Nanoindentation

1 Nanoindentation In the nanoindentation technique (Fig. 5.16), with the AFM operating in the force mode the tip is brought into contact with the fiber surface and a force is applied to indent the surface of the nanofiher. The indentation is force-controlled, with a maximum load of Fmax apphed to the sample. For the technique to yield meaningful data, the tip diameter needs to be very much smaller than that of the nanofiher and all data must be 

Differential scanning calorimetry (DSC) allows the measurement of the glass transition temperature. This is needed especially when aerogels contain viscoelastic components such as organic polymers. To determine the stiffness as a function of temperature, a dynamic 

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In AFM, the relative approach of sample and tip is nonnally stopped after contact is reached. Flowever, the instrument may also be used as a nanoindenter, measuring the penetration deptli of the tip as it is pressed into the surface of the material under test. Infomiation such as the elastic modulus at a given point on the surface may be obtained in tliis way [114], altliough producing enough points to synthesize an elastic modulus image is very time consuming. 

Figure Bl.19.35. Experimental nanoindentation eurves obtained with the AFM showing the loading and unloading behaviour of (a) an elastomer and highly oriented pyrolytie graphite and (b) a gold foil. (Taken from [183]. figure 4.)...

As is true for macroscopic adhesion and mechanical testing experiments, nanoscale measurements do not a priori sense the intrinsic properties of surfaces or adhesive junctions. Instead, the measurements reflect a combination of interfacial chemistry (surface energy, covalent bonding), mechanics (elastic modulus, Poisson s ratio), and contact geometry (probe shape, radius). Furthermore, the probe/sample interaction may not only consist of elastic deformations, but may also include energy dissipation at the surface and/or in the bulk of the sample (or even within the measurement apparatus). Study of rate-dependent adhesion and mechanical properties is possible with both nanoindentation and... 

Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

In this chapter, we overview basic techniques for making nanoscale adhesion and mechanical property measurements. Both quasi-static and dynamic measurements are addressed. In Section 2 of this chapter, we overview basic AFM instrumentation and techniques, while depth-sensing nanoindentation is overviewed in Section 3. Section 4 addresses recent advances in instrumentation and techniques... 

Indentation has been used for over 100 years to determine hardness of materials [8J. For a given indenter geometry (e.g. spherical or pyramidal), hardness is determined by the ratio of the applied load to the projected area of contact, which was determined optically after indentation. For low loads and contacts with small dimensionality (e.g. when indenting thin films or composites), a new way to determine the contact size was needed. Depth-sensing nanoindentation [2] was developed to eliminate the need to visualize the indents, and resulted in the added capability of measuring properties like elastic modulus and creep. 

Fig. 9. (a) Depth-sensing nanoindenter model and (b) simple mechanical model for force controlled indentation assuming purely elastic contact mechanics. 

Like the AFM, load-displacement curves from nanoindentation can also be used to measure tip-sample adhesion. However, because the force resolution of nanoindentation is typically of order tens to hundreds of nanoNewtons, such experiments... 

Depth-sensing nanoindentation is one of the primary tools for nanomechanical mechanical properties measurements. Major advantages to this technique over AFM include (1) simultaneous measurement of force and displacement (2) perpendicular tip-sample approach and (3) well-modeled mechanics for dynamic measurements. Also, the ability to quantitatively infer contact area during force-displacement measurements provides a very useful approach to explore adhesion mechanics and models. Disadvantages relative to AFM include lower force resolution, as well as far lower spatial resolution, both from the larger tip radii employed and a lack of sample positioning and imaging capabilities provided by piezoelectric scanners. 

Fig. 15. Dynamic stiffness images of alternating layers of polyethylene (PE) of two molecular weights at (a) 105 Hz and (b) 200 Hz. The contrast is due to changes in contact compliance (1/stiffness) of the nanoindenter probe in contact with each of the two polymers. The probe-sample respon.se (1/stiffness) as a function of frequency shown in (c) is consistent with the dynamic stiffness images.

Israelachvili, J.N., Intermolecular and Surface Forces. Academic Press, San Diego, 1992. Landman, U., Luedtke, W.D., Burnham, N.A. and Colton, R.J., Atomistic mechanisms and dynamics of adhesion, nanoindentation, and fracture. Science, 248(4954), 454-461 (1990). 

Pharr, G.M., Harding, D.S. and Oliver, W.C., Measurement of fracture toughness in thin films and small volumes using nanoindentation methods. MRS, 1995, pp. 663-675. 

Asif, S.A.S., Colton, R.J. and Wahl, K.J., Nanoscale surface mechanical property measurements Force modulation techniques applied to nanoindentation. In Ovemey, R.M. and Frommer, J.E. (Eds.), Interfacial Properties on the Submicron Scale. ACS/Oxford Press, Oxford, 2001. 

Asif, S.A.S., Wahl, K.J. and Colton, R.J., Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer. Rev. Sci. Instrum., 70, 2408-2413 (1999). 

Bhushan, B., Kulkami, A.V., Bonin, W. and Wyrobek, J.T., Nanoindentation and picoin dentation measurements using a capacitive transducer system in atomic force microscopy. Philos. Mag. A Phys. Condens. Matter Struct. Defects Mech. Prop., 74(5), 1117-1128 (1996). 

Since the early 1980s, the study of mechanical properties of materials on the nanometre scale has received much attention, as these properties are size dependent. The nanoindentation and nanoscratch are the important techniques for probing mechanical properties of materials in small volumes. Indentation load-displacement data contain a wealth of information. From the load-displacement data, many mechanical properties such as hardness and elastic modulus can be determined. The nanoindenter has also been used to measure the fracture toughness and fatigue properties of ul-... 

Fig. 29—Scanning elecron micrograph of a smaii nanoindentation made with a Berkovich indenter in a 500 nm aiuminum fiim deposited on giass (from Ref. [64]).

In order to study microscale friction and wear, scientists have developed the friction force microscope (FFM), nanoindentation and nanoscratch tester which serve as excellent tools in micro tribological research [1,6-9]. In this chapter, we first compare the differences between macro and micro friction and wear, and then introduce some results of our research group on microscale friction and wear of ordered films, thin solid films, and multilayers. 

Nanoindentations were carried out by using a commercial AFM (AutoProbe CP Research, Park Scientific Instruments) equipped with a commercial capacitance transducer (TriboScope, Hysitron) with a three-sided pyramidal dia-... 

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