Microstructure indentation

In another example, amorphization by dislocation accumulation in shear bands in a microcrystalline c-Y2Si207 was investigated by TEM in the vicinity of the resulting microstructural indent obtained by indentation deformation at RT. Figures 3.84 and 3.85 are illustrations of pile-ups in c-Y2Si207-... 

Visually, the sites resemble mechanically induced gouges or indentions in the tube wall. However, examinations of the microstructure at these sites revealed no distortion of the metal, which would certainly occur had the indentions been mechanically induced. The erosive character of the highly localized turbulent flow was the predominant aspect responsible for the metal loss, there being little or perhaps no contribution from corrosion of the metal. 

The present review shows how the microhardness technique can be used to elucidate the dependence of a variety of local deformational processes upon polymer texture and morphology. Microhardness is a rather elusive quantity, that is really a combination of other mechanical properties. It is most suitably defined in terms of the pyramid indentation test. Hardness is primarily taken as a measure of the irreversible deformation mechanisms which characterize a polymeric material, though it also involves elastic and time dependent effects which depend on microstructural details. In isotropic lamellar polymers a hardness depression from ideal values, due to the finite crystal thickness, occurs. The interlamellar non-crystalline layer introduces an additional weak component which contributes further to a lowering of the hardness value. Annealing effects and chemical etching are shown to produce, on the contrary, a significant hardening of the material. The prevalent mechanisms for plastic deformation are proposed. Anisotropy behaviour for several oriented materials is critically discussed. 

Microindentation hardness normally is measured by static penetration of the specimen with a standard indenter at a known force. After loading with a sharp indenter a residual surface impression is left on the flat test specimen. An adequate measure of the material hardness may be computed by dividing the peak contact load, P, by the projected area of impression1. The hardness, so defined, may be considered as an indicator of the irreversible deformation processes which characterize the material. The strain boundaries for plastic deformation, below the indenter are sensibly dependent, as we shall show below, on microstructural factors (crystal size and perfection, degree of crystallinity, etc). Indentation during a hardness test deforms only a small volumen element of the specimen (V 1011 nm3) (non destructive test). The rest acts as a constraint. Thus the contact stress between the indenter and the specimen is much greater than the compressive yield stress of the specimen (a factor of 3 higher). 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

It follows from the above that deviations from parallel variation in abrasiveness and hardness are the outcome of the inaccuracy of measurement, increasing with hardness. Attention should be paid in particular to the microstructure of the material under test in the area of the indentation, and the degree of brittleness should be estimated on morphological analysis... 

The phase composition from the surface to the interior of the samples was determined by X-ray diffractometry (XRD) through successive grinding of the surface at 100 pm intervals. The microstructural characterization of the sintered specimens was achieved by scanning electron microscopy (SEM) in backscattered mode. The hardness change from the surface to the interior of the sample was measured by the Vickers indentation method at 19.6 N load. 

In summary, it is clear that there are substantial effects that vary systematically with the wavelength of the multilayer due both to internal stresses and the microstructure of the coatings. It has also been seen that deformation can occur not just by dislocation flow, as the initial analyses have assumed, but by mechanisms such as lattice rotations and shear along column boundaries. In addition, the use of indentation complicates the deformation field, so that the assumption that equal strains in both layers are required need not be correct. These effects all influence the hardness but have not so far been included in analyses. 

It can be seen that ceramic multilayer structures have been produced with increments of the hardness of up to 60 GPa, increasing the hardness by up to a factor of almost 3. Initial work in this area has developed a number of ideas, such as the effect of modulus mismatch, which in some cases give good agreement with the models suggested but in many others do not. It is suggested that at least some of this discrepancy can be accounted for by differences in the microstructure and residual stress-state of the film, both of which are often poorly characterized. Furthermore there is very little direct evidence about how these structures deform and in particular about how different layers must be strained in order to accommodate the indenter when it is pressed into the sample. Further advances in this area will require the greater use of numerical techniques to analyse the complex stress and strain behaviour under the indentation, coupled with the use of recently developed techniques that allow the localized deformation behaviour to be observed in detail. 

