Abrasive grits formed of ceramic, impregnation method of making the same and products made therewith

Abstract: Ceramic abrasive grits are formed by an impregnation process that involves preparing an alumina hydrate sol, drying the sol to form a porous solid, crushing the solid to produce particles, calcining the dried particles, preparing a mixture of an additive metal oxide or its precursor in a liquid vehicle, impregnating the mixture into the calcined particles, drying the impregnated particles, calcining the dried impregnated particles, and firing the particles to produce ceramic abrasive grits. The concentration of the modifying additive metal is greater at or near the surface of the grit than at the interior of the grit.

Description: This invention relates to the production of alumina-based ceramic abrasive grits, a method of making the same, and abrasive products made with the abrasive grits. The preparation by a sol-gel process of dense, alumina-based ceramic abrasive grain is described, for example, in Leitheiser et al, U.S. Pat. No. 4,314,827, assigned to the assignee of the present application. This patent teaches making abrasive grains by employing chemical ceramic technology by gelling alumina monohydrate with a precursor of at least one modifying component followed by dehydration and firing. The modifying component is selected from zirconia, hafnia, a combination of zirconia and hafnia, and a spinel derived from alumina and at least one oxide of cobalt, nickel, zinc, or magnesium. Abrasive products containing ceramic abrasive grits made by a sol gel process have been found to perform in a superior manner as compared- to the best fused synthetic abrasive mineral in many applications. A typical example of a high performance fused synthetic abrasive mineral is formed of fused alumina-zirconia available, for example, under the trade designation "NorZon" from the Norton Company. Other references which disclose the preparation of alumina-based ceramic abrasive grains include the following: Cottringer et al, U.S. Patent No. 4,623,364, issued November 18, 1986, entitled Abrasive Material and Method For Preparing The Same. Gerk, U.S. Patent No. 4,574,003, issued March 4, 1986, entitled Process For Improved Densification of Sol-Gel Produced Alumina-Based Ceramics. Amero, U.S. Patent No. 3,450,515 discloses a method of making impregnated sintered bauxite abrasive grains. The grains are prepared by impregnating sized particles of calcined bauxite with an aqueous solution of manganese, iron or copper ions, and firing the impregnated particles for at least two hours at 1600°C. The resultant abrasive grains are said to contain agglomerated alpha alumina crystals of a size between 50 and 200 microns, with the preferred crystal size of about 100 microns and higher being preferred. Bauxite is an impure form of alumina containing other oxides including, for example, iron oxide, titania, and silica. Instead of producing abrasive grains of higher strength, the resultant grains are said to be weaker, a result which the patentee appears to desire for stainless steel snagging. References which disclose the preparation of ceramics include the following: Church et al, U.S. Patent No. 4,007,020 discloses a method of producing a refractory abrasive body by forming a porous skeletal body which is impregnated with a compound of a metal capable of being converted to an oxide in situ at relatively low temperatures, heating the body so impregnated at a temperature well above the normal vitrification to a temperature of at least 316°C (600°F) and for a time sufficient to convert the compound impregnated therein to an oxide and repeating the impregnation and heating steps until the desired degree of hardness is obtained. The porous body can be made from relatively finely divided materials which may be relatively pure powders, mixtures of powders, or impure powders, including additives in the form of discrete particles, fibers, fillers and the like. The powders are molded and bound together or bound together and molded with a binder which may comprise the impregnating compound or other suitable binder prior to treatment. Such refractory materials include alumina, beryllia, magnesia, titania, and zirconia. Berneberg et al., U.S. Patent No. 4,552,786, discloses a method for densification of ceramic materials which involves dissolving a ceramic precursor in a supercritical fluid, infiltrating the low density ceramic material with the ceramic precursor-laden fluid, and reducing the solubility of the ceramic precursor in the fluid to impregnate the ceramic precursor in the void spaces of the ceramic material. Bailey et al., U.S. Patent No. 3,859,399 discloses a method for making dense composite ceramic bodies of titanium diboride, boron carbide, silicon carbide and silicon. The ceramic bodies are produced by forming a mixture of titanium diboride, boron carbide and a temporary binder into a desired shape to obtain a coherent green body which is siliconized by heating in contact with silicon to a temperature near the melting point of silicon, where upon the molten silicon infiltrates the body and reacts with some of the boron carbide therein to produce silicon carbide in situ. Bugosh, U.S. Patent No. 3,108,888, discloses processes for producing colloidal, anisodiametric transition aluminas by heating colloidal, anisodiametric boehmite at a temperature in the range of 300 to 1000°C until the desired conversion has occurred, and is further directed to the process for making strong, shaped bodies by forming a mass of such boehmite particles and so heating until the boehmite is converted into a transition alumina, and optionally to the alpha form. The temperature of heating could be above the sintering point. Another method involves the introduction of grain-growth inhibitors and/or sintering-promoting substances such as iron oxide, manganese oxide, copper oxide and titanium oxide to impregnate a porous object of gamma alumina derived from fibrous colloidal boehmite alumina powder. Impregnation by aqueous solutions of soluble precursors of the desired oxide modifier are suggested, followed by drying to fix the modifier on the surface of the individual gamma alumina powders. This patent does not, however, teach the preparation of abrasive grits by this method. EP-A-200,487 discloses ceramic abrasive grits comprising alpha alumina and a modifying additive metal selected from magnesium, zinc, cobalt, nickel, zirconium, hafnium, chromium and titanium. The disclosed ceramic abrasive grits are formed by a method comprising: (1) preparing a sol of alumina hydrate containing nucleating sites, produced e.g., by ball-milling the sol with ceramic balls; (2) introducing the modifying additive metal; (3) drying the sol to a solid; (4) crushing the dried solid to produce particles; and (5) calcining the particles to form a ceramic abrasive grit. In a first aspect the invention provides ceramic abrasive grits comprising alpha alumina comprising at least one modifying additive metal selected from the group consisting of zirconium, hafnium, cobalt, nickel, zinc, magnesium, yttrium, praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium, and combinations of two or more of such metals, wherein the concentration of said modifying additive metal is greater at or near the surface of said grit than at the interior of said grit. Such ceramic abrasive grits have superior abrasive performance in abrading certain workpieces such as those made of stainless steel, titanium, high nickel alloys, aluminum and others, and excellent performance on more conventional workpieces such as mild steel. The ceramic abrasive grits comprise alpha alumina and desirably at least about 0.5% (preferably about 1% to about 30%) by weight of the oxide of the modifying additive metal. The preferred modifying additive is an oxide of yttrium, magnesium, and a rare earth metal selected from praseodymium, samarium, ytterbium, neodymium, lanthanium, gadolinium, cerium, dysprosium, and erbium. The additive metal oxide will usually react with aluminum oxide to form an additive metal-aluminum oxide reaction product. The oxide of the reaction product of cobalt, nickel, zinc and magnesium will generally be a spinel. The oxide of the reaction product dysprosium and gadolinium with aluminum oxide will generally be a garnet, while the oxide of the reaction product of praseodymium, ytterbium, erbium and samarium with aluminum oxide will generally be a perovskite which may include some garnet. The reaction between lanthanum and aluminum oxides results in beta alumina. The alumina in each case which has not reacted with the additive metal oxide will be substantially in the alpha form. In another aspect, the invention provides a method of making ceramic abrasive grits, said method comprising the steps of: (1) preparing a sol of alumina hydrate; (2) drying the sol to form dried porous solid shapes comprised of the dried sol; (3) calcining the dried shapes to substantially remove water of hydration and convert the alumina hydrate to an alumina form which is insoluble in a liquid vehicle; (4) impregnating a homogenous mixture of an additive metal oxide or its precursor in said liquid vehicle into the calcined