cracking above 1300 °C may lead to premature structural failure, restricting further improvement in long-term service reliability.

3.2.3. Pore Structure Design

Pore structure design primarily enhances sintering resistance through multiscale porosity construction and stable pore morphology regulation.

In 2020, East China University of Science and Technology [42] embedded agglomerated micrometer-sized 8YSZ powders with unique intraparticle pores (unmelted/semimelted) into conventional coatings, forming a composite structure with porous particles uniformly dispersed in the lamellar matrix. After exposure at 1300 °C for 100 h, the elastic modulus of conventional YSZ increased sharply from 15 GPa to 60 GPa (approximately 3 times the initial value), accompanied by significant stiffening and rapid performance degradation. In contrast, the novel porous composite coating maintained an elastic modulus of only 10–15 GPa and a hardness of ~500–700 HV, comparable to the as-sprayed state of conventional coatings. Meanwhile, the thermal shock life at 1450 °C was prolonged from 15 cycles to more than 60 cycles (over 4 times improvement), and at 1320 °C, the composite coating remained intact after 1225 cycles without spalling. The significantly improved durability was attributed to suppressed sintering-induced stiffening and reduced crack driving force. In 2022, South China University of Technology [43] further optimized pore architecture by injecting YSZ powders with controllable fine/coarse pores into the plasma plume tail, preparing porous coatings with multiscale pore distributions. After 1300 °C/100 h sintering, conventional coatings exhibited rapid pore healing, with porosity decreasing from 7% to 5% and elastic modulus surging to 50–60 GPa. In comparison, coatings embedded with micron-scale porous particles maintained porosity above 25% and limited the elastic modulus to only 10–20 GPa, with thermal diffusivity 0.10–0.15 mm²·s⁻¹ lower than conventional coatings. Quasi-in situ observations confirmed that micron-scale intraparticle pores resisted sintering-induced healing, whereas nanoscale pores densified rapidly and generated interfacial cracks. Only coarse porous structures provided persistent, stable sintering resistance.

Pore structure design effectively suppresses sintering-induced stiffening, pore closure and thermal insulation degradation by constructing thermally stable multiscale pores, especially micron-scale intraparticle pores. This strategy is fully compatible with conventional APS processes and remarkably extends thermal shock life by reducing modulus growth. Nevertheless, excessive porosity inevitably sacrifices mechanical strength and structural integrity. Precise regulation of pore size, morphology and distribution remains technically challenging, and the trade-off between high porosity/low thermal conductivity and mechanical reliability constitutes the primary bottleneck for engineering applications.

3.2.4. Multilayered Structural Design

Multilayered structural design transforms conventional continuous stacking into bilayer, multilayer, and functionally graded configurations, which optimizes thermal gradient distribution, relieves interfacial thermal stress, and inhibits sintering-induced densification via complementary material properties.

In 2012, the Beijing Institute of Aeronautical Materials [44] fabricated La₂(Zr_0.7Ce_0.3)₂O₇ (LZ7C3)/YSZ double-ceramic-layer (DCL) TBCs via EB-PVD. Burner rig tests at 1573 K showed that the DCL system exhibited a 27% longer thermal cycling lifetime (up to 2507 cycles) than single-layer YSZ, owing to the excellent sintering resistance of the LZ7C3 top layer and stable columnar microstructure. After long-term thermal cycling, the DCL coating maintained clear columnar gaps, while conventional YSZ suffered serious column bridging and densification. In 2016, Universitat Politècnica de València [45] prepared multilayered and functionally graded YSZ/Gd₂Zr₂O₇ coatings using dual-feed APS. After annealing at 1050 °C for 100 h, the porosity of multilayer coatings decreased from 14% to 6.8%, while functionally graded coatings maintained porosity above 8.5% and presented the lowest microhardness increase (from 450 HV_0.2 to 780 HV_0.2). Thermal cycling tests confirmed that graded coatings survived over 1500 cycles, far exceeding the 950 cycles of multilayer coatings, due to homogeneous stress distribution and gradual composition transition. In 2018, Xi’an Jiaotong University [46] proposed compositionally graded porous TBCs with porosity decreasing gradually from top to bottom to suppress sintering. After annealing at 1200 °C for 50 h, the Young’s modulus increment of graded coatings was 13.23%–16.45% lower than that of homogeneous coatings, and the thermal conductivity rise was suppressed by over 10%. A new sintering resistance parameter R_s was established, confirming that the graded structure effectively retarded modulus stiffening and thermal insulation degradation. In 2022, Université de Bordeaux [47] optimized environmental barrier coatings (EBCs) using a bilayer design: a coarse-grained Y₂SiO₅ (YMS) outer layer suppressed volatilization with a recession rate of 15 nm·h⁻¹ at 1400 °C; a submicron Y₂Si₂O₇ (YDS) middle layer acted as a diffusion barrier, reducing oxygen ionic conductivity to 2.1 × 10⁻⁷ S·cm⁻¹ (five orders lower than YSZ) and lowering the intermediate layer temperature to restrain grain coalescence and ion diffusion. In 2025, East China University of Science and Technology [10] fabricated Gd/Yb co-doped YSZ (RYSZ)/8YSZ DCL coatings via APS. After annealing at 1150 °C for 200 h, the porosity of DCL decreased by only 29.89% (from 15.2% to 10.7%), whereas that of single 8YSZ decreased by 73.94% (from 14.2% to 3.7%). The DCL survived 60 thermal cycles without failure, superior to single RYSZ (35 cycles) and single 8YSZ (40 cycles).

