PSI - Issue 32

R.I. Izyumov et al. / Procedia Structural Integrity 32 (2021) 87–92 Author name / Structural Integrity Procedia 00 (2019) 000–000

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of the material, magnetic or electrical properties, imaging of subsurface layers not only in air, but also in a liquid medium, determination of many other characteristics (local elastic moduli, parameters of hardening, creep, residual stresses, appearance of dislocations, phase transitions and other phenomena [Reggente 2015, Gadelrab 2012, Zhang 2017]). These technologies open up new horizons for understanding the structural, mechanical and physical processes that occur in a material at the nanoscale. During the development of AFM, many modes and applications of this technique were developed [Garcia 2002, Garcia 2020]. The most common mode is nanomechanical mapping [Herruzo 2014, Magerle 2020, Stühn]. Dynamic modes of materials research are of great interest [Haviland 2017]. One of the most effective modes is the Force modulation method (FMM) [Maivald 1991]. Nowadays, it can be used to obtain maps of the mechanical properties of a material on nanoscale with high resolution [Killgore 2011]. The obtained AFM experimental data represent the dependences between the coordinates of the scanning points, the indentation force (cantilever bending), and the depth of the probe penetration into the material under study. If the material is structurally inhomogeneous, then for each point on the scanned surface we will obtain dependences F ( u ), the form of which will be different. These differences are due to the structure of the material: the presence or absence of inclusions near the point of contact, the shape and orientation of the particles. In the dynamic AFM mode, the presence of filler particles also affects the phase portrait of the surface (Fig. 1). The phase shift can depend on many characteristics of the surface (adhesive properties, relief geometry, mechanical properties, surface static charge). However, the surface of the considering material is homogeneous and sufficiently smooth. Thus, the result (Fig. 1) is a consequence of the inhomogeneity of the subsurface layer (due to the presence of filler particles). The effect of particles on the phase shift can be explained by the fact that the particles under the surface make an additional contribution to the reaction of the material to the penetration, so the probe penetrates into the sample to a lesser depth. Consequently, the contact area between the probe and the material becomes smaller, the effects of adhesion reduce, and the phase shift between the oscillations of the probe and the base of the cantilever reduces too. Correct interpretation of the obtained experimental data is impossible without the use of mathematical modeling. This technique is quite often used to study the influence of particles, filler fibers of soft composite materials, or any rigid internal structures on the response of an AFM probe. FE modeling is also a very useful tool in research aimed at investigating the features of atomic force microscopy and developing new experimental techniques [Li 2018, Tang 2018, Tang 2019, Wang 2019, Zhang 2018, Zheng 2016].

Fig. 1. Phase portrait superimposed on the relief of a nanocomposite (butadiene-methylstyrene rubber filled with carbon black and single-walled carbon nanotubes). The size of the area is 1x1 microns, resolution is 500x500 points. The scale indicates the range for the phase shift in degrees.