shapes to achieve an average concentration of modifying additive in the resultant ceramic after firing of at least about 0.5% by weight on a fired solids basis; (5) drying the impregnated shapes; (6) calcining the dried impregnated shapes to substantially remove bound volatile materials, and (7) firing the shapes to provide ceramic abrasive shapes. The shapes can be formed by partially drying the sol, shaping and completing the drying or by drying the sol and then crushing to produce particles. The liquid vehicle may be water, for example. The metal oxide precursor may be a water-soluble salt of the additive metal, for example. In a third aspect the invention provides an abrasive product comprising ceramic abrasive grits according to the first aspect of the invention, preferred abrasive articles include coated abrasive products, bonded abrasive products such as grinding wheels, and nonwoven abrasive products in a form similar to that sold under trade designation Scotchbrite® by the 3M company. The first step of the prepartion process involves the preparation of a sol of alumina hydrate (eg. boehmite), usually comprising about 10 to about 60 weight percent aluminium oxide monohydrate. The boehmite can either be prepared from various techniques well known in the art or can be acquired commercially from a number of suppliers. Examples of commercially available materials include that available under the trade designation Disperal R produced by Condea Chemie, GMBH and that available under the trade designation Catapal R S.B., produced by Vista Chemical Company. These aluminum oxide monohydrates are typically in the alpha-form, are relatively pure, include relatively little, if any, hydrate phases other than the monohydrate, and have a high surface area. A peptizing agent is usually added to the boehmite dispersion to produce a more stable hydrosol or colloidal dispersion. Monoprotic acids or acid compounds which may be used as the peptizing agent include hydrochloric, acetic, and nitric acid. Nitric acid is a preferred peptizing agent. Multiprotic acids are normally avoided since they rapidly gel the dispersion, making it difficult to handle or mix with additional components. Some commercial sources of boehmite contain acid titer, such as absorbed formic or nitric acid, to assist in forming a stable dispersion. The dispersion may optionally contain a nucleating agent to enhance the transformation to alpha alumina. Suitable nucleating agents include fine particles of alpha alumina, alpha ferric oxide or its percursor and any other material which will nucleate the transformation. The amount of nucleating agent is sufficient to effect nucleation. Nucleating such dispersions is disclosed in assignee's European Patent No. 0 200 487, published November 5, 1986. The sol is then dried by conventional means to form a porous solid. Drying may be accomplished in steps, first forming a plastic mass of the partially dried dispersion. Once sufficient water has been removed from the alumina sol or dispersion, the partially dried plastic mass may be shaped by any convenient method such as pressing, molding or extrusion and then carefully dried to produce the desired shape such as a rod, pyramid, diamond, cone and the like. Irregularly shaped abrasive grits are conveniently formed by simply depositing the sol or dispersion in any convenient size of drying vessel such as one in the shape of a cake pan and drying, usually at a temperature below the frothing temperature of the sol or dispersion. Drying may be accomplished by simply air drying or using any of several other dewatering methods that are known in the art to remove the free water of the sol to form a solid. After the solid is dried, it can be crushed or broken by any suitable means, such as a hammer or ball mill to form grits or particles. Any method for comminuting the solid can be used and the term "crushing" is used to include all of such methods. After crushing, the dried shaped grits or particles, after crushing if appropriate, are then calcined to remove essentially all volatiles. The dry grits are preferably heated to a temperature between 400°C and about 800°C and held within this temperature range until the free water and a substantial amount of the bound water is removed, preferably over 90 weight percent of the total water. The calcined grits may then be screened and grits of a particular size or the entire batch may be used in the next steps. The metal oxide additive or its precursor, typically a salt of the metal, is added to a liquid vehicle such as water to provide an impregnation mixture or solution, if the