Toughness Characterization Methods for Diamond-like Carbon Films

Abstract

Diamond-like carbon (DLC) films exhibit superior tribological properties; however, their widespread adoption in precision manufacturing is hampered by inherent brittleness and a lack of reliable toughness characterization methods at the micrometer scale. This review critically examines existing techniques for evaluating DLC film toughness, highlighting limitations due to film thickness constraints and subjective failure definitions. We focus on two prominent micro-scale methods: impact testing and scratch testing. Impact toughness is assessed through energy absorption analysis based on impact crater morphology, including crack patterns and delamination areas. Scratch toughness is evaluated using critical loads (Lc₁, Lc₂) and the derived Crack Propagation Resistance (CPRS) parameter, complemented by microscopic failure analysis. We argue that neither method alone suffices for comprehensive toughness assessment. Instead, we propose a synergistic strategy integrating both techniques to provide a practical and comprehensive evaluation encompassing energy- and stress-based failure mechanisms under varying loading conditions. This approach offers a practical framework for developing tougher DLC coatings.

1. Introduction

Diamond-like carbon (DLC) films have garnered extensive attention in surface engineering over recent decades due to their good combination of properties, including high hardness, low friction coefficient, good wear resistance, and good chemical inertness [1,2]. These attributes make them advisable protective coatings for various tribological applications, particularly in precision manufacturing, such as precision bearings, cutting tools, and automotive components [3,4]. However, the pursuit of super-hardness in DLC films has often come at the expense of toughness, a property critical to their practical viability [5].

The inherent brittleness of DLC films represents a critical bottleneck, severely limiting their service life and reliability under mechanical loading [6]. In practical applications, coatings are frequently subjected to impact, fatigue, and concentrated stresses. Insufficient toughness can lead to the initiation and propagation of micro-cracks, ultimately resulting in catastrophic failure modes such as coating delamination, spalling, and premature wear [7]. The consequences of such failures extend beyond operational disruptions to significant economic losses. For example, in critical applications such as deep-well drilling tools, the failure of a coated component can necessitate a complete halt of operations, resulting in financial losses amounting to hundreds of thousands of dollars per day [8]. Therefore, improving the toughness of DLC films is equally as crucial as improving their hardness to ensure long-term durability and operational safety.

Despite its paramount importance, research on the toughness of DLC films has progressed slowly compared to other properties. A primary reason for this stagnation is the formidable challenge associated with its accurate measurement. The thickness of DLC films typically ranges from a few hundred nanometers to several micrometers, placing them in a dimensional regime where conventional standardized toughness evaluation methods for bulk materials (e.g., Plane Strain Fracture Toughness) become inapplicable [9,10]. The substrate’s influence, the difficulty in generating and measuring well-defined cracks, and the ambiguity in defining failure criteria all contribute to this complexity [11].

Several indirect methods have been adapted for thin film toughness assessment, including bending tests, buckling tests, nanoindentation, and tensile testing [12]. Each method possesses inherent limitations. For example, nanoindentation-based methods often rely on measuring crack lengths from radial fractures, which can be subjective and challenging to reproduce accurately [13]. Furthermore, no single method has gained universal acceptance, leading to a lack of comparable and reliable data across different studies.

Among the available techniques, impact testing and scratch testing have emerged as two of the most practical and widely used methods for evaluating the toughness of DLC films. Impact testing simulates repetitive contact fatigue, assessing the film’s ability to absorb energy and resist crack initiation and propagation under dynamic loading [14]. Scratch testing, on the other hand, evaluates the film’s response to a progressively increasing lateral stress, providing insights into adhesion strength and crack propagation resistance [15]. While valuable, both methods have drawbacks; impact test results can be difficult to quantify reliably, and scratch toughness is often considered an indicative rather than an absolute measure [16].

While previous reviews have touched upon thin film toughness characterization in general, e.g., [2,17], a systematic review that critically focuses on DLC films, with an in-depth evaluation of practical micro-mechanical methods like impact and scratch testing, and proposes a synergistic combined strategy, is still lacking. This review aims to fill this gap by providing a comprehensive and critical overview of current methodologies for characterizing the toughness of DLC films. It examines the principles, applications, advantages, and limitations of the predominant techniques, with a focused discussion on impact and scratch testing. Furthermore, we advocate for a combined characterization strategy that integrates these two methods to overcome their individual shortcomings. By providing a more holistic and robust framework for toughness evaluation, this review seeks to facilitate the development of tougher, more reliable DLC coatings and accelerate their adoption in demanding industrial applications.