Concurrently (2023–2025), this paper’s authors developed bionic shell-inspired “brick–mud” layered YSZ seal coatings [48,49,50,51] via high-temperature adhesives and cyclic APS deposition. Thermal cycling tests demonstrated that low-fracture-toughness “mortar” layers introduced controllable microcracks to release sintering-induced stress, reducing interfacial stress by 25%–35% and delaying microstructure densification. After 1100 °C thermal cycling, the layered coatings maintained porosity above 11% and prolonged thermal shock life by >50% compared with conventional monolithic coatings (Figure 1).

Multilayered and graded structural design achieves synergistic optimization of thermal insulation and sintering resistance by constructing dual/multilayer or gradient architectures, combining the complementary advantages of different material layers. This strategy shows obvious superiority in relieving sintering stress, slowing down modulus stiffening, and prolonging thermal cycle life, and is compatible with mainstream EB-PVD and APS preparation processes. However, these multilayered structures typically face critical challenges: serious interfacial stress concentration caused by thermal expansion mismatch, easy delamination failure induced by inconsistent sintering behaviors between adjacent layers, complicated construction of multi-interface architectures, and extremely demanding process control and parameter optimization during mass manufacturing. In addition, long-term interfacial diffusion and phase segregation above 1300 °C further deteriorate structural stability, restricting its long-term reliability under ultra-high-temperature and heavy thermal cycle conditions.

3.2.5. Quantitative Performance Comparison of Structure Optimization Strategies

To intuitively compare and evaluate the anti-sintering effects of different coating structure optimization strategies, key quantitative indicators, including temperature, holding time, porosity change, thermal conductivity, elastic modulus, thermal cycling life, and failure mode, are summarized in

Table 2. Quantitative comparison of anti-sintering properties for ceramic coating structure optimization strategies [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52].

Strategy	Temperature	Time	Porosity Change	Thermal Conductivity/Diffusivity	Elastic Modulus	Thermal Cycling Life	Failure Mode
Conventional YSZ coating	1400 °C	20 h	Decrease from 8% to 4.1%	0.58 mm2/s	80–120 GPa	<200 cycles	Sintering densification, spallation
Nano-bimodal structure	1400 °C	20 h	Increase from 8% to 14.4%	0.32 mm2/s	40–60 GPa	>500 cycles	No obvious stiffening, stable porosity
Core–shell structure (LZO + YSZ + LZO)	1200 °C	200 h	Decrease from 28% to 18%	Low and stable	Slight increase	Significantly improved	Inhibited diffusion bridging, slow densification
Pore structure design (microporous embedded)	1300 °C	100 h	Remain >25%	Low thermal diffusivity	10–15 GPa	>1225 cycles	No modulus stiffening, no spallation
Double ceramic layer (DCL)	1150 °C	200 h	Decrease by 29.89%	Low thermal conductivity	Moderate increase	Up to 2507 cycles	No column bridging, delayed densification
Graded porous structure	1200 °C	50 h	Well retained	Increase suppressed by >10%	Increment reduced by 13.23%–16.45%	>1500 cycles	Homogeneous stress, no delamination
“Brick–mud” layered structure	1100 °C	Thermal cycling	Remain >11%	Low and stable	Low increment	>50% improved	Stress released, no interfacial cracking

[10,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51].

