The Green Lubricant Coatings Deposited by Physical Vapor Deposition for Demanding Tribological Applications: A Review

Abstract

The emergence of nanotechnology and surface engineering techniques provides new opportunities for designing self-lubricant coatings with enhanced properties. In recent years, green coating technologies have played a vital role in environmental preservation. This article mainly reviews five typical types of self-lubricant coatings including MoN coatings, VN coatings, WN coatings and TMN (Transition Metal Nitride) soft-metal coatings, and DLC (Diamond-like Carbon) with lubricant agents deposited by PVD (Physical Vapor Deposition) for the demanding tribological applications, which is the latest research into the green lubricant coatings. Furthermore, it is of great significance for designing the green self-lubricant coatings to adapt the demanding tribological applications to meet the industrial requirements.

1. Introduction

In recent decades, there has been a growing focus on emerging green technologies that aim to utilize natural resources effectively to minimize waste and pollution. This shift towards sustainability was catalyzed by the introduction of the 2030 Agenda UN Sustainable Development Goals (SDGs), gaining significant support from the research and development (R&D) community [1,2]. According to the SDG 7: Affordable and Clean Energy, the green lubricant coatings can reduce friction and wear in machinery, leading to increased energy efficiency and lower energy consumption. Green coating technologies are particularly notable for their emphasis on environmental conservation [3]. The design and implementation of eco-friendly coatings are playing an increasingly crucial role on a global scale. Holmberg and Erdemir [4] investigated the fact that tribological contacts account for approximately 23% of total energy consumption worldwide. Moreover, roughly 20% is expended solely to overcome friction, while the remaining 3% is allocated towards the refurbishing of worn components and providing spare equipment due to wear and associated malfunctions.

Coatings refer to layers of material applied to surfaces to alter their properties or appearance [5,6,7,8,9]. The self-lubricant coatings are rooted in the broader context of sustainability and environmental consciousness within the field of materials science and engineering and have been developed during recent decades for demanding tribological applications, for energy saving and friction reduction. Several studies have explored the field of self-lubricant coatings. For instance, the self-lubricating MoS₂/C sputtered coating demonstrated a characteristic amorphous structure and high hardness when different carbon contents were incorporated [10]. Additionally, the investigation delved into the intricate aspects of synthesizing, understanding the microstructure, and assessing the mechanical properties of W–S–C self-lubricant thin films, and these films were meticulously deposited using magnetron sputtering, revealing insights into their behavior and performance. [11]. Pimentel et al. [12] studied the tribological properties of W–S–C–Cr self-lubricant coatings, whereas Mauafov et al. [13] and his team explored the impact of deposition conditions on the chemical composition, structure, and mechanical properties of self-lubricant W–S–N coatings. Moreover, the inclusion of amorphous SiN_x in self-lubricant Mo₂N-Ag composite films was found to enhance their mechanical and tribological properties [14]. The study meticulously examined how the introduction of novel self-lubricating TiSiVN films influenced the intricate dynamics of topographical changes, diffusion mechanisms, and oxidation phenomena occurring at the interface between the chip and the tool during the dry machining of the Ti₆Al₄V alloy. [15]. Lastly, the investigation meticulously explored the tribological properties of the WC₁₀Co₄Cr₄CaF₂ wear-resistant self-lubricating coating, carefully assessing its performance under diverse test temperatures to gain comprehensive insights into its behavior across a spectrum of thermal conditions [16]. Self-lubricant coatings draw inspiration from the concept of a ‘chameleon’. Previous research has explored adaptive ‘chameleon’ nanocomposite coatings, demonstrating excellent tribological properties across different environments [17,18,19,20,21,22]. These coatings earn their ‘chameleon’ moniker due to a capability for self-guided, complex adaptive behaviors, which empowers them to seamlessly acclimate to a multitude of environments and temperatures, demonstrating exceptional versatility in their responses to varying conditions.

Research focusing on green coatings has been relatively limited in the past. Gautam and colleagues [23] explored the applications of green nanomaterials in coatings. Buch and Oza [24] conducted a comparative study on the machining parameters of High-Speed Steel (HSS) cutting tools, their focus centered on tools coated with multi-walled carbon nanotubes, using an eco-conscious coating technique. Through their rigorous examination, they uncovered compelling evidence: the coated Multiwall Carbon Nanotube (MWCNT) tool surpassed its uncoated counterpart, demonstrating extended lifetime, enhanced wear resistance, increased hardness, and superior surface quality. In a recent development, a new environmentally friendly film was developed by incorporating silver nanoparticles into an amorphous SiN_x matrix via magnetron sputtering. This film demonstrated long-term lubrication capabilities at temperature cycling conditions ranging from room temperature to 500 °C [25]. Jian et al. [26] introduced a pioneering method aimed at crafting durable and environmentally sustainable coatings for safeguarding magnesium alloy substrates. This approach offers a green and eco-friendly solution specifically tailored for fabricating corrosion protection coatings on AZ91D magnesium alloy. Furthermore, Fan and his team [27] developed a brand-new green coating technology aimed at achieving the controllable release of energy from nitrocellulose-based propellants. Their innovative approach provides a means to finely tune the spherical propellant energy release process with precision.

When evaluating tribological properties, we can consider factors such as the coefficient of friction (COF) [28], wear rate [29], wear track [30], and wear mechanism [31]. According to Vitu et al. [32], a tribolayer forms on the surface of the wear track. During friction, coatings composed of transition metal dichalcogenides (TMDs) reorient themselves [33]. This reorientation allows the material to slide more easily under shear forces, forming a shear-friendly layer with self-lubricating properties on the surface of the wear track. The presence of this tribolayer can significantly reduce the friction coefficient and wear rate, thereby improving the wear resistance and extending the service life of the coating. In this paper, we mainly discuss the tribological applications of coatings from COF, wear rate and wear track.

Green self-lubricant coatings represent an environmentally friendly surface treatment that offers lubrication to mitigate friction and wear between moving surfaces. These coatings present numerous advantages over conventional lubricants, such as oils and greases, including reduced environmental impact, enhanced efficiency, and increased durability. This work provides a comprehensive review of previous studies, categorized into five parts: MoN coatings, VN coatings, WN coatings, TMN soft-metal coatings, and DLC with lubricant agents. Therefore, the research on green lubricant coatings deposited by PVD for demanding tribological applications holds significant importance.

