Wear Testing Glass with Acoustic Emissions Monitoring

Acoustic emission (AE) is the high-frequency elastic waves that travel through a dense medium because of dynamic processes occurring in the material¹. It is usually generated when a material is subjected to external stress and rapidly releases energy in the form of transient stress waves, causing localized small surface displacements. The AE technique is widely used in inspection, quality control, system feedback, process monitoring and others.

AE has been commonly used as an additional component for scratch tests. The AE generated during a scratch test can be related to failures taking place at different critical loads, e.g. crack formation and propagation, as well as buckling, spallation and complete delamination of hard coatings, allowing users to detect premature adhesive or cohesive failures of the coating and substrate system in real-life applications. During wear tests, the constant rubbing of the counterface against the test sample creates a series of events, such as deformation, cracking, abrasion, phase transformation and material removal. These rapid stress-releasing events are the sources of AE. A quantifiable and reliable measurement of AE can provide more insight into the wear failure mechanism.

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The NANOVEA Tribometer equipped with an AE detector was applied to evaluate the tribological behavior, e.g. coefficient of friction, COF, and wear resistance. An Al₂O₃ ball tip (6 mm diameter) was applied against the tested glasses. The COF and AE were recorded in situ. The test parameters are summarized in Table 1. The wear track morphology was examined using an optical microscope.

The pin-on-disk wear tests were conducted on the regular glass, S3 lens and sapphire coating samples. The COF and AE were recorded in situ during the wear tests, as shown in Fig. 2 and Fig. 3. Fig. 4 compares the wear tracks after the tests.

As shown in Fig. 2, the regular glass exhibits a high COF of above 0.7 in the first 100 s of the wear test. In comparison, the S3 lens shows a low COF of ~0.1 in the first 20 s, followed by an increase of COF to ~0.55 until ~200 s. The sapphire coating maintains a low COF of ~0.1 in the first 200 s. Such a trend in the evolution of COF at the beginning of the wear tests shows a strong correlation with the evolution of AE in Fig. 3. High peaks of AE signals are recorded in the first 100 s of the wear test on regular glass, corresponding to the high value of COF in this period. The S3 lens starts to generate AE at ~20 s, while the sapphire coating shows no AE under the constant rubbing movement against the Al₂O₃ ball at a low COF in the first 200 s of the wear test.

The AE signal is indicative of the change at the sliding surface and possible wear failures of the test sample. In this particular study, we observe signs of wear scar formation as the AE detector records high signals. As more AE spikes are detected, the localized wear scars grow longer and wider, eventually leading to the formation of a complete wear ring. Such a wear ring occurs quickly on the regular glass, but it is not observed on the sapphire coating in the first 200 s of the wear process.

The COF of the regular glass and S3 lens progressively decreases and reaches a constant value of ~0.35 and ~0.25, respectively, which indicates the establishment of a stable sliding contact; in comparison, the sapphire coating shows a substantial increase of COF to ~0.8 after 200 s of wear. This corresponds to the evolution of AE in Fig. 3: AE signals start to be detected at ~200 s, and high AE spikes occur from ~300 to 500 s, indicating initiation and propagation of wear scars during this period. The relatively high AE throughout the rest of the wear test of the sapphire coating compared to the other two samples is in agreement with its high COF. The higher COF and AE of the sapphire coating after the run-in period are possibly related to the Al₂O₃ counterface ball used in this study, which possesses a similar composition and serves as a self-mated couple.

Fig. 4 displays the wear tracks of the test glass samples under a 200x microscope. Severe wear took place on the regular glass surface. A lot of debris was formed and ground in the wear track, which acted as abrasive media and accelerated the wear process. The S3 lens exhibits a track of mild wear with traces of shallow scratches, which demonstrates its substantially enhanced wear resistance compared to regular glass. The sapphire coating shows a relatively narrow wear track and improves the wear resistance of the glass substrate.

In this application, we show that the NANOVEA Tribometer is a superior tool for the evaluation and quality control of commercial glass. An acoustic emission (AE) sensor can detect the initiation and propagation of the wear scar and is a useful additional component to the tribometer. The capacity for in-situ AE and COF measurement allows users to correlate different stages of the wear process with the evolution of AE and COF, which is critical in improving the fundamental understanding of the wear mechanism and tribological characteristics of various materials.

Based on the comprehensive tribological analysis of the glass samples in this study, we show that the Galaxy S3 lens and sapphire coating possess enhanced wear resistance compared with the regular glass sample. This assessment allows us to evaluate and select the best candidate to fulfill the needs of targeted applications.

The NANOVEA Tribometer offers 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. Liptai, R. G.; Harris, D. O.; Tatro, C. A. (1972), Acoustic Emission, Special Technical Publication 505, American Society for Testing and Materials, Philadelphia, PA, USA.