Abrasive grain, abrasive articles, and methods of making and...

Details Abrasive grain, abrasive articles, and methods of making and... Abrasive grain, abrasive articles, and methods of making and...

1999-09-28

2001-09-11

Marcheschi, Michael (Department: 1755)

Abrasive tool making process, material, or composition

With inorganic material

Metal or metal oxide

C051S295000, C051S308000, C501S128000, C451S028000

Reexamination Certificate

active

06287353

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to abrasive grain and a method of making abrasive grain. The abrasive grain can be incorporated into a variety of abrasive articles, including bonded abrasives, coated abrasives, nonwoven abrasives, and abrasive brushes.
DESCRIPTION OF RELATED ART
In the early 1980's a new and substantially improved type of alumina abrasive grain, commonly referred to as “sol gel” or “sol gel-derived” abrasive grain, was commercialized. This new type of alpha alumina abrasive grain had a microstructure made up of very fine alpha alumina crystallites. The grinding performance of the new abrasive grain on metal, as measured, for example, by life of abrasive products made with the grain was dramatically longer than such products made from conventional, fused alumina abrasive grain.
In general, sol gel abrasive grain are typically made by preparing a dispersion or sol comprising water, alumina monohydrate (boehmite), and optionally peptizing agent (e.g., an acid such as nitric acid), gelling the dispersion, drying the gelled dispersion, crushing the dried dispersion into particles, calcining the particles to remove volatiles, and sintering the calcined particles at a temperature below the melting point of alumina. Frequently, the dispersion also includes one or more oxide modifiers (e.g., CeO
2
, Cr
2
O
3
, CoO, Dy
2
O
3
, Er
2
O
3
, Eu
2
O
3
, Fe
2
O
3
, Gd
2
O
3
, HfO
2
, La
2
O
3
, Li
2
O, MgO, MnO, Na
2
O, Nd
2
O
3
, NiO, Pr
2
O
3
, Sm
2
O
3
, SiO
2
, SnO
2
, TiO
2
, Y
2
O
3
, Yb
2
O
3
, ZnO, and ZrO
2
), nucleating agents (e.g., &agr;-Al
2
O
3
, &agr;-Cr
2
O
3
, and &agr;-Fe
2
O
3
) and/or precursors thereof. Such additions are typically made to alter or otherwise modify the physical properties and/or microstructure of the sintered abrasive grain. In addition, or alternatively, such oxide modifiers, nucleating agents, and/or precursors thereof may be impregnated into the dried or calcined material (typically calcined particles). Further details regarding sol gel abrasive grain, including methods for making them, can be found, for example, in U.S. Pat. No. 4,314,827 (Leitheiser et al.), U.S. Pat. No. 4,518,397 (Leitheiser et al.), U.S. Pat. No. 4,623,364 (Cottringer et al.), U.S. Pat. No. 4,744,802 (Schwabel), U.S. Pat. No. 4,770,671 (Monroe et al.), U.S. Pat. No. 4,881,951 (Wood et al.), U.S. Pat. No. 4,960,441 (Pellow et al.) U.S. Pat. No. 5,011,508 (Wald et al.), U.S. Pat. No. 5,090,968 (Pellow), U.S. Pat. No. 5,139,978 (Wood), U.S. Pat. No. 5,201,916 (Berg et al.), U.S. Pat. No. 5,227,104 (Bauer), U.S. Pat. No. 5,366,523 (Rowenhorst et al.), U.S. Pat. No. 5,429,647 (Larmie), U.S. Pat. No. 5,547,479 (Conwell et al.), U.S. Pat. No. 5,498,269 (Larmie), U.S. Pat. No. 5,551,963 (Larmie), U.S. Pat. No. 5,725,162 (Garg et al.), and U.S. Pat. No. 5,776,214 (Wood).
Certain preferred alpha alumina-based abrasive grains are highly dense (i.e., greater than 95% of theoretical) and have a fine (e.g., submicrometer), uniform alpha alumina microstructure. Further, some preferred alpha alumina-based abrasive grains include oxide modifiers, as discussed above, which may, in some cases may also include submicrometer oxides other than alpha alumina, wherein the latter may or may not be submicrometer. The grain size of the alpha alumina and other oxides, the oxide phases present in the abrasive grain, as well as the physical properties (e.g., density, hardness, and toughness) or characteristics may depend, for example, on the particular composition and/or process (including sintering time and temperature) used to make the abrasive grain. For example, longer sintering times and higher temperatures tend to provide higher density abrasive grains. However, longer sintering times and higher temperatures also tend to undesirably increase grain growth.