2. Conventional Toughness Testing Methods and Their Limitations for Thin Films

A summary of the principles, advantages, and—most critically—the limitations of these conventional methods, particularly in the context of DLC films, is provided in

Table 1. Summary of conventional toughness testing methods for thin films.

Method	Parameters	Main Advantages	Limitations for DLC Films	Refs.
Bending test	Critical strain (G c)	Mature model e.g., Suo-Hutchinson	Difficult pre-crack creation. Sensitive to substrate and residual stress.	[12,19]
Nanoindentation	Crack length (c), load (P)	Simple sample preparation	Inapplicable to tough DLCs (no cracks). Subjective crack measurement.	[20,21,22]
Tensile test	Crack density vs. strain	Well-defined stress state	Complex setup Challenging to isolate film fracture	[23,24]
Buckling test	Interfacial energy (γ)	Measures adhesion strength	Measures interface energy, not bulk film toughness	[25]

. Evaluating the fracture toughness of thin films poses a significant challenge because most standardized methods are designed for bulk materials. The substrate’s influence, the difficulty in initiating and measuring controlled cracks, and the minute scale of the forces and displacements involved render conventional techniques problematic [17,18]. Below is a critical overview of adapted methods and their inherent limitations for DLC films.

2.1. Bending Tests

Bending tests of film-substrate systems can estimate fracture toughness by propagating a pre-crack (see Table 1). However, this method is particularly problematic for DLC films. The experimental difficulty of introducing a well-defined pre-crack into a thin, hard DLC coating is a major obstacle [19]. Furthermore, the analysis is highly sensitive to substrate properties and the significant residual stresses inherent in many DLC films, often making it impossible to decouple the film’s true toughness from these extrinsic factors [12]. Consequently, despite its theoretical foundation, the bending test is impractical for providing a reliable or absolute measure of fracture toughness in DLC coatings, serving at best as a comparative tool.

2.2. Nanoindentation

Nanoindentation is a prevalent method for fracture toughness estimation due to its simplicity (see Table 1). However, its application to DLC films is fundamentally limited. The method relies on measuring crack lengths from indentations, creating a severe paradox: it fails precisely for tough, ductile DLC variants (e.g., metal-doped) which are designed to resist cracking and may only form a plastic zone [6,20,21]. Even when cracks are present, the results are compromised by the subjective measurement of crack lengths and the material-dependent empirical constant, introducing significant uncertainty [22]. Consequently, nanoindentation provides, at best, a semi-quantitative and often misleading assessment of toughness for the very DLC coatings that aim for enhanced fracture resistance.

2.3. Tensile Tests

Tensile testing derives fracture toughness from the crack density evolution in a film on a ductile substrate under strain (see Table 1). However, its application to hard DLC films on typically softer substrates is fraught with fundamental challenges. It is exceptionally difficult to isolate the film’s fracture strain from the substrate’s yielding and to ensure perfect interfacial adhesion, which is critical for valid data [10]. Moreover, the analysis is ultimately limited by the crack saturation regime, the modeling of which requires an accurate interfacial shear strength—a parameter notoriously difficult to quantify for DLC-substrate systems due to their complex interface chemistry and residual stresses [23,24]. Consequently, the method struggles to provide more than a comparative ranking of toughness for DLC films.

2.4. Buckling Tests

Buckling tests quantify the interfacial adhesion energy from the geometry of stress-induced delaminations (see Table 1). While valuable for assessing interfacial integrity, this method suffers from a critical conceptual limitation for evaluating DLC film toughness: it exclusively measures the energy to propagate a crack at the interface (adhesion) [25], rather than through the bulk material (cohesive toughness). This distinction is paramount, as the resistance to in-plane cracking within the DLC coating itself is often the dominant factor governing mechanical durability in service. Therefore, buckling tests cannot characterize the intrinsic fracture resistance of the DLC film, rendering the data irrelevant for evaluating this key property [26].

Consequently, no single conventional method provides a straightforward, reliable, and universally accepted measure of the fracture toughness of DLC films. The limitations of these techniques have necessitated the development and adoption of more practical, albeit sometimes more qualitative, methods like impact and scratch testing. The following sections focus on these two micro-mechanical methods, which have emerged as the most prominent and practical approaches for evaluating the toughness of DLC coatings under simulated service conditions.

