Corundum hardness

Ail hardness values rounded and calculated assuming corundum hardness to be 1000... 

The hardness of a mineral as measured by the Mohs scale is a criterion of its resistance to crushing [Fahrenwald, Trans. Am. In.st. Min. Metall. Pet. Eng., 112, 88 (1934)]. It is a fairly good indication of the abrasive character of the mineral, a factor that determines the wear on the grinding media. Arranged in increasing order or hardness, the Mohs scale is as fohows 1, talc 2, gypsum 3, calcite 4, fluoride 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum and 10, diamond. 

Hard Materials (7) Quartz, granite (8) topaz (9) corundum, sapphire, emeiy (10) diamond. 

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. 

Diamant-glanz, m. adamantine luster, -kitt, m. diamond cement. -mOrser, m, diamond mortar (small steel mortar), -pulver, n. diamond dust, -schwarz, n. diamond black, -spat, m. adamantine spar (corundum). -Stahl, m, very hard steel, tool steel, Diamido-. diamino-, diamido- (see Amido-), -benzol, n. diaminobenzene, -toluol, n. diaminotoluene. 

Aluminum Oxide (Alumina, oAlumina, Corundum, Alandum). A1203, mw 101.96, v hard white cryst powd, mp 2045°, bp 2980°, d 3.965g/cc. Insol in w, v si sol in aq alkaline solns. Found in nature as a mineral such as bauxite. Lab prepn is by heating aluminum hydroxide to above 1100°. 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

Excellent (hardness above 7 on Mohs scale) Diamond Corundum... 

Aluminum oxide, which has the mineral name corundum, is a solid that has several important uses. Because it will withstand very high temperatures, it is a refractory material, and because of its hardness it is commonly used in abrasives. Corundum often contains traces of other metals that impart a color to the crystals, making them valuable as gemstones. For example, ruby contains a small amount of chromium oxide, which causes the crystal to have a red color. By adding a small amount of a suitable metal oxide, it is possible to produce gemstones having a range of colors. 

Tungsten carbide (WC) is extremely hard and resistant to high temperatures. When cemented to tools, it is as hard as corundum (aluminum oxide) and makes excellent grinding surfaces and cutting edges for machine tools. 

Chromium(III) oxide crystallizes in the rhombohedral structure of the corundum type space group D3d-R3c, Q 5.2 g/cm3. Because of its high hardness (ca. 9 on the Mohs scale) the abrasive properties of the pigment must be taken into account in certain applications [3.44], It melts at 2435 °C but starts to evaporate at 2000 °C. Depending on the manufacturing conditions, the particle sizes of chromium oxide pigments are in the range 0.1-3 pm with mean values of 0.3-0.6 pm. Most of the particles are isometric. Coarser chromium oxides are produced for special applications, e.g., for applications in the refractory area. 

The effects of high hardness are important in many fields. Abrasives are used to form and polish many substances. Diamonds are an important mineral component in cutting tools for the manufacturing of metals and other substances, forming dies for the drawing of wires, and for cutting cores in oil wells and mineral exploration. Emery - a variety of corundum, is used in many abrasive products that do not require the hardness (or expense) of diamond tools. Garnets were used as an abrasive in sandpaper. Talc is an extremely soft mineral that has been used in bath powders (talcum powder). 

A grinding wheel, synthetic corundum, grain size 30, conventional hardness M and structure 6. 

Fig. 5.1. Variation of hardness in corundum a-Al202 with elementary cell parameters, being a function of Ti203 content computed to Ti02 (from data of Schrevelius, 1948).

Effect of modifying admixtures on hardness of corundum ceramics (after Pavlushkin, 1961)... 

The influence of atmospheric air on the properties of mineral materials manufactured in thermal processes is generally known. An example of the nature of this phenomenon as regards hardness, is a series of Vickers hardness tests of a material made of sintered corundum modified with 0.6% MgO sintered at 1950-2050 K in various environments. The sintering process is accelerated in the presence of hydrogen and is slowest in air thus allowing a material with optimum parameters to be obtained at a significantly lower temperature. The results, specified in Table 6.2.4, show the gases used as... 

Hardness of synthetic corundum measured with Grodzinski double cone in different directions in relation to opitical axis C (1961 mN, 30 s)... 

Once the hardness of standards has been determined from the Mohs scale by more precise methods, it became apparent that the Mohs scale in the interval 1-9 (talc-corundum) represents a series of minerals with a uniformly increasing hardness whose magnitudes are spread roughly on a curve corresponding to a mathematical function approximating a parabola of third degree (y = 3) (Fig. 4.5.1). As a result, the introduction of a quantitative method of hardness measurement with diamond indenters has not reduced the importance of the Mohs scale in any way on the contrary, its position has been enhanced. It will continue to be the basic macrodiagnostic method. 

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