Tip indentation

Using lithography in contact mode the surface is modified by increasing the apphed normal load during the scan. The tip indents the sample and scratches the surface. This lithographic method is called also static plowing. 

Dynamic plowing lithography (DPL), i.e. the lithography technique in tapping mode in which the force between tip and sample is increased by suddenly increasing the amplitude of the cantilever oscillations, has been developed by Klehn and coworkers [257-260]. The topography of the sample is acquired with a normal, small amplitude of the cantilever oscillations. When the amplitude is increased, the feedback makes the sample approach to the tip, in order to keep constant the oscillation amplitude, and the tip indents the sample. 

Kourtesis G, Renwick GM, Fischer-Cripps AC and Swain MV (1997) Mechanical property characterization of a number of polymers using uniaxial compression and spherical tipped indentation tests. J Mater Sci 32 4493-500. 

In many cases, because of the large adhesion between the sample and the tip, the effective force that applies at the tip sample contact, ranges between the nanoNewton to hundredth nanoNewton. As the contact area between the tip and a soft sample can be of the order of 100 nm, such an applied effective load leads to a very large pressure on the sample. In other words, for a sample softer than the tip, indentation of the tip in the surface will occur. 

Che Cheng, L., Xia, X., Scriven, L.E., Gerberich, W.W. Spherical-tip indentation of viscoelastic material. Mech. Mater. 37 (2005) 213-226. 

Swain, M. V. Mencik, J. Mechanical property characterization of thin films using spherical tipped indenters. Thin Solid Films 253, 204-211, doi 10.1016/0040-... 

Special attention is required when selecting the correct indenter tip. Sharp indenters such as the Berkovich tip indenter have been used by most researchers to measure the hardness and Young s modulus. However, the assumption of the transition from elastic to plastic behavior of the material is not permissible with a sharp-tipped indenter because these indenters create a nominally constant plastic strain impression. With a spherical tip, on the other hand, the depth of penetration increases as the contact stress increases therefore, the response of the elastic to plastic transition and the contact stress—strain property of a material can be determined (He and Swain, 2007). 

Figure 7. Tip indentation, a) 2.41 x 2.41 image of a whole formed in the film by...

ELASTO-PLASTIC BEHAVIOR OF GLASSY CARBON AND SILICA GLASS BY NANO-INDENTATION WITH SPHERICAL TIPPED INDENTER... 

Field and Swain have studied elasto-plastic response on a number of brittle materials, including silicate glass, silicon single crystal and single crystalline sapphire, by the indentation with small micron sized spherical tipped indenters. The analysis of indentation stress-strain curve was performed from the indentation on... 

Field and Swain found that the indentation force-displacement behavior for GC exhibited almost complete recovery with a significant hysteresis between loading and unloading when indented with 3 and 10 pm radius spherical tipped indenters. And also we have studied on elasto-plastic deformation of silica glass and glassy carbons with different type of indenters ... 

In the present work, indentation of GCs heat-treated at different temperatures was carried out using a spherical tipped indenter. For comparison, fused silica glass was also used. Quantitative evaluation of elasticity and plasticity of elasto-plastic deformation by the spherical tipped indentation on GC and silica was also attempted. The dependence of hysteresis and elasto-plastic deformation of GCs on their crystal structure are discussed from stress-strain curve. 

Figure 1. Load penetration curve of fused silica glass with 3 pm radius spherical tipped indenter.

The effective strain associated with a spherical tipped indenter may be written as the ratio of radius of contact circle to the radius of indenter, ajR ... 

Table 2. Elastic modulus, yield stress, yield strain, constants of power law fitting and indentation elasticity index obtained from stress strain curves with 3 xm radius spherical tipped indenter on GCs and silica glass.

The value of k, which is the stress at strain a/R= 1, corresponds to hardness for spherical tipped indenter, although the extrapolation of the power law fitting to such strains will invariably introduce errors into the present estimation of the hardness. 

As shown in the present work, elastic and elasto-plastic deformations derived by spherical tipped indentation on glassy carbons depended strongly on their crystal structure. The elastic modulus and the yield stress of glassy carbons decreased with the elevation of HTT of glassy carbons. 

M. V. Swain, J. Menak, Mechanical Property Characterization of Thin Films Lfsing Spherical Tipped Indenters, Thin Solid Films, 253 204 211 (1994). 

For all samples the penetration depth is determined at 27 C and 67 C at a given load of 20 nN. PS2 and PS6 at 27 °C exhibited similar values of about 2.5 nm. For the PSIOO sample, the tip indentation is 3.5 nm. Assuming a spherical tip with a radius of 20 nm, the difference in penetration depth is equal to a difference in contact area of about 30 %. Comparable penetration depths of 3 to 3.5 nm have been found for all samples upon heating to 67 °C. The change in contact radius can influence the measured adhesive force and must be taken into account for the following discussion. 

The area-depth function depends on the shape of the indenter used, and there are standard relationships for specific indenters. For example, a Berkovich indenter (typical of a diamond tip indenter in many AFM systems) has A = 24.5hc, while for a spherical indenter, A = Tv(2Rhc + h/), where R is the indenter radius. There are corrections made for the depth, h, to become h to account for the actual contact area because A is less than what would be calculated for h (see Figure 33.1). The following equation finds h. ... 

Most force curves can be fit well by the above equations, especially since tip-indentation never exceeds tens of nan-ometers. However, in the case where indentation probes beyond the shell layer of the microgel, a two-section model can be utilized in which the load is carried by two layers that experience indentation of a conical tip ... 

When a sharp tip indents into a network, whether the con-tinurrm mechanics can hold is always a concern. The PDMS samples studied in this work have highly coiled chains between networks and can hold their continuum mechanical property rmder large deformation. However, this may not have necessarily remained trae rrsing the indentation method, in which high loading forces can break stress-strain linearity or fracture the sample. 

The most common measure of hardness is the distance a steel ball penetrates into the material under a specified load. A spring-loaded, ball-tipped indenter may be used so that the stress is not a linear function of penetration. Since the measurement is basically a compressive modulus, one expects stiff materials to be hard and flexible materials to be soft. In choosing a material for a gasket, hardness often is the only specification listed. 

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