Why do we still need nanoindenters to measure small material volumes mechanical properties?

Post publication date:

Tuesday, November 18, 2014

Scanning techniques for the determination of mechanical properties (mainly the Elastic modulus) of nano-materials and thin films have emerged over the past few years. The high throughput of such scanning techniques have drawn high interest within the nanoscale scientists communities worldwide. Scanning techniques are inherently fast and, in most commercially available implementations, integrated with AFM imaging capabilities. In particular, AFM-like material properties images can be obtained, with unrivaled space resolution, but are such maps providing truly quantitative mechanical properties information?

Scanning techniques require assumptions

It is in facts widely accepted that state-of-the-art scanning techniques allows creating materials stiffness maps in well-controlled ways, in which either sensitivity or accuracy can be optimized. However, looking at the continuous solid mechanics, the assumptions to be met in order to derive an Elastic Modulus map from a stiffness map, are:

The surfaces are continuous and non-conforming; absolutely smooth, flat surface
The strains are small
Each solid can be considered as an elastic half-space: no plasticity allowed
The surfaces are frictionless

First condition assumes the scan load is held constant very accurately at very low values for long time, second one assume no tip wear, third one: the contact area should not change significantly during one harmonic cycle.

Pros and Cons

On one side, avoiding any local onset of plasticity may be not as trivial as it sounds, considering for instance that, the minimum tip radius of curvature to keep a completely elastic contact on e.g. copper with a controlled load of just 100nN should be >10um (smaller local radius would require a smaller controlled force to keep purely elastic contact).
On the other side: measurement of hardness and materials yield strength , conversely, require achieving full plasticity (i.e. full plasticity must be achieved to measure yield stress, for materials for which yield stress has meaning).

The good news: faster instrumented indenters exist on the market

The good news are: nowadays faster instrumented indenters exist on the market, capable of performing nanoindentation cycles at speed of one or few seconds/cycle, thus enabling nanomechanical mapping of elastic modulus and hardness.
Please don't hesitate to contact us @ Schaefer-Tec for more information or a state-of-the art nanoindenter quote!