Novel Features, Applications, and Recent Developments of High-Entropy Ceramic Coatings: A State-of-the-Art Review

Abstract

This state-of-the-art review provides a comprehensive, critical synthesis of the rapidly expanding field of HECCs, emphasizing the unique scientific challenges that distinguish these materials from conventional ceramics and high-entropy alloys. Key challenges of HECCs include accurately predicting stable phases and quantifying resultant material properties, optimizing complex fabrication and processing techniques, and establishing a robust correlation between the intricate microstructural characteristics and macroscopic performance. Unlike previous reviews that focus on individual ceramic families, this article integrates the novel features, diverse applications, and recent developmental breakthroughs across carbides, nitrides, borides, and oxides to reveal the unifying principles governing configurational disorder, phase stability, and microstructure property relationships in HECCs. A key novelty of this review work is the systematic mapping of fabrication pathways, including CTR, PAS, SPS, and reactive sintering, against the underlying thermodynamic and kinetic constraints specific to multicomponent ceramic systems. The review introduces emerging ideas such as HEDFT, machine-learning-assisted phase prediction, and entropy–enthalpy competition as foundational tools for next-generation HECC design and performance analysis. Additionally, it uniquely presents densification behavior, diffusion barriers, defect chemistry, and residual stress evolution with mechanical, thermal, and tribological performance across the coating classes. By consolidating theoretical intuitions with experimental developments, this article provides a novel roadmap for predictive compositional design, development, microstructural engineering, and targeted application of HECCs in extreme environments. This work aims to support researchers and coating industries toward the rational development of high-performance HECCs and establish a unified framework for future research in high-entropy ceramic technologies.

1. Introduction

In recent years, high-entropy alloys (HEAs) have been studied in materials science and engineering practices because of their special features and composition. HEAs are made up of several principal elements as compared to the usually used alloys, which contain one or two base elements [1,2]. This expanded composition space allows for a significantly larger number of possible HEA compositions compared to conventional alloys [3,4,5]. The exploration of HEAs has raised basic questions that cast doubt on theories, models, and procedures that have been established for conventional alloys [6,7,8,9]. Researchers have been actively investigating these issues to enhance understanding of phase formation in HEAs [10,11,12]. HEAs are recognized in the fields of biomedical engineering, aerospace and the energy sector for their exceptional mechanical strength and corrosion resistance. By studying the phase stability and transformation mechanisms in HEAs, scientists aim to develop predictive models and guidelines for alloy design, processing, and performance optimization [13,14,15,16,17]. In addition to addressing fundamental issues, researchers have also discovered several novel properties exhibited by HEAs [18,19,20,21,22]. Figure 1a, developed according to the information from Refs. [23,24], shows the rising trend of chemical complexity of the alloy and innovation of HEAs, and Figure 1b depicts the historical overview of HEMs in detail [25].

These characteristics include superparamagnetism, superconductivity, outstanding ductility, extraordinary mechanical performance at high temperatures, and fracture toughness at cryogenic temperatures [25,26,27,28]. These unique characteristics make HEAs attractive for various applications [29,30,31,32]. HEAs offer considerable structural and functional potential, making them promising candidates for new applications, as shown in Figure 2 [33,34,35,36,37,38,39,40]. The richness of design possibilities in HEAs allows for tailoring their properties to meet specific requirements [41,42,43]. However, further studies are necessary to explore the potential of HEAs fully and uncover additional applications in different fields [44,45,46,47,48]. This study on HEAs has provided valuable insights into phase formation, alloy design, and the unique properties exhibited by these alloys. The article offers insights into the challenges and developments of high-entropy ceramics (HEC), emphasizing the importance of understanding the operating conditions and the contest between entropy and enthalpy in these materials. It also highlights the transition from thin-film analogs to advanced HEC materials.

