Research Progress of High-Entropy Ceramic Films via Arc Ion Plating

Abstract

High-entropy ceramic (HEC) thin films generally refer to multi-component solid solutions composed of multiple metallic and non-metallic elements, existing in forms such as carbides, nitrides, and borides. Benefiting from the high-entropy effect, lattice distortion, sluggish diffusion, and cocktail effect of high-entropy systems, HEC thin films form simple amorphous or nanocrystalline structures while exhibiting high hardness/elastic modulus, excellent tribological properties, and thermal stability. Although the mixing entropy increases with the number of elements in the system, a higher number of elements does not guarantee improved performance. In addition to system configuration, the regulation of preparation methods and processes is also a key factor in enhancing performance. Arc ion plating (AIP) has emerged as one of the mainstream techniques for fabricating high-entropy ceramic (HEC) thin films, which is attributed to its high ionization efficiency, flexible multi-target configuration, precise control over process parameters, and high deposition rate. Through rational design of the compositional system and optimization of key process parameters—such as the substrate bias voltage, gas flow rates, and arc current—HEC thin films with high hardness/toughness, wear resistance, high-temperature oxidation resistance, and electrochemical performance can be fabricated, and several of these properties can even be simultaneously achieved. Against the backdrop of AIP deposition, this review focuses on discussions grounded in the thermodynamic principles of high-entropy systems. It systematically discusses how process parameters influence the microstructure and, consequently, the mechanical, tribological, electrochemical, and high-temperature oxidation behaviors of HEC thin films under various complex service conditions. Finally, the review outlines prospective research directions for advancing the AIP-based synthesis of high-entropy ceramic coatings.

1. Introduction

High-entropy materials have emerged as a research hotspot in materials science, which is attributed to their multi-principal-element composition and unique properties. High-entropy ceramic (HEC) thin films, derived from high-entropy alloys [1] but containing five or more cations/anions (instead of pure metals) to form nitrides/borides/oxides, exhibit distinctive amorphous/nanocrystalline microstructures and superior performance (e.g., thermal stability, diffusion barrier capability, and balanced mechanical/tribological/corrosion properties) [2,3]. To realize their structural control and performance optimization, physical vapor deposition (PVD) dominates HEC fabrication, among which arc ion plating (AIP) is outstanding for its mature process, high deposition efficiency, and precise compositional tunability [4,5]. AIP enables simultaneous evaporation of multi-principal-element alloy targets (avoiding complex multi-target switching), achieves ~tens of nm/min deposition rates while maintaining densification, and suppresses large droplets via magnetic filtering/pulsed bias—key to improving HEC hardness–toughness synergy [6,7,8].

This review focuses on HEC thin films fabricated by AIP, with three core objectives: First, it systematically clarifies the thermodynamic mechanisms (high-entropy, lattice-distortion, sluggish-diffusion, and cocktail effects) governing HEC microstructural formation. Second, it elaborates how critical AIP parameters (substrate bias, reactive gas flow, arc current) regulate HEC microstructures (grain size, phase composition, density) and further tailor their mechanical, tribological, electrochemical, and high-temperature oxidation properties. Finally, it outlines prospective directions for advancing AIP-based HEC synthesis, such as multi-nonmetal sublattice design and integration with 2D materials. This work aims to provide a comprehensive “process–structure–property” framework for HEC thin film research and industrial applications.

2. High-Entropy Ceramic Systems and Their Thermodynamic Principles

Since the late twentieth century, alloy design philosophy has transitioned from a “single-principal-element” approach to a “multiple-principal-elements” strategy. The intrinsic driving force behind this shift is the enhanced thermodynamic stability achieved by maximizing the configurational entropy of multicomponent systems. In 1993, Greer and co-workers introduced the “confusion principle,” which states that increasing the number of constituent species raises the system’s configurational entropy, thereby suppressing intermetallic compound precipitation and promoting amorphous phase formation [9]. While the “confusion principle” focused on amorphous systems, Yeh and the Cantor group subsequently extended entropy-driven design to crystalline solid solutions by independently introducing the concept of “high-entropy alloys (HEAs)” in 2004. They proposed that when the configurational mixing entropy $\Delta Smix\ge 1.5 R$, the alloy tends to form simple single-phase FCC (face-centered cubic) or BCC (body-centered cubic) structures, accompanied by severe lattice distortion and sluggish diffusion effects—collectively endowing the material with high strength, high hardness, and excellent softening resistance [1,10]. Additionally, the tunability of high-entropy materials—achieved by adjusting the number and concentration of constituent elements—enables the tailoring of mechanical, thermal, and functional properties [11].

In 2013, Otto and co-workers validated these concepts via multi-scale characterization techniques: scanning electron microscopy/electron backscatter diffraction (SEM/EBSD) for grain and twin evolution, transmission electron microscopy (TEM) for dislocation density quantification, and in situ neutron diffraction for lattice strain tracking during tensile loading of the CoCrFeMnNi alloy (77 K to 1073 K). Their results showed that the alloy retained a single-phase FCC structure without long-range ordering or secondary-phase precipitation, directly confirming the “entropy-stabilized single-phase” hypothesis. This work established the thermodynamic foundation that later supported the emergence of high-entropy ceramics [12,13]. Leveraging this foundation, Yeh et al. [14] extended the HEA concept to high-entropy ceramic systems in 2014, proposing a novel design paradigm: when five or more principal elements coexist at near-equimolar ratios, the high-entropy effect suppresses intermetallic formation and stabilizes simple solid solutions.

Building on this paradigm, combining four or more transition metals (e.g., Ti, Zr, Hf, Nb, Ta) with non-metallic elements (C, N, B) enables the synthesis of high-entropy ceramics with rock salt or wurtzite structures (e.g., nitrides, carbides, borides) [15,16,17]. This overturns the traditional “dominant principal element + minor dopants” design, enabling performance breakthroughs via synergistic near-equimolar composition. From a material system perspective, high-entropy ceramics cover nitride, carbide, oxide, and boride anion systems, with applications in high-temperature impact, marine corrosion, nuclear irradiation, lightweight structures, and flexible electronics [18,19].

2.1. High Entropy Effect

The high entropy effect describes the tendency of high-entropy systems—composed of multiple near-equimolar components—to form simple structures (e.g., solid solutions) instead of complex intermetallic compounds. Thermodynamically, mixing entropy significantly modulates Gibbs free energy (∆G), stabilizing disordered phases while suppressing ordered phase formation. Unlike traditional alloys, where high-proportion principal elements dominate and minor alloying elements readily form brittle intermetallic compounds or precipitates, the elevated mixing entropy in high-entropy systems reduces ∆G substantially. This renders disordered solid solutions (e.g., body-centered cubic, face-centered cubic, or rock salt structures) thermodynamically more stable than their ordered counterparts [5], as described by Equation (1).

$$∆Gmix=∆Hmix-T∆Smix$$

(1)

where $∆Gmix$: Gibbs free energy change of mixing, $∆Hmix$: enthalpy of mixing, T: absolute temperature, and $∆Smix$: configurational (mixing) entropy. An increase in $∆Smix$ reduces $∆Gmix$, thereby stabilizing the solid solution phase. The configurational mixing entropy is quantified via the Boltzmann entropy relation:

$$∆Smix=-R{\sum }_N^{i=1}{x}_i{lnx}_i$$

(2)

where R: universal gas constant, $x_i$: atomic (mole) fraction of the i-th element, and N: total number of constituent elements. For equiatomic compositions ( $x_i={\displaystyle \frac{1}{N}}$), Equation (2) simplifies to the following:

$$∆Smix=RlnN$$

(3)

Figure 1a presents the entropy-stability map of two-dimensional high-entropy materials (2D HEMs), tracking the evolution from 3D bulk high-entropy alloys (HEAs, 2004) to 3D high-entropy ceramics (HECs, post-2015) and finally to 2D HEMs (post-2020). These 2D systems (thickness < 5 nm, ≥5 distinct basal plane metals) have interfacial entropy densities that are three orders of magnitude higher than bulk, validating the “dimensional reduction” trajectory—lower dimensionality enhances surface/interfacial entropy density for entropy-driven stabilization at lower temperatures, which is critical for protective coatings and flexible electronics. Figure 1b schematically depicts the atomic scale formation of entropy-stabilized solid solutions as a continuous transition (dilution → interface formation → short-range ordering → entropy-stabilized solid solution), with composition/temperature/deposition rate tuning enabling ordered-to-disordered transformations to support the design of high-strength/toughness high-entropy thin films. Figure 1c clarifies the relationship between $∆Smix$, atomic fractions, and N: low N leads to $∆Hmix$ dominance (phase separation/intermetallics) while high N leads to ∆Smix dominance (single-phase solid solutions, shaded “entropy-stabilized zone”). Figure 1d illustrates the random distribution of metallic/non-metallic elements, linking high configurational entropy to material properties [20,21,22].

