Scratch Hardness Anisotropy

Khrushchev (1957) considers that the need to measure the force T has not been sufficiently well substantiated, nor has a sufficiently precise and easy in service hardness tester been developed yet for determinations of this type. However, he appreciates the usefulness of scratch hardness tests, especially at low loads, as a non-destructive technique. He recommends these methods as very useful for hardness determination of metallic layers or of materials exposed to abrasive wear under operating conditions (plastics, organic coatings, such as varnishes and paints, etc.). Scratch methods are especially important in tests of anisotropic materials where a change in scratch width is the measure of anisotropy. In static indentation methods, the indentations obtained in anisotropic materials are misformed, varying... 

Comparison of results of crystal hardness anisotropy determination by Vickers and scratch methods... 

Boyarskaya (1972) carried out comparative tests to determine hardness anisotropy by indentation with a Vickers pyramid and by scratch in a number of minerals and crystals (Table 7.7, Figs. 7.4, 7.5). It is seen from the table that for most of the crystals tested the results for scratch hardness are clearer. Interesting is her finding that where K,1V is lowest, KIiR is highest. This appears to be due to a more legible test, which no doubt facilitates correct hardness measurement. 

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. 

Penetration depth hardness scale, 46-47 and Rockwell scales, 47 and scratch hardness, 62-63 of various diamonds, 41 Penny crack growth, 156, 170 Penny shaped flaw, 150-161 Periclase, 264 Perovskites, 286-290 cracked indent in, 66 hardness anisotropy, 77, 93, 288-289 structure, 286-287 tolerance factor equation, 287 Peters equation, 176 pH... 

It is obvious by now that the mismatch of symmetry between the Knoop indenter and 111 planes is a weakness when exploring hardness anisotropy of such planes. However, since scratch hardness does not suffer from such a mismatch, the resolved shear stress curves for the (111) plane in cubic crystals with the three commonly found slip systems shown in Figure 3.29 may be useful in anisotropy and slip system investigations. Clearly more detailed anisotropy is predicted but the small anisotropy factors implied in the scale-of Figure 3.29 must be remembered. Generally speaking, only the main features of the predicted curves have ever been established and experimental uncertainty makes it unlikely that the fine detail will be found. 

Grabko et al. (1977) studying the anisotropy of the mechanical properties of admixed ZnTe monocrystals used the scratch method to reveal the reorientation of blocks in crystals ZnTe Mg, and ZnTe Be (Fig. 4.3.7). The minimum-hardness direction within one block would change... 

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