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Review

Toughening Mechanisms of Diamond-like Carbon Films via Non-Carbide Metal Doping

by
Jiahao Liu
1,
Zhifang Yang
2,
Guangying Cui
2,
Jiayin Lv
2 and
Xiang Yu
2,*
1
Programme of Sustainable Energy Technology and Management, Faculty of Science and Technology, Beijing Normal-Hong Kong Baptist University, Zhuhai 519087, China
2
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China
*
Author to whom correspondence should be addressed.
Lubricants 2025, 13(11), 496; https://doi.org/10.3390/lubricants13110496
Submission received: 28 September 2025 / Revised: 30 October 2025 / Accepted: 12 November 2025 / Published: 14 November 2025
(This article belongs to the Special Issue Recent Advances in Lubricated Tribological Contacts)
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Abstract

The inherent brittleness and poor fracture toughness of diamond-like carbon (DLC) films significantly limit their long-term reliability in mechanical and tribological applications. Among various strategies to enhance toughness, doping with non-carbide-forming metals (e.g., Ag, Cu) has emerged as a highly effective approach due to their ductile properties and compatibility with carbon matrices. This review comprehensively examines the underlying toughening mechanisms induced by non-carbide metal doping in DLC films. We systematically analyze how metal incorporation influences film microstructure, stress state, and crack behavior throughout the entire lifecycle—from deposition to mechanical testing. Five primary toughening mechanisms are identified and discussed: (I) bombardment-induced compressive stress relaxation during film growth; (II) refinement of carbon atomic clusters and enhancement of grain boundary sliding; (III) inhibition of dislocation accumulation through moderated carbon atom repulsion; (IV) plastic deformation, crack bridging, and strain field relaxation at crack tips; (V) shear-induced stress relief via soft metal particles. Among these, Mechanism IV (ductile phase toughening) is identified as the dominant contributor, and their synergistic action can lead to orders of magnitude improvement in wear resistance and a significant increase in crack propagation resistance. Furthermore, the critical role of doping content is emphasized, revealing an optimal concentration range (e.g., ~10–15 at.% for Ag and Cu) beyond which toughness may deteriorate due to excessive boundary formation or hardness loss. This work provides a mechanistic framework for designing toughened DLC films and guides future efforts in developing high-performance, durable carbon-based coatings.