1.1. High-Entropy Ceramics Coating

In recent years, high-entropy effects’ significance in metal alloys and, more prominently, in high-entropy ceramics has become a key topic, as it is not limited to oxides but has since grown to encompass silicides, borides, carbides, nitrides, and sulfides [16,49,50]. Figure 3 shows different structures of high-entropy ceramics (HECs). These materials exhibit remarkable properties with various applications, such as thermal and energy storage, water splitting, thermoelectricity, environmental preservation, and catalysis [51,52,53,54]. HECs are stable and resilient materials that remain single-phase even under extreme conditions, such as high temperatures, pressures, and challenging chemical environments [55,56,57,58]. Their fundamental properties and characteristics include resistance to corrosion, low thermal conductivity, mechanical enhancement through various mechanisms, inhibition of grain coarsening, and potential dependence of elastic modulus and hardness on valence electron concentration [59,60,61,62,63]. These properties make HEC suitable for a broad assortment of applications [64]. Furthermore, the complex mechanisms underlying these properties provide opportunities for computational modeling and further research in this field. Standard ab initio techniques, which rely on theoretical calculations, are generally unreliable in ceramics [65,66,67]. This unreliability is attributed to the substantial differences in the chemical character of the ceramic constituents [68,69]. Empirical corrections have been applied to these techniques to make quantitative analyses more accurate [70]. On the other hand, cation and anion sublattices are different in reciprocal high-entropy ceramics [71,72]. These unique sublattices provide separate stabilizations and solubilities. Anions aid in the creation of configurational disorder by acting as an obstacle between metal cations [73]. As a result, more compositions form single phases, and their stability ranges are expanded [74,75]. In these cases, knowledge of the high-entropy ceramics’ operating circumstances is essential to their advancement [76]. This entails parameterizing elements such as melting temperatures and miscibility gaps [77]. The impact of entropy on these materials is unavoidable, but the complexities of how entropy and enthalpy compete as the number of species increases must be addressed to understand its impact correctly [78]. High-entropy ceramic coatings (HECCs) were initially investigated as thin-film analogs to unstable metal alloys before entropy-stabilized oxides were synthesized [79,80]. HEA targets were sputtered with magnetrons in an Ar + N₂ atmosphere in the initial attempts to create high-entropy nitride (HEN) thin films, which produced amorphous or multiphase films. Compositions like (AlCrTaTiZr)N and AlCoCrCuFeNi were among the first to create crystalline formations in a single phase [81]. The exploration of high-entropy ceramic coatings continues to be an active area of research, and this review can contribute to providing insight into their application potential and advancing the field of materials science and engineering.

1.2. High-Entropy Carbide Coating

The potential and challenges associated with high-entropy carbide ceramics (HECCs) outline a roadmap for advancing the design and fabrication of these materials. HECCs are a class of materials with several desirable properties, including excellent oxidation resistance, high hardness, and a variable thermal conductivity range [82,83,84]. These qualities make them attractive options for structural materials under harsh service circumstances [85,86,87]. Current research on HECCs has limitations in providing comprehensive guidance for designing HECC materials with stable structures and promising properties [33]. This indicates a gap in the knowledge and understanding of these materials [4,5]. Researchers propose using high-throughput density functional theory calculations (HTDFT) to theoretically identify the rules governing the formation of single-phase HECC materials [88]. This is crucial for achieving stable structures and desired properties. Whereas, developing a highly accurate predictive model for rapid compositional design is recommended [89,90,91]. This model would likely combine HTDFT and machine-learning studies to guide the formulation of HECC materials with specific properties [92]. It emphasized that there is a deficiency of theoretical support and guidelines for the synthesis of highly dense and pure HECC materials [8,93,94]. Understanding the fundamentals of producing HECC pre-alloy powders and the mechanics behind powder densification during high-temperature sintering are the main goals of recent research [95]. For HECC materials to progress, a better grasp of their mechanical, oxidation, and heat conduction behaviors is crucial [96]. This includes elucidating the toughening and strengthening mechanisms [97]. The approach is to examine the correlations between the composition of HECC, dislocation behavior, bonding state, and energy distribution of piling failures [98,99]. In this article, a more comprehensive approach for advancing the field of high-entropy carbide ceramics, encompassing theoretical investigations, compositional design, synthesis, densification, and mechanistic understanding, has been discussed. Researchers aim to develop high-performance HECC materials for use in demanding applications by addressing these aspects.