2.2. Lattice Distortion Effect

The lattice distortion effect, a hallmark of high-entropy materials (HEMs, including high-entropy alloys (HEAs) and high-entropy ceramics (HECs)), arises from atomic size mismatch and valence electron differences among multiple components. This induces local lattice stresses (compressive/tensile), twisting, or deformation, which microscopically correlate with HEMs’ mechanical, thermal, and electrical properties [25]. In HEAs, solute atom diffusion/clustering causes lattice distortion that acts as dislocation motion barriers—elevating strength/hardness by increasing dislocation bypass energy. Additionally, local stress fields absorb energy to delay crack propagation (improving toughness) and suppress ordered phases, stabilizing the solid solution structure.

He et al. [26] investigated lattice distortion in HEAs. Figure 2a verifies that disparities in atomic radius and valence electrons induce distortion, which enhances the mechanical properties via solid solution strengthening. Figure 2b shows that random element distribution causes heterogeneous interatomic interactions, generating local strains that alter atomic behavior and phase transformation, thereby affecting thermal stability and corrosion resistance—quantifying such distortion is essential for explaining the macroscopic performance. Figure 2c presents the special quasi-random structure (SQS) of FeCoCrNiMn HEA [27], with black arrows marking neighboring bonds related to interatomic interactions and properties. Figure 2d displays Mn-Mn bond statistics, where the consistency between theoretical predictions and experimental data validates the computational model and highlights local distortion’s role in regulating HEA performance. Stress concentrations from distortion affect strength and toughness, making its control critical for high-performance HEA design. Figure 2e compares the full width at half maximum (FWHM) of radial distribution functions (g(r)) for pure Ni and FeNiCrCoCu HEA [28]; the alloy’s larger FWHM directly demonstrates size mismatch-induced distortion. Figure 2f illustrates atomic size differences between the alloy and pure Ni, which induce lattice perturbations and stress fields to enhance solid solution strengthening (improving hardness and toughness). In HECs, lattice distortion further improves the structural stability by relieving internal stress and reducing formation/mixing enthalpies.

2.3. Sluggish Diffusion Effect

Thermal diffusivity is a key metric for evaluating heat transfer performance in high-entropy alloys (HEAs). The “sluggish diffusion effect”—prominent in high-entropy materials (HEMs, including HEAs and high-entropy ceramics (HECs))—arises from atomic size disparities (lengthening diffusion pathways) and random element distribution (creating complex chemical environments) under thermal activation. These factors increase atomic migration energy barriers, significantly reducing diffusion rates [29]. Sluggish diffusion primarily regulates phase transformation, high-temperature stability, and mechanical properties: diverse interelement bonding raises atomic jump activation energy [30,31], suppressing grain growth and phase transformations to retain high strength/hardness, enhance creep resistance, and extend service life, endowing HEMs with superior high-temperature stability.

Figure 3a,b compare (a) temperature-dependent thermal diffusivity of pure Al vs. an HEA and (b) Ni atom migration energy in pure metal, conventional alloy, and HEA matrices. The HEA exhibits consistently lower thermal diffusivity across all temperatures and higher atomic migration barriers than pure metals and traditional alloys, which are attributed to sluggish diffusion-induced intricate atomic interactions and convoluted diffusion pathways. These features enhance high-temperature thermal stability and creep resistance by mitigating thermal stresses/damage, though they may affect hot working and heat treatment, requiring balanced consideration in practical applications [32]. Figure 3c presents interdiffusion coefficients ( $\widetilde{D_{ij}}$) of Al, Co, Cr, and Ni (with Fe as reference) in FCC AlCoCrFeNi HEA after 46 h annealing at 1273 K, 1323 K, and 1373 K. Al shows the highest interdiffusion coefficient (attributed to its small atomic radius and high chemical activity). All elements exhibit temperature-dependent diffusion (coefficients increase with temperature), which is consistent with conventional behavior, indicating that HEA diffusion characteristics can be tuned via heat treatment—critical for processing and applications. Jiang et al. [33] demonstrated via first-principles calculations that high-entropy carbides (HfTaZrNb)C and (HfTaZrTi)C possess higher Gibbs free energies at elevated temperatures, exhibiting superior thermal/mechanical stability compared to conventional carbides. (HfTaZrNb)C offers strength advantages (higher bulk modulus) for engineering applications, while (HfTaZrTi)C (larger thermal expansion coefficient) is more suitable for coatings. Future research should explore the effects of composition and heat treatment on HEM performance to achieve rational design of high-performance stable materials.

2.4. Cocktail Effect

The “cocktail effect,” a core feature distinguishing high-entropy materials (HEMs) from traditional counterparts, refers to synergistic performance enhancement by multiple principal elements, exceeding the linear superposition of individual component properties. Originating from HEMs’ high mixing entropy, significant lattice distortion, and sluggish diffusion, its mechanisms involve atomic size mismatch, electronic structure modulation, and lattice strain—enabling precise control over microscopic phase composition, crystal orientation, and defect structures to form stable solid solutions with complex substructures [37]. Tokarewicz et al. [38] showed that AlCoCrFeNiCu alloy hardness increases with Al content, accompanied by phase evolution (FCC → FCC + BCC → BCC), along with enhanced strength and reduced ductility, demonstrating the nonlinear compositional effect on properties. When four or more cations/anions occupy lattice sites in near-equimolar ratios, interelement electronic interactions and lattice strain synergism are amplified. This endows HEMs with excellent thermodynamic stability and cooperative enhancements in macroscopic hardness, strength, and other functional properties, establishing a unique “composition–structure–property” relationship that underpins the design of high-performance high-entropy ceramics (HECs) [39]. HECs extend the cocktail effect from metallic alloys to inorganic ceramics, typically existing as single-phase solid solutions [2].

Figure 4 presents representative ball-and-stick models of typical high-entropy ceramic (HEC) structures, including rock salt, fluorite, and perovskite types. Benefiting from electron–phonon coupling among multi-component elements, such structural diversity not only enables the modulation of quantum behaviors (e.g., superconductivity) but also realizes synergistic enhancements in mechanical strength, high-temperature stability, electrical performance, radiation resistance, and corrosion resistance of the materials. Consequently, HECs are endowed with highly tunable bandgaps, phonon spectra, and ionic transport properties, thereby opening up a broad design space for their applications in fields such as ultra-high-temperature protection, thermoelectric conversion, solid-state electrolytes, and electromagnetic absorption. Thus, the cocktail effect is not only the intrinsic driving force behind the multifunctional integration and performance breakthroughs of high-entropy ceramics, but it also inaugurates a new paradigm for target-oriented material design based on elemental combination strategies [8,40,41].

3. Influence of AIP Process Parameters on the Microstructure of High-Entropy Ceramic Films

Arc ion plating (AIP), also known as multi-arc ion plating (MAIP), is a widely used physical vapor deposition (PVD) technique in industry. It evaporates target material via a cathodic arc to generate high-density plasma (ionization degree > 80%) in a vacuum chamber; metal ions are accelerated by an external electric field to strike the substrate, enabling rapid deposition of target atoms/ions, and even stable deposition on non-metallic substrates with rational parameter adjustment [42,43].

Compared with magnetron sputtering (MS), AIP features a high deposition rate (10–20 nm·s⁻¹, suitable for thick films ≥ 5 µm), low energy consumption, simple equipment, and high target utilization—making it a mainstream method for high entropy coatings. For high-entropy ceramic (HEC) thin films, its high ionization rate effectively suppresses elemental segregation (atomic fraction deviation < 5%). Via single-target, alloy-target, or multi-target co-deposition, efficient HEC preparation is achieved, while substrate holder rotation ensures uniform film growth and high entropy system stability [44,45,46,47].

