Abrasive tool making process – material – or composition – With inorganic material – Metal or metal oxide
Reexamination Certificate
2000-06-12
2001-07-10
Marcheschi, Michael (Department: 1755)
Abrasive tool making process, material, or composition
With inorganic material
Metal or metal oxide
C051S293000, C051S307000
Reexamination Certificate
active
06258141
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns sol-gel alumina abrasive grits and particularly abrasive grits that are intended for incorporation in bonded abrasives.
Sol-gel alumina abrasive grits, particularly those that have an alumina crystal size that is essentially sub-micron are extremely effective in many applications especially when incorporated in vitreous bonded abrasive wheels. In the production of such wheels, the grits are mixed with a vitreous bond material and various organic temporary binders, burnout materials and/or lubricants and the mixture is placed in a mold. The temporary binders and/or lubricants are needed to facilitate the mixing and molding operation and the burnout materials are needed to ensure that the wheel has a desired degree of porosity when completed. The burnout material is of course intended to be completely removed during the firing of the wheel.
During the production process the temperature of the mixture is raised to the point at which the bond components mix, (if a raw bond is used), and flow until the abrasive grits are coated with the vitreous bond material and the molten bond forms a bond post connecting the points on adjacent grits that contact or are in close proximity. After allowing the maturing of the bond post structures, the temperature is allowed to ramp down until the wheel can be removed from the mold.
The temperature at which the bond is fired is rather critical because it is found that sol-gel alumina abrasive grits with alumina crystal sizes of the order of one micron or less are deteriorated by exposure to temperatures higher than about 1150° C. for the prolonged periods required for formation of a vitreous bonded wheel. This is because the very fine crystal structure, which is closely connected to the excellent grinding performance, begins to be coarsened. For this reason it is strongly recommended that, when using sol-gel alumina abrasive grits, particularly with seeded sol-gel alumina abrasive grits, a bond should be used that is matured at a temperature below 1150° C. and preferably below about 1000° C.
In addition, the higher the temperature at which the bond is formed, the greater the extent of the penetration of the abrasive grits by silica from the vitreous bond system. This interaction, while not a serious problem with low temperature-matured bonds, is something that needs to be considered in developing optimum performance.
It is found however that, using such low temperature-matured vitreous bond materials, the organic materials that are supposed to be completely burned out in fact leave a residue of carbon. This residue collects in surface pores on the grits, which are typically white or colorless, such that the wheel produced may have black spots.
A process has now been developed that permits the use of low temperature vitreous bonds with sol-gel alumina abrasive grits without the development of the unsightly blemishes, or black spots, on the surface of the vitreous bonded product.
The process has also been found to have a beneficial effect on reducing the interaction between the bond and the abrasive grits thereby allowing the development of stronger bonds.
DESCRIPTION OF THE INVENTION
The present invention provides a coated sol-gel alumina abrasive grit in which the alumina grits are coated with a ceramic oxide and comprise crystals of alpha alumina that are two microns or smaller in size.
By the term “coated” is meant that the alpha alumina abrasive grits have been treated with a solution of a precursor of the ceramic oxide and then calcined at a temperature sufficient to drive off the solvent in which the precursor was dissolved and to form the ceramic oxide from the precursor. In practice the most usual effect is for the ceramic oxide to diffuse into the surface layer of the grit and reduce the porosity of the abrasive grit. This reduction can be by as much as 50%, though more commonly the surface area reduction is found to be from about 10% to 40%. The “surface area” as the term is used herein is understood to refer to the surface area as measured by the BET (Breuner, Emmet, Teller) equation and method. In some cases there is evidence that the ceramic oxide reacts, at least in part, with the alumina to form an aluminate, (which would include spinels in the case of certain oxides such as magnesium oxide). For the purposes of this invention such aluminates, where formed, are considered to be ceramic oxides, except for the purpose of calculating the amount of the ceramic oxide added when the ceramic oxide in the highest oxidation state is used as the calculation basis.
The invention therefore also provides a process for the production of sol-gel alumina abrasive grit which comprises forming alpha alumina abrasive grits with a crystal size of about 2 microns or less and then treating the grits with a solution or dispersion of a ceramic oxide precursor and subsequently heating the treated grits at a temperature sufficient to drive off the solvent and convert the precursor to a ceramic oxide.
The temperature of the treatment is preferably from 900 to 1300° C. and more preferably from 1000 to 1200° C. though higher or lower temperatures can be used provided that the alumina crystal size is not materially altered and providing the ceramic oxide is formed. It is also highly preferred that the temperature should be high enough to result in the BET surface area of the treated grits being reduced as indicated above.
The ceramic oxide precursor is preferably a salt that is soluble in water and which, upon heating, decomposes to the oxide and gases with no residual material. The salt can be for example a nitrate of an organic acid salt. The ceramic oxide itself is understood to refer to a metal oxide stable at temperatures over 1500° C. Typical examples include magnesium oxide, titanium dioxide, cobalt oxide, nickel oxide and chromium oxide. As indicated above the ceramic oxide may be in the form of the corresponding aluminates where these are formed by reaction of the oxide with the alumina. The amount of the ceramic oxide added is sufficient to provide up to one percent by weight, (measured as the oxide), of the total alumina grit weight.
The ceramic oxide may be added, as indicated above, as an aqueous solution of a soluble salt that yields the oxide when heated to temperatures to which the treated grain is above about at which alumina is exposed during processing. Alternatively and often preferably the ceramic oxide salt is dissolved or dispersed in a dispersion or sol of an alpha alumina precursor such as for example a boehmite sol or an aqueous dispersion of alumina trihydrate. This mode of addition can also place a layer of an alpha alumina or an alpha alumina precursor such as gamma alumina around the abrasive grits. More usually however the bulk of the alumina reacts to form an aluminate. This coating or layer also adds protection for the grits against attack from a vitreous bond when the grits are used to make vitreous bonded materials.
The sol-gel alumina from which the abrasive grits are made is preferably a seeded sol-gel alumina with a sub-micron crystal size. In the context of this invention, the crystal size is measured by the “intercept method” in which an SEM micrograph is analyzed by tracing a plurality of diagonal lines across the micrograph and dividing the total length of the lines by the number of crystals crossed by the lines and then multiplying the value obtained by a correction factor of 1.5.
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patent: 6080216 (2000-06-01), Erickson
Bauer Ralph
Sung Jason
Bennett David
Marcheschi Michael
Saint-Gobain Industrial Ceramics, Inc.
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