Wear Resistance of Magnetic Stripes Using Tribometer

Magnetic stripes are widely used in a variety of products, including credit cards, identity cards, transportation tickets and many others. The magnetic stripes on the cards encode information that can be read by swiping past a magnetic reading head. Such repeated swiping on a daily basis may gradually create wear scratches on the magnetic stripes and eventually lead to failure of the cards. Due to the importance of the information stored in the cards, good wear resistance of the magnetic stripes becomes a critical factor.

A reliable and repeatable technique is needed to simulate the wear process and evaluate the wear resistance. Due to the complex nature of the wear process, which involves a variety of factors such as unidirectional sliding, reciprocating, rolling, as well as impact loads, speed, temperature and others, it is important to simplify the wear test setup by using a linear reciprocating sliding motion to simulate the real-life wear process of magnetic card stripes. The well controlled and monitored wear measurement allows users to follow the evolution of the coefficient of friction and correlate it with different stages of the wear process, which provides more insight into the wear mechanism and failure modes.

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The tribological behavior, e.g. coefficient of friction, COF, and wear resistance, of the magnetic stripes of two different cards was evaluated by the NANOVEA Tribometer using a linear reciprocating setup. A flat tip was applied against the tested samples. The COF was monitored in situ. The test parameters are summarized in Table 1.

Fig. 2 shows the raw COF between the pin and the fidelity card sample over the course of the test. The COF goes back and forth from positive to negative because of the back and forth motion of the stage during this test. The pin slid on the sample for ~1200 revolutions without causing visible damage, after which the pin started creating debris and the full wear occurred over a couple of strokes, as shown in Fig. 3. For the remainder of the test, no significant further wear was observed; however, an increase in the amplitude of the COF can be observed, due to the failure of the surface layer and exposure of the substrate, which possesses a higher COF.

In comparison, the automobile club card exhibits surface layer failure after a much higher number of cycles of ~14,000 revolutions. Fig. 4 compares the COF (squared) against the number of cycles during the tests for both samples. It clearly shows that the automobile club card has much superior wear resistance. The sliding motions in this test simulate the actions of swiping these cards in readers or sliding them in and out of a wallet. The result indicates that the airline fidelity card could exhibit damage to its magnetic stripe earlier than the automobile club card. The wear test of different cards using an identical experimental setup and test conditions allows us to compare and improve the material and manufacturing process in order to obtain the best performance in real-life applications.

Although the tribometer is often used in conjunction with the profilometer to measure the wear rate of coated or uncoated samples, the standard tribometer can provide reliable evaluation of wear resistance on its own in some applications where coatings or specific properties of materials have a certain lifetime. In this study, the magnetic stripes would not show evident wear until the point of failure, allowing users to compare the wear resistance of the magnetic stripes on different cards using the same test conditions. The wear test can also be applied to samples where a top material layer wears gradually over time, uncovering a second material with a different COF, or where a surface texture is worn through. The change in COF also serves as a useful indicator of the coating lifetime.

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 tribocorrosion modules available in one pre-integrated system. An optional 3D non-contact profiler is available for high resolution 3D imaging of the wear track in addition to other surface measurements such as roughness.