Application Note | Coating Scratch Resistance
Macro Scratch Testing of Anodized Coatings
Critical Loads Reveal How the Anodizing Process Sets Scratch Resistance
Research & Experimental Testing
Duanjie Li & Andrea Herrmann
Visual Design & Editorial
Andrew Shore
Introduction
Anodizing is an electrolytic passivation process commonly applied to convert aluminum to aluminum oxide. It can modify the texture of the surface and change the microstructure of the metal near the surface. The formation of an anodized aluminum oxide layer can enhance corrosion and wear resistance, improve cosmetic effects and provide better adhesion for paint primers. Anodized aluminum is widely used on electronic devices, such as mp3 players, flashlights, cameras and many other products.
Importance of Macro Scratch Testing for Quality Control
Anodized aluminum surfaces have a higher hardness than aluminum; however, their wear resistance is moderate due to their porous nature. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but quality control of such films is still of great significance. For example, when one of the most popular cell phones with a dyed aluminum frame was first introduced in 2012, the anodizing process did not create sufficient layers, leaving the aluminum frame vulnerable to chips, dents and scratches. The scratch test allows us to detect premature adhesive or cohesive failure of the anodized aluminum layer in real-life applications.
ℹ️ Learn more about scratch testing lab services.
Measurement Objective
The process of scratching is simulated in a controlled and monitored manner to observe adhesive or cohesive failures. In this application, the NANOVEA Mechanical Tester in its macro scratch testing mode (see Fig. 1) is used to measure the load required to cause failure of anodized aluminum films produced by different processes.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Parameters
The NANOVEA Mechanical Tester equipped with a Rockwell C diamond stylus (200 µm radius) was used to perform progressive load scratch tests on three anodized aluminum samples using the Macro Scratch Tester mode. Three tests were repeated under the same testing conditions on each sample to ensure reproducibility of the results. The test parameters are shown in Table 1.
| Parameter | Value |
|---|---|
| Load type | Progressive |
| Initial load | 0.03 N |
| Final load | 100 N and 150 N |
| Loading rate | 200 N/min |
| Scratch length | 10 mm |
| Scratching speed, dx/dt | 20 mm/min |
| Indenter geometry | 120° cone |
| Indenter material (tip) | Diamond |
| Indenter tip radius | 200 µm |
Table 1: Test parameters.
Test Results
Fig. 2 shows the plot of normal force, frictional force and penetration as a function of scratch length for Sample A as an example. An optional acoustic emission module can be installed to provide more information. It is clearly seen that as the normal load progressively increases, the indenter tip gradually sinks into the tested sample, as reflected by the nearly linear change of penetration at the beginning of the test. At a scratch length of ~3.8 mm, which corresponds to a normal load of ~37 N, a sudden change in the slopes of the frictional force and penetration curves takes place, followed by fluctuation of these two values. Such behavior can be used as an indication that coating failure starts to occur.
The above finding is in agreement with further optical observation under the microscope, as displayed in Fig. 3, where clear exposure of the substrate is shown at the corresponding load of ~37 N. The NANOVEA Mechanical Tester software includes a function that can automatically generate an image of the whole scratch (panorama) and correlate the different areas of the scratch with the load applied. This allows the user to perform analysis at any time, rather than having to determine the critical load under the microscope immediately after the scratch tests.
Table 2 summarizes the scratch test results of all three anodized aluminum samples (Samples A, B and C) produced using different process parameters. It is evident that optimization of the anodization process is critical to the mechanical properties, more specifically the scratch resistance, of the anodized aluminum oxide layer in this case. Sample C possesses substantially enhanced scratch resistance, exhibiting a critical load of 118.9 N, compared with only 37.3 N for Sample A. Moreover, it is demonstrated that the NANOVEA Mechanical Tester can provide reproducible quantitative analysis of the scratch resistance of anodized coatings: the scratch tests on all three anodized aluminum oxide coatings show low standard deviations.
| Scratch | Sample A: critical load (N) | Sample B: critical load (N) | Sample C: critical load (N) |
|---|---|---|---|
| 1st | 37.2 | 67.5 | 118.7 |
| 2nd | 37.0 | 68.1 | 119.8 |
| 3rd | 37.7 | 68.1 | 118.3 |
| Average | 37.3 | 67.9 | 118.9 |
| Standard deviation | 0.34 | 0.34 | 0.78 |
Table 2: Summary of critical loads of the tested samples.
Conclusion
The NANOVEA Mechanical Tester in Macro Scratch Tester mode is a superior tool for quality control of anodized aluminum oxide coatings. Scratch testing can detect adhesion and cohesion problems in the coating system before parts are actually put to use. By applying loads in a controlled and closely monitored fashion, the tool allows users to identify quantitative and reproducible critical load failures. This type of information can help manufacturers improve and control coating quality.
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, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of indentation, scratch and wear track in addition to other surface measurements such as roughness.
Frequently Asked Questions About Scratch Testing Anodized Coatings
Why scratch test anodized aluminum?
Anodized layers are harder than aluminum but porous, so wear resistance is moderate; an insufficient anodized layer leaves parts vulnerable to chips, dents and scratches, as happened with a popular phone frame in 2012. Scratch testing detects premature adhesive or cohesive failure.
How were the coatings tested?
Progressive load scratches from 0.03 N with a 200 µm Rockwell C diamond stylus at 200 N/min over 10 mm, repeated three times on each sample.
How is coating failure identified?
A sudden change in the slope of the frictional force and penetration curves, followed by fluctuation, marks the start of failure; microscope images of the scratch confirm substrate exposure at that load.
What critical loads were measured?
Sample A failed at 37.3 N, Sample B at 67.9 N and Sample C at 118.9 N on average, so the process used for Sample C gave substantially better scratch resistance.
How repeatable were the results?
Standard deviations were 0.34 N for Samples A and B and 0.78 N for Sample C.
Can the critical load be checked later?
Yes. The software builds a panorama image of the whole scratch correlated with the load, so the analysis can be done at any time instead of immediately at the microscope.
