Improving Wear Resistance of DLC-Coated Metal Components During Service: A Review

Abstract

Diamond-like carbon (DLC) coatings have emerged as a focal point in advanced carbon materials research due to exceptional tribological properties, including ultralow friction coefficient, exceptional wear resistance, ultrahigh hardness, and chemical inertness. Deposition of DLC coatings on metal components represents an innovative solution to enhance wear resistance in engineering applications. However, suboptimal adhesion strength between coatings and substrates, coupled with inherent material limitations, critically compromises the tribological performance. This review systematically examines recent advances in improving the wear resistance of DLC-coated metal components. First, the fundamental wear mechanisms governing both metallic substrates and DLC coatings under service conditions are elucidated. Next, three pivotal technologies, substrate material treatment/strengthening, coating structure design, and elemental doping, all demonstrating significant efficacy in wear resistance enhancement, are critically analyzed. Furthermore, a comparative assessment of these techniques reveals the synergistic potential in hybrid approaches. Finally, a concise summary of the outlook is presented.

1. Introduction

Tribology is the discipline that investigates friction, wear, and lubrication at interacting surfaces. Wear, a critical tribological phenomenon characterized by progressive material deformation, damage, and removal at contacting surfaces, significantly governs the service life of mechanical components across scales, spanning from the nanoscale to the macroscale [1]. Statistical analyses reveal that 23% of global energy consumption originates from friction-related activities, with 40% of such losses being preventable through the implementation of advanced surface engineering, materials, and lubrication technologies [2]. Friction and wear not only induce substantial energy dissipation but also pose risks of structural failure. Consequently, in engineering applications, surface lubrication and protective coatings are indispensable for enhancing system performance and durability. In terms of energy conservation, environmental protection, safety, and economic viability, optimizing tribological performance in mechanical systems remains of paramount importance. As this paper is specifically focused on the ability of a coating (DLC, Diamond-like carbon) to improve tribological performance, it would be beneficial to discuss surface engineering generally to reduce energy dissipation (wastage) and consequently save money.

Surface engineering techniques serve as effective methods for controlling friction and wear by altering surface characteristics at dynamic contact points through surface deposition or modification. Conventional surface hardening methods, such as gas carburizing [3] and plasma nitrocarburizing [4], can enhance surface mechanical properties, including hardness and wear resistance, thereby mitigating common failure modes. However, these treatments typically fail to reduce the coefficient of friction, which limits their efficacy in improving tribological performance. With advancements in industrial development and material synthesis technologies, novel carbon-based thin-coating materials have been continuously developed, significantly expanding the scope of research in thin-coating materials science [5,6,7,8,9,10,11]. DLC coatings are widely adopted in industrial applications due to their exceptional properties, such as high hardness [12], superior wear resistance [13], low friction coefficient [14], chemical inertness [15], anti-reflection [16], and thermal stability. DLC coatings are predominantly fabricated via physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma-enhanced chemical vapor deposition (PECVD) techniques, encompassing over 10 methodologies, including sputtering, cathodic arc deposition, pulsed laser processing, electron/ion beam-assisted systems, microwave plasma variants, and their hybrid derivatives [17,18]. DLC coatings have emerged as a pivotal enabler of green tribotechnology, synergizing exceptional tribological properties with sustainable manufacturing paradigms [19]. The application of DLC coatings aligns with the fundamental objectives of green tribotechnology by reducing friction and wear, thereby minimizing energy consumption and material loss. This tribological mitigation directly contributes to lowering the ecological footprint of mechanical systems. DLC coatings can also serve as a model approach in which not only green lubricants but also the surfaces, with all their many different properties, can contribute to a novel functionality [20].

1.1. DLC Coatings

The first published reports of hard amorphous carbon coatings were by Schmellenmeier in 1953 [21]. In 1971, Aisenberg and Chabot pioneered the preparation of DLC coatings at room temperature using ion beam deposition (IBD), which triggered a surge in DLC research interest [22]. DLC coatings are a class of metastable amorphous materials that contain both diamond structures (sp³ hybridized bonds) and graphite structures (sp² hybridized bonds). The carbon atoms are primarily bonded through sp³ and sp² hybridized bonds, and this atomic composition categorizes them as diamond-like (higher sp³ bonds) or graphite-like (higher sp² bonds) coatings [23]. DLC coatings encompass diverse compositions and structures governed by deposition techniques and parameters. According to the Carbon Coatings standard published by the German Institute of Engineers (VDI) [24], DLC coatings are categorized as follows:

a-C: Amorphous carbon;

ta-C: Tetrahedral amorphous carbon;

a-C:Me (Me=W, Ti, Mo, Al, etc.): Metal-doped amorphous carbon;

a-C:H: Hydrogenated amorphous carbon;

ta-C:H: Tetrahedral hydrogenated amorphous carbon;

a-C:H:Me (Me=W, Ti, Mo, Al, etc.): Metal-doped hydrogenated amorphous carbon;

a-C:H:X (X=Si, O, N, F, B, etc.): Modified hydrogenated amorphous carbon.

The structure of amorphous carbon-based coatings is highly sensitive to deposition conditions. Variations in these conditions directly influence the ratio of sp² to sp³ hybridized bonds and hydrogen content, ultimately governing the physicochemical properties of the coatings [25]. J. Robertson elucidated the critical factors governing sp³-hybridized carbon atom formation. Key studies demonstrate that energetic ion bombardment facilitates sp³-bond formation, while hydrogen content regulates film microstructure and functional properties. These discoveries establish theoretical foundations for amorphous carbon materials. J. Robertson’s work further provides experimental frameworks to guide their design and application [26,27,28].

The optimization of DLC coating can be classified into four regimes, i.e., inception regime before 2003, first generation between 2006 and 2010, second generation from 2010 to 2014, and third generation from 2014 onwards [29]. Multi-parameter optimization of DLC coatings is now evolving in an ongoing fourth-generation optimization regime where data analytics and AI tools for multi-parameter prediction and optimization studies [30]. The evolution of DLC technology can be categorized into distinct phases: Pioneering Exploration (1950s–1970s), Technological Breakthroughs and Industrialization (1980s–1990s), Functionalization and Diversification (2000s–2010s), and Smart Technologies and Sustainable Transitions (2020s–Present). Figure 1 illustrates the DLC technologies’ evolution roadmap.

