Nanoindentation, measuring techniques

Li, X., and B. Bhushan, A review of nanoindentation continuous stiffness measurement technique and its applications. Mater. Char., 48, 11-36 (2002). 

Flaws dominate the mechanical properties of ceramics. They determine how we test them and how we design components from them. Flaws are also the reason why ceramics are stronger in compression than tension. In this chapter we described the methods used to measure mechanical properties of ceramics. The important ones are bend testing, compression testing, and indentation. To determine the mechanical properties of small volumes we use nanoindentation. This technique is especially important for thin hlms, surfaces, and nanomaterials. An understanding of statistics is particularly important when using ceramics in load-bearing applications. The Weibull approach is the one most widely used for ceramics. 

Mechanical and thermal characterization of ultra low k dielecteics is very similar to the characterization of dense low k dielectrics however the introduction of porosity requires the development of new characterization techniques in order to understand the pore structure. The dielectric constant, dielectric breakdown and coefficient of diermal expansion can be measured using the same techniques used for dense low k electrics. Modulus and hardness can also be measured by die same techniques, however, if using nanoindentation, measurements firom porous dielectrics may have larger substrate contributions at equivalent film thicknesses. Therefore, the modulus values... 

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... 

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. 

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. 

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-... 

Wang, S.W., Kahn, A, Sands, R. and Vasudevan, A.K. (1992). A novel nanoindenter technique for measuring fiber-matrix interfacial strength in composites. J. Mater. Sci. Lett. 11, 739-741. 

One of the most popular techniques used for determining the hardness of a material is the Mohs scale that consists of a qualitative but an arbitrary hardness index scheme ranging from extremely soft materials (value of 1 Moh) to very hard materials such as diamond (10 Moh). Other techniques that are often employed for measuring hardness of substances are developed by Rockwell [72], Brinell [72], Knoop, and Vickers [73]. Over the years, more quantitative methods such as nanoindentation [74] have been developed. This technique applies a small and a controllable load on to the substrate with a probe. The depth of penetration along with a known geometry of the probe provides an indirect way to measure the area of contact at full penetration, which is then used to determine the hardness. The hardness is determined by the ratio of the total force to the contact area. Table 7.2 lists the bulk hardness of different materials, metal films, and abrasive particles, in both Moh and microhardness scales [75]. 

Nanoindentation techniques were used to determine the hardness of Cu, Ta W metal discs and thin films on silicon substrates as a function of load or indentation depth. Cu films exposed to oxidizing solutions containing H2O2 exhibited a higher hardness at the surface while no such change was observed for W exposed to ferric nitrate. The implication of these measurements and their relationship to chemical-mechanical polishing rates are discussed. 

Hardness measurements of as-deposited Cu film and those exposed to H2O2 containing solution, using nanoindentation techniques reveal a harder surface film in the treated copper films. W films were harder but their hardness did not change after exposure to ferric nitrate. Ta films had the highest hardness. Thin films of W Ta were found to be harder than their corresponding metallic discs. However, the relation between the relative polish rates of these materials and their hardness values remains unresolved. 

Asif SAS, Wahl KJ, Colton RJ Nanoindentation and contact stiffness measurement using force modulation with a capacitive load-displacement transducer. Rev Sci Instrum 1999 70 2408-2413. Loubet JL, Oliver WC, Lucas BN Measurement of the loss tangent of low-density polyethylene with a nanoindentation technique. J Mater Res 2000 15 1195-1198. 

To measure the hardness and elastic modulus of thin films while avoiding the influence of the substrate, peak indentation depth cannot exceed about 30% of the film thickness.Because commercial nanoindenters can make a minimum penetration depth of 10-15 nm, hardness and elastic modulus of films thinner than 30 nm cannot be measured. Clearly, new techniques for fabricating sharper indenters and new nanoindentation theories are needed to extend this technique. For film thicknesses less than 30 nm, nanoscratch tests are widely accepted to evaluate the mechanical properties (discussed later). Alternatively, assuming the hardness and elastic modulus of a film do not change with thickness, thicker films can be used. 

Modern hardware and testing protocols are capable of superimposing a small oscillatory component to the apply load. This effectively introduces a series of unloading cycles that can be used to extract the modulus this technique is commonly referred to as the dynamic contact method (DCM) and is a standard option in commercial nanoindentation systems. The result is that one can measure... 

The technique has led to various insights in cell mechanics. It has been shown that cells are able to adjust the strength of their traction forces in response to physical cues like rigidity or adhesivity of their environment. - " How a cell responds to its physical environment with its traction forces has been implicated as a hallmark of cancer. The technique has been used in combination with nanoscale manipulations like magnetic beads, laser nanoscissors, and nanoindentation to examine the mechanics of the cytoskeleton to verify different theories about its solidity or fluidity. Overall, traction force microscopy is a relatively popular technique for measuring traction forces because the polyacrylamide gels are able to be fabricated with equipment commonly available in most laboratories. 

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