High-Temperature Oxidation-Resistant Composite Coatings for Extreme Environments: Material Systems, Design Strategies, Preparation Technologies, Performance Characterizations, and Research Challenges

Abstract

With the development of aerospace, energy and power, nuclear energy, and chemical industries, hot-end components of various key equipment are gradually facing more severe high-temperature challenges. The high-temperature oxidation failure of key thermal structural materials in hot-end components has become a critical bottleneck restricting their service life. Consequently, there is an urgent need for oxidation protection of these components. Oxidation-resistant composite coatings are widely recognized as one of the most effective approaches to mitigating high-temperature oxidation. This review initially outlines the characteristics and anti-oxidation mechanisms of various coating materials, followed by an in-depth examination of the impact of structural modifications such as multi-layer/gradient design, diffusion barriers, and self-healing structures on the anti-oxidation efficacy of coatings. Furthermore, it discusses the fundamental principles and key features of advanced coating fabrication techniques, as well as summarizes the methods for characterizing the performance of anti-oxidation composite coatings under real operating conditions. Lastly, the review analyzes the current limitations and challenges facing anti-oxidation coatings in practical applications and provides insights into future development prospects.

1. Introduction

Currently, the aerospace, energy and power, nuclear energy, and chemical engineering industries, as well as other related fields, are developing rapidly. Against this backdrop, the hot-end components of various high-temperature equipments are confronting increasingly severe high-temperature challenges. Such equipment includes aerospace vehicles, aero-engines, gas turbines, industrial high-temperature pipelines, and nuclear fuel claddings [1,2]. The performance of hot-end components is a key factor of the service capability of high-temperature operating equipment [3]. Nickel-based superalloys, titanium alloys, refractory metals, ceramic composites, and carbon-based composites, among others, have become irreplaceable thermal structural materials in hot-end components due to their excellent mechanical properties and high-temperature stability. However, these materials all face a common problem: high-temperature oxidation, as shown in Figure 1a. For instance, when the temperature of nickel-based superalloys exceeds 1000 °C, the alloys undergo rapid oxidation, leading to severe degradation of mechanical properties and thus significantly shortening their service life [4]; titanium alloys, which have a strong ability to absorb oxygen, tend to undergo surface oxidation at high temperatures, and oxygen can further diffuse inward, which causes the alloy to become brittle and reduces its strength [5]; carbon-based composites undergo oxidation at temperatures above 400 °C, which affects their long-term use [6,7]. It is evident that the problem of high-temperature oxidation of hot-end components seriously impacts the service performance of equipment in multiple fields such as aerospace, energy, and chemical engineering. Therefore, establishing an effective thermal protection system is an urgent priority [8].

Applying an oxidation-resistant composite coating on the surface of hot-end components is currently one of the most effective approaches. This method can significantly alleviate the high-temperature oxidation of thermal structural materials. Additionally, it helps improve the service performance of key equipment such as aero-engines and gas turbines. Oxidation-resistant coatings typically need to have the following characteristics: (1) high melting point, (2) good oxidation resistance and low oxygen permeability, (3) stable mechanical properties at high temperatures, (4) thermal expansion coefficient matching the substrate, (5) high bonding strength with the substrate. Based on these requirements, many materials based on metals, ceramics, or metal–ceramic composites are potential candidates for preparing oxidation-resistant coatings [9]. However, several key issues restrict the long-term protective performance of coatings on hot-end components. For instance, in metal materials, pure MoSi₂ silicide coatings undergo spontaneous powdering and cracking (“Pesting” failure) within the temperature range of 400–750 °C. For MCrAlY coatings, severe element diffusion occurs between the coating and the substrate. Furthermore, in ceramic materials, boride coatings generate a B₂O₃ glass phase during high-temperature oxidation, which volatilizes violently above 1000 °C. Moreover, ceramic coatings inherently exhibit high brittleness, further limiting their practical application [10,11,12,13]. Therefore, developing composite coatings with better oxidation resistance through rational material system design, coating structure regulation, and advanced preparation technologies is a current research hotspot.

This paper focuses on oxidation-resistant composite coatings for extreme high-temperature applications. It mainly summarizes various coating material systems, structural regulation methods, advanced coating preparation technologies, and coating system characterization methods in recent years. Additionally, it puts forward the current challenges faced by high-temperature oxidation-resistant coatings and their future development directions.

2. Oxidation-Resistant Composite Coating Systems

High-temperature oxidation-resistant composite coatings are important barriers ensuring long-term use of thermal structural materials. They evolved from single aluminide coatings and have developed into a series of coatings. Currently, high-temperature oxidation-resistant composite coatings are mainly divided into aluminide-based composite coatings, silicide-based composite coatings, MCrAlY-based composite coatings, ceramic-based composite coatings, enamel composite coatings, and inorganic paint composite coatings. Aluminide-based composite coatings can quickly form protective Al₂O₃ ceramic film under high-temperature conditions, but the coating is easy to fall off when facing thermal shock. Silicide-based composite coatings rely on SiO₂ glass phases for oxygen blocking and self-healing, but heavy Si consumption during ultra-high-temperature long-term service will shorten the service life of the coatings. MCrAlY-based composite coatings have good compatibility with superalloys. But when used at high temperatures for a long time, elements between the coating and the base material mix together, leading to thickened oxide layers and coating spallation. Ceramic-based coatings, with dense structures and excellent high-temperature stability, show great potential in ultra-high-temperature oxidation resistance but are limited by poor toughness [14,15,16,17,18]. Enamel composite coatings rely on dense glass phases formed by multi-component oxidation for oxygen blocking, with simple preparation but insufficient high-temperature oxidation resistance. Inorganic paint composite coatings, composed of refractory fillers and binders, are economical and convenient but require further research for high-temperature applications. The following will explain the characteristics and oxidation resistance mechanisms of various anti-oxidation coating materials, with a brief summary of different composite coatings in

Table 1. Comparison of different types of antioxidant composite coatings.

Types of Coatings	Representative Material	Oxygen Barrier Phase
Aluminide-Based Composite Coatings	TiAl, Ni3Al, (Ni,Pt)Al	Al2O3
Silicide-Based Composite Coatings	MoSi2, NbSi2, TaSi2	SiO2
MCrAlY-Based Composite Coatings	NiCrAlY, NiCoCrAlY	Al2O3
Boride-Based Composite Coatings	ZrB2, HfB2, MAB	B2O3, ZrO2, Al2O3
Carbide-Based Composite Coatings	SiC, ZrC, HfC, TaC	SiO2
Nitride-Based Composite Coatings	TiAlN, TiAlSiN, Si3N4	Al2O3, SiO2
Oxide Ceramic Composite Coatings	Al2O3, ZrO2, HfO2	Dense oxide layer
MAX Phase Composite Coatings	Ti2AlC, Cr2AlC	Al2O3, Cr2O3
Enamel Composite Coatings	silicate glass and multi-phase composite glass	Dense glassy phase
Inorganic Paint Composite Coatings	Al, SiC	Al2O3, SiO2

.

2.1. Metal-Based Composite Coatings

2.1.1. Aluminide-Based Composite Coatings

Aluminide-based composite coatings are thermally diffused composite systems formed by chemical reactions with the substrate. Due to aluminum’s low melting point, high thermal expansion coefficient, and high reactivity, aluminide-based composite coatings exhibit oxidation resistance by forming dense Al₂O₃ ceramic films on the surface through reaction of high-activity Al with oxygen at high temperatures [19]. Currently, the core matrices of widely studied aluminide-based composite coatings include TiAl, Ni₃Al, Fe₃Al, NiAl, and FeAl, among which Ni-Al- and Ti-Al-based composite systems exhibit superior high-temperature oxidation resistance, capable of protecting substrates at temperatures above 1000 °C. However, aluminide coatings have high brittleness, easily generating defects or even spallation during long-term high-temperature service. Additionally, their different compositions from the substrate causes element mixing, speeding up aluminum loss [20]. These limitations are be effectively addressed through the design of aluminide-based composite coatings. The fabrication of aluminide-based composite coatings is commonly realized via elemental doping (e.g., Pt, Si, Cr, Co, Hf, Y) [21,22,23], which can not only reduce the critical Al content required for selective oxidation but also enhance the adhesion of the Al₂O₃ film, thereby prolonging the service life of the coatings. For example, Yang et al. [24] effectively improved the oxidation resistance of aluminide coatings by adding an amount of element Y. They prepared Y-modified aluminide-based composite coatings on Ni-based superalloy via chemical vapor deposition (CVD). After 100 h cyclic oxidation at 1100 °C, the mass gain of single aluminide coatings was 0.55 mg/cm², while that of Y-modified ones was 0.3 mg/cm², representing a roughly 45% improvement in oxidation resistance. Huang et al. [25] prepared Ti(Al,Si)₃ coatings on TiAl alloys by cold spraying. After 1000 h isothermal oxidation at 950 °C, the coating gradually transformed into a layered structure: oxide layer, Ti(Al,Si)₃ layer with reticular Ti₅Si₃, continuous Ti₅Si₃ layer, and TiAl₂ layer. During long-term oxidation, the thickness and morphology of the continuous Ti₅Si₃ diffusion barrier remained almost unchanged, effectively inhibiting inward Al diffusion and ensuring long-term oxidation resistance. Among various modified aluminide coatings, Pt has proven the most effective. The addition of Pt can not only reduce the amount of Al needed for selective growth and help form Al₂O₃, but also improve the adhesion between Al₂O₃ film and coating. Angenete et al. [26] prepared a Pt-modified Ni-based superalloy via CVD and Pt plating. Long-term high-temperature oxidation tests at 1050 °C showed that Pt-modified ones only started to peel off over 10% of the surface after 10,000 h, significantly outperforming regular aluminide coatings which spalled after 1000 h, demonstrating dramatically improved oxidation resistance. Li et al. [27] studied the cyclic oxidation performance of Hf, Y-doped (Ni,Pt) Al coatings at 1150 °C. As shown in Figure 2i, after 500 cycles, the mass gains of normal, Y-modified, Hf-modified, and Hf-Y co-modified aluminide coatings were 0.89, 0.75, 0.58, and 0.50 mg/cm², respectively. It is clear that adding just Hf or Y made the coating gain less weight. Notably, adding both Hf and Y exhibited the most promising potential in slowing the rate of scale growth. Figure 2j shows XRD patterns of the four coatings after 500 cyclic oxidations at 1150 °C, indicating that all formed pure α-Al₂O₃, suggesting good oxidation resistance for both the regular (Ni,Pt)Al coatings and the ones with Y or Hf added. Additionally, Figure 2a–h show surface and cross-sectional morphologies of the four aluminide coatings after 500 oxidation cycles. The (Ni,Pt) Al coating surface exhibited numerous cracks and even slight oxide spallation at the edges. Y-doped and Hf-doped (Ni,Pt) Al coatings also showed varying degrees of defects, but the oxide scale formed on the (Ni,Pt) Al coating with both Hf and Y added was completely intact, with no damage at all, exhibiting the best oxygen-blocking performance, which explains why it gained the least mass.

In conclusion, high Al content endows aluminide-based composite coatings with excellent high-temperature oxidation and corrosion resistance. Ni-Al and Ti-Al system coatings show better high-temperature oxygen-blocking ability than Fe-Al systems. Synergistic modification with various elements has significantly improved the oxidation resistance of aluminide-based composite coatings. However, issues like rapid Al consumption at high temperatures and coating spallation under thermal shock limit their application in complex working conditions.

