Alumina-zirconia abrasive

Zirconium oxide is fused with alurnina in electric-arc furnaces to make alumina—zirconia abrasive grains for use in grinding wheels, coated-abrasive disks, and belts (104) (see Abrasives). The addition of zirconia improves the shock resistance of brittle alurnina and toughens the abrasive. Most of the baddeleyite imported is used for this appHcation, as is zirconia produced by burning zirconium carbide nitride. 

The wear rates of alumina and alumina-zirconia abrasive belts have been compared (43). Alumina—zirconia was found to be much freer-cutting and requires less energy7 to remove a given volume of metal. Under moderate grinding conditions the alumina —zirconia abrasive removed 10 times more metal than the alumina abrasive and under severe conditions about 30% more. The newly developed sintered sol—gel alumina abrasives are more effective in some applications than alumina-zirconia in increasing cut-rate, reducing power requirements, and increasing belt or disk life. 

Fig. 2. SEM photomicrograph of polished section of near eutectic alumina-zirconia abrasive grain showing white zirconia in dark alumina matrix.

Each abrasive type has its own area of prime importance. For example, silicon carbide is used to grind glass, wood, and certain metal alloys, but is generally not recommended for ferrous alloy applications, except for certain white cast irons. Alumina-zirconia abrasives, on the other hand, can be used on a broad range of materials. 

The ability of organically modified ceramics based on alumina, zirconia, titania, or silica (and mixtures of each) to function as abrasion-resistant coatings has also been studied (62). For example, polycarbonate, when coated with an epoxy—aluminosilicate system, experiences a significant reduction in the degree of hazing induced by an abrader, as compared to uncoated polycarbonate. 

Coated abrasives consist of a flexible backing on which films of adhesive hold a coating of abrasive grains. The backing may be paper, doth, open-mesh cloth, vulcanized fiber (a specially treated cotton rag base paper), or any combination of these materials. The abrasives most generally used are fused aluminum oxide, sol—gel alumina, alumina-zirconia, silicon carbide, garnet, emery, and flint... 

In recent years a wide variety of inorganic, non-metallic materials has been developed for the electrical, nuclear power, and engineering industries. In the shaping and processing of these products some form of heat treatment is involved, and they too are regarded as ceramic materials. Examples are rutile, a form of titanium dioxide used for making ferroelectric materials steatite or talc, for electrical insulators alumina, zirconia, thoria and beryllia as refractories and electrical insulators, uranium oxide as a nuclear-fuel element, and nitrides and carbides as abrasives or insulators. 

Aluminum silicate, zirconium silicate, alumina, zirconia, ceria, magnesia, tin oxide, silicon carbide, boron carbide, tungsten carbide Hard coatings, abrasives, polishing media... 

Modem day abrasive grits have also progressed from natural minerals to synthetic abrasives, which include alumina-zirconia arid ceramic aluminum oxide abrasives. Conventional aluminum oxides are still widely used however, these new abrasive materials are... 

Other potential applications are ceramic powders coated with their sintering aids, zirconia coated withyttria stabilizer, tungsten carbide coated with cobalt, or nickel, alumina abrasive powders coated with a relatively brittle second phase such as MgAl204 and plasma spray powders without the segregation of alloying elements. 

Zinc oxide (ZnO) is widely used as an active filler in rubber and as a weatherability improver in polyolefins and polyesters. Titanium dioxide (TiOj) is widely used as a white pigment and as a weatherability improver in many polymers. Ground barites (BaS04) yield x-ray-opaque plastics with controlled densities. The addition of finely divided calcined alumina or silicon carbide produces abrasive composites. Zirconia, zirconium silicate, and iron oxide, which have specific gravities greater than 4.5, are used to produce plastics with controlled high densities. 

A wide variety of materials have been implemented as abrasive particles in CMP processes. They include alumina, silica, ceria, zirconia, titania, and diamond. The effectiveness and suitability of these particles in CMP with particular applications are greatly influenced by their bulk properties (density, hardness, particle size, crystallinity etc.) and the surface properties (surface area, isoelectric electric point (lEP), OH content, etc.). This section will focus on the evaluation of alumina, silica, diamond, and ceria as the major abrasives used for the CMP of metals. 

Abrasive particles are a key component in CMP slurry. The most commonly used abrasive particles include silica, alumina, ceria, zirconia, titania, and diamond. Table 21.1 listed a set of information on each type of abrasive particles such as density, microhardness, and isoelectric points (lEP). It is important to point out that the specific values for these properties depend highly on the preparation techniques and the specific states of the samples. The values listed in the table represent an average of the most commonly reported data. For example, the isoelectric point for silica is a function of the number of hydroxyl groups, type and level of adsorbed species, metal impurity in the solid matrix, and the treatment history of the materials [1]. There are three major types of silica according to their preparation methods fumed, colloidal, and precipitated. The common sources for obtaining these abrasive particles are listed in Table 21.2. As examples, some of the more specific information on... 

Zirconia combines with carbon to form a carbide which is also an abrasive. Combining zirconia with small quantities of silica, beryllia, magnesia and alumina lowers the melting point (alumina being the least detrimental), while thoria and yttria raise... 

Femoral ball heads of hip endoprostheses made from bioinert ceramics such as alumina or zirconia have to sustain high mechanical stresses, resorp-tion/corrosion by aggressive body fluid and abrasive wear over the lifetime of the implant in the human body of 15-20years. Some important properties of ceramic femoral ball heads are listed in Table 2.3 (Willmann, 1995). Mechanical properties of alumina and zirconia are discussed in Chapter 4.1. 

Alumina is the most important oxidic abrasion-resistant material. Metal carbides are in some ways superior to oxides with respect to hardness and melting point, but they are much more brittle than the oxides and are only used in isolated instances as wearing bodies. Silicon carbide is characterized by its low thermal expansion and high thermal conductivity and has proved to be more resistant to thermal shock than oxides. Zirconia is tougher than alumina its modulus of elasticity is only about half as large, and it is comparable with that of steel. Zirconia is therefore very suitable for compound structures with steel. At present, the applications of ceramic sintered materials in chemical plant construction are slide rings, pump parts, and slide bearings. 

---