3.3. Limitations of Existing Technologies

Structural densification, manifested by reduced porosity and diminished pore size, is the most prominent indicator of sintering-induced degradation in ceramic coatings. Conventional abradable seal coatings (porous ceramics) rely on pre-incorporated pore formers that burn out during service, inevitably undergoing sintering densification, increased stiffness, and degraded high-temperature performance at elevated temperatures. This severely endangers rotor blades and overall engine operational safety. Previous research has passively mitigated porosity loss through material system optimization and multidimensional structural design (“passive pore retention”), achieving considerable progress in sintering resistance. However, such anti-sintering mechanisms merely delay the densification process; their efficacy declines with extended service duration, exposing inherent limitations in resisting the densification of ceramic coatings. Furthermore, these strategies suffer from persistent drawbacks, as summarized in

Table 3. Key characteristics of current anti-sintering technologies.

Category	Representative Technology	Key Advantages	Key Limitations
1 Material system innovation	1.1 Rare earth oxide doping	(1) Induces lattice distortion and regulates oxygen vacancy concentration, reduces atomic diffusion rate and suppresses high-temperature sintering densification.	(1) High doping content easily reduces fracture toughness and causes thermal expansion mismatch, deteriorating thermal cycling performance.
		(2) Exhibits good compatibility with existing YSZ fabrication routes, simple process and easy engineering implementation.	(2) High-purity rare earth raw materials are costly; single doping shows limited effect and relies on optimized co-doping composition.
	1.2 Second-phase introduction	(1) Refines grains and hinders grain boundary migration via a grain boundary pinning effect, achieving remarkable high-temperature structural stability.	(1) Secondary phase particles tend to agglomerate and distribute unevenly, causing local stress concentration and scattered performance.
		(2) Enables synergistic regulation of microstructure and thermophysical properties; compatible with EB-PVD, APS and other mainstream processes.	(2) Thermal expansion matching between the second phase and matrix is difficult; interfacial debonding and delamination easily occur under long-term thermal cycling.
	1.3 High-entropy design	(1) Inhibits grain growth effectively by virtue of the sluggish diffusion effect, far superior to traditional modification methods.	(1) The complex multi-component solid-state synthesis process will greatly increase the raw material cost. The cost of rare earth oxide raw materials with a purity of more than 99.9% is 8–10 times that of YSZ.
		(2) The multi-component entropy effect guarantees excellent ultra-high-temperature phase stability and low thermal conductivity with wide performance adjustability.	(2) The kinetic imbalance of multi-element diffusion easily induces compositional segregation of the material, resulting in poor preparation stability.
	1.4 Novel ceramic matrix design	(1) Possesses excellent high-temperature thermal stability derived from an intrinsic crystal structure, restraining microcrack healing, particle sintering and grain coarsening spontaneously.	(1) The mechanisms of phase evolution, element volatilization and corrosion failure under long-term high-temperature service are not fully clarified.
		(2) Shows lower thermal diffusivity and sintering densification rate than conventional YSZ, presenting outstanding thermal insulation and structural stability under ultra-high temperature.	(2) Rare-earth-related raw materials suffer from high cost; extensive process optimization is required for matching with existing coating technologies and substrate thermal expansion.
		(3) Some systems can maintain or even increase porosity during high-temperature service, guaranteeing superior long-term microstructural stability.	(3) Insufficient fundamental theory and engineering application experience make it difficult to replace mature YSZ systems on a large scale in the short term.
2 Multidimensional coating structure optimization	2.1 Nano-bimodal structure design	(1) The bimodal microstructure can introduce a large number of spherical pores, microcracks and lamellar structures, significantly improving the sintering resistance of the coating.	(1) The high porosity in the quench zone will destroy the lamellar continuity of the coating and significantly reduce the interfacial bonding strength of the coating.
		(2) It can simultaneously regulate the thermal conductivity of the coating, reduce the heat conduction efficiency of the coating, and adapt to the service requirements of thermal barrier coatings.	(2) Random pore aggregation will form “loose channels”, accelerate the growth of the TGO layer, and severely deteriorate the high-temperature corrosion resistance of the coating.
	2.2 Core–shell structure design	(1) The heterogeneous interface can effectively block the diffusion bridging between YSZ lamellae, inhibit the mass transfer process, and greatly improve the sintering resistance of the coating.	(1) It is extremely difficult to achieve uniform encapsulation of the powder, the preparation process is complex, and the batch stability is difficult to control.
		(2) The core–shell structure can realize uniform encapsulation of the core phase and simultaneously optimize the mechanical properties and high-temperature stability of the coating.	(2) It is easy to introduce new failure mechanisms, such as interfacial thermal mismatch and elemental interdiffusion, which may induce catastrophic delamination failure of the coating in an ultra-high temperature environment above 1300 °C.
	2.3 Pore structure design	(1) The micropore and stress concentration structure can effectively reduce the thermal conductivity of the coating and improve the thermal barrier effect.	(1) Large-scale pores and high porosity will significantly reduce the mechanical properties of the coating and easily induce coating cracking and spalling.
		(2) It can release the internal stress during the sintering process through pore structure design, delaying the sintering densification process of the coating.	(2) It is extremely difficult to precisely control the size, distribution and porosity of the pore structure, which has extremely high requirements for the preparation process.
	2.4 Bilayer structure design	(1) It can achieve an excellent thermal barrier effect and reduce the heat conduction efficiency of the coating through the low thermal conductivity design of the top layer.	(1) The bilayer structure is prone to interfacial stress concentration, and the difference in sintering behavior between the upper and lower layers easily induces delamination failure of the coating.
		(2) It can ensure the mechanical properties and interfacial bonding stability of the coating through the high fracture toughness and thermal expansion matching design of the bottom layer.	(2) It is difficult to match the thermal expansion coefficients of the two layers of materials, and interfacial cracking easily occurs during high-temperature service
	2.5 Multilayered/graded structure design	(1) The multilayer structure can realize independent design of parameters, such as porosity and composition of each layer, and optimize the stress gradient in the coating at the same time.	(1) The preparation process of the multilayer/multi-interface structure is complex, the preparation cycle is long, and the cost is significantly increased.
		(2) The graded structure can realize continuous compositional gradient and multifunctional integration, construct a synergistic anti-sintering mechanism, and greatly improve the high-temperature service performance of the coating.	(2) During high-temperature service, the redistribution of elements easily causes severe degradation of the gradient structure, and a structure without a clear interface will greatly increase the difficulty of characterization.
	2.6 “Brick–mud” structure design	(1) In the novel “brick–mud” structure, the “mud” layer can undergo preferential viscoplastic deformation to effectively release the internal stress induced by sintering.	(1) The preparation process of a “brick–mud” structured coating is complex, with extremely high requirements for the control of the preparation process and poor batch stability.
		(2) It can significantly delay the sintering densification process of the coating, and greatly improve the sintering resistance and high-temperature structural stability of the coating.	(2) It is difficult to control the interfacial bonding strength of the two phases in the structure, and interfacial debonding and cracking easily occur during high-temperature service.