2.1.2. Silicide-Based Composite Coatings

In high-temperature oxidizing environments, silicides form a continuous, dense, and fluid SiO₂ glass phase on their surface. This glass phase effectively blocks oxygen from diffusing into the substrate. Additionally, when thermal stress-induced cracks form at high temperatures, the SiO₂ glass phase can automatically flow into these cracks under capillary force, repairing the coating [28,29]. Thus, silicide-based composite coatings are promising high-temperature oxidation-resistant coatings. Silicide-based composite coatings are mainly constructed with refractory metal silicides such as MoSi₂, NbSi₂, and TaSi₂ as the matrix, combined with functional additives. Among them, MoSi₂ has high melting point, moderate density, good thermal stability, excellent oxidation and corrosion resistance, and effective protection over a wide temperature range, showing great potential in silicide-based composite coatings. But MoSi₂ suffers from “pesting” at 400–600 °C, affecting oxidation resistance. During long-term high-temperature service, Si volatilizes as SiO₂, damaging coating’s integrity and service life [30,31,32,33,34]. Therefore, extensive research has been performed to enhance MoSi₂ coating oxidation resistance and extend service life. Zhu et al. [35] designed MoSi₂ and MoSi₂-bentonite oxidation-resistant coatings on Mo-based alloys by spark plasma discharge. Isothermal oxidation tests at 1500 °C for 300 h exhibited final mass gains of 1.14 mg/cm² and 0.49 mg/cm² for MoSi₂ and MoSi₂-bentonite coatings, respectively, with the oxidation rate of the bentonite-containing composite coating much lower than that of the MoSi₂ coating, significantly improving high-temperature oxidation resistance. Tao et al. [36] created Si-MoSi₂ composite coating on Mo substrate and studied its isothermal oxidation at 1200 °C. After 20 h exposure at 1200 °C, the coating mass gain was only 0.183 mg/cm², and the oxide layer thickness was 6.72 μm. Heavy Si doping resulted in a smooth, compact surface structure, ensuring excellent oxidation resistance. Wang et al. [37] fabricated Y₂O₃-modified MoSi₂-based coatings with different Y₂O₃ contents on C/C substrates via plasma spraying and studied their microstructure, oxidation behavior, and protection mechanism in air at 1500 °C. Experimental results demonstrated that among all Y₂O₃-modified MoSi₂ coatings, the composite coating with 20 wt% Y₂O₃ exhibited the best oxidation resistance, with a weight loss rate of only 1.92% after 100 h oxidation at 1500 °C, a significant improvement compared to the unmodified silicide coating, which showed an oxidation weight loss rate of 8.98% after 88 h. The reason for this is that the appropriate amount of Y₂O₃ provided yttrium ions, which strengthened the Si-O bonds in the SiO₂ network, increasing the density and viscosity of the Si-O-Y multicomponent glass layer, thus effectively blocking the inward spread of active oxygen. Zhai et al. [38] prepared MoSi₂-MoB bilayer coating on Mo substrate; after 15 h at 1300 °C under isothermal oxidation conditions, a weight loss of 6.382 mg/cm² was observed for the coating, exhibiting excellent oxygen-blocking ability due to the formation of a dense, continuous SiO₂ layer during oxidation. Ou et al. [39] prepared MoSi₂-HfO₂ coating on niobium alloy. The coating with 20 wt% HfO₂ doping (M20H) exhibited the best oxidation resistance, with a service life of up to 200 h at 1700 °C. Figure 3 shows surface, cross-sectional images, and element distribution maps of the M20H coating after 50 h, 100 h, and 200 h oxidation. As seen in Figure 3a–c, the thickness of the MoSi₂-rich layer gradually decreased with increasing oxidation time, and numerous voids appeared after 100 h and 200 h, indicating significant Si consumption, but the coating did not fail, demonstrating that HfO₂ doping significantly enhanced the high-temperature oxidation resistance of the MoSi₂ coating. Zhang et al. [40] fabricated MoSi₂ coatings with different mullite addition amounts on silicided niobium alloys. The addition of an appropriate amount of mullite can inhibit the crystallization of SiO₂ during the oxidation process of the coating, obtain crack-free oxide scales, and prevent the inward diffusion of oxygen. In the team’s experiments, the coating with 10 wt% mullite had excellent oxidation resistance. After 140 h at 1500 °C, the coating exhibited minimal mass loss of 4.06 mg/cm². In addition, the team [41] also prepared WSi₂-mullite-MoSi₂ coatings on silicided Nb alloys by atmospheric plasma spraying, which demonstrated a mass loss of only 4.41 mg/cm² after oxidation at 1500 °C for 500 h. Moreover, Zhu et al. [42] utilized atmospheric plasma spraying to prepare a ZrB₂-MoSi₂ coating on tantalum via 20 h isothermal oxidation treatment at 1750 °C, which produced a mass gain of 1.75 mg/cm² in the coating, exhibiting significant oxidation resistance.

TaSi₂ and NbSi₂ are widely used as core matrix materials for silicide-based composite coatings, favored for their excellent high-temperature oxidation resistance in extreme environments [43,44]. TaSi₂ mainly blocks oxygen from getting into the substrate by forming protective layers of low-viscosity Ta₂O₅ and dense SiO₂. However, single TaSi₂ is hard and brittle, and cracks formed in the coating at high temperatures are difficult to heal, so it is often used in combination with other thermal protection materials. NbSi₂ has low density, good thermal stability, and a suitable thermal expansion coefficient. To achieve better high-temperature oxidation resistance, NbSi₂ coatings are also often used as composite coatings [45,46]. For example, Tao et al. [47] created Si-NbSi₂ composite coating with high Si content in niobium alloy, which demonstrated a mass gain per unit area of 45.68 mg/cm² after 8 h exposure at 1200 °C. Wang et al. [48] constructed a NbSi₂/Nb₂O₅-SiO₂/SiC silicide-based composite coating on niobium alloy by incorporating nano-SiC particles, which displayed a mass gain of 2.49 mg/cm² after 50 h oxidation at 1250 °C, significantly lower than the 6.49 mg/cm² of the single-layer NbSi₂ coating, for reasons detailed in Figure 4. As oxidation time increases, SiO₂ gradually turns into a nanocrystalline or amorphous state, which is beneficial for improving the high-temperature oxidation resistance of the coating. Additionally, SiC provides an additional Si source to form a self-healing film, and the SiO₂ phase keeps the oxide layer thickness within a stable range, avoiding oxide layer spallation and cracking caused by high growth stress. Ni et al. [49] applied low-pressure plasma spraying to prepare a TaSi₂/MoSi₂ bilayer coating on graphite substrate. After 30 min of high-temperature oxidation in static air at 1650 °C, the coating structure remained intact without macro-cracks, with a uniform and dense microstructure. A large amount of dense glassy SiO₂ on the coating surface provided good protection for the substrate. Liu [25] invented a multi-layer SiB₆-TaSi₂-MoSi₂ coating. After 200 h of isothermal oxidation at 1500 °C, the coating’s mass gain was about 1.41%, and its oxygen permeability was only 1.09%, exhibiting excellent oxidation resistance.

In summary, silicide-based composite coatings can generate SiO₂ glass phases with good fluidity under high-temperature oxidizing conditions, which not only block oxygen, but also give the coating with certain self-healing ability, significantly extending the service life of the coating at high temperatures. In the field of high-temperature oxidation resistance, MoSi₂ shows the most application potential, but the oxidation resistance of silicide-based composite coatings at temperatures above 1700 °C and their long-term service performance over a wide temperature range need further research and improvement.

2.1.3. MCrAlY-Based Composite Coatings

MCrAlY-based composite coatings are high-performance, high-temperature, oxidation-resistant composite systems, where M denotes Ni, Cr, or a combination of Ni and Cr. Al is the main element responsible for high-temperature oxidation resistance in MCrAlY-based composite coatings; it can form a continuous, dense Al₂O₃ protective film when oxidized at high temperatures. Cr reduces the critical Al content required for forming a continuous Al₂O₃ film and improves the coating’s resistance to hot corrosion. Y significantly enhances the spallation resistance of the oxide film [50,51].

The composition of MCrAlY-based composite coatings can be flexibly adjusted according to specific service environments and substrate alloy compositions, with NiCrAlY and NiCoCrAlY as the most widely used composite systems [52]. They are commonly applied as overlay coatings for hot-end components of gas turbines and aero-engines or bond coats for thermal barrier coatings, thanks to their excellent compatibility with Ni-based superalloys and synergistic oxidation/corrosion resistance. However, when MCrAlY coatings are exposed to environments above 1000 °C for a long time, elements such as Al and Cr in the coating interdiffuse with the substrate, forming destructive substances, affecting the interfacial bonding between the substrate and the coating, and leading to rapid coating failure [14,53]. Many researchers have made efforts to improve coating performance through element doping, structural regulation, and preparation technology optimization [54,55,56]. For example, Duan et al. [57] added Hf and Ta to the NiCoCrAlY coating. The composite coating sample was tested with cyclic oxidation tests in air at 1050 °C and failed after 2664 h, significantly extending the service life compared to pure NiCoCrAlY, which failed after 240 h. Wang et al. [58] fabricated an ODS AlCoCrFeNi2.1 composite coating on Ni-based superalloy via laser cladding. After prolonged oxidation at 1000 °C and 1100 °C, the composite coating demonstrated superior oxidation resistance compared with conventional NiCoCrAlY coatings. As revealed by TEM results (Figure 5), this enhancement is primarily attributed to the formation of Y₂Hf₂O₇ nanoparticles during oxidation, which effectively reduced the growth rate of the thermally grown oxide (TGO), alleviated the thermal expansion mismatch stress, and improved the toughness of the coating/TGO interface. Li et al. [59] combined arc ion plating and vacuum annealing to create MCrAlY-AlSiY composite coatings with different thicknesses. C1 was a 10 μm AlSiY layer, C2 consisted of a 20 μm NiCrAlYSi layer and a 10 μm AlSiY layer, and C3 was composed of a 30 μm NiCrAlYSi layer and a 10 μm AlSiY layer. The isothermal oxidation behavior of the coatings at 1100 °C for 300 h was studied (as shown in Figure 6). The results showed that the C2 coating had the lowest oxidation rate with a total mass gain of only 0.73 mg/cm², exhibiting better high-temperature oxidation resistance. Kang et al. [60] invented gradient NiCrAlYSi coating, whose gradient design effectively extended the service life at 1200 °C.

In summary, MCrAlY-based composite coatings are mainly used for high-temperature oxidation protection of Ni-based superalloy components due to their good thermal matching with Ni-based superalloys. Adding small amounts of alloying elements to MCrAlY-based composite coatings primarily enhances the adhesion of the protective oxide film formed on the coating surface through the synergistic effect between the doping elements and the MCrAlY matrix, thereby significantly extending the high-temperature service life of the composite system. Nanocrystalline MCrAlY-based composite coatings, due to increased grain boundary density and reduced lattice diffusion distance, promote the formation of protective oxide scales during high-temperature exposure. Preparation technology optimization mainly improves coating oxidation resistance and service life by reducing defects such as microcracks and pores in the coating and preparing multi-layer/gradient coatings. However, when MCrAlY-based composite coatings are used in oxidizing environments above 1100 °C for a long time, stress tends to build up at the interface with the substrate, and once it exceeds the bonding strength limit, microcracks form, eventually leading to coating spallation. Therefore, slowing down the consumption of Al and Cr, which have oxygen-blocking effects, and the thickening of oxide growth layers is key to extending the application of MCrAlY-based composite coatings in higher-temperature oxidation resistance fields.