1. Introduction

Diamond-like carbon (DLC) films are a class of metastable amorphous carbon materials that possess a unique combination of properties, including high hardness, low friction coefficient, excellent chemical inertness, and superior wear resistance [1,2]. These characteristics have made them indispensable protective coatings in numerous applications, from automotive components and precision cutting tools to biomedical implants [3,4]. However, the pursuit of extreme hardness in DLC films has often been achieved at the expense of fracture toughness [5].
The inherent brittleness of many DLC variants constitutes a significant technological bottleneck, severely limiting their reliability and service life under mechanical loads [6]. Under concentrated or cyclic stresses, the low fracture toughness of DLC coatings can lead to rapid initiation and propagation of micro-cracks, resulting in catastrophic failure modes such as coating delamination, spalling, and abrasive wear [7]. In critical systems like deep-well drilling machinery or aerospace actuators, such failures can lead to operational shutdowns and substantial economic losses [8]. Therefore, enhancing the toughness of DLC films without drastically compromising their hardness is essential for expanding their use in demanding tribological applications.
In the context of protective coatings, “toughness” specifically refers to the fracture toughness—the material’s resistance to crack propagation under stress. For DLC films, this is not an intrinsic bulk property but a system property profoundly influenced by the film’s microstructure, residual stress state, and interface adhesion to the substrate [7,9]. The primary challenge and goal are to improve this crack propagation resistance without sacrificing the hallmark high hardness of DLC. As this review will elucidate, doping with non-carbide metals provides a unique pathway to achieve this by introducing nanoscale ductile phases that dissipate energy through mechanisms such as crack bridging and deflection.
Various strategies have been explored to address this challenge, including the design of multilayer architectures [10], gradient compositions [11], and the incorporation of dopant elements [12]. Among these, doping with metal elements has proven to be one of the most effective approaches for toughening DLC films [13]. Metal dopants can be broadly categorized into carbide-forming metals (e.g., Ti, Cr, W) and metals that are typically non-carbide-forming under standard DLC synthesis conditions, such as Ag, Cu, and Au (Al is a more complex case, as it can form Al4C3 under certain conditions) [9,14]. While carbide-forming metals can enhance hardness and thermal stability through the formation of nanocrystalline carbides embedded in the amorphous carbon matrix, they often exacerbate brittleness due to the intrinsic hardness of the carbide phases [15].
In contrast, metals like Ag, Cu, and Au, which predominantly remain in their metallic state within the DLC matrix under typical deposition conditions, offer a distinct pathway to toughness enhancement. Their low reactivity and solubility with carbon lead to the formation of nanosized metallic particles or clusters uniformly dispersed within the DLC matrix [14,16]. This microstructure leverages the inherent ductility and plasticity of the metal phase to impede crack propagation through mechanisms such as crack bridging, deflection, and energy dissipation via plastic deformation [17,18]. The resulting metal-doped DLC (Me-DLC) composites often exhibit a more favorable combination of hardness and toughness, leading to improved resistance to contact fatigue and wear [19].
Despite the demonstrated efficacy of non-carbide metal doping, a systematic and comprehensive review of the underlying toughening mechanisms is still lacking. The current understanding is fragmented across studies focusing on specific metal elements or deposition techniques. A holistic framework that synthesizes these mechanisms, from the film growth stage to mechanical failure, is needed to guide the rational design of next-generation tough DLC coatings [20].
This review aims to fill this gap by introducing a unified framework that categorizes and links five primary toughening mechanisms operating across the film’s lifecycle. The framework systematically links metal doping to the inhibition of crack initiation and propagation through: (I) deposition-stage stress relief, (II) microstructural refinement, (III) inhibition of dislocation accumulation, (IV) ductile phase toughening during loading, and (V) shear-induced stress relief. This work provides a thorough and mechanistic overview of these effects induced by non-carbid e metal doping in DLC films. We systematically explore and categorize these five primary mechanisms that operate at different stages, from deposition (e.g., stress modulation, nanostructuring) to mechanical loading (e.g., crack bridging, shear yielding). Furthermore, we discuss the pivotal role of doping content and distribution in optimizing toughness, highlighting the existence of an optimal range beyond which properties may deteriorate. By integrating findings from recent studies, this framework provides a foundational structure for understanding the synergistic, rather than isolated, action of these mechanisms, thereby accelerating the development of high-toughness DLC films for advanced industrial applications.