The exceptional mechanical properties of DLC coatings stem from their exceptionally high atomic density, the shortest bond lengths observed in carbon-based materials, and the robust covalent bonding (sp³ hybridization) between carbon atoms. Figure 2 shows crystalline and disordered atomic configurations of diamond (a) and typical ta-C structures (b) [36,37].

When two surfaces undergo relative sliding, the softer material conventionally experiences greater wear. However, despite being significantly harder than the opposing material, DLC coatings still undergo wear during friction processes, leading to the formation of a protective layer known as the transfer layer on the mating surface [38]. Composed of wear products from the DLC coating, this transfer layer exhibits graphite-like characteristics that effectively shield the counterpart surface from further wear while substantially reducing the coefficient of friction (CoF) [39]. The transfer layer primarily consists of a graphitic structure with unique lamellar arrangements, held together only by weak van der Waals forces, which facilitate easy interlayer sliding [40]. This structural configuration endows the transfer layer with superior lubricity, ultimately yielding exceptionally low CoF values. It is worth emphasizing that while all DLC coating variants significantly improve friction performance compared to uncoated components, their specific CoF values and wear rates can vary greatly. These variations depend on the coating architecture, chemical composition, and operational parameters [41].

Consequently, systematic investigation and optimization of DLC coating materials composition and tribological characteristics demonstrate substantial academic value and practical potential for advancing their dual functionality in friction mitigation and wear prevention, while enhancing operational reliability under multifaceted service conditions.

As illustrated in Figure 3, the knowledge map delineates four major research frontiers in the DLC coatings domain. The first cluster focuses on deposition techniques and characterization methods, primarily investigating performance optimization of DLC coatings through systematic tailoring of deposition parameters and process configurations. The green zone highlights research keywords related to deposition process optimization and microstructural characterization, aiming to enhance coating quality, homogeneity, and adhesion strength. The blue zone centers on mechanical properties, with an emphasis on hardness, microstructure, adhesion strength, and surface toughness. These properties play a crucial role in governing wear resistance, inhibiting crack propagation and preventing delamination. The red zone addresses tribological performance, emphasizing the CoF and wear resistance under varying operational conditions, particularly in high-load and high-speed applications. Within the red zone, critical parameters include the CoF, wear mechanisms, operational temperature ranges, and contact kinematics. These metrics directly determine DLC coatings’ performance in practical implementations, especially in critical components such as ball screws and metallic transmission systems. The yellow zone explores corrosion resistance, analyzing the environmental degradation behavior of DLC coatings across diverse corrosive media.

1.2. Purpose of the Review

In recent years, although many scholars have investigated the friction and wear behavior as well as wear inhibition technologies of DLC coatings, few reviews have addressed topics such as sustainable/green manufacturing, multi-scale wear mechanisms, and the synergistic effects of wear inhibition technologies. Under the premise of green and sustainable manufacturing, this paper critically discusses the research progress of wear mechanism and wear inhibition technologies of DLC-coated metal components. Firstly, the wear mechanism of DLC-coated metal components is reviewed and the wear mechanism of metal components is summarized from different scales. Secondly, the wear inhibition technologies of DLC-coated metal components are reviewed and discussed critically. Finally, the synergistic effect of wear inhibition technologies is reviewed and the future improvement in the wear resistance of DLC coatings on metal components is prospected.

2. Wear Mechanisms of DLC-Coated Metal Components

Tribological processes are inherently complex, involving multi-scale interactions and diverse physicochemical mechanisms. A comprehensive understanding of contact tribological processes not only facilitates the elucidation of frictional and wear behaviors at interacting surfaces but also establishes theoretical foundations for advanced material design and coating development. Practical implementation strategies involving engineered optimization of surface topography coupled with strategic modulation of hardness and roughness gradients between coatings and substrates have demonstrated efficacy in enhancing tribological performance, thereby significantly extending the service life of engineered components.

2.1. Wear Mechanisms of Metallic Substrates

Wear, as a critical tribological phenomenon encompassing material deformation, transfer, and loss at metallic interfaces, detrimentally affects energy efficiency and shortens the service life of mechanical components [42,43]. Therefore, its study and mitigation are crucial for improving performance and durability. Relative motion between contacting surfaces initiates frictional interactions, while wear manifests as an inevitable consequence of accumulated frictional energy dissipation. Elucidating wear mechanisms across multiple scales is essential for developing effective anti-wear strategies. Distinct from instantaneous friction events, wear constitutes a progressive process involving material detachment from parent surfaces over sustained sliding contact periods. At atomic and nanoscopic scales, wear mechanisms involve discrete interactions between individual atoms or localized atomic clusters. Molecular dynamics (MD) simulations constitute a critical methodology for probing atomic-scale wear mechanisms, providing quantitative characterization of interfacial sliding dynamics via explicit modeling of atomic displacement and detachment phenomena [44,45].

In MD simulations, atomic forces and trajectories are governed by Newton’s equations of motion [46]

$$m_i{\ddot{r}}_i=-{\nabla }_{r_i}U(r_1,r_2,\dots ,r_N),$$

(1)

where m_i and r_i denote the mass and position vector of the i-th atom, respectively, with $-{\nabla }_{r_i}U$ representing the potential energy gradient of the system.

Typical MD simulations operate on nanosecond timescales (spanning tens to hundreds of nanoseconds), enabling atomic-level resolution of phenomena arising from interatomic interactions and configurational changes. This temporal resolution permits detailed observation of interfacial atomic rearrangement evolution and localized wear processes during frictional sliding.

Recent advancements in nanoscale tribology have provided unprecedented insights into fundamental wear mechanisms. Through integrated in situ high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM), He’s research team revealed an atomic diffusion-mediated low-friction mechanism by capturing real-time structural evolution at metal contact interfaces during sliding processes [47]. Atomic-scale friction manifests as stick-slip motion, characterized by alternating periods of atomic adhesion and sliding that produce oscillatory friction forces [48]. AFM-based studies further demonstrate that nanoscale material removal (typically 1–100 nm³ per cycle) predominantly involves discrete atomic/cluster ejection events, with the process being highly sensitive to interfacial adsorbates and lubricant boundary layers.