While both AIP and high-power impulse magnetron sputtering (HiPIMS) provide high-density plasma for high-quality thin film deposition, AIP holds distinct advantages in industrial scalability and cost-efficiency for HEC coatings. Unlike HiPIMS, which suffers from significantly lower deposition rates due to its high-voltage pulse characteristics, AIP leverages cathodic arc evaporation to maintain exceptional deposition efficiency, making it more viable for thick film preparation. Additionally, despite HiPIMS’ intense ion bombardment, improving surface smoothness and hardness, AIP’s unique mechanism achieves superior control over multi-component segregation (atomic fraction deviation < 5%) through intense ion fluxes that promote grain refinement and interfacial bonding [48,49]. AIP systems also exhibit lower energy consumption and simpler equipment architectures compared to HiPIMS’ complex power supplies, solidifying its position as the preferred choice for large-scale industrial production of HECs.

Increased ion bombardment energy in AIP promotes in-film atomic diffusion, forming dense diffusion layers or nano-layered/gradient structures. This refines grains, controls phase composition, and enhances interfacial bonding—endowing AIP-deposited films with higher adhesion than conventional evaporation coatings and underscoring the key role of ion bombardment [50,51]. Key process parameters for HEC films include substrate bias, reactive gas flow rate, and arc current; coordinating duty cycle and substrate temperature enables control of ion beam power and surface diffusion, yielding fine-grained, dense HEC films with suppressed segregation. These parameters directly regulate grain size, film quality, microstructure, and, ultimately, the macroscopic performance of the coatings [52,53,54] (Figure 5).

3.1. Substrate Bias

By regulating ion energy, substrate bias directly affects the ion-bombardment intensity, thereby controlling four key microstructural aspects of high-entropy ceramic (HEC) films: crystal structure, grain size, elemental distribution, and density [60,61]. Specifically, under low bias, weak ion-bombardment energy leads to high atomic disorder and partial amorphous phases; as bias increases, elevated ion kinetic energy promotes ordered atomic arrangement, forming stable crystalline solid solution structures (e.g., FCC, BCC). For grain size, low bias limits atomic mobility, causing grain coarsening and uneven distribution; with further bias enhancement, the intensified ion bombardment fragments the oversized grains and suppresses preferential growth, resulting in finer grains. In terms of elemental distribution, high-energy ion bombardment (under moderate-to-high bias) accelerates the diffusion and mixing of different atoms, effectively reducing segregation; it also enhances metal–nonmetal bonding (e.g., nitride formation), optimizing the film’s bonding state. Regarding density and morphology, low bias results in loose atomic stacking (prone to pores, defects, and columnar grains), while higher bias forces tighter packing—suppressing columnar growth, reducing porosity, and forming a smooth, dense surface. From a macroscopic perspective, the most direct effect of bias voltage variation on thin films is the change in the number of surface pitting particles and defects, as illustrated in Figure 6a–e. Meanwhile, the control of droplet particles and surface pitting can be optimized by rationally regulating the bias voltage, i.e., by adjusting the intensity of ion bombardment.

The regulatory role of substrate bias has been validated in simpler systems first. Kamaneva et al. [62] studied TiN films by AIP and found that increasing bias from −80 V to −150 V enhanced ion bombardment, leading to finer grains, higher (111)-oriented crystallinity, and improved density/hardness (Figure 6f–i). For HEC systems (the core focus here), substrate bias exhibits more complex, element-dependent regulatory behavior, due to multi-component interactions. Kuczyk et al. [63] deposited six high-entropy nitrides—(AlCrTaTiZr)N, (AlCrMoTaTiZr)N, (AlCrNbSiTiV)N, (HfNbTiVZr)N, (CrHfTiVZr)N, and (HfNbTaTiVZr)N—under biases of 0 V to −300 V via cathodic DC vacuum arc deposition. They observed that bias-driven ion bombardment affects the sputtering behavior and deposition rate, with distinct element responses: lighter elements (Ti, V) (lower boiling points, higher volatility) show more pronounced grain refinement and lattice distortion (due to higher mobility), while heavier elements (Ta, Hf) are mainly affected by lattice distortion (lower mobility limits grain refinement). Importantly, films were dense and uniform up to −200 V; beyond this bias, excessive ion energy caused surface pitting, large particle formation, temperature rise, and stress concentration—diminishing the strengthening effect.

Jia et al. [64] further revealed a non-monotonic relationship between bias and grain size in (FeCoCrNiAl)N HEC films (bias range: −20 V to −200 V). At low bias (−20 V), weak bombardment limited atomic mobility, leading to grain coarsening (average size: 5.9 nm); raising bias to −100 V intensified the bombardment, fragmenting the oversized grains and suppressing preferential growth, refining grains to 2.8 nm. However, further increasing bias to −150 V and −200 V strengthened localized thermal effects, restoring atomic mobility and causing slight coarsening (5.2–5.3 nm). This non-monotonic trend highlights the critical role of “optimal bias windows” in HEC fabrication, as excessive energy can offset refinement benefits via thermal effects.

In summary, substrate bias regulates the HEC film microstructure by tuning ion-bombardment intensity: moderate bias is optimal for grain refinement, segregation suppression, and densification. For HECs, multi-element interactions introduce unique behaviors (element-dependent responses, non-monotonic grain size), which must be considered for process optimization.

Figure 6. Surface morphologies of the AlCrSiN coatings, deposited at different pulse bias voltages. (a) 0 V; (b) −50 V; (c) −100 V; (d) −200 V; (e) −300 V. [65] (f,g) Formation of space globular structures: substrate surface wetting stimulation, increase in globule-to-surface contact area and decrease in globule height as substrate bias voltage increases. (f) 80 V–1.0 × 1.7 µm, and (g) 200 V–ϕ1.2 µm. (h,i) Formation of 3D structures with granular substructure: influence of bias voltage on the shape, the orientation with respect to the substrate, and the size of the structure. (h) 150 V–ϕ1.4 μm, H ≈ 2.0 μm, and (i) 200 V–ϕ720 nm to 2.5 µm, H ≈ 1.0 µm [62].

Figure 6. Surface morphologies of the AlCrSiN coatings, deposited at different pulse bias voltages. (a) 0 V; (b) −50 V; (c) −100 V; (d) −200 V; (e) −300 V. [65] (f,g) Formation of space globular structures: substrate surface wetting stimulation, increase in globule-to-surface contact area and decrease in globule height as substrate bias voltage increases. (f) 80 V–1.0 × 1.7 µm, and (g) 200 V–ϕ1.2 µm. (h,i) Formation of 3D structures with granular substructure: influence of bias voltage on the shape, the orientation with respect to the substrate, and the size of the structure. (h) 150 V–ϕ1.4 μm, H ≈ 2.0 μm, and (i) 200 V–ϕ720 nm to 2.5 µm, H ≈ 1.0 µm [62].

3.2. Reactive Gas Flow Rate

The flow rate of reactive gas (e.g., N₂, C₂H₂) is a core parameter governing the fabrication of high-entropy ceramic (HEC) thin films, as it directly dictates the concentration of non-metallic elements (N, C) incorporated into the film and thereby modulates the proportion of metal–nonmetal (Me-N/Me-C) bonds—an essential factor for tailoring microstructures and properties. This regulatory role follows a well-defined “three-state” mechanism [66,67]: an insufficient reactive gas supply leads to incomplete reaction of sputtered metal ions, resulting in dominant metal–metal (Me-Me) bonds, sub-stoichiometric phases (sub-nitrides or sub-carbides), and poor structural stability; in contrast, adequate gas supply facilitates sufficient Me-N/Me-C bond formation, driving the film toward stoichiometry and stabilizing the high-entropy solid solution structure; excessive gas flow, however, induces target poisoning (a dense nitride/carbide layer forms on the target surface), suppressing metal ion sputtering and reducing the deposition rate, while surplus non-metallic species (e.g., free N, amorphous carbon) may cause supersaturation and disrupt solid solution continuity. Taking HfNbTaTiVZrN films as an example, Figure 7a,b reveal the effects of nitrogen partial pressure and flow ratio on the film elemental content and deposition rate. A rational nitrogen flow rate facilitates the formation of nitrides, enhances film density and phase structure, and thereby achieves the dual goals of reducing surface defects and improving performance. The structural evolution of surface morphology and film thickness is illustrated in Figure 8.