2.2. Ceramic-Based Composite Coatings

Ceramic-based composite coatings have dense structures, excellent high-temperature stability, and superior oxidation resistance [61,62]. These characteristics make them suitable for thermal protection of hot-end components operating under high-temperature and ultra-high-temperature conditions. Additionally, they are highly promising as high-temperature oxidation-resistant coatings. Common ceramic protective coatings include inert oxides like Al₂O₃, ZrO₂, and YSZ. In recent years, borides, carbides, and nitrides of refractory metals (Hf, Zr, Ta, etc.) with high melting points, excellent high-temperature oxidation resistance, and ablation resistance and MAX phase ceramic coatings have also attracted much attention.

2.2.1. Boride-Based Composite Coatings

The oxidation resistance mechanism of boride-based composite coatings mainly involves forming a protective B₂O₃ glass phase in high-temperature oxidizing environments [63]. In the boride family, ZrB₂ and HfB₂ are the most promising oxidation-resistant ceramic materials. ZrB₂ is a material with low density, high strength, ablation resistance, wear resistance, and excellent medium-to-low temperature oxidation resistance. Both ZrO₂ and B₂O₃ formed after ZrB₂ oxidation can play an antioxidant role. Additionally, ZrO₂ formed after ZrB₂ oxidation can react with SiO₂ to form ZrSiO₄, which has an antioxidant effect at 1700 °C. Meanwhile, Zr compound particles distributed in the SiO₂ glass phase play a pinning role, improving coating stability. HfB₂ also has excellent high-temperature oxidation resistance, but its natural ability to resist oxidation is not enough. To better exert the oxidation resistance of boride materials, they are usually compounded with SiC, silicides, rare earth oxides, and other thermal protection materials to prepare composite coatings [64,65,66]. Ma et al. [67] prepared Y₂O₃-modified ZrB₂-SiC composite coating on C/C composites via atmospheric plasma spraying. The coating with 10% Y₂O₃ lost 5.77% of its mass after isothermal oxidation at 1450 °C for 10 h, while the undoped Y₂O₃ coating lost 16.79%. The improved thermal protection performance was mainly due to the presence of Y₂O₃ inhibiting the phase transformation of ZrO₂, thereby avoiding cracks caused by coating volume expansion. Wang et al. [68] designed a ZrB₂-HfB₂-SiC-TaSi₂ composite coating on the C/C substrate by plasma spraying. TaSi₂ can form dense Zr-Ta-O and Hf-Ta-O oxides with Zr and Hf elements, effectively blocking oxygen. And the formation of a Ta-Si-O glass phase can effectively seal coating pores, preventing oxygen penetration. The coating exhibited excellent high-temperature oxidation resistance and ablation resistance. Duan et al. [69] fabricated a ZrB₂-SiC coating on a graphite substrate via slurry brushing. After 120 min oxidation at 1200 °C, the weight loss rate was −0.38%. Its excellent high-temperature oxidation resistance was attributed to the low oxygen permeability of the borosilicate glass layer formed by B₂O₃ and SiO₂, as well as the self-healing ability of molten B₂O₃ glass and borosilicate glass to automatically fill some pores and cracks in the coating. Li et al. [70] prepared multi-layer SiC/SiC-MoSi₂-ZrB₂ coating through a novel three-step method. The composite glass phase with good crack-sealing ability and the formation of thermally stable ZrSiO₄ enabled the coating to effectively protect the C/C substrate at 1500 °C for 305 h.

In addition, there are many studies on HfB₂-based composite coatings. For example, Zhang et al. [71] prepared a SiC-HfB₂-Si ternary oxidation-resistant coating on C/C composites via slurry spraying and vapor silicon infiltration. The coating could protect the substrate for more than 1507 h under isothermal oxidation at 1773 K, with linear and mass ablation rates of −0.72 μm/s and 0.07 mg/s, respectively, significantly improving the oxidation resistance of C/C composites. Ren et al. [61] developed a HfB₂-MoSi₂-SiC coating with controllable composition and thickness through in situ reaction and slurry sintering. Due to the formation of a highly stable Hf-Si-O glass layer, the sample exhibited minimal weight loss of 0.08% at 1500 °C for 200 h. Lv et al. [72] prepared HfB₂-SiC/SiC coating on C/C composite substrate through PC, slurry spraying, and partial reaction sintering. Due to the formation of a SiO₂/HfSiO₄/HfO₂ protective layer with extremely high thermal stability, it could protect the substrate in air at 1700 °C for 100 h, with only 2.4% mass loss. Zhu et al. [73] utilized slurry impregnation to produce HfB₂-SiC coatings with different HfB₂ contents on SiC-coated C/C composites. The HfB₂-SiC coating with 50 wt% HfB₂ could protect C/C composites from oxidation for 494 h at 900 °C in air, exhibiting excellent oxidation resistance, which was attributed to the formation of a highly thermally stable and dense borosilicate glass layer on the coating surface during high-temperature oxidation. Zhang et al. [74] studied a CeO₂-modified HfB₂-MoSi₂-SiC coating. Compared with the pure HfB₂-MoSi₂-SiC coating, the addition of CeO₂ to the Hf-B-Si-O system formed a stable Hf-Ce-B-Si-O complex phase glass, greatly improving the viscosity, stability, and self-healing densification performance of the glass layer and reducing the sample mass gain rate from 0.53% to 0.41% after 100 min isothermal oxidation at 1700 °C. Ding et al. [75] developed MoSi₂-modified HfB₂-SiC-Si/SiC-Si coating on Cf/C substrate through slurry impregnation and gaseous silicon infiltration. The pinning effect of HfO₂, formed MoB, and Hf, Mo co-doped silicon-based glass layer successfully prevented oxygen migration to the substrate, enabling the coating to protect Cf/C composites in air at 1700 °C for 276 h. Zhang et al. [76] prepared a HfB₂-MoSi₂-TaB₂ coating via SPS technology. Compared with the HfB₂-MoSi₂ coating system, the addition of an appropriate amount of TaB₂ reduced the oxidation mass gain by 78.56% at 900 °C and by 63.14% at 1700 °C, significantly enhancing the high-temperature oxidation resistance of the coating. Zhang et al. [77] utilized the LFT technology to produce high-quality HfB₂-SiC-ZrSi₂ oxidation-resistant coatings, which showed low oxygen permeability of only 0.28% during oxidation at 1700 °C, exhibiting excellent oxygen-blocking performance. Furthermore, MAB-based materials have also received a lot of attention in the field of oxidation-resistant coatings [78,79]. MAB phase materials are layered transition metal borides, where M represents a transition metal and A denotes aluminum [80,81]. These materials exhibit high thermal stability and excellent oxidation resistance. Commonly employed MAB phase systems primarily include Cr₂AlB₂, Cr₃AlB₄, and MoAlB. Their oxidation resistance mechanism primarily depends on the formation of continuous, dense protective films (such as Al₂O₃ and Cr₂O₃) at high temperatures [82]. Zhang et al. [83] prepared MoAlB composite coatings via magnetron sputtering. The presence of MoAlB nanocrystals promoted the selective formation of a dense α-Al₂O₃ scale through their oxygen-blocking effect, thereby imparting excellent oxidation resistance to the coatings. MAB materials show promise for high-temperature oxidation resistance. However, further exploration of different MAB phases and their high-temperature oxidation behavior in relation to their elemental and structural characteristics is still needed.

Boride-based ceramic coatings, through multi-phase composite and structural regulation, have not only significantly improved their high-temperature oxidation resistance, but also effectively enhanced the repair ability of self-healing glass phases under high-temperature oxidizing conditions, highlighting their application value in high-temperature oxidation resistance fields. However, issues such as rapid loss of glass phases during long-term high-temperature service and the ability of coatings to be applied over a wide temperature range require further research.

2.2.2. Carbide-Based Composite Coatings

Similarly to boride-based composite coatings, carbide-based composite coatings have also attracted extensive research attention in the field of high-temperature oxidation protection [84]. They are anchored on core matrix phases of high-melting-point carbides such as SiC, ZrC, HfC, and TaC, owing to their exceptional thermal stability and chemical inertness. The difference is that carbides have higher melting points and excellent high-temperature stability, showing great potential in ultra-high-temperature oxidation protection [85,86]. Among these carbides, SiC is widely used because it can form SiO₂ glass phases under high-temperature oxidizing conditions. However, single carbides are difficult to meet the long-term oxygen-blocking requirements at high temperatures. Therefore, in the actual design of high-temperature oxidation-resistant coatings, SiC is mainly compounded with other carbide ceramics or thermal protection materials, such as common systems like HfC-SiC, SiC/ZrB₂-SiC, ZrB₂-ZrC-SiC, and ZrC-SiC-TiC [87,88,89]. Additionally, Du et al. [90] prepared a HfC-TaC-B₄C-SiC/ZrSiO₄-glass composite coating on C/C composites through slurry sintering, which extended the oxidation resistance time by two to three times compared to traditional coatings. The long-term oxidation resistance of the coating was attributed to the fixing effect of ZrSiO₄ in the outer layer, the increase in Al₂O₃ content, and the formation of a dense Hf-Ta-Si-O/Si layer during oxidation. Zhu et al. [91] designed SiC/SiC-ZrSi₂ coating with a microporous structure on C/C substrate through simple filler cementation and slurry methods, as shown in Figure 7. This structured coating could oxidize at 1500 °C for 1194 h and only gained 1.42% in mass. This was mainly because the pores between SiC and ZrSi₂ powders increased the oxidizable area, generating a large amount of SiO₂ glass phase in the coating. The SiO₂ glass phase not only repaired cracks on the coating surface, but also embedded into the ceramic particles of the SiC-ZrSi₂ coating, forming a SiO₂-ceramic embedding structure with extremely low oxygen permeability, and effectively alleviated the CTE mismatch. Huang et al. [92] prepared a ZrB₂-ZrC-SiC high-temperature oxidation-resistant ceramic composite coating on C/C composites via laser cladding. The formation of high-temperature phases ZrO₂ and ZrSiO₄ effectively protected the C/C substrate at 1600 °C for 80 min. Liu et al. [93] employed liquid silicon infiltration to create a dense SiC-Si composite coating on graphite substrate. The network-structured Si promoted the development of SiO₂ phases with excellent oxygen-blocking ability, resulting in a mass gain of only 3.018% after 300 h isothermal oxidation at 1600 °C. Yang et al. [94] generated TiSi₂-Si-SiC/SiC bilayer coating on graphite via two-step filling and cementation technique. Small TiSi₂ particles were embedded in SiC particles. In high-temperature oxidizing environments, small TiSi₂ particles were oxidized firstly, reducing SiC consumption. Thus, after 200 h oxidation in static air at 1500 °C, the coating exhibited a mass gain of only 0.96%.

Overall, carbide composite coatings based on SiC exhibit excellent properties such as high melting point, excellent high-temperature oxidation resistance, and high-temperature strength of carbides. Additionally, they can utilize flowing glass phases to repair cracks, micropores, and other defects in the coating, achieving improved thermal stability of the coating. But carbide ceramic coatings have poor toughness, and further research is needed to address this mechanical performance deficiency.