2. Effect of Metal Doping on DLC Properties

2.1. Distinction Between Carbide-Forming and Non-Carbide-Forming Metals

The choice of metal dopant is paramount, as it fundamentally dictates the resulting microstructure and mechanical properties of the composite film. The following discussion, supported by the comprehensive data summarized in Table 1 and Table 2, delineates the distinct pathways and outcomes associated with these two categories of metal dopants. The efficacy of this approach is first illustrated by the enhanced performance of architectured coatings, as shown in Figure 1 for a CrN/DLC/Cr-DLC multilayer system [21].
Carbide-forming Metals (e.g., Ti, Cr, W, Mo). These elements have a high chemical affinity for carbon. During deposition, they react with carbon to form nanocrystalline carbide phases (e.g., TiC, Cr3C2) that are embedded within the amorphous carbon matrix [22]. This creates a nanocomposite structure. While these carbides significantly enhance hardness and thermal stability, they are inherently brittle. Their formation consumes sp3-hybridized carbon, often reducing the cross-link density of the amorphous network and potentially increasing residual stress [23,24]. Consequently, while hardness increases, fracture toughness may not improve substantially and can even deteriorate.
To provide a comprehensive overview of the trade-offs associated with carbide-forming metal doping, Table 1 summarizes key findings from representative studies on Ti-, Cr-, W-, and Mo-doped DLC films. The data corroborate the general trend that while these dopants are highly effective in enhancing hardness and wear resistance, their contribution to fracture toughness is often limited and highly sensitive to doping content and microstructure. The formation of hard carbide phases typically does not introduce the energy-dissipating, ductile deformation mechanisms crucial for substantial toughness improvement, as reflected in the toughness evaluation methods and main findings listed.
As a specific illustration of the trade-off discussed above and reflected in Table 1, this trade-off is clearly demonstrated in Figure 2, where excessive Ti doping leads to the formation of large TiC grains and a sharp increase in wear rate [25].
Table 1. Summary of representative studies on carbide-forming metal-doped DLC films.
Table 1. Summary of representative studies on carbide-forming metal-doped DLC films.
Deposition
Method		Metal Content
(at.%)		Thickness
(μm)		Hardness
(GPa)		Wear Rate
(×10−7 mm3/N·m)		Toughness Evaluation MethodToughness/
Mechanism
Magnetron
Sputtering [21]		Ti/101.5285.0Scratch
Test
(Critical Load)		TiC nanocrystals refined structure and moderately improved adhesion/toughness; excessive doping led to brittleness.
Unbalanced
Magnetron
Sputtering [26]		Cr/15~2.0223.2Scratch
Test,
Impact Test		Nanocomposite (nc-CrCx/a-C) showed high hardness but limited toughness gain; failure often occurred via brittle fracture of carbides.
Filtered Cathodic
Vacuum Arc
[27]		W/51.0–1.535–400.5–1.5Nanoindentation (Fracture toughness model)Ultra-high hardness maintained; moderate toughness improvement attributed to compressive stress and nanocrystalline WC formation, but ductile deformation is minimal.
Closed Field
Unbalanced
Magnetron
Sputtering
[28]		Mo/8~1.8252.8Rockwell-C Adhesion,
Wear
Track
Analysis		MoCx formation increased hardness and thermal stability.
Adhesion was improved, but the fracture toughness improvement was marginal compared to non-carbide metals.
Note: The wear rates and specific values are highly dependent on the test conditions (e.g., counter body, load, environment) and should be compared qualitatively.
Non-Carbide-forming Metals (e.g., Ag, Cu, Au, Al). These metals have very low solubility and reactivity with carbon. They do not form chemical bonds with the carbon matrix but instead precipitate as nanosized metallic particles or clusters [29]. While elements like aluminum (Al) can form carbides (e.g., Al4C3) and even Ag/Cu may show specific interfacial interactions with carbon nanostructures, the predominant behavior in Me-DLC composites, which is central to the toughening mechanisms discussed here, is their physical dispersion as a metallic phase. This physical dispersion preserves the ductile, metallic nature of the dopant. The toughening effect is therefore achieved not through hardening, but through the introduction of a ductile phase that can plastically deform, bridge cracks, and absorb energy, leading to a more robust combination of properties [30,31]. The superior performance enabled by this approach is evident in Figure 3, where Ag doping reduces the wear rate of the substrate by four orders of magnitude [32]. As illustrated in Figure 3a, Ag-DLC/Cr coating exhibits significantly lower COF and wear rate than surgical stainless steel. The surface profile analyses (Figure 3(b1,b2)) reveal that Ag-DLC/Cr coating only shows slight microadhesion and furrows induced by abrasives. The SEM micrographs (Figure 3(c1–c3)) further demonstrate that Ag-DLC/Cr coating mainly presents tearing and minor oxide islands, indicating a much milder wear behavior.
The superior toughness enhancement enabled by this ductile phase approach is substantiated by a growing body of experimental work, as summarized in Table 2. This table compiles key studies on Ag-, Cu-, Au-, and Al-doped DLC films, highlighting how the preservation of the metallic nature of these dopants directly contributes to improved crack propagation resistance through the mechanisms outlined in Section 3 (e.g., crack bridging, plastic deformation).
The data in Table 2 consistently demonstrate that a well-dispersed metallic phase within the DLC matrix can dramatically improve fracture toughness, albeit often with some sacrifice in hardness. This establishes a clear contrast with the data for carbide-forming metals in Table 1 and sets the stage for the detailed discussion of the underlying toughening mechanisms in the following section.
Table 2. Summary of representative studies on non-carbide-forming metal-doped DLC films for toughness enhancement.
Table 2. Summary of representative studies on non-carbide-forming metal-doped DLC films for toughness enhancement.
Metal Content
(at.%)		DepositionThickness
(μm)		Hardness
(GPa)		Wear Rate
(×10−7 mm3/N·m)		Toughness Evaluation MethodToughness/
Mechanism
Ag/15 [20]Ion Beam Assisted Deposition1.0150.2Impact Test &
Scratch Test		Optimal Ag content resulted in significant toughness improvement via crack bridging
and plastic
deformation of Ag particles.
Cu/10.5 [33]Magnetron Sputtering1.2180.8Scratch Test (CPRS)Maximum crack propagation resistance achieved; Cu
particles act as
bridges and blunt crack tips
Au/6
[34]		Pulsed Laser Deposition0.8121.0NanoindentationDuctile Au nanoparticles improved toughness and reduced friction,
but caused
significant hardness loss.
Al/5
[35]		Plasma-Enhanced CVD1.5202.5Scratch TestModerate toughness improvement; complex behavior
due to potential for very fine
AlxCy formation influencing interface strength.
Note: Al is included here based on its predominant toughening behavior in several DLC studies, though its capacity to form carbides is acknowledged. The wear rates are highly dependent on test conditions.

