Knoop scale

Corundum is an aluminum oxide that possesses a hexagonal crystal structure, The compound is extremely hard (2000 on the Knoop scale), sp gr 3.95, and is widely used in abrasives and refractories. Corundum is manufactured by fusing alumina or bauxite in an electric arc furnace operated at about SIOO C. 

Knoop scale. Comparative hardness scale, ranges from glass (300-600) to diamond (6000-6500). 

See 90 Ed., p. 12-222, for hardness of minerals and ceramics (Mohs and Knoop scales) can be sorted in electronic versions. 

TABLE 8.2. Hardness Values of Various Materials on the Knoop Scale... 

Table 1. Hardness ranking of minerals and some prominent synthetic ceramic materials according to F. Mohs. In the case of the synthetic materials microhardness values are given in units of the Knoop scale. The microhardness variations result from variations in the grain size, the load of indentation, the phase composition and the used densification techniques. Modified after [152].

Hardness The ability of a material to scratch or penetrate another material. This is measured with Knoop scale (Krar 1995 Salmon 1992). 

Diamond is the hardest material known, with a value of 10 on the Mohs scale, which is a scratch hardness test or, on the Knoop scale, which is an indentation test dependent on the load, indenter shape and the crystal face, giving a value of 5,700-10,400 kgmm . The hardness is attributed to the strength of bonding of the atoms in conjunction with the uniformity. There is, however, a possibility that there are new materials, sueh as carbon nitride (C3N4) and compressed Ceo that may eventually be shown to be harder than diamond. 

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

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. 

Mineral Grain size, No. (mm) Hardness, on Mohs scale //M Hardness determined by Knoop method HK Cleav- ability Degree of brittleness... 

Mohs scale after Franz, 1850 1 after Pfaff, 1884 after Poschl, 1909 m ON -14 O CO u 3 C3 after Hodge and McKay, 1934 after Auerbach, 1936 i after Knoop et al., 1939 Corundum after Winchell, 1945 after Khrushchev, 1947 j after Taylor, 1949 after Dinritreva, 1949... 

Hardness measured in units of Mohs scale Mineral Chemical formula Vickers ffv (with micro- measurement) Knoop Hk tropy Khrushchev coeffi- 1 hardness cient, classes r -H 1 H° " h2 J = 0.7 p Nv Hlbcoc Optimum calculated, oad of by ova- j indeixter, rennykh (1963) ... 

Hardness measured in units of Mohs scale Mineral Chemical formula Measured ha Vickers Hv (with micro- measurement) rdness Knoop If Aniso- tropy coeffi- cient h2 H0 in Khrushchev hardness classes Ho = 0.7 J/fly Hi heor calculated by Pova-rennykh (1963) Optimum load of indenter P, mN... 

With the rapid advances now observed in hardness test methods (Vickers, Knoop, Grodzinski, Berkovich and Hanneman diamond indenters or Brinell and Rockwell hardness testers, etc.), the 10-degree Mohs scale poses a number of problems owing to the low distinction of hardness... 

Material Mohs scale Ridgeway s scale Wooilell s scale Knoop hardness3, kN/m b... 

Alaterial CAS Registry Number Alohs scale Ridgeway s3 scale WoodeU sb scale Knoop hardness, kN/m2d... 

One of the most popular techniques used for determining the hardness of a material is the Mohs scale that consists of a qualitative but an arbitrary hardness index scheme ranging from extremely soft materials (value of 1 Moh) to very hard materials such as diamond (10 Moh). Other techniques that are often employed for measuring hardness of substances are developed by Rockwell [72], Brinell [72], Knoop, and Vickers [73]. Over the years, more quantitative methods such as nanoindentation [74] have been developed. This technique applies a small and a controllable load on to the substrate with a probe. The depth of penetration along with a known geometry of the probe provides an indirect way to measure the area of contact at full penetration, which is then used to determine the hardness. The hardness is determined by the ratio of the total force to the contact area. Table 7.2 lists the bulk hardness of different materials, metal films, and abrasive particles, in both Moh and microhardness scales [75]. 

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... 

Vicker s hardness number (VHN). Hardness on Rockwell and Brinell scales are also reported for metals and alloys. In both KHN and VHN measurements, the approach is to make an indentation on the surface of the sample using a diamond micro-indenter. In the Vicker s method the indenter has the shape of a square pyramid, while in the Knoop s method it has the shape of an elongated pyramid. The indentation is generally of the order of few microns and a load of 5 to 1000 gm are used (in order to determine fracture toughness, it becomes necessary to use higher values of load (due to the necessity of initiating cracks)), VHN is given by. 

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