Elastic modulus and fracture strength evaluation on the nanoscale by scanning force microscope experiments.

This work first reviews the capability of scanning force microscopy (SFM) to perform experiments with forces in a wide range, from low non-contact forces to high contact forces which induce mechanical deformations in the substrate. In analogy to fracture strength evaluation, as established in materials science, SFM is used to exert forces on pillars with nanometer dimensions while the cantilever deformations are monitored quantitatively. Hence, it is possible to bend the pillars until the threshold for triggering fracture is reached, and to determine the mechanical properties at the different stages of this process. Using this novel approach, in combination with 'state of the art' nanofabrication to produce nanopillar arrays on silicon and silicon dioxide substrates, a number of experiments are performed. Furthermore, quantitative measurements of the fracture strength of Si and of the SiO2/Si interface and E-modulus are presented. To analyze the experimental data obtained in the different experimental procedures and modes, finite element method calculations were used. The methods introduced herein provide a versatile toolbox for addressing a wide range of scientific problems and for applications in materials science and technology.

An international round-robin calibration protocol for nanoindentation measurements.

Nanoindentation has become a common technique for measuring the hardness and elastic-plastic properties of materials, including coatings and thin films. In recent years, different nanoindenter instruments have been commercialised and used for this purpose. Each instrument is equipped with its own analysis software for the derivation of the hardness and reduced Young's modulus from the raw data. These data are mostly analysed through the Oliver and Pharr method. In all cases, the calibration of compliance and area function is mandatory. The present work illustrates and describes a calibration procedure and an approach to raw data analysis carried out for six different nanoindentation instruments through several round-robin experiments. Three different indenters were used, Berkovich, cube corner, spherical, and three standardised reference samples were chosen, hard fused quartz, soft polycarbonate, and sapphire. It was clearly shown that the use of these common procedures consistently limited the hardness and reduced the Young's modulus data spread compared to the same measurements performed using instrument-specific procedures. The following recommendations for nanoindentation calibration must be followed: (a) use only sharp indenters, (b) set an upper cut-off value for the penetration depth below which measurements must be considered unreliable, (c) perform nanoindentation measurements with limited thermal drift, (d) ensure that the load-displacement curves are as smooth as possible, (e) perform stiffness measurements specific to each instrument/indenter couple, (f) use Fq and Sa as calibration reference samples for stiffness and area function determination, (g) use a function, rather than a single value, for the stiffness and (h) adopt a unique protocol and software for raw data analysis in order to limit the data spread related to the instruments (i.e. the level of drift or noise, defects of a given probe) and to make the H and E(r) data intercomparable.

Experimental investigation of the contact mechanics of rough fractal surfaces.

The nonstationary character of roughness is a widely recognized property of surface morphology and suggests modeling several solid surfaces by fractal geometry. In the field of contact mechanics, this demands novel investigations attempting to clarify the role of multiscale roughness during physical contact. Here we review the results we recently obtained in the characterization of the contact mechanics of fractal surfaces by depth-sensing indentation. One class of experiments was conducted on organic thin films, load-displacement curves being acquired by atomic force microscopy using custom-designed tips. Another class of experiments focused on well-defined crystalline and mechanically polished ceramic substrates probed by a traditional nanoindenter. We observed the first-loading cycle to be considerably affected by surface roughness. Plastic failure was found to dominate incipient contact while contact stiffness increased on decreasing fractal dimension and roughness. Our findings suggest fractal parameters to drive contact mechanics whenever the penetration depth is kept below the interface width.