This three-state regulation has been extensively validated in high-entropy ceramic (HEC) systems, particularly high-entropy nitrides controlled by N₂ flow. Cui et al. [68] verified that N₂ flow rate directly affects the elemental uniformity and film thickness of FeCoCrNiAlN and TiVCrNiSiN films, with adequate N₂ reducing compositional fluctuation, increasing film thickness, and promoting densification. Li et al. [69] further established a direct correlation between N₂ flow, nitrogen content (x, atomic fraction in (AlCrMoTiNi)_1−xN_x), microstructure, and performance via co-filtered cathodic vacuum arc deposition: as N₂ flow increased (x from 0 to 0.45), the film structure evolved sequentially from an amorphous phase (x = 0, no N) to an amorphous matrix with metallic Ni (x = 0.29, insufficient N), and finally to a nanocrystalline face-centered cubic (FCC) solid solution with residual amorphous phases (x ≥ 0.37, adequate N), achieving optimal hardness, toughness, wear resistance, and corrosion resistance at x = 0.45 (defined as the “optimal N₂ flow window” for saturated solid solution formation). Jia et al. [64] supplemented the N₂ regulation mechanism in FeCoCrNiAlN films, noting that Cr preferentially bonds with N (enhancing hardness but impairing toughness if excessive) while Al lowers the alloy melting point to improve coating uniformity and densification; increasing Al content also drives an FCC-to-BCC phase transition to accommodate larger Al atoms, relieve lattice strain, and stabilize the system. Yan et al. [70] corroborated N₂’s structural regulation effect in multi-arc ion plating-prepared (ZrTiNbV)N films: at a nitrogen flow ratio (R_n) of 0, the film was a body-centered cubic (BCC) alloy with dense large particles, numerous pits, and high surface roughness, whereas increased N₂ flow induced a transformation to an FCC-type nitride solid solution with reduced roughness and enhanced uniformity, density, and substrate adhesion.

The regulatory effect of reactive gas flow is not limited to nitrides; it also extends to high-entropy carbides controlled by C₂H₂ flow, with system-specific characteristics. Xu et al. [71] fabricated (TiCrZrVNb)C films via multi-arc ion plating, adjusting C₂H₂ flow rate to control the acetylene ratio (Rc). Their resultsshowed a sequential structural evolution with increasing Rc: an amorphous state at Rc = 0 (no C₂H₂, due to elemental crystal structure differences and atomic size mismatch inhibiting long-range ordering), a single-phase FCC solid solution at Rc > 1 (sufficient C₂H₂ enabling Ti/Cr to react with C and form FCC Me-C phases), and a composite of FCC solid solution and amorphous carbon at Rc > 2 (excessive C₂H₂ leading to surplus carbon precipitation). This distinguishes carbides from nitrides: while excessive N₂ primarily causes target poisoning, carbides are more prone to secondary phase formation, requiring stricter C₂H₂ flow control. Beyond N₂ and C₂H₂, oxygen can serve as an auxiliary reactive gas to functionalize HEC films. Li et al. [72] incorporated O₂ into Cr-based HEC coatings and found that an O₂ flow rate of 130 sccm promotes Cr₂O₃ formation—this oxide phase imparts lubricity and chemical stability, significantly reducing frictional adhesion and enhancing working performance. This finding suggests multi-gas co-regulation (e.g., N₂ + O₂) may extend HEC functionality, though it remains a supplementary strategy to N₂/C₂H₂ control, which is primary for solid solution formation.

Figure 7. (a) Metal content in terms of the dependence of the nitrogen flow ratio and a working pressure p = 2 Pa and p = 5 Pa, (b) deposition rate and nitrogen atomic content in terms of the dependence of the N₂/(N₂ + Ar) ratio and the working pressure [73].

Figure 7. (a) Metal content in terms of the dependence of the nitrogen flow ratio and a working pressure p = 2 Pa and p = 5 Pa, (b) deposition rate and nitrogen atomic content in terms of the dependence of the N₂/(N₂ + Ar) ratio and the working pressure [73].

Figure 8. (a–l) Surfaces for (HfNbTaTiVZr)N coatings, deposited at working pressures of p = 2 Pa (a–f) and p = 5 Pa (g–l) and varying nitrogen flow ratios R_N2. (m–p) Cross sections of (HfNbTaTiVZr)N coatings deposited at p = 5 Pa with different nitrogen flow ratios: (m) 0.09; (n) 0.12; (o) 0.25, and (p) 0.33 [73].

Figure 8. (a–l) Surfaces for (HfNbTaTiVZr)N coatings, deposited at working pressures of p = 2 Pa (a–f) and p = 5 Pa (g–l) and varying nitrogen flow ratios R_N2. (m–p) Cross sections of (HfNbTaTiVZr)N coatings deposited at p = 5 Pa with different nitrogen flow ratios: (m) 0.09; (n) 0.12; (o) 0.25, and (p) 0.33 [73].

In conclusion, reactive gas flow rate modulates HEC thin films by tuning the plasma activity, stoichiometry, and deposition rate, thereby governing compositional uniformity, phase orientation, grain size, and density—all of which directly impact hardness, elastic modulus, and wear/corrosion resistance. A universal “optimal flow window” exists across HEC systems: moderate flow is indispensable for achieving single-phase solid solutions and balanced properties, while insufficient or excessive flow degrades performance. System-specific differences (e.g., nitrides’ tolerance to excessive gas vs. carbides’ sensitivity to secondary phases) and element competition effects (e.g., Cr’s preferential reaction with N) must be considered in process optimization. For practical applications, systematic flow-gradient experiments combined with characterization techniques (XRD, XPS, TEM, nano-indentation) are necessary to identify the optimal gas flow window for each HEC system, enabling synergistic optimization of structure and properties.

3.3. Arc Current

The arc current influences plasma characteristics, target sputtering behavior, and film-growth kinetics, thereby exerting multidimensional effects on the composition, microstructure, and mechanical properties of high-entropy ceramic (HEC) thin films. Specifically, the arc current directly determines the energy and stability of cathode arc spots: at low arc currents, plasma energy is low and particle migration on the substrate is weak, resulting in porous films, coarse grains, or even amorphous structures; as the arc current increases, the cathode spot temperature rises, the metal atom ionization degree (up to 90%) increases, and both the concentration and kinetic energy of plasma metal ions rise markedly. This high-energy ion bombardment induces a “kinetic energy injection effect,” promoting dense atomic packing, single solid solution phase formation, and suppression of excessive grain growth—ultimately yielding a nanocrystalline structure [74,75]. Highly ionized plasma further enables complete reactions between multi-metallic ions in HECs and non-metallic species (N, C), facilitating uniform solid solution phases (e.g., FCC-type high-entropy nitrides) and preventing phase separation caused by insufficient ion concentration. Conversely, an excessively high arc current causes rapid target evaporation, generating abundant metal droplets (a typical arc ion deposition defect); these droplets form surface protrusions or internal pores, reducing film densification and inducing stress concentration [76]. For refractory metal targets (Nb, Ta, Hf) with low sputtering yields, moderate arc current elevation enhances the sputtering intensity, ensuring near-equimolar ratios of these elements in the film; however, mismatched arc currents across multiple targets may cause over-sputtering of low-melting-point metals, compromising compositional uniformity and film performance.

The regulatory role of the arc current has been extensively validated across diverse HEC and ceramic systems, with clear evidence of target-material dependence. Zhou et al. [77] fabricated TiAlSiN coatings via arc ion deposition, varying the arc current on a TiAlSi composite target. As shown in Figure 9a–c, increasing the multi-arc current induced systematic changes: surface roughness increased due to more large surface particles, while the internal microstructure improved—the coating density rose, porosity decreased, Al content increased continuously, and phase composition (dominated by Ti₃AlN, AlN, Ti₂N) showed enhanced hexagonal AlN formation at 70 A. Film growth was also modulated: thickness increased from 4.47 µm (50 A) to 8.84 µm (70 A), with microstructure transitioning from loose fibrous grains to dense equiaxed grains.

