Characterization of material surface properties – Nanovea Nanoindentation…

The competitive field of Nanoindentation measurement is commonly decided upon depth and load specifications. Competing Nanoindentation instruments list resolution capabilities from the Pico to the Micro ranges. Unfortunately, these specifications are sometimes intentionally misleading. When Depth Resolution is specified in the Pico range, it suggests that depth is controlled and measured at a hundredth of an Angstrom. Keeping in mind that an atom is between 62 and 520 pm in diameter, it is doubtful that any Nanoindentation instruments can measure in this range taking in to account the normal vibration movement of an atom and adding a minimum vibration noise coming from the environment caused by electrical or mechanical noises.

Let’s take the example of a 1m ruler, the 1m would be the full depth range of an instrument. An acquisition card of an instrument would define the capability to create lines on the ruler. So an acquisition card of 10bits would be 210 lines or 1024 lines and a 16bits would be 216 lines or 65536 lines. Dividing the ruler by these will give respective resolutions of 0.00097m or 0.97mm and 0.000015m or 0.015mm. In this example, our eye would be the sensor and in the case of the Nanoindentation instrument, it would be the capacitor. For the case of the ruler, it is clear that our eye will be able to see the split line of 1mm but will not be able to read the 0.015mm.Therefore, even if we were able to increase the number of line to let’s say 20bits, we would not add any improvement to the usefulness of the ruler. Now with powerful acquisition cards, it is often the case that these are not the limit factor in terms of what can be reached. The sensors sometimes are, but in the case of a capacitor we can also get to the physical limit and environmental conditions. Going back to our ruler,  even if our eyes were better and we were able to read to 0.9micron (20bit), a part vibrating at 10micron would not allow us to measure better than this 10micron  “noise”. This logic is valid also for the load resolution listed for Nanoindentation instruments.

There is strong incentive for manufacturers to list the best “even if not based on reality” resolutions. For example, competitive bids where purchasing agents don’t have the technical knowledge to understand that these resolutions are not indications of actual sensitivity, resulting in the elimination of some bidders based solely on skewed resolutions. Unfortunately, there is not much that can be done about this. The concern is when the main deciding engineer or scientist is not aware of this truth on specifications, they may ignore the best choice for their projects. Therefore it is crucial to understand standard noise level, also called resolution noise level, actual point of contact and minimum “maximum” load on hard and soft materials. This will give a much better account of the capability of a nanoindentation instrument. In the end, nothing overshadows a demonstration on your own samples and the comparative testing data.

Therefore, in review, a Nanoindentation instrument provider listing Pico through Micro range depth resolution should be questioned. The next time you are in the market for a Nanoindentation instrument, it is crucial that your chosen provider clearly demonstrates  the ability to control depth and noise at the given range required for your applications.

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