Indenter tips

Penetration—Indentation. Penetration and indentation tests have long been used to characterize viscoelastic materials such as asphalt, mbber, plastics, and coatings. The basic test consists of pressing an indentor of prescribed geometry against the test surface. Most instmments have an indenting tip, eg, cone, needle, or hemisphere, attached to a short rod that is held vertically. The load is controlled at some constant value, and the time of indentation is specified the size or depth of the indentation is measured. Instmments have been built which allow loads as low as 10 N with penetration depths less than mm. The entire experiment is carried out in the vacuum chamber of a scanning electron microscope with which the penetration is monitored (248). 

We test the hardness of polymers by applying an indenter to their surface with a known force and noting the depth to which the tip penetrates the sample. These tests typically fall into one of two categories. In the first, the depth of penetration is read directly from a dial on the instrument, calibrated in arbitrary hardness units. The farther the tip penetrates the sample, the lower is its hardness. The second type of test involves impressing a pyramidal indenter tip against the sample with a known force and measuring the depth to which it penetrates. In practice we measure the dimensions of the indentation and calculate the depth of penetration and compressive modulus based on the tip geometry. 

K.W. McElhaney et al Determination of indenter tip geometry and indentation contact area for depth-sensing indentation experiments. J. Mater. Res. 13, 1300-1306 (1998)... 

The H values measured along the crack zone are lower than the microhardness of the bulk. In this case the indentations were made close to the notch tip. The indenter tip was placed into the crack and the outer part of the indenter pressed on the bulk polymer. Assuming a two-phase system consisting of the crack and the bulk material, a lower H value than that of the bulk must be measured because the microhardness of the cracked portion (Hcrack) is equal to zero (eqs. (3.9) and (3.10)). 

The hardening and embrittlement of polyimides by ion implantation has been also studied (Pivin, 1994). Nanoindentation tests performed using a sharp diamond pyramid of apical angle 35° provided very quantitative depth profiles of microhardness in polyimides implanted with C, N, O, Ne or Si ions. In all cases the microhardness increased steeply when the amount of deposited energy reached the order of 20 eV atom". For energies of 200 eV atom" the polymer is transformed into an amorphous hydrocarbon and the microhardening factor saturates at a value of 13-20. However, the carbonized layer is poorly adherent, as is evidenced by reproducible discontinuities in the depth vs load curves, when the indenter tip reached the interface. 

Pellicle and tea-immersed pellicle were analyzed using nanoDMA (dynamic mechanical analysis) to see if the tannins had an effect on the viscoelasticity of the pellicle. NanoDMA is a technique used to study and characterize mechanical properties in viscoelastic materials. The method is an extension of nanoindentation testing [58, 59], An analysis of the nanoindentation load-depth curve gives the hardness (H) and reduced elastic modulus (E ), provided the area of contact, A, between the indenter tip and the sample is known [ 13]. By... 

The accuracy of the test method relies critically on the accurate determination of the area function, which varies with tip geometry. At the scale of many nanoindentation experiments, minute differences in tip geometry—such as those caused by wear induced by repeated experiments—can have a large effect on the area function. For this reason, the area function is typically measured by indenting a known material to several depths and computing the function from Equation 39.37. This experimentally determined tip function is then used to interpret further tests on unknown materials. Fused silica (or quartz) is commonly used to calibrate the tip, that is, determine the area tip function, because its material response is nearly perfectly elastic and it does not exhibit adhesion with diamond indenter tips. 

Stress distribution in indentation is largely affected by the indenter tip geometry, which is a vital factor in determining the boundary conditions for the field. The major types of indenter tips shown schematically in Figure 3 may be separated into two groups, viz. point-force (pyramidal and conical) and spherical indenters. Correspondingly, Boussinesq and Hertzian stress fields will describe point-force and spherical indentation in the case of purely elastic loading (Fig. 4). To account for possible elastic compliance of the indenter, a reduced elastic modulus Er is... 

Fig. 3. Schematic of the various types of indenter tips used in indentation testing (a) Vickers (b) Berkovich (c) Knoop (d) conical (e) Rockwell and (f) spherical.

