AFM image of a residual imprint produced by a nanoindenter

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

As a challenge, the rule of thumb says that the indentation depth should not exceed 10% of the coating’s thickness to avoid influences of the underlying substrate. For a 1-μm thick 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 indentation depth should therefore be 50 nm.

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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