1. Introduction

Contact mechanical testing methods are the oldest techniques to characterize the mechanical response of materials [1]. The hardness of a material describes its resistance to the penetration of a harder indenter and correlates to its strength. Similarly, scratch hardness testing has long been used to describe the response of a material to the relative motion of a harder indenter sliding at the velocity v and under the action of a load Fn, thus enabling the study of friction and wear. According to Bowden and Tabor, friction and wear of metals are mediated either by the formation and shearing of junctions between surface asperities leading to their de-bonding or the plowing of a surface by a harder asperity leading to debris formation (see, e.g., Ref. [2]).

With the development of atomic force microscopy (AFM), the investigation of friction and wear between a smooth surface and a single asperity has become possible. This has allowed bridging the gap between macroscale experiments and the underlying tribological mechanisms that typically take place at the nm scale. At low load, single-asperity sliding friction of

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

metals has been observed to be governed by the dragging of nanoscale metallic junction giving rise to atomic stick-slip [3, 4]. The effects of surface-assembled monolayer (SAM) and oxidation on the nanotribology of Au(111) have been investigated and compared to the sliding friction behavior of an Au(111) surface [5]. It was shown that the formation of an Au neck at the Au (111)/tip interface determines the nanotribology of gold. Further, the authors have shown how the formation of such a neck can be suppressed by SAM and how the friction response of a gold surface can be switched by applying an electrochemical potential. In Refs. [6, 7], friction between Au islands and graphite was studied. AF<sup>2</sup>=<sup>3</sup> <sup>n</sup> dependence of the friction force on gold islands measured in ambient conditions was observed, where Fn is the normal force [6]. These contrasts with results in Refs. [3, 4], where almost no frictional energy dissipation was measured. In this load regime also, the authors recently showed how the shear strength of such junctions can be tuned by changing the metallurgical affinity between the contact materials [7]. Also, nanoscale wear experiments by AFM demonstrated the determinant role of plastic deformation mechanisms [8, 9]. AFM indentation has proven to be a capable experimental method to resolve the atomistic mechanisms of plastic deformation [10–14]. For example, this method has been applied to study single dislocation activation in KBr(100) single crystals [10], Cu(100) [11], and Au(111) [12–14]. There, atomistic plasticity events were observed in the shape of pop-ins, with lengths in the range of 1 Å. More recently, AFM indentation has been combined with noncontact AFM to quantitatively determine the hardness and the fundamental mechanisms of plastic deformation of Au(111) [14], and Pt(111), and Pt-based metallic glass surfaces [15].

Fn <sup>¼</sup> CnSVAB and Fl <sup>¼</sup> <sup>3</sup>

the cantilever length.

beam theory, Cn <sup>¼</sup> Ewt<sup>3</sup>

<sup>4</sup>L<sup>3</sup> and Cl <sup>¼</sup> Gwt<sup>3</sup>

typical diamond-coated Si single-crystalline AFM cantilever and its tip.

3h<sup>2</sup>

from the first bending resonance frequency of the cantilever <sup>f</sup>0, with <sup>t</sup> <sup>¼</sup> <sup>2</sup> ffiffiffiffi

sample.

where S is the sensitivity of the photodiode, which we assume to be isotropic; VAB and VCD are the sum voltages for the photodiode segments indicated in the subscripts; Cn and Cl are, respectively, the bending and torsion stiffnesses of the cantilever; h is the tip height; and L is

The setups illustrated in Figure 1 mostly differ in the arrangement of their piezoelectric scanners. For the measurements in UHV, a sample tube xyz-scanner was used to both scan the sample surface and control the height of the cantilever or the interaction force between tip and sample. In the setup used for measurements in ambient conditions, a linear xy nanopositioning stage was used to scan the sample surface, while a separate linear z-scanner was used to control the height of the cantilever or the interaction forces between tip and

In this work, the cantilever stiffnesses were determined either according to the geometrical beam theory [17] or following the thermal noise analysis [18]. According to the geometrical

the width of the cantilever, and t its thickness. The length and the width of the cantilever can be measured by means of optical or electron microscopy. The thickness is usually determined

is the mass density. Alternatively, the normal stiffness can be determined from the mean square

Figure 1. Experimental setups: instrumental setup used in (a) UHV and (b) ambient conditions; (c) TEM images of a

<sup>L</sup>, where E is Young's modulus, G is the shear modulus, w is

<sup>12</sup> <sup>p</sup> <sup>π</sup> 1:8752

ffiffi r E q <sup>f</sup> <sup>0</sup>L<sup>2</sup>

, where r

2 Cl h

Experimental Studies of Nanometer-Scaled Single-Asperity Contacts with Metal Surfaces

<sup>L</sup> SVCD (1)

http://dx.doi.org/10.5772/intechopen.72990

29

In this chapter, we describe three experimental methods based on atomic force microscopy and corresponding methods for statistical data analysis to determine the hardness and the mechanisms governing wear and friction of metallic surfaces.
