Application Note | Coating Adhesion
DLC Coating Failure Using Micro Scratch Testing
Chipping and Delamination Critical Loads of Three Diamond-Like Carbon Coatings
Research & Experimental Testing
Andrea Herrmann
Visual Design & Editorial
Andrew Shore
Introduction
Diamond-like carbon (DLC) coatings are well known for their use in creating surfaces with diamond-like properties on nearly any material. The primary properties of interest are extreme hardness, wear resistance and a low friction coefficient. These favorable tribological properties of DLC coatings are in high demand in industries such as aerospace, automotive and tooling.
Importance of Micro Scratch Testing for Quality Control
A major concern for DLC coatings is ensuring strong coating adhesion and the ability to withstand marring and cracking. The same internal stress that enhances the hardness of DLC coatings conversely makes them difficult to bond to the substrate being coated. For this reason, DLC coatings require quality assurance when establishing a reliable coating process. Although adhesion or cracking failure may be inevitable over time, it is crucial that these failures are investigated, known and controlled. Using the micro scratch test, precisely controlled loads can be used to investigate DLC coatings for these very failures.
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Measurement Objective
We simulate the process of scratching in a controlled and monitored manner to observe the sample’s behavior. In this application, the NANOVEA Mechanical Tester, in its micro scratch mode, is used to measure the load required to cause cracking and adhesive failure of three separately processed DLC coatings. A 90° cone, 20 µm diamond-tipped stylus is used at a progressive load ranging from 0.01 mN to 15 N to scratch the DLC coating. The point where the coating fails by cracking is taken as the point of failure.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Parameters
| Parameter | Value |
|---|---|
| Load type | Progressive |
| Initial load | 0.01 mN |
| Final load | 15 N |
| Loading rate | 30 N/min |
| Scratch length | 3 mm |
| Indenter geometry | 90° cone |
| Indenter tip radius | 20 µm |
| Indenter material (tip) | Diamond |
Table 1: Scratch test parameters.
Results
The main numerical results are summarized in Table 2 and the individual scratches in Table 3. Chipping is the first evidence of damage to the coating; here the coating is chipped on the right side of the scratch. Complete delamination is the point where the coating is completely removed to the substrate.
| Sample | Chipping (N) | Complete delamination (N) |
|---|---|---|
| A | 3.52 ± 0.10 | 7.59 ± 0.13 |
| B | 4.60 ± 0.09 | 5.81 ± 0.26 |
| C | 3.51 ± 0.33 | 10.15 ± 0.05 |
Table 2: Summary of main numerical results (average ± standard deviation of three scratches).
| Sample | Chipping, scratches 1 to 3 (N) | Complete delamination, scratches 1 to 3 (N) |
|---|---|---|
| A | 3.41, 3.57, 3.59 | 7.63, 7.44, 7.70 |
| B | 4.70, 4.53, 4.57 | 6.11, 5.68, 5.65 |
| C | 3.14, 3.79, 3.61 | 10.18, 10.17, 10.10 |
Table 3: Critical loads of the individual scratches.
Sample A
Sample B
Sample C
Conclusion
Using the NANOVEA Mechanical Tester in micro scratch mode, the cohesive and adhesive failures of the three DLC samples were comparatively evaluated. The samples exhibit a large variation in adhesion strength compared to their cracking properties. Micro scratch testing has the ability to quantify adhesive and cohesive failures of DLC coatings with high repeatability. Many indenter tips can be used to simulate various levels of scratches.
Additionally, the NANOVEA Mechanical Tester could also have been used to measure hardness, elastic modulus, wear, friction coefficient, fracture toughness, roughness and many others. It is a complete and powerful tool for DLC coating research and control.
Frequently Asked Questions About DLC Scratch Testing
Why scratch test DLC coatings?
The internal stress that gives DLC its hardness also makes it hard to bond to the substrate, so adhesion and resistance to cracking must be verified when establishing a reliable coating process.
How were the coatings tested?
Progressive load micro scratch tests from 0.01 mN to 15 N at 30 N/min over 3 mm, with a 90° cone diamond stylus of 20 µm tip radius, three scratches per sample.
What failures were recorded?
Chipping, the first evidence of damage at the edge of the scratch, and complete delamination, where the coating is removed down to the substrate.
How did the three coatings compare?
Chipping occurred at similar loads (3.52, 4.60 and 3.51 N for A, B and C), but complete delamination varied widely: 7.59 N for A, 5.81 N for B and 10.15 N for C.
How repeatable is the method?
Standard deviations were 0.05 to 0.33 N over three scratches per sample.
What else can be measured on DLC?
Hardness, elastic modulus, wear, friction coefficient, fracture toughness and roughness, with the same mechanical tester and its modules.