H—Hardness. There are different types of hardness. Why Because the value of a material s hardness depends on how it is tested. The hardness of a material is its resistance to the formation of a permanent surface impression by an indenter. You will also see it defined as resistance of a material to deformation, scratching, and erosion. So the geometry of the indenter tip and the crystal orientation (and therefore the microstructure) will affect the hardness. In ceramics, there tends to be wide variations in hardness because it involves plastic deformation and cracking. Table 16.4 lists hardness values on the Mohs hardness scale, a scratch test that can be used to compare hardness of different minerals. For example, quartz has a Mohs hardness of 7, which made flint (a cryptocrystalline quartz) particularly useful in prehistoric times for shaping bone (the mineral component is apatite with hardness 5) and shell (the mineral component is calcite with hardness 3). Mohs hardness scale was not the first scratch hardness technique. As long ago as 1690, Christian Huygens, the famous astronomer, had noticed anisotropy in scratch hardness. 

Load (P) versus depth of penetration (h) curves, also called compliance curves, are the output from a nanoindentation test. The curves are obtained as load is applied to the indenter tip up to some maximum value and then back to zero. 

The Vicat softening temperature (VST) is the temperature (in °C), at which an indentation tip has penetrated 1 mm deep into the test specimen surface. The indentation tip has a circular cross-section area of 1 mm. The testing method is standardized in ISO 306. It defines four different methods by varying heating rate (50 K h and 120 K h ) and load (A 10 N and B 50 N). The most common method is the B50-method (50 N and 50 K h ). The specimens have to be plane and parallel with a thickness between 3 mm and 6.5 mm and a minimum diameter resp. edge length of 10 mm (Fig. 3.5). 

Figure 10. Distribution of normalized maximum principal stress in the vicinity of indenter tip.

The elastic moduli of the as-sintered porous LSCF cathode film samples were measured using a NanoTest nanoindentation platform (Micromaterials, UK) with a spherical diamond indenter tip of 50pm diameter. Compared with sharp indenters like Berkovich tips, benefits of using spherical tips include less sensitivity to surface condition. At least 20 measurements were conducted in different locations for each sample in order to measure the variability of the mechanical response of the sample. Prior to nanoindentation tests, the NanoTest platform was precisely calibrated using a standard sihca sample to establish the system frame compliance. 

Figure 4. Effect of surface roughness on the real contact area with the indenter tip, leading to errors in indentation results. Here the sample shown was sintered at 1000 °C...

Apparatus The indenting tip shall preferably be of hardened steel 3 mm long, of circular cross section 1.000 0.015 mm fixed at the bottom of the rod. The lower surface of the indenting tip shall be plane and perpendicular to the axis of the rod and free frnm burrs. A flat-tipped hardened steel needle with a cross-sectional area of 1.000 0.015 mm shall be used. The needle shall protrude at least 2 mm at the end of the loading rod. 

Mount the test specimen horizontally under the indenting tip of the unloaded rod. The indenting tip shall at no point be nearer than 3 mm to the edge of the test specimen. Place the specimen on the support so that it is approximately centered under the needle. The needle should not be nearer than 3 mm. 

After 5 min with the indenting tip still in position, add the weights to the load-carrying plate so that the total thrust on the test specimen is 50 1N. Apply the extra mass required to increase the load on the specimen to 10 0.2 N (loading 1) or 50 1.0 N (loading 2). 

Note the temperature at which the indenting tip has penetrated in to the test specimen hy 1 0.01 mm heyond the starting position, and record it as the Vicat softening temperature of the test specimen. Record the temperature at which the penetration depth is 1 mm. If the range of the temperatures recorded for each specimen exceeds 2°C, then record the individual temperatures and rerun the test. 

Nanoindentation is considered a simple and effective way to obtain meaningfiil values of coating hardness and Young s modulus on the micro- and nanoscales. To obtain the best possible results during nanoindentation, key requirements are sample preparation, calibration of equipment and corrections for thermal drift, initial penetration, frame compliance, indenter tip shape, and penetration depth (Field and Swain, 1993, 1995 Gan et al., 1996 Fischer-Cripps, 2002). 

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). 

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