Mohs scratch hardness numbers

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. 

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. 

The hardness of any solid can be assessed with testing equipment that measures the relative ease with which a surface can be either scratched or penetrated by another solid of known hardness. A long-used classification of the hardness of minerals, the Mohs scale of hardness, is based on the scratch test ten rather common minerals are arranged in the scale in order of their increasing relative hardness and listed in a scale varying from 1 to 10 (see Table 20). Each mineral in the Mohs scale scratches those with lower hardness numbers but does not scratch higher-hardness minerals. If a... 

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. 

A mineral s hardness is defined as its ability to scratch another mineral. This is usually measured using a comparative scale devised about 200 years ago by Friedrich Mohs. The Mohs scale lists ten common minerals, assigning to each a hardness from 1 (talc) to 10 (diamond). A mineral can scratch all those minerals having a lower Mohs hardness number. For example, calcite (hardness three) can scratch gypsum (hardness two) and talc (hardness one), but it cannot scratch fluorite (hardness four). 

The softest mineral, talc, can be used in body powder. The hardest, diamond, is used in drill bits to cut through the most dense crustal materials. Mohs scale is a relative index scale, meaning that a determination of Mohs hardness number for a mineral is based upon scratch tests. For example, gypsum (Mohs hardness number = 2) will scratch talc (Mohs hardness number = 1). Talc, however, will not scratch gypsum. Glass is assigned a Mohs hardness number of 5.5 because it will scratch apatite (Mohs hardness number = 5) but will not scratch orthoclase feldspar (Mohs hardness number = 6). 

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. 

Hardness characteristics—especially in calcite crystals— may vary as a property dependent upon the direction of the scratch (i.e., able show evidence of a particular Mohs number if scratched along one face or direction as opposed to a different hardness number if scratched in a different direction. 

A closely related mechanical property which has been used extensively in glass literature is the microhardness. Micro in microhardness only indicates that the hardness measurements have been made on a micron scale. Microhardness actually measures only the scratch resistance of the material and thus a scale of microhardness is a scale of the scratch resistances - harder material can scratch the surface of the softer material. One of the widely used scales is Mohs scale of hardness calibrated with the hardness of the hardest material, namely diamond, marked with a value of 10 and with the hardness of the softest material, namely talc, marked with a value of 1. On this scale most oxide glasses register microhardnesses between 5 and 7. In scientific investigations two other scales are used, namely Knoop s hardness number (KHN) and... 

Scratch hardness may be measured on Mohs scale, which ranges from 1 for talc to 10 for diamonds, or by scratching with pencils of specified hardness (ASTM-D-3363). Hardness may also be measured by the number of bounces of a ball or the amount of rocking by a Sward hardness rocker. Abrasion resistance may be measured by the loss in weight caused by the rubbing of the wheels of a Taber abraser (ASTM-D-1044). 

Basically, scratch hardness is a measure of the resistance the test sample has to being scratched by other materials. The most common way of qualifying this property is by means of the Mohs scale. On this scale various materials are classified from 1 to 10. The materials used, as shown in Figure 3.46, range from talc (1) to diamond (10). Each material on the scale can scratch the materials that have a lower Mohs number however, the Mohs scale is not of much value for classifying plastic materials, because most common plastics fall in the 2-3 Mohs range. However, the basic technique of scratch hardness may be used to establish the relative merits of different plastic materials fi om their ability to scratch one another. 

Another mechanical property that may be important to consider is hardness, which is a measure of a material s resistance to localized plastic deformation (e.g., a small dent or a scratch). Early hardness tests were based on natmal minerals with a scale constructed solely on the ability of one material to scratch another that was softer. A qualitative and somewhat arbitrary hardness indexing scheme was devised, termed the Mohs scale, which ranged from 1 on the soft end for talc to 10 for diamond. Quantitative hardness techniques have been developed over the years in which a small indenter is forced into the surface of a material to be tested under controlled conditions of load and rate of application. The depth or size of the resulting indentation is measured and related to a hardness number the softer the material, the larger and deeper the indentation, and the lower the hardness index number. Measured hardnesses are only relative (rather than absolute), and care should be exercised when comparing values determined by different techniques. 

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. 

Mohs hardness It is a measure of the scratch resistance of a material. The higher the number, the greater scratch resistance with number 10 being termed diamond. 

Despite its tremendous strength, glass can be fairly easy to abrade. On the Mohs scale of hardness,f glass is between 5 (apatite) and 7 (quartz). So, glass can scratch materials with lower numbers (for example, copper, aluminum, and talc). Likewise, glass can be scratched by materials that have higher numbers (for example, sand, hard steel, and diamond). 

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 often considered to be relative rather than an absolute property. A qualitative, indirect hardness test exists for powders based on the ability of particles of one material to scratch particles of another. This test is based on the work of Mohs the Mohs hardness scale (below) lists ten selected minerals in the order of increasing hardness from talc to diamond so that material of a given Mohs number cannot scratch any substance of a higher number but will scratch those of lower numbers. 

There are several hardness scales for describing the resistance of a material to indentation or scratching. This table lists a number of common materials in order of increasing hardness. Values are given, when available, on three different hardness scales the original Mohs Scale (range 1 to 10) the modified Mohs Scale (range 1... 

The use of the Moh hardness scale for minerals may be useful. This is a listing in which a material lower in the table with a higher number will scratch the material above it. Refer to Table 8.2. 

Mohs scale A scale that measures the hardness of minerals by their ability to scratch one another. A mineral is given a number on Mohs scale according to its ability to scratch one of the reference materials. In order of increasing hardness 1, talc 2, gypsum 3, calcite 4, fluorite 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum 10, diamond. 

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 (see Fig. 2-25). 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 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. 

Initially 10 scratch materials were used, ranging from talc at the soft end to diamond at the hard end. The scale was later expanded to 15, with five materials added between the original numbers 9 and 10, corundum and diamond. AH are fisted in Table 1.4." Although the Mohs scale has the advantages of simplicity and ease of use, it also has... 

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