In contrast, Peng et al. [78] observed opposing trends in Cr coatings on a zirconium alloy (multi-arc ion plating). As shown in Figure 9d,e, increasing the arc current from 120 A to 150 A led to more porous columnar crystals and higher structural defect density—directly differing from Zhou et al.’s TiAlSiN results and highlighting system-specific arc current effects. Zhu et al. [79] further confirmed this dependence in AlCrN coatings (AlCr target): arc current elevation drove phase evolution from Al₈Cr₅ intermetallic compound to FCC CrAlN, alongside reduced surface metal particles/droplets and surface roughness (from 98.206 nm at 120 A to 51.463 nm at 180 A), yielding more uniform dense coatings (Figure 9f–i).

Collectively, these studies demonstrate that the arc current exerts non-linear, system-dependent effects on coatings: its influence on surface morphology (large particles/droplets), densification, and phase composition varies with the target material, and performance does not universally improve with an increasing current. Consequently, optimizing arc ion deposition processes for HECs requires a systematic, material-specific assessment of arc current parameters—ensuring alignment with the target system’s unique response to current modulation.

3.4. Discussion on Film Failure and Durability

Notably, despite the highly controllable process conditions and excellent film properties offered by AIP, the prepared films still suffer from failure issues such as large droplets and defects. Therefore, it is necessary to discuss the durability of AIP-deposited films. From the perspective of failure mechanisms, film failure does not occur instantaneously but is a progressive process involving the initiation of surface defects, crack propagation, and eventual delamination and spallation. During the corrosion or wear processes, corrosion products (e.g., oxides, sulfides) tend to accumulate at the coating/substrate interface or grain boundaries. Mismatch in thermal expansion coefficients or volume expansion of corrosion products induce microcracks at the interface; as these cracks propagate, the coating detaches along the interface, forming a distinct step-like delamination structure. For high-temperature protective coatings (e.g., Cr-Al, NiCrAlY), a dense oxide protective layer (e.g., Al₂O₃ or Cr₂O₃) forms on the surface. When the oxide layer thickness exceeds a critical value or is subjected to mechanical impact, the brittle oxide layer undergoes fracture. After the spallation of oxide fragments, the underlying metal substrate is directly exposed to the corrosive environment, leading to a sharp increase in the corrosion rate [80,81,82].

In addition to controlling optimal process parameters during film deposition (as discussed in the preceding sections) to minimize the number of particles and defects and achieve a sufficiently dense and uniform film, we propose that monitoring three key parameters—corrosion potential, interfacial adhesion, and surface roughness—during film service is conducive to evaluating the failure state of the film. The service life of AIP-deposited films typically consists of three stages: an initial stable period, a performance degradation period, and a final failure period. A decrease in corrosion potential indicates reduced corrosion resistance; the weakening of interfacial adhesion signifies the risk of film delamination; and an increase in surface roughness leads to enhanced friction and wear. Changes in these three properties all expose the coating to failure risks such as spallation and wear. Monitoring and optimizing these three properties before and after film service can extend the durability of the film to a certain extent, thereby creating greater value. Figure 10 illustrates several failure modes of the films.

4. Primary Properties of High-Entropy Ceramic Films Deposited by AIP

Structure determines performance. For high-entropy ceramic (HEC) thin films fabricated by arc ion plating (AIP), the key process parameters summarized earlier regulate the microstructure in a targeted manner: substrate bias dominates grain refinement and texture by adjusting ion incident energy; reactive gas flow rate and working pressure modulate phase composition and densification via controlling plasma density and collision frequency; and the arc current sets the deposition rate and microstructural uniformity through regulating target evaporation rate and metal atom ionization degree. This forms a tightly connected “process–structure–property” triad—yet the multi-element nature of HECs increases parameter optimization complexity, driving research from traditional trial-and-error toward a rational design paradigm [8,37,85].

This paradigm follows three core steps: first, thermodynamic models of high-entropy systems are used for preliminary compositional and structural screening; second, first principles (FP) calculations (e.g., density functional theory, DFT), machine learning (ML) techniques, and other informatics tools are integrated to conduct in-depth theoretical analysis and property prediction; finally, efficient orthogonal experiments explore optimal processing windows for different HEC systems in a multidimensional parameter space. Kretschmer et al. [86] exemplified this paradigm by applying DFT to predict the structural stability of (Hf, Ta, Ti, V, Zr)BN HEC systems (with varying B:N ratios): they investigated the stability of FCC (NaCl-type), hexagonal AlB₂-type, and orthorhombic FeB-type structures, and validated their predictions by fabricating (Hf, Ta, Ti, V, Zr)BN composite coatings via magnetron sputtering. Building on this, Jia et al. [64] developed ML-based predictive models for (FeCoCrNiAl)N HEC films—taking key process parameters (N/Ar flow ratio, deposition bias, substrate temperature) and compositional variables (atomic fractions of Fe, Co, Cr, Ni, Al) as inputs to estimate the tribological performance, enabling a direct “parameter to property” prediction.

Overall, the core objective of this theoretical–experimental design is to retain AIP’s high ionization advantage while fabricating an ideal HEC microstructure: ultrafine grains, high density, near absence of defects, and a high-entropy-driven single-phase solid solution. This microstructure serves as the physical basis for the outstanding hardness, wear resistance, oxidation resistance, and corrosion resistance of HEC thin films. The following sections provide a detailed discussion of these key properties.

4.1. Mechanical Properties

High-entropy ceramic (HEC) films exhibit exceptional mechanical performance—maintaining ultrahigh hardness (≥30 GPa) and elastic modulus (≥380 GPa) while significantly improving fracture toughness—which is driven by the synergistic effects of multi-element solid solution strengthening, lattice distortion, and high configurational entropy. Specifically, high configurational entropy stabilizes the single-phase solid solution structure (providing a basis for the other two effects); solid solution strengthening arises from solute atoms impeding dislocation motion; and lattice distortion (from atomic size-mismatches) further increases dislocation resistance. Their mechanical performance also shows good adaptability to temperature, deposition atmosphere, and post-treatment processes; the entropy-driven design even enables flexible bending, broadening application prospects in high-temperature, high-load scenarios (e.g., aerospace engines, turbine blade components, micro-electronic packaging) [8,87].

This performance is supported by experimental data. Wan et al. [6] investigated FeCoCrNiAl_0.1N HEC films, quantifying the effects of gas pressure and substrate bias on hardness (34.3 GPa) and Young’s modulus (>600 GPa). Both properties increased with nitrogen pressure—indicating that a higher N₂ pressure promotes harder, stiffer coatings—while the maximum hardness was achieved at bias B4 and peak Young’s modulus at bias B5, highlighting substrate bias as a key regulator of mechanical performance. The H/E and H³/E² ratios further enable a quantitative evaluation of HEC film performance. The H/E ratio (often called the “elastic strain limit”) represents the maximum elastic strain before plastic yielding—high values indicate excellent damage tolerance (microcracks are less likely to nucleate/propagate and elastic recovery buffers impact energy), challenging the “high hardness = brittleness” misconception. The H³/E² ratio is proportional to the indentation plastic deformation yield pressure; high values suppress surface indentation and plastic deformation, which is critical for tribological applications (e.g., reducing wear from surface plowing) [88]. These metrics are validated in other HEC systems: Lin et al. [89] and Li et al. [69] reported hardness, elastic modulus, H/E, and H³/E² data for Cr_0.35Al_0.25Nb_0.12Si_0.08V_0.20N and (AlCrMoTiNi)_1−xN_x films. Cross-comparison shows that HEC films outperform their high-entropy alloy (HEA) counterparts (N = 0) by severalfold in hardness, modulus, toughness, and plastic deformation resistance. The reported maximum hardness, elastic modulus, and corresponding process parameters of some high-entropy ceramic (HEC) films are presented in Table 1. In addition to the high-entropy ceramic nitride films commonly used in industry, Figure 11 also presents the mechanical properties of high-entropy ceramic oxide films with different structures for reference.