2. Physical Vapor Deposition Techniques

Multilayered films are typically fabricated through diverse methods, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). PVD, a widely utilized manufacturing method, is employed in producing thin films adapted to diverse applications, encompassing optical, mechanical, electrical, acoustic, or chemical functionalities [34]. Among PVD techniques, magnetron sputtering and arc ion plating are commonly employed methods [35]. Meanwhile, CVD is a vacuum deposition technique renowned for generating high-quality, high-performance solid materials. It finds extensive use in the semiconductor industry for thin film production [36]. Additionally, electroplating, alternatively termed electrochemical deposition, is achieved by reducing metal cations in a solution using direct electric current [37].

In this study, emphasis is placed on coatings fabricated through PVD techniques. Figure 1 gives the main types of physical vapor deposition. In this part, we will highlight two types of PVD, magnetron sputtering and arc ion plating techniques.

2.1. Magnetron Sputtering

Magnetron sputtering, a prominent example of PVD, offers precise control over film composition by adjusting the conditions [38,39]. Typically, sputtering equipment consists of essential components including vacuum-pumping equipment, the systems of water-cooling, sputtering-gas supply, power supply and mechanisms for positioning both substrates and targets. The target functions as the supplier of the film-forming material, while the substrate gathers the sputtered material to produce the film. During magnetron sputtering, an electric field is created through the application of voltage between the cathode (target) and the anode (either an independent electrode or the grounded wall of the vacuum chamber) [40]. Typically, the cathode target is linked to a power supply with a negative charge (Direct Current: DC or Radio Frequency: RF). The substrate is positioned on a substrate holder, which offers the flexibility to either be grounded, left floating, or biased, as needed [41]. Permanent magnets or electromagnets are strategically positioned adjacent to the target, generating a magnetic field that extends above the target surface through the alignment of magnetic field lines [42,43].

2.2. Arc Ion Plating

As an advanced surface treatment method, arc ion-plating leverages arc-discharge plasma technology to precisely deposit metal ions onto a substrate surface. This meticulously controlled process culminates in the creation of a thin film, enhancing the substrate’s properties. This process initiates with high-energy arc discharge, creating plasma that evaporates the metal source. Subsequently, the plasma interacts with ions in the vapor, facilitated by ions from the reaction gas, inducing ionization and the consequent deposition onto the substrate surface [44,45,46]. Arc ion plating is capable of producing high-quality multilayered nanofilms.

Magnetron sputtering excels in producing coatings with excellent adhesion and smooth surfaces, which are crucial for applications like optical lenses, where even minor surface imperfections can significantly impact performance. This technology is also advantageous for creating multilayered structures, often required in advanced electronic devices. The precise control over deposition parameters allows for fine-tuning the film’s properties, such as thickness, composition, and crystal structure, to meet specific application needs.

Arc ion plating is particularly beneficial for producing coatings for high-performance applications such as aerospace components, automotive parts, and medical devices. The method’s ability to produce coatings with superior mechanical properties, such as enhanced hardness and resistance to oxidation and corrosion, makes it invaluable in environments demanding the utmost reliability and longevity. Additionally, the process can create aesthetically pleasing finishes, a significant advantage for decorative applications in consumer goods and architectural elements.

In the applications, including those demanding high-quality, low-temperature, and uniform coatings, magnetron sputtering is more suitable for green lubricant coatings. Conversely, in situations where high hardness and rapid deposition are essential, arc ion plating proves more advantageous. Green lubricating coatings deposited by PVD are typically designed to reduce the coefficient of friction and reduce energy losses [47,48]. Magnetron-sputtering and arc ion-plating technologies provide a high-quality, uniform and dense substrate for these coatings, helping to ensure the durability and efficiency of the lubricating coating.

3. Self-Lubricant Coatings for Tribological Applications

3.1. MoN Coatings

Molybdenum nitride (MoN) films, a representative type of transition metal nitride (TMN), can be applied for diverse applications due to their outstanding mechanical performance and the easy formation of self-lubricating phases such as Mo₄O₁₁ and MoO₃. Consequently, MoN-based films have been the subject of extensive study [49,50,51].

The MoN multilayer, marked by a combination of crystalline and amorphous traits during deposition, displayed robust interfaces, ensuring impeccable adhesion and remarkable crack resistance [52]. Conversely, the M_CP film demonstrated a capability to suppress the preferred orientation through the strategic stacking of crystalline layers in alternation. Similar effects were observed in the M_CA film, which combined crystalline and amorphous building layers. However, a slight overgrowth of grains into adjacent layers was evident in both films. In the M_PA film, a prominent issue arose due to the excessive overgrowth of crystalline grains spanning multiple layers. This phenomenon led to extensive grain extrusion in the later stages of deposition, ultimately resulting in the disruption of intact layer interfaces. Molybdenum nitrides have the capability to crystallize into diverse crystal structures, such as γ-Mo₂N (cubic) and δ-MoN (hexagonal). Jauberteau and his team [53] demonstrated that the stable, non-stoichiometric γ-Mo₂N1 ± x phase, with a NaCl-B1-type cubic structure, displays favorable catalytic characteristics.

Deposited by the PVD process, molybdenum nitrides and carbides exhibit numerous distinctive physical and mechanical attributes. These include high hardness, low average friction coefficients, excellent adherence to steel substrates, and low electrical resistivity, making them highly desirable for various applications [54,55,56,57]. Abboudi et al. [58] conducted a study to analyze how variations in nitrogen content and film thickness affect the structural and tribo-mechanical properties of MoN thin films deposited using reactive sputtering. In the MoN film, they observed a significant compressive residual stress of −5.7 GPa with a thickness of 0.3 μm, which progressively decreased as the film thickness increased and the friction coefficient of MoN films, tested with Si₃N₄ balls, varied within the range of 0.55 to 0.93. The Mo_xN coating, deposited by high-power impulse magnetron sputtering with a peak current of 260 A, stands out for its notably low friction coefficient of 0.28, while the same coating applied with a peak current of 300 A showcases exceptional wear resistance, characterized by achieving the lowest wear rate, observed at 5 × 10⁻⁸ mm³/(Nm) [59].