oxide or salt dissolves. The impregnation mixture is then imbibed or doped into the porous grits of calcined alumina. Preferably sufficient impregnation mixture is added to the calcined alumina grits to provide at least about 0.5% by weight and more preferably about 1 to about 30% by weight metal oxide additive, as calculated on a fired solids basis. It should be understood that this concentration of metal oxide additive will be the goal of the impregnation process, but the concentration will be the average concentration throughout each fired impregnated grit. The method of the invention produces impregnated grits with higher concentrations of metal oxide additive at or near the surface than in the interior of the impregnated structure. The concentration differential is thought to be a result of greater concentrations of metal ions being at the surface caused by lack of penetration of the liquid vehicle into the interior of the porous grits or by outward migration of the metal ions once impregnated into the interior as drying occurs. The concentration differential caused by migration may be reduced by employing a liquid vehicle which is modified to have increased viscosity or by reaction to cause precipitation of the metal ions within the interior of the porous grit. Conventional viscosity modifiers may be used to increase the viscosity. Such increase of viscosity should not be so great, however, so as to prevent adequate penetration of the solution. Reaction of some of the impregnation solutions with NH 4 OH can cause the metal ions to precipitate within the porous grit. Other similar reactants will also he useful. Reactants and viscosity modifiers must be selected so as not to leave a residue on calcining or firing which will adversely affect these steps or the fired product. The preferred metal precursor is a salt of a volatile anion. Metal salts having volatile anions include, for example, the metal nitrate, formate, acetate or the like. The most readily available chemical form of metal is typically the oxide which is easily converted to a salt with a volatile anion by reaction with an excess of concentrated nitric acid to produce a nitrate solution which can conveniently be introduced into the porous alumina body in the desired amount. Metal salts and compounds which remain stable and have anions which do not volatilize at least at the firing temperature of the ceramic material should be avoided since they generally do not react with alumina to form the desired metal-aluminum oxide reaction product, as will hereinafter be explained. The metal may also be introduced as the oxide, for example, as finely divided hydrated particles as in a sol. The impregnating solution may contain other additives, e.g., a nucleating agent to enhance the transformation of the alumina hydrate to alpha alumina. The nucleating agent may also be contained in the alumina hydrate dispersion or it may be solely contained in the dispersion or the impregnating solution. Impregnation may be accomplished in a single impregnation, i.e., using only one impregnation solution a single time or it may be accomplished in several impregnation steps. That is, more than one impregnation solution may be applied to a particular porous structure. For example, the same impregnation solution may be applied once, and, after the drying and calcining steps, applied again to increase the concentration in the porous structure of the solids being carried in the impregnation solution. The subsequent impregnation, solution may also have a different concentration of solids and/or a combination of different solids. For example, the first solution may contain one metal salt and the second solution may contain a different one. After impregnation, the impregnated grits are dried and calcined to remove bound volatile materials. Calcining is usually accomplished at a temperature of between about 400-800°C. The impregnated calcined grits are then sintered e.g., by heating to a temperature between about 1200°C to about 1650°C and holding within this temperature range until substantially all of the additive metal oxide reacts with alumina under firing conditions to thereby be converted to a reaction product and until substantially all of the remaining alumina is converted to a fired form, typically alpha alumina. Of course, the length of time to which the calcined material must be exposed to the sintering temperature to achieve this level of conversion will depend upon various factors but usually will be accomplished within seconds to about 30 minutes. Other steps can be included in this process, such as rapidly heating