Arc ion plating (AIP) is pivotal for enhancing HEC film mechanical properties, with two core mechanisms: First, high-energy particle bombardment promotes abundant Me-N/Me-C covalent bond formation—these bonds have much higher energies than metallic bonds, impeding atomic slip and increasing hardness/modulus [90]. Second, CAIP induces grain refinement (increasing the grain boundary area) and lattice distortion (from atomic size mismatches generating internal strain); both hinder dislocation motion, further elevating hardness [91]. Additionally, sufficient reactive gases enable multi-element ceramic phases (e.g., TiN, CrN) to form a single FCC solid solution via the high-entropy effect—its dense, isotropic packing ensures high hardness, while multiple slip systems enable plastic coordination, mitigating conventional single-phase ceramic brittleness.

Table 1. Reported maximum hardness and elastic modulus of some high-entropy ceramic films.

Film System	Parameters	Hardness (GPa)	Elastic Modulus (GPa)	Ref.
(AlCrMoTiNi)1−xNx	AIP. x = 0, −50 V, 0.15 Pa	13.3	212.7	[69]
(AlCrMoTiNi)1−xNx	AIP. x = 0.45, −50 V, 0.15 Pa	32.26	333.7
(ZrTiNbV)N	AIP. Rn = 0, −200V, 200 °C, 0.28–0.31 Pa	7.67	176	[70]
(ZrTiNbV)N	AIP. Rn = 3, −200V, 200 °C, 0.54–0.59 Pa	33.3	411
(MoNbTaVW)1−xNx	AIP/MS. x = 0, 120 A, 5 Pa	17	-	[92]
(MoNbTaVW)1−xNx	AIP/MS. x = 0.35, 0.15 A, 0.5 Pa	28	-
(AlCrTaTiZr)N	MS. −100 V, 150 W	30	277	[63]
(AlCrMoTaTiZr)N	MS. Rn = 40%, 150 W, 0.8 Pa	40.2	420
(AlCrNbSiTiV)N	MS. −100 V, 200 W	41	410
(HfNbTiVZr)N	AIP. −110 V, 5 Pa	44.3	460
(CrHfTiVZr)N	MS. −100 V, 350 W	34.1	316
(HfNbTaTiVZr)N	AIP. −70 V	45.32	314
(TiCrZrVAl)N	AIP. Rn = 0%, −200 V	9.5	176	[93]
(TiCrZrVAl)N	AIP. Rn = 80%, −200 V	32.9	400
(NbMoCrTiAl)N	AIP. Cr-150 A, TiAl-180 A, Mo-90 A, Nb-120 A	43	326	[94]
(TiCrZrVNb)C	AIP. Rc = 0, −200 V, 350 °C	11.8	200	[71]
(TiCrZrVNb)C	AIP. Rc = 1, −200 V, 350 °C	27.7	384

Table 1. Reported maximum hardness and elastic modulus of some high-entropy ceramic films.

Film System	Parameters	Hardness (GPa)	Elastic Modulus (GPa)	Ref.
(AlCrMoTiNi)1−xNx	AIP. x = 0, −50 V, 0.15 Pa	13.3	212.7	[69]
(AlCrMoTiNi)1−xNx	AIP. x = 0.45, −50 V, 0.15 Pa	32.26	333.7
(ZrTiNbV)N	AIP. Rn = 0, −200V, 200 °C, 0.28–0.31 Pa	7.67	176	[70]
(ZrTiNbV)N	AIP. Rn = 3, −200V, 200 °C, 0.54–0.59 Pa	33.3	411
(MoNbTaVW)1−xNx	AIP/MS. x = 0, 120 A, 5 Pa	17	-	[92]
(MoNbTaVW)1−xNx	AIP/MS. x = 0.35, 0.15 A, 0.5 Pa	28	-
(AlCrTaTiZr)N	MS. −100 V, 150 W	30	277	[63]
(AlCrMoTaTiZr)N	MS. Rn = 40%, 150 W, 0.8 Pa	40.2	420
(AlCrNbSiTiV)N	MS. −100 V, 200 W	41	410
(HfNbTiVZr)N	AIP. −110 V, 5 Pa	44.3	460
(CrHfTiVZr)N	MS. −100 V, 350 W	34.1	316
(HfNbTaTiVZr)N	AIP. −70 V	45.32	314
(TiCrZrVAl)N	AIP. Rn = 0%, −200 V	9.5	176	[93]
(TiCrZrVAl)N	AIP. Rn = 80%, −200 V	32.9	400
(NbMoCrTiAl)N	AIP. Cr-150 A, TiAl-180 A, Mo-90 A, Nb-120 A	43	326	[94]
(TiCrZrVNb)C	AIP. Rc = 0, −200 V, 350 °C	11.8	200	[71]
(TiCrZrVNb)C	AIP. Rc = 1, −200 V, 350 °C	27.7	384

Figure 11. Summary of mechanical properties of HEOs from a literature survey: (a) elastic modulus and hardness, (b) H/E ratio, (c) hardness, and (d) elastic modulus with distortion across different HEO structures. The hardness and elastic modulus of several important engineering metals are also included for comparison, which are common substrate materials for such coatings [95].

Figure 11. Summary of mechanical properties of HEOs from a literature survey: (a) elastic modulus and hardness, (b) H/E ratio, (c) hardness, and (d) elastic modulus with distortion across different HEO structures. The hardness and elastic modulus of several important engineering metals are also included for comparison, which are common substrate materials for such coatings [95].

4.2. Tribological Performance

Tribological performance is a longstanding focus in thin film research. Conventional ceramics (e.g., SiC) exhibit poor tribological behavior due to their inherent brittleness [96,97]: under sliding contact, surface defects such as pores and grain boundaries easily induce stress concentration, triggering crack initiation. These cracks either propagate along grain boundaries, causing entire grains to detach from the matrix, or directly through grains, resulting in material spallation. At elevated temperatures or under frictional heating, SiC oxidizes to form a SiO₂ glass layer; while this oxide layer provides limited lubrication and protection, its weak bonding with the underlying SiC matrix often leads to delamination under mechanical stress. This delamination exposes fresh substrate, which undergoes brittle fracture again, establishing a cyclic “oxidation–delamination–reoxidation” process that ultimately makes brittle removal the dominant wear mechanism of SiC.

In sharp contrast, high-entropy ceramic (HEC) films achieve superior tribological performance through the synergistic effects of microstructural and chemical features that are inherent to high-entropy systems. Severe lattice distortion, caused by atomic size mismatches among multiple constituent elements, significantly impedes dislocation motion and crack propagation, thereby reducing the risk of brittle fracture during sliding contact. Under external loads, HEC films do not undergo immediate brittle fracture; instead, they undergo slight localized plastic deformation via mechanisms such as grain boundary sliding and nanograin rotation. This plastic deformation effectively blunts crack tips, relieves local stress concentration, and thus delays coating spallation [98]. Notably, the multi-component nature of HECs facilitates the in situ formation of complex, dense, and well-bonded mixed oxide layers (e.g., Cr₂O₃, Al₂O₃) during tribological processes, and the comprehensive action mechanisms of these oxide layers are crucial for enhancing tribological performance. First, the dense oxide layer acts as a physical barrier, which tightly adheres to the film surface and effectively blocks the inward diffusion of oxygen from the external environment and the outward diffusion of metallic elements from the film matrix, thereby suppressing further oxidative wear and chemical degradation of the substrate. Second, the mixed oxide system exhibits excellent chemical stability and mechanical compatibility with the HEC matrix; it can withstand cyclic shear stress during sliding without premature delamination, maintaining long-term protective efficacy. Moreover, in certain cases, specific elements in HECs (e.g., V, Mo) preferentially oxidize to form oxides (V₂O₅, MoO₃) with intrinsic lamellar or glassy lubricating properties. These oxides can form a continuous lubricating film at the friction interface, reducing direct contact between the HEC film and the counterbody, thereby significantly lowering the frictional resistance and achieving a “self-lubricating” effect [99]. Additionally, most HEC thin films possess a nanocrystalline/amorphous composite structure, which further optimizes their tribological behavior: the hard nanocrystalline phase serves as the load-bearing skeleton, effectively resisting the applied contact stress, while the surrounding amorphous phase (typically distributed at grain boundaries) relaxes stress through viscous flow under frictional heating and hinders the transgranular and intergranular propagation of cracks. This synergistic effect of the dual-phase structure makes brittle material removal exceptionally difficult for these coatings, thus enhancing their wear resistance.