The tribological properties of Mo₂N films are profoundly influenced by temperature variations [60]. As the temperature rises from 25 °C to 200 °C, there is a reduction in moisture absorption and the weight fraction of MoO₃. Consequently, this causes an elevation in the average friction coefficient and a reduced wear rate, with the lowest average friction coefficient being 0.29 (550 °C). Gassner and his team found that by suppressing the formation of the volatile MoO₃ phase, such as limiting the maximum testing temperatures to 500 °C, Mo–N coatings demonstrate excellent lubricious properties and this leads to significantly lower friction coefficients of 0.4 [61]. Furthermore, heat treatment has been shown to enhance the mechanical and tribological performance of MoN-based films and the COF of the MoN film stabilized at approximately 0.34 before annealing; it then experienced a slight decrease and stabilized at about 0.30, after annealing [62]. As for the wear track, MoN film has large scratches, while the wear scar morphology shows itself to be smoother than before, after annealing. MoN/VN multilayer films have demonstrated improved tribological properties, particularly at room temperature (25 °C) and 700 °C, due to the occurrence of sliding wear and the formation of self-lubricating oxide films [63]. The MoN/VN multilayer films with Λ = 22.66 nm displayed exceptional performance, with the lowest recorded average friction coefficient of 0.29 and a minimal wear rate of 1.37 × 10⁻⁶ mm³/Nm at 700 °C. MoN coatings deposited by ion beam-assisted deposition achieve the lowest COF of 0.27 at a deposition temperature of 600 °C [64].

Apart from magnetron sputtering, MoN coatings are also studied by other methods. MoN coatings via CVD (chemical vapor deposition) exhibited exceptional properties [65]. MoN coatings were prepared by the cathodic arc evaporation method and the specific wear rates varied among the coatings and ranged between (1 × 10⁻⁷ and 2.1 × 10⁻⁶) mm³ N⁻¹ m⁻¹ [66]. MoN coatings for pistons in a diesel engine were analyzed by Hazar [67], who found that the MoN-coated piston exhibited reduced deformation and fewer wear-induced scratches in comparison to the uncoated piston; the elevated surface hardness of the MoN-coated surface effectively minimizes wear traces.

Later, the researchers endeavored to design the multilayer coatings to enhance their mechanical or tribological properties. According to Guojun Zhang [68], the MoN_x/SiN_x multilayer coatings show a friction coefficient of around 0.53. For MoN/MoSiN coatings, it was found that the substantial volumetric ratio of c-MoN to a-MoSiN, along with the discontinuities within the crystalline phases, were fundamental factors contributing to their advantageous mechanical properties [69]. Moreover, self-lubricant nano-multilayer coatings such as TiN/MoN and TiAlN/MoN, produced using a closed-field unbalanced magnetron sputtering system on high-speed steel drills, have been shown to enhance tool durability and surface quality during drilling operations [70].

Table 1. Mechanical/Tribological properties of MoN coatings.

System	Techniques	Mechanical/Tribological Properties	References
MoN	Reactive magnetron sputtering	Hardness: ~9.5 GPa–~35 GPa; coefficient of friction: 0.55	[58]
MoxN	High-power impulse magnetron sputtering	Coefficient of friction: 0.28; wear rate: 5 × 10−8 mm3/(Nm)	[59]
Mo2N	Reactive magnetron sputtering	Coefficient of friction: 0.29~0.53; wear rate: 2.1 × 10−6 mm3/Nmm~ 5.3 × 10−7 mm3/Nmm	[60]
MoNx	Reactive magnetron sputtering	Hardness: ~10.6 GPa–~31.4 GPa; coefficient of friction: 0.5~0.93	[71]
MoN	Vacuum arc plasma-assisted	Hardness: ~36 GPa; wear rate: 2.08 × 10−7 mm3N−1 m−1	[72]
MoNx	DC magnetron sputtering	Hardness: ~27 GPa	[73]
MoNx	DC magnetron sputtering	Hardness: ~23 Gpa	[74]
MoN	High-power impulse magnetron sputtering	Hardness: >~22 GPa; elastic modulus: >300 GPa; coefficient of friction: 0.7	[75]
MoNx	DC magnetron sputtering	Hardness: ~18 GPa–~24 GPa; coefficient of friction: 0.04~0.08	[76]
MoNx	DC pulsed magnetron sputtering	Hardness: ~16 GPa; elastic modulus: ~180 GPa	[77]
MoN	Vacuum arc plasma-assisted	Coefficient of friction: 0.3~0.7	[78]

shows part of the mechanical/tribological properties of MoN coatings.

The previous studies discovered that MoN coatings exhibit significant compressive stress and that their friction coefficients vary, based on nitrogen content and thickness. Various deposition methods, such as high-power pulsed magnetron sputtering, ion beam-assisted deposition, and vacuum arc deposition, influence the tribological and mechanical properties of MoN coatings. MoN-based multilayer coatings demonstrate excellent self-lubricating properties at high temperatures, with their performance further enhanced by heat treatment. Overall, MoN coatings show great potential in enhancing the wear resistance of mechanical equipment and reducing friction losses.

In the future, we will further investigate the self-lubricating properties of MoN-based multilayer coatings under high-temperature conditions. We will also explore methods to enhance their high-temperature performance by incorporating other lubricating materials or chemical elements. Additionally, we plan to develop composite coating systems that include MoN, such as those combining MoN with other transition metal nitrides or carbides. This approach aims to further improve the overall performance of the coatings and enable them to meet more diversified application requirements. The tribological properties of MoN coatings are of great significance in engineering and applications. MoN coatings usually have high hardness and excellent wear resistance, which can effectively reduce friction and surface wear. This is crucial to improving the working efficiency and extending the service life of mechanical parts, especially in equipment with a high-load, high-speed or harsh environment.

3.2. VN Coatings

As one of the most promising options for tribological hard coatings, vanadium nitride (VN) coating has attracted considerable interest; it is susceptible to oxidation at elevated temperatures and can melt, leading to the creation of a lubricant layer [79,80].

The integration of transition metal vanadium (V) into nitride coatings catalyzes the emergence of Magnéli oxide lubricant phases, characterized by their inherently sheared planes, in the context of machining or sliding applications. These phases serve as efficacious agents in reducing both the coefficient of friction and the wear rate of the coatings, thereby bolstering their durability and operational efficiency [81,82]. Optimizing the nitrogen flow rate to 2.5 sccm yields a VN film characterized by exceptional properties: elevated hardness, diminished electrical resistivity, and minimal residual stress, and this fine-tuned parameter underscores the film’s superior performance and stability [83]. The fracture toughness of VN coatings is dependent on texture, and specimens displaying random texture and (200) texture manifest Gc values of 11.2 and 19.2 J/m², respectively [84]. However, Ge and his team [85] enhanced the orientation of constituent columns and discovered that suppressing fracture-dominated wear mechanisms in dense VN coatings can significantly reduce wear rates. Moreover, denser VN coatings were obtained with higher hardness (22.9 GPa) at 0.29 Pa, through the influence of nitrogen partial pressure and substrate bias on the mechanical properties of VN coatings [86].