2.2.3. Nitride-Based Composite Coatings

Nitride-based composite coatings have high hardness, thermal stability, excellent wear resistance, high oxidation resistance, and corrosion resistance, so they are also favored in the field of oxidation resistance. Materials such as TiAlN, TiAlSiN, and Si₃N₄ rely on Al and Si elements in the coating materials to form dense Al₂O₃ and SiO₂ layers in high-temperature oxidizing environments, blocking further oxygen infiltration into the substrate [95,96]. Fan et al. [97] deposited a TiAlN/CrAlON/CrAlO composite coating via cathodic arc evaporation. It exhibited good oxidation resistance at 950 °C because the outer CrAlON and CrAlO can effectively slow down the spread of oxidation. However, Si₃N₄ performs even better in higher temperature fields, not only for its excellent high-temperature stability, but also for its ability to rapidly form a continuous, dense glass layer in high-temperature oxidizing atmospheres. Therefore, many researchers have developed a series of high-temperature oxidation-resistant coatings based on Si₃N₄. For example, Wang et al. [98] prepared a Al-doped Ti-12Si₃N₄ coating on a Ti-6Al-4V alloy surface via laser cladding. Following 100 h of isothermal oxidation at 700 °C, the coating gained only 3.9% in mass, much lower than the 32.3% of the uncoated substrate. This design skillfully alleviated the compatibility between Si₃N₄ and the titanium alloy substrate and improved coating oxidation resistance. Du et al. [99] combined embedding and slurry brushing to prepare AO20/Si₃N₄ bilayer coating on C/C substrate, as shown in Figure 8. The porous Si₃N₄ coating could effectively alleviate thermal stress in the coating, resulting in a weight loss of only 0.32% after 36 thermal shocks at 1400 °C and a mass loss rate of 2.86% after 134 h oxidation at 1400 °C.

It is evident that nitride-based composite coatings hold considerable promise for high-temperature oxidation protection. Porous Si₃N₄ coatings have also shown great application potential in alleviating thermal stress between coatings and substrates. However, compared with boride-based and carbide-based composite coatings, nitride-based composite coatings exhibit insufficient protection ability in high-temperature oxidation resistance fields due to their lower melting points, requiring further in-depth research.

2.2.4. Oxide Ceramic Composite Coatings

Oxide ceramic composite coatings consist of one or more inert oxides that hardly react with the substrate [100,101]. They take advantage of the high melting point, good chemical inertness, and high-temperature stability of oxide ceramics to mechanically block oxygen diffusion into the substrate, improving the high-temperature oxidation resistance of the substrate. Common ones include Al₂O₃, ZrO₂, HfO₂, etc. To avoid large cracks between the coating and the substrate, reasonable coating composition design and appropriate coating preparation processes are required [102,103]. For instance, Zhou et al. [104] fabricated a continuous, dense 8YSZ-Al₂O₃ composite coating on GH3039 alloy surface by sol–gel. After 100 h isothermal oxidation at 1000 °C, the coating showed no obvious spallation with a mass gain of 0.50 mg/cm², only 14% of that of the substrate. It skillfully used YSZ as a transition coating to alleviate the negative impact of mismatched thermal expansion coefficients between the coating and the substrate. Mao et al. [105] prepared Yb₂O₃-Si coatings with different Yb₂O₃ contents through vacuum plasma spraying. The coating of 15 wt% Yb₂O₃ (S15Yb) offered the best high-temperature oxidation resistance, which can protect the substrate in oxidizing conditions at 1350 °C for 300 h, mainly attributed to the formation of a TGO layer composed of Yb₂Si₂O₇ and an mTGO layer during high-temperature oxidation of the coating (as shown in Figure 9). Both thick and uneven oxide layers showed good oxygen diffusion resistance, greatly protecting the substrate. Zhou et al. [106] employed liquid plasma-assisted particle deposition and sintering to prepare a HfSi₂-HfO₂-SiO₂ nanocomposite coating on a TiAl alloy. The introduction of SiO₂ formed a dense HfO₂-SiO₂ oxide layer, which could effectively delay crack initiation and block oxygen diffusion into the substrate, exhibiting good oxidation and ablation resistance.

Coating systems composed of a variety of inert oxide components using their respective melting points, thermal expansion coefficients, thermal stability and other characteristics, or oxide ceramic coating systems that rely on high temperature, as well as Si, SiO₂, and other components, to form dense layers all have their own shining points in the field of high temperature oxidation. Due to the inherent brittleness of oxide ceramic coatings and the difference in thermal expansion coefficients between high-temperature alloys and carbon-based composite substrates, the bonding strength between the coating and the substrate under greater thermal stress conditions requires close attention.

2.2.5. MAX Phase Composite Coatings

MAX phases are ternary carbides/nitrides or carbonitrides with a layered structure, with the general formula being M_n+1AX_n (where M is an early transition metal, A is an A-group element, X is C, N, or both, and n = 1–3). They combine the advantages of metals and ceramics, such as good electrical and thermal conductivity, good oxidation resistance, excellent thermal shock resistance, and processability. High-temperature oxidation-resistant composite coatings developed based on MAX phases have great potential in the field of oxidation protection for thermal structures [107,108,109,110,111]. Aluminide MAX phase coatings have been studied a lot in the field of oxidation resistance, mainly because Al atoms can easily diffuse to the coating surface and form a continuous, dense α-Al₂O₃ protective layer on the coating surface. Therefore, current research on the high-temperature oxidation resistance of MAX phase composite coatings mainly focuses on Cr-based Cr₂AlC and Ti-based Ti₂AlC coatings. When exposed to high-temperature air, Cr₂AlC coatings can form Cr₂O₃ and Al₂O₃ oxide layers, which exhibit excellent thermal and chemical stability, effectively preventing further oxidation of Cr₂AlC coatings and substrates by oxygen or other oxidizing media [112,113,114,115,116]. Zhu et al. [117] prepared two types of Ti-Al-N MAX phase composite coatings (T1 and TA4) on TC4 alloy surfaces using laser cladding and laser heat treatment, respectively. Under high-temperature oxidation conditions ranging from 600 °C to 1000 °C, the oxide scale thickness of the T1 and TA4 coatings was reduced to approximately 0.93–0.5 and 0.74–0.49 times that of the pure TC4 substrate, respectively, demonstrating the excellent oxidation resistance of the composite coatings. According to XPS analysis (as shown in Figure 10), the oxidation resistance of both coatings and the difference between them are mainly attributed to the aluminum content available for forming a dense Al₂O₃/TiO₂ composite protective layer. Mengis et al. [118] prepared Cr₂AlC composite coating on titanium alloy substrate by magnetron sputtering. Due to the formation of protective Al₂O₃ and Cr₂O₃ oxide films during oxidation, the coating displayed a mass gain per unit area of only 0.70 mg/cm² after 300 h isothermal oxidation at 800 °C. Wang et al. [119] designed a Sn-doped Ti₂(Al_0.6Sn_0.4)C oxidation-resistant composite coating on a TC4 substrate by magnetron sputtering. Following 60 h of isothermal oxidation at 800 °C, the coating exhibited a mass gain of 1.75 mg/cm². Doping Sn to replace part of Al enabled crack self-healing at 700 °C. Figure 11 shows the microstructure and main mechanism of the Ti₂(Al_0.6Sn_0.4)C oxidation-resistant coating after self-healing. It can be seen that the main self-healing mechanism of the coating is the filling of cracks with SnO₂ and TiO₂, and the filled oxides show a dense structure, while the interface between the coating and the oxides is not completely healed, possibly due to limited healing time.

MAX phase composite coatings based on Cr₂AlC and Ti₂AlC provide a novel approach for oxidation resistance of substrates at 1000 °C by their ability to rapidly form protective films and self-healing mechanisms. However, when the temperature exceeds 1000 °C or even higher, severe interdiffusion occurs between Al-containing MAX coatings and substrates, which can lead to the formation of brittle phases such as Cr₇C₃ and Cr₂Al, seriously affecting coating performance. Therefore, the diffusion mechanism of Al in coatings and service at higher temperatures are key areas for future research.

2.3. Enamel Composite Coatings

Currently, enamel-based composite coatings for high-temperature oxidation protection are mainly constructed with silicate glass coatings and multi-phase composite glass. By adjusting the oxide content, the thermal expansion coefficient of the coating can be matched to that of the substrate, further enhancing oxidation resistance [120]. For example, Yu et al. [121] prepared an oxidation-resistant enamel composite coating on the surface of Ti-6Al-4V alloy by slurry impregnation using SiO₂, BaO, kaolin, and quartz particles as raw materials. The addition of kaolin and quartz ceramic particles improved the stability of the alloy/Ti₅Si₃/Al₂O₃/enamel layered structure formed at the interface and increased the viscosity of the glass phase. It effectively reduced oxygen permeability, providing 5 h of effective protection for the alloy substrate at 1200 °C and improving the oxidation resistance of the titanium alloy by 50%. Li et al. [122] created SiO₂-MO-K₂O-ZrO₂ (M = Sr or Ca)-based enamel composite coatings with different Al₂O₃ contents on K444 superalloy via spraying. The coating with 10 wt% Al₂O₃ exhibited the best oxidation resistance, with almost no mass loss after cyclic oxidation tests at 1000 °C, mainly because enamel coatings with more than 6 wt% Al₂O₃ promoted the formation of a continuous alumina scale at the enamel/alloy interface, thereby improving the oxidation resistance and bonding strength of the enamel. Gu et al. [123] fabricated a CaO-Al₂O₃-SiO₂/ZrO₂-borosilicate multi-layer oxidation-resistant glass coating (CAS/BG) on a C/C substrate through the slurry method. The weight loss rates of the coated C/C samples after 50 h oxidation at 1073 K, 1173 K, and 1273 K were 21.70%, 1.63%, and 0.035%, respectively, showing an increasing trend in oxidation resistance with increasing temperature. According to the surface/cross-sectional morphologies of the coating after oxidation at different temperatures in Figure 12, as well as the analysis of the viscosity and activation energy of the inner and outer layers of the coating, this was mainly attributed to the improved glass flow healing ability at high temperatures. Fan et al. [124] applied PDC technology and slurry brushing to create a SiCN/borosilicate glass-B₄C-Al₂O₃ coating on C/C composites. After 10 h oxidation in air at 700 °C, 800 °C, and 900 °C, the mass losses were 0.12%, 0.51%, and 0.29%, respectively. This was mainly because B₂O₃ could repair cracks in the coating, improving coating density, and Al₂O₃ present in the glass layer could inhibit B₂O₃ volatilization, synergistically endowing the coating with excellent oxidation resistance. Zhou et al. [125] used the PADD method to prepare a new β-Y₂Si₂O₇NWs-Y₂SiO₅/YAS multi-layer microcrystalline composite coating on a SiC-coated C/C composite material. The dense microcrystalline glass coating effectively hindered the diffusion of oxygen to the matrix and inhibited crack propagation. It oxidized in air at 1400 °C for 176 h, and the mass loss of the coating sample is only 0.317 mg/cm². Gao et al. [126] invented ZrO₂/ZrSiO₄-glass composite coating for high-temperature oxidation protection of C/C substrates. Due to the self-healing ability of the glass phase and the slow bubble growth rate, the coating with 20 wt% ZrO₂ demonstrated the best performance, losing only 0.6% weight after 650 h at 1300 °C.

