Vitrified grinding wheel

Abstract: Bonding of sintered sol-gel aluminous abrasive grits, comprising sub-micron sized alpha alumina crystals, in vitrified (ceramic) wheels results in abrasive performance superior to premium grade fused alumina in grinding tool-steels. Viscosity of the bond and/or temperature of firing must be controlled to avoid reaction between the bond and the abrasive.

Description: This invention relates to vitrified (glass) bonded grinding wheels containing aluminous abrasive. Heretofore, the best abrasive (other than cubic boron nitride) for use in vitrified grinding wheels for grinding tool steels, e.g. toolroom grinding, have been very high purity alumina grits preferably produced by fusion of alumina in a sulfide matrix, and are essentially mono-crystalline particles. Recently alumina-spinel abrasives produced by the process of U.S. Patent 4,314,827 have appeared on the market in coated abrasive products such as flexible cloth-bonded belts and fiber discs. Such abrasives, although they can be used in making conventional vitrified bonded grinding wheels, perform no better than the best alumina abrasive in dry grinding and are markedly less effective in wet grinding than the premium fused alumina abrasive. The EP-A-0 152 768 (falling under Article 54(3) EPC thus not relevant for inventive step) discloses a ceramic body such as a vitrified grinding wheel containing equiaxed alpha alumina submicron crystallites, whereby the alumina has a hardness of at least 16 GPa and a density of 90% or greater without specifying the used bond material and the firing and maturing temperatures. Said alumina has been made by sintering an alpha alumina precursor below 1400°C. It would be desirable to produce a vitrified grinding wheel containing an aluminous abrasive, made by sintering of an alumina gel, which is superior to the conventional fused alumina in vitrified grinding wheels. We have discovered that, by the use of sintered alumina grits made up of submicron alpha-alumina particles having a non-cellular microstructure, vitrified bonded grinding wheels can be made which are markedly superior to such wheels which contain fused alumina as the abrasive. Accordingly, the above object is met by a vitrified grinding wheel comprising aluminous abrasive grits and an inorganic glassy bond characterized in that said abrasive grits comprise sintered polycrystalline alumina grits made up of submicron alpha alumina particles having a non-cellular microstructure, said grits having a density of at least 95% of theoretical, the bond is a high alumina and silica bond which contains a molar ratio of alkali to alumina to silica of 1 to 1 to 6, with 70% of the alkaline being potassium and the wheel is fired at a temperature of 1220°C or less and matured at a temperature of 1220°C or below. The abrasive employed in this invention differs from the commercially available sintered gel type abrasive in microstructure in that the commercial abrasives contain the alumina in the form of porous cells or grains having sometimes typically the appearance of sunbursts. The cells are generally coarser than 5 µm , typically 8 to 10 µm , and are single crystals. In the abrasive employed in this invention the cellular or sunburst structure is absent, and the alpha alumina particles are all submicron. The abrasive useful in the present invention is made by firing at 1250°C. to 1400°C., dried alumina gel particles which contain submicron alpha-alumina seed particles in the amount, typically, of about 0.6% by weight. Such alumina gels need not, but may, contain magnesia and need not, but may, contain other additives such as zirconia. First attempts to use abrasives in vitrified wheels resulted in total failure due to reaction between the bond and the abrasive. It was then discovered that if the firing temperature of the wheels was reduced to 1100°C. or less, for conventional bonds, or 1220°C. or less for more viscous high alumina and silica bonds, such adverse reaction was avoided. In other words the bond is matured at a temperature at which the integrity of the grits is not adversely affected by the bond. Such extreme sensitivity to firing temperature of the wheel is not found when the prior art sintered gel type alumina or fused alumina grits are employed. Alumina grits were prepared by crushing and firing a dried gel containing sub-micron alpha alumina seed particles. The gel was produced by mixing alumina monohydrate powder (microcrystalline boehmite) with water, nitric acid, and about 1 weight % (0.1 to 2%) of submicron alpha alumina particles. After drying of the gel it was broken up and fired at 1350°C. for about 5 minutes. Various examples of production of the abrasive can be found in co-pending Application Serial No. 85 100 506.6 filed Jan. 18, 1985 , and in co-pending Application Serial Number 85 106 752.0 filed May 31, 1985 Wheels were made from the non-cellular, at least 95% dense, abrasive made from seeded gels and compared to wheels made from premium commercially available fused alumina (sulfide process) and from abrasive grits corresponding to Example 41 (alumina and 5% magnesia) of U.S. Patent 4,314,827. After mixing the abrasive grits with glass bond ingredients, grinding wheels were pressed to shape and to the desired porosity of 45% (the mix including 48 volume % abrasive and 7.1 volume %, as fired, of bond). The wheels were then fired to 1100°C. at a rate of 60°C. per hour, soaked at 1100°C. for 6 hours and then the power was shut off and the wheels allowed to cool in the kiln to room temperature. The bond composition, on an oxide basis was 1 mole of alkali and alkaline earth oxides of composition A, below, and SiO₂, TiO₂ and R₂O₃ oxides identified as composition B, below. Composition A (mole fractions). Na₂O, 0.47; K₂O, 0.07; MgO, 0.25; CaO, 0.21 Composition B (moles per mole of A) Al₂O₃, 0.58; Fe₂O₃, 0.01; B₂O₃, 0.65; SiO₂, 3.77; Ti₂O₃, 0.03. The composition of the glass can thus be represented by the empirical formula: (.47 NA₂O, 0.07 K₂O, 0.25 MgO, 0.21 CaO) 0.58 Al₂O₃, 0.01 Fe₂O₃, 0.65 B₂O₃, 3.77 SiO₂, 0.03 TiO₂. In grinding tests the following results were achieved for the 45% porosity (J8) wheels in grinding tool-steel employing 60 grit size (about 400 µm(microns)) abrasive. Similar wheels made from commercially available sol gel abrasives containing about 5% MgO were inferior to the fused alumina in both dry and wet grinding. When fired at 1150°C., wheels containing the abrasive of this invention suffered a volume change (increase) of 7.9% and were inferior in grinding performance to the wheels containing the fused alumina (in which no volume change was observed in firing to 1150°C.). The commercial sol-gel abrasives suffered only about 2% linear growth on firing, but were still inferior to the fused abrasive wheels in grinding performance. It is believed that the growth of the wheels on firing is caused by absorption of the glassy bond into the abrasive grits during firing. The submicron crystal size of the alumina in the grits, apparently being responsible for such attack by the bond, which attack occurs much less in the case of abrasive grits having larger crystal size. While with the conventional glass bond described above, the highest allowable temperature is about 1100°C., variation of the composition of the bond, to produce a more viscous material at the firing temperature, can permit the use of higher firing temperature. A glassy bond containing a molar ratio of alkali to alumina to silica of 1 to 1 to 6, with 70% of the alkali being potassium, could be fired to 1220°C. without damage to the abrasive.