There is some research focused on tribological properties and a wide range of temperatures. The tribological properties of VN films are greatly improved after thermal treatment, with the VN film featuring a VO₂ thin pre-oxide layer exhibiting a significantly lower coefficient of friction of approximately 0.46 and a wear rate of (2.27 ± 0.14) × 10⁻¹⁵ m³ N⁻¹ m⁻¹, compared to the VN film without the pre-oxide layer, highlighting the critical role of the pre-oxide layer in enhancing the film’s properties [87]. VN coatings were deposited by reactive unbalanced magnetron sputtering, and exhibited self-lubricating properties due to the liquid/solid lubrication, particularly effective at high temperatures, resulting in low friction coefficients during dry-sliding against various counterpart materials [88]. There are studies which have also focused on the performance of VN coatings across a wide temperature range. The width of the wear track for the VN coating initially increases up to 300 °C, after which it decreases until reaching 500 °C [89]. However, at 700 °C, significant delamination and the presence of abundant wear debris can be observed on the wear tracks of the VN coatings. Regarding the COF, it shows a tendency to decline for the VN coatings until 500 °C, reaching its lowest value at (0.36). This decrease can be attributed to the formation of a lubricious phase, likely V₂O₅. However, the COF remains relatively constant at higher-friction temperatures (700 °C). Zhou et al. [90] pointed out that the melting point of the V₂O₅ phase is approximately 690 °C, which is significantly lower than the friction temperature of 700 °C, resulting in a notable decline in the concentration of the V₂O₅ phase as the temperature reaches 700 °C. As the friction temperature rises, the wear rates of VN coatings initially increase, reaching peak values around 8.732 × 10⁻⁶ mm³/Nm, at approximately 500 °C. This initial rise in wear rate is caused by the evaporation or removal of a condensed water-vapor film present on the sliding surface, which provides a lubricating effect [91]. As temperatures continue to increase beyond 500 °C, extensive surface oxidation occurs due to prolonged exposure to high temperatures and sliding wear [92]. In Fateh’s study [93], the friction coefficient of the VN coating continues to decrease above 400 °C and reaches 0.25 at 700 °C. This decrease is attributed to the formation and subsequent melting of V₂O₅, which facilitates liquid lubrication. Moreover, the VN coating exhibits self-lubricating properties at elevated temperatures.

As for the VN-based coatings, the cracking of V_xN coatings induced by scratching is primarily confined to larger droplets exhibiting weak interfacial bonding with the surrounding coating matrix [94]. When comparing VN/VCN coatings, researchers noted that the multilayer VN/C coating displayed markedly superior properties in both wear resistance and corrosion resistance compared to the monolayer VN coatings [95]. Furthermore, the VCN coating exhibited consistently lower friction coefficients and reduced wear rates across a spectrum of temperatures when compared directly with the VN coating [96,97]. These findings underscore the advantages of multilayer structures and the beneficial effects of carbon incorporation in enhancing the performance of these coatings in demanding tribological applications. Cai et al. [98] made a study of tribological properties of VN-based coatings and obtained friction coefficients for the VCN, VAlN, VCN and VAlCN/Ag coatings which decreased significantly to 0.3, 0.37/0.42, and 0.25, respectively, at 550 °C. This notable reduction can be attributed to the formation of the V₂O₅ Magnéli phase, known for its low shear strength, which initiates above 500 °C [99,100]. Moreover, at elevated temperatures, silver atoms have the capacity to migrate towards the surface of the coating through grain boundaries. Upon reaching the surface, these atoms undergo reactions to form a lubricious layer of reaction products with low shear strength. This layer, deposited onto the worn surface, plays a significant role in further decreasing the friction coefficient of the VAlCN/Ag coating [101,102]. The following

Table 2. Mechanical/Tribological properties of VN coatings.

System	Techniques	Mechanical/Tribological Properties	References
VN	Reactive magnetron sputtering	Hardness: ~25–~30 GPa; wear rate: <5 × 10−17 m3/N m	[85]
VN	Reactive magnetron sputtering	Hardness: ~22.9 GPa;	[86]
VN	Arc ion plating method	Coefficient of friction: 0.23/0.46; wear rate: (11.41 ± 0.22) × 10–15 m3 N−1 m−1 /(2.27 ± 0.14) × 10–15 m3 N−1 m−1	[87]
VN	Reactive magnetron sputtering	Coefficient of friction: 0.5	[88]
VxN	Arc evaporated	Coefficient of friction: 0.15–0.20;	[94]
VN	Reactive magnetron sputtering	Hardness: ~11 GPa–~20 GPa	[103]
VN	Inductively coupled plasma (ICP)-assisted sputtering	Hardness: ~28.2 GPa	[104]
VN	Arc evaporation method	Hardness: ~19.00 ± 1.26 GPa; coefficient of friction: 0.30; wear rate: 1.55 × 10−6 mm3/Nm	[105]
VN	Arc evaporation method	Hardness: ~37 GPa; wear rate: 1.7 × 10−6 mm3/Nm	[106]

shows the mechanical/tribological properties of VN coatings.

Based on the research findings, the tribological properties of VN coatings vary significantly with temperature changes, demonstrating notably lower friction coefficients and wear rates after heat treatment, particularly due to the beneficial impact of the pre-oxidation layer. At high temperatures, VN coatings exhibit self-lubricating properties, primarily resulting from the liquid/solid lubrication effect. The friction coefficient decreases steadily up to 500 °C, achieving a minimum value of 0.36, mainly attributable to the formation of the V₂O₅ lubricating phase. However, at 700 °C, the friction coefficient stabilizes while the wear rate peaks at 500 °C, with surface oxidation intensifying as the temperature increases further. Additionally, the previous studies reveal that VN/VCN multilayer coatings offer superior wear and corrosion resistance compared to single-layer VN coatings. VN-based coatings demonstrate significantly reduced friction coefficients and wear rates at high temperatures, with VAlCN/Ag coatings, in particular, exhibiting excellent self-lubricating properties. The lubricating layer formed by the migration of silver atoms further contributes to reducing the friction coefficient. However, the urgent technological demand lies in the development of vanadium nitride coatings that are hard and resilient yet possess self-lubricating characteristics. This advancement is crucial for mitigating mechanical friction losses and curbing energy consumption. In addition, the green multilayer self-lubricant coatings design has great prospects for the future.