1.3. High-Entropy Nitride Coating

The concept of high-entropy materials, particularly high-entropy nitride coatings, and their potential applications in wear-resistant protective coatings has become a major aspect of HEA. The concept of this was initially observed in bulk alloys and later extended to various fields, including metal or ceramic physical vapor deposition (PVD) coatings [100]. Initially, high-entropy systems were primarily equimolar, meaning they had nearly equal atomic concentrations of different elements [101]. However, more recent research has explored non-equimolar high-entropy systems having atomic concentrations and at least five constituent elements ranging from 5% to 35% [102]. High-entropy nitride systems have gained attention for their outstanding qualities, including resistance to wear, high-temperature oxidation resistance, wear resistance, and anti-corrosion capabilities [103,104]. These properties make them attractive for use in protective hard coatings [105]. To evaluate the wear resistance of high-entropy nitride coatings using an indicator called H3/E2, which, in a ball-on-plane system, is connected to the load that determines the change from elastic to plastic contact [68]. The primary objective is to enhance the hardness of HEN coatings by adjusting deposition parameters [106]. This is considered a more efficient method than reducing the elastic modulus to improve the H3/E2 value [107]. The focus is to understand the impact of substrate temperature and bias on residual stress, grain size, and densification, all of which influence the hardness of the coatings [108,109,110]. In addition to hardness, another parameter, i.e., measurement of crystal structure and chemical composition to identify any effects on hardness, must also be considered [33]. To assess the wear performance of the high H3/E2 coatings, ball-on-disc wear tests can be performed, as they act as a bridge to address the gap between existing literature and the latest developments in HENC. Overall, the goal is to enhance the hardness and wear resistance of HENC and explore the effects of various deposition parameters on their properties.

1.4. High-Entropy Boride Coatings (HEBC)

High-entropy boride coatings refer to a class of advanced materials used as protective coatings in various industrial applications. These coatings are composed of boron-rich compounds and other elements, typically transition metals that form a complex microstructure with high entropy. Borides are compounds that contain boron, and they often exhibit excellent hardness, wear resistance, and high melting points [111]. The addition of other elements, especially those with high melting points and desirable properties, creates a high-entropy alloy or compound. This combination of boron and other elements in high-entropy boride coatings results in unique mechanical and thermal properties. Borides are known for their exceptional hardness, and the addition of high-entropy elements further enhances this property [112]. This makes the coatings suitable for applications where wear resistance is crucial. It is also designed to withstand abrasive wear, making it useful in industries such as manufacturing, mining, and aerospace, where components are subjected to harsh environments. The high melting points of borides and high-entropy alloys contribute to the thermal stability of these coatings. This makes them suitable for applications involving high temperatures, such as cutting tools or components in high-temperature environments. Moreover, depending on the specific composition, high-entropy boride coatings exhibit good corrosion resistance, enhancing the lifespan of coated components [113]. The random and disordered atomic structure resulting from high-entropy compositions can lead to unique microstructural features, influencing the overall performance of the coating. HENC can be tailored by adjusting the composition of the alloy, allowing for customization based on specific application requirements. Applications of high-entropy boride coatings can effectively be found in various industries, including aerospace, automotive, cutting tools, and manufacturing, where the properties of these coatings can improve the performance and longevity of components subjected to harsh conditions.

1.5. High-Entropy Oxide Coatings (HEOC)

HEOCs can be described as a class of advanced materials that are composed of multiple metallic elements in roughly equal atomic proportions, forming a stable oxide structure. High-entropy materials typically consist of at least four or more elements in roughly equal proportions [114]. This composition is designed to maximize configurational entropy and promote unique structural and mechanical properties. The high-entropy composition often leads to increased stability compared to traditional alloys or compounds. This can result in improved resistance to oxidation, corrosion, and wear. These high-entropy oxide coatings specifically involve the formation of stable oxide phases. These oxides can have desirable characteristics, including resistance, high hardness, and thermal stability in harsh environments. HEOC has potential applications in various industries [115]. For example, they can be used as protective coatings to enhance the durability and lifespan of materials exposed to extreme conditions, such as high temperatures, corrosive environments, or aggressive chemicals. The combination of multiple elements in high-entropy oxides can result in unique mechanical properties, including high hardness and strength. These properties make them suitable for applications where wear resistance and mechanical robustness are crucial. A high-entropy oxide coating exhibits favorable thermal properties, such as high melting points and thermal stability. This makes them suitable for applications where resistance to high temperatures is essential. The field of high-entropy materials, including oxides, is still in the early stages of research and development. Scientists and engineers are exploring different compositions and fabrication techniques to optimize the properties of these materials for specific applications [63]. Moreover, various methods, such as chemical and physical vapor deposition and sol–gel methods, are employed to fabricate high-entropy oxide coatings. The choice of technique depends on the specific requirements of the application and the desired properties of the coating. High-entropy oxide coatings represent an exciting area of research in materials science, offering the potential for enhanced performance and durability in challenging environments. Ongoing research aims to further understand and optimize the properties of these materials for practical applications in industries such as aerospace, energy, and electronics.