To facilitate mixing, mixers based on chaotic advection in special microstructures were constructed. For instance, in a PDMS fluidic mixer, small chevron-shaped indentations, which were not centered, were constructed in the channel. Such an arrangement forced the fluid to recirculate in order to achieve a mixing effect [184],... 

Fig. 2.9. Microstructure of the stainless steel-aluminium transition zone.197 Temperature 700°C, dipping time 3000 s, melt A1 + 2.5 mass % Fe and corresponding amounts of other elements from the steel. Microhardness indentations were made at a load of 0.196 N (20 g). 

Polymers showing a viscoelastic behaviour occupy the intermediate range. Out of all the existing hardness tests, the pyramid indenters are best suited for research on small specimens and microstructurally inhomogeneous samples (Tabor, 1951). Pyramid indenters provide, in addition, a contact pressure which is nearly independent of indent size and are less affected by elastic release than other indenters. 

Cracks were observed on the top surface and the section surface of the disk with an optical microscope. The microstructure was observed by SEM. The surfaces were polished with diamond paste. The Vickers indentation technique was used to evaluate the direction of residual stress. The residual stresses were calculated by an analytical technique on the assumption of elastic condition [2]. For electrical measurement, Pt electrode was formed on the disk surfaces with Pt paste. The electrical resistivity was measured by two-probe method. 

Figure 7.55 a Schematic of (A) different indent location and microstructural characteristics and (B) dense and porous areas of the top surface and the cross-section, b SEM... 

Bulk density of specimens was measured by the Archimedes principle. The Vickers hardness as well as indentation fracture toughness were determined at room temperature using a Vickers diamond indenter at a 10 kg load for 10 s. Microstructure observation was performed under a transmission electron microscope (TEM JEOL, Tokyo, Japan JEM-2010/200CX/21 OOF). Optical transmissions of the translucent samples in wavelength of 2.5-6.5pm were measured by FTIR spectrometer (EQU1NOX55, Bruker, Billerica, MA). 

Note that while the average hardness of the stir zone was nearly identical to that of the base metal, the point-to-point variation in hardness was much smaller for the stir zone region than for the base metal. The smaller variations of the stir zone apparently derived from the greater local uniformity of the microstructure relative to the base metal. For example, indents in the base metal may have encountered different amounts of a and p phase depending on location (to give different hardness values), while indents in the stir zone always sampled the same amounts of each phase. 

Flardness tests are widely used as a non-destructive method of estimating the yield stress of metal products, to check whether heat or surface treatments have been carried out correctly. The test is less common for plastics, partly because such treatments are not used, and partly because viscoelasticity makes the indentation size decrease with time. Recently, nano-indentation has been used to examine microstructural variation in polymers. This section considers the case where the indentation depth is much smaller than the product thickness, whereas Section 8.2.6 considers the case of the indenter penetrating the product. 

Raman microspectroscopy studies indicate that the spectra from hardness impressions in SiC and those from the pristine surface outside the indentation area are significantly different [4, 134], This is illustrated in Figure 48 for a polycrystalline chemical-vapor deposited (CVD) 3C SiC film. The results for a single crystal 2H poly type of SiC are essentially the same [134], except for the extra line at 770 cm" in the Raman spectrum of pristine 2H SiC, related to the splitting of the TO(T) modes in hexagonal 2H as compared to the cubic 3C SiC [226]. This indicates that the deformation mechanism during indentation of SiC is independent of its microstructure prior to loading. Comparison of a typical spectrum... 

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. 

The repeated measurement of the size of indents, and the interpretation of indent geometry for the purposes of calculation, may be tedious, and operator bias is almost unavoidable. The edge of the impression is not always well defined, and misleading edge effects may be associated with anisotropic plasticity or plastic recovery. Faceted and elongated grains, or other microstructural features, together with the limitations of contrast and resolution in the optical microscope, complicate the interpretation, while the shape of the indent may differ in different materials so-called pin-cushion or barreled indents, associated with different constitutive relations and frictional shear on the faces of the indentor in contact with the plastic zone [3]. Mismeasurement of indent size is a major source of scatter in the experimental data and the relative errors in the results of different operators. 

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