Transitioning to the micrometer scale, wear mechanisms retain qualitative similarities to macroscopic phenomena but exhibit distinct quantitative scaling relationships. Four principal wear modes have been systematically categorized as adhesive wear [49] driven by interfacial adhesion and material transfer, abrasive wear [50] initiated by hard asperity plowing, fatigue wear [51] featuring progressive crack propagation along grain boundaries, and corrosive wear [52] involving synergistic chemical-mechanical degradation. Particularly, fatigue wear demonstrates nonlinear damage accumulation, where cyclic loading induces subsurface crack nucleation followed by rapid, catastrophic failure, a behavior that defies accurate representation through linear damage models [53]. The inherent multiscale nature of tribological processes necessitates cross-scale analysis, as atomic-level interactions such as bond formation and rupture directly influence macroscopic wear rates through dislocation-mediated plasticity and surface energy modifications [54].

In 1946, using the views of Holm, who was the pioneer in introducing the mechanism of wear, Archard [49] proposed a relation to express the adhesive wear of two surfaces in contact. Abrasive wear initiates through mechanical interactions with hard particulates originating from either surface degradation processes or environmental contamination. The CoF [55] constitutes a specific parameter of the friction system controlled by the interface load distribution [56], material performance gradient, and surface energy landscape [57]. The current empirical amalgamation of these factors into unified wear coefficients imposes fundamental limitations on the Archard model’s predictive capacity. Addressing this constraint, Frérot et al. [58] developed two mechanistic derivation frameworks incorporating critical length scale concepts specifically the probabilistic reformulation of Archard’s hypothesis and the mono-asperity contact physics extension. Their theoretical framework revealed wear coefficient evolution featuring distinct power-law regimes and load-range stability, demonstrating remarkable consistency with experimental datasets. The inherent complexity arises from synergistic interactions among multiscale contact geometries such as surface roughness and abrasive particle distributions, heterogeneous material responses under dynamic loading, and transient sliding conditions. Consequently, surface engineering strategies focusing on controlled interfacial property modulation emerge as essential approaches for achieving predetermined tribological performance targets.

2.2. Wear Mechanisms of DLC Coatings

The origin of friction between solid surfaces is fundamentally attributed to three primary mechanisms: abrasive wear, shear deformation, and adhesive interactions, as depicted in Figure 4 [59].

Abrasion arises from plowing effects induced by interfacial micro-asperities or hard particulates entrapped between sliding surfaces. Shearing involves energy dissipation through plastic or viscous flow of materials at the friction interface. Adhesion arises from the rupture of micro-junctions that bridge contacting surfaces and is governed by bonding force, electrostatic force, capillary force, and polarization force. DLC coatings uniquely suppress these three friction contributors to ultralow levels, collectively elucidating the multiscale synergistic principles underlying their low-friction and high wear-resistance characteristics. The shear behavior involves the transfer layer, graphitization, and shear localization of the transport layer. The adhesion effect is related to surface passivation, including the regulation of dangling bond, π bond interaction, hydrogen bond, and van der Waals force. These mechanisms jointly determine the friction properties of DLC coatings.

As archetypal low-friction coatings, DLC systems demonstrate superior wear resistance compared to conventional materials. Their extreme hardness inhibits abrasive penetration and material removal, while nanoscale smoothness minimizes the real contact area. Although hardness generally correlates with wear resistance, phase transformations under elevated temperatures necessitate careful compositional design. Zhang et al. [60] systematically investigated B/Cr co-doped DLC coatings under high-temperature tribological conditions (Figure 5), revealing critical doping dependent performance transitions. Consequently, optimizing Cr doping content enables a balanced interplay between graphitization efficacy and oxidation resistance, ultimately regulating the tribological adaptability of the coatings.

Khan, S.A. et al. demonstrated that DLC-Ar coatings exhibit low friction/wear on both coating and titanium counterparts under minimal load, attributed to spontaneous graphitic transfer-layer formation at the interface [61]. Under increased loading, accelerated shearing progressively degrades this tribofilm, ultimately exposing the substrate material. According to the study by Kim, D. et al., higher sliding velocity and normal load enhance graphitization and transfer layer formation on the surface [62]. The rate of wear initially increases with velocity then decreases due to the protective role of the transfer layer and changes in surface hardness.

The tribological behavior of DLC coatings is predominantly influenced by environmental factors through their modulation of surface chemical states. Environment-dependent wear mechanisms are governed by humidity [63], temperature [64], and lubrication media [65]. The viscoelastic behavior of coatings can dissipate partial energy to mitigate wear, while shear behavior correlates with interfacial slip motion or tangential stress distribution. Transfer layers formed during friction processes act as lubricating coatings, effectively reducing shear resistance and friction coefficients [38]. Under high-temperature or high-pressure conditions, DLC coatings may undergo graphitization to form graphite-like structures [66]. Adhesion behavior originates from chemical or physical adsorption between contact surfaces, with the intensity of chemical interactions determined by the type and density of surface dangling bonds [67]. π-electron interactions and hydrogen bonding may enhance interfacial adsorption, while surface passivation suppresses undesired interactions by reducing surface reactivity [68,69]. Weak intermolecular forces can also contribute to adhesion. In ultrahigh vacuum environments, DLC coatings exhibit exceptionally high friction and wear due to abundant unsaturated dangling bonds caused by hydrogen termination deficiency [70]. These chemically active bonds readily form covalent bonds, resulting in elevated friction coefficients. The absence of reactive gas participation in surface passivation under vacuum conditions may further exacerbate high-friction behavior [71].

Friction coefficients and wear rates of DLC coatings in a vacuum typically increase with sliding distance due to hydrogen termination depletion, whereas hydrogen molecule supply can maintain surface passivation, prolonging superior lubricity and reducing wear [72]. During friction processes, hydrogen molecules undergo continuous dissociative adsorption on DLC surfaces, ensuring fully passivated and chemically inert interfaces. Comparative studies reveal that DLC coatings exhibit lower wear in water or humid air environments than in dry air [37].

