Diamond Knoop hardness

Diamond. Diamond [7782 0-3] is the hardest substance known (see Carbon, diamond, natural). It has a Knoop hardness of 78—80 kN/m (8000—8200 kgf/m ). The next hardest substance is cubic boron nitride with a Knoop value of 46 kN/m, and its inventor, Wentorf, beheves that no manufactured material will ever exceed diamond s hardness (17). In 1987 the world production of natural industrial diamonds (4) was about 110 t (1 g = 5 carats). It should be noted that whereas the United States was the leading consumer of industrial diamonds in 1987 (140 t) only 260 kg of natural industrial diamonds were consumed this is the lowest figure in 48 years (4), illustrating the impact that synthetic diamonds have made on the natural diamond abrasive market. 

Sihcon carbide is well known as a hard material occupying a relative position on Mohs scale between alumina at 9 and diamond at 10 (see Hardness). The average values for Knoop hardness under a load of 100 g are... 

On ihe Mohs scale diamond is 10 and quartz 7. An abemative measure is the Knoop hardness (kgmm ) as measured with a 100-g load typical values on this scale are diamond 7000, boron carbide 2750, corundum 2100. topaz 1340, quartz 820. hardened tool steel 740. 

Knoop hardness It is a measure of hardness is measured by a calibrated machine that forces a rhomb-shape, pyramidal diamond indenter having specified edge angles under specific small loading conditions into the surface of the test material the long diagonal in the material is measured after removal of the load. 

The Knoop test is a microhardness test. In microhardness testing the indentation dimensions are comparable to microstructural ones. Thus, this testing method becomes useful for assessing the relative hardnesses of various phases or microconstituents in two phase or multiphase alloys. It can also be used to monitor hardness gradients that may exist in a solid, e.g., in a surface hardened part. The Knoop test employs a skewed diamond indentor shaped so that the long and short diagonals of the indentation are approximately in the ratio 7 1. The Knoop hardness number (KHN) is calculated as the force divided by the projected indentation area. The test uses low loads to provide small indentations required for microhardness studies. Since the indentations are very small their dimensions have to be measured under an optical microscope. This implies that the surface of the material is prepared approximately. For those reasons, microhardness assessments are not as often used industrially as are other hardness tests. However, the use of microhardness testing is undisputed in research and development situations. 

The hardness of a material typically represents its ability to resist indentation. Macroscopic hardness is usually measured by pressing a diamond tip (of known area A) into a solid, with a given load (force). The hardness //, often called Vicker s or Knoop hardness (which differ by the diamond tip geometry), is defined as // = W//4, where W is the load and A is the indented area. This is a destructive test The permanent indentation caused by the diamond is produced by breaking bonds and displacing atoms. A typical indentation profile is shown in Figure 8.1, and the Vicker s and Knoop hardnesses of several materials are listed in Tables 8.1 and 8.2. 

The indentor used in the Knoop hardness is a diamond pyramid, but the lengths of the two diagonals, as shown in Figure 3.42, are different. If the long diagonal of the indent is measured as D, the hardness number is obtained from... 

According to ASTM D1474 the Knoop hardness number is measured with the pyramidal Knoop diamond. According to ISO 2815 the Buchholz instrument is used. This method cannot detect elastic deformation of the coating that disappears immediately after the load is removed. 

Other common hardness tests involve the use of diamond pyramids. In the Vickers hardness test, a square pyramid is used and in the Knoop hardness test, the pyramid is elongated. The area term in the former test is the actual indentation area and in the latter, the projected area. From the impression geometries, shown in Fig. 6.30, the Vickers Hardness Number (VHN) and Knoop Hardness Number (KHN) can be shown to be VHN=1.854F/a and KHN=14.2F/L, respectively. A common hardness test in the USA is the RockweO hardness test, which uses various indenter types and loads. The result of these tests is a dimensionless number and leads to the use of various hardness scales (e.g., Rockwell B, Rockwell C). 

