Indentation Process

Hardness is determined by hardness tests which involve the measurement of a material s resistance to surface penetration by an indentor with a force applied to it The indentation process occurs by plastic deformation of metals and alloys. Hardness is therefore inherently related to plastic flow resistance of these materials. Brittle materials, such as glass and ceramics at room temperature, can also be subjected to hardness testing by indentation. This implies that these materials are capable of plastic flow, at least at the microscopic level. However, hardness testing of brittle materials is frequently accompanied by unicrack formation, and this fact makes the relationship between hardness and flow strength less direct than it is for metals. 

The indentation process is driven by the applied load, and resisted by two principal factors the resistance of the specimen to plastic deformation (and elastic deformation) plus the frictional resistance at the indenter/specimen interface. The ratio of these resistances changes with the size of the indentation because the plastic resistance is proportional to the volume of the indentation, while the frictional resistance is proportional to the surface area of the indentation. Therefore, the ratio varies as the reciprocal indentation size. This interpretation has been tested and found to be valid by Bystrzycki and Varin (1993). 

Lawn et al. (1975, 1978), and Lawn and Marshall (1978) distinguish two types of indenter whose action on the tested surface differs significantly (1) a blunt indenter (e.g., a hard ball) distinguished by an ideal elastic contact, so that the crack initiation is controlled by previously present defects (usually on the sample surface), and (2) a sharp indenter (e.g., a cone or pyramid) distinguished by partially plastic contact, so that the original defects start to grow as the result of the indentation process itself. In practice, the contact situations can therefore be seen as intermediate between the two cases. Within this area all typical indenters used for hardness measurement are contained. 

Only in the last three years have some researchers begun to pay their attention to the structured surface and to parameters influencing the lithography with force-displacement curves (FDI) [266-268]. The big advantage of FDI is the possibility of gaining knowledge about the whole indentation process during FDI, the force and the indentation are known at every point, and not only stiffness and hardness, but also other important properties such as density, elasto-plastic behaviour, adhesion, time behaviour, etc. can be measured and calculated. 

Since the force-displacement curve contains information about the whole indentation process, the elastic deformation of the sample can be measured and used to calculate the stiffness S=dFldh at h=hmax, where F is the force and h is the indentation. As already explained in Sect. 3.1.1., in order to relate the stiffness to the Young s modulus, it is necessary to make assumptions about the contact area. The depth of the permanent indentation (plastic deformation), i.e. the depth DFdi shown in Fig. 26b, and the maximum indentation (sum of the plastic and of the elastic deformation) can be used to calculate a parameter that describes the relative weight of the elastic and of the plastic response. 

The mechanism of the indentation process has been clearly defined by Tabor (1947). When a ball presses on a metal surface, the material deforms elastically. As the load increases, the stresses soon exceed the elastic limit and plastic flow starts. By increasing the load still further the material directly beneath the penetrator becomes completely plastic. On release of the load there is an amount of elastic recovery. 

Figure 2.35. Examples of indentation processes to determine surface hardness. Shown are (a) Vickers indentation on a SiC-BN composite, (b) atomic force microscope images of the nanoindentation of a silver nanowire, and (c) height profile and load-displacement curve for an indent on the nanowire. Reproduced with permission fromNanoLett. 2003, 3(11), 1495. Copyright 2003 American Chemical Society.

Baer et al (1961) later considered the indentation process in which large loads are placed on a spherical penetrator and the material beneath the indenter becomes permanently displaced. In addition, he defined the recovery process which occurs immediately after the load is released analysing it in terms of the elastic concepts developed by Hertz (Love, 1927). 

As pointed out above, the semicrystalline polymer can be considered as a two-phase composite of amorphous regions sandwiched between hard crystalline lamellae (Fig. 4.2(a)). Crystal lamellae ( c) are normally 10-25 nm thick and have transverse dimensions of 0.1-1 pm while the amorphous layer thickness, a, is 5-10 nm. As mentioned in the previous section, melt-crystallized polymers generally exhibit a spherulitic morphology in which ribbon-like lamellae are arranged radially in the polycrystalline aggregate (Bassett, 1981). Since the indentation process involves plastic yielding under the stress field of the indenter, microhardness is correlated to the modes of deformation of the semicrystalline polymers (see Chapter 2). These... 

Nonequilibrium molecular dynamics studies of the indentation process show a transition to the amorphous phase in a region a few atomic layers thick surrounding the lateral faces of the indentor [42], as has been suggested by experimental results [43]. This possibility has also been suggested by modeling of the crystalline-amorphous interface [40]... 

FIGURE 1.10 The schematic image of a Berkovich indentor and nano-indentation process. 

FIGURE 1.11 The dependence of density fluctuation (y) on volume of deformed nanoparticles in the nano-indentation process material in logarithmic coordinates for... 

In situ optical microscopy observation of the indentation process demonstrated that diamond becomes nontransparent to visible light in the loaded zone [196] (Fig. 40 white regions show the reflection of light). This is consistent with a narrowing optical window in a diamond anvil that was observed at much higher... 

Fig. 8.14c, this nano-indentation process prior to solvent exposure provides a fine control of the spatial layout of the wrinkled domains. Furthermore, since the wrinkle orientation is determined by the diffusion front, it becomes possible to generate tailor-made wrinkled patterns by tuning the geometry of the carving. Wrinkles initiated at the comers and tips of the engraved area (Fig. 8.14b) exhibit radial orientation while parallel wrinkles develop from the linear parts. 

