Alumina toughness

Sintered abrasives made from bauxite and calcined alumina are heavy-duty abrasives they are much too strong and tough for precision grinding. 

In order to extead appHcatioas of cBN to iaclude machining of medium-hardness steels, modifications of the cBN were iatroduced. An example is the fabrication of siatered cBN tools by the same HP—HT process, but usiag biader and second phase (either metallic or nonmetaUic) such as TiN or TiC to iacrease toughness (171). In regard to phase distribution, cBN tools resemble cemented-carbide or alumina—TiC ceramic tools, but are tougher and have greater chemical stabUity. 

SiC whisker-reinforced alumina is a major advance in tool material development, as it provides a means to increase the fracture toughness of the material via the composite material approach. It is entirely possible that in the next century many new whiskers of refractory, hard materials will be made... 

In the glass (qv) and ceramic industry (see Ceramics), barite can be used both as a flux, to promote melting at a lower temperature or to increase the production rate, and as an additive to increase the refractive index of glass. The viscosity of barite-containing glass often needs to be raised. Alumina in the form of feldspar is sometimes used. To offset any color produced by iron from the barite addition, more decolorizer may be needed. When properly used, barytes help reduce seed, increase toughness and brilliancy, and reduce annealing time. Barite is also a raw material for the manufacture of other barium chemicals. 

Alumina (AI2O3) was one of the first pure oxides to be produced in complex shapes, but its combination of high expansion coefficient, poor conductivity and low toughness gives it bad thermal-shock resistance. 

There are, of course, many more ceramics available than those listed here alumina is available in many densities, silicon carbide in many qualities. As before, the structure-insensitive properties (density, modulus and melting point) depend little on quality -they do not vary by more than 10%. But the structure-sensitive properties (fracture toughness, modulus of rupture and some thermal properties including expansion) are much more variable. For these, it is essential to consult manufacturers data sheets or conduct your own tests. 

Cutting tools Hardness, toughness Alumina, siolons... 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

Michalske, T.A. and Hellmann, J. (1988). Strength and toughness of continuous alumina fiber reinforced glass matrix composites. J. Am. Ceram. Soc. 71, 725 731. 

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