the material from the calcining temperature to the sintering temperature, sizing granular material, centrifuging the dispersion to remove sludge waste, etc. Moreover, this process may be modified by combining two or more of the individually described steps, if desired. The firing steps are more fully described in U.S. Pat. No. 4,314,827. The ceramic materials according to the invention typically have a density varying from near their theoretical density, e.g., 95% or greater, to about 75%. The ceramic material may be substantially void free or it may be characterized by including porosity, typically in the form of internal vermicular or equiaxial pores which are for the most part on the interior of the ceramic with a minor part of the pores extending to the surface. Porosity is very difficult to measure accurately by conventional porosity measuring techniques because the porosity is a mix of closed pores which do not extend to the surface and open pores which do. The ceramic abrasive grits according to the present invention may be utilized in conventional abrasive products, preferably as a blend with less expensive conventional abrasive grits such as fused alumina, silicon carbide, garnet, fused alumina-zirconia and the like. It may also be blended with minerals or materials which are not noted as abrasives such as glass and the like. Because of the relatively high cost of the rare earth metal compounds, it is preferred to blend ceramic abrasive grits which contain expensive starting materials with less expensive abrasive minerals. Such blending of abrasive grits is known. A preferred method of blending is described in assignee's European Patent No. 0 161 869, published November 21, 1985, involving a method known as selective mineral substitution wherein the coarse abrasive is removed from an inexpensive abrasive grit charge that is to be utilized in an abrasive product such as a coated abrasive and is substituted with coarse mineral of the invention. It is recognized in that patent application that in any coated abrasive the coarse abrasive grits are substantially responsible for a major portion of the abrading of a workpiece. By such substitution, the improved abrasive grits of the present invention are interposed in an abrasive product between smaller grits of conventional abrasive mineral to permit the improved coarse abrasive grits to do the bulk of the abrading with such product. The ceramic abrasive grits of the present invention are conveniently handled and incorporated into various abrasive products according to well-known techniques to make, for example, coated abrasive products, bonded abrasive products, and lofty non-woven abrasive products. The methods of making such abrasive products are well-known to those skilled in the art. A coated abrasive product includes a backing, for example, formed of fabric (e.g., woven or non-woven fabric such as paper) which may be impregnated with a filled binder material, a polymer film such as that formed of oriented heat-set polypropylene or polyethylene terephthalate which may be first primed, if needed, with a priming material, or any other conventional backing material. The coated abrasive also includes a binder material, typically in layers including a make or maker coat, a size or sizing coat and possibly a supersize coat. Conventional binder materials include phenolic resins. It has been found that the addition of a grinding aid over the surface of the abrasive grits typically in the supersize coating provides an improved grinding performance when using a coated abrasive product containing the ceramic abrasive grits of the present invention. Grinding aids may also be added to the size coat or as particulate material. The preferred grinding aid is KBF 4 , although other grinding aids are also believed to be useful. Other useful grinding aids include NaCl, sulfur, K 2 TiF 6 , polyvinylidene chloride, polyvinyl chloride, cryolite and combinations and mixtures thereof. The preferred amount of grinding aid is on the order of 50 to 300 g., preferably 80 to 160 g. per square meter of coated abrasive product. Non-woven abrasive products typically include an open porous lofty polymer filament structure having the ceramic abrasive grits distributed throughout the structure and adherently bonded therein by an adhesive material. The method of making such non-woven abrasive products is well known. Bonded abrasive products typically consist of a shaped mass of abrasive grits held together by an organic or ceramic binder material. The shaped mass is preferably in the form of a grinding wheel. The preferred binder