Poking material to probe nanomechanical properties
I haven’t written much about nanoindentation yet. The field itself is pretty confusing because of a lot of ‘AFMers’ who conduct force spectroscopy measurements call these measurements nanoindentation. And then there is a whole area of instrumented nanoindenters, which are systems designed and dedicated exclusively to nanoindentation but that really don’t have much to do with AFM (unless they add on an AFM for imaging purposes, not nanoindentation purposes). So what is actual nanoindentation? This is the field of poking materials on a very small length scale and measuring the load (or force) vs displacement as you poke. See an example below of 2 nanoindentation measurements by a diamond Berkovich tip at 2 different loads – 5500 uN and 10,000uN on fused quartz, a favorite calibration sample.
2 nanoindentation measurements by a diamond Berkovich tip at 2 different loads – 5500 uN and 10,000uN on fused quartz
Why is this useful? You can extract both elastic (e.g. Youngs modulus) and plastic (e.g. hardness) properties on your sample from this data. There are certainly bulk and even micro-indenters that do this measurement on a larger scale. Nanoindenters came around about 40 years ago as the need to get this information on laterally small domains or thin films emerged. Current nanoindenters can exert forces as low as a few uN and measure displacements in the few nanometers. The lateral size of indents is limited by the diamond indenter tip dimensions, which are roughly an order of magnitude larger than AFM silicon tips. See below an AFM image of a series of 8 indents in an inorganic film made with a Berkovich indenter where the dark triangular spots are the indents into the sample.
AFM image of a series of 8 indents in an inorganic film made with a Berkovich indenter
What all these indenters have in common is that they poke a material, measure the force exerted on the material, and then somehow measure the displacement. An important parameter for nanoindentation that is also tricky to measure is the contact area which is the actual area between indenter and tip while they are in contact. Traditional hardness measurements done by Vickers hardness testers measure the size of the indent optically. But clearly optical measurements of indents on this length scale won’t work, so they figured out a different approach.
The Masters of Nanoindentation Warren Oliver (currently president of Nanomechanics Inc) and George Pharr (at Texas A&M) published one of the most important and most cited works in nanoindentation in 1992 where they derived how to measure the indent size without needing to image it (like in traditional methods) by continuously monitoring force and displacement throughout the actual indent: An Improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. Once this piece was figured out, nanoindentation really took off as a technique to measure elastic and plastic properties of materials on such a small length scale.
How does AFM come in? Briefly, a force curve where you measure force vs. displacement is exactly like the measurement performed by the nanoindenter. See example of a force curve of a silicon tip on a silicon wafer below. Note it also measures force vs. displacement
A force curve image, typical of AFM
Though the measurement is the same, the differences in hardware and probe are substantial and lead to a lot of differences in the capabilities between the two instruments - I’ll save that for the next blog. In the meantime, for those of you who want to delve more into nanoindentation, I can offer 2 excellent resources:
- Nanoindentation by Anthony Fischer-Cripps (Springer – 2011)
- A truly excellent free, monthly webinar series that was started in 2016 and is still ongoing by Jenny Hays of Nanomechanics. I have been thoroughly enjoying these webinars which offers a really comprehensive and thorough introduction to all the different experimental and theoretical aspects of nanoindentation. Jenny is also an excellent lecturer who explains concepts and math very clearly. The first in this series can be found here and is entitled “Instrumented Indentation Session 1: Introduction and Overview). We just completed Session 8 – Nanoindentation of thin films.
So to answer the question: is nanoindentation an AFM technique? Sort of. I would say AFM can perform nanoindentation-type measurements like in a force curve, but it comes with a lot of caveats that don’t exist for instruments designed for this measurement (instrumented nanoindenters.) We will explore these caveats more in the next blog.
Dalia Yablon, Ph.D.