Scratch hardness

Scrap rubber fuel Scrap tires Scratch hardness... 

Scratch Te.st. The scratch microhardness test is a refinement of the Mohs test. The corner of a cubic diamond is drawn across the surface of a metaHographicaHy poHshed sample under a constant load, usuaHy 29.4 N (3 kgf). The width of the resultant Vee groove scratch varies inversely with the hardness of the material displaced where H = scratch hardness number and A = groove width in micrometers. 

This test finds appHcation on finely poHshed and/or etched metaHographic samples and mineralogical samples. It is useful for distinguishing variations in hardness between adjacent microconstituents. The test is extremely deHcate and therefore is Httle used commercially. Use is largely restricted to research institutions. There is no estabHshed means of converting scratch hardness data to other hardness scales. 

Ritzel, n. (Mach.) pinion, ritzen, v.t. crack slit scratch etch cut. RitzhlLrte, /. hardness to scratching, -probe, /. abrasive hardness test, scratch test, -priifer, m. abrasive hardness tester, aclerometer. -verfahren, n. = Ritzharteprobe. -zahl, /. (scratch) hardness number. 

Foundry core oil. CNSL resins are known to impart good scratch hardness to sand core after they are baked [133,134,137,167]. It also provided resistance to moisture and weathering and good green strength, and surface finish to molded articles. 

Figure 1.3 Correlation between the Moh scratch hardness and Vickers indentation hardness scales.

However, to develop some intuitive sense of hardness it is useful to consider the Moh scratch hardness scale. This is a rank-hle scale consisting of ten levels. Each level has been assigned to a particular mineral such that the mineral at level n is capable of scratching the one at level (n - l).The mineral at the lowest level (designated 1) is talc, and the highest (designated 10) is diamond. 

Gerk (1976) has shown that indentation and scratch hardnesses are equivalent if the time of indentation and the velocity of scratching are taken into account. Figure 2.10 shows the numerical relationship between the two types of measurement indicating that they are essentially the same. 

Correlations between differing measures of hardness are discussed by Mott (1957). A correlation diagram for Mohs scratch hardness and indentation hardness in the case of minerals is given in Figure 2.11. 

G. Chin, Strong and Hard Solids ,Trans. Amer. Crystallographic Assoc., 11,1 (1975). A. P. Gerk, The Relationship of Time-dependent Hardness and Scratch Hardness ,... 

Indentation data for the sulfides could not be found in the literature. However, Mohs scratch hardness numbers were found (Winkler, 1955). They were converted to Vickers numbers using a correlation chart. The hardnesses are shown in Figure 9.10. Since they all have the same number of valence electrons, this is the same as plotting the hardnesses versus the valence electron densities. 

Physical hardness can be defined to be proportional, and sometimes equal, to the chemical hardness (Parr and Yang, 1989). The relationship between the two types of hardness depends on the type of chemical bonding. For simple metals, where the bonding is nonlocal, the bulk modulus is proportional to the chemical hardness density. The same is true for non-local ionic bonding. However, for covalent crystals, where the bonding is local, the bulk moduli may be less appropriate measures of stability than the octahedral shear moduli. In this case, it is also found that the indentation hardness—and therefore the Mohs scratch hardness—are monotonic functions of the chemical hardness density. 

Table 14.3 contains comparative hardness values for five hardness scales including the classical Mohs scale, which ranges from the force necessary to indent talc given a value of 1 to that needed to scratch diamond given a Mohs value of 10. In the field, a number of relative tests have been developed to measure relative hardness. The easiest test for scratch hardness is to simply see how hard you have to push your fingernail into a material to indent it. A more reliable approach involves scratching the material with pencils of specified hardness (ASTM-D-3363) and noting the pencil hardness necessary to indent the material. 

The test bars of the Nile mud were reddish brown (5YR 5/4) in the Munsell color notation at the 600°C zone of the gradient firing, red (2.5YR 5/6) at 850°C, and darkened with incipient vitrification to weak red (2.5YR 4/3) at 1100°C after a holding period of 30 minutes. The scratch hardness, using Mohs scale, increased from 3.0 to 6.5 for the male (better working clay in the potters terminology), and from 2.3 to 5.0 for the female mud. The Nile mud shrank far more when fired... 

Any fertilizer materials are minerals and have certain hardness values and therefore cause wear. Other plant nutrient salts or chemicals, particularly in the presence of moisture, will additionally produce corrosion. It is apt in this context to know some details of the scratch hardness of minerals which cause wear and abrasion, and this is normally measured in terms of Mohs scale [8]. 

Mohs hardness is a measure of the relative hardness and resistance to scratching between minerals. Other hardness scales rely on the ability to create an indentation into the tested mineral (such as the Rockwell, Vickers, and Brinell hardness - these are used mainly to determine hardness in metals and metal alloys). The scratch hardness is related to the breaking of the chemical bonds in the material, creation of micro fractures on the surface, or displacing atoms in the metals of the mineral. Generally, minerals with covalent bonds are the hardest while minerals with ionic, metallic, or van der Waals bonding are much softer. 

The most practical tests specifically followed for rubber lining are hardness test, spark test and immersion test. Hardness test is invariably the most frequently prescribed test for rubber lining quality by clients, though it is an inadequate test. The term hardness is a vague one the different expressions such as "Scratch Hardness","Cutting Hardness" "Abrasion Hardness" etc. illustrate that different concepts of hardness exist. 

Fig. 4.3.1. (b) Scratch Hardness Tester after Sikkens, Model 601, manufactured by Erichsen GmbH. 

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... 

Characteristics of diamond indenters used for scratch hardness measurement... 

Some investigators believe that the best course to follow in scratch hardness determination is to find the tangential force acting on the surface of material under test required to obtain a scratch of width b. In this case, hardness is expressed as the ratio of that force to the scratch cross-section or a magnitude proportional to it, or else as a quotient of the work necessary to produce the scratch and its volume. Proponents of this method (Yushkin, 1971) consider that scratch hardness tests in the present form play only a marginal role. Unlike Shreyner (1949), who contended that the results obtained under these methods are less accurate than those for mineral-bymineral scratch after Mohs, they accept that these tests have certain usefulness, but only of a complementary nature. The differences of opinion arise from the different approach to the question of accuracy of the method of determination itself, since as can readily be proved, a strict relationship exists between hardness defined as the ratio of load P to square of... 

Fig. 4.3.6. Force distribution in scratch hardness tests. (After Grigorovich, 1965)...

Fig. 4.3.21. Scratch hardness measured with DURIMET hardness tester, E. Leitz,...

Similar investigations on dislocation effects in crystals during scratch hardness tests were conducted by Boyarskaya (1972). 

Swain (1978) analysed in detail the critical load inducing crack formation around a scratch in scratch hardness test, and Veldkamp et ah (1978) determined this critical point from the equation... 

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