2.2. Influence on Structure, Hardness, Stress, and Tribological Properties

The incorporation of non-carbide metals induces profound changes in the film’s characteristics: (i) Structure. The metallic nanoparticles disrupt the continuous amorphous carbon network, refining the “atomic clusters”. This creates a high density of interfaces between the metal particles and the carbon matrix, which are critical for deflecting cracks and dissipating energy [36]. (ii) Hardness. There is typically a trade-off. The soft metal particles can cause a decrease in the overall film hardness. The extent of this reduction depends on the metal type, size, and volume fraction. An optimal doping content exists that maximizes toughness with an acceptable loss in hardness [37]. (iii) Residual Stress. A significant benefit of metal doping is the reduction in high intrinsic compressive stresses commonly found in hard DLC films. The incorporation of soft metal particles can relieve these stresses through mechanisms like grain boundary relaxation and altered deposition kinetics, thereby improving adhesion and reducing the driving force for delamination [38]. (iv) Tribological Performance. The ductile metal particles can act as solid lubricants, especially metals like Ag and Au that have low shear strength. This leads to a lower friction coefficient. Furthermore, the enhanced toughness directly translates to superior wear resistance, as the coating is better able to withstand abrasive and fatigue wear without cracking or spalling [39,40].

3. Toughening Mechanisms of Non-Carbide Metal Doping

Having established the microstructural basis for toughness enhancement (Section 2), this section delineates the fundamental toughening mechanisms induced by non-carbide metal doping in DLC films.
  • Mechanism I: Bombardment-Induced Compressive Stress Relief (Figure 4a)
During deposition, energetic particles (ions, atoms) bombard the growing film. This “atomic peening” effect is a primary source of high compressive stress in DLC films [41]. The incorporation of ductile metal nanoparticles can absorb and dissipate this impact energy through plastic deformation and interface sliding, thereby mitigating the buildup of detrimental internal stresses. A lower residual stress state directly reduces the driving force for crack initiation and delamination [42,43]. However, the efficacy of this mechanism is highly dependent on the deposition technique and energy. For instance, in high-energy processes like magnetron sputtering, the stress-relieving effect of metals like Cu is more pronounced compared to lower-energy CVD methods [44]. Conversely, an excessive reduction in compressive stress can sometimes diminish the beneficial ‘load-bearing’ capacity of the coating, indicating a need for precise balance.
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Figure 4. Toughening mechanisms of DLC film doped with non-carbide metal.
Figure 4. Toughening mechanisms of DLC film doped with non-carbide metal.
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  • Mechanism II: Atomic Cluster Refinement and Boundary Multiplication (Figure 4b)
The amorphous carbon matrix can be thought of as consisting of nanoscale sp2/sp3 “atomic clusters”. Undoped DLC has larger clusters with fewer boundaries. Dopant metal particles disrupt the carbon network, acting as nucleation sites for new, smaller carbon clusters. This refinement has two consequences:
(i) Reduced Stress Concentration. Smaller cluster size minimizes stress concentration at potential flaw sites, making crack initiation more difficult [45]. (ii) Enhanced Boundary Sliding: The drastically increased density of cluster boundaries and metal-carbon interfaces provides more paths for energy dissipation through micro-slip and plastic flow at these boundaries, inhibiting crack propagation [46,47]. Recent molecular dynamics simulations have provided atomic-scale evidence for this, showing that Ag nanoparticles effectively pin shear transformation zones in the amorphous carbon, forcing more distributed plastic flow [48]. It is worth noting that a critical debate surrounds the optimal interface strength: a weak interface promotes energy dissipation via sliding but may facilitate particle debonding and void formation, while a strong interface ensures load transfer but may lead to a more brittle fracture.