In conclusion, compared with other coatings, enamel composite coatings have simple preparation processes, low costs, and are suitable for parts with complex shapes. When the temperature is higher than the softening temperature of the glass components, the highly fluid glass migrates to defects such as microcracks and pores under surface tension and capillary force, thereby self-repairing damaged coatings and extending service life. However, enamel composite coatings tend to evaporate and degrade at very high temperatures and are easy to cracking during thermal cycling. These issues currently limit their broader application.

2.4. Inorganic Paint Composite Coatings

High-temperature oxidation-resistant inorganic paint composite coatings are typically formed by mixing high-temperature-resistant fillers into high-temperature-resistant inorganic binders, applying or spraying them onto the substrate surface at room temperature, followed by heating and curing. Common inorganic binders mainly include phosphate and silicate categories, among which aluminum phosphate binders are most widely used, benefiting from their excellent high- and low-temperature resistance, low curing temperature, adjustable curing temperature, good bonding performance, and simple processing and forming [127]. In recent years, high-temperature oxidation-resistant inorganic paint composite coatings based on inorganic aluminum phosphate binders have attracted much attention. For instance, Han et al. [128] used an aluminum phosphate binder mixed with Al₂O₃ and SiC as a slurry to fabricate an oxidation-resistant aluminum phosphate composite coating on a Ti65 titanium alloy substrate. Due to the high density, excellent thermal stability, and chemical stability of the coating, following 60 h of isothermal oxidation at 800 °C, the mass gain of the coating was 1.75 mg/cm². Xu et al. [129] designed Y³⁺- and Zr⁴⁺-modified aluminum phosphate coatings on graphite substrate by solution impregnation. Y³⁺ could improve the wettability of the aluminum phosphate impregnating solution on the graphite surface, Zr⁴⁺ could reduce the thermal expansion coefficient of the aluminum phosphate coating, and the formation of small amounts of Y₂O₃ and ZrO₂ resulted in a mass gain of approximately 6.23 mg/cm² after 5 h isothermal oxidation at 1000 °C. Zhao et al. [130] employed slurry spraying to produce an Al/AlPO₄ + SiC/AlPO₄ multi-layer composite coating on Ti65 alloy. In the initial oxidation stage, a protective Al₂O₃ film formed rapidly and an intermetallic oxide diffusion layer also developed between the coating and the alloy substrate. As a result, after 400 h of isothermal oxidation at 650 °C, the coating exhibited a mass gain of only 0.25 mg/cm², demonstrating excellent oxidation resistance. Li et al. [131] designed phenolic resin/phosphate organic–inorganic hybrid paint oxidation-resistant coating through a combination of sol–gel and air spraying. The addition of phenolic resin formed a heat-stable network structure with aluminum phosphate, and the carbonized resin provided structural support for the phosphate coating (as shown in the coating morphology after oxidation in Figure 13). After 100 h of isothermal oxidation at 600 °C, the mass gain was measured at 0.085 mg/cm².

Overall, phosphate coatings themselves have good heat resistance. High-temperature oxidation-resistant inorganic paint composite coatings developed by doping Al, SiC, and other thermal protection fillers into phosphate or modifying their structure with resins have shown unique advantages in the field of oxidation resistance, achieving long-term oxidation protection at medium and low temperatures while simplifying preparation methods and enabling oxidation protection for various thermal structural substrates. However, coating systems for higher-temperature applications of inorganic paint composite coatings and effective measures to alleviate thermal expansion coefficient mismatch caused by the brittleness of phosphate-based coatings need further development.

3. Structural Regulation Design

Oxidation-resistant composite coatings are important barriers ensuring continuous use of thermal structural materials in high-temperature oxidation environments. The design of coating material compositions determines their ability to block oxygen under high-temperature conditions. As oxidation environments become more severe and complex, coatings face issues such as accumulated interfacial stress between coating and substrate as well as reduced service life, leading to decreased stability. Therefore, optimizing coating structures to improve their high-temperature oxidation resistance, thermal stability, and durability has become a research hotspot in the field of high-temperature oxidation resistance.

3.1. Multi-Layer/Gradient Structures

Under high-temperature oxidation conditions, once a single-layer coating is damaged, the substrate is exposed, affecting the service life of hot-end components. Designing multi-layer/gradient structures can significantly improve the high-temperature oxidation resistance of coatings and extend their service life. Multi-layer composite coatings usually consist of two or three of a buffer layer, an oxygen-blocking layer, and a sealing layer. The buffer layer alleviates thermal stress caused by mismatched thermal expansion coefficients between the substrate and the coating, avoiding crack formation between coatings. The oxygen-blocking layer acts as a barrier to prevent external oxygen from entering the substrate, and the sealing layer densifies the coating surface, reducing oxidation active sites [132,133,134]. Hu et al. [135] prepared a multi-layer MoSi₂-Mullite composite coating on a C/C composite substrate via atmospheric plasma spraying, as shown in Figure 14. The design of the multi-layer structure effectively alleviated thermal stress in the coating, improved its high-temperature oxidation resistance, and after 226 h of isothermal oxidation at 1500 °C, a mass gain of 1.75 mg/cm² was observed for the coating, which was only half the mass loss of the single-layer structure coating. Additionally, Tang et al. [136] combined the slurry method and vapor deposition to prepare a HfSi₂-ZrSi₂-modified SiC/ZrB₂-SiC/SiC multi-layer coating on a Cf/SiC composite. The composite coating not only produced SiO₂ glass phases that repaired defects, achieving self-densification of the coating, but also had excellent interfacial bonding strength and ablation resistance. Chen et al. [137] created a Yb₂Si₂O₇/SiC bilayer oxidation-resistant coating on the surface of a C/SiC composite via combination of chemical vapor deposition (CVD), sol–gel, and air spraying. After 100 h isothermal oxidation at 1500 °C, the weight loss rate was only 0.81 mg/cm², significantly improving the oxidation resistance of the C/SiC composite. To improve the oxidation resistance of C/C composites, Quan et al. [138] designed a multi-layer SiC/ZrB₂-CrSi₂-Si/SiC composite coating at low temperature through a combination of chemical vapor deposition, brush sintering, and pre-oxidation. Due to the formation of borosilicate glass and thermally stable ZrSiO₄ phases after high-temperature oxidation, as well as the synergistic effect of self-healing mechanisms in inner and outer layers, the long-term oxidation life of the composite coating was effectively extended, exhibiting minimal mass loss of 8.05 mg/cm² after 340 h isothermal oxidation at 1400 °C. Gai et al. [139] developed an alternating SiC/HfB₂ multi-layer coating on a C/C composite by chemical vapor deposition. The alternating multi-layer structure effectively prevented stress concentration, enhanced the overall stability of the coating, and showed significant linear and mass ablation rates of −0.26 μm/s and 0.14 mg/s, respectively, after 30 s ablation by an oxyacetylene flame with a heat flux of 2.4 MW/m, exhibiting good high-temperature oxidation resistance and ablation resistance.

Gradient coatings refer to coating systems where composition and/or structure transition continuously and smoothly from the base material to the surface functional layer. Through gradient design, multiple thermal protection functions can be integrated into the coating, significantly improving its thermal protection performance. For example, Zhu et al. [140] designed a gradient structure coating with a dense outer layer and a microporous inner layer, where the dense layer had excellent oxygen diffusion resistance, and the microporous layer reduced CTE mismatch between the inner SiC layer and the glass layer on the coating surface. Compared with SiC-coated samples, the mass retention rate of gradient composite coating-coated samples increased from 62.9% to 97.94%. After oxidation at 1473 K for 810 h, 1573 K for 815 h, and 1773 K for 901 h, mass gains of the gradient coating were 1.62%, 1.48%, and 1.14%, respectively. The oxidation resistance mechanism of the gradient coating at different temperatures is shown in Figure 15. As oxidation temperature increased, SiC particles in the microporous layer were oxidized, generating SiO₂ that filled pores and thickened the dense layer. After oxidation at 1573 K and 1773 K, although bubbles formed in the dense layer, a glass layer of a certain thickness still protected the substrate; ZrSiO₄ particles also deflected and arrested cracks. Yang et al. [141] also prepared gradient ZrB₂-MoSi₂-SiC dense layer in porous 3D needled carbon fiber composite, which significantly improved both emissivity and oxidation resistance. Guan et al. [142] designed a gradient MoSi₂-borosilicate glass composite coating on a mullite fiber insulation tile via in situ reaction. The excellent oxidation resistance of the coating is mainly due to the interface layer of the bilayer gradient coating, which improved mechanical and thermal compatibility with the substrate and effectively inhibited interfacial radial cracking. Additionally, the coating had excellent thermal shock resistance and high emissivity. Zhang et al. [143] fabricated a novel gradient CrSi₂-ZrSi₂-SiC-Si composite coating on a C/C composite via optimized pre-oxidation and two-step pack cementation. The SiO₂-ZrO₂-Cr₂O₃-ZrSiO₄ oxide layer formed under high-temperature oxidation conditions had good structural stability and oxygen-blocking ability, and the final coating exhibited a weight gain of only 4.83 mg/cm² at 1500 °C for 604 h. Wang et al. [144] utilized vacuum infiltration sintering to prepare a NiCoCrAlYTa/Al₂O₃ gradient coating on single-crystal superalloy. Al₂O₃ reacted with the matrix to form YAlO₃, which inhibited intergranular crack propagation and improved thermal shock and oxidation resistance.

In summary, establishment of multi-layer/gradient structures can not only alleviate thermal stress between coating and substrate, but also further improve high-temperature service performance. However, further research is needed on the long-term service stability of multi-layer composite coatings and their performance under complex working conditions. The core advantage of gradient coatings lies in achieving adaptation to interfacial thermal stress between coating and substrate and improving oxygen-blocking performance through smooth transitions in structure and function. But in ultra-high-temperature environments, the bonding status of different layers in gradient coatings under higher thermal stress, and their adaptability to multi-field coupling require further research.

3.2. Diffusion Barriers

Diffusion barriers are primarily an added diffusion-blocking layer between the coating and the substrate, which can prevent element interdiffusion between the coating and the substrate, thereby increasing the service life of the coating. Due to severe element diffusion in metal substrates, especially superalloys, diffusion barriers are mainly applied to them. The primary function of a diffusion barrier is to reduce the rate of atomic migration near the interface, limiting mutual diffusion between the coating and the substrate. Based on material properties, they can be divided into metal diffusion barriers and ceramic diffusion barriers. Metal diffusion barriers have high compatibility with coatings or substrates but have poor stability and degrade quickly. In contrast, ceramic diffusion barriers have high stability and have attracted increasing research interest [145,146,147]. For example, Li et al. [148] prepared a Cr₂AlC diffusion barrier between an NiCrAlY coating and an Ni-based superalloy, which underwent long-term cyclic oxidation at 1000 °C. As shown in Figure 16, after adding the diffusion barrier, Cr₂AlC decomposed into Cr₇C₃ and Cr₂₃C₆ in sequence. The generated Cr₂₃C₆ could effectively block diffusion between the NiCrAlY coating and the substrate, and the Cr₂AlC transition layer also provided additional Al for the NiCrAlY coating, significantly extending the service life of the NiCrAlY coating. Moreover, Cr₂AlC, Cr₂₃C₆, and Cr₇C₃ all exhibited good thermal matching with both the substrate and coating, reducing the risk of spallation during thermal cycling. In another study, Li et al. [149] employed magnetron sputtering to prepare a Cr₂AlC diffusion barrier on the surfaces of NiCrAlY and TiAl titanium alloy substrates. After being oxidized at 900 °C for 1000 h, the coating demonstrated a mass gain of 1.51 mg/cm², exhibiting better oxidation resistance than the standalone NiCrAlY coating. Xu et al. [150] combined embedding and slurry sintering to produce a TaB₂ diffusion barrier between the tantalum alloy substrate and MoSi₂ coating. A porous layer formed between the boride diffusion barrier and the silicide layer, reducing the driving force for Si atom diffusion and slowing MoSi₂ degradation. The coating provided oxidation protection to the alloy substrate at 1700 °C for 14 h and survived 1021 thermal cycles between room temperature and 1700 °C. Xu et al. [151] designed a Pt-modified, Re-based diffusion barrier NiCoCrAlY coating. The Re-based diffusion barrier can effectively inhibit element diffusion between the NiCoCrAlY coating and the substrate, thereby reducing the thickness of the oxide layer.