2. Synthesis and Fabrication

HECCs are advanced materials known for their mechanical properties and high-temperature stability. The process requires pressure-assisted sintering (PAS) and high-energy ball milling (HEBM) for combining binary carbide particles in a non-single-phase powder mixture, as shown in Figure 4 [116]. Sintering is then carried out using methods like Spark Plasma Sintering (SPS) and hot pressing (HP). Researchers have used this approach to synthesize various HECCs, including (HfNbTaTiZr)C, (HfNbTaZr)C, (NbTaZr)C, (NbTaTiZr)C, and more [117,118]. In the second route, i.e., Carbothermal Reaction (CTR), researchers used carbothermal reactions to produce pre-powders, the majority of which were single-phase [119]. These pre-powders were then sintered using similar techniques. HECCs like (NbTaTiZrW)C, (NbTaTiZrMo)C, and (HfNbTaTiW)C were synthesized using this method [120,121]. However, graphite and pure metal powders are also utilized as raw materials, and these were ball-milled to create HECC pre-powders [122,123]. Carbides such as (HfTaTiNbZr)C, (NbTaTiZrW)C, (MoNbTaWV)C, and (HfTaTiNbMo)C were synthesized because of this method [73,124,125,126]. These various methods and precursor materials allowed researchers to create a range of HECCs with different compositions and properties [127,128,129]. These materials have potential applications in high-temperature and high-stress environments due to their unique properties [130].

Several studies have focused on optimizing the processes for producing HECCs. The use of Spark Plasma Sintering (SPS) to produce (NbTaZrTiW)C bulks through three different routes. Figure 5 shows the schematic representation of co-deposition for HEAs. The letters A to E describe different distances between the substrate and the target of co-deposition. Various targets provide different concentration gradients on the substrate during co-deposition to produce an HEA film, which becomes a homogeneous concentration of film distribution. The choice of the route was found to influence both densification and the phase components of the resulting materials. The combination of high-throughput characterization techniques helps to achieve rapid screening of HEAs, followed by the bulk-like selected components. As per the design requirements of HEAs, alloy or single-element targets can be prepared, and this is controlled by the atomic percentages of elements.

The mechanical properties and microstructure of HECCs can be significantly improved through multi-step sintering processes [131,132]. However, it was also discovered that High-Energy Ball Milling (HEBM) treatment before conventional transient liquid phase sintering (CTR) is beneficial in order to obtain fine pre-powders at reduced heat [133,134,135]. Moreover, Gild et al. [128] and Wei et al. [136] discovered that reactive sintering can greatly improve the ease of densification and the purity of pre-powders. However, theoretical studies in this area are relatively scarce compared to the experimental investigations into HECC fabrication and optimization [137,138,139]. Most studies are empirical and lack a strong theoretical foundation. A theoretical study by Feng et al. [140] identified the critical temperature to complete CTR. This was achieved by calculating changes in Gibbs’ free energy utilizing Thermodynamic Calculations (FactSage software, Version: FactSage 7.2) [140] for reactions involving various binary carbide types (HfC, ZrC, TiC, TaC, and NbC) as a function of temperature and partial pressure of CO (P_CO). The synthesis of pure single-phase rock-salt structured (HfZrTi-TaNb)C with a certain particle size and oxygen content at a comparatively low temperature (1600 °C) was led by the theoretical study that produced the insights.