  • Mechanism III: Carbon Atom Rearrangement and Dislocation Accumulation Inhibition (Figure 4c)
The presence of metal particles alters the local atomic environment. Carbon atoms tend to distribute around the metal particles, which can shield the strong repulsive interactions between carbon atoms. Under load, this allows for a more gradual and distributed rearrangement of atoms, slowing down the accumulation of dislocations at defect sites. This delayed plastic instability helps to postpone the onset of crack nucleation [49]. While this mechanism is supported by computational studies, its direct experimental observation remains challenging. Its contribution is often inferred indirectly from the improved nanoindentation creep resistance and more homogeneous deformation morphology observed in metal-doped DLC films, as opposed to the localized catastrophic failure in pure DLC.
The toughening effect is not a result of a single mechanism but a synergy of several that operate from deposition to mechanical failure. Figure 4 provides a visual guide to these mechanisms (I–V) and influential factors (a–g). The relative contribution of each mechanism varies with the mechanical stimulus (e.g., nanoimpact vs. scratch testing) and the metal species.
  • Mechanism IV: Plastic Deformation and Crack Bridging (Figure 4d–f)
This is the most direct and dominant group of mechanisms during mechanical loading: (i) Plastic Deformation and Energy Absorption (Figure 4d). Ductile metal particles undergo plastic deformation (e.g., changing from circular to oval shape) under localized stress. This process absorbs a significant amount of energy that would otherwise be used to propagate a crack, effectively blunting the stress field [50]. (ii) Crack Tip Blunting and Strain Field Relaxation (Figure 4e). When a propagating crack tip encounters a metal particle, the particle yields and deforms. This blunts the sharp crack tip and relaxes the intense strain field at the tip, dramatically reducing the local stress intensity and arresting further propagation [51].Crack Bridging (Figure 4f). Metal particles that span the crack wake can exert closing forces behind the crack tip, effectively “bridging” the crack. This shielding effect reduces the effective stress intensity factor at the tip, making further advance more difficult [52,53]. In situ SEM scratch tests on Ag-DLC films have vividly captured these events, showing elongated Ag particles in the crack wake and deflected crack paths [54]. It is important to note that this mechanism is most effective when the metal particle size is comparable to or larger than the crack opening displacement. Excessively small nanoparticles may be bypassed by the crack without significant deformation.
  • Mechanism V: Shear-Induced Stress Relief (Figure 4g).
Under high shear stresses, soft metal particles with low shear strength (e.g., Ag) can be sheared off. This process provides a localized stress-relief mechanism, effectively creating a micro-shear zone that prevents the stress from being transferred to the brittle carbon matrix and causing large-scale fracture [55]. This mechanism is particularly crucial for achieving super-low friction. However, a potential downside is the smearing and eventual depletion of the soft metal, which can lead to the long-term degradation of the coating’s tribological performance in certain environments.

4. Influential Factors and Optimal Doping Content

The efficacy of the toughening mechanisms described in Section 3 is governed by several key factors, which also determine the optimal doping content.

4.1. Seven Influential Factors (a-g)

The efficacy of the toughening mechanisms depends on several key factors, denoted in red in Figure 4: a. Bombardment Energy. Determines the initial residual stress state and interface bonding. b. Metal-Carbon Interface Nature. The strength of the interface controls stress transfer and particle debonding. c. Repulsive Interaction between Carbon Atoms. Influences the ease of atomic rearrangement under load. d. Friction at Interfaces. Affects energy dissipation through interface sliding. e. Strain Field at Crack Tip. The intensity dictates the required energy for particle deformation. f. Ligament (Particle) Strength. Determines the effectiveness of crack bridging. g. Shear Strength of Metal Particle. Dictates the ease of the shear-induced stress relief mechanism.

