In Situ Non-Contact Measurement of Wear Rate Evolution

The anodizing process is widely applied on aluminum to modify the texture of the surface and change the microstructure of the metal near the surface. The formation of such an anodized aluminum oxide layer can enhance corrosion and wear resistance and improve cosmetic effects. Anodized aluminum is widely used on electronic devices, such as mp3 players, flashlights and cameras.

Anodic films are generally much stronger and more adherent than most types of paint and metal plating. Anodized aluminum coatings grown on different types of aluminum alloy substrates possess significantly different wear resistance. Taber abrasion tests were conventionally used to evaluate the wear resistance of anodized aluminum coatings according to the ASTM D4060 standard. However, as mentioned in the ASTM D4060 standard, abrasion tests with the Taber Abraser may vary for some materials as the abrasive characteristics of the wheel change during testing¹. Moreover, the aggressive abrasion process of the Taber Abraser can quickly wear through anodized aluminum coatings and create mass loss in the substrate, which leads to substantial errors in the calculation of the coating weight loss.

A more reliable and repeatable technique for wear evaluation is needed. In this study, we developed a tribometer implemented with a non-contact profilometer that allows us to closely monitor the evolution of the wear process of anodized aluminum coatings, determine the failure mode of the different anodized aluminum layers, and select the best candidate for real-life applications involving wear protection.

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The NANOVEA Tribometer equipped with a non-contact profilometer was applied to evaluate the tribological behavior, e.g. coefficient of friction, COF, and wear resistance, of an anodized Al2036 sample. A normal load of 10 N was applied by an Al₂O₃ ball tip (6 mm diameter) against the sample. The COF was recorded in situ. Wear track profiles were measured and the wear track morphology was examined using an optical microscope every 2000 revolutions. The test parameters are summarized in Table 1.

The COF recorded in situ during the wear test is shown in Fig. 2. The COF exhibits a high value above 0.75 at the beginning of the wear process. It gradually decreases in the first 8000 cycles and then maintains a relatively constant value of ~0.6 throughout a long sliding distance until close to 30000 revolutions. This is followed by an abrupt decrease of COF to ~0.35, indicating a change of the tribological behavior at the contact surface at this point.

Fig. 3 displays the images of the wear tracks and Fig. 4 presents the cross-section profiles after 2000, 4000, 8000, 16000, 24000 and 32000 cycles. These results are in agreement with the evolution of the COF during the wear test. The initial spike of COF at the beginning of the wear process is primarily attributed to the rough morphology of the anodized surface layer. As the asperities at the surface are removed progressively in the first 8000 revolutions, as shown in Fig. 3a–c, the COF gradually decreases to a value of ~0.6. At this stage, the tested anodized Al sample shows a relatively high wear rate, as reflected by the fast increase of wear volume from 2000 to 8000 cycles shown in the cross-section profiles of the wear tracks in Fig. 4.

As the wear process continues through ~8000 to ~30000 revolutions, the wear track maintains a fairly smooth surface (see Fig. 3d–e), while the wear volume does not show any significant change, as indicated in Fig. 4. Such an observation demonstrates that the anodized Al surface layer possesses very different wear resistance at different distances from the top surface: the shallow surface has a poorer wear performance due to its porous nature, while the dense layer at a depth of ~30 to 50 µm exhibits significantly enhanced wear resistance.

When the Al₂O₃ ball tip wears through the anodized Al protective layer at ~30000 revolutions, the wear process dramatically accelerates, as indicated by the large wear track and exposure of the metal substrate after 32000 cycles (see Fig. 3f and Fig. 4). This also leads to the decrease of COF shown in Fig. 2.

The evolution of the average wear rates after different numbers of revolutions is shown in Fig. 5. It is calculated using the formula K = V/(F × s) = A/(F × n), where V is the total worn volume, F is the normal load, s is the sliding distance, A is the cross-sectional area of the wear track as displayed in Fig. 4, and n is the number of revolutions. It is evident from Fig. 5 that the calculated wear rate of a material strongly depends on the number of revolutions, which is directly related to the different stages of the wear mechanism. In this particular study, the wear resistance of the tested sample (anodized Al2036) varies along the thickness of the coating, resulting in significantly different wear rates calculated after different numbers of revolutions.

Therefore, it is extremely important to be cautious in comparing wear rates measured using different test parameters or reported from different labs. This study also demonstrates that the NANOVEA Tribometer with an integrated non-contact profilometer is an ideal tool for monitoring and studying the evolution of the wear process at different stages. It cuts the cost and time for the preparation of multiple samples, and eliminates the possibility of test result fluctuation due to property variation between samples. Moreover, compared to conventional pin-on-disc or Taber testers, it considerably facilitates the initial investigation of the test parameters, which in many cases is the most energy and time consuming process. For example, the Taber tester is widely used to study the wear resistance of anodized coatings. However, because of the aggressive wear nature of the Taber abraser, the coatings could be worn out in a short time; and if the test time is too short, the more wear resistant sublayer will not be reached, resulting in an underestimated calculation of the wear rate.

In this study, we showcased that the NANOVEA Tribometer equipped with a non-contact profilometer is a superior tool for wear evaluation. Compared to conventional Taber abrasion measurements, the NANOVEA Tribometer provides well-controlled, quantifiable and reliable wear assessment that can ensure reproducible evaluation and quality control of anodized Al coatings. Moreover, the capacity for in-situ COF and wear rate measurements allows users to correlate different stages of the wear process with the evolution of COF and wear rate, which is critical in improving the fundamental understanding of the wear mechanism.

Based on the comprehensive tribological analyses, we show that anodized Al2036 possesses different wear resistance along the thickness of the coating. This assessment allows us to evaluate and select the best candidate for environments that involve wear. Moreover, the capacity of evaluating the wear rates after different numbers of revolutions substantially facilitates the initial test parameter setup.

NANOVEA Tribometers offer precise and repeatable wear and friction testing using ISO and ASTM compliant rotative and linear modes, with optional high temperature wear, lubrication and tribo-corrosion modules available in one pre-integrated system.

1. ASTM D4060-10, Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser.