DD296025A5 - FIBER BASED ON SINTERED SOL-GEL ALUMINUM OXIDE AND METHOD AND USE - Google Patents

Abstract

The invention relates to a fiber based on sintered sol-gel alumina and a process for their preparation and use thereof. The inventive solution is to be achieved that microcrystalline sol-gel sintered materials with significantly improved properties directly in a predetermined form, which requires no further change and can be used in this form. According to the invention, this is achieved by the fiber consisting of alpha alumina sintered crystals having a grain of not more than 2 mm, preferably 1 mm, the fiber having a substantially uniform cross-section of at most 2 mm in diameter, an average length ratio of at least 1: 1 and a hardness of at least 16 GPa. The preferred applications are abrasive and reinforcing fibers. {Fiber; sintered sol-gel alumina; Method; Use; grit; uniform cross section; Diameter; average length ratio; Hardness; Abrasive fiber; Verstaerkungsfaser}

Description

The invention relates to a fiber based on sintered sol-gel alumina and a process for their preparation and use thereof.

It has been known that sol-gel aluminas, particularly seeded sol-gel abrasives, have demonstrated considerable advantages over other superabrasives since their introduction several years ago in broad applications in the form of abrasive sheet material and bonded abrasive articles. These are prepared in a known manner by drying and sintering hydrated alumina, which may also contain varying amounts of additives such as MgO or ZrO ₂ . The dried material is coarsely crushed either before or after sintering to obtain irregular, block-shaped polycrystalline abrasive bodies in a desired grain size range. The grains may later be incorporated in a bonded abrasive article or abrasive sheet material, such as an abrasive article. As a grinding wheel or a Segmentschleifkörper, a grinding belt or a round abrasive sheet are stored.

U.S. Patent No. 4,314,827 discloses abrasive grains made by such a process wherein the sintered grains contain irregular "snowflake" shaped alpha-Al crystals having a diameter of the order of 5 to 10 microns. The spaces between the arms of a "snowflake" and between adjacent "snowflakes" are occupied by other phases, for example a finely crystalline alumina magnesia finial.

U.S. Patent No. 4,623,364 discloses a sol-gel process for making alumina-containing abrasive grain, as well as other products such as coatings, laminas, fibers, rods, or small moldings having improved properties. Here, the conversion of the hydrated alumina into alpha alumina is facilitated by introducing seed into the gel or gel precursor prior to drying. This can be accomplished either by wet-milling the gel or gel precursor with alpha alumina media in the vibratory mill or by directly adding very fine-grained seed particles in powder or other form. To make abrasive grains, the seeded gel is dried, crushed and fired. The abrasive grains thus produced can be used in the manufacture of products such as round abrasive sheets and grinding wheels. For the production of moldings or rods, the material can also be molded or molded prior to firing, for example by extrusion. In the case of extrusion, the resulting bars are later cut or broken to suitable lengths. Once the gel has formed, it may be shaped in a known manner, such as by pressing, compression molding or extrusion, and then dried thoroughly to produce a crack-free body of the desired shape. If an abrasive is made, then the gel can be extruded or simply applied in any form and dried. After drying, the solid body or material can be cut or worked to obtain a desired shape, or it can be crushed or broken by suitable means, such as a hammer or ball mill, to form abrasive particles or granules.

Such seeded sol-gel abrasives have a much stronger alpha-Al ₂ O ₃ crystal structure and higher density than the non-seeded sol-gel material according to US-P 4,314,827. The alpha-Al ₂ O ₃ crystals of the seeded sol Gel abrasives have submicron grain sizes, and usually on the order of about 0.4 pm and below; however, a slightly larger structure may result if inoculation is not performed optimally or the material is fired at too high a temperature or too long.

Other materials, such. Fe ₂ O ₃ can also be used as seed additives to facilitate the transformation into АІрІіа-АІ ₂ Оз. It is expedient that such inoculants should be structurally identical to Al ₂ O ₃ and have similar crystal parameters (within about 15%) in order to function well.

The known sol-gel abrasives thus had to be made by pulverizing the largest particles and classifying the product to the desired grain size. However, this method is very uneconomical because it creates a broad range of particle sizes that are not all useful. Thus, a significant portion of production must be eliminated, returned or used in a substandard application.

The invention has for its object to provide a fiber based on sintered sol-gel alumina and a process for their preparation and for the use thereof, wherein microcrystalline sol-gel sintered materials having significantly improved properties directly in a predetermined form, which no further change needs to be generated and used in this form.

According to the invention the object is achieved in that the fiber consists of sintered alpha alumina crystals with a grain size of at most 2 pm, preferably of 1 μιτι, wherein the fiber has a substantially uniform cross section, a diameter of at most 2 mm, an average aspect ratio of at least 1: 1 and has a hardness of at least 16GPa.

A preferred aspect ratio of the fiber is between 1.5: 1 to 25: 1. Here, an expedient embodiment is that the fiber is a particle having a length of less than 10 mm and / or a diameter of 0.0001 mm to 2 mm. The fiber may consist of at least 80% of generally equiaxed crystals with a maximum dimension of 0.4 цt.

In a preferred embodiment, the fiber has a density of at least 95% of the theoretical density, at 1 to 50 parts by weight in% of an oxide selected from zirconia, titania, magnesia, hafnia, ceria, spinel, mullite, manganese dioxide, precursors and Contains mixtures of these oxides.

In the sense of the solution according to the invention, the alpha alumina crystals may be seeded alpha alumina crystals having a dimension of less than 1 pm.

The fiber may be curved along its longitudinal axis. This is especially beneficial when used as abrasive fiber.

This is also true when the fiber is twisted along its longitudinal axis.

For the use of the fiber, it is particularly favorable if it has a substantial freedom from vitreous constituents and a breaking strength of at least 8000 kp / cm ² .

In a further aspect of the invention, this comprises a process for producing a sintered sol-gel alumina based fiber, wherein the fiber is prepared from a gelled dispersion of hydrated alumina particles in the submicron range and the gelled dispersion is formed into fibers which have been dried and thereafter fired at a temperature between 1090 "C and 1500 ° C, during a firing period in which the fibers are converted into a ceramic body, which are formed predominantly of alpha alumina crystals having a size of 2 pm and below.

It is advantageous if the gelled dispersion contains an effective amount of sub-micron seed particles to facilitate rapid conversion of the hydrated alumina particles to alpha alumina upon sintering to form alpha alumina crystals having a crystal size of about 1 pm or below.

Conveniently, the alpha alumina crystals have a crystal size of about 0.4μπΊ or less. Preferably, the seed particles are selected from the group consisting of alpha alumina, alpha iron (III) oxide, gamma alumina, chromia, precursors of alpha alumina, gamma alumina, chromia or alpha ferric oxide, and mixtures of these substances, and the seed particles in an amount of 0.5 to 10 parts by weight in%.

In order to achieve fibers of desired dimensions in a continuous and economical manner, they can be formed from the gelled dispersion by extrusion or spinning.

With the method according to the invention, it is possible effectively to produce a fiber of predetermined dimensions and properties, such as alpha alumina sintered crystals having a grain size of at most 2 μm, preferably 1 liter and below, having a substantially uniform cross-section with a diameter of at most 2 mm, an aspect ratio of at least 1: 1 and a hardness of at least 16GPa.

A particularly preferred form of use of the fiber according to the invention, which is produced by the process according to the invention, is that the fiber as

a) an abrasive fiber in a grinding wheel made of a bonded abrasive, in particular in the form of grinding wheels, Segmentschleifkörpern or Abziehsteinen and a grinding sheet material in particular in the form of round abrasive sheets or abrasive belts,

b) a reinforcing fiber is used in a metallic and / or ceramic composite material.

The inventive solution has been achieved that microcrystalline sol-gel sintered materials can be produced with significantly improved properties directly in a particularly advantageous configuration, which require no further crushing before use. The improved properties are particularly surprising since it was believed that the properties of microcrystalline sol-gel alumina abrasives are derived from crystal size and purity, particularly from the absence of impurities, e.g. B. occur in bauxite and lead to the formation of glassy substance at the formation temperature of alpha alumina.

In addition, the use of a direct manufacturing process results in much lower production of materials for which there is no immediate need. In fact, essentially all the material produced has the desired characteristics, and as a result the process is considerably more productive.

In a further embodiment of the invention, there is provided a sintered fibrous sol-gel alumina-based abrasive consisting of sintered alpha alumina crystals. This abrasive has an average aspect ratio of at least 1: 1 and preferably 1.5: 1 and a hardness of at least 16GPa, with the size of the alpha alumina crystals not exceeding 10pm and preferably 2pm.

