Abrasives alumina

The frictional characteristics of abrasive alumina and silica particles were investigated and are shown in Fig. 16.28. The alumina slurry was very sensitive to the slurry chemistry. The highest frictional force of 9 kgf was observed in DI water, and the lowest frictional force of 4 kgf was measured when citric acid was added into the alumina slurry. The frictional forces of silica particles (6 kgf) were about the same with or without citric acid during CMP. This is consistent with the fact that citrate has little adsorption onto silica particles. Yoon et al. [69] reported that higher adhesion force between two surfaces caused higher friction force on them. 

To quantify the effects of handling on slurries, a series of experiments were performed. Tests 16 included handling and filtration experiments performed using silica-M slurry, whereas Tests 7-13 included experiments with silica-H slurry. Tests 14-15 and 16-17 involved handling and filtration tests conducted using low wt% abrasive alumina and ceria slurries, respectively. LPC and PSD from Tests 1-11 will be presented and discussed, and key findings of Tests 12-17 will be summarized as follows. 

Abrasive Alumina abrasive a-alumina particles ctiwabel, 1991 Bange, 2001)... 

Refractoriness (Melting Temperature). Instantaneous grinding temperatures may exceed 3500°C at the interface between an abrasive and the workpiece being ground (14). Hence melting temperature is an important property. Additionady, for alumina, sdicon carbide, B C, and many other materials, hardness decreases rapidly with increasing temperature (7). Fortunately, ferrous metals also soften with increasing temperatures and do so even more rapidly than abrasives (15). 

Special pink or mby variations of the white abrasive are produced by adding small amounts of chromium compounds to the melt. The color is dependent on the amount of chromium added. A green alumina, developed by Simonds Abrasives, results from small additions of vanadia [11099-11-9]. Each was developed to improve on the suitabiUty of white abrasive for tool and precision grinding. 

In 1989, 164,200 t of regular fused alumina abrasive and 30,630 t of high purity fused alumina were produced in the United States and Canada, valued at U.S. 62.3 and 19.4 million, respectively (20). 

One advantage of sintering is the close control of si2e and shape of the abrasive particle. Extmded, cylindricaHy shaped, sintered abrasives of circular cross section were produced from bauxite (24) and from calcined alumina (25). The Uelt2 sintered bauxite was also later produced in extmded cylinder form and designated as 76A. Extmded sintered abrasives of a wide variety of cross-sectional configurations, eg, square and triangular, were later patented (26). 

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

Sol—Gel Sintered Aluminum Oxide. A new and much more versatile sintered alumina abrasive is now produced from aluminum monohydrate, with or without small additions of modifiers such as magnesia, by the sol—gel process (see Sol-gel technology). The first modified sol—gel abrasive on the market, Cubitron, was patented (27) and produced by the 3M Corporation for products such as coated belts and disks. The success of this material promoted intensive research into sol—gel abrasives. 

A higher density sol—gel abrasive, produced by the introduction of seed crystaUites formed by wet-milling with high alumina media or by introduction of submicrometer a-alumina particles, was patented (28) and designated Norton SG. The microstmcture of this abrasive consists of submicrometer a-alumina crystals (Fig. 1) and its bulk density approaches that of fused alumina. Norton SG has proven to be an exceptional performer in coated and bonded abrasive products it was awarded the 1989 ASM Engineering Materials Achievement Award (29). 

Fig. 1. SEM photomicrograph of polished and thermally etched section of Norton SG sol—gel alumina abrasive grain.

Eig. 2. SEM photomicrograph of poHshed section of neat eutectic alumina-2inconia abrasive grain showiag white 2inconia ia dark alumina matrix. 

Testing. Chemical analyses are done on all manufactured abrasives, as well as physical tests such as sieve analyses, specific gravity, impact strength, and loose poured density (a rough measure of particle shape). Special abrasives such as sintered sol—gel aluminas require more sophisticated tests such as electron microscope measurement of a-alumina crystal si2e, and indentation microhardness. 

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

An important appHcation of MMCs in the automotive area is in diesel piston crowns (53). This appHcation involves incorporation of short fibers of alumina or alumina—siHca in the crown of the piston. The conventional diesel engine piston has an Al—Si casting alloy with a crown made of a nickel cast iron. The replacement of the nickel cast iron by aluminum matrix composite results in a lighter, more abrasion resistant, and cheaper product. Another appHcation in the automotive sector involves the use of carbon fiber and alumina particles in an aluminum matrix for use as cylinder liners in the Prelude model of Honda Motor Co. 

Nuclear utiUties have sharply reduced the volume of low level radioactive waste over the years. In addition to treating wastes, utiUties avoid contamination of bulk material by limiting the contact with radioactive materials. Decontamination of used equipment and materials is also carried out. For example, lead used for shielding can be successfully decontaminated and recycled using an abrasive mixture of low pressure air, water, and alumina. 

Calcined alumina markets consume slightly less than 50% of the specialty alumina chemicals production (1—8,20,22—115). Worldwide usage is estimated to be about 50% for refractories (qv), 20% for abrasives, and 25% for ceramics (qv). Calcined aluminas are also used in the manufacture of tabular alumina and calcium aluminate cements (CAC). Quantities are estimated to be over 200,000 and 100,000 t, respectively (7). 

Refractories. Calcined alumina is used in the bond matrix to improve the refractoriness, high temperature strength/creep resistance, and abrasion/corrosion resistance of refractories (1,2,4,7). The normal, coarse (2 to 5 )J.m median) crystalline, nominally 100% a-Al202, calcined aluminas ground to 95% —325 mesh mesh are used to extend the particle size distribution of refractory mixes, for alumina enrichment, and for reaction with... 

Pulpstones. Improvements have been made in the composition and speed of the grinding wheel, in methods of feeding the wood and pressing it against the stone, in control of power to the stones, and in the size and capacity of the units. The first pulpstones were manufactured from quarried sandstone, but have been replaced by carbide and alumina embedded in a softer ceramic matrix, in which the harder grit particles project from the surface of the wheel (see Abrasives). The abrasive segments ate made up of three basic manufactured abrasive siUcon carbide, aluminum oxide, or a modified aluminum oxide. Synthetic stones have the mechanical strength to operate at peripheral surface speeds of about 1200—1400 m /min (3900 to 4600 ft/min) under conditions that consume 0.37—3.7 MJ/s (500—5000 hp) pet stone. 

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. 

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