, highlighting an urgent demand for innovative anti-sintering design strategies for thermal protective coatings.

4. Outlook

In 2025, during thermal cycling tests of SiC-based abradable environmental barrier coatings (1.5 h isothermal holding at 1350 °C, followed by 0.5 h cooling to room temperature), this paper’s authors observed distinctive “large-scale chain-like pores” within the Si coating after failure, as shown in Figure 2e.

Figure 2a shows the as-sprayed abradable environmental barrier coating. The Si layer acts as a bond coating to mitigate thermal expansion mismatch, exhibiting a dense structure with minimal porosity. After 20 thermal cycles, Figure 2b reveals the emergence of finely dispersed micropores within the Si layer, at the Si/SiC substrate interface, and at the Si/EBC interface. Upon increasing thermal cycles to 40, Figure 2e shows significant growth and coalescence of these pores, evolving into macroscopic, chained cavities. Combined analysis of Figure 2c,d indicates that under thermally induced fatigue stresses and TGO growth-induced stresses (which intensify with increasing TGO thickness), severe cracking occurred not only within the abradable coating and EBC but also prominently within the Si layer and along its interfaces with the SiC substrate and EBC. Oxygen permeates not only through cracks in the abradable ceramic layer and EBC but also diffuses along cracks within the Si layer and interfacial regions. This promotes a direct in situ reaction between oxygen and the Si layer, accelerating oxidation of the bond coating. Consequently, during thermal cycling, the bond coating consists of a mixture of unoxidized Si and the TGO.