HECCs are materials with complex compositions at their cation sites, which make their synthesis and sintering unique. In conventional binary carbides, the synthesis typically involves the reduction of a single type of oxide (e.g., HfO₂) to form a specific metal carbide (e.g., HfC) [141]. In contrast, synthesizing HECCs involves simultaneous reduction and solid solution reactions. Multiple types of oxides are reduced, corresponding metals are carbonized, and solid solutions are formed through elemental interdiffusion and mass migration [142]. After high-energy ball milling (HEBM), particles in conventional binary carbides encounter the same type of carbide. In HECCs, particles are surrounded by different types of particles, making elemental interdiffusion and mass migration more complex. On the other hand, the sintering process of densifying the material is complicated in HECCs due to the configurational differences and the expected retarded diffusion. High-entropy materials are known for their slow diffusion, which poses difficulties in achieving homogenization and densification. Systematic experimental and theoretical research is necessary to guide the fabrication of high-quality HECCs [143]. Experimental efforts are necessary to quantify diffusion coefficients between binary carbides, study the role of reactive sintering, and investigate phase evolution and mass migration during synthesis. Additionally, systematic studies are needed to understand the relationship between temperature, time, and pressure during sintering, densification, and grain structure. The physical principles underlying diffusion coefficients and vacancy formation energies for metallic elements in cation sites can be better understood theoretically through studies such as first principles and thermodynamics calculations. It is possible to create fundamental theories about bulk sintering and HECC powder synthesis by combining theoretical and experimental efforts [144]. These theories can serve as the basis for producing HECC pre-powders with low impurities and controllable particle sizes and for achieving highly dense HECC bulk materials with fine microstructures. In essence, the article emphasizes the unique challenges and complexities involved in the synthesis and sintering of HECC, and it highlights the need for comprehensive research to advance the understanding and production of these materials.

3. Properties

3.1. Thermal Conductivity and Wettability

The thermal conductivity of HECC was found to increase as the volumetric concentration of nanoparticles increased along with the temperature increment. It was observed that at 50 °C, the thermal conductivity was maximum and a concentration of 1.25 vol.% for Al-MWCNT hybrid ceramic coatings [145]. The blending of multi-walled carbon nanotubes (MWCNT) with alumina improved thermal conductivity by approximately 2.6% [146]. On the other hand, the contact angle, which measures the wettability of the nanofluids, was significantly affected by the concentration of the nanoparticles [147]. The contact angle first reduced and subsequently increased as the concentration of nanoparticles increased for both alumina and Al-MWCNT hybrid nanofluids [148]. MWCNT effectively increased the wettability of alumina-based nanofluids, improving their capacity to extract heat and serve as lubricants. In addition, compared to alumina nanofluids, the hybrid nanofluids of Al-MWCNT demonstrated a decreased friction coefficient between the disc and the pin [149]. The friction coefficients for Al-MWCNT and alumina nanofluids were roughly 0.07 and 0.18 at 1.25 vol% concentration, respectively [150]. Friction force and pin wear are decreased with a reduced friction coefficient. For both alumina and Al-MWCNT hybrid nanofluids, an increase in nanoparticle concentration resulted in less pin wear. For Al-MWCNT hybrid nanofluids, the lowest wear value was 1.25 vol.% [151]. The decrease in wear was probably caused by the nanoparticles’ creation of a nanolayer between the disc and the pin’s sliding surface [152]. However, the surface quality of various nanofluids and base fluids varied significantly, as observed by field emission scanning electron microscopy (FESEM) pictures. In comparison to alumina nanofluids, the surface of Al-MWCNT hybrid nanofluids was smoother, signifying that the hybrid nanofluids are better lubricants [153]. The study on the thermal conductivity and wettability of HECC suggests that the addition of MWCNT to alumina-based nanofluids can enhance their thermal properties, wettability, and lubricating characteristics, leading to reduced wear and improved surface quality in frictional applications.

3.2. Transition from Elastic to Plastic Contact (H³/E²)

It is very important to focus on improving the wear resistance of high-entropy nitride coatings (HENC). HENC are materials designed to resist wear and friction in various applications [145]. The H3/E2 ratio is used as an indicator to evaluate the anti-wear capability of the high-entropy nitride coating [154]. It helps to measure how the material responds to stress and wear [155]. The main objective is to improve the hardness of the coatings [156]. This is considered a more efficient and general approach to improving wear resistance than reducing the elastic modulus [157]. There are mainly two variable parameters that were considered, such as bias and substrate temperature [158]. These parameters are chosen for their impact on residual stress, grain size, and densification, which in turn strongly influence the hardness of the coating [159]. Apart from hardness, material properties like crystal structure and chemical composition also need to be taken as key parameters to identify any effects on hardness.