3.3. WN Coatings

Another notable class of materials for tribological applications is tungsten nitride (WN)-based coatings, primarily utilized in machining tools [107]. Lately, a series of tungsten nitride coatings with nitrogen content ranging from 0 to 55 atomic percent were fabricated and subsequently analyzed [108]. Generally, the trend of increasing hardness was observed with higher nitrogen content, particularly evident in the α-W and β-W structures associated with lower nitrogen levels. Conversely, for the β-W₂N phase, the opposite trend was noted. Boukhris et al. [109] found that high-quality W2N films can be achieved with a dense structure and a resistivity of approximately 200 μΩ cm.

In terms of mechanical properties, the suitability of tungsten nitride coatings, produced through High Power Impulse Magnetron Sputtering (HiPIMS), was examined for their potential application as plasma-facing materials in fusion environments [110]. Furthermore, a first-principle study was conducted to assess how ultrathin WN films behave regarding surface stability under exposure to oxygen (O₂) and water vapor (H₂O). Patra et al. [111] found that optimal conditions are required to achieve defect-free surfaces in energetically stable ultrathin films terminated with tungsten (W), thereby ensuring surfaces with remarkable mechanical strength.

As a protective layer, WN coatings, such as TiN HEC (electrode contact), safeguard PCM (phase-change memory) electrodes from oxidation during manufacturing and fatigue processes [112]. Among these coatings, the most resilient W/W–N coating, featuring 5-nm layers, demonstrates superior protective properties [113]. Furthermore, W and WN_x films were examined at temperatures reaching 500 °C, and films with x ≤ 0.20 revealed an α-W structure and columnar microstructure, while those with x ≥ 0.27 exhibited a β-W₂N or δ-WN structure and fine-grained microstructure [114]. In addition, the incorporation of carbon into columnar WN coatings has been shown to enhance oxidation resistance [115].

In 2007, Polar et al. investigated the tribological properties of WN coatings and found that the coefficient of friction was 0.61 (12 at.%N) and the wear rate was 0.09 × 10⁻⁶ mm³/Nm (55 at.% N); with the increase in N content, the key factor influencing the change in the tribological behavior of W–N coatings appears to be the presence of the NaCl-type W₂N phase [116]. WN_x coatings were applied to silicon (Si) and AISI 304 stainless steel (SS) substrates using a combined HiPIMS technology and MF (mid-frequency) impulse. The WN_0.12 coating showed a low friction coefficient of 0.33, a high hardness of 31.5 GPa, and high H/E and H³/E² ratios of 0.104 and 0.342 GPa [117]. These excellent properties are attributed to solution hardening, the influence of substrate bias, and the higher peak power densities achieved during the deposition process. According to Dinesh Kumara [118], the average friction coefficient for the WN coatings deposited on silicon substrates was found to be 0.3, whereas for those deposited on steel substrates, the average value was 0.21. Based on the previous studies, the lowest COF we can achieve is 0.21.

As for the multilayer WN coatings, the relation between friction coefficient and wear rate of the WN coatings under different testing temperatures was revealed, and it was found that an increase in the nitrogen content within the coatings directly corresponds to improved wear resistance at elevated temperatures [119]. It is reported that the friction coefficient of the CrN monolayer coating is 0.63, whereas the CrN/WN multilayer film with a bilayer period of 30 nm exhibits a significantly lower friction coefficient of 0.31; the enhancement in tribo-mechanical properties of CrN/WN multilayer coatings can be attributed to the presence of the superlattice structure [120]. Moreover, the combination of ZrN/WN multilayers has demonstrated high hardness, low residual stress, high elastic modulus, and good fracture resistance [121]. The addition of tungsten (W) to TiAlCrN/WN nano-multilayered coatings has been shown to improve tool life [122]. (TiZrNbHfTa)N/WN multicomponent coatings were developed and the results revealed that the (TiZrNbHfTa)N layer exhibits the formation of a simple disordered solid solution, while the WN layer demonstrates a β-W₂N phase with an fcc crystal structure. In addition, the high-entropy alloy phases display highly disordered bcc orientations along (110) and (220) planes [123].

Table 3. Mechanical/Tribological properties of WN coatings.

System	Techniques	Mechanical/Tribological Properties	References
WNx	High-power impulse magnetron sputtering	Coefficient of friction: 0.33; hardness: ~31.5 GPa	[117]
WN	DC magnetron sputtering	Coefficient of friction: 0.21–0.3	[118]
WN	Reactive magnetron sputtering	Coefficient of friction: 0.61 (12 at.%N); wear rate: 0.09 × 10−6 mm3/Nm (55 at.%N); hardness: ~40 GPa	[119]
WNx	Reactive magnetron sputtering	Hardness: ~28 GPa	[124]
WNx	Reactive magnetron sputtering	Hardness: ~27.7/38.7 GPa	[125]
WN	Reactive magnetron sputtering	Hardness: ~23 GPa	[126]

shows the mechanical/tribological properties of WN coatings.

Previous studies have indicated a lack of research mainly focused on the tribological properties of WN coatings, with most of the research emphasizing mechanical properties or oxidation properties. Polar et al. studied the tribological properties of WN coatings and found that with the increase in nitrogen content, the friction coefficient was 0.61 (12 at.%N) and the wear rate was 0.09 × 10⁻⁶ mm³/Nm (55 at.%N), mainly due to the presence of the NaCl-type W₂N phase. WN coatings were applied on silicon and AISI 304 stainless steel substrates using HiPIMS and MF systems. The WN_0.12 coating exhibited a low friction coefficient of 0.33, a high hardness of 31.5 GPa, and a high H/E and H³/E² ratio, with these excellent properties being ascribed to solid solution strengthening, substrate bias, and the high peak-power density during deposition. Additionally, Dinesh Kumar noted that the friction coefficient of WN coatings was 0.3 on silicon substrates and 0.21 on steel substrates. Furthermore, multilayer WN coatings demonstrated that increasing nitrogen content at various temperatures can enhance high-temperature wear resistance. Hence, it is of profound significance to investigate environmentally friendly WN-based multilayer coatings deposited by PVD for demanding tribological applications. Future research on WN coatings should include that can still maintain excellent performance under high-temperature conditions, especially coating materials that can maintain low-friction and high-wear resistance at high temperatures; it should develop multilayer-structure and composite-material coatings, alternately superimpose WN with other high-performance materials (such as TiN, CrN, etc.), combine the advantages of different materials, and significantly improve the comprehensive performance of the coating, including wear resistance, corrosion resistance and self-lubricating properties. WN coatings in high-temperature applications include cutting tools for machining hardened steels, turbine components in aerospace engines, and dies and molds in metal-forming processes. Ongoing research aims to optimize WN coatings further for specific high-temperature applications, enhancing their performance and expanding their use in various industries.