Claims: A vitrified grinding wheel comprising aluminous abrasive grits and an inorganic glassy bond characterized in that said abrasive grits comprise sintered polycrystalline alumina grits made up of submicron alpha alumina particles having a non-cellular microstructure, said grits having a density of at least 95% of theoretical, the bond is a high alumina and silica bond which contains a molar ratio of alkali to alumina to silica of 1 to 1 to 6, with 70% of the alkaline being potassium and the wheel is fired at a temperature of 1220°C or less and matured at a temperature of 1220°C or below. A vitrified grinding wheel as in claim 1, wherein said abrasive grits are prepared by firing dried alumina seeded gel particles. The vitrified grinding wheel of claim 1, in which the gel contains 0.1 to 2.0 wt % of alpha alumina seed particles. The vitrified grinding wheel of claim 3, in which the gel contains about 0.6 wt% of alpha alumina seed particles. The vitrified grinding wheel of one of the preceding claims, in which the gel contains magnesia and/or zirconia. A process of making vitrified grinding wheels comprising polycrystalline aluminous abrasive grits in an inorganic glassy bond, characterized in that said grits are made of submicron alpha alumina particles having a non-cellular microstructure and having a density of at least 95% of theoretical, and in that the wheel is fired with the highest allowable temperature being about 1220°C and matured at a temperature of 1220°C, or below and in that the bond is a high alumina and silica bond which contains a molar ratio of alkali to alumina to silica of 1 to 1 to 6, with 70% of the alkaline being potassium. The process of claim 6 characterized in that said abrasive grits are prepared by firing dried alumina seeded gel particles. The process of claims 6 or 7, characterized in that the firing of the grits is effected between 1250°C and 1400°C. The process of claim 8, characterized in that the gel contains magnesia and/or zirconia.

Inventors: RUE CHARLES VICTOR, RUE, CHARLES VICTOR
Applicants: NORTON CO, NORTON COMPANY
Application year: 1985
Main classification: B24D3
IPC codes: B24D3, C04B35