materials for the ceramic abrasive grits of the invention are organic binders. Ceramic or vitrified binders may be used if they are curable at temperatures and under conditions which will not adversely affect the ceramic abrasive grits of the present invention. The following examples are illustrative of certain specific embodiments of this invention; however, these examples are for illustrative purposes only and are not to be construed as limitations upon the invention. All parts are by weight, unless otherwise specified. EXAMPLES 1-32 Room temperature deionized water (2600 ml), 48 g of 16N analytical reagent grade nitric acid and 800 g alpha alumninum oxide monohydrate powder sold under the trade designation Disperal R were charged into an 18.9 liter polyethylene-lined steel vessel. The charge was dispersed at high speed for five minutes using a Gifford-Wood Homogenizer Mixed (Greeco Corp., Hudson, NH). The resulting sol was poured into a 46 cm x 66 cm x 5 cm polyester-lined aluminum tray where it was dried in a forced air oven at 100°C to a friable solid. The resultant dried material was crushed using a "Braun" type UD pulverizer having 1.1 mm gap between the steel plates. The crushed material was screened and the 0.125 mm to about 1 mm screen size material was retained and was fed into the end of a calciner which was a 23 cm diameter 4.3 meter long stainless steel tube having a 2.9 meter hot zone, the tube being inclined at 2.4 degrees with respect to the horizontal, and rotating at 7 rpm, to provide residence time therein of about 15 minutes. The calciner had a hot zone feed end temperature of 350°C and exit end temperature of 800°C. The prefired material (100 grams) was added to 300 ml of the rare earth nitrate solution of the concentration given in Table I contained in a 500 ml glass filtering flask. An aspirator was used to pull a partial vacuum above the solution which allowed air trapped within the porosity of the grits to escape, and the rare earth nitrate solution to completely infiltrate the porosity. The partial vacuum was maintained for about one minute, after which the excess nitrate solution was removed by filtering the saturated grits over No. 4 filter paper. The grits were dried in a forced air oven at 100°C, then fed through a rotary calciner as described previously. For multiple impregnations, the prefired material is allowed to cool and is then impregnated again in the the desired solution. Excess solution is removed, the material is dried and prefired again. This process may be repeated as often as necessary to obtain the desired concentration of rare earth oxide. The fired product from the calciner was fed into a 1380°C kiln which was a 8.9 cm diameter 1.32 meter long silicon carbide tube inclined at 4.4 degrees with respect to the horizontal and having a 76 cm hot zone, rotating at 10.5 rpm, to provide a residence time therein of about 5 minutes. The product exited the kiln into room temperature air where it was collected in a metal container and allowed to cool to room temperature. Table I sets forth details about the preparation of Examples 1-32. These examples involve the use of only one rare earth oxide modifier. Table I reveals the relative amounts of alumina and modifier and the type of modifier. The modifier is shown as the rare earth metal oxide for convenience but it will typically be present as a complex crystal structure with the alumina, as previously explained. This crystal structure is also set forth in Table I. The concentration of aqueous solution of rare earth metal nitrate employed in each case is also given. Where two concentrations appear for a particular example, e.g., "23/23" for Example 1, this means that the porous alumina hydrate body was impregnated twice. In Example 1, the concentration of the rare earth nitrate was the same in each impregnation. In Example 6, however, the first impregnation was with a 23% solution while the second impregnation was with a 15% solution. Ex. No. Al 2 O 3 % Rare Earth Metal Oxide Crystal Structure Rare Earth Metal Nitrate Solution solids (%) (Type) (%) 1 74 Dy 2 O 3 26 Garnet 23/23 2 84 " 16 Garnet 23 3 91 " 9 Garnet 15 4 95 " 5 Garnet 10 5 97 " 2 Garnet 5 6 83 " 17 Garnet 23/15 7 83 " 17 Garnet 23/15 8 94 La 2 O 3 6 Beta Al 2 O 3 22.6 9 94 " 6 Beta Al 2 O 3 22.6 10 85 Gd 2 O 3 15 Garnet 22.9 11 85 " 15 Garnet 22.9 12 89.4 Pr 2 O 3 10.6 Perovskite 22.5 13 89.6 " 10.4 Perovskite 20 14 90.9 " 9.1 Perovskite 17.5 15 91.8 " 8.2 Perovskite 15 16 94.6 " 5.4 Perovskite 12.5 17 95. " 5 Perovskite 10 18 87 Yb 2 O 3 13 Perovskite and garnet 22.5 19 88.6 " 11.4 Perovskite and garnet 17.5 20 91.3 " 8.7 Perovskite and garnet 12.5 21 87.7 Er 2 O 3 12.3 Perovskite and garnet 22.5 22 91.4 " 8.6 Perovskite and garnet 17.5 23 95 " 5 Perovskite and garnet 12.5 24 ?? Sm 2 O 3 ?? Perovskite 17.4 25 92.4 " 7.6 Perovskite 13.1 26 95.7 " 4.3 Perovskite 8.7 27 88.2 " 11.8 Perovskite 17.4/8.7 28 18 " ? Perovskite 18.4 29 18 La 2 O 3 ? Beta-Al 2 O 3 18.1 30 38 " ? Beta-Al 2 O 3 11.3 31 38 " ? Beta-Al 2 O 3 15.1 32 38 " ? Beta-Al 2 O 3 18.8 Abrasive grits of each of the examples were made into coated abrasive products which were tested for abrasiveness. The coated abrasive products were made according to conventional coated abrasive making procedures. The abrasive grits were screened to yield various grain sizes or abrasive grit grades and the desired grade selected for the particular construction. The abrasive grits were bonded to polyester or vulcanized fiber backings using conventional make, size, and optionally supersize adhesive resin compositions. Table II reveals the grit size (grade), the composition of the abrasive grits, and grinding aid, if used, the total amount of metal removed in a ginding test (Total Cut) and the relative grinding performance according to a "disc test" (unless otherwise specified) as described below when compared to the performance of a standard consisting of commercial grade fused alumina-zirconia abrasive grits available under tile trade designation NorZon R . In Table II, the given grade size refers to abrasive grit having an average diameter as follows: Grade Average Diameter (micrometers) 36 710 40 600 50 430 The term "disc" test refers to disc test hereinafter described. The disc test involved the testing of 17.8 cm diameter abrasive discs having the following approximate coating weights: Grade Make Resin Mineral Size Resin Supersize 36 4.2 g 18.8 g 13.6 g 8 g 40 4.2 g 18.0 g 13.0 g 8 g 50 4.2 g 13.2 g 8.7 g 6 g Disc Test The discs were prepared using conventional coated abrasive making procedures, conventional 0.76 mm vulcanized fiber backings and conventional calcium carbonate-filled phenolic resin make and size resins, without adjusting for mineral density differences. The make resin was precured for 90 minutes at 88°C and the size resin precured for 90 minutes at 88°C followed by a final cure of 100°C for 10 hours. The coating was done using conventional techniques in a one-trip operation with curing in a forced air oven. The cured discs were first conventionally flexed to controllably break the hard bonding resins, mounted on a beveled aluminum back-up pad, and used to grind the face of a 2.5 cm by 18 cm 304 stainless steel workpiece. The disc was driven at 5,500 rpm while the portion of the disc overlaying the beveled edge of the back-up pad contacted the workpiece at 5.91 kg pressure generating a disc wear path of about 140 cm 2 . Each disc was used to grind a separate workpiece for one minute each for a total time of 12 minutes each or for sufficient one minute time segments until no more than 5 grams of metal were removed in any one minute grinding cut. The total cut for each disc is reported in Table II. The relative cumulative cut of each of the 12 cuts (or for a lesser number of cuts, depending upon performance) for each disc, using the cumulative cut of a disc made of a control abrasive grain as 100%, was calculated and is also tabulated in Table II. All of the grinding data was generated using the Disc Test unless otherwise indicated in which case the grinding data was generated using an Abrasive Belt Test described below. Abrasive Belt Test Grade 50 coated abrasive sheets were converted to 7.6 cm x 335 cm endless abrasive belts and tested on a constant load surface grinder, abrading the 2 1/2 cm x 18.4 cm face of a 304 stainless steel workpiece with 30 successive 60 second grinding passes, weighing and cooling after each pass, employing 111N (25 lb) pressure, a 2000 m/min belt speed, and 1.8 m/min workpiece feed rate. Ex. No. Additive Abrasive Grade Grinding Aid Total Cut (Grams) Relative Cut (%) (Type) (%) 1 Dy 2 O 3 26 40 - 46 107 2 " 16 40 - 36 84 3 " 9 40 - 22 52 4 " 5 40 - 18 41 5 " 2 40 - 20 45 6 " 17 40 - 105 64 7 " 17 40 KBF 4 354 215 8 La 2 O 3 6 36 - 57 88 9 " 6 36 KBF 4 127 102 10 Gd 2 O 3 15 36 - 137 110 11 " 15 36 KBF 4 231 186 12 Pr 2 O 3 10.6 50 KBF 4 54 88 13 " 10.4 50 KBF 4 46 76 14 " 9.1 50 KBF 4 47 76 15 " 8.2 50 KBF 4 45 74 16 " 5.4 50 KBF 4 118 194 17 " 5 50 KBF 4 147 241 18 Yb 2 O 3 13 50 KBF 4 125 204 19 " 11.4 50 KBF 4 100 164 20 " 8.7 50 KBF 4 85 140 21 Er 2 O 3 12.3 50 KBF 4 197 322 22 " 8.6 50 KBF 4 152 249 23 " 5 50 KBF 4 114 186 24 Sm 2 O 3 ? 50 KBF 4 196 182 25 " 7.6 50 KBF 4 232 214 26 " 4.3 50 KBF 4 177 164 27 " 11.8 50 KBF 4 71 71 28 " ? 50 - 64 - 29 La 2 O 3 ? 50 - 85 - 30 " ? 50 - 510 84 31 " ? 50 - 429 70 32 " ? 50 - 105 17