Numerous experiments support these advantages. Jia et al. [64] studied (FeCoCrNiAl)N films and found that low nitrogen pressure led to insufficient ceramic phase, low hardness, deep wear tracks, and high COF (similar to conventional ceramics’ brittle wear). Optimized nitrogen pressure improved the hardness and wear resistance; excessive Cr reduced the toughness and wear resistance, while a higher critical load (Lc1) correlated with stronger film–substrate adhesion and lower COF. Mishra et al. [100] observed that high-entropy silicide (TiVNbMoW)Si₂ had better wear resistance than conventional alloys, as the silicide phase absorbed frictional energy and acted as a lubricant, with wear dominated by mild adhesion and abrasion. Lothrop et al. [101] validated HECs’ superiority by comparing (AlCrTiMoV)N and TiN coatings (Figure 12): under the same conditions, (AlCrTiMoV)N had a comparable COF to TiN but half the wear rate, and SEM analysis of the wear debris confirmed its milder wear mechanism.

Optimizing the tribological performance of HEC films relies on the synergistic enhancement of microstructural densification, surface smoothness, and film–substrate adhesion. In cathodic arc ion plating (CAIP), a key fabrication technique for HECs, high-energy particle bombardment not only cleans the substrate surface but also promotes interfacial diffusion between the film and substrate, forming a robust bonding layer that strengthens film–substrate adhesion [102]. Additionally, the Me-N and Me-C covalent bonds in HECs, with bond energies far exceeding those of pure metallic bonds, make the films less prone to adhesive interaction with counter faces (e.g., Si₃N₄ balls or WC-Co balls) during sliding, thereby effectively reducing the COF. Furthermore, the multiple metallic elements in HEC systems can form an ultra-thin oxide film (e.g., Al₂O₃, Cr₂O₃) on the sliding surface; these oxides are both hard and lubricious, further lowering frictional resistance while protecting the interior of the coating from excessive wear and preventing adhesive wear-induced delamination.

4.3. Electrochemical Performance

High-entropy ceramic (HEC) films exhibit multiple advantages in electrochemical applications. Among them, high-entropy carbide films exhibit excellent electrochemical corrosion resistance due to their low porosity and uniform microstructure, which significantly reduces open-circuit potential and corrosion current density in corrosive media [103,104]. Their low leakage current and temperature-stable dielectric constant remain constant over a wide temperature range, endowing HEC films with superior energy storage and capacitance performance; for instance, flexible capacitors retain a stable dielectric response even after repeated bending cycles [105,106,107].

Figure 13a–c presents the morphological and defect analyses of TiN/ZrN composite films deposited on Ti-6Al-4V substrates, conducted by Geng et al. [83] via long-term salt spray corrosion tests. Combined with Figure 13e, the physicochemical processes of corrosion can be intuitively understood, where the formation and propagation of pits and droplets are identified as the key factors leading to the corrosion failure of the material. A comparison of the surface morphologies of Cr/TiAlN and Cr/TiAlSiN multilayers before and after corrosion tests simulating a nuclear reactor environment, conducted by Yang et al. [108], reveals that a large number of pores and irregularly sized particles appear on the surface of the Cr/TiAlN coating. This phenomenon is most likely attributed to the early-stage erosion of large particles adhering to the coating surface by high-pressure water and high temperature. These defects cause an uneven distribution of nitrogen within the film, which in turn induces microporous corrosion and degrades the corrosion resistance of the Cr/TiAlN coating. In contrast, the Cr/TiAlSiN coating remains dense and uniform under the same experimental conditions, with no obvious corrosion damage observed. This experiment further validates that the entropy-increased system contributes to the enhancement of the corrosion resistance of thin films.

Xia et al. [92] fabricated (MoNbTaVW)N high-entropy nitride coatings through cathodic arc deposition. By adjusting the nitrogen flow, they intensified electron scattering at grain boundaries and lattice distortions, which raised the resistivity from 7 × 10⁻⁷ Ω·m to 4 × 10⁻⁶ Ω·m. Additionally, Yan et al. [109] deposited (AlTiVCrMo)N high-entropy ceramic films with different nitrogen contents by using a co-filtering cathodic vacuum arc system. The interface contact resistance (ICR) of these coatings was significantly better than that of titanium substrates. Under a PEMFC assembly pressure of 1.4 MPa, a nitrogen-free high-entropy alloy (HEA) coating had an ICR of approximately 40 mΩ·m·cm⁻². In contrast, a nitrogen-rich (28.12 at.%) HEC coating achieved a low ICR of 9 mΩ·m·cm⁻², a corrosion potential of 0.193 V, and a corrosion current density of 3.11 × 10⁻⁷ A·cm⁻². For comparison, the Ti substrate showed an ICR of 608 mΩ·m·cm⁻², a corrosion potential of −0.276 V, and a corrosion current density of 1.84 × 10⁻⁵ A·cm⁻². The introduction of nitrogen improves interfacial conductivity. Nitrogen-free coatings have a higher ICR because columnar grains and disordered grain-boundary atoms cause electron scattering. On the other hand, nitrogen incorporation forms nitride phases, mitigates lattice distortion, and introduces defect-mediated conductive electrons when the nitrogen content is optimal (≈28.12 at.%). This process enhances both corrosion resistance and electrical performance. Furthermore, Zhu et al. [79] reported that AlCrN films show significantly improved corrosion resistance as the current density increases. At 180 A, the corrosion potential reaches −165 mV (compared with −346 mV for the substrate), and the corrosion current density drops to 0.05 µA·cm⁻². This results in a protection efficiency of 98.25%. The main reasons for this improvement are a higher Al content, which generates more Al₂O₃, and the dense coating structure that suppresses the diffusion of corrosive ions.

The high-energy particle bombardment that is inherent to cathodic arc ion plating produces highly dense, defect-scarce films. These films block electrolyte-infiltration pathways, which prevents interfacial corrosion or side reactions between the coating and substrate [110,111]. For example, they can suppress electrolyte decomposition in lithium-ion batteries or electrolyte leakage in fuel cells [112]. The disordered solid solution structure of HECs is characterized by substantial lattice distortion and defect sites (vacancies, interstitials). This structure provides rapid ion transport channels, lowers ion migration barriers, enhances ionic conductivity, and reduces interfacial resistance. Moreover, these defect pathways reduce film delamination or cracking caused by volumetric expansion/contraction during electrochemical cycling. A typical example is the volume effect in lithium-ion battery electrodes. This helps preserve the long-term integrity of the electrode/electrolyte interface [113]. By doping conductive elements such as Ti or Nb, HECs can form a “solid solution conductive network,” and this network tunes electronic conductivity. Additionally, adjusting deposition parameters to control the grain size, crystallographic orientation, and surface roughness further customizes the physical pathways for ion adsorption and diffusion.

4.4. Oxidation Resistance

High-entropy ceramic (HEC) films exhibit superior high-temperature oxidation resistance, which is attributed to the synergistic effects of multi-component high-configurational entropy and sluggish diffusion behavior [114,115]. These effects promote the formation of dense and stable oxide layers at elevated temperatures, which inhibit oxygen penetration and internal oxidation. Consequently, some HEC films retain structural integrity and hardness even above 1100 °C, extending the service life of substrate components (e.g., turbine blades) and overcoming the limitation of conventional yttria-stabilized zirconia (YSZ) coatings that fail at ~1200 °C.