In summary, the diffusion barriers serve to enhance high-temperature coating stability. They achieve this by forming stable phases that block elemental interdiffusion, thereby slowing the generation of harmful phases at the interface. However, ideal diffusion barriers need to be good at stopping diffusion and stick strongly to both the substrate and the coating, but these requirements often conflict in practice. A barrier layer that completely blocks element diffusion may disrupt the formation of protective films at the interface and weaken coating–substrate adhesion. Additionally, harmful phases may form during long-term high-temperature diffusion reactions. Thus, further development is needed for research and application of diffusion barriers.

3.3. Self-Healing Structure Design

Designing self-healing structures in coatings is currently an extremely effective way to extend their service life. The self-healing mechanism of oxidation-resistant coatings primarily relies on low-viscosity glass phases formed at high temperatures, which can flow to fill small cracks or micropores generated in the coating during high-temperature service, repairing the coating and thus improving its oxidation resistance [152]. Among these, B₂O₃ and SiO₂ are the most popular glass phases. B₂O₃ is typically generated by the oxidation of borides such as B₄C, HfB₂, and ZrB₂, while SiO₂ is provided by the oxidation of materials like SiC, SiB₆, and MoSi₂. It should be noted that pure B₂O₃ coatings tend to volatilize above 1000 °C, and SiC generates SiO₂ slowly below 1500 °C with high viscosity, failing to effectively heal coating cracks. Therefore, to enhance coating self-healing ability, multiple self-healing system materials are usually combined to prepare oxidation-resistant coatings [153,154]. For example, Wang et al. [155] designed a self-healing ZrSi₂-MoSi₂ composite coating on SiC-coated C/C composites using ultrasonic plasma spraying. ZrO₂/ZrSiO₄ particles formed by the preferential oxidation of ZrSi₂ reduced crack formation, and these particles pinned the surface protective SiO₂ layer, inhibiting the re-cracking of the coating. Subjected to isothermal oxidation for 42 h at 1450 °C, the coating displayed a mass gain of 1.89%, exhibiting excellent oxidation resistance and self-healing ability. Wang et al. [156] prepared a SiB₆-MoSi₂ composite coating on a C/C composite substrate via plasma spraying. The rapid oxidation of SiB₆ generated a low-viscosity B₂O₃-SiO₂ borosilicate glass protective layer, reducing coating oxygen permeability and significantly extending service life and oxidation resistance. The coating exhibited only a 0.93% mass gain after 120 h isothermal oxidation at 1200 °C. Zhu et al. [154] applied low-temperature slurry impregnation densification to prepared HfO₂-B₂O₃-SiC/SiC composite coating on C/C substrate. The coating was tested under isothermal oxidation at 1173 K for 403 h, 1473 K for 723 h, and 1773 K for 403 h, exhibiting mass gains of 0.38 mg/cm², 2.14 mg/cm², and 0.04 mg/cm², respectively. As shown in Figure 17, the coating demonstrated broad-temperature self-healing capacity: it relied on B-O-Si glass for sealing cracks at lower temperatures and on HfSiO₄-SiO₂ glass at higher temperatures. Furthermore, HfO₂ and HfSiO₄ ceramic particles improved glass thermal stability and enhanced glass layer durability.

As a current research hotspot in high-temperature oxidation-resistant coatings, there are many composite coating systems that enhance high-temperature oxidation performance through self-healing structures listed in

Table 2. Coating systems with self-healing mechanisms.

Coating Materials	Temperature/°C	Time/h	Mass Loss/%	Healing Phase
HfB2-B4C-SiC/SiC [157]	800	10	14.8	B-rich glass
	1200	104	5.45	B-O-Si glass
MoSi2-Mullite/SiC [158]	1500	80	−2.60	Al-Si-O glass
TiB2-SiC-Si/SiC-Si [153]	1300	300	−0.03	B-O-Si glass
SiC/ZrB2-SiC-Y2O3 [159]	1450	10	1.39	B-O-Si glass
ZrB2-SiC-Y2O3 [67]	1450	10	5.77	ZrSiO4-SiO2 glass
ZrO2/ZrSiO4-glass [126]	1300	650	0.60	ZrSiO4-SiO2 glass
ZrB2-MoSi2 [42]	1750	20	−15.09	SiO2 glass

. Self-healing structure designs have effectively improved high-temperature oxygen-blocking ability and service life of coatings, complex compositional designs can even achieve wide-temperature-range protection. However, studies on how coatings behave at ultra-high temperatures and how well their mechanical properties are retained after self-healing require further attention.

3.4. Other Microstructures

In addition to multi-layer/gradient designs, diffusion barriers, and self-healing structures, nanocrystalline structures, porous structures, and whisker introduction can also effectively enhance high-temperature oxidation resistance of coatings. Nanocrystalline coatings help oxidation-resistant components spread to the surface quickly, promoting dense protective film formation and improving oxidation resistance. Porous structures enhance oxidation resistance by increasing oxygen diffusion paths and whiskers improve the bonding strength between the coating and reduce thermal stress between coating and substrate [160,161,162,163,164]. For example, Yang et al. [165] developed three new nanocrystalline composite coatings with different Al contents on single-crystal superalloy. After 500 oxidation cycles between room temperature and 1100 °C in air, the Al-9 coating (with 8 wt% Al) displayed a dense, intact surface without defects (as shown in Figure 18), exhibiting the best oxidation resistance with a final mass gain of 0.51 mg/cm². Zhu et al. [91] fabricated a SiC/SiC-ZrSi₂ composite coating with microporous structure on a C/C substrate via filler cementation and slurry methods. The porous structure increased the oxidizable area, enabling rapid generation of more glass phases and significantly improving oxidation resistance. Thus, after 1194 h of isothermal oxidation at 1500 °C, the mass gain was 1.42%. Chen et al. [166] employed filler cementation and slurry brushing to prepare a HfC-HfB₂-SiC composite coating with a SiC-SiC_w interlayer on a C/C substrate, as shown in Figure 19. The SiC-SiC_w interlayer strengthened the bond between the coating and the substrate’s surface, effectively anchored SiC-SiO₂ oxidation products, promoted continuous protective glass layer formation, improved high-temperature oxidation resistance, and enhanced ablation resistance. Xie et al. [167] embedded silicon carbide whiskers (SiC_w) into a SiC layer, creating a SiC-SiC_w transition layer as an inner coating on the C/C substrate, significantly improving thermal expansion matching between the coating and the substrate and ensuring long-term service of the coating.

In summary, nanocrystalline structures are mainly applied to Al-containing coatings for oxidation resistance. Their excellent performance stems from extremely fine grain sizes providing abundant rapid diffusion paths for Al, reducing the critical Al concentration for Al₂O₃ formation, and high-density grain boundaries and dislocations promoting θ-Al₂O₃ to α-Al₂O₃ transformation. Porous structures increase oxygen diffusion paths, while whiskers reduce thermal stress between the coating and the substrate. However, before these structures can be reliably applied in real hot-end components, further research is needed on high-temperature phase stability, interface durability, adaptability to complex environments, and process controllability.

3.5. Coating Preparation Technologies

Rational structural regulation can enhance coating oxidation resistance, but structural designs require proper preparation technologies for implementation. Selecting suitable preparation technologies based on service conditions ensures coating quality and influences service performance. This section lists common preparation technologies for oxidation-resistant coatings, with their key characteristics summarized and applications in

Table 3. Comparison of technologies for preparing high-temperature oxidation-resistant coatings.

Technology	Characteristic	Coating Quality	Disadvantages
APS	High coating quality Suitable for complex shapes	High bonding strength Excellent coating uniformity	Complex equipment Complex parameters
Cold spraying	Small thermal impact Simple equipment	Dense	Low bonding strength Relatively high porosity
Laser cladding	Adjustable thickness	High bonding strength	High cost
Magnetron sputtering	Controllable components and thickness	High density High bonding strength High purity	Complex equipment Slow deposition rate
PEO	High efficiency Simple operation	High bonding strength	Metal substrates only More pores
Embedding method	Simple operation Flexible design	Excellent substrate coverage	Low bonding strength Poor durability
Slurry method	Low cost Simple operation	High coverage	Low efficiency Low bonding strength

.

3.5.1. Atmospheric Plasma Spraying

Atmospheric plasma spraying (APS) uses a plasma arc as the heat source to fully or partially melt the feed materials, which are then sprayed at high speed to form coatings on substrate’s surface. It can melt high-melting-point metal alloys or ceramics, ensuring uniform particle integration. APS produces high-quality coatings with strong bonding. This technology is suitable for complex-shaped substrates, offering significant advantages in oxidation-resistant coating production. However, APS requires complex equipment and careful control of parameters [168].

3.5.2. Cold Spraying

Cold spraying is a coating deposition technique in which feed materials are accelerated by compressed gas and impacted at high velocity onto a substrate to form a coating. This process generates minimal thermal effect, thereby minimizing or avoiding oxidation and phase transformation of the sprayed material, so it is particularly suitable for applying coatings to temperature-sensitive substrates, such as titanium alloys. However, coatings produced by cold spraying generally exhibit limited bond strength and relatively high porosity, which restricts their broader application [169,170,171].

3.5.3. Laser Cladding

This technology mainly utilizes high-power-density laser beam to melt pre-placed coating materials on the substrate, which then rapidly solidify to bond with the substrate. This technique produces coatings with high bond strength and the thickness of coating is adjustable, but costs are high [172].

3.5.4. Magnetron Sputtering

Magnetron sputtering operates on the basis of electron collision with an inert gas, typically argon, within an electric field, leading to the generation of argon ions that then bombard a target material. Subsequently, target atoms acquire energy, enabling their release from the surface and deposition onto a substrate to create a coating. The coatings produced are uniform, dense, high-purity, and adhere well to the substrate. Consequently, this method is well-suited for creating antioxidant coatings on metal and ceramic surfaces [173].

3.5.5. Plasma-Electrolyte Oxidation

Plasma-Electrolyte Oxidation (PEO) employs plasma discharge to generate instantaneous high temperature and pressure on metal substrates. This environment causes components within the electrolyte to react and grow in situ into ceramic coatings. The advantages of PEO include strong coating adhesion, operational simplicity, and high process efficiency, but its application is restricted to metal substrates, and the resulting coatings are often porous [174,175,176,177].