Claims: Ceramic abrasive grits comprising alpha alumina comprising at least one modifying additive metal selected from the group consisting of zirconium, hafnium, cobalt, nickel, zinc, magnesium, yttrium, praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium, and combinations of two or more of such metals, wherein the concentration of said modifying additive metal is greater at or near the surface of said grit than at the interior of said grit. The ceramic abrasive grits according to claim 1 wherein the modifying additive metal oxide content is from about 0.5 to about 30% by weight on a fired solids basis. The ceramic abrasive grits of claim 1, made by a process which includes forming a dried porous shape of an alumina hydrate sol selected from rods, pyramids, diamonds or cones, impregnating the shape with a modifying additive metal salt in a liquid vehicle, and then firing the shape to convert it to ceramic. The ceramic abrasive grains of claim 1, made by a process which includes an impregnating liquid containing a nucleating agent that is impregnated into a dried porous shape formed of an alumina hydrate sol. An abrasive product comprising ceramic abrasive grits according to any one of claims 1 to 4. An abrasive product according to claim 5 which is a coated abrasive product comprising a backing having adhesively bonded to one side thereof said ceramic abrasive grits. An abrasive product according to claim 5 which is a bonded abrasive product comprising a shaped mass of said ceramic abrasive grits and binder materials. An abrasive product according to claim 5 which is a lofty non-woven abrasive product comprising an open porous lofty polymer filament structure having said ceramic abrasive grits distributed throughout said structure and adhesively bonded therein by an adhesive material. An abrasive product according to claim 6, including a grinding aid. An abrasive product according to claim 9, wherein said grinding aid is KBF 4 . An abrasive product according to claim 7 further comprising particulate minerals or materials which are not noted as abrasives. An abrasive product according to claim 11 wherein said particulate material not noted as an abrasive is glass. A method of making ceramic abrasive grits, said method comprising the steps of: (1) preparing a sol of alumina hydrate; (2) drying the sol to form dried porous solid shapes comprised of the dried sol; (3) calcining the dried shapes to substantially remove water of hydration and convert the alumina hydrate to an alumina form which is insoluble in a liquid vehicle; (4) impregnating a homogenous mixture of an additive metal oxide or its precursor in said liquid vehicle into the calcined shapes to achieve an average concentration of modifying additive in the resultant ceramic after firing of at least about 0.5% by weight on a fired solids basis; (5) drying the impregnated shapes; (6) calcining the dried impregnated shapes to substantially remove bound volatile materials, and (7) firing the shapes to provide ceramic abrasive shapes. A method according to claim 13, in which the shapes are formed by crushing the dried porous solid to produce particles. A method according to claim 13, in which the sol is partially dried and shaped and drying is completed. A method according to any one of claims 13 to 15, wherein said precursor compound is a salt of a rare earth metal selected from the group consisting of yttrium, praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium and combinations of two or more of such metals. A method according to any one of claims 13 to 16, wherein said impregnation is accomplished by impregnating, drying and calcining said porous shapes more than one time. A method according to any one of claims 13 to 15, wherein the concentration of metal oxide or its precursor in the impregnation solution is the same in each impregnation. A method according to any one of claims 13 to 17, wherein the concentration of impregnation solution is different for each impregnation. A method according to any one of claims 13 to 19, wherein the composition of the impregnating solution is the same for each impregnation. A method according to any of claims 13 to 19, wherein the composition of the impregnating solution is different for each impregnation.

Inventors: WOOD WILLIAM P, WOOD, WILLIAM P., WOOD, WILLIAM P. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1988
Main classification: C04B35
IPC codes: C04B35, C04B41, C01F7, B24D3, B24D11, C09K3