Method for Improving the Quality of Abrasive Grains

1. Field of the Invention

The invention relates to a method for improving the quality of abrasive grains of Al.sub.2 O.sub.3 comprising titanium oxide, preferably Ti.sub.2 O.sub.3, whereby a ceramic coating is applied, e.g. melted and/or sintered, onto the abrasive grain.

The invention also relates to the treatment of conventional abrasive grains of molten or sintered electrocorundum, which are commercially available as normal corundum or microcrystalline corundum and contain between 0.5 and 5.0 weight percent of titanium oxide (Ti.sub.2 O.sub.3 and TiO.sub.2), the chief constituent being Ti.sub.2 O.sub.3.

2. Description of the Prior Art

Corundum, which is to be used as an abrasive means in abrasive articles, such as abrasive disks, is, according to prior art methods, occasionally annealed at a temperature of about 1350.degree. C., whereby the grain structure is changed and the abrasive properties are improved. This treatment shows in the blue colour which the corundum takes on in the course of this treatment.

It is further known in the art to provide corundum grains with a coating, e.g. of ceramic materials and ferric oxides, producing an improved adhesion between the abrasive grain and the bond in the abrasive article resp. having filler properties and taking an active part in the abrasive process, e.g. by improving the heat removal.

In prior art methods the abrasive grain is annealed and coated in two separate operating cycles. First the abrasive grain is heated in oxidizing atmosphere to a temperature of about 1350.degree. C. until it takes on blue colour and then the abrasive grain is cooled off again to room temperature. After cooling off, a ceramic coating is applied onto the abrasive grain in a separate operating cycle, said coating being based on phosphates or silicates with low-melting, ceramic frits (glass frits) and fine ground metallic oxides--preferably Fe.sub.2 O.sub.3 for cost saving reasons. For this purpose the abrasive grain is heated again to a temperature of about 600.degree. C. to 800.degree. C.

The thus obtained coating creates a greater surface and a better surface wettability due to its rough, granular surface structure. Consequently, a greater and improved adhesive surface is provided for the resinoid bond of the abrasive article, whereas the adhesion between the abrasive grain and the coating is frequently not as intense as required. When abrasive grains treated in the above-described manner are inserted into the abrasive article, grains break off easily, which have not or only insufficiently been utilized in the abrasive process, particularly in the case of abrasive disks of high density. A further reason for the breaking off of abrasive grains is the porous and relatively thick (15-50 .mu.m) coating itself which is only of low strength. The liquid constituent of the resinoid bond is not always able to soak the thick coating entirely, i.e. to its bottom. Therefore, the porous coating itself forms a preferred fracture zone in the course of subsequent stress. Even if good adhesion is obtained between the coating and the grain and between the coating and the bond, the coating itself can be torn, whereby one part of the coating adheres to the grain, and the other part adheres as corresponding part to the binding material of the abrasive article.

The above-mentioned annealing process, i.e. the heating of the abrasive grain to a temperature of about 1350.degree. C., alters the fracture behaviour and the strength properties of the abrasive grain and increases the abrasive capacity of the finished abrasive tool. It also guarantees a cooler grinding. The change from brown into blue and the modified mechanical properties are due to the change of the titanium from the trivalent into the quadrivalent degree of oxidation. A maximum of 5% by weight of titanium in the form of oxides (particularly Ti.sub.2 O.sub.3) is contained in the abrasive grain of the present invention. This rise in the degree of oxidation from Ti.sup.3+ to Ti.sup.4+ effects the "precipitation" of the Ti.sub.2 O.sub.3, which is "dissolved" in the Al.sub.2 O.sub.3 in the form of TiO.sub.2.

It is the object of the present invention to provide a method in which the annealing process, which effects the change in the grain structure, and the coating of the grain, e.g. with a layer improving adhesion, are carried out in one operating cycle and which allows a reduction in the consumption of energy.

According to the invention this is achieved by exposing the abrasive grain to a temperature between 1250.degree. and 1350.degree. C. over a period of between 15 minutes and 2 hours, whereby the ceramic coating is sintered resp. melted onto the abrasive grain simultaneously with the change of the Ti-oxides from the trivalent into the quadrivalent degree of oxidation, and thus simultaneously with the change in the grain structure.

The coating of the abrasive grain treated in accordance with the method of the present invention has a thickness of only 2-5 .mu.m. Said coating is characterized by an extremely great adhesion to the grain surface and great internal strength, which is due to an intermediate layer made of spinel, for example. Apart from the energy-saving effect as compared to conventional methods, the coating applied onto the abrasive grain in accordance with the present invention has the great advantage of an extraordinary adhesion to the grain, high internal strength and reduced thickness.

It is preferably provided that the period of the heat treatment lies between 20 minutes and 45 minutes.

It is further preferably provided that the coating materials are silicates, clay, kaolin and/or high-melting glass frits.