4.2. Determination of Optimal Doping Content and Negative Effects of Over-Doping

The relationship between metal content and toughness is non-monotonic. An optimal doping window exists (e.g., ~10–15 at.% for Ag, Cu).
Optimal Content. Within this window, the metal particles are well-dispersed, providing a high density of toughening sites without significantly compromising the hardness and continuity of the carbon matrix. The mechanisms of cluster refinement (II), crack bridging (IV), and stress relief (I, V) operate synergistically at peak efficiency.
Negative Effects of Over-doping. Excessive metal content leads to: (i) Overlap of Metal Particles. Creates easy propagation paths for cracks through the interconnected soft phase (leap of boundary, as mentioned in Mechanism IV). (ii) Significant Hardness Loss. The film becomes too soft, losing its wear resistance. (iii) Formation of Large Agglomerates. Large particles act as critical flaws, promoting crack initiation. (iv) Weakened Matrix. The carbon network becomes too diluted to effectively transfer load.
Thus, achieving superior toughness requires a precise balance, maximizing the positive mechanisms while avoiding the detrimental effects of excessive softening and microstructural coarsening.

5. Case Studies: Experimental Validation of Toughening Mechanisms

The proposed toughening framework, comprising the mechanisms (Section 3) and influential factors (Section 4), is strongly supported by experimental evidence across the literature (as summarized in Table 1 and Table 2). The following case studies are presented as paradigmatic examples to validate this framework and vividly illustrate how these mechanisms operate synergistically in specific metal-doped DLC systems.

5.1. Ag-DLC: The Paradigm of Ductile Phase Toughening

The study by Yu et al. [56] on Ag-DLC films serves as a quintessential example of Mechanisms II, IV, and V. The dramatic improvement in wear resistance achieved by Ag incorporation, as shown previously in Figure 3, provides macroscopic evidence for its efficacy.
Experimental Findings. A gradient of Ag content (0–31.8 at.%) was synthesized. Films with intermediate Ag content (~15 at.%) exhibited the best performance in scratch testing (highest critical loads, smoothest scratch tracks) and impact testing (minimal cracking and delamination). In contrast, films with low or excessive Ag content showed brittle failure.
Mechanism Validation. Cluster Refinement (II). Ag nanoparticles disrupted the carbon network, creating a refined nanocomposite structure. Crack Bridging and Plastic Deformation (IV): SEM analysis of scratch tracks and impact cavities showed clear evidence of plastic deformation and microvoid formation around Ag particles, indicating energy absorption. Crack paths were visibly deflected and bridged by Ag particles. Shear-Induced Relief (V): The low shear strength of Ag facilitated interfacial sliding, contributing to a lower friction coefficient and stress relief at crack tips.
This study clearly demonstrates the non-monotonic relationship between doping content and toughness, showcasing the optimal window where ductile phase toughening is most effective.

5.2. Cu-DLC: Balancing Toughness and Hardness

Research on Cu-DLC, such as that by Zhang et al. [57], provides further evidence for Mechanism IV and the critical role of the optimal doping content. This highlights a key advantage over carbide-forming metals; for instance, as shown in Figure 2, a macro amount of Ti leads to the formation of large TiC grains that destroy the continuous carbon network, resulting in a drastic reduction in both hardness and toughness.
(i) Experimental Findings. The fracture toughness, quantified by the CPRS parameter, increased with Cu content up to an optimum (~10.5 at.%), after which it decreased. The film with the highest CPRS showed minimal cracking in scratch tests.
(ii) Mechanism Validation. Crack Bridging and Blunting (IV): The quantitative CPRS metric directly correlates to the work done to propagate a crack, which is increased by the energy-dissipating processes of plastic deformation and crack bridging by Cu particles.
(iii) Optimal Content. The decline in CPRS beyond 10.5 at.% validates the negative impact of over-doping, where the formation of larger Cu agglomerates and the excessive softening of the matrix begin to dominate, creating easy paths for crack propagation.
Cu-DLC studies highlight the effectiveness of non-carbide metals in improving crack propagation resistance and provide a quantitative framework (CPRS) for assessing it.