Oxygen environments demonstrate irregular wear behavior, with some studies reporting oxygen molecule adsorption on DLC surfaces forming robust C-O terminal groups that resist mechanical disruption under external loads, thereby reducing friction [73]. Oxygen adsorption transitions from physical adsorption during initial sliding to chemical adsorption in steady-state stages. This interfacial modification facilitates the formation of easily shearable lamellar structures composed of carbon and oxygen atoms, contributing to friction reduction. Multiple investigations confirm that oxygen-induced wear rate reduction in DLC coatings highlights the critical role of chemical reactions between DLC surfaces and environmental molecules in governing tribological performance [74].

The reactive molecular dynamics (RMD) simulations reveal that DLC wear is primarily influenced by two mechanisms: chemical wear due to the tribochemical emission of C_xH_y and C_xH_yO_z compounds and mechanical wear caused by the formation of interfacial C–C bonds. The environment significantly affects DLC tribological behavior by altering the surface chemical state, specifically the quantity and type of surface terminations. In water, the presence of additional surface terminations leads to increased chemical wear and reduced mechanical wear compared to vacuum conditions. Oxygen-containing terminations, such as C-OH and C=O, are found to be more resistant to mechanical wear than hydrogen terminations [75].

Table 1. Friction coefficient and wear rate of different types of DLC coating in different environmental conditions [76].

	Diamond Coatings [77,78,79,80,81,82]	Hydrogen Free DLC (H-Free DLC) [83,84,85,86,87,88,89,90,91,92,93,94]	Hydrogenated DLC (H-DLC) [77,85,86,89,91,92]	Modified/Doped DLC [84,89,90,91,92,95]
Structure	CVD diamond	a-C ta-C	a-C:H ta-C:H	a-C:Me a-C:H:Me a-C:H:X
Atomic structure	sp3	sp2 and sp3	sp2 and sp3	sp2 and sp3
Hydrogen content	-	>1%	10–50%
µ in vacuum	0.02–1	0.3–0.8	0.007–0.005	0.03
µ in dry N2	0.03	0.6–0.7	0.01–0.15	0.007
µ in dry N2 5–15% RH	0.08–0.1	0.6	0.025–0.22	0.03
µ in humid air 15–95%	0.05–0.15	0.05–0.23	0.02–0.5	0.03–0.4
µ in water	0.002–0.08	0.007–0.1	0.01–0.7	0.06
µ in oil	-	0.03	0.1	0.1
K in vacuum	1–1000	60–400	0.0001	-
K in dry N2	0.1–0.2	0.1–0.7	0.00001–0.1	-
K in dry air 5–15% RH	1–5	0.3	0.01–0.4	-
K in humid air 15–95%	0.04–0.06	0.0001–400	0.01–1	0.1–1
K in water	0.0001–1	-	0.002–0.2	0.15
K in oil	-	-	-	0.1

µ refers to coefficient of friction. K refers to wear rate [10⁻⁶ mm³ (Nm)⁻¹]. Me = W, Ti…. X = Si, O, N, F, B.

presents a comprehensive database of CoF and wear rates for various DLC coatings under different environmental conditions. The counterfaces for the coefficient of friction include AISI 52100 steel balls, Si₃N₄ balls, Al₂O₃ balls, and natural diamond.

Yin, Zhiyuan et al. [96] developed an atomic-scale method to analyze material wear mechanisms. Their study found that under low loads and sliding speeds, average friction variations depended solely on interfacial bonds or contact area. However, under high loads and speeds, friction mechanisms correlated with both interfacial bond quantities and transfer layers. Huang, Tao et al. [97] employed MD simulations to model sliding interface evolution and tribological performance under varying pressures and graphitization states. Parametric correlations between MD simulations and experiments remained challenging due to significant time and length scale differences [98].

The tribological behavior of DLC coatings emerges from the synergistic integration of compositional design, interfacial structural evolution, and dynamic environmental chemical interactions. To advance this field, future efforts must prioritize molecular-scale simulations coupled with experimental validation to unravel the competitive passivation dynamics governing hydrogen-terminated surfaces and dangling bonds across environments including vacuum, humidity, and elevated temperatures. Such insights will drive the rational development of DLC coatings with tailored transfer coating formation and stability. Concurrently, constructing quantitative frameworks that correlate environmental variables such as humidity and oxygen partial pressure with friction coefficients will empower adaptive optimization strategies for DLC coatings under varying operational scenarios, ensuring robust performance in extreme service conditions.

3. Wear Inhibition Technologies of DLC-Coated Metal Components

Green tribotechnology is crucial for advancing low-carbon economies and addressing global environmental pollution, energy crises, and climate change, making it an essential pathway to achieving a sustainable society [99]. Wear minimization is the significant tribology issue that applies to green tribotechnology [100]. DLC coating is the core carrier of green tribotechnology. But a critical challenge in the fabrication of DLC coatings arises from the coefficient of thermal expansion (CTE) mismatch between the coating and substrate materials. This discrepancy induces significant residual stress accumulation, which severely constrains the achievable coating thickness. The microstrain [101] generated within the amorphous matrix structure promotes the formation of residual compressive stresses, ultimately leading to coating delamination through interfacial failure. Such insufficient interlayer bonding strength represents a predominant defect in DLC coatings processing, consequently resulting in substantially compromised wear resistance of coated metallic components.

Current research has established three principal methodologies to enhance the wear resistance of DLC coatings. Surface modification prior to coating deposition, aimed at increasing substrate hardness and mitigating the CTE mismatch between the coating and substrate. The structural design of the coating is carried out, the interlayer is applied between the substrate and the coating, the realization of gradient interlayer makes the stress distribution uniform, and the thickness of the coating is designed. Elemental doping is carried out in the preparation of the coating, and the performance of the coating is improved by doping metal or non-metal elements. The summary of various methods is shown in Figure 6.

3.1. Substrate Material Treatment/Strengthening

The superior adhesion of DLC coatings to substrates plays a critical role in reducing friction and wear, making interfacial bonding a key consideration in the deposition on metal components. Robust adhesion between the tribological counterparts and substrate ensures operational stability and durability, effectively preventing delamination-induced mechanical failures. Prior to DLC coating deposition, substrate surfaces typically undergo ion cleaning treatment in vacuum chambers. This process involves argon gas ionization to generate Ar⁺ ions or plasma, which are accelerated toward the substrate surface via applied negative bias, removing surface contaminants and trace substrate material through kinetic energy transfer [103].

Surface roughness significantly influences coating wear performance [104]. Khan et al. [105] systematically investigated this relationship by depositing DLC coatings on WC-Co substrates, as shown in Figure 7. Their findings demonstrate that reduced surface roughness enhances coating adhesion, lowers friction coefficients, and improves load-bearing capacity. Smooth surfaces promote stable transfer layer formation, whereas high roughness induces localized pressure peaks that initiate coating spallation and substrate exposure. The plowing effect of asperities on rough surfaces destabilizes transfer layers, causing rapid delamination even when transient layers form.

The hardened interlayer effectively suppresses subsurface plastic deformation beneath DLC coatings, while strong interfacial adhesion inhibits surface crack initiation [106]. Pretreatment via plasma nitriding prior to DLC deposition facilitates the formation of lamellar coating architectures, significantly enhancing load-bearing capacity, mechanical properties, and tribological performance. Morita et al. [107] demonstrated that plasma nitriding outperforms plasma carburizing as a pretreatment for stainless steel substrates, achieving lower friction coefficients through enhanced load support from the hardened nitride layer. This nitride interphase, formed during plasma nitriding, serves dual functions such as improving coating–substrate adhesion through chemical bonding and mitigating residual stress to prevent delamination failure. Yetim et al. [108] investigated the effects of dual surface treatments combining plasma nitriding and DLC coating on the fatigue properties of Ti6Al4V alloy, demonstrating that the DLC coating significantly enhances the material’s fatigue resistance. Kovacı et al. [106] deposited DLC coatings on plasma-nitrided AISI 4140 steel, revealing a notable improvement in wear resistance and load-bearing capacity of the treated steel. Yamauchi et al. [109] showed that pretreatment with SiC and C media via shot peening effectively enhances the adhesion of DLC coatings on magnesium alloys. Wu et al. [110] applied shot peening followed by DLC coating to 18CrNiMo7-6 gear surface, achieving a substantial extension in fatigue life. The developed equipment, coating structure, and elemental analysis are illustrated in Figure 8, with the transition layer identified as Cr and CrC.

Surface texturing technology enables the engineering of micro/nanoscale surface structures with precisely controlled geometries, periodic arrays, and tailored dimensions while preserving the bulk material composition. This strategy enhances functional performance beyond the intrinsic properties of the base material, particularly in tribological and interfacial applications. In 1994, Suh et al. [111] proposed the concept of integrating surface texturing with low elastic modulus coatings as a method to control friction. Surface texturing not only enhances the effective bonding strength between coatings and substrates but also reduces the actual contact area, thereby diminishing the adhesive component of friction. In most studies, surface texturing is implemented prior to DLC coating deposition. Comparative studies under 15 N, 25 N, and 35 N loads revealed that laser-textured tungsten-doped DLC coatings exhibit lower friction coefficients than their non-textured counterparts. Notably, non-textured coatings demonstrated higher graphitization degrees under 35 N loading [112]. Research indicates that interfacial texturing improves the tribological performance of coatings by strengthening coating-substrate adhesion, which is particularly critical for high-contact-pressure applications where conventional DLC coatings face limitations. Liu et al. [113] fabricated laser-textured steel substrates with sequentially deposited multilayer Cr/CrN/DLC coatings. Vacuum arc-deposited DLC coatings exhibited reduced delamination at lower area densities, achieving conformal filling of surface features without spalling. Enhanced interfacial adhesion was further confirmed through scratch testing. Geng et al. [114] demonstrated that nano-depth interfacial texturing increases the transition layer thickness between DLC coatings and silicon substrates by approximately 47.62% while reducing internal coating stress by 30%. The experimental results showed that DLC coatings with interfacial texturing maintain superior tribological performance under contact pressures up to 1 GPa, with wear life significantly exceeding non-textured coatings—particularly pronounced in thicker coatings. Molecular dynamics simulations (Figure 9a–d) and the experimental results (Figure 9e–i) collectively validate the stress-relieving and tribological-enhancing effects of interfacial texturing. These advancements suggest the feasibility of deploying DLC coatings under higher contact pressures and achieving thicker coating deposition in future applications.

3.2. Coating Structure Design

Recent advancements in material design and surface engineering have unveiled novel possibilities for surface protection through the development of functionally graded and hierarchical coatings. The classical multilayer coating concept, which involves creating smooth interfacial transitions to gradually tailor properties from substrate to coating surface, has witnessed rapid experimental progress in recent decades [115,116].

Wu et al. [117] fabricated DLC multilayer coatings with varying bilayer periods (228 nm to 970 nm) using magnetron sputtering with Ni interlayers. The coating with a 710 nm bilayer period exhibited densified microstructure, refined grain size, and enhanced mechanical properties. Incorporation of Ni reduced the sp² content, which in turn decreased surface roughness, increased hardness, and lowered the friction coefficient. Nemati et al. [118] proposed a functional multilayer coating (FMC) design comprising alternating amorphous carbon (a:C) and WC nanolayers deposited on 304 stainless steel. Process optimization yielded coatings with exceptional elasticity, shear modulus, wear resistance, and corrosion resistance. As shown in Figure 10, the a:C/WC multilayer structure with 100 alternating layers exhibited ultra-low wear rates under a contact pressure of 0.88 GPa, while maintaining stable low friction during prolonged sliding. Ramírez-Reyna et al. [119] deposited three coating systems via High Power Impulse Magnetron Sputtering (HiPIMS) technology on AISI 52100 steel: System A (DLC/WC/W), System B (DLC/WCN/W), and System C (DLC/WC/WCN/W). All systems formed uniform, dense layers with distinct interfacial transitions. System C exhibited superior performance due to WC/WCN bilayer synergy-achieving peak hardness, highest elastic modulus, and minimal penetration depth. This intermediate bilayer configuration reduced wear damage compared to monolayer counterparts through stress delocalization and interfacial strain accommodation mechanisms.

Bertran et al. [120] fabricated Mo/a-C and W/a-C multilayered coatings with 30 modulation periods via magnetron sputtering. The results show that the multilayer design enhances coating hardness and elastic modulus, reduces compressive stress, and further improves the adhesion between DLC coatings and substrates. The multilayered interface structure effectively restricts defect mobility and prevents their propagation, thereby acting as a barrier to defect coalescence [121]. Haneef et al. [102] deposited two sets of nanolayered Ti-DLC/DLC coatings on AISI M2 steel substrates at different rotational speeds using magnetron sputtering. Figure 11 shows schematic diagrams of both coating sets, which share similar chemical compositions. The results indicate that the sp²/sp³ increases with Ti concentration. Coatings deposited at a substrate rotation speed of 1 rpm exhibit superior performance, and their properties can be tailored by alternating DLC and Ti-doped DLC layers.

Thicker coatings generally demonstrate higher wear resistance and surface hardness, improving tribological system durability. However, excessive thickness may reduce adhesion strength [122]. Sahoo et al. [123] found that DLC coatings significantly enhance the hardness and wear resistance of cutting tools while reducing the friction coefficient. Among the DLC coatings of varying thicknesses, the 640 nm thick coating exhibited optimal cutting performance, effectively reducing friction and size effects. However, adhesion decreases with increasing coating thickness, potentially causing delamination during cutting and shortening tool lifespan. In summary, the multilayer structural design of DLC coatings improves hardness and toughness, reduces internal stress, and enhances tribological performance. Additionally, multilayer interfaces act as barriers to defects, suppressing crack initiation and propagation under complex stress fields, thereby improving coating toughness. Finally, layer thickness, modulation period, and material selection are critical parameters for tailoring DLC coating performance.

3.3. Elemental Doping

The fabrication of high-performance DLC coatings is constrained by internal stresses. Elevated internal stresses reduce the adhesion strength between DLC coatings and substrate materials while limiting the deposition thickness [124,125]. Studies demonstrate that doping with specific metallic or nonmetallic elements effectively reduces internal stresses, enhances adhesion strength, and improves toughness [126,127]. Recent research reveals that co-doping with two or more elements synergistically enhances comprehensive properties, including hardness, toughness, tribological performance, and corrosion resistance [128,129].

One of the most challenging issues in DLC coatings arises from the thermal expansion coefficient mismatch between the coating and substrate, which generates substantial residual stresses [130]. Accumulation of high residual stresses restricts coating thickness [131]. Microstrain formation within the amorphous matrix of DLC coatings induces residual stresses, leading to coating delamination. The roles of substrate treatment and structural design techniques have been discussed in the preceding sections. Additionally, incorporating metal or nonmetal elements into DLC coatings can mitigate stresses. Elemental doping modifies the coating structure, particularly the sp³/sp² ratio [132]. Moreover, residual stresses and hardness critically influence coating-substrate adhesion and tribological behavior during service [110]. As shown in Figure 12, doping with selected elements allows precise modulation of the microstructure and chemical composition of DLC coatings, significantly influencing both their structural and functional properties. This section reviews the fundamental principles and recent advancements in the wear resistance behavior of DLC coatings doped with selected nonmetallic and metallic elements.

H-DLC coatings are characterized by hardness values spanning 15–50 GPa [133]. In contrast, H-free DLC coatings achieve superior hardness (30–80 GPa) through a dense network of tetrahedrally coordinated carbon atoms, predominantly sp³-bonded [134]. In H-DLC coatings, solid surface interactions are predominantly governed by hydrogen presence, which inhibits C–C bonding at friction interfaces [135]. Figure 13a depicts H-free DLC coatings surfaces under varying sliding parameters. Dry sliding combined with elevated humidity and temperature triggers graphitization and oxidative reactions (e.g., C–O, COO, C–OH bond formation) mediated by atmospheric water vapor. The presence of H atoms terminating carbon bonds on highly hydrogenated DLC surfaces (Figure 13b) reduces surface interactions, significantly lowering contact stress. However, elevated operational temperatures, sliding velocities, or humidity trigger chemical reactions that elevate the friction coefficient.

Nitrogen (N) doping enhances fracture resistance and adhesion of Si-DLC coatings through strong covalent bonding with carbon atoms, promoting Si-C and C-N bond formation [136]. Si and N-co-doped DLC coatings exhibit exceptional thermal stability with wear rates reaching 10⁻⁷ mm³/mN after 450 °C annealing [137,138,139,140]. Introducing fluorine (F) into DLC coatings terminates dangling bond formation and reduces friction coefficients, with optimal F content yielding superior tribological performance [141]. B₄C doping significantly improves DLC wear resistance by enhancing mechanical properties and facilitating lubricious graphitic layer formation at elevated temperatures. Chen et al. [127] demonstrated that B₄C-DLC coatings exhibit superior high-temperature lubrication and wear resistance between 400 and 500 °C. B₄C doping additionally increases coating hardness and adhesion while reducing residual stresses, with B₄C-DLC containing 4.57 at.% B showing peak hardness and optimal mechanical properties.

Introducing metal atoms into DLC coatings alters their structure through reactions with carbon atoms, generating nanocrystalline metals and metal carbides. Zhou et al. [142] systematically investigated microstructure evolution, mechanical properties, and tribological behavior of Ti-DLC coatings on 304 stainless steel. Figure 14 compares wear mechanisms across Ti concentrations, revealing that moderate Ti doping relieves residual stresses and enhances practical coating-substrate adhesion. Ti doping improves DLC coating deformability, mitigating stress accumulation while suppressing brittle fatigue and fracture.

