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Hi all, I'm starting to make indentations with the AFM. I obtain Force-piezo displacement curves, but I would like to know how to relate the piezo displacement to the actual penetration depth of the tip in order to obtain the Force-displacement curves mentioned in the literature and used to draw information about the elasto-plastic behaviour of the samples (e.g. Oliver, W. C., & Pharr, G. M. (2004). Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology. Journal Of Materials Research).
What I have in mind is that in a very soft sample, the piezo displacement is equal to the sample deformation (penetration depth), while if the sample is very hard it does not deform for any piezo displacement. It seems that I must have information about the sample in order to relate piezo displacement to penetration depth.
I appreciate any comments about this issue,
Luis
Hi Luis,
in my understanding, penetration depth/indentation is the same general quantity as separation, just calculated for situations, where the tip is contact with the sample.
So basically, this is calculated from the difference between piezo displacement and cantilever deflection. If you set the piezo displacement to 0 at the contact point, and put the positive values of dispacement in direction towards the sample, you should get positive indentation/penetration depth, when you subtract deflection (referenced to the baseline of the force curve in non-contact) from z displacement.
In case of a hard sample, deflection equals piezo displacement, means indentation gets zero. In case of a soft sample, deflection is smaller than piezo displacement, and you get a indentation larger than 0.
Regards, Hartmut.
Hi Harmut, thanks for you explanation! It helped a lot!
I am now doing some indentations on an anealed polymer with an Antimony doped Si tip (diamond doped coating and nominal cantilever spring constant 20-80 N/m); this are typical Load-piezo displacement and corresponding Load-depth curves (after calibrating the sensitivity on a Si sample) that I obtain:
The curvature of the indentation is opposite to the one that I have seen in the literature for elasto-plastic deformations (like the Oliver-Pahr paper cited above), where one expects Deflection~(separation)^m with 'm' greater than one . The idea I have is that when you apply a certain load, the tip indents and the area of contact increases. A bigger area of contact means that a greater load has to be made on the sample to indent the same amount as before. However, this idea explains the opposite curvature than the one I see. I guess that the polymer is viscoelastic, but I don't know how to interpret the load-depth curve.
I was wondering if anyone in the forum has obtained such force-curves and would like to share their experience or if anyone has any comments about them.
Thanks in advance!
Hi Luís,
these curves really strange. I believe, you see some artifact here: the deflection voltage gets very high, such that the non-linearity of the photodiode comes into play. How do the curves on your hard calibration sample look like at similarly high deflection voltages? Do they show a similar curvature?
Maybe you need to consider lower load or a stiffer probe.
Regards,
Hartmut.