3.3. Hardness and Elastic Modulus

The hardness of thin films is influenced by multiple factors, including residual stress, grain size, and densification. The relationship between these factors and hardness can be complex, with different factors coming into play under different conditions, such as bias and substrate temperature [160]. Understanding and controlling these factors is crucial in materials science and engineering, especially for optimizing the properties of thin films [161]. The hardness of a material is affected by its grain size, and this relationship is commonly described by the Hall–Petch effect. According to this effect, smaller grain sizes typically result in higher hardness. Also, densification is an important factor in improving hardness. Higher density in a thin film leads to increased hardness [145]. A lower thin-film density with a higher void concentration generally results in lower hardness. Residual stress also plays a significant role in determining hardness [104,162,163]. When residual stress is more compressive, it often leads to higher hardness. However, this relationship is influenced by various parameters, such as substrate bias and temperature. Initially, increasing substrate temperature improves hardness due to densification, which eliminates internal cracks [164]. However, as substrate temperature continues to rise, the reduction in residual stress is accompanied by an increase in grain size [165]. Larger grain sizes typically result in lower hardness, according to the Hall–Petch relationship. The combination of these effects can result in hardness remaining relatively constant within a certain temperature range [166]. On the contrary, residual stress and hardness can be increased by point defects, such as metal atoms occupying nitrogen sites or nitrogen atoms occupying interstitial sites [167]. Defect formation is thought to be a major element that effectively influences hardness.

3.4. Tribological Characteristic (Tool Flank Wear, Average Friction Coefficient, Wear Rate)

Sharma et al. [145] reported the use of nanofluids, specifically alumina-based and Al-MWCNT (aluminum-multiwalled carbon nanotube) hybrid nanofluids, in machining processes and their effects on tool wear and friction between sliding surfaces. They showed that Al-MWCNT hybrid nanofluids resulted in lower tool flank wear than alumina-based nanofluids. This reduction in tool wear is attributed to the decreased nodal temperature, primarily caused by the temperature generated in the primary and secondary shear zones during machining. Increasing the concentration of nanoparticles in the nanofluids further reduces tool flank wear [168]. An increased number of nanoparticles causes the nodal temperature to drop, which in turn reduces tool wear. Cutting fluids containing alumina nanoparticles produce a ball-bearing effect between the sliding surfaces, which lessens friction between them. Because of the synergistic effect of the hybrid nanoparticles, the friction coefficient is further reduced when MWCNTs and alumina nanoparticles are combined. On the other hand, the weak structure of MWCNTs exfoliates due to the chip’s shearing action on the tool surface. The thickness and effectiveness of these films are enhanced with a higher concentration of nanoparticles, leading to reduced friction and wear. The lower friction force, attributed to the presence of MWCNT nanoparticles, leads to a reduction in the cutting force and nodal temperature [169]. Hybrid nanofluids are better at extracting heat from the tool due to their advanced wettability on the affected tool surface and better thermal conductivity than alumina nanofluids. This, in turn, helps retain tool hardness for longer periods. The presence of MWCNTs and their synergistic effects with alumina nanoparticles play a crucial role in improving the tribological behavior and overall performance of the nanofluids in machining applications.

