Nanoindentation

Nanoindentation is one of the most important techniques for the quantitative characterization of mechanical properties of materials. Essentially it works by of pushing a hard, sharpened indenter tip with a well-defined shape against the surface of a sample. This tensile testing technique is used to precisely and locally characterize materials of all sorts (thin coatings, metals, ceramics, polymers, biomaterials etc.) at a nanoscale level, and can also be of great interest for heterogeneous surfaces (different phases, porous materials, depth sensing, defects vs. intact surface, etc.). By analyzing force-displacement curves, it is possible to extract sample hardness and elastic modulus without measuring the residual imprint as it is done with conventional macroscale hardness techniques.

AFM image of a residual imprint produced by a nanoindenter shows the three-sided pyramid

As a challenge, the rule of thumb says that the depth of penetration should not exceed 10% of the coating’s thickness to avoid influences of the underlying substrate. For a 1-μm thin film, this corresponds to a maximum indent of 100 nm. Moreover, to avoid the influence of surface roughness on the measurement, it should be smaller than 20% of the indentation depth. For a roughness of 10 nm, the minimum depth of indentation should therefore be 50 nm.

AFM and Nanoindentation combined

Nanoindentation and atomic force microscopy (AFM) can be coupled in a single system with an accurate repositioning stage to allow a comprehensive and (semi)automated analysis. In a first step, the atomic force microscope measures surface roughness to help define the minimum indentation depth. Then the sample is precisely positioned under the nanoindenter to perform a mechanical analysis on the same location. In a last step, this location can again be moved under the AFM to characterize and understand stress-induced features such as material pile-up, sink-in, or cracks induced around the indentation. If observed, these effects might have an influence on the values obtained for hardness and elastic modulus.

In a publication by Perfler et al. (Inorg. Chem. 54 (2015) 683), the Nanosurf NaniteAFM was used together with a Micro Materials nanoindentation system to obtain the image shown above (courtesy of the authors). It shows the residual imprint of a nanoindenter without any of the above-mentioned stress-induced features.

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NaniteAFM — The leading mountable AFM

Most atomic force microscopes are limited in the type and size of samples they can handle. The NaniteAFM by Nanosurf is the market leading solution for AFM integration with least restriction to the sample dimensions.