5.3. Ti-DLC: A Contrasting Case of Carbide-Forming Metal

The work by Zhou et al. [25] on Ti-DLC offers a valuable contrast, illustrating the different toughening pathway of carbide-forming metals and its limitations.
(i) Experimental Findings. A micro amount of Ti (1.82 wt%) formed TiC nanocrystallites, which increased hardness and moderately improved toughness by refining the matrix. However, a macro amount of Ti led to the formation of large TiC grains that destroyed the continuous carbon network, resulting in a drastic reduction in both hardness and toughness.
(ii) Mechanism Validation. This case contrasts with Mechanism II. While TiC nanocrystallites also refine the microstructure, the resulting hard, brittle phase does not offer the ductile energy-absorption mechanisms (IV, V) that non-carbide metals do. The toughening is limited and easily negated by over-doping.
Consequently, Ti-DLC exemplifies the inherent trade-off with carbide-forming metals: toughness improvement is limited and highly sensitive to doping content, as it relies on microstructural refinement without the benefit of ductile phase deformation.
These case studies collectively underscore the superiority of non-carbide metals for effective toughening and provide concrete experimental proof for the multi-mechanism framework proposed in this review. The significance of this framework, however, extends beyond merely validating known phenomena. While the individual mechanisms have been reported in scattered studies, this review’s contribution lies in their integration into a cohesive lifecycle model. This unified framework provides a universal lens to interpret, compare, and predict the toughening behavior of diverse Me-DLC systems. It elucidates, for example, why a carbide-former like Ti (Section 5.3) fails to achieve the same level of toughness as Cu or Ag, by highlighting its inability to activate the crucial ductile phase toughening mechanisms (IV and V). Thus, the framework serves not only as a summary of past research but, more importantly, as a foundational and predictive tool for the rational design of next-generation, high-toughness DLC coatings.

6. Conclusions and Outlook

6.1. Summary of Mechanisms

This review has established a comprehensive framework for understanding the toughening of DLC films via non-carbide metal doping. The enhancement in fracture toughness is not attributable to a single mechanism but is the result of a synergistic interplay of five primary mechanisms operating across different stages: Deposition-Induced Stress Relief (I): Mitigates the internal driving force for delamination. Microstructural Refinement (II): Creates a nanostructured composite with a high density of interfaces to hinder crack initiation. Inhibition of Dislocation Accumulation (III): Promotes a more homogeneous plastic response at the nanoscale. Ductile Phase Toughening (IV): Provides active, large-scale energy dissipation through crack bridging, tip blunting, and particle deformation. Shear Yield-Induced Stress Relief (V): Offers a localized mechanism to relax stress concentrations under shear.
The optimal toughening effect is achieved within a specific window of metal content, balancing the positive effects of these mechanisms against the negative consequences of matrix weakening and particle agglomeration.

6.2. Future Research Directions

Despite the significant progress, several challenges remain and future research should focus on the following avenues to further advance the field:
(1) Multi-Dopant and Composite Design: The exploration of co-doping with two or more metal elements (e.g., Ag + Cu, Ag + Al) is largely untapped. This strategy could unlock synergistic effects, where one metal optimizes the microstructure (Mechanism II), while another enhances plastic deformation (Mechanism IV). Furthermore, incorporating secondary nano-reinforcements like hexagonal BN or MoS2 could create hybrid composites that further tailor tribological and mechanical properties.
(2) Architected Multilayer and Gradient Structures: Combining metal doping with sophisticated layered architectures is a highly promising path. For instance, a metal-doped layer could be engineered to reside at a critical depth below the surface (e.g., where maximum tensile stress occurs) to act as a dedicated “crack-arresting” zone, while the top layer remains hard and wear-resistant. Functionally graded designs could seamlessly transition from a tough, ductile interface to a hard, low-friction surface, mitigating interfacial stress concentrations.
(3) Advanced In Situ and Operando Characterization: Utilizing techniques such as in situ TEM nanomechanical testing and synchrotron X-ray diffraction during mechanical loading is crucial. These methods can directly observe the dynamic sequence of activation of the toughening mechanisms (e.g., visualizing crack bridging and particle deformation in real-time), moving from post-mortem analysis to direct experimental proof and enabling the validation of theoretical models.
(4) AI-Guided Design and Multi-Scale Modeling: Implementing machine learning to analyze the vast parameter space (dopant type, content, deposition energy, etc.) can accelerate the discovery of optimal compositions. Coupling this with multi-scale modeling—from atomic-scale molecular dynamics simulations of metal-carbon interfaces to continuum-level predictions of fracture toughness—will create a powerful predictive design tool, reducing reliance on trial-and-error experimental approaches.
(5) Environment-Specific Performance Validation: Future studies must extend beyond standard laboratory conditions to evaluate toughness and wear performance under environments mimicking real applications. This includes testing at elevated temperatures, in humid atmospheres, and under corrosive conditions to ensure the proposed toughening mechanisms remain effective and the coatings possess long-term durability.
In conclusion, the strategy of doping with non-carbide metals provides a powerful and versatile toolkit for overcoming the brittleness of DLC films. By understanding the underlying mechanisms and strategically designing microstructures, the development of next-generation coatings that are truly both hard and tough is within reach, promising to significantly extend the lifespan and reliability of critical mechanical components.

