Application Note | DLC Scratch Testing
Cohesive and Adhesive Strength of DLC on Different Steel Substrates
How Substrate Hardness Changes the Critical Loads of Diamond-Like Carbon Coatings
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
The combination of excellent wear resistance and very low friction makes diamond-like carbon (DLC) an ideal protective coating for a variety of industrial applications, such as razor blades, metal cutting tools, bearings, motorcycle engines, and medical devices. For example, high quality tooling components, such as drill bits, dies and molds are often coated with DLC to reduce abrasive wear. The low friction and great wear resistance can significantly increase the work efficiency and extend the lifespan of the tool. Its excellent tribological properties and bio-compatibility make it an ideal material for medical application.
Importance of Macro Scratch Testing on DLC Coatings
DLC coated tools are often used under very high load and concentrated stresses, e.g. drill bits and bearings. Under such extreme conditions, sufficient cohesive and adhesive strength of the coating/substrate system becomes vital. In order to select the best metal substrate for the target application and to establish a consistent coating process for DLC, it is critical to develop a reliable technique to quantitatively assess cohesive and adhesive strength of different DLC coating systems.
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Measurement Objective
In this study, we showcase that the macro scratch mode of the NANOVEA Mechanical Tester provides an ideal tool to evaluate the cohesive/adhesive strength of the protective DLC coatings under a high pressure in a controlled and quantitative manner. Instrumented scratch testing is carried out to assess the failure modes of two DLC coatings on different steel substrates.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Parameters
The NANOVEA Mechanical Tester was used to perform the scratch tests on two DLC coatings on different steel substrates using the same test parameters as summarized in Table 1.
| Test parameter | Value |
|---|---|
| Loading mode | Progressive linear |
| Initial load | 0.1 N |
| Final load | 100 N |
| Loading rate | 100 N/min |
| Scratch speed | 3 mm/min |
| Scratch length | 3 mm |
| Indenter type | Sphero-conical |
| Indenter radius | 200 µm |
Table 1: Test parameters of the scratch tests on the DLC samples.
Results and Discussion
The following graph shows the recorded evolution of depth and coefficient of friction (COF) as a function of applied normal load during the test. It can be observed that DLC1, which has a steel substrate of higher hardness, exhibits a penetration depth of ~14 µm, compared to ~36 µm for DLC2. Meanwhile, the COF progressively increases as the stylus penetrates into the sample. The DLC1 and DLC2 exhibit a low COF of ~0.1 at the beginning of the scratch test, due to the superior self-lubrication property of DLC. The steel substrate starts to be exposed as the DLC coating fails under the high pressure applied by the stylus, leading to gradually increased COF as the scratch test continues.
The full scratch tracks on the DLC samples are shown in Fig. 3. The failure behaviors at different critical loads are displayed in Fig. 4, where critical load Lc1 is defined as the load at which the first sign of cohesive crack occurs in the scratch track, Lc2 is the load after which adhesive failures take place and the steel substrate starts to be exposed, and Lc3 is the load at which the coating is completely removed from the substrate. It can be observed that DLC1 shows a smaller scratch track compared to DLC2. DLC1 exhibits lower critical loads at different stages of the scratch tests compared to DLC2. It demonstrates the importance of proper substrate support on the cohesive and adhesive strength of the coating/substrate system. A substrate with a higher hardness is critical to resist deformation under a high external load and concentrated stress.
Fig. 4: DLC failures under different critical loads, Lc.
| Critical load | DLC1 | DLC2 |
|---|---|---|
| Lc1 (first cohesive crack) | 24.2 N, D = 4.5 µm | 21.7 N, D = 8.5 µm |
| Lc2 (substrate exposed) | 32.3 N, D = 5.5 µm | 29.3 N, D = 12.0 µm |
| Lc3 (coating removed) | 50.5 N, D = 7.7 µm | 40.6 N, D = 15.6 µm |
Table 2: Critical loads (N) and corresponding depths (D) of the DLC coatings (from Fig. 4).
Conclusion
In this study, we showcased that the NANOVEA Mechanical Tester performs reliable and accurate macro scratch tests on the DLC coated steel sample in a controlled and closely monitored manner. The scratch measurement allows users to identify the critical load at which typical cohesive and adhesive coating failure occurs. It provides a superior tool to quantitatively evaluate and compare the intrinsic quality of the coating and the interfacial integrity of the coating/substrate system for the DLC coated metal materials for high load applications. A substrate with a higher hardness can resist large deformation under a high external load and concentrated stress, which is vital for the cohesive and adhesive strength of the coating/substrate system.
The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. In addition, an optional 3D non-contact profiler and AFM module are available for high resolution 3D imaging of indentation, scratch and wear tracks.
Frequently Asked Questions About DLC Scratch Testing
Why scratch-test DLC coatings at high load?
DLC-coated drill bits, bearings and tools work under very high loads and concentrated stresses, so the cohesive and adhesive strength of the coating/substrate system must be quantified to choose substrates and qualify the coating process.
What do Lc1, Lc2 and Lc3 mean?
Lc1 is the load at the first cohesive crack in the scratch track, Lc2 the load after which adhesive failure exposes the steel substrate, and Lc3 the load at which the coating is completely removed.
What critical loads were measured?
DLC1 failed at Lc1 24.2 N, Lc2 32.3 N and Lc3 50.5 N; DLC2 at 21.7, 29.3 and 40.6 N.
Why did the coatings behave differently?
DLC1 sits on a harder steel substrate, which penetrated only about 14 µm against about 36 µm for DLC2. A harder substrate resists deformation under high load and concentrated stress, supporting the coating.
How does friction change during the scratch?
Both coatings start at a low COF of about 0.1 thanks to DLC’s self-lubrication; the COF rises gradually once the coating fails and the steel substrate is exposed.
Which scratch parameters were used?
A progressive linear load from 0.1 to 100 N at 100 N/min over 3 mm at 3 mm/min, with a 200 µm sphero-conical indenter.