For some higher pressure grinding applications, it is preferred that the sintered alumina abrasive grain be relatively tough to withstand the high grinding forces. Such increased toughness may be achieved through the addition of various metal oxides to the alumina crystal structure. Alternatively, in some lower pressure grinding applications, it is preferred that the sintered alumina abrasive grain be more friable so that the abrasive grain can “breakdown” during grinding. In order to achieve the optimum grinding performance under these wide ranges of grinding conditions, a variety of sintered alpha alumina abrasive grains have been developed and commercialized.
Over the past fifteen years sintered alumina abrasive grain, in particular sol gel-derived alpha alumina-based sintered abrasive grain, have been used in a wide variety of abrasive products (e.g., bonded abrasives, coated abrasives, and abrasive brushes) and abrading applications, including both low and high pressure grinding applications. For example sol gel-derived abrasive grain have been incorporated into resin bonded grinding wheels, and have been found to be particularly useful in high pressure, high stock removal grinding applications. Such abrasive grain have been used in vitrified grinding wheels for the precision grinding of camshafts. Sol gel-derived abrasive grain have also been incorporated into medium grade coated abrasive products that are used to sand wood cabinet panels. In addition, coated abrasive discs that include sol gel-derived abrasive grain are used under relatively light pressure to abrade painted automotive parts.
SUMMARY OF THE INVENTION
In one aspect, the present invention surprisingly provides sintered alpha alumina-based abrasive grain comprising MgO, SiO
2
, and ZrO
2
(typically at least 0.1 percent (preferably, at least 0.2, 0.25, 0.3, or even 0.5 percent) by weight of each of MgO, SiO
2
, and ZrO
2
, based on the total metal oxide content of the abrasive grain), at least 0.1 percent by weight of each of MgO, SiO
2
, and ZrO
2
, based on the total metal oxide content of the abrasive grain, wherein the alpha alumina of the abrasive grain has an average crystallite size of less than 1 (preferably, less than 0.8, 0.7 0.6, 0.5, 0.4, or even 0.3) micrometer, and wherein the ZrO
2
that is present as crystalline zirconia has an average crystallite size of less than 0.25 micrometer. Typically, at least a majority of the alpha alumina was nucleated with a nucleating agent Preferably the average crystallite size of the alpha alumina is less than 0.75 micrometer, more preferably, less than 0.5 micrometer, and even more preferably, less than 0.3 micrometer.
Although not wanting to be bound by theory, it is believed that the presence of the Al
2
O
3
, MgO, SiO
2
, and ZrO
2
and the crystallite size of the Al
2
O
3
and ZrO
2
have a significant affect on its grinding performance. The small alpha alumina crystals are believed to result in a fast cutting, long lasting abrasive; the presence of zirconia to result in an abrasive grain that works well at higher pressures; and the presence of silica, as well as the zirconia, aiding in the densification to ensure small alpha alumina crystals. In addition, it is believed that the presence of the zirconia may toughen the abrasive grain. Further, the MgO is believed to aid during sintering in obtaining finer grained alpha alumina microstructure. In addition, it believed that the presence of the MgO is believed to aid in the toughening of the abrasive grain via the formation of a spinel phase during the sintering process.
One preferred sintered alpha alumina-based abrasive grain according to the present invention comprises at least 0.1 percent (preferably, at least 0.2, 0.25, 0.3, or even 0.5 percent; more preferably at least 1 percent; even more preferably in the range from 1 to 3 percent) by weight SiO
2
, at least 0.1 percent (preferably, at least 0.2, 0.25, 0.3, or even 0.5 percent; more preferably at least 1 percent; even more preferably in the range from 1 to 14 percent, or even from 1 to 7 percent) by weight ZrO
2
, and at least 0.1 percent (preferably, at least 0.2, 0.25, 0.3, or even 0.5 percent; more preferably at least 1 percent; even more preferably in the range from 3 to 7 percent) b

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