The primary reaction between Si and O₂ at 1350 °C formed SiO₂ [52,53]. Upon cooling to approximately 220 °C, this SiO₂ typically crystallized as β-cristobalite (high-temperature phase). Subsequent cooling below 220 °C triggered a phase transformation from β-cristobalite to α-cristobalite (low-temperature phase), accompanied by a well-documented volumetric contraction of approximately 5% [52,54,55,56,57,58]. This contraction may generate initial pores [59,60]. During reheating, the reverse transformation (α- to β-cristobalite) induced a volumetric expansion of also about 5%. However, this expansion does not fully compensate for the prior contraction for two main reasons. First, the 5% expansion occurs abruptly at the phase transformation temperature (approximately 220 °C), whereas the thermal expansion coefficient of cristobalite is about 10⁻⁵ K⁻¹ [61], leading to a continuous dimensional change of less than 1% over the temperature range below 220 °C. The stark mismatch between the abrupt 5% transformative expansion and the gradual thermal contraction upon cooling makes perfect compensation impossible. Second, in ceramic sintering theory, gases trapped within closed pores generate an internal counter-pressure that opposes the surface tension-driven driving force for pore shrinkage. Consequently, during the expansion half-cycle, this internal gas pressure prevents complete pore closure, leading to net porosity accumulation under thermal cycling. This gas entrapment effect is fundamentally recognized in sintering kinetics [62,63]. Thus, due to trapped gases [62,63], initial pores [59,60], and anisotropic expansion [64], the expansion failed to fully heal defects and could even enlarge them. Ultimately, repeated thermal cycling leads to pore accumulation and interconnection, forming the macroscopic chained cavities observed in Figure 2e.

The reversible β ⇄ α cristobalite phase transformation in the Si layer, with its significant cyclic contraction/expansion, drives the nucleation, growth, and accumulation of pores. This mechanism not only offers a novel pore-engineering strategy for porous ceramic coatings but also inspires innovative design approaches to counteract high-temperature sintering-induced densification in ceramic coatings. Based on this analysis, the authors suggest that the following research directions merit further investigation:

(1) In situ active pore-forming technology. Thermal cycling between high and low temperatures can trigger reversible phase transformations in ceramic systems, accompanied by obvious volume contraction or expansion. Typical systems include: zirconia (ZrO₂): t-ZrO₂ → m-ZrO₂ with 3%–5% volume expansion; cristobalite (SiO₂): β-cristobalite → α-cristobalite with ~5% volume contraction; and alumina (Al₂O₃): γ-Al₂O₃ → α-Al₂O₃ with ~15% volume contraction. Volume contraction directly generates primary pores, while volume expansion cannot fully heal defects due to pre-existing pores, anisotropic volume change, and trapped gas. Repeated thermal cycling thus promotes continuous pore formation and accumulation. However, long-term phase stability, cyclic phase reversibility, and elemental interdiffusion between phase-change components and the matrix must be carefully addressed to avoid unexpected structural degradation.

(2) Composite pore-control technology. During thermal cycling, reversible phase transformations induce periodic volume changes, resulting in in situ dynamic pore formation. Although accumulated pores can relieve thermal stress and phase-transformation stress, the released strain energy may promote grain boundary migration, accelerate pore coalescence and grain growth, and eventually reduce porosity, which is detrimental to sintering resistance. Therefore, suppressing pore coarsening and grain growth is essential, such as introducing nanoparticles for grain boundary pinning or constructing nano-bimodal structures. On this basis, composite pore-control technology that integrates “in situ pore formation” and “pore stabilization” can be developed. Nevertheless, thermal expansion mismatch between multiscale components and increased complexity of microstructure control remain key issues that must be resolved.