3. Impact Toughness Characterization

3.1. Principle of Impact Testing

Impact testing simulates the repetitive contact fatigue a coating might experience in service (e.g., in bearings or gears). A hard indenter (commonly a Si₃N₄ or WC ball) is cyclically impacted onto the coated surface with a controlled force and frequency. Typical testing parameters encompass a force range of 10–200 mN, a frequency of 1–50 Hz, and a number of cycles that can extend up to 10⁶, depending on the specific test setup and coating system [27]. As illustrated in Figure 1, the test subjects the film to high-strain-rate loading, culminating in fatigue, crack initiation, and ultimately, failure through stages of elastic deformation (I), ring crack formation (II), radial crack propagation (III), and final delamination (IV). This schematic provides a critical visual framework for understanding the progressive failure mechanisms that are central to the qualitative assessment of impact toughness, directly linking the observed crater morphology (Section 3.2) to the underlying energy-absorption and crack-evolution processes. The film’s impact toughness is qualitatively evaluated by its ability to absorb energy and resist these cracking and delamination processes over many cycles [27,28].

3.2. Morphology Analysis: Cracks and Spallation Areas

The primary evaluation method is post-mortem analysis of the impact crater using scanning electron microscopy (SEM) or optical profilometry [29]. Key features analyzed include: (i) Ring Cracks: Concentric cracks around the impact site indicate brittle failure and poor resistance to crack initiation. (ii) Radial Cracks: Cracks propagating outward from the center signify the progression of failure. (iii) Delamination/Spallation: The extent of the peeled-off area (often appearing bright in SEM-BSE mode due to substrate exposure) is the most direct indicator of poor toughness. A larger spallation area corresponds to lower energy absorption and poorer impact toughness. (iv) Cavity Profile: The sharpness of the crater edge and the presence of pile-up can also provide insights into the film’s ductility [30].

3.3. Case Study: Impact Morphology Comparison of Ag-DLC Films

Zak et al. [28] effectively demonstrated this method by testing DLC films with gradient Ag contents. As shown in their work (Figure 2):

(i)

Low/High Ag content (Figure 2a,b,f): Exhibited large white spallation zones around the crater, severe delamination, and extensive radial and circular cracking. This morphology indicates poor impact toughness, as cracks readily initiated and propagated through Stages I–III.

(ii)

Medium Ag content (Figure 2c–e): Showed significantly reduced spallation and cracking. The impact cavities had clearer profiles with minimal or no visible cracking, indicating that the films could absorb the impact energy through mechanisms other than brittle fracture, thus exhibiting good to excellent impact toughness.

This case study highlights the utility of impact testing as a comparative tool for ranking the fatigue and impact resistance of different films under development [31].

4. Scratch Toughness Characterization

4.1. Scratch Test and Critical Loads (Lc₁, Lc₂)

In contrast to the dynamic loading in impact tests, a diamond stylus (Rockwell C) is drawn across the coated surface under a progressively increasing normal load. Acoustic emission and friction force are monitored. The defined critical loads’ mark transitions in failure mode are as follows [32]:

(i)

First Critical Load (Lc₁): The load at which the first cohesive failures (e.g., conformal cracking, chipping) occur within the film.

(ii)

Second Critical Load (Lc₂): The load at which the first adhesive failures (complete exposure of the substrate) occur.

While L_C1 is often associated with coating cohesion and Lc₂ with adhesion, a higher Lc₁ also indicates a greater resistance to initial cracking, which is a measure of toughness.

4.2. Definition and Calculation of CPRS

Wang et al. [33] introduced the Crack Propagation Resistance (CPRS) parameter to provide a more quantitative measure of scratch toughness. CPRS is defined as the work done by the scratching stylus to propagate a crack through the film and is calculated by integrating the frictional force from Lc₁ to Lc₂:

CPRS = ∫[Lc₁ to Lc₂] μ(L) × dL

(1)

where μ is the coefficient of friction and L is the normal load. A higher CPRS value indicates that the film requires more energy to propagate a crack from initial damage to complete failure, signifying better fracture toughness [34]. The utility of the CPRS parameter lies in its ability to provide a quantitative framework that links the frictional work done by the stylus directly to the crack propagation process. This approach has been demonstrated to effectively differentiate and rank the scratch toughness of DLC films with varying compositions and microstructures, offering a more nuanced understanding than critical load values alone [34]. The typical ranges of these critical loads and the derived CPRS parameter for various DLC film types are summarized in

Table 2. Typical ranges of scratch and impact test parameters for DLC films.

DLC Type	Scratch Test	Typical Parameters	References
Standard a-C:H or a-C	Lc1: 10–25 N CPRS: 50–200 μJ	Cycles to Failure 0: 104–105	[35]
Metal-doped DLC (e.g., Cr-DLC)	Lc1: 20–40 N CPRS: 200–500 μJ	Cycles to Failure 0: 105–5 × 105	[33]
Metal-doped DLC (e.g., Ag-DLC) Toughness Optimized	Lc1: 10–30 N CPRS: 150–400 μJ	Cycles to Failure 0: >106	[28]
Multilayer/Graphitic DLC (Enhanced Toughness)	Lc1: 25–50 N CPRS: 300–800 μJ	Cycles to Failure 0: 5 × 105–>106	[36]

⁰ Impact testing conditions: load range ~20–50 mN, frequency ~10–50 Hz. The values below are representative examples compiled from the literature. Actual values are highly dependent on specific deposition conditions, substrate, film thickness, and exact test parameters.

