Microhardness measurements

The complexity of the apparatus needed for ion implantation makes this method of case hardening of limited application. Further, the case depth is considerably lower than that produced by carburizing or nitriding. The depth of implantation of nitrogen in a steel is about 0.00006 cm (19), ie, so thin that it is difficult to measure the hardness profile by conventional microhardness measurements. 

In general, the water uptake of D films tended to be higher than that of / films, but a more significant difference was shown by microhardness measurements. The results obtained with all three vehicles showed that the D areas were significantly softer than the / areas and that the distribution of the hardness values corresponded to that of the resistances. It was concluded that these films have a very heterogeneous structure and that / and D areas are brought about by differences in crosslinking density within the film. 

All experiments were carried out with an ultra-microhardness measuring device (Vickers-hardness) produced by Anton-Paar-Company. It is attached to a microscope with an adapted photo camera. The force of indentation, the time of indentation and the time-gradient of the indentation force can be selected and adjusted separatly. 

BN-67/7001-05. Szkliwa ceramiczne. Pomiar mikrotwardosci (Ceramic Glazes. Microhardness Measurement). 

After cooling, the bimetallic specimen obtained was cut along the cy-lindrical axis, ground flat and polished electrolytically. The cross-sections prepared in such a way were examined metallographically, by x-ray techniques and electron probe microanalysis. Microhardness measurements in the transition zone between initial phases were also made. 

FICXJRE 13.35 (A) Packing denaly measured by a quantimet on rectangular sections fran a transfer section of a powder pressed in a die from both sides. Data from Afanasjev et al. [79]. (b) Density distribution in a sample pressed from one side. Density has been inferred by microhardness measurements. Data taken from Torre et al. [80]. 

Scratch tests have been used for microhardness measurements of polymeric materials (Bierbaum Scratch Hardness Test (ASTM D 1526)). These tests are related to cuts and scratches, and, to some extent, to the wear resistance of materials. Scratch tests are not always related to the resistance to local deformation and they are now being replaced by the preferred indentation test. 

In the last two decades the value of microhardness measurement as a technique capable of detecting a variety of morphological and textural changes in crystalline polymers has been amply emphasized leading to an extensive research programme in several laboratories. This is because microindentation hardness is based on plastic straining and, consequently, is directly correlated to molecular and supermolecular... 

As we have seen in Section 1.1, Vickers microhardness measurement uses as the indenter a square-based diamond pyramid about 100 qm in height. The included angles between opposite faces are a = 136°. This corresponds to the tangential angle of an ideal ball impression, considered to have a diameter equal to 0.375 times that of the ball (Tabor, 1951). The pyramid is pressed under a load P into the polymer surface to form a plastic deformation. When the indenter is removed the diameter of the indentation is measured and its area determined. The mean pressure p over the indentation is then... 

Specimen preparation is important in microhardness measurements and care must be taken to ensure that the microhardness recorded is representative of that material and morphology. 

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). 

The kinetics of the OSO4 treatment as revealed by microhardness measurements has been examined. The increase in microhardness has been explained in terms of the large reduction in molecular mobility of the amorphous, interlamellar layers, Chlorosulphonation produces an initial microhardening of the amorphous phase, in which reaction time, temperature and the molecular weight play an important role. The OSO4 reaction induces an additional microhardening at the surface of crystalline lamellae. Results reveal that Ha increases, after the above chemical reactions, from nearly zero up to values of 300 MPa (Baltd Calleja et ai, 1997). 

The influence of polymorphism on hardness can be illustrated using the case of PP. i-PP can crystallize into several polymorphic forms. Microhardness measurements have been carried out on samples prepared by the temperature-gradient method of Fujiwara Asano (Fujiwara, 1968 Asano Fujiwara, 1978), which allows a distinct separation of the a (monoclinic) and (hexagonal) phases within the sample. Results show that H decreases from 111 MPa to 90 MPa when passing from the a to the )8 phase (Balta Calleja et al, 1988). The H decrease is partially connected with a crystallinity decrease from 0.72 to 0.62. However, according to eq. (4.7) the microhardness of each phase depends not only on crystallinity but also on the crystal microhardness. He, itself. Thus, we can write for both crystalline phases ... 

In Section 4.5 we demonstrated that microhardness measurement is a technique capable of detecting polymorphic changes in polymers. In particular, the study of the transition from the a to the p form in i-PP confirmed that the changes in H can be explained in terms of an additive contribution to the H of independent phase components // , He and Ha (Balta Calleja etal. 1988). This approach opens up the possibility of characterizing f-PP samples consisting of a mixture of a and p phases by means of H measurements. 

The starting material represents a polyblock PEE comprising PBT as the hard segments and PEO (with a molecular weight of 1000 and polydispersity of 1.3, according to GPC analysis) (Fakirov et al, 1991) as the soft segments in a ratio of 57/43 wt%. The sample was a bristle, drawn at room temperature to five times its initial length, and then annealed with fixed ends for 6 h in vacuum at a temperature of 170 °C. WAXS and microhardness measurements were performed in the same way as for the homo-PBT. 

The study of the strain-induced polymorphic transitions by microhardness measurement offers the opportunity to gain additional information on the deformation behaviour of more complex polymer systems such as polymer blends. Since polymer blends are usually multicomponent and multiphase systems the question arises of how the independent components and phases react under the external load. The polymorphic transition will reflect the behaviour of the crystalline phases provided strain-induced polymorphic transition is possible. 

As mentioned above, PEE, is well characterized mainly by SAXS in the deformation regime under consideration (Fakirov et al, 1991, 1992, 1993, 1994 Stribeck et al., 1997). In addition to these morphological investigations a study of the strain-induced polymorphic transition in PEE using microhardness measurement will shed additional light on the stress- and strain-induced structural reorganization of this new class of polymeric materials. 

This chapter presents selected examples of the application of microhardness measurement for the characterization of polymeric materials after various physical treatments. 

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 7.7. Microhardness measured at the inner yz surface (see Fig. 7.2) on PET bars processed at various mould temperatures samples as received (I) and samples dried at 150 °C under vacuum ( ). (From Balta Calleja et al, 1993.)...

Microhardness tests upon the same worn hip cups have provided direct supporting evidence for the changed nature of PE adjacent to the wear surface (Flores et al, 2000). An increased microhardness of the used cups, at their wear surfaces, in comparison to the control was apparent. Attention turned, accordingly, to the lateral surfaces of the cups (Fig. 7.16) which did possess the smooth planar condition required for optimum microhardness measurements. Figure 7.16 shows microhardness measured as a function of the radial distance, h, from the indent to the edge of the concave surface. The microhardness of the control sample H 57 MPa) does not vary with h. however, H for the hip cups, after implantation and removal from... 

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