Scratch hardness tests

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... 

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)...

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... 

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. 

Diamond is the hardest material known, with a value of 10 on the Mohs scale, which is a scratch hardness test or, on the Knoop scale, which is an indentation test dependent on the load, indenter shape and the crystal face, giving a value of 5,700-10,400 kgmm . The hardness is attributed to the strength of bonding of the atoms in conjunction with the uniformity. There is, however, a possibility that there are new materials, sueh as carbon nitride (C3N4) and compressed Ceo that may eventually be shown to be harder than diamond. 

Scratch Hardness Tests. The method is based on the ability of one solid to scratch another. A comer of a diamond cube is drawn across the sample surface under a force of 29.4 mN applied to the body diagonal of the cube this creates a V-shaped groove of width X, which is measured microscopically (34). The hardness is given by... 

Microcharacter Hardness. A scratch hardness test in which a loaded diamond is used it has been applied to the testing of glazes. The Microcharacter instrument was developed by C. H. Bierbaum (Trans. Amer. Soc. Mech. Engrs., p. 1099,1920 p. 1273,1921). Mkrocline. See feldspar. Mkrocracking See fracture, crack nucleation crack propagation ... 

Equation (1.52) can be used to predict the size of debris particles formed in scratch hardness testing. Figure 1.9 contains some typical scratch widths obtained at different loads for three important ceramics. 

Other hardness-testing techniques are frequently employed but will not be discussed here these include ultrasonic microhardness, dynamic (Scleroscope), durometer (for plastic and elastomeric materials), and scratch hardness tests. These are described in references provided at the end of the chapter. 

Scratch Hardness Tests. The method is based on the ability of one solid to scratch another. A comer of a diamond cube is drawn across the sample surface... 

Hardness. Glass hardness tests usually measure the resistance to abrasion by grinding or grit-blasting, resistance to scratching, or penetration by an indenter. The method to be used depends on expected service conditions. Knoop hardness (Table 4) is commonly used, because other methods usually fracture the glass. 

MohsAn early (1822) hardness comparison test involved assigning a relative number to aH known materials (usuaHy minerals and pure metals) by virtue of their relative abHity to scratch one another. The results of this classification are not relatable to other properties of materials or to other measures of hardness. As a result of this limited useflilness, the Mohs hardness test is primarily used for mineral identification. Some examples of the Mohs hardness scale, which ranks materials from 1 to 10, are Hsted in Table 6. 

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. 

Hardness is closely related to strength, stiffness, scratch resistance, wear resistance, and brittleness. The opposite characteristic, softness, is associated with ductility. There are different kinds of hardness that measure a number of different properties (Fig. 5-5). The usual hardness tests are listed in three categories (a) to measure the resistance of a material to indentation by an indentor some measure indentation with the load applied, some the residual indentation after it is removed, such as tests using Brinell hardness,... 

Vickers and Knoop indentors, Barcol hardness, and Shore durometers (2) (b) to measure the resistance of a material to scratching by another material or by a sharp point, such as the Bierbaum hardness or scratch-resistance test and the Moh one for hardness and (c) to measure rebound efficiency or resilience, such as the various Rockwell hardness tests. The various tests provide different behavior characteristics for plastics, as described by different ASTM standards such as D 785. The ASTM and other sources provide different degrees of comparison for some of these tests. 

Although hardness is a somewhat nebulous term, it can be defined in terms of the tensile modulus of elasticity. From a more practical side, it is usually characterized by a combination of three measurable parameters (1) scratch resistance (2) abrasion or mar resistance and (3) indentation under load. To measure scratch resistance or hardness, an approach is where a specimen is moved laterally under a loaded diamond point. The hardness value is expressed as the load divided by the width of the scratch. In other tests, especially in the paint industry, the surface is scratched with lead pencils of different hardnesses. The hardness of the surface is defined by the pencil hardness that first causes a visible scratch. Other tests include a sand-blast spray evaluation. 

One simple hardness test is the Moh hardness test it is based on the fact that a harder material will scratch a softer material. Geologists and mineralogists frequently use this test. The Moh scale is an arbitrary scale of hardness based on the ability of ten selected minerals to scratch each other. The relative Moh hardness for several substances is given in Table 15.6. 

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... 

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. 

In hardness tests of metals and their alloys, Davidenkov s scratch point method has found use through a strict relationship of the result to true tensile strength [Pg.31]

In hardness tests by scratch methods, apart from the previously mentioned diamond cones with included angle of 90° or 120°, trihedral pyramid-shaped points (Bierbaum pyramid and Berkovich pyramid) and Vickers tetrahedral pyramid (Fig. 4.3.3) are now widely used their main... 

Other formulae describing hardness tested by the scratch-with-edge-to-the-front method are also based on the ratio of load to contact surface of indenter with material under test, or to that surface projected to the sample plane. For the tetrahedral pyramid in the widely used Vickers hardness testers and in PMT-3... 

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