Nanoindentation Testing of Nanomaterials

Abstract:

The properties of ultra-nanocrystalline diamond (UNCD) films, a patent of Argonne National Laboratory (ANL), provide great possibilities for the applications in micro electromechanical systems (MEMS) technology. Measuring the mechanical properties of UNCD thin films is a collaborative project between MEMS/Nanotechnology group, Northwestern University and Material Science Division (MSD) of ANL.

Basically, there are three methods to study the mechanical properties of micro scale thin films:

1. Fabricate thin film cantilevers, apply load on them using an indenter and analyze the load - displacement data.
2. Fabricate thin film bridges, apply load on them using an indenter and analyze the load - displacement data (MDE, membrane deflection experiment).
3. The last method is depositing the thin film on a substrate and doing indentation tests.

For UNCD thin films, the Young's Modulus is around 936 GPa as determined with the first method. Results from second method show a modulus around 1050 GPa. The contribution presented here consists of growing UNCD thin films on silicon substrates under guidance of Dr. D. Gruen, Dr. J. Carlisle and Dr. O. Auciello in ANL, and doing nanoindentation tests under guidance of Prof. H. Espinosa and Dr. B. Prorok.

Although nanoindentation has the advantage that it has no special requirements regarding the specimen shape and preparation, strong influences on the measured data are coming from the roughness of the thin film, the effects from substrate, the imperfect shape of Berkovich nanoindenter and great difference between the UNCD film modulus and the silicon substrate modulus. It is a challenge to properly interpret load - displacement raw data from the indenter system and extract the mechanical properties of the thin film. For this purpose, several models were investigated, as following:

1. Oliver and Nix developed different methods to get the projected contact area and reduced modulus from raw load - displacement data. This is the equivalent modulus from moduli of the sample and indenter (normally diamond).
2. Gao and King set up their own models to explain thin film/substrate system with different moduli: the modulus of thin film, modulus of indenter, and modulus of substrate on which the thin film is deposited.

If we do not consider any model to address the thin film/substrate system effects, a UNCD modulus of 600 GPa was obtained. Our results show that King and Gao's models work well in explaining the modulus of UNCD/silicon composite system: Young's modulus of UNCD is around 700 to 800 GPa in Gao's model and 950 GPa in King's model. These results are very close to what have obtained from UNCD cantilever and MDE experiments.

Also, the surface roughness of UNCD is usually about 20 - 40 nm. If the nanoindentation depth is comparable with this scale, it is hard to rule out the effects from surface roughness. And the typical Berkovich tip radius is about 100 - 200 nm, due to the limitation in current micromachining techniques. The nanoindentation theory assumes the tip shape is sharp at the end. These effects are under study.

The modulus/hardness properties of UNCD explored by three different methods are encouraging. It would be an interesting case to study the properties of such a hard film in micro scale.

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