4. Influencing Parameters

4.1. Crystal Structure and Grain Size

It was found that substrate temperature and bias conditions significantly influenced the crystal structure and grain size of high-entropy nitride coatings, with higher temperatures and specific bias conditions leading to changes in crystal orientation and grain size. These findings are important for understanding the properties and potential applications of such coatings. The grain sizes were estimated using the Scherrer formula, primarily based on the full width at half maximum (FWHM) of (111) and (200) peaks. The actual grain size may be slightly larger due to lattice strain broadening [170]. They also reported that grain sizes increased as the substrate temperature increased, reflecting enhanced mobility and diffusivity at higher temperatures [171,172,173]. The addition of bias caused a shift in the preferred orientation from (200) to (111), and finally to (220). The shift from (200) to (111) was attributed to re-nucleation, likely caused by defects induced by ion bombardment. These defects served as nucleation sites for secondary growth [174,175,176]. The shift from (111) to (220) was attributed to the channeling effect, where there are fewer atom columns per unit area in the (200) plane compared to the (111) plane. Applying a small bias of −50 V initially led to a decrease in grain size, consistent with the idea of re-nucleation in defect-rich areas [177]. Increasing the bias to −200 V resulted in larger grain sizes [130], likely due to enhanced adatom mobility. This encouraged bigger grain development and atomic migration towards the grain borders. Similarly, Wang et al. [178] performed EBSD analysis of the microstructure to investigate the effect of grain size on the laser-directed energy deposition (LDED) of Hf–Nb–Ta–Ti–Zr refractory HEAs. The grain size was found to be larger at higher energy input, with values of 143 µm at 3500 W (specimen #5), 116 µm at 3000 W (specimen #4), and 38 µm at 2500 W (specimen #1), as shown in the inverse pole figure (IPF) maps in Figure 6.

4.2. Substrate Temperature and Bias

Residual stress in coatings is influenced by two main components, i.e., intrinsic and thermal stress [179]. Defects produced during the deposition process, such as voids, lattice defects, and atoms in non-equilibrium locations, are the source of intrinsic stress [180]. However, because of the different thermal expansion coefficients of the deposited films and the substrate, thermal stress really varies with substrate temperatures [157,181]. As a result, it was found that residual stress in the thin-film coating was much less compressive, especially at higher substrate temperatures [182]. The difference in residual stress from 200 °C to 400 °C was attributed to the decrease in the intrinsic stress. At lower deposition temperatures, such as room temperature (RT) and 200 °C, intrinsic stress was expected to result mainly from defects like vacancies or voids due to low adatom mobility during deposition. The enhancement in the substrate temperature to 400 °C effectively reduced these defects because it allowed for surface diffusion during film growth. Residual stress was found to become more compressive as the substrate bias (ion energy) became more negative [183]. The residual stress was primarily influenced by intrinsic stress in the case of fixed thermal stress and substrate temperature. Increasing the bias (higher incident ion energy) led to the generation of more defects, like metal atoms occupying nitrogen sites and nitrogen atoms occupying metal sites [184,185]. The enhanced compressive residual stress resulted from the atom-peening effect due to the higher incident ion energy. Moreover, factors like substrate temperature and bias influence the residual stress in thin-film coatings [186]. Residual stress can be affected by both thermal stress and intrinsic stress, and understanding these factors is essential for controlling the properties of thin-film coatings.

4.3. Densification and Compressive Residual Stress

The influence of densification and compressive residual stress on HECC is an important aspect of material science and surface engineering. HECCs are coatings made from materials that have multiple elements in nearly equal proportions, which can lead to unique properties and performance [187,188,189]. Densification and compressive residual stress play crucial roles in determining the quality and functionality of these coatings. Proper densification of the ceramic coatings is essential for achieving high mechanical properties [190]. A well-densified coating is less likely to have porosity and voids, leading to improved hardness, wear resistance, and durability. It also helps in creating a more thermally stable coating. A dense coating is less prone to cracking or delamination at high temperatures [191]. On the other hand, compressive residual stress can improve the coating’s adhesion property, which helps it to adhere to the substrate more. This is particularly important for preventing spallation or flaking of the coating under mechanical or thermal loading [192]. Compressive stress also leads to an increase in the hardness of the coating, making it more resistant to abrasion and wear. It counteracts the tensile stresses that can develop during the coating process or subsequent thermal cycling. This helps prevent cracking or debonding [193]. The relationship between densification and compressive residual stress is complex and interconnected. Achieving a high degree of densification is often necessary to create compressive residual stress within the coating. This is because, during the cooling phase of the coating process, the coating contracts more than the substrate, resulting in compressive stresses [194]. However, if the densification is not sufficient, it can lead to the formation of voids or defects, negating the benefits of compressive stress [195]. The influence of densification and compressive residual stress on high-entropy ceramic coatings is vital for achieving coatings with superior mechanical properties, adhesion, and thermal stability [158,196]. Finding the right balance between these factors is essential for optimizing the performance and longevity of such coatings in various applications, including in high-temperature and corrosive environments. The challenges in developing HECCs and the expected computational approach for the solution and optimization of composition for the fabrication of HECCs are presented in Figure 7.