, providing a benchmark for expected performance.

4.3. Case Study: Scratch Morphology and CPRS Analysis of Cu-DLC and Ag-DLC

In Figure 3 for Cu-DLC, Wang et al. [33] showed that for Cu-DLC films, Lc₁, (Lc₂-Lc₁), and CPRS values all increased with Cu content up to an optimum (e.g., 10.5 at.%), after which they decreased. The film with the highest CPRS value exhibited a smooth scratch track with minimal crack in Figure 3c and peeling, directly correlating the quantitative CPRS metric with superior scratch toughness observed morphologically. This direct correlation between the high quantitative CPRS metric and the superior morphological integrity observed in the scratch track provides strong visual validation for the use of CPRS as a reliable parameter for evaluating scratch toughness [37].

As shown in Figure 4, a comparative study of Ag-DLC films with different contents revealed morphological differences [28]. Low (5.3%) and high (31.8%) Ag content films failed in a brittle manner at low loads. An intermediate content film (15.2%) showed a smooth scratch track with no apparent cracks under optical microscopy, indicating the best scratch toughness. Critically, this method could distinguish between the 15.2% and 24.6% Ag films, which the impact test could not, demonstrating the complementary nature of the two techniques.

The scratch test, especially when combined with CPRS analysis, provides a more quantitative framework for assessing the toughness of DLC films under sliding contact stress [38].

5. Combined Characterization Strategy

5.1. Complementarity of Impact and Scratch Methods

The inherent limitations of using either impact or scratch testing in isolation underscore the necessity for a combined approach. These two methods are not redundant but rather highly complementary, as they probe the film’s mechanical response under fundamentally different loading conditions and stress states:

(i)

Impact Testing primarily assesses the film’s resistance to dynamic, repetitive normal loading and its ability to absorb energy over time. It is excellent for simulating and evaluating fatigue wear resistance and impact toughness. Its weakness lies in its qualitative nature and difficulty in distinguishing between films with similarly good (or poor) morphologies [39].

(ii)

Scratch Testing primarily assesses the film’s response to continuously increasing lateral shear stress and its resistance to crack initiation and propagation under a single pass. It provides quantitative parameters (Lc₁, Lc₂, CPRS) that are more readily comparable. Its weakness is that it is an indicative measure, and the stress field is complex, mixing compression, tension, and shear.

A film may perform well in one test but poorly in the other. For instance, a very hard film might have good scratch resistance (high Lc₁) but poor impact toughness due to its brittleness. Conversely, a softer, more ductile film might withstand impact well but exhibit low scratch resistance. Therefore, a comprehensive evaluation must consider both aspects [40].

5.2. A Holistic Toughness Evaluation Framework

We propose a synergistic strategy for evaluating the toughness of DLC films by integrating both methods. This synergistic strategy is illustrated in the flowchart presented in Figure 5, which outlines the step-by-step process from initial screening to final decision-making. a practical and comprehensive framework.

•

Step 1: Initial Quantitative Screening via Scratch Test

Utilize the scratch test as a first pass to obtain quantitative data (Lc₁, CPRS) and quickly rank coating compositions or architectures. The scratch morphology (e.g., conformal cracking, chipping) provides an initial qualitative assessment of cracking behavior [41]. Candidates exhibiting low Lc₁ and CPRS values can be deprioritized at this stage. Utilize the scratch test to obtain quantitative data (Lc₁, CPRS) and quickly identify promising coating compositions or architectures. The measured values should be contextualized against typical performance ranges for similar DLC types, as provided in Table 2.

•

Step 2: Qualitative Fatigue Assessment via Impact Test

Subject the top-performing candidates identified from the scratch test to impact testing. This step evaluates their long-term durability and resistance to failure under repetitive dynamic loading, which is critical for many applications [42]. Post-impact morphology analysis (e.g., spallation area, crack patterns) serves as the key qualitative metric.

