Atomic force microscope nanoindentation

Ultrahard fullerites with disordered structure and hardness of more than 200 GPa are already used as material for heavy-duty probes of scanning atomic force microscopes-nanoindenters. In combination with electrically conducting properties it enables one to investigate not only the mechanical properties of solid bodies by the AFM method, but also the electrical properties. Application of an ultrahard fullerite as an indentor material has allowed one to measure correctly for the first time the hardness of diamond at room temperature. 

Figure 2.35. Examples of indentation processes to determine surface hardness. Shown are (a) Vickers indentation on a SiC-BN composite, (b) atomic force microscope images of the nanoindentation of a silver nanowire, and (c) height profile and load-displacement curve for an indent on the nanowire. Reproduced with permission fromNanoLett. 2003, 3(11), 1495. Copyright 2003 American Chemical Society.

Nanoindentation is a powerful technique because the shape of the load-displacement curve can be used to identify effects such as phase transformations, cracking, and film delamination dining indentation. It is also important in studying the mechanical properties of nanomaterials, such as carbon nanotubes. There is reference now to a picoindenter, which is a combination of a nanoindenter and an atomic force microscope (AFM). 

The actual trend in hardness testing is to use the nanoindentation instruments in conjimction with atomic force microscopes (45). Load-displacement measurements are used to derive hardness and elastic modulus data while the atomic force microscope yields additional topographic information of the indentation area. Measurements at depths of 1 nm can be performed. 

Nanoindentation and nanotribology have been active research topics since the introduction of experimental tools such as the surface forces apparatus (SFA) and the atomic force microscope (AFM). These tools allow for detailed investigations of frictional properties of confined molecular systems at nanometer length scales(i,2,5,, 5). 

Fig. 9.12. (a) Atomic force microscope image of the impression created on a Zr— 17.9Cu-14.6Ni-10Al-5Ti (atomic percent) bulk metallic glass alloy which was subjected to nanoindentation at a maximum load of 60 mN. Discontinuous shear bands encompass the indent, (b) SAD patterns showing diffraction spots which were produced by the formation of nanocrystalline particles at the indents and in the shear bands. The inset schematically shows six diffraction spots which were associated with the (111) plane of tetragonal Zr2Ni particles, (c) A small distance away from the indent only halo ring patterns characteristic of a fully amorphous structure are seen. Reproduced with permission from Kim et al. (2002). 

For characterization of the mechanical properties, a nanoindentational Atomic Force Microscope (AFM) was applied to check the local hardness of the structured surface directly within intervals of 200 nm. The hardness in the laser-treated area, where laser intensity exceeds a certain threshold level, is significantly higher than that of the area in between, where laser intensity is below this threshold level. The average hardness in the laser-treated area is close to 10 GPa, while for the in between area it is around 4 GPa, which is close to the untreated state (Fig. 10). 

The direet determination of the mechanical properties of monolayer graphene has been undertaken by Lee et al. through the nanoindentation of graphene membranes, suspended over holes of 1.0-1.5 pm in diameter on a silicon substrate, using an atomic force microscope (AFM). The variation of force with indentation depth was determined and stress-strain curves derived by assuming that the graphene of thickness 0.335 nm behaved mechanically... 

Many techniques have been developed to measure the Young s modulus and the stress of the mesoscopic systems [12, 13]. Besides the traditional Vickers microhardness test, techniques mostly used for nanostructures are tensile test using an atomic force microscope (AFM) cantilever, a nanotensile tester, a transmission electron microscopy (TEM)-based tensile tester, an AFM nanoindenter, an AFM three-point bending tester, an AFM wire free-end displacement tester, an AFM elastic-plastic indentation tester, and a nanoindentation tester. Surface acoustic waves (SAWs), ultrasonic waves, atomic force acoustic microscopy (AFAM), and electric field-induced oscillations in AFM and in TEM are also used. Comparatively, the methods of SAWs, ultrasonic waves, field-induced oscillations, and an AFAM could minimize the artifacts because of their nondestructive nature though these techniques collect statistic information from responses of all the chemical bonds involved [14]. 

The force microscope is also well suited for atomic and molecular manipulation as it allows the measurement and control of forces involved in the manipulation process. In fact, the force needed to move a Co atom or a CO molecule across a Cu(lll) surface has been quantified in a combined NC-AFM/STM experiment [238]. This experiment and other NC-AFM manipulation experiments have initially been performed at cryogenic temperatures in analogy to procedures known from STM manipulation. However, sophisticated experimental methods of atom tracking and feed-forward techniques also allow imaging, manipulation, and spectroscopy with atomic precision at room temperature [239-242]. Controlled vertical manipulation has been demonstrated by displacement of individual silicon atoms on a Si(lll)7x7 surface by soft nanoindentation [243] and lateral manipulation for adsorbates on a Ge(lll)-c(2x8) surface [244]. The concept of lateral manipulation has further been developed to create atomic structures on semiconductor surfaces at room temperature by using sophisticated manipulation protocols [245, 246]. Room-temperature, atomic-scale manipulation has also been achieved on insulating surfaces [247, 248] however, the processes involved are more complicated and the degree of control is lower in this case. 

The nanofiller structure was studied on force-atomic microscope Nano-DST (Pacific Nanotechnology, USA) by a semicontact method in the force modulation regime. The received CNT size and polydispersity analysis was made with the aid of the analytieal disk centrifuge (CPS Instrument, Inc., USA), allowing to determine with high precision the size and distribution by sizes in range from 2 run up to 5 mcm. The nanocomposites BSR/CNT elasticity modulus was determined by nanoindentation method on apparatus Nano-test 600 (Great Britain). 

---