The superior oxidation resistance of HECs has been verified by numerous experimental studies. Fu et al. [116] prepared CrAlSiWN coatings on SKH-51 steel via multi-arc ion plating, which exhibited excellent thermodynamic stability below 1100 °C and a low oxidation mass gain rate of only 0.08 mg/mm² at 1100 °C. The study on TiAlSiN coatings by Zhou et al. [77] further revealed the underlying mechanism: Figure 14a shows that coatings prepared under different arc currents all exhibited low oxidation rates at 800 °C, with the arc current having a negligible effect on oxidation resistance. This is attributed to the synergistic effect of lattice distortion and sluggish diffusion, which inhibits oxidation kinetics and endows the coatings with high chemical stability. Figure 14b–f present the microstructure of the oxide layer formed on the AlMo_0.5NbTa_0.5TiZr alloy after oxidation at 1000 °C for 10 h [117]: surface roughness and pores accelerate oxidant penetration, while surface elemental enrichment (e.g., Mo-rich regions) can regulate local stability; the synergistic effect of differences in physical and chemical properties among various sublayers retards the diffusion and oxidation reactions. Studies on other systems also confirm the advantages of HECs: Ding et al. [118] reports that lattice distortion in TiZrHfN reduces the diffusion energy barrier and optimizes thermal performance; Peng et al. [119] demonstrated that (TiVCrZrHf)N maintains good thermal stability at 700 °C when deposited on copper substrates; and Yan et al. [120] synthesized (HfZrTaNbTi)C by spark plasma sintering, retaining a single-phase structure below 1140 °C. Zhou et al. [77] also found the regulatory effect of the arc current on oxidation resistance: the oxidation resistance improved with an increasing arc current, reaching the optimal level at 70 A. At this current, the formed Al₂O₃ barrier layer exists in a solid solution state due to the high-entropy effect, achieving grain refinement, increased density, and enhanced interface bonding. The core advantage of HECs stems from their multi-component disordered solid solution structure: it significantly increases the activation energy of atomic diffusion, reducing the diffusion rates of oxygen and metal ions by several orders of magnitude. Additionally, in coatings with 22.8% ZrO₂ content (zirconium alloy substrate), elements such as Al regulate the thermal expansion coefficient of oxidation products, alleviating thermal cycling stress and avoiding spallation to ensure effective oxygen barrier performance.

The cathodic arc ion plating process further enhances the performance: high-energy ion bombardment fills pores and promotes atomic-level bonding between the film and substrate (e.g., metallurgical bonding), resulting in a density that is close to the theoretical maximum and eliminating the risk of blistering and delamination during oxidation. Combined with the sluggish diffusion effect (reducing the high-temperature diffusion rate of elements by 1–2 orders of magnitude), the (TiZrHfNbTa)C films prepared by Jin et al. [121] maintained integrity in air at 1600 °C, confirming that the HEC structure can shorten the diffusion path of O₂/O²⁻ and physically block the penetration of oxidizing media [122,123]. This verifies that the exceptional high-temperature oxidation resistance of HEC films originates from the synergistic effect of the intrinsic material properties (high configurational entropy and sluggish diffusion) and advanced preparation processes (cathodic arc ion plating), which together construct a robust oxidation barrier.

5. Prospects

Given the current development status of high-entropy ceramic (HEC) coatings deposited by arc ion plating (AIP), future research efforts should prioritize two core fundamental directions to overcome technical bottlenecks, while simultaneously addressing key challenges in industrial applications. On this basis, further expansion of technical boundaries and application scenarios can be realized. At the fundamental research level, two critical knowledge gaps remain to be filled: first, the dynamic coupling mechanism between multi-component metal ions and reactive gases within the plasma is not yet fully understood, and the regulatory effects of plasma parameters on coatings’ compositional uniformity and phase structure evolution require further quantitative investigation; second, research on the failure mechanisms of HEC coatings under extreme service conditions (e.g., high-temperature oxidation, corrosion, and wear) is insufficient, and the correlation between interfacial stress evolution and microstructural damage urgently demands in-depth analysis.

At the industrial application level, pressing challenges to be addressed include the following: achieving uniform deposition of coatings on large-sized and complex-shaped substrates and overcoming the limitations of target configuration and tooling design in existing AIP equipment; balancing the synergistic optimization of coating hardness, toughness, and adhesion, and developing low-cost, high-stability preparation processes that meet engineering requirements; establishing a standardized coating performance evaluation system; and bridging the critical gap between laboratory research findings and industrial-scale production.

Building on this foundation, future research on HEC thin films prepared by cathodic arc ion plating (CAIP, a technology belonging to the same arc ion plating system as AIP) can further break through the traditional “multiple metals + single non-metal” structural configuration. By precisely regulating core process parameters such as substrate bias voltage, arc current, and gas flow rate, researchers can introduce functional non-metallic elements (e.g., B, Si, O) to construct “metal–non-metal” mixed sublattice structures, while maintaining the thermodynamic stability of the high-entropy system. Such sublattice structures enable the diversification of chemical bonding at the atomic scale, thereby synchronously enhancing multiple properties of the films, including hardness, high-temperature oxidation resistance, and corrosion resistance [124,125,126,127].

Currently, the rapidly evolving industrial demands impose increasingly stringent requirements on the comprehensive performance of materials, and extreme service environments such as high temperature, radiation, and severe corrosion present new challenges for material development. Leveraging the synergistic effects of multiple principal elements in HEC thin films, systematic integration technologies (e.g., graded architectures, functional composite layers) can be developed. Conducting service life and reliability assessments, including high-temperature cycling tests, thermal shock tests, corrosion immersion experiments, and failure analysis (focused ion beam–transmission electron microscopy (FIB-TEM), X-ray photoelectron spectroscopy (XPS) depth profiling), will facilitate the application of such coatings in fields with rigorous performance requirements, such as aerospace thermal protection, cutting tools and molds, nuclear radiation environments, optoelectronic devices, electromagnetic wave absorption and shielding, catalysis, and energy storage/conversion. In addition, co-deposition of two-dimensional materials (e.g., graphene, MXene) with HEC thin films enables the construction of metal–non-metal 2D ternary composite systems [128,129]. Investigations into the preparation processes and phase stability of these novel composite films are expected to achieve synergistic improvements in multiple performance metrics of HEC coatings.

To advance the aforementioned research efficiently, high-throughput computation and machine learning screening techniques (density functional theory (DFT), CALPHAD phase diagram calculation method) can be incorporated into a systematic multiscale modeling framework (spanning atomic, mesoscopic, and macroscopic scales) to predict the coupling effects of composition on hardness, thermal stability, and oxidation behavior. In situ real-time characterization techniques (e.g., in situ TEM, synchrotron radiation) can be employed to verify the quantitative relationships between lattice distortion, the electronic band structure, and macroscopic properties induced by the multi-component system [130]. Meanwhile, process optimization can be driven by advanced diagnostic technologies: designing orthogonal experiments combined with real-time plasma monitoring to achieve precise regulation of bias voltage, gas flow rate, and arc current, thereby identifying the phase transition threshold of HEC thin films—a critical node for the comprehensive performance enhancement of coatings. Adopting auxiliary strategies such as low-power pulsed bias, plasma filtration, and target material recycling can reduce energy consumption, improve material utilization efficiency, and further enhance the economic feasibility of this technology, providing robust support for the translation of laboratory achievements into industrial production.

6. Conclusions

This paper systematically reviews the research progress in fabricating high-entropy ceramic (HEC) thin films via arc ion plating (AIP), with a primary focus on the interrelationships among “thermodynamic effects–process parameters–microstructure–material properties”. Firstly, it outlines the fundamental thermodynamic effects of HEC thin films, namely the high-entropy effect, lattice distortion effect, sluggish diffusion effect, and cocktail effect, which lay the foundation for the formation of simple solid solution structures and the films’ exceptional performance. Subsequently, the influences of several key AIP process parameters on the films are discussed in detail: the substrate bias voltage regulates grain size and film density; the reactive gas flow rate determines the content of non-metallic elements and phase composition; and the arc current governs the plasma characteristics and deposition rate. The combined action of these parameters dictates the final microstructure of HEC thin films. The review then describes the core properties of HEC coatings. Mechanically, they can simultaneously achieve high hardness and reliable toughness. Tribologically, they display low coefficients of friction and reduced wear rates. Electrochemically, the films show excellent corrosion resistance and favorable electrical conductivity. Moreover, they possess outstanding high-temperature oxidation resistance. Finally, the article looks ahead to future research directions. Prospective work includes the design of multicomponent non-metal sub-lattices; the integration of two-dimensional materials such as graphene and MXene into HEC composites; reliability-oriented modeling combined with machine-learning-driven optimization; and the expansion of applications into extreme environment fields, such as aerospace thermal protection, nuclear and radiation environments, and advanced energy systems. These perspectives offer feasible pathways for continued advancement in the field.