•

Step 3: Correlative Analysis and Final Decision-Making

Correlate the quantitative scratch data with the qualitative impact performance to gain a multifaceted understanding of the film’s toughness profile and guide further development:

• Scenario A: High SCRATCH performance (High CPRS/Lc₁) & High IMPACT performance (Minimal Damage). The film possesses excellent overall toughness and is a prime candidate for demanding applications.
• Scenario B: High SCRATCH performance and Poor IMPACT performance. The film has good crack propagation resistance under sliding stress but poor energy absorption under impact. This indicates high brittleness, and strategies to improve ductility (e.g., doping, multilayer design) should be pursued.
• Scenario C: Poor SCRATCH performance and High IMPACT performance. The film is ductile and can absorb impact energy well but is susceptible to abrasive wear and scratch-induced damage. Enhancing hardness and cohesive strength may be required.
• Scenario D: Poor SCRATCH performance and Poor IMPACT performance. The film exhibits insufficient toughness. A fundamental reformulation of the coating composition or architecture is necessary [36].

This integrated strategy moves beyond a single-number assessment and provides a comprehensive understanding of a film’s toughness under different service conditions, effectively guiding the development of robust DLC coatings.

6. Conclusions and Outlook

This review has evaluated the primary methods for characterizing the toughness of DLC films, addressing both conventional approaches and advanced techniques. (i) Conventional Methods (Bending, Indentation, etc.): While providing a foundation, they are often hampered by substrate effects, complex analysis, and impracticality for routine assessment of thin, hard films. (ii) Impact Testing: Excellent simulator of contact fatigue; provides a direct qualitative visualization of energy absorption and failure progression. Results are primarily qualitative and comparative; difficult to quantify precisely; and can lack resolution for ranking films with similar performance. (iii) Scratch Testing: Provides quantitative parameters (Lc₁, Lc₂, CPRS); standard and relatively easy to perform; and good for assessing crack initiation resistance. The stress state is complex and not pure; “scratch toughness” is an engineering parameter, not a fundamental fracture property; and results can be influenced by friction and substrate hardness.

The insights gained from this analysis allow for the formulation of practical recommendations to guide the selection of characterization strategies for specific substrate-film systems:

(1)

For hard DLC films on hard substrates (e.g., tool steels, cemented carbides), where cohesive fracture within the film often dominates failure, the combination of scratch testing (providing quantitative CPRS and Lc₁) and impact testing (assessing fatigue crack resistance) is highly recommended. This duo effectively probes both the crack initiation/propagation resistance and the dynamic load-bearing capacity.

(2)

For hard DLC films on compliant substrates (e.g., aluminum, titanium alloys, or polymers), where interfacial adhesion and film flexibility are critical, the scratch test (for Lc₂ and adhesion failure mode) is indispensable. This should be complemented by impact testing, which is particularly sensitive to the delamination driven by the large elastic mismatch in such systems.

(3)

When the primary concern is specifically interfacial adhesion, scratch testing should be the primary method, with buckling tests offering a valuable supplementary perspective on the interfacial fracture energy under a different stress state.

In all cases, the synergistic framework of initial scratch screening followed by impact validation provides a robust and comprehensive assessment, moving beyond a single-number metric to a multi-faceted understanding of coating toughness.

Future advancements in toughness characterization for DLC films are likely to focus on the following: (i) In Situ and Operando Techniques: The development of in situ SEM or TEM scratch/impact setups will allow for the real-time observation of crack initiation and propagation, providing unprecedented insight into the fundamental failure mechanisms. (ii) High-Throughput and Automated Analysis: Machine learning algorithms for the automatic analysis of impact craters and scratch tracks (e.g., crack length counting, spallation area measurement) could remove human subjectivity and significantly increase throughput and reproducibility. (iii) Advanced Modeling and Simulation: Multi-scale modeling, from first-principles calculations of grain boundary strength to finite element analysis (FEA) of the entire impact/scratch process, can help bridge the gap between qualitative observations and quantitative predictions of toughness, ultimately guiding material design in silico. (iv) Standardization of Methods: The field would greatly benefit from community-wide efforts to standardize testing protocols and analysis criteria. Building upon existing standards for related techniques, such as ISO 20502 [43] for scratch testing and ASTM E2546 [44] for instrumented indentation, similar rigorous protocols need to be established and widely adopted for critical yet less-standardized methods like impact testing. This is essential to enable direct and reliable comparison of data across different laboratories. (v) Application-Specific Testing: Developing characterization methods that more closely mimic the actual service environment of the coating, such as combined impact-slide tests or tests in corrosive environments, will provide even more relevant toughness data.

In conclusion, while the challenge of measuring thin film toughness remains, the combined use of scratch and impact testing provides a powerful and practical framework for the comparative assessment and development of tougher, more durable DLC coatings. Embracing new technologies and working towards standardization will be key to unlocking further progress in this critical field.