Scratch test anisotropy

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. 

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... 

Sclerometry finds ever wider uses. The cited example of the scratch method exploited in structural tests is just one of many. Among others, interesting studies of the anisotropy of crystals, chiefly semiconducting, were carried by this method by Soviet investigators, including Boyarskaya (1972, 1977), Pasko et al. (1977), Zhitaru et al. (1977), Lazarenko et al. (1978), Simashko et al. (1978). 

A comparison of this number with the test results of Boyarskaya (1972), listed in Table 7.7, shows that practical measurements of anisotropy, both with Vickers indenter and by scratch, depart widely from the theoretical result which is too low. Similarly, results which are too low, are given by Boyarskaya for disthene, 1.8 instead of 3.13 (Lebedeva, 1963), also diverging from the results reported by Winchell (1946), who determined the ratio 1120/420 = 2.68. 

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. 

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