(3) Feasibility validation and technical challenges in practical coating systems. It should be noted that the phenomenon of “cyclic phase transformation-induced pore formation” reported in this study has only been verified so far between β-cristobalite and α-cristobalite in a silicon bond coating and has not yet been experimentally investigated in typical thermal/environmental barrier coating ceramic topcoat materials such as YSZ, zirconates, or ytterbium silicates. Therefore, the current results cannot be directly extrapolated to these ceramic coating systems. This study aims to report this new phenomenon and preliminarily explore its mechanism, providing a possible new reference direction for anti-sintering methods rather than drawing definitive and universal conclusions. To validate the feasibility of the “active pore generation via reversible phase transformation” concept in ceramic coatings, future work should systematically investigate: (i) the feasibility of the “active pore generation via reversible phase transformation” concept in ceramic coatings by focusing on selecting typical ceramic layer materials, such as YSZ, La₂Zr₂O₇, and Yb₂Si₂O₇, introducing cyclic phase transformation components through compositional tuning or doping, designing thermal cycling experiments to characterize pore evolution and anti-sintering performance, and conducting comparative validation to determine whether this mechanism holds true in different material systems; (ii) how to introduce phase-change components into target ceramic matrices without severely degrading the coefficient of thermal expansion matching, fracture toughness, and thermal cycling life and accelerating faster oxygen/CMAS penetration; (iii) the effect of phase-change cycles on overall coating integrity; and (iv) the optimization of phase-change particle size, distribution, and volume fraction to balance pore-generation efficiency and mechanical properties.

5. Conclusions

(1) Based on the sintering densification mechanism of ceramic materials, researchers are currently focusing on anti-sintering research primarily from two aspects: optimization of ceramic material systems and multidimensional design of coating structures. In terms of material system optimization, the main representative technologies are rare earth oxide doping and high-entropy design. Rare earth oxide doping primarily suppresses diffusion through mechanisms such as lattice distortion, oxygen vacancy regulation, and grain boundary pinning. However, rare earth oxides are expensive and reduce fracture toughness and thermal cycling performance. High-entropy design mainly delays densification through the sluggish diffusion effect, but the preparation process of high-entropy materials is complex and extremely challenging. Moreover, in long-term high-temperature environments, the kinetic imbalance in multi-element diffusion can lead to compositional segregation while also facing the issue of high raw material costs.

(2) By regulating structural features such as porosity, pore morphology, and crack networks, the mass transport rate and sintering driving force can be reduced, thereby delaying densification. Among these approaches, the nano-bimodal structure utilizes differential sintering between unmelted nano-zones and fully melted lamellar zones to generate a “self-compensation effect” but suffers from low bonding strength and poor corrosion resistance. The core–shell structure blocks mass transport paths through a diffusion barrier, yet it is difficult to control coating uniformity and interfacial failure. The porous structure introduces stable micron-scale pores, albeit at the cost of reduced mechanical properties. Lamellar structures (e.g., gradient and brick-and-mortar biomimetic designs) enhance sintering resistance by releasing sintering stresses but face challenges such as interfacial stress concentration and complex processing. These multidimensional design strategies provide important routes for anti-sintering, though performance trade-offs and preparation difficulties remain to be resolved.

(3) Most material system optimizations and structural optimizations belong to traditional “passive pore-retaining” anti-sintering methods, which inevitably suffer from irreversible degradation of anti-sintering function with prolonged high-temperature exposure. In contrast, based on the theory of cyclic solid-state phase transformation in polymorphic ceramics, this study proposes a novel anti-sintering method with the potential for “active pore generation”. The fundamental principle of this method is that polymorphic ceramic particles undergo reversible phase transformation under high- and low-temperature cyclic impact (consistent with actual service conditions, corresponding to multiple start-up and shut-down cycles of aero-engines), accompanied by volume expansion/contraction effects, which continuously induce the initiation and growth of pores, thereby achieving an in situ active and continuous “pore-generation” function for thermal protection coatings. This provides a new reference direction for solving the failure problems caused by sintering densification of thermal protection coatings under high-temperature/ultra-high-temperature service environments.

(4) Overall, current anti-sintering technologies for ceramic thermal protective coatings have made remarkable progress in laboratory research, but they still face prominent bottlenecks in engineering applications related to aero-engines and gas turbines. Key unresolved challenges include long-term phase stability under ultra-high temperatures, thermal expansion matching between multi-component systems, trade-offs between thermal insulation and mechanical reliability, resistance to CMAS corrosion and oxygen penetration, and controllable and repeatable preparation processes. For engine-relevant coating systems, future developments must fully consider practical requirements, such as high-temperature durability, thermal cycle stability, economic feasibility, and engineering process adaptability, so as to promote the real-world application of high-performance anti-sintering thermal protection coatings.