Hardness Knoop

Material CAS Registry Number Mohs scale Ridgeway s scale WoodeU s scale Knoop hardness, kN/m " ... 

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. 

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. 

The Knoop hardness number is computed from the measured long diagonal by the following formula where HK = Knoop hardness,... 

Material Dielectric constant at high frequency Density, kg/m Knoop hardness, kg/mm Thermal conductivity, W/(m-K) Melting point, °C... 

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... 

Table 9 Hsts select properties of Co—Cr alloys. It is generally conceded that the casting shrinkage of the cobalt—chromium alloys is greater than that of the gold alloys. The lower density of the base metal alloys provides a weight advantage over the higher-density gold alloys in certain types of bulky restorations. Cobalt—chromium alloys have Knoop hardnesses of 310—415.

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. 

Pure crystalline silicon is a brittle material with a gray metallic appearance. Its mechanical properties, such as Knoop hardness (950-1150 kg mm-2), Young s modulus (190 GPa for (111), 170 GPa for (110), 130 GPa for (100)), torsion modulus (4050 kg mm-2) and compression breaking strength (5000 kg cm-2) vary slightly with crystal orientation. Silicon has a low thermal expansion coefficient (2.33x 1(T6 K-1) and a high thermal conductivity (148 W K-1m-1). Crystalline silicon melts at 1413 °C (1686 K). 

In general, one can expect systems like these to have properties different from those in bulk materials, where most ions are in a crystalline enviromnent that is, they do not see the surface. Here, most atoms know they are near an interface specifically, in a noncrystaUine, nonperiodic enviromnent. As shown in Table 17.1 and Figure 17.4, the Knoop hardness of these films is increased from 2.9 to 5.6 times the value for the harder of the two comp)onents, the electrodeposited nickel. 

TABLE 17.1. Cu/Ni Knoop Hardness Data for Electrodeposited Multilayers ... 

In interpreting the information in Tables 3 and 4 it is useful to recall that a Knoop hardness of 12 KHN is adequate for acrylic auto enamels. Thus the 40/60 and 50/50 enamels (21-26 KHN) are much harder than auto enamels. It is quite probable that they are also more flexible, although direct comparisons are not possible from the data at hand. 

The enamels with Knoop hardness above 20 KHN and hardness of 3H or higher had excellent solvent (methyl ethyl ketone) resistance. 

Hardness. The Antarctic coals have Knoop hardnesses that range from 27 kg./sq. mm. to 188.6 kg./sq. cm. (Table I). Other natural cokes are generally the hardest, and the semianthracites are generally the softest. Hardness increases as the distance from the sill decreases for the coals from the Terrace Ridge area (Figure 11). However, when all the samples are considered, the hardness does not necessarily correlate in a completely regular manner with another single set of test data. 

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