Cubic hardness

Tabor relationship, 13, 238-239 Tantalum nitride (TaN), cubic, hardness, 297... 

Fig. 2. PMMA template used for growth of face-centered cubic hard-sphere coUoids in the (100)...

Figure Bl.21.1. Atomic hard-ball models of low-Miller-index bulk-temiinated surfaces of simple metals with face-centred close-packed (fee), hexagonal close-packed (licp) and body-centred cubic (bcc) lattices (a) fee (lll)-(l X 1) (b)fcc(lO -(l X l) (c)fcc(110)-(l X 1) (d)hcp(0001)-(l x 1) (e) hcp(l0-10)-(l X 1), usually written as hcp(l010)-(l x 1) (f) bcc(l 10)-(1 x ]) (g) bcc(100)-(l x 1) and (li) bcc(l 11)-(1 x 1). The atomic spheres are drawn with radii that are smaller than touching-sphere radii, in order to give better depth views. The arrows are unit cell vectors. These figures were produced by the software program BALSAC [35]-...

Charged particles in polar solvents have soft-repulsive interactions (see section C2.6.4). Just as hard spheres, such particles also undergo an ordering transition. Important differences, however, are that tire transition takes place at (much) lower particle volume fractions, and at low ionic strengtli (low k) tire solid phase may be body centred cubic (bee), ratlier tlian tire more compact fee stmcture (see [69, 73, 84]). For tire interactions, a Yukawa potential (equation (C2.6.11)1 is often used. The phase diagram for the Yukawa potential was calculated using computer simulations by Robbins et al [851. 

Natural titanium is reported to become very radioactive after bombardment with deuterons. The emitted radiations are mostly positrons and hard gamma rays. The metal is dimorphic. The hexagonal alpha form changes to the cubic beta form very slowly at about 88O0C. The metal combines with oxygen at red heat, and with chlorine at 550oC. 

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. 

Ferrites can be classified according to crystal stmcture, ie, cubic vs hexagonal, or magnetic behavior, ie, soft vs hard ferrites. A systematic classification as well as some appHcations ate given in Table 1 (see also Magnetic materials, bulk Magnetic materials, thin film). 

Diamond is supreme among natural gemstones ia H, RI, and DISP. Table 3 shows the steady improvement ia the sequence of diamond imitations, the aim being to produce a colorless, adequately hard material having closely matching optical properties. The iatroduction of synthetic cubic 2irconia ia 1976 brought about a sufficiently close match. 

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. 

Lead Telluride. Lead teUuride [1314-91 -6] PbTe, forms white cubic crystals, mol wt 334.79, sp gr 8.16, and has a hardness of 3 on the Mohs scale. It is very slightly soluble in water, melts at 917°C, and is prepared by melting lead and tellurium together. Lead teUuride has semiconductive and photoconductive properties. It is used in pyrometry, in heat-sensing instmments such as bolometers and infrared spectroscopes (see Infrared technology AND RAMAN SPECTROSCOPY), and in thermoelectric elements to convert heat directly to electricity (33,34,83). Lead teUuride is also used in catalysts for oxygen reduction in fuel ceUs (qv) (84), as cathodes in primary batteries with lithium anodes (85), in electrical contacts for vacuum switches (86), in lead-ion selective electrodes (87), in tunable lasers (qv) (88), and in thermistors (89). 

The diboride has a hexagonal stmcture and melts at 3245°C (136) it is considered to have the best oxidation resistance of all the refractory hard metals. The dodecaboride has a cubic stmcture. 

The greatest use of cubic boron nitride is as an abrasive under the name Bora2on, in the form of small crystals, 1—500 p.m in si2e. Usually these crystals are incorporated in abrasive wheels and used to grind hard ferrous and nickel-based alloys, ranging from high speed steel tools and chilled cast-iron to gas turbine parts. The extreme hardness of the crystals and their resistance to attack by air and hot metal make the wheels very durable, and close tolerances can be maintained on the workpieces. 

The cubic BN crystals may also be bonded into strong bodies that make excellent cutting tools for hard iron and nickel-based alloys. Such tools produce red-hot chips and permit the wider use of tough, high temperature alloys which would otherwise be prohibitively difficult to shape (12,20,21) (see... 

Table 3 summarizes the properties of the so-called nonmetallic hard materials, including diamond and the diamondlike carbides B C, SiC, and Be2C. Also iacluded ia this category are comadum, AI2O2, cubic boroa nitride, BN, aluminum nitride, AIN, siUcon nitride, Si N, and siUcon boride, SiB (12). 

Fig. 5.1. The close packing of hard-sphere atoms. The ABC slacking gives the face-centred cubic (f.c.c.) structure.

This "packing" argument may seem an unnecessary complication. But its advantage comes now. Consider cubic zirconia, ZrOj, an engineering ceramic of growing importance. The structure (Fig. 16.1c) looks hard to describe, but it isn t. It is simply an f.c.c. packing of zirconium with the ions in the tetrahedral holes. Since there are two tetrahedral holes for each atom of the f.c.c. structure, the formula works out at ZrOj. 

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