Testing methods 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... 

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... 

ASTM G171-03 Standard Test Method for Scratch Hardness of Materials Using a Diamond Stylus... 

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... 

Pencil hardness measurements were performed as per the protocol specified in the ASTM D3363 standard. A pencil scratch hardness tester from PPH-I was used to test samples (color chips) of different resins under a load of Ikgf at a temperature of 23°C. Mitsubishi-UNI pencils specified by the JIS S6006 standard were used in the evaluation. The hardness ratings, going from softer to harder, range from 6B to 7H and take the form 6B.3B, 2B, B, HB, F, H, 2H, 3H...7H (B=Black, F=Firm and H=Hard). Scratch resistance was also evaluated using a different method compliant with the ISO 1518 standard. An Erichson Scratch Tester (Type 413) was used to measure the resistance of the material to penetration when scratched with a spherical needle 1mm in diameter under a load of 6N. 

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. 

Scratch methods are useful above all in the determination of the conventional hardness number which is linked to a specified, standardized scratch width. This follows from the absence of a directly proportional relation, as one would expect, between scratch width and square root of load, observable in the case of some materials. When such materials are tested, the scratch width increases at a slower pace than does the square root of the load. Since the relation varies with load, the scratch-width variation curves for various materials may intersect, thus making it impossible to arrange them by scratch-determined hardness. The order in which they are ranked may vary according to applied load. 

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... 

The physical meaning of hardness determined by Bierbaum s method is exactly equal to load P divided by the projected contact surface of pyramid with material under test in the scratch process. It should be mentioned here that the result obtained under Bierbaum s method is roughly directly proportional to the hardness number obtained by the pyramid indenter method, and the relation has the form... 

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... 

Since from the outset attempts to determine the hardness of minerals have involved various techniques, ranging from scratch through abrasion to indentation, it will be useful to examine these three measurement groups, and to distinguish them according to the nature of action on the material under test, and also to present attempts to relate them to the results obtained by different methods. Specified in Table 4.1.1 are the most important methods, classified according to the mode of action on the sample under test. 

Scratch tests are a common method used to identify mineral hardness relative to Mohs scale. Streak tests are often carried out on streak plates. Mineral hardness is a fundamental property of minerals and can be used to identify unknown minerals, hi the absence of comparative minerals, geologists often resort to common objects with a relatively well-established Mohs hardness number. In addition to glass (5.5), copper pennies measure 3.5, and the average human fingernail averages a Mohs hardness of 2.5. 

Oddly enough, there has been no exact definition of hardness. Its value for a given sample is usually determined by very empirical methods, such as the scratch test, which gives the Moh scale of hardness, or the effects of dropping a weight on the sample. Such numbers are very useful, but difficult to interpret in a fundamental way. Also, the results are very dependent on the past history of the sample and its purity. 

The standard methods to determine the coating microhardness by indentation of a pyramid-shaped diamond indenter are described in ASTM E384-07 (2007). Depending on the shape of the indenter, Knoop- and Vickers-type diamonds as well as Berkovich indenters can be distinguished. The reported hardness number expressed in N mm-2 (MPa) is the force exerted on the specimen surface by the diamond indenter used to produce the impression. In principle, the technique is less affected by porosity than the scratch tests based on measuring the indenter travel caused by a specific increase in load. Microhardness tests are usually made... 

It is clear that the scratch test is the only viable method of determining the adhesion of thin hard coatings which adhere well to hard glass substrates. It is equally clear that the test cannot be applied without great care being used in the interpretation of the results. Irrespective of the particular coating-substrate combination under examination, a number of points need to be kept in mind. 

Hardness of a material may be determined in several ways (1) resistance to indentation, (2) rebound efficiency, and (3) resistance to scratching. The first method is the most commonly used technique for plastics. Numerous test methods are available for measuring the resistance of a material to indentation, but they differ only in detail. Basically they all use the size of an indent produced by a hardened steel or diamond indentor in the material as an indication of its hardness—the smaller the indent produced, the harder the material, and so the greater the hardness number. Hardness tests are simple, quick, and nondestructive, which account for their wide use for quality control purposes. 

Hardness, scratch and abrasion resistance, and wear (friction) properties are characteristics which can be important for the selection of the most suitable material for a specific job. For the purpose of comparison, the designer must make sure that the data shown in the various charts have been produced using the same test methods. 

The wear mechanism of multilayer coatings is assumed to occur layer by layer and the cracks are deflected by the interfaces between the sublayers. Besides cutting test the performance of coatings can be tested (at least compared) by scratch tests, in which a critical load is obtained at which the coatings are removed. The critical loads are typically 50-100 N and are influenced by the type of the deposition process. Also the obtained hardness is dependent on the preparation method magnetron sputtered Ti(C, N) layers reach up to about 37GPa [108]. 

For hardness determination, different methods are possible scratching the surface, penetration of an indenter with static or dynamic loads, or rebound as a result of elastic material behavior. The methods with a penetrating indenter are the most important ones. The applied methods are distinguished, e.g., by the shape of the indenter. Brinell hardness is determined by a ball-shaped indenter, while Vickers hardness applies a pyramid-shaped one. After the indenting test with a certain load, the surface area of the indentation is measured which delivers a value for material hardness. Determination of Rockwell hardness uses the depth of the indentation instead of the surface area (Bargel and Schulze 1988). Independent of the method, the so-called surface hardness... 

Hardness is defined as the resistance of a material to indentation. Since this resistance strongly depends on the shape of the indenter and the force level, there are a large number of different testing methods. These different methods can be classified in three groups, scratch tests, indentation tests, and rehound tests. In general, it is not possible to calculate a hardness value given a value measured with another method - however, conversion tables for common materials are available. 

Historically, scratch tests are of some importance, for they were the first hardness tests employed. In a scratch test, it is tested whether a material can be scratched using a needle made of another material. Either relative scales that allow to sort materials by their scratchability are used, or the size of the scratch is measured to determine the hardness. Although the method can yield quantitative results, it is not easy to perform precise measurements. 

---