3.4. TMN-SMe Coatings

Transition metal nitrides (TMNs) encompass various types, including VN, TiN, and chromium nitride (CrN), where nitrogen atoms occupy interstitial positions within the lattice metal crystal network [127]. TMN exhibits three primary structural forms: face-centered cubic (fcc), hexagonal close-packed (hcp), and simple hexagonal (hex) [128]. In particular, transition metal nitrides generally exhibit more covalent and metallic properties compared to corresponding oxides and sulfides, endowing them with superior characteristics across multiple domains [129]. These include remarkable hardness, high melting points, high Young’s modulus, excellent thermal and chemical stabilities, high electrical conductivity, and wide band gaps.

There are many studies on TMN adding Ag content. The incorporation of Ag into TiN films has been identified as a substantial enhancer of their fracture toughness and critical load, markedly improving their mechanical characteristics [130]. With the introduction of Ag (0.20 at 41.1 at.% Ag) into TiN film, the mean friction coefficient was reduced from 0.78 to 0. Nevertheless, the wear rate of TiN/Ag films showed an initial decrease, reaching its nadir at approximately 0.8 at.% Ag, with a value of about 1.3 × 10⁻⁷ mm³/(mm N), before subsequently increasing. The tribological characteristics of the films were notably affected by both the presence of the lubricant Ag phase and the ratios of H/E and H³/E². The introduction of Ag into the ZrN matrix resulted in a different change in the average friction coefficient of the films in a wide range of temperatures. At an Ag content of 26.6 atomic percent (at.%) at room temperature, the film achieves its lowest average friction coefficient of 0.62, yet this condition simultaneously results in the highest wear rate, peaking at 1.3 × 10⁻⁷ mm³/N·mm, whereas at an elevated temperature of 600 °C, the average friction coefficient further decreases to 0.29, although the wear rate markedly increases to 2.1 × 10⁻⁶ mm³/N·mm [131]. Significant friction reductions for CrN/Ag coating are achieved, with the friction coefficient reaching its lowest point of 0.05 during testing at a temperature of 500 °C [132]. The TaN films doped with a small amount of solute Ag atoms (1.2 at.%) demonstrate an impressive combination of properties, including higher hardness (36.1 GPa), enhanced toughness (K_IC = 1.48 MPa·m^1/2), and superior wear resistance, as evidenced by a minimum wear rate of approximately 1.9 × 10⁻⁶ mm³/N·m and a significantly reduced coefficient of friction of about 0.21, making them exceptionally well-suited for applications that demand both durability and low friction [133]. A VN/Ag solid-solution coating, with up to about 5 atomic percent of Ag, was created via magnetron sputtering to enhance its self-lubricating behavior in both dry-friction and oil-lubrication conditions, showing higher hardness (~24.5 GPa) and a lower coefficient of friction (~0.04) [134]. Based on Wu’s [135] study, MoN/Ag coatings containing 2.2% Ag, prepared using a hybrid PVD technology involving DC and RF magnetron sputtering, exhibited outstanding tribological properties, characterized by an exceptionally low friction coefficient of 0.27 even at a high temperature of 700 °C. The addition of Ag into NbN was studied by many researchers. Hongbo Ju and Junhua Xu [136] found that the average friction coefficient and wear rate of NbN-Ag films is influenced by the Ag content; the optimal Ag content, ranging from 9.2 to 13.5 atomic percent (at.%), was determined to exhibit low average friction coefficients ranging from 0.46 to 0.40, along with minimal wear rates ranging from 1.1 × 10⁻⁸ to 1.7 × 10⁻⁸ mm³/(mm·N) [137]. Ping Ren et al. [138] put forward a novel method to integrate a small number of Ag atoms (~1.5 at.%) into NbN film, creating a NbN/Ag solid-solution structure, resulting in increased hardness and toughness, as well as enhanced wear-resistance ability and a significant reduction in coefficient of friction. NbN films (∼1.5 at.% solute Ag) exhibited a highest hardness of 28 GPa, a minimal wear rate of 4.1 × 10⁻⁹ mm³/(Nm), and a low coefficient of friction of 0.22.

Another commonly used addition is the Cu element, in TMN. The addition of Cu to TiN films has been shown to significantly enhance wear resistance, with TiN/Cu films containing 1.6 atomic percent Cu achieving a maximum hardness value of 36 GPa under a pulse bias voltage of −200 V [139]. The addition of Cu to the MoN coating significantly reduces its wear rate under both dry and oil-lubricated conditions. Specifically, the wear rates for the MoN/Cu coating are 3.2 × 10⁻⁷ mm³/(Nm) and 2.1 × 10⁻⁷ mm³/(Nm) under dry and oil-lubricated conditions, respectively. In dry conditions, the friction coefficient of the MoN coating exhibits a brief running-in period before stabilizing at approximately 0.58. Conversely, in oil-lubricated conditions, the friction coefficient of the MoN/Cu coating initially increases gradually and eventually stabilizes around 0.053 [140]. As for the Nbn/Cu coatings, the COF was observed to rise with the incorporation of copper into the NbN coatings, and NbN/Cu coatings with 24.23 at.% Cu showed a high wear rate of 0.93 [141].

There are other soft metals to apply in TMN coatings. The TiN/Au coatings initially demonstrate a notably low friction coefficient (μ = 0.07), which undergoes a gradual increase over the course of operation until it stabilizes at a slightly higher value of μ = 0.12; this frictional behavior is attributed to intra-film sliding serving as the predominant lubrication mechanism within the TiN/Au coating [142]. Compared with the pure TiN coatings, TiN/Pt multilayer toughness was higher [143]. The following

Table 4. Mechanical/Tribological properties of TMN soft-metal coatings.