Knoop hardness test n. An indentation hardness test using calibrated machines to force a rhombic-based pyramidal diamond... 

There are a variety of types of hardness tests, but the most popular in the case of rare earth metals seems to have been the Brinell test and the Vickers, or Diamond Pyramid Hardness (DPH) test as it is most commonly called. The DPH test is perhaps the most reliable hardness test and, in fact, should give hardness values in close agreement with the results of properly conducted Brinell and Knoop (when loads are >500 g) hardness tests. Consequently, the emphasis in this section is on DPH data, but Brinell data are also presented where they are instructive. Frequently, microhardness data are quoted in the rare earth literature. Since they are determined with a diamond pyramid indenter using lower loads than macrohardness, the author includes them with DPH values when they are in agreement with macroscopic values although it is recognized that microhardness results can be load sensitive. 

The Knoop hardness (HK) of a material is a measure of the residual surface changes after the application of pressure with a test diamond. The standard ISO 9385 describes the measurement procedure for glasses. In accordance with this standard, values for Knoop hardness HK are listed in the data sheets for a test force of 0.9807 N (corresponds to 0.1 kp) and an effective test period of 20 s. The test was performed on polished glass surfaces at room temperature. The data for hardness values are rounded to 10 HK 0.1/20. The microhardness is a function of the magnitude of the test force and decreases with increasing test force. 

Knoop Hardness. A pyramidal-diamond indentation test. The result is expressed in kg/mm2, the applied load being indicated, eg. Knoop 100 or KlOO a US abbreviation is KHN(Knoop Hardness Number) The test was introduced by F. Knoop, C G Peters and W. B. Emerson (/. Res. Nat. Bur. Stand., 23,39,1939). ASTM C849 describes its application to ceramic whitewares C730 to glass. 

The hardness of diamond is compared with that of other hard materials in Fig.11.16. The test method is the Knoop hardness test which is considered the most accurate for crystalline materials. The hardness is also a function of the crystal orientation as shown in Table 11.7. 

Radiation damage can, in some samples, introduce a degree of plasticity for example, in diamond the overall hardness is reduced after irradiation but there are no data showing what this does to the Knoop hardness anisotropy. We can, however, note that radiation damage in MgO increases the overall hardness but does not affect the anisotropy, a fact that seems to be general for the rock-salt structure. ... 

Barium titanate (BaTi03), strength after indentation, 185 Barrel indents, 44-126 BeBg Knoop hardness, 303 Beevers-Ross sites, 280 Berkovich hardness anisotropy, 94 diamond, 11, 39-40 equation, 11... 

Knoop Hardness Test n An indentation hardness test using calibrated machines to force a rhombic-based pyramidal diamond indenter having specific edge angles, under specified conditions, into the surface of the material under test and to measure the long diagonal after removal of the load. (Handbook of physical polymer testing, vol 50. Brown R (ed). Marcel Dekker, New York, 1999 www.astm.org). 

Knoop hardness. A Knoop indenter is impressed on a specimen, and the depth of penetration determines the Knoop hardness. The Knoop indenter is a diamond pyramid with a rhombic base. The diagonals are in the ratio of 1 7, and included apical angles are 130° and 172° 30. ... 

Ceramics are among the hardest substances known, and their hardness is correspondingly difficult to measure. Most methods rely on the ability of one material to scratch another and the measurement is presented on a relative scale. Of the available methods, the Knoop method is the most frequently used. In this approach, the surface is highly polished and a pointed diamond stylus under a light load is allowed to impact on the material. The depth of the indentation formed by the stylus is measured and converted to a qualitative scale called the Knoop or HK scale. The Knoop hardness of selected ceramics is given in Table 4.8. 

Knoop hardness (HK) number The expression derived from the force used and the projected area of an imprint obtained by a specifically shaped (ASTM E 384) diamond indenter forced into a surface. Abbreviated to HK (formally KHN). HK = 14229P/d where P = grams force and d = length of long diagonal in microns. See also Vickers hardness (HV) number. 

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