Microhardness defined

The accuracy of microhardness testing has been questioned a wide range of values appears in the Hterature for plated deposits, especially in hardness extremes. ASTM B8.10 is involved in intedaboratory testing to define the precision and bias of the Specification B576 Microhardness of Electroplated Coatings (55,56). 

J. Homer, "Microhardness Testing of Plating Coatings Defining Precision and Bias," Inf/Tech Conf. Proc., AESF SUR/FIN, Atianta, Ga., 1992. 

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. 

Durometer hardness is defined as the measure of resistance to indentation using either a macro- or microhardness tester. To the pharmaceutical drug manufacturer, hardness is important because of its relationship to ultimate mechanical properties— particularly modulus. In general, softer compounds of the same elastomer base have better coring and reseal properties, whereas harder compounds tend to process better on high-speed filling lines. 

Since all practical methods of measuring the hardness of mbber involve measuring the resistance to indentation, hardness may be defined simply as resistance to indentation . Hardness is an expression of the elastic modulus of the mbber. More specifically, the load required to press a ball of given diameter to a given depth into the mbber is proportional to its elastic modulus. See Hardness Testing, Pusey and Jones Plastometer, Microhardness Testing. 

Microindentation hardness is currently 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 impression (Tabor, 1951). The microhardness, 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 critically dependent, as we shall show in the next chapter, on microstructural factors (crystal size and perfection, degree of crystallinity, etc.). Indentation during a microhardness test permanently deforms only a small volume element of the specimen (V 10 -10 nm ) (non-destructive test). The rest of the specimen acts as a constraint. Thus the contact stress between the indenter and the specimen is much larger than the compressive yield stress of the specimen (about a factor of 3 higher). 

Nanoindentation testing by CDR does not give values of absolute microhardness directly. This is because microhardness is usually defined as load divided by indent area projected onto the plane of the surface, and this area is not explicitly measured in nanoindentation testing. However, the data can be processed on the basis of well established assumptions (Loubet et al, 1984) to yield relatively direct information that is of value in quality control. 

Where the prime requirement is to obtain absolute values of microhardness in the sense of resistance to plastic deformation, a logical approach is to replace the optical microscope of a microindenter by an electron microscope. A nanoindentation attachment that can be used inside a SEM has been the basis of patents and is commercially available (Bangert Wagendistrel, 1985). In principle, this approach makes it possible to establish a reliable comparison between nanoindentation microhardness values and established scales of microhardness numbers, such as those defined in national standards specifications. It is necessary to overcome the difficulties of imaging small indentations with sufficient contrast, and, at the smallest depths, to correct for the deformation of the required conductive layer of soft metal (Wagendristel et al., 1987). 

The values obtained are summarized in Table 6.1. The strain variation as a function of the H value is also presented in Fig. 6.2. One can see a very well defined decrease in H (from 150 to 120 MPa) in a rather narrow deformation interval (e between 5 and 8%). Before this drop in H in the deformation range of = 0-5%, the microhardness is relatively constant at about 150 MPa, which is typical for semicrystalline PBT (Giri et al., 1997). After reaching its lowest value (H = 118 MPa) around s = 10%, H starts to increase almost linearly up... 

It is a well-known fact that the hardness of polycrystaUine structures is defined by their microstruc-ture. In order to have an increased hardness, the structure should be able to oppose formation and motion of dislocations and appearance of microcracks. This problem can be solved by several ways, in particular grain refinement, cold strain, alloying, etc. However, these methods are unacceptable for nanocrystalline objects, because alloying atoms leave the grain volume and segregate at its boundaries and dislocations are not formed at all. Therefore, the microhardness of films obtained in the ion bombardment environment is defined by process parameters and by an opportunity to transform grain boundaries of single-phase material by the second phase. This subsection will delve into consideration of the first component. 

The papers [16,18] used higher energies, which defined the increase in microhardness up to 18-30 GPa. The coatings with the finest grain of 5-8 nm [18] demonstrated the highest increase in... 

The microhardness of the ceramic samples with large fraction of YSZ (A40 and A45) being 17 GPa is close to the values of the individual 2.8YSZ. This is defined probably by the presence of YSZ matrix where the corundum grains are distributed. Fracture toughness of the ceramics containing t-YSZ was in 5 - 6 MPa m range. 

Hence, in the present chapter the Hill and Marsch equations fractal analogs are obtained, which has shown, that cross-linked epoxy polymers microhardness is defined by their structure only, characterized by its fractal dimension. Tabor s criterion is only fulfilled for Euclidean (or close to them) solids. The cross-linking degree enhancement results to loosely packed matrix loosening and corresponding reduction of cross-linked epoxy polymers microhardness. The similar results obtained for linear polyethylene and nanocomposites on its basis, filled with organoclay [16]. 

We denote it as in this appendix (H in the main text) in order to compare microhardness p and nanohardness H as extracted from nanoindentation curves. As discussed in detail, p and H may differ. Macro-, micro- and nanoindentation domains have been defined and hardness values are to be compared in the respective domain since an ISE is reported for many types of materials (Gao et al., 1999 Sangwal, 2000 Elmustafo and Stone, 2003). In the macro- and microindentation domains, the surface is generally determined after unloading by optical microscopy. We shall refer to this area as Ap (projected area defined above). Going to the nanoindentation domain, hardness is determined under load and is defined as the ratio of the force to the projected contact area Ac (Section H.l) ... 

The ratio of microhardness and yield stress for particulate-filled polymer nanocomposites is defined by this polymer nanomaterials stmctural state only. Tabor criterion is correct for Euclidean (or close to them) solids only. 

The notion was developed earlier that the microhardness of glassy solids is defined by the fluctuation free volume microvoid formation (or collapse) work, ascribed to the microvoid volume unit [1]. Such an approach corresponds to the results in the present section since, as has been shown above, the fluctuation free volume is concentrated in the loosely packed matrix of polymer structure. The relative fraction of the fluctuation free volume can be estimated according to Equation 1.33. As was to be expected, the linear correlation between the values of and calculated according to Equations... 

Thus, the fractal analogues of the Hill and Marsch equations obtained above have shown that the microhardness of crosslinked epoxy polymers is defined only by their structure, characterised by its fractal dimension. Tabor s criterion is correct for Euclidean (or close to Euclidean) solids only. The degree of increase in crosslinking results in loosening of the loosely packed matrix and to a corresponding reduction of the microhardness of crosslinked epoxy polymers [60]. 

The microhardness was measured again at room temperature using a Leitz tester supplied with a square-pyramidal diamond indentor and a stretching device. The microhardness measurement was carried out at a deformation step of e = 5%. Loads of 147 and 245 mN were employed (loading cycle of 0.1 min) to eliminate the instant elastic contribution. Ten indentations were made for each point [69]. H measurements were performed under strain up to 20% relative deformation, e, defined as e = I - Iq)/Iq where Iq and I are the starting and a given length of the sample, respectively. 

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