Systems	Techniques	Mechanical/Tribological Properties	References
TiN/Ag	Reactive magnetron sputtering	Hardness: ~29 GPa (Ag 0.8 at.%); wear rate: 1.3 × 10−7 mm3/(mm.N)(Ag 0.8 at.%)	[130]
ZrN/Ag	Reactive magnetron sputtering	Hardness: ~29 GPa; coefficient of friction: 0.62 (Ag 26.6 at.%); wear rate: 1.3 × 10−7 mm3 N−1mm−1 (Ag 26.6 at.%)	[131]
TaN/Ag	Reactive magnetron sputtering	Hardness: ~36.1 GPa (Ag1.2 at. %); wear rate: 1.9 × 10−6 mm3/Nm	[133]
VN/Ag	Magnetron sputtering	Hardness: ~24.5 GPa (≤~5 at.% Ag); Coefficient of friction: 0.04	[134]
NbN/Ag	Reactive magnetron sputtering	Coefficient of friction: 0.4–046 (Ag 9.2–13.5 at.%); wear rate: 1.1 × 10−8~1.7 × 10−8 mm3/(mm N)	[136]
TiN/Cu	Reactive magnetron sputtering	Hardness: 40.4 GPa	[139]
MoN/Cu	DC magnetron sputtering	Coefficient of friction: 0.58; wear rate: 2.1 × 10−7 mm3(Nm)−1/3.2 × 10−7 mm3(Nm)−1	[140]
NbN/Cu	Reactive magnetron sputtering	Hardness: 40 GPa (Cu 1 at.%); coefficient of friction: 0.93	[141]
TiN/Au	Magnetron sputtering	Coefficient of friction: 0.12	[142]

shows the mechanical/tribological properties of TMN soft-metal coatings.

Based on the above research, TMN presents a set of notable characteristics, including high hardness, a high melting point, a robust elastic modulus, outstanding thermochemical stability, high conductivity, and a wide bandgap. Moreover, investigations have explored the integration of elements into TMN to improve its tribological performance. This strategic introduction of diverse elements, such as Ag, Cu, and Au, among others, serves to enhance various attributes of the TMN coating. When Ag is added to TMN coatings, the friction coefficient will be different when a different content of the Ag element is added and a different temperature is used. For instance, for TiN/Ag coatings with the Ag content ranging between 0.20 and 41.1, the friction coefficient decreases significantly from 0.78 to 0.29, while the friction coefficient of CrN/Ag coatings reaches a remarkably low minimum of 0.05 at 500 °C.

When Cu is added to TMN coatings, it generally enhances the hardness and wear resistance of the TMN coating, but it may also lead to an increase in the friction coefficient and wear rate. When other elements are added to TMN coatings, such as in the case of TiN/Au films, the initial friction coefficient starts at 0.07 and gradually increases to 0.12.

Consequently, the future development we should consider is that the alternating superposition of TMN layers and lubricating layers containing soft metals can provide better friction and wear performance under high-temperature and high-load conditions. Additionally, research is being conducted to maintain the stability and lubrication effect of TMN soft-metal coatings under extreme temperature conditions. With increasingly stringent environmental regulations, the development of non-toxic and environmentally friendly TMN soft-metal coatings will become an important trend.

3.5. Lubricant Agent-Doped DLC Coatings

Diamond-like carbon (DLC) coatings were comprehensively studied for their performance in gear bending fatigue, contact fatigue, and resistance to scuffing. Classified as a metastable form of amorphous carbon, these coatings exhibit properties that bear resemblance to both diamond and graphite. These include remarkable wear resistance and a low coefficient of friction. It was found that the combination of shot peening and DLC coating remains effective in enhancing the bending fatigue performance of the composite material, by Wu and his team [144]. Marian et al. [145] demonstrated that the combination of 3D (three-dimensional) printing’s high flexibility and cost efficiency with DLC’s exceptional mechanical and tribological properties creates synergies that culminate in outstanding performance under dry sliding conditions. DLC films, characterized as a metastable form of amorphous carbon, sharing traits of both diamond and graphite, have attracted considerable attention due to their outstanding chemical and mechanical properties and they offer high wear resistance alongside a low friction coefficient, making them highly desirable for various applications [146,147,148,149,150,151,152,153,154,155,156]. The remarkable stability of DLC films in both acidic and alkaline solutions make them an excellent choice for effectively inhibiting corrosion in metallic products [157,158].

The graphene/DLC coating, applied with three layers of spraying, exhibits outstanding tribological properties under vacuum conditions, featuring an average coefficient of friction of approximately 0.08 and a low wear rate of 4.59 × 10⁻⁷ mm³/N·m [159]. In comparison with mineral-based lubricants, synthetic saturated base esters effectively reduce friction under extreme boundary conditions [160]. Tasdemir et al. [161] discussed the fact that the organic friction modifier glycerol-mono-oleate demonstrated the ability to maintain low friction properties of the coating even at elevated temperatures, with enhanced durability. Conversely, the anti-wear additive Zinc Dialkyldithiophosphate (ZnDTP) exhibited exceptional wear protection for the ta-C coating by generating a thick pad-like tribofilm on the steel counterpart across a wide range of temperatures; however, the friction coefficient increased when the coated surfaces were rubbed in ZnDTP-additivated oil. In addition, Kuznetsova et al. [162] utilized silicon addition to fine-tune the properties of tribofilms and enhance the tribological properties of DLC coatings and found that the DLC/Si(0.8%) formulation stands out, with the highest observed coefficient of friction (COF) at 0.135, whereas the DLC/Si(10%) variant displays the lowest COF along with a specific volumetric wear rate of 0.6 × 10⁻³ mm³/N·m, indicating its superior performance in terms of frictional characteristics and wear resistance.