3.5.6. Embedding Method

This method involves immersing composite components in a liquid or solid medium and heat-treating them in a high-temperature, protective atmosphere. During this process, reactants diffuse inward from the surface to form coatings. The technique is simple and allows for the flexible design of gradient structure formation. However, the adhesion and durability of the antioxidant coatings produced by this modified method remain unsatisfactory [178,179].

3.5.7. Slurry Method

In this process, a slurry with controlled rheological and adhesive properties is applied to a substrate via brushing, then cured and sintered to form dense coatings [180]. The primary advantages of slurry brushing are its simplicity, low cost, and applicability to large parts. However, its drawbacks include the requirement for multiple deposition steps and the production of coatings with lower bonding strength.

In summary, every technology has its own unique characteristics and application ranges, so the choice of which to use should align with actual service requirements. To ensure the bonding strength between the coating and the substrate under complex conditions, as well as to produce denser oxygen barrier structures, future research must explore new high-efficiency technologies and multi-technique combinations.

4. Coating Performance Evaluation Methods

Diverse coating material systems, structural designs, and advancing preparation technologies provide abundant solutions for high-temperature oxidation protection. However, when coatings are subjected to long-term service in extreme environments, they still face the challenges of multiple factors, including thermal shock, water–oxygen corrosion, molten salt corrosion, particle scouring, and so on. Systematic evaluation of coating performance can not only provide data support for coating optimization but also establish qualification standards for their service applications under complex working conditions. Therefore, this section will focus on the key evaluation methods and main indicators of coating performance during service in extreme high-temperature service.

4.1. Static Environment

Static environmental evaluation of composite coatings can simulate their service behavior in constant temperature and environment. It enables effective analysis of structural and compositional changes in coatings under extreme high-temperature conditions, thereby providing an important basis for the optimization of coating systems and structures.

4.1.1. Isothermal Oxidation

Isothermal oxidation mainly involves oxidation testing of coatings in air, which is the most basic method to characterize the high-temperature oxidation resistance of composite coatings. The evolution of the coating compositions and oxidation kinetic equations under this testing condition not only provide strong support for revealing oxidation resistance mechanisms but also offer ideas for further improving the long-term stability of coatings [181]. Wang et al. [182] combined the halide-activated pack cementation method and liquid plasma-assisted particle deposition and sintering technology to form a ZrSi₂/SiO₂-Nb₂O₅/NbSi₂ multi-layer coating on niobium alloys. As shown in Figure 20, compared with single-layer NbSi₂ and double-layer SiO₂-Nb₂O₅/NbSi₂, the multi-layer coating displayed the best high-temperature oxidation resistance due to the formation of a SiO₂-ZrO₂-ZrSiO₄ framework. After oxidation at 1200 °C for 100 h, tahe weight gain of the coating was merely 4.86 mg/cm². In addition, Wang et al. [183] invented a NbSi₂/SiO₂-Nb₂O₅/MoSi₂-Yb₂O₃ multi-layer coating. The addition of appropriate amount of Yb³⁺ can effectively fill the gaps in the SiO₂ network, thereby further inhibiting the diffusion of oxygen, with an oxidation weight gain of 4.86 mg/cm² after 100 h of isothermal oxidation at 1200 °C, achieving high-temperature oxidation protection of niobium alloys. Wang et al. [184] also designed a NbSi₂/SiO₂-Nb₂O₅/HfSi₂-HfO₂ composite coating. HfO₂ and HfSiO₄ formed at high temperatures are anchored in the SiO₂ layer, which can not only effectively block oxygen penetration, but also enhance the structural stability of the ceramic phase. The coating exhibited minimal mass gain of 5.583 mg/cm² after static oxidation at 1200 °C for 100 h, greatly improving the high-temperature oxidation resistance of the coating. Furthermore, by depositing Al₂O₃/SiO₂ composite coating on stainless steel, Ma et al. [185] achieved effective protection for the substrate at 900 °C for 100 h.

In conclusion, static isothermal oxidation serves as an effective way to evaluate the oxidation behavior of coatings under specific temperature field conditions and the oxidation kinetic equations offer the most direct and efficient method for screening high-temperature oxidation-resistant coatings. Nevertheless, the conditions of a single temperature field are difficult to simulate the temperature fluctuations that the coatings face during actual service.

4.1.2. Water–Oxygen Corrosion

In the service environment of hot-end components, water vapor has high chemical activity at high temperatures, and the water–oxygen coupling environment causes more severe damage to coatings, making it necessary to test their resistance to water–oxygen corrosion [186,187]. To study the service performance of coatings under water–oxygen conditions, Fan et al. [188] prepared a MoSi₂/mullite composite coating on niobium alloys via plasma spraying. The team compared the water vapor corrosion behavior and dry oxygen oxidation behavior of the composite coating at 1500 °C. It was found that the mass change in the sample in the water vapor environment was −4.31 mg/cm², and the oxidation rate was −0.39 mg/(cm²·h¹/²), approximately 4 times higher than that in dry oxygen. The research results showed that high-temperature water vapor accelerates O₂ penetration and exacerbates substrate oxidation. To improve the water–oxygen corrosion resistance of coatings, Ou et al. [189] utilized supersonic plasma spraying to prepare an Al₂O₃-modified Yb₂SiO₅ coating (YbMS/Al₂O₃) on niobium alloys. Coating oxidation tests were conducted in an environment with 50% oxygen and 50% water vapor at 1500 °C. The service life of the modified coating reached 30 h, 6 times that of the unmodified coating, with the cross-sectional condition of the coating after testing shown in Figure 21. This improvement is mainly because the addition of Al₂O₃ makes Yb₂O₃ tend to react with Al₂O₃ in situ to form Yb₃Al₅O₁₂, which inhibited the inward diffusion of Yb₂O₃, thereby increasing the thickness of the diffusion layer and reducing micropores near the interface. Ou et al. [190] also fabricated the ZrO₂-toughened Yb₂SiO₅ coating, which greatly improved corrosion resistance by enhancing atomic bonding and could effectively protect the substrate for 50 h under water vapor conditions at 1500 °C.

In summary, in the water–oxygen coupling environment, highly active water vapor enhances oxygen permeability and exacerbates coating damage. However, dynamic water–oxygen coupling field testing conditions are currently lacking. To better simulate the actual service environment of coatings, it is necessary to improve these testing conditions.

4.1.3. Molten Salt Corrosion

For power devices such as turbine engines and gas turbines, some impurity elements in fuel will form NaCl and Na₂SO₄ molten salt deposits under high-temperature conditions, which continuously dissolve the protective oxide film formed on the surface of hot-end components, thereby damaging the substrate. Notably, molten salt corrosion is more severe than oxidation [191,192,193,194]. Sun et al. [195] comparatively studied the hot corrosion behavior of niobium alloy substrates with MoSi₂-based composite coatings exposed to air at 1000 °C and Na₂SO₄ molten salt at 1000 °C. They found that after 2 h of oxidation in air, the mass loss was 1.59 mg/cm²; while after 2 h of hot corrosion in Na₂SO₄ molten salt, corrosion products such as cristobalite scales and sodium molybdate appeared, resulting in a mass loss of 2.5 mg/cm². Sun et al. [196] prepared SiO₂-Ta₂O₅ oxide films with different Ta₂O₅ contents on MoSi₂-based ceramics through pre-oxidation, and then tested these coatings in Na₂SO₄ molten salt at 1000 °C for 8 h. The cross-sectional micrographs of oxide layers formed with different Ta₂O₅ contents after corrosion are shown in Figure 22. The results demonstrated that the oxide scale with 10% Ta₂O₅ (T10) has excellent hot corrosion resistance, mainly because Ta₂O₅ preferentially reacts with Na₂SO₄ to form solid sodium tantalate, delaying the dissolution of the surface SiO₂ dense layer.

Overall, high-temperature molten salt corrosion destroys the integrity of the protective film on the coating surface, thereby providing paths for oxygen diffusion to the substrate and exacerbating the damage process of hot-end components. However, there are few studies on long-term high-temperature molten salt corrosion, coatings designed for service in high-temperature molten salt environments require further development to meet the demands of complex operational conditions.

4.1.4. Thermal Radiation Performance

Radiation heat dissipation plays an important role in cooling hot-end components in high-temperature or ultra-high-temperature environments. The radiative heat dissipation capability of hot-end components primarily relies on the level of their surface infrared emissivity (ε). In particular, the spectral emissivity value in the near-to-mid infrared range is a key performance indicator that determines heat transfer, and it plays a crucial role in reducing the surface thermal equilibrium temperature of hot-end components. Therefore, adding high-emissivity components in the design of high-temperature oxidation-resistant coatings is an effective way to improve coating thermal protection performance [197,198]. Ye et al. [199] combined the embedding method and liquid plasma-assisted particle deposition and sintering technology to prepare a NbSi₂/Nb₂O₅-SiO₂/MoSi₂ multi-layer coating on niobium alloys. The coating demonstrated minimal mass gain of 0.37 mg/cm² after 50 h of isothermal oxidation in air at 1250 °C, and the thickness of the oxide layer of the multi-layer coating hardly changed after oxidation testing, showing high chemical stability. At the same time, the multi-layer coating also had excellent thermal radiation performance, with emissivity values above 0.88 in the 3–20 μm thermal infrared band (as shown in Figure 23). In addition, the team [200] developed a NbSi₂/Nb₂O₅-SiO₂/SiC multi-layer coating on niobium alloys to simultaneously improve thermal corrosion resistance and infrared thermal radiation performance. When tested under molten salt conditions (75% Na₂SO₄ + 25% NaCl) at 700 °C, the weight change after 100 h of the coating was −0.56 mg/cm². The addition of SiC made the coating still have a high emissivity above 0.9 in the entire 3–20 μm wavelength range after oxidation at 700 °C.

In summary, radiation heat dissipation can effectively reduce the temperature of the coating system, thereby significantly improving the high-temperature thermal protection performance of the coating. However, as the temperature rises, the thermal radiation capacity of the coating will continuously decline, and there is an urgent need to develop coatings that can maintain high emissivity under high-temperature conditions.