Chromium (Cr) demonstrates exceptional wear resistance and oxidation stability with superior steel substrate compatibility. Similarly to Ti doping, Cr forms carbide precipitates in the amorphous carbon matrix, alleviating internal stresses and enhancing adhesion to mitigate DLC brittleness and improve mechanical-tribological behavior [143]. Nickel (Ni) doping effectively suppresses interfacial oxidation/corrosion during DLC preparation, stabilizing friction coefficients and improving wear resistance [144]. Vilius et al. [145] conducted a comparative study on Cr- and Ni-doped DLC coatings, demonstrating that the Ni-containing counterparts exhibited superior adhesion strength and nano hardness. The enhanced performance was attributed to Ni doping-induced structural modifications, including an elevated sp²/sp³ that promotes the growth of nanocrystalline graphite domains in metal-doped DLC systems, coupled with diminished surface defect density [146]. H-free DLC coatings exhibit inadequate thermal resistance at high temperatures. Deng et al. observed that ta-C coatings undergo crack initiation and delamination during tribological testing at 500 °C [147]. Yu, W et al. conducted in situ high-temperature tests at temperatures ranging from room temperature to 500 °C [148]. The results showed that Si-DLC coatings with different Si content exhibited different lubrication mechanisms in different temperature ranges. With the increase in test temperature and Si content, the lubrication mechanism gradually changed from graphitization caused by high temperature to abrasive wear dominated by Si carbide (SiC) and formed Si dioxide (SiO₂) abrasive particles. However, Beake et al. [149] found that Si-doped DLC coatings show inferior reciprocating wear resistance but enhanced crack resistance during impact testing. Tungsten (W)-doped DLC coatings demonstrate exceptional impact resistance with high crack resistance and low wear resistance in reciprocating tests. This suggests synergistic wear resistance improvement through W/Si gradient doping architectures.

Recent studies have shown that doping with other elements can enhance the performance of DLC coatings. For example, Ag doping significantly improves wear resistance due to the formation of Ag-rich rod-like wear debris [150]. Co-doping with Ag and TiO₂ not only enhances the coating’s properties but also maintains good adhesion to the substrate [151]. Additionally, lead doping effectively reduces the damage caused by oxygen atom erosion on the coating surface [152]. Li, N. et al. suggested that hydrogenated DLC films offer a promising approach for improving the performance of graphene-DLC solid–liquid composite systems [153].

In summary, appropriate elemental doping not only reduces coating stresses but also optimizes mechanical-tribological properties. Distinct doping concentrations, existing forms, and bonding configurations yield varying performance outcomes. Future research should prioritize precise doping concentration control via advanced deposition techniques and multi-condition elemental ratio optimization to address industrial challenges like poor wear resistance.

4. Synergistic Effect of Wear Inhibition Technologies for DLC-Coated Metal Components

4.1. Advantages and Limitations of Inhibition Technologies

The fabrication of DLC coatings on metallic components for wear resistance enhancement relies on three pivotal technological approaches: substrate pretreatment/strengthening, coating structural design, and elemental doping. While each technique exhibits distinct advantages, practical implementation requires careful balancing of process complexity, cost-effectiveness, and performance compatibility. The synergistic integration of substrate treatment/strengthening, coating architecture design, and elemental doping proves critical for optimizing DLC-coated metallic surface wear resistance. Substrate pretreatment enhances mechanical interlocking and stress buffering through surface topography modification (e.g., laser micro-texturing, high-temperature carburizing), providing high-adhesion foundations for coatings. Coating structural design employs functionally graded layers, hierarchical micro or nano textures, and composite interfacial architectures to optimize mechanical compatibility and thermal expansion matching. Elemental doping tailors metal/nonmetal atomic ratios in carbon matrices, endowing coatings with high hardness, low friction, and environmental adaptability. However, synergistic implementation faces challenges including substrate thermal deformation and carbon gradient imbalance from pretreatment, defect-stress tradeoffs in multilayer deposition, and brittle phase formation from doping. Additional constraints arise from high costs and complex service environment requirements.

Table 2. Advantages and limitations of inhibition technologies.

Technologies	Advantages and Limitations	Reference
Substrate Material Treatment/Strengthening	Advantages: Enhanced load-bearing capacity of the substrate and reduced plastic deformation of coatings under mechanical loading. Limitations: High-temperature treatments (e.g., carburization) may induce substrate deformation or grain coarsening.	[110,154,155]
Coating Structure Design	Advantages: Stress distribution optimization and mitigation of thermal expansion mismatch at interfaces via gradient coatings. Limitations: Balancing high hardness with high toughness remains challenging.	[156,157]
Element Doping	Advantages: Internal stress regulation to improve coating adhesion and reduce friction coefficients. Limitations: Difficulty in achieving uniform doping, leading to localized performance degradation (e.g., brittle phase formation).	[126]

summarizes the advantages and limitations of different mitigation techniques based on preceding discussions and the contemporary literature.

4.2. Synergetic Effects of Inhibition Technologies

The choice of a suitable interlayer, multilayer structure, processing parameters, substrate preparation, and addition of dopants is crucial for improving the adhesion and requires more technical research. The combined strategy of structural design and elemental doping optimizes coating architectures and metallic dopant ratios/species, effectively reducing friction and enhancing wear resistance. Luo, G. et al. [158] addressed weak interfacial bonding in soft AISI 1045 steel by developing multilayer coatings to overcome thickness limitations and friction/wear reduction inefficiencies. CrN/W-DLC/DLC multilayer coatings with optimized CrN interlayer thickness significantly improved coating-substrate adhesion and wear resistance. Figure 15a–d illustrate the microstructure, mechanical properties, and tribological performance of CrN/W-DLC/DLC coatings on AISI 1045 steel. Figure 15a demonstrates excellent interlayer bonding, with increased CrN interlayer thickness promoting DLC layer growth and surface roughening. Scratch testing in Figure 15b reveals enhanced critical load with thicker CrN interlayers, where 3CrN/W-DLC/DLC exhibited optimal performance despite brittle fracture failure modes. Tribological tests in Figure 15c,d show 3CrN/W-DLC/DLC coatings achieving the lowest friction coefficient and minimal wear volume/depth through superior self-lubrication and wear resistance. CrN interlayer thickness critically influences microstructural, mechanical, and tribological properties, confirming that optimized structural-doping synergy enhances comprehensive coating performance.

Yamada et al. [159] demonstrated that rough DLC surfaces fail via brittle fracture, while smooth DLC wear rates remain independent of fracture toughness. Specifically, high-roughness DLC coatings show wear rate reduction with increased toughness, whereas low-roughness coatings maintain low wear rates despite limited toughness. Substrate roughness elevation significantly increases coating wear rates and alters wear mechanisms [104]. He et al. [154] observed lower CoF in laser-textured specimens due to reduced real contact area during running-in. Post-running-in, textured DLC surfaces maintained stable CoF through wear debris entrapment (Figure 16a). Wear rates exhibit threshold behavior: low-to-medium texture coverage reduces wear, while excessive coverage accelerates wear via surface polishing/oxidation (Figure 16b). Textured surfaces show enhanced graphitization during sliding, potentially due to stress concentration and microcrack-induced carbon chain rupture at texture grooves.