When adding Perfluoropolyether (PFPE) lubricants, in simulations involving HDI (head–disk interface) contacts, it was observed that PFPE lubricants with greater quantities and stronger functional end groups tended to generate higher friction forces and exhibit reduced head contamination due to lubricant transfer. Conversely, as temperatures increased, head contamination increased while friction forces decreased [163]. DLC/biodegradable lubricants were studied, and it was found that the properties of the coating itself were found to have a more pronounced impact on wear compared with the influence of additives in the lubricant, with the wear coefficient generally remaining around the order of 0.1 × 10⁻¹⁸ m² N⁻¹ across all observed cases, except in instances where coating spallation occurred [164]. As for DLC/Ti-synthetic bio-lubricant coating, it is noteworthy that the friction of DLC/Ti exceeds that of pure DLC, and there is a significant reduction in wear observed when using pure DLC compared to DLC/Ti [165]. A tungsten carbide-doped DLC coating (WC/C) deposited by PVD on bearing steel was studied, and it was found that the tribological properties using water-based lubricants can be significantly enhanced with the application of this type of DLC coating [166]. When it comes to the tribological properties of DLC/MoDTC (Molybdenum Dithiocarbamate)-containing oils, Ueda et al. [167] revealed that the presence of other surface-active additives can decrease DLC wear in PAO + Mo (Mo in Polyalphaolefin base oil) through three mechanisms: the formation of thick anti-wear tribofilms, an increase in the ratio of MoS₂:MoO₃, and competitive adsorption with other surface-active additives. According to Yoshida et al. [168], the density of the DLC film decreased as hydrogen concentration increased, which in turn increased the wear coefficient of the sample, regardless of whether MoDTC was present in the oil, and an increase in the sp²/sp³ ratio of the DLC also increased the wear coefficient when immersed in MoDTC-containing oil. The following

Table 5. Tribological properties of lubricant agent-doped DLC coatings.

Systems	Technique	Mechanical/Tribological Properties	Reference
W/DLC	Magnetron sputtering	Coefficient of friction: 0.2	[169]
Ni/DLC	Magnetron Sputtering	Coefficient of friction: 0.17; wear rate: 1.6 × 10−17 mm3/Nm	[170]
Cr-DLC	Magnetron Sputtering	Coefficient of friction: 0.17; wear rate: 1.6 × 10−17 mm3/Nm	[171]
Ti/DLC	PECVD(Plasma-Enhanced Chemical Deposition)	Coefficient of friction: 0.15 (19 at.% of Ti)	[172]
W-DLC/Ti-DLC	PECVD	Coefficient of friction: 0.21 for Ti-DLC coefficient of friction: 0.22 for W-DLC	[173]
Ag/DLC	Magnetron sputtering	Coefficient of friction: 0.16; wear rate: 14 × 10−17 mm3/Nm (2.99 at.% of Ag)	[174]

illustrates the tribological properties of lubricant agent-doped DLC coatings.

On the basis of the previous studies, DLC coating has undergone extensive examination for its efficacy in enhancing gear bending fatigue, contact fatigue, and wear resistance. Studies have revealed that the three-layer sprayed graphene/DLC coatings demonstrated excellent tribological properties under vacuum conditions, achieving a friction coefficient of approximately 0.08 and a wear rate of 4.59 × 10⁻⁷ mm³/N·m. Synthetic ester-based lubricants were effective in reducing friction under extreme boundary conditions. Coatings containing organic friction modifiers maintained low friction at high temperatures, while ZnDTP provided excellent anti-wear protection across a broad temperature range, albeit with an increase in friction coefficient. Silicon-containing DLC coatings exhibited varied friction and wear characteristics, with DLC/Si (10%) achieving the lowest friction coefficient and a wear rate of 0.6 × 10⁻³ mm³/N·m. Although PFPE lubricants reduced friction, they increased contamination at high temperatures. The wear coefficient of DLC/biodegradable lubricants was approximately 0.1 × 10⁻¹⁸ m²/N, and DLC/Ti coatings exhibited lower wear compared to pure-DLC coatings. In future development, by incorporating a variety of lubricants into the DLC coating and combining the advantages of different materials, a composite coating with excellent tribological properties and multifunctionality will be developed. Additionally, a DLC coating with self-healing ability is being developed, allowing it to self-repair through the migration and reconstruction of the internal lubricant when damaged, thereby extending the service life and reliability of the coating.

4. Conclusions and Perspectives

This paper provides a comprehensive review of five common types of self-lubricating coatings, namely MoN coatings, VN coatings, WN coatings, TMN soft-metal coatings, and DLC coatings with lubricant agents. These coatings are deposited using the PVD technique to meet the rigorous demands of tribological applications.

MoN coatings exhibit high hardness (10.6–31.4 GPa) and excellent wear resistance, particularly due to the formation of a self-lubricating oxide layer at high temperatures. VN coatings also demonstrate excellent self-lubricating properties at high temperatures, with a hardness range of 11–37 GPa. WN coatings offer good wear resistance and hardness (23–38.7 GPa), maintaining a low friction coefficient at elevated temperatures.

TMN-SMe coatings, which incorporate soft metals such as Ag, Cu, and Au, significantly enhance the self-lubricity and wear resistance of the coating. These coatings leverage the inherent lubricating properties of the soft metals to reduce friction and improve overall performance under demanding conditions.

Lubricant-doped DLC (diamond-like carbon) coatings are renowned for their low friction coefficients and excellent wear resistance. For example, W/DLC, Ni/DLC, and Cr-DLC coatings have friction coefficients of 0.2, 0.17, and 0.17, respectively. These coatings combine the hard, wear-resistant nature of DLC with the lubricating properties of the added elements, resulting in a synergistic effect that enhances their tribological performance.

Overall, each of these coatings exhibits specific advantages in different applications, particularly under high-temperature and high-load conditions. The high hardness and wear resistance of MoN, VN, and WN coatings make them suitable for demanding environments where durability is critical. TMN-SMe and lubricant-doped DLC coatings offer exceptional self-lubricating properties, making them ideal for applications where reducing friction is essential. The development and optimization of these coatings continue to advance, with research focusing on fine-tuning their compositions and structures to meet the evolving demands of various industries. By leveraging the unique properties of each coating type, it is possible to enhance the performance and longevity of mechanical components, reduce maintenance needs, and improve overall efficiency in a wide range of applications.

Designing coatings presents several challenges, including (i) achieving a balance between green technology and optimizing exceptional tribological properties; (ii) maintaining specific properties under demanding tribological conditions; and (iii) controlling relevant parameters to design improved green self-lubricating coatings.

Different views can be considered, with respect to the future development of this type of coatings. To begin with, green technology will be widely used in surface engineering; secondly, the tribological properties of the self-lubricant coatings in the field of cutting tools will be optimized and the lifespan of the equipment prolonged; finally, it is essential to analyze the feasibility of application in real industry.

However, although this type of coating has demonstrated excellent tribological properties, further research is still needed to improve its structure, and enhance environmental adaptability and functional diversity to meet the needs of a wider range of applications. This includes improved performance under different operating conditions, such as contact pressure, temperature changes, etc., as well as its reduced sensitivity to environmental changes. In addition, an in-depth understanding of the wear mechanism of coatings and their long-term stability in practical applications is also a key direction for future research.