4.2. Dynamic Environment

Evaluation and testing methods in dynamic environments can reflect the stability of coatings under fluctuating conditions, which is a more severe testing condition for coatings and can provide a more reliable basis for the application of coatings in complex environments

4.2.1. Cyclic Oxidation and Thermal Shock Resistance

During service in extreme high-temperature environments, coatings are subjected to cyclic thermal shocks and severe temperature fluctuations, which can cause crack propagation and coating spallation [201,202]. Excellent thermal shock resistance can effectively improve the stability of coatings when facing temperature fluctuations. Therefore, many researchers have improved coating composition and structure to enhance thermal shock resistance [203]. Yang et al. [204] prepared Pt-modified (Ni,Pt)Al composite coating on Ni-based superalloys through low-activity high-temperature aluminizing technology. After 300 cyclic oxidations at 1100 °C, the unmodified NiAl coating suffered significant mass loss, while the modified coating maintained mass gain, with partial wrinkles on the surface, exhibiting excellent thermal shock resistance. Zhang et al. [205] designed three types of composite coatings on Ni-based superalloys: NiAl coatings, Hf-modified 5Hf-NiAl coatings, and 15Hf-NiAl coatings. As shown in Figure 24, after 200 cyclic oxidation tests at 1100 °C, the weight gain of the NiAl coating was 0.51 mg/cm², while those of the 5Hf-NiAl coating and 15Hf-NiAl coating were 0.63 mg/cm² and 0.74 mg/cm², respectively, close to their weight gains in static isothermal oxidation tests (0.65 mg/cm² and 0.77 mg/cm²). The main reason for this lies in the addition of Hf, which inhibited interdiffusion between the coating and the substrate, thus helping maintain the integrity of the oxide scale. Wang et al. [206] synthesized a HfB₂-modified SiC coating on a C/C substrate via in situ synthesis. After 30 thermal cycles, the optimized coating with 18.2 wt% HfB₂ exhibited 25.8% lower mass loss compared to the unmodified coating. This enhancement is due to the formation of a Hf-Si-O glass layer during oxidation, which improved coating stability, reduced oxygen permeability, and thus enhanced oxygen-blocking performance. He et al. [4] fabricated a Ti₃SiC₂/MoSi₂ composite coating on Nb-Si-based alloys. Among the samples tested, the composite coating with 10% Ti₃SiC₂ displayed the highest stability, gaining only −2.7 mg/cm² after 100 thermal cycles, attributed to the formation of a compact and dense oxide scale in this type of composite coating. To design coatings with excellent thermal shock resistance, Wang et al. [207] prepared a multi-layer Cr/CrN/Cr/CrAlN coating via magnetron sputtering. During high-temperature oxidation, Cr₂O₃ and α-Al₂O₃ formed in the coating endowed the coating with excellent oxidation resistance and, at the same time, showed excellent thermal shock resistance within 700 °C. Li et al. [208] utilized the slurry method to form a ZrSiO₄-glass coating on the C/C substrate. Due to the pinning effect of ZrSiO₄ in the glass layer, the coating showed excellent thermal shock resistance, with a mass loss of only 0.33% after thermal shock testing at 1200 °C. Guan et al. [142] synthesized gradient MoSi₂-borosilicate glass coating with excellent thermal shock resistance through in situ reaction. The coating can withstand 25 thermal shock tests between 1500 and 20 °C. The gradient interface layer reduced the CTE difference between the substrate and the dense coating, resulting in an interface bonding strength of over 0.67 ± 0.15 MPa, which can withstand higher thermal stress. Wu et al. [209] prepared plasma electrolytic oxidation (PEO) coatings and laser-assisted PEO (LPEO) coatings in NaAlO₂-based electrolyte to enhance the high-temperature oxidation resistance and thermal shock resistance of Ti6Al4V alloys. They found that the LPEO-60 coating with a laser irradiation energy of 60 W had the best oxidation resistance and thermal shock resistance. Compared with other coatings, after 70 thermal shocks, although some cracks appeared on the surface, no spallation occurred in any area. The coating exhibited minimal mass gain of 0.211 mg/cm² after 100 h of oxidation in air at 600 °C. In addition, the coating also showed high bonding strength after scratch testing.

Cyclic oxidation and thermal shock resistance tests provide important evaluation criteria for the synergistic performance of coatings in thermal shock resistance and oxidation resistance. However, current methods cannot adequately assess the synergy between thermal shock and multi-field coupling conditions, highlighting the need for more comprehensive evaluation systems.

4.2.2. Ablation Resistance

High-temperature ablation is also a challenge for coatings serving in extreme high-temperature environments. Ablation resistance tests can simulate environments where high-temperature oxidation and ablation are synergistic and reflect coating performance through mass ablation rate and linear ablation rate [210,211]. To improve coating ablation resistance, Xu [212] designed a ZrB₂-SiC-TaSi₂ coating. Plasma flame ablation tests showed that the coating exhibited excellent ablation resistance and oxidation resistance under a heat flux of 3.0 MW/m² for 120 s. Ye et al. [213] developed a NbSi₂/Nb₂O₅-SiO₂/HfC-HfO₂-MoSi₂-Yb₂O₃ multi-functional thermal protection coating. The multi-particle components in the coating not only helped reduce oxygen permeability, but also improved the thermal stability of the oxide scale by forming a HfSiO₄-Yb₂O₃-SiO₂ framework. After 120 h of isothermal oxidation at 1200 °C, the oxide film thickness was only 128 μm, a 67% reduction compared to the uncoated sample (192 μm), exhibiting excellent oxidation resistance. Furthermore, the mass ablation rate and linear ablation rate were −0.116 mg/s and −0.29 μm/s during short-term ablation at 1800 °C with oxyacetylene, demonstrating good ablation resistance. Wang et al. [214] combined halide-activated pack cementation and liquid plasma-assisted particle deposition and sintering methods to prepare three types of multi-layer ceramic coatings on niobium alloys: NbSi₂/SiO₂-Nb₂O₅/MoSi₂ (Mo), NbSi₂/SiO₂-Nb₂O₅/MoSi₂-Yb₂O₃-SiO₂ (MoYb), and NbSi₂/SiO₂-Nb₂O₅/MoSi₂-Yb₂O₃-ZrO₂-ZrC (MoYbZr). All three coatings remained intact after propane ablation at 1500 °C, demonstrating excellent ablation resistance. Under a more severe 1800 °C oxyacetylene flame test, as shown in Figure 25, their performances are significantly different: the Mo coating had long cracks, blisters, and local spallation, while the MoYb coating had large cracks but no spallation, and the MoYbZr coating remained intact without cracks. The mass ablation rate and linear ablation rate of the MoYbZr coating after oxyacetylene ablation were only 0.137 mg/s and 0.196 μm/s, 25.14% and 49.61% lower than those of the MoYb coating, and 88.01% and 75.37% lower than those of the Mo coating, respectively. The excellent ablation resistance of the MoYbZr coating is mainly due to the doping of Yb₂O₃ and ZrC. On the one hand, the introduction of Yb₂O₃ strengthened the Si-O bonds in the SiO₂ network, increased the viscosity of the SiO₂ oxide scale, and thus enhanced the self-healing ability of the oxide scale; on the other hand, ZrSiO₄ generated during high-temperature oxidation of the coating can inhibit the generation and propagation of microcracks and play a pinning role in the oxide scale, enhancing the high-temperature structural stability of the coating. Qiang et al. [215] used chemical vapor deposition to in situ prepare a multi-layer silicon carbide nanowire-reinforced silicon carbide (SiC_nws-SiC) coating on carbon/carbon composites. The C/C composites with the coating had a weight loss of only 1.77% after oxidation at 1500 °C for 361 h, and the coating completely failed after 130 h of testing in a 1600 °C wind tunnel, demonstrating excellent high-temperature oxidation resistance and ablation resistance.

In conclusion, oxyacetylene and wind tunnel tests effectively simulate the high-temperature ablation conditions that coatings face in actual service environments, and mass ablation rate and linear ablation rate provide data support for analyzing coating ablation resistance. However, characterization methods for instantaneous high-temperature ablation conditions need further development.

4.2.3. Erosion Resistance

Hot-end parts such as the front ends of aerospace vehicle wings and engines are usually subjected to erosion by particles in air or gas during service. As a result, coatings with excellent erosion resistance can better protect these hot-end parts. Erosion resistance is typically evaluated by testing with erosion media at different angles and distances, and finally through mass loss after erosion [216,217,218]. For example, Qiang et al. [215] employed chemical vapor deposition to synthesize a multi-layer SiC_nws-SiC coating on carbon/carbon composites. After 130 h of thermal flux erosion at 1600 °C, the weight loss of the coated sample was only 1.1%, which is attributed to the introduction of SiCnws, which enhanced the coating’s toughness and thereby greatly improved its erosion resistance.

The erosion resistance test significantly improves the working conditions and data reliability of the evaluation of the erosion resistance performance of the coating, but the effects of microcrack propagation and interfacial stress accumulation caused by the erosion medium after scouring need to be further studied.

5. Summary and Outlooks

Currently, the application of oxidation-resistant composite coatings on the surface of hot-end components can significantly enhance their service performance. Commonly used high-temperature oxidation-resistant composite coating systems mainly include aluminide-based composite coatings, silicide-based composite coatings, MCrAlY coatings, boride-based composite coatings, carbide-based composite coatings, nitride-based composite coatings, oxide ceramic composite coatings, MAX phase composite coatings, as well as enamel and inorganic paint composite coatings. To further improve the high-temperature oxidation resistance of composite coatings, various effective structural regulation methods have emerged, mainly including the construction of multi-layer/gradient structures, the addition of diffusion barrier structures, the design of self-healing structures, and porous structures. At the same time, to adapt to different working conditions, advanced coating preparation technologies such as atmospheric plasma spraying, laser cladding, magnetron sputtering, and slurry methods have been continuously developed and applied. In view of the harsh and complex actual service environment of coatings, various evaluation methods for coatings under static and dynamic environments have emerged, such as isothermal oxidation, water–oxygen corrosion, molten salt corrosion, cyclic oxidation, thermal shock resistance, ablation resistance, and erosion resistance. This series of coating performance evaluation methods provides an important basis for coatings to move from laboratory to engineering applications.

At present, although the construction of high-temperature oxidation-resistant composite coatings has achieved certain results in solving the extreme high-temperature oxidation problem of hot-end components, the application of high-temperature oxidation-resistant coatings still faces many challenges, such as insufficient long-term service capability of coatings above 1700 °C, insufficient adaptability to multi-field coupling environments, and lack of evaluation methods for coating performance in complex environments. To address the above issues, some promising improvement measures are proposed.

(1)

Using first-principles, machine learning, finite element simulation, and other methods to establish the relationship between the physical properties of coating materials and coating structure design, screen and optimize coating systems with high-temperature oxidation resistance. At the same time, from the perspective of coating preparation, the combination of multiple efficient coating preparation technologies, such as embedding method and slurry sintering, chemical vapor deposition, sol–gel combined with air spraying, can be used to optimize the interface between the coating and the substrate, improve the bonding strength and stability of the coating during ultra-high-temperature service.

(2)

The actual service environment of coatings is mostly the coupling of multiple complex factors such as high temperature, oxidation, ablation, and thermal shock. Therefore, multiple functional layers can be combined to develop coatings adapted to multi-field coupling environments, such as gradient structure construction of high-emissivity layer–oxidation resistance layer, erosion resistance layer–heat insulation layer–oxidation resistance layer, etc. Meanwhile, by combining atomic-scale and macro-scale and conducting cross-scale regulation, we can guide the development of adaptive coatings that can independently adjust their structure and performance in response to changes in service environment.

(3)

Coating performance evaluation should focus on the development of evaluation systems under multi-factor coupling environments. This could be achieved by building a multi-field coupling simulation test platform to simulate the actual service environment of hot-end components, such as water–thermal–oxygen and thermal–force–oxygen coupling environments, and collecting oxidation kinetics, corrosion rates, and instantaneous high-temperature damage of coatings under simulated conditions. In addition, performance testing standards under multi-field coupling environments should be established as soon as possible to promote the transformation of excellent thermal protection coatings from laboratory to engineering applications.

In summary, selecting appropriate coating materials according to different working conditions, then conducting reasonable structural regulation and preparation, and finally undergoing strict performance evaluation to obtain high-quality coatings with excellent high-temperature oxidation resistance is of great significance for the development of hot-end components in aerospace, energy power, nuclear energy chemical engineering, and other fields towards high performance.