As shown in

Table 3. Wear rate performance of three inhibition technologies.

	Coating Type	Deposition Technique/ Substrate	Wear Test Conditions	Wear Rate	Reference
Substrate material treatment/strengthening	Cr/CrN-DLC	HiPIMS/ cemented carbide, WC-Co	A total of 100 K cycles, under dry condition with a stroke length of 2 mm and applied force of 10 N using a 100Cr6 steel ball of 10 mm diameter as a counterbody	Ra = 0.260 µm, 4.73 × 10⁻7 mm3/(N·m) Ra = 0.017 µm, 2.41 × 10⁻8 mm3/(N·m) Ra = 0.008 µm, 1.74 × 10⁻8 mm3/(N·m)	[105]
Coating structure design	WC-Cr-DLC	DC magnetron sputtering/304 stainless steel	A load of 10–50 mN, speed set to 2 mm/s with a stroke of 2 mm, counter surface is a stainless steel ball with a diameter of 1 mm	10−9 mm3/(N·m)	[118]
Elemental doping	Ti-DLC (Ti, 1.82 wt%)	DC magnetron sputtering/304 stainless steel	A load of 5 N, reciprocating distance of 4 mm (frequency = 5 Hz), and a GCr15 grinding pair steel ball (diameter = 3 mm)	1.95 × 10⁻3 mm3/(N·m)	[142]

, the wear rates corresponding to different suppression technologies are summarized.

Thus, wear-resistant DLC design requires both fracture toughness improvement and stress intensity factor reduction via surface morphology control. Appropriate substrate surface pretreatment remains essential prior to coating deposition.

5. Conclusions and Outlook

5.1. Conclusions

The wear mechanisms of metallic components constitute a complex multi-scale, multi-factor coupled process. At micrometer scales, wear manifests as adhesive, abrasive, fatigue, and corrosive wear, each governed by contact surface microgeometry, roughness, load, velocity, temperature, and chemical environment. The wear process involves localized plastic deformation and microcrack propagation, synergistically accelerated by hard particle impacts and environmental corrosion, ultimately causing material loss or delamination. Optimizing wear resistance requires comprehensive control of surface properties, energy transfer, and microstructural evolution. DLC coating wear behavior depends on hardness, superlubricity, and viscoplasticity. DLC coatings function as interfacial intermediate layers at relative motion interfaces, effectively suppressing wear initiation. The performance optimization of DLC coatings centers on structural refinement, interface engineering, and tribological modulation to achieve stable friction and wear characteristics under complex service conditions.

Substrate pretreatment/strengthening, coating structural design, and elemental doping have demonstrated significant efficacy in improving DLC wear resistance. Substrate engineering enhances coating adhesion, reduces interfacial delamination, and improves overall durability. Rational coating architectures optimize friction patterns and delay wear failure. Elemental doping tailors DLC chemical, physical, and tribological characteristics for wear resistance enhancement. Continuous technological innovations are further advancing these methodologies.

Distinct mitigation strategies exhibit complementary advantages: substrate strengthening improves interfacial bonding and load-bearing capacity; structural optimization relieves stresses while enhancing toughness and stability; and elemental doping modulates physicochemical properties to optimize tribological responses. Their synergistic integration enables comprehensive wear resistance improvement in DLC-coated metallic components.

This review systematically compiles metallic component wear mechanisms across scales and DLC coating failure modes, addressing literature gaps in multi-scale wear mechanism integration. Substrate engineering, coating structure design, and doping technologies are critically evaluated, clarifying their effectiveness, limitations, and future research trajectories. Synergistic analysis reveals combinatorial technology potential for wear resistance enhancement, establishing foundations for integrated mitigation strategies. Guided by inhibition technologies, advancements in deposition methods are driving the transition of DLC coatings from lab-scale research to industrial-scale production. This shift requires addressing three critical factors including process cost optimization, production efficiency enhancement, and standardized manufacturing protocols.

5.2. Outlook

Despite the significant progress in DLC-coated metallic component wear resistance, unresolved scientific challenges persist, particularly regarding the environmental sensitivity of tribological performance. Future research must prioritize service-condition-specific wear mechanism analysis and technology optimization. Current knowledge gaps suggest five critical research directions (as shown in Figure 17).

Fundamental wear mechanisms: The multi-scale coupling effects, dynamic sp³/sp² phase transitions, transfer layer stability, and environmental dependencies of DLC wear remain inadequately understood. Current experimental-computational approaches lack systematic characterization of interfacial chemical kinetics during wear. Advanced in situ techniques AFM, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) combined with multi-scale modeling MD, density functional theory (DFT), and Finite Element Analysis (FEA) should elucidate atomic-scale wear processes to guide coating optimization for extreme environments.

Green tribotechnology: Developing eco-friendly strategies—low-emission deposition processes and adaptive friction control—is imperative. Synergistic evaluation of coating architecture, doping, and external field modulation will establish new paradigms for sustainable high-performance DLC systems.

Predictive wear modeling: The current models inadequately predict temperature-dependent interfacial reactions and wear rate quantification. Integrating advanced diagnostics with multi-scale computational frameworks will enable accurate service life prediction and adaptive control strategies.

Interdisciplinary innovation: Hybridizing plasma-enhanced CVD with energy beam processing enables microstructural precision control. Computational materials’ science-guided doping optimization will accelerate DLC performance breakthroughs. AI/ML methods like genetic algorithms and neural networks can be employed to predict and optimize DLC properties. In terms of deposition routes, HiPIMS offers high ionization rates and improved coating adhesion Cross-domain convergence promises revolutionary advances in extreme-condition tribological solutions.

Industrial scalability: Addressing cost-efficiency and large-area deposition challenges through automated manufacturing and smart process control will accelerate industrial adoption. Standardization and supply chain integration will drive DLC technology penetration across automotive, aerospace, and energy sectors.