Microhardness anisotropy

Next we would like to mention the mechanical properties of these compounds. The microhardness anisotropy of a single crystal induces a slip system because the hardness maxima and... 

Figure 7 Vickers microhardness anisotropy of BP (100) wafers. (From Ref 47.)...

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. 

Fig. 19a. Microhardness values parallel ( ) and perpendicular (X) to the orientation axis as a function of draw ratio, for PE drawn at different temperatures 60 °C (O), 100 °C ( ) 120 °C (A) 130 °C ( )12 b Microindentation anisotropy of the above drawn samples vs. draw ratio...

A. Sasaki and M. Iwata, Anisotropy of Microhardness in Anthracene Single Crystals, Phys. Stat. Sol. A, 85, K105 (1984). 

The effects involved, from an optical point of view, are an increase of reflectivity, and both opacity and anisotropy. These effects, and others—e.g., insolubility or microhardness—appear to be related to an increase in chemical and physical stability. If one considers that these effects are stable towards further geological agents, we now realize that we have in hand a "geological photographic plate/ This optimistic approach should not hide the fact that data and information obtained are rather complex and that our understanding of the basic phenomena is still inadequate. We shall thus confine ourselves to describing and interpreting a few typical examples. 

The Vickers microhardness test gives a smaller indentation for a given force than the Knoop test and is less sensitive to material anisotropy. The Knoop test is easier to use because the impression is longer for a given load and usually only one measurement per test is required. The microhardness measurement with the Knoop diamond is quite sensitive to material anisotropy because of the twofold symmetry of indentation. Nevertheless, the Vickers diamond, as we shall show below, also detects anisotropy conveniently (Balta Calleja Bassett, 1977 Balta Calleja et al, 1980a). 

Table 6.1. Microhardness perpendicular to the chain orientation (strain) direction H , indentation anisotropy at room temperature (25°C) AH, X-ray crystallinity Wc(WAXS), percentage of a and modifications (according to Tashiro et al, 1980) and crystal hardness of homo-PBT He stretched at various strains, s. 

It will be shown that the level of uniaxial orientation and the variation of local mechanical properties generated by controlling the injection temperature can be conveniently characterized by microhardness measurement in combination with the measurement of optical birefringence An and DSC. In Section 2.7 we saw that microhardness is a very useful mechanical property, which can provide direct information about the anisotropy developed within highly oriented polymers. 

Figure 7.2 schematically shows the geometry of an injection-moulded polymer bar in which z is the injection direction. Indentations were made on the yz plane. In all cases an indentation anisotropy arises because the microhardness is maximum when the indentation diagonal is parallel to the injection direction (Hj) and minimum when the diagonal is normal to it (Hy). The large value corresponds to... 

Anisotropy in mechanical properties, as well as electrical properties, is associated with the presence of the mirror planes. Easy cleavage on (00.1) conduction planes is observed. The microhardness method has been found convenient for probing this mechanical anisotropy and producing data that... 

Rueda D R, Bayer R K and Balta Calleja F J (1989) Microhardness and mechanical anisotropy of elongatioual flow-injection molded polyethylene, J Macromol Sci Phys 828 267-284. 

The Vickers microhardness of the BP wafers varies from 3000 to 4000 kg/mm depending on orientation (47,51). The periodicity of the hardness curve of the (100) plane shows fourfold symmetry (Pig. 7). Minimum and maximum hardnesses in the (100) plane correspond to the (110) and (001) directions, respectively. By analogy to the anisotropy of hardness in the (001) plane of cubic boron nitride (52) compared with resolved shear stress curves, the primary slip systems of BP are 111 (110). The elastic constants Cu, Cu, and C44 were determined by Brillouin scattering (18) as given in Table 1. 

Loboda, P. I. (1999). Anisotropy of microhardness of directionally crystallized materials based on lanthanum hexaboride. Materials Science, 35(4), 552-560. doi 10.1007/BF02365754. 

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