Author Contributions

J.L. (Jiahao Liu): Writing—Original draft and Writing—review & editing; Z.Y.: Investigation and Writing—review & editing; G.C.: Resources; J.L. (Jiayin Lv): Writing—review & editing; X.Y.: Writing—Supervision and Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Lubricants 13 00496 g001
Figure 1. CrN/DLC/Cr-DLC multilayer coating with improved adhesive wear resistance. (a) Schematic diagrams of wear mechanism of the coatings; (b) 3D image and cross sectional profile of wear tracks: sliding; (c) Friction coefficient of the coatings with different friction pair; (d) Wear rate of the deposited coatings [21].
Figure 1. CrN/DLC/Cr-DLC multilayer coating with improved adhesive wear resistance. (a) Schematic diagrams of wear mechanism of the coatings; (b) 3D image and cross sectional profile of wear tracks: sliding; (c) Friction coefficient of the coatings with different friction pair; (d) Wear rate of the deposited coatings [21].
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Lubricants 13 00496 g002
Figure 2. Mechanical and tribological behaviors of Ti-DLC films (A) The schematics diagram of frictional mechanism for the films; (B) Wear rates and wear morphology results of DLC and Ti-DLC films with different doped Ti contents [25].
Figure 2. Mechanical and tribological behaviors of Ti-DLC films (A) The schematics diagram of frictional mechanism for the films; (B) Wear rates and wear morphology results of DLC and Ti-DLC films with different doped Ti contents [25].
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Lubricants 13 00496 g003
Figure 3. Comprehensive structural and tribological evaluation of silver-doped DLC thin film coating with chromium interlayer (Ag-DLC/Cr) (a) Wear scar profile on surgical SS surface and Ag-DLC/Cr-coated surface after tribotest and illustration of friction coefficient and wear rate obtained in tribotests; (b1,b2) Plastic deformation in surgical SS and worn surface at 5 N load and worn surface at 10 N load; (c1–c3) SEM micrographs of wear track on Ag-DLC/Cr coating [32].
Figure 3. Comprehensive structural and tribological evaluation of silver-doped DLC thin film coating with chromium interlayer (Ag-DLC/Cr) (a) Wear scar profile on surgical SS surface and Ag-DLC/Cr-coated surface after tribotest and illustration of friction coefficient and wear rate obtained in tribotests; (b1,b2) Plastic deformation in surgical SS and worn surface at 5 N load and worn surface at 10 N load; (c1–c3) SEM micrographs of wear track on Ag-DLC/Cr coating [32].
Lubricants 13 00496 g003
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Liu, J.; Yang, Z.; Cui, G.; Lv, J.; Yu, X. Toughening Mechanisms of Diamond-like Carbon Films via Non-Carbide Metal Doping. Lubricants 2025, 13, 496. https://doi.org/10.3390/lubricants13110496

AMA Style

Liu J, Yang Z, Cui G, Lv J, Yu X. Toughening Mechanisms of Diamond-like Carbon Films via Non-Carbide Metal Doping. Lubricants. 2025; 13(11):496. https://doi.org/10.3390/lubricants13110496

Chicago/Turabian Style

Liu, Jiahao, Zhifang Yang, Guangying Cui, Jiayin Lv, and Xiang Yu. 2025. "Toughening Mechanisms of Diamond-like Carbon Films via Non-Carbide Metal Doping" Lubricants 13, no. 11: 496. https://doi.org/10.3390/lubricants13110496

APA Style

Liu, J., Yang, Z., Cui, G., Lv, J., & Yu, X. (2025). Toughening Mechanisms of Diamond-like Carbon Films via Non-Carbide Metal Doping. Lubricants, 13(11), 496. https://doi.org/10.3390/lubricants13110496

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