Hence, the stated above results have shown, that elasticity modulus change at nanoindentation for particulate-filled elastomeric nanocomposites is due to a number of causes, which can be elucidated within the frameworks of an harmonicity conception and density fluctuation theory. Application of the first from the indicated conceptions assumes, that in nanocomposites during nano indentation process local strain is realized, affecting polymer matrix only, and the transition to macrosystems means nanocomposite deformation as homogeneous system. The second from the mentioned conceptions has shown, that nano- and micro systems differ by density fluctuation absence in the first and availability of ones in the second. The last circumstance assumes that for the considered nanocomposites density fluctuations take into account nanofiller and polymer matrix density difference. The transition from nano to Microsystems is realized in the case, when the deformed material volume exceeds nanofiller particles aggregate and surrounding it layers of polymer matrix combined volume [49]. 

Influence of the friction and the geometry in indentation processes MARIN Marta, CAMACHO Ana - and SEBASTIAN Miguel Anger d... 

The technological advances and the recent researches provide new alternatives for manufacturing processes based on conventional processes [1], This paper studies the indentation process from the point of view of manufacturing. There are a variety of studies on the indentation process. However in most of them, indentation is used to obtain mechanical properties of the material such as hardness [2, 3, 4], In the present study, the indentation process is analyzed as a unitary compression process [5], This unitary operation studies the influence of technological parameters such as the friction and the workpiece geometry [6],... 

In this study, it has been employed a ductile material. The aluminum alloy AA 6082 has been chosen due to its good mechanical properties, its light weight and its capacity of being recycled [7].The indentation process has been studied under axial symmetric conditions. 

The Finite Element Method (FEM) is used to analyze the indentation process. All cases have been analyzed by the Finite Element Method (FEM) using a general purpose code of implicit methodology (ABAQUS/Standard) [8]. 

The indentation process is studied under axisymmetric conditions. Both geometries of the punch and the workpiece have got circular section and the application of load through the punch is axial kind. Initially, the punch is in contact with the top-surface of the workpiece. In this situation, the punch does not apply forces on the workpiece. Below, the punch compresses the workpiece until the punch displaces 2 mm in all cases. Fig. 1 has represented the axisimmetric model where the stroke of the punch is named with the variable p. A cylindrical geometry of the punch is considered, where B is the diameter. The workpiece dimensions, diameter and height, have been defined by the variables d and h, respectively. 

In general terms, the higher the friction coefficient, the higher the forces required. However, this increase is not significant compared to the change of the friction coefficient. Due to this, the forces to carry out indentation process under axisymmetric conditions do not seem to depend on the friction coefficient. 

Influence of shape factor. In Fig. 5 the obtained forces versus the shape factor are represented. In the figure it is observed that with a constant height of the workpiece, the higher the width, the higher the forces to carry out the indentation process. Nevertheless, if the width of the workpiece remains constant and the height increases, the obtained forces are almost the same. Therefore, in the indentation process under axisymmetric conditions it has more influence a variation of width that a change of the workpiece height. 

In this work technological parameters such as the Coulomb friction and the geometry of the workpiece (shape factor) in indentation processes have been studied. To analyze the influence of them, it has been obtained the forces to carry out the indentation process and the contact pressures between the punch and workpiece. 

The indentation process has been analyzed by the finite element method. It has been used the implicit methodology. The studied models have been analyzed under axisymmetric conditions. It has been demonstrated that apparently the forces do not depend on the friction between the contact surfaces of the punch and workpiece. The forces in axisymmetric models are influenced by the width of the workpiece while the height is not so relevant. 

In Fig. 3.14, the changes in several ceramics are shown at the same fluence and at an irradiation temperature of 300 K. The radiation hardening at stage I in AI2O3 is attributed to both plastic and elastic hardening. This interpretation by Izumi et al. [11] is based on the dissipation of the elastic and plastic energies. We and Wp, respectively during the indentation process. 

The material removal mechanism of the CMP process was relatively well explained by the previous scientists. The material removal mechanism of dielectric CMP is further well explained by Cook in his paper pubhshed in 1990 [7]. It was explained that the rate of mass transportation during glass pohshing is determined by five factors the rate of water diffusion into the glass surface, the dissolution of the glass under the applied load, the adsorption rate of the dissolved material onto the abrasive surface, the redeposition of the dissolved material onto the surface of the work piece, and the aqueous corrosion between particle impacts. Water diffuses into sUoxane bonding (Si—O—Si) and the diffusion rate is controlled by multiple process conditions such as pressure or temperature. This hydrated oxide surface is removed by an abrasion process. The indentation process by each abrasive was modeled by Hertzian contact and their contact stress was calculated from the theory of elasticity. 

Figure 2 shows the contact geometry for a pyramid indenter at zero load, at maximum load, and after unloading. The material under the indenter consists of a zone of plastic deformation (a few times the penetration depth distance) surrounded by a larger outer zone of elastic deformation. Several effects can be distinguished during the indentation process ... 

ThGOry of Sliding Contact. Based on Hertzian contact model, Hamilton and Goodman (11), which was later elaborated by Hamilton (12), presented the scratch process as a combination of an indentation process and a sliding process. Figure 3 shows the schematic of the scratch process in this model. At the contact surface, where z = 0,... 

The nature of molecular dynamics simulations allows for the quantification of the number of defects formed and the determination of their exact location. These simulations show that gauche defects are formed as a result of the indentation process, as predicted by Salmeron and coworkers (4). Due to the geometry of the nanotube, these defects are localized to the region below and surrounding the tube. In addition, due to the small contact area of the nanotube used in these simulations, the number of defects formed is a function of penetration depth into the monolayer (39). Because the flexible nanotube compresses slightly as it interacts with the C13 monolayer, it does not penetrate the monolayer as deeply as a rigid nanotube. As a result, the flexible tube generates fewer defects as it indents the monolayer. 

Figure 1.7. Atomistic representation of hardness indentation process.

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