5. Recent Developments in High-Entropy Ceramic Coatings

The field of high-entropy materials, including HECCs, is dynamic and evolving. New developments and breakthroughs may have occurred in recent years. High-entropy ceramic coatings are being explored for various applications due to their unique properties. Their recent developments and prospects are discussed below:

5.1. Improved Processing Techniques

Researchers are working on refining the fabrication methods of high-entropy ceramic coatings to enhance their quality and properties. Techniques such as thermal spray, physical vapor deposition, and chemical vapor deposition are being explored to optimize the coating deposition process.

5.2. Material Combinations

Scientists are investigating new combinations of ceramic materials to create high-entropy coatings. These combinations are designed to provide enhanced mechanical, thermal, and chemical properties.

5.3. Application in Extreme Environments

High-entropy ceramic coatings are being considered for use in extreme environments, such as aerospace and gas turbine engines, where they can offer improved resistance to high temperatures, corrosion, and wear.

6. Future Prospects and Applications

6.1. Advanced Thermal Barrier Coatings

HECCs have the potential to serve as advanced thermal barrier coatings in gas turbines, providing improved thermal insulation and protection against high-temperature corrosion. This could lead to more efficient and durable turbine engines.

6.2. Enhanced Wear Resistance

HECCs can be applied to components in various industries to increase their wear resistance. This includes applications in manufacturing, transportation, and energy production.

6.3. Corrosion Resistance

HECCs could be used in environments with corrosive gases or liquids, such as chemical processing plants and offshore oil platforms, where they can provide improved protection against corrosion.

6.4. Biomedical Applications

These coatings could be explored for medical devices, implants, and instruments due to their biocompatibility and resistance to wear and corrosion in physiological environments.

6.5. Energy Storage and Conversion

HECCs could be used in solid oxide fuel cells and other energy storage and conversion devices, where they can enhance durability and performance.

6.6. Advancements in Material Design

Continued research in material design and selection could effectively lead to high-entropy coatings with tailored properties for specific applications, further expanding their use in various industries.

6.7. Environmental Considerations

Research on developing more environmentally friendly processes for creating high-entropy ceramic coatings and their end-of-life disposal can also be performed in the foreseeable future.

7. Conclusions

HEA has emerged as a fascinating area of materials science with the potential to create new and improved materials for a broad assortment of applications due to their unique properties and the use of advanced research methods, including artificial intelligence. The potential of HECC materials, their early stage of development, and the need for comprehensive research efforts to understand and optimize their properties and applications have been discussed in this article. This involves predictive models, advanced synthesis techniques, microstructural analysis, and a combination of theoretical and experimental studies. HECC materials are noted for their promising properties. These properties make them attractive for various applications. Despite their potential, HECC materials are still in the early stages of development. This suggests that there is much room for further exploration and improvement. There are three key aspects of research on HECC materials, as discussed below:

7.1. Phase/Property Prediction

Researchers aim to predict the phases and properties of HECC materials. Developing predictive models and synthesis rules for single-phase HECC materials is a crucial challenge.

7.2. Fabrication

Achieving highly pure HECC powders with desirable particle sizes and dense bulk materials with fine microstructures is a primary concern in fabrication.

7.3. Microstructure Property Relation

Understanding the microstructural process by which the HECC materials influence their mechanical, oxidation, and thermal conduction behavior is crucial. This involves experimental studies and theoretical investigations.

On the other hand, HDFT and its several calculations using machine-learning methods are proposed as tools to bridge the gaps in the synthesis and property prediction of HECC materials. Moreover, the diffusion and mass migration processes during the synthesis and sintering of HECC materials are highlighted as distinct from those in conventional binary carbides. This difference requires further research in both experimental and theoretical ways to identify and understand the microstructural origins of HECC materials. Techniques such as First Principles Calculations combined with the Reverse Monte Carlo method can be considered significant in this regard. The importance of understanding the correlation between local structures in HECC materials and properties like stacking fault energy distribution, bonding states, oxygen diffusion, and thermal conduction of electrons and phonons is key. Integrated efforts combining various research approaches can be valuable for efficiently designing HECC materials and ensuring high-quality fabrication for high-performance applications.