Most common microindentation measurements in both bulk and thin film systems focus on the determination of hardness and elastic modulus of the material. However, in many material systems, discontinuities in the load-depth relationship can often be observed, especially in materials where film failure, delamination, dislocation movement or phase change may have occurred. Characterization of certain physical phenomena using acoustic emission can provide an accurate in-situ measurement of both the magnitude and type of event.
Previous studies on the acoustic emission behaviour during indentation of a variety of materials have shown that the speed at which an event occurs can be correlated to the type of event which led to the acoustic release of energy. Since microindentation causes discrete, localized events, the ability to identify each physical event and correlate them to the acoustic behaviour allows a direct comparison between the event and the individual acoustic emission signature.
What is Acoustic Emission
Acoustic emission is the sudden release of elastic energy into acoustic waves that travel through the material. Traditionally, such waves have been separated into two types of behaviour: burst emission and continuous emission. A burst emission is a discrete packet of waves associated with a single event, whereas continuous emission tends to be an agglomeration of many small interlinked events. The Anton Paar Microindentation Tester (MHT) incorporates an acoustic emission sensor operating with a frequency of 150 kHz over a dynamic range of 65 dB with amplification up to 200,000x. Such a wide dynamic response enables the sensor to resolve acoustic events in most engineering materials when subjected to instrumented indentation over the applied load range 0.01 - 30 N. The sensor is mounted directly on the indenter housing to minimize losses and its signal is acquired simultaneously with the load and depth signals to give a complete picture of a compressive fracture event.
Advantages of Acquiring Acoustic Signal During Microindentation
One of the distinct advantages of acquiring the acoustic signal during microindentation is that it provides an indication of when the acoustic event actually occurs during the experiment. Fig. 1 shows a range of examples of acoustic signatures for microindentations made on a Si wafer with a Vickers indenter. In each case, brittle fracture (cracking) has occurred during the loading portion only. This is an interesting observation because cracking can sometimes also occur during the unloading phase in some materials. In these eight examples, the maximum load (15 N) has been maintained in each case, but the loading rate has been varied over the range 1 - 250 N/min. in order to investigate the influence of loading rate on the severity of cracking.
Figure 1. Typical acoustic emission signatures for microindentations made on a Si wafer with applied load of 15 N. Loading rates of 1, 10, 20, 40, 100, 150, 200 and 250 N/min. are shown.
It can clearly be seen that the fastest loading rate results in the severest cracking, observed both from the level of the acoustic signal and subsequent optical microscopy of the residual indentation. An example of a progressive load multicycle on a Si wafer is shown in Fig. 2. This confirms that cracking occurs only during the loading portion even though the material is being progressively fatigued by increasing the applied load through five steps. Fracture in brittle materials is usually more significant when the load is progressively applied than if a single load-unload cycle (to the same maximum load) was applied.
Figure 2. Progressive load multicycle (5 cycles over range 1 - 10 N) with a Vickers indenter on a Si wafer. Acoustic signal confirms cracking during the loading portion of each cycle.
This is because more energy is being channelled into the material in the former case. In some cases, the subsequent acoustic bursts can be stronger than the original burst, and the time between bursts is much greater than the time in which a sound wave can travel across the sample. This leads to the conclusion that the multiple events observed are not merely reflections of the acoustic waves, but are individual events. It should also be remembered that only a fraction of the acoustic energy is picked up by the detector, and only a fraction of the released elastic energy is converted to acoustic energy. Even though the signal is limited by the bandwidth of the sensor used, the ability to arrive at a semiquantitative measurement of event strength makes it an appealing method of analysis.
Figure 3 shows the acoustic signature for a microindentation on a Titanium Nitride thin film of thickness 3 µm. Again, the main acoustic events are observed during the loading phase and corresponding cracking is observed around the residual imprint. In the case of a coating, the acoustic signal may give an indication of the bond strength between the coating and the substrate: if the coating is poorly bonded, then little energy may be released during delamination.
Figure 3. Acoustic signature for a microindentation with applied load of 10 N on a Titanium Nitride (TiN) coating (thickness 3 µm) on a steel substrate.
Since the elastic energy released during coating delamination can be quantitatively measured (from the load-depth curve) it could be possible to calibrate the energy output of a given sensor to the elastic energy released.
In any case, acoustic emission measurement shows great promise for revealing the relationship between physical phenomena and the corresponding acoustic emission signal. Such measurement capability will be able to shed some light on the magnitude of a brittle failure event as well as the precise moment when it was initiated.
This information has been sourced, reviewed and adapted from materials provided by Anton Par.
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