A further preferred embodiment of the present invention provides that metallic oxides, e.g. ferric oxides and/or manganese oxides and/or chromium oxides, are admixed to the coating material resp. contained therein.

At burning temperatures of between 1250.degree. and 1350.degree. C. the controlled use of metallic oxides effects the separation of oxygen in accordance with the following example:

In the present embodiment the oxygen is always set free immediately at the grain surface, and consequently the oxidation of the trivalent titanium is considerably accelerated by the nascent oxygen. This additional oxygen supply allows a reduction of the burning period by up to 50% as compared to conventional annealing methods.

It is preferably provided that the metallic oxide constituent amounts to about 1-5% by weight referred to the amount of abrasive grain.

A further embodiment of the present invention provides that substances, e.g. ZrO.sub.2, are admixed to the coating material, which have a coefficient of expansion varying from the coefficient of expansion of the ceramic material (and the abrasive grain) as much as possible.

By adding zirconium corundum, for example, whose high thermal change in volume, when being cooled off after the burning process, produces a fine mesh of microcracks, a greater coating surface and an increase in the tensile- and bending strength of the coating and, thus, occasionally of the abrasive grain, are obtained, as said microcracks prevent the formation of great cracks.

A particularly preferred embodiment of the present invention in which the adhesion between the abrasive grain and the resinoid bond of the abrasive disk is substantially improved provides that between 0.5 and 1.5 weight percent, preferably 1.1 weight percent, of SiC referred to the amount of grain are admixed to the coating material.

Said admixture effects that the SiC decomposes resp. oxidizes already at temperatures from 900.degree. C. onwards. An optimum degree of decomposition is obtained at the temperatures applied in the treatment of the grain in accordance with the present invention, i.e. 1250.degree.-1350.degree. C. Gaseous carbon dioxide resp. carbon monoxide and SiO resp. SiO.sub.2 are produced in the course of said decomposition, whereby the gaseous carbon oxides create blisters in the molten material of the coating. Said blisters produce an obvious improvement in the coating surface and, consequently, an increased external adhesive surface on the abrasive grain and an improved adhesion of the coated abrasive grain in the binding material.

In accordance with the present invention the following substances, either alone or in mixtures, are, for example, used as a coating material:

Further coating materials used in accordance with the present invention are phosphates, borates and silicates, which are applied individually or in a mixture.

When applying the coating material onto the grain surface in accordance with the present invention it has proved advantageous to moisten the abrasive grain.

The following wetting agents are used by the present invention:

aqueous solutions of silicates, phosphates and borates,

phosphoric acids,

boric acids,

silicic acids,

organic Si-compounds, such as ethyl- and methyl polysiloxane.

It is essential to choose coating and wetting materials which are suitable for producing on the abrasive grain a rough, macroscopically closed coating which sporadically contains the above-mentioned microcracks. It is further essential that in spite of the high temperatures during the heat treatment no undesired intense edge corrosion occurs on the abrasive grain, which would considerably deteriorate its abrasive properties. A slight surface etching of the abrasive grains is indispensable, however, as such etching allows the formation of a chemical layer (cf. drawing). The wetting agents have to be used in suitably low concentrations.

In the following five embodiments of the present invention will be described:

The FIGURE of the drawing shows in accordance with embodiment 4 the abrasive grain 1 (normal corundum, Al.sub.2 O.sub.3), an adjacent galaxite intermediate layer MnAl.sub.2 O.sub.4 2 and a sintered layer of manganese oxide grains 3 in a glass matrix 4. The intermediate layer 2 forms a chemical as well as a mechanical bond with the corundum 1 and the actual coating layer 3. Microcracks due to ZrO.sub.2, for example, are shown by the number 5.

The FIGURE of the drawing shows a schematic sectional view of the boundary layer of an abrasive grain.

Preferably after being moistened the abrasive grain is mixed with the fine ground coating material in a conventional mixer. It is then coated and subsequently burned in a rotary tubular kiln at a temperature of between 1250.degree. C. and 1350.degree. C., whereby the heat treatment should not exceed 2 hours and last over a period of between 20 and 45 minutes, for example.

In order to find out the influence of the treatment of the abrasive grain on the strength behaviour of a finished abrasive article, specimens of the below-indicated composition were made. Their dimensions were 120.times.10.times.15 mm for testing the bending yield strength and 10.times.10.times.15 mm for testing the compressive yield strength. They were compressed to a compressive density of d.sub.o =. . . 2,79.

Hardening was carried out at a temperature of 190.degree. C. over 54 hours.

Specimens (severing disks--400.times.45.times.32 mm) composed as indicated above were made of untreated, conventionally annealed and coated normal corundum and of normal corundum treated in accordance with the present invention and then tested. In the tests round profiles .phi. 30 mm of constructional steel were severed. The results obtained in these comparative tests are listed in the following table: