Mohs scratch method

Hardness It is not possible to obtain a reliable figure for the hardness of anodic coatings with either the indentation or scratch methods, because of the influence of the relatively soft metal beneath the anodic film, and the presence of a soft outer layer on thick films. On Moh s Scale, the hardness of normal anodic films lies between 7 and 8, i.e. between quartz and topaz. 

There are other, less commonly used, methods for measuring hardness. One is an impact method in which an indenter is dropped from a known height onto a specimen, and either the size of the indentation, or the coefficient of restitution, is measured. Another is the pendulum method in which a rocking pendulum is applied to a specimen surface. The damping of the pendulum s oscillations is a measure of the hardness. Still another is Moh s scratch method in which the ability of one specimen to scratch another is observed. These methods are described in various books (McColm, 1990), but only the... 

Comparative values of standards on old Mohs scale obtained by scratch and abrasion methods and by indentation methods compiled by Povarennykh (1963)... 

Mackensen-Zeiss blower method, 39,101 Martens sclerometer, 48, 49 McGee s method, 92 Mean Mohs-Mackensen hardness, 106 Method, Blondel s 92 Bohme s disc, 90 Brinell, 36, 58 Davidenkov scratch point, 50 Davidenkov s, 50 dry friction without balls, 92 Harvey s 92... 

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... 

Two kinds of hardness can be measured scratch hardness and indentation hardness. Special styluses are available to measure the scratch hardness. These are provided with a crystal tip which has a certain hardness. When a material is scratched with a certain stylus and no scratch is made, then the material has a higher hardness than the tip of the stylus. By using different styluses you can accurately determine the hardness. This method is often applied in mineralogy, e.g. by mineral collectors. Hardness is ranked along the Mohs scale, which is based on the hardness of minerals. In this scale talc is the softest mineral, which becomes clear when you rub it between your fingers. 

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. 

Scratch hardness is the oldest form of hardness measurement and was probably first developed by mineralogists. It depends on the ability of one solid to scratch another or to be scratched by another solid. The method was first put on a semiquantitative basis by Mohs (1882) who selected ten minerals as standards, beginning with talc (scratch hardness 1) and ending with diamond (scratch hardness 10). 

Another type of scratch hardness which is a logical development of the Mohs scale consists of drawing a diamond stylus, under a definite load, across the surface to be examined. The hardness is determined by the width or depth of the resulting scratch the harder the material the smaller the scratch. This method has some value as a means of measuring the variation in hardness across a grain boundary. In general, however, the scratch sclerometer is a difficult instrument to operate. 

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. 

Hardness is the most important characteristic of superhard materials. It indicates the potential of the cutting tool this is the field of application where superhard materials are used the most. Cutting (or scratching) is the basis of the first primitive Mohs hardness scale. Hardness is defined as the load on an indenter divided by the projected area. Several hardness measurement methods are used in practice, the major ones being indentation and sclerometry (scratch at a constant indenter load). A detailed comparison of the indentation and sclerometry methods is given in [15]. These methods conform well with each other. Sclerometry implies a greater plastic deformation as compared with indentation [15],... 

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