The invention will be explained in more detail in some embodiments. The polycrystalline abrasive fibers of the present invention are based on sintered alumina. The alpha alumina crystallites in the abrasive fibers can be as large as 2 microns when the gel is not seeded; however, they are less than 1 pm in the preferred vaccinated gels and less than about 0.4 pm when optimally processed. It has been found that, generally speaking, the abrasive action is much better at the smaller crystallite sizes, so that particles on the order of about 0.05 to 1 micron are often particularly advantageous. The fibers may be prepared by preparing a sol gel of aluminum hydroxide, spinning or extruding the gel into filaments, drying the filaments, and firing the dried filaments at a temperature of at most about 1500 ^° C. Preferably, the process involves adding to the starting sol or gel an effective amount of submicron seed material which promotes rapid conversion of the alumina in the gel to very fine alpha alumina crystals when the extruded and dried sol gel is fired. Examples of such seed are beta alumina, gamma alumina, chromia, alpha iron (III) oxide, alpha alumina, and precursors of these oxides, as well as other fine crystal debris that act as nucleants for alpha alumina.

The microcrystals are formed by a growth process from a sol-gel, as this leads to a fine uniform microstructure, especially when the sol-gel was inoculated. This growth process is very important and leads to significant differences between products of the invention and products resulting from sintering of predominantly or wholly crystalline alpha alumina particle compositions. Unless relatively high temperatures are used (resulting in crystal growth), these latter products usually have weak sintered bonds between adjacent crystal particles, and the grains consisting of such particles usually have low strength. In contrast, bred from a SoI-GeI and sintered in situ

Alpha alumina crystals, especially those made by a seed process, are fired at much lower sintering temperatures to obtain the fine crystalline structure.

It is also advantageous if the microcrystalline structure is substantially free of impurities that would lead to the formation of vitreous material during firing. By glassy material is meant an amorphous, noncrystalline material without a long term molecular order. Such a material significantly weakens the overall structure of the particles and reduces their effectiveness as abrasive grain. Therefore, the particles according to the invention contain less than 5 parts by weight in% and preferably less than 2 parts by weight in% of such a glassy component. The abrasive fibers of the present invention are formed as an elongated ceramic body. They each have an over the entire length approximately constant cross-section with a maximum cross section of 2 mm and preferably less than 0.5 mm. The length corresponds at least to the dimension of the cross section. However, it is preferably twice as large as the largest cross-sectional dimension. The abrasive fibers of this invention may be bent or twisted so that the length is not necessarily straight, but is measured along the body. The abrasive fibers are produced, preferably by extruding or spinning a seeded possible gel of aluminum hydroxide to continuous filaments, drying the filaments so obtained, cutting or breaking the filaments followed to desired lengths from the burning of the filaments at a temperature of not more than 1500 ⁰ C. U.S. Patent Nos. 4,314,827, 4,623,364, and 4,797,139 disclose various sol-gel processes for making and firing gels of aluminum hydroxide. In addition to the aluminum hydroxide disclosed in said patents, the sol may contain up to 10-15 parts by weight of additives such as titanium dioxide, spinel, mullite, manganese dioxide, magnesium dioxide, ceria, zirconia in powder form or in the form of a precursor these substances or other compatible additives or their precursors provided that they do not form vitreous materials at the alpha alumina forming temperatures and do not interfere with the stability and integrity of the gel in fiber formation. These additives can be incorporated to modify properties such as fracture toughness, hardness, friability, fracture mechanics or drying behavior. In its most preferred embodiment, the sol or gel contains a finely divided submicron crystalline seed or its precursor in an amount effective to facilitate conversion of the aluminum hydroxide to alpha alumina upon sintering. The amount of inoculum should not exceed about 10 parts by weight in% of the aluminum hydroxide and is normally not useful in amounts greater than about 5 parts by weight in%. If the seed additive is sufficiently fine-grained (preferably 6OmVg or above) then amounts of about 0.5 to 10% may be employed, with 1-5% being preferred. The addition of much larger amounts usually interferes with the stability of the sol and makes fiber formation more difficult. In addition, the strength of the inter-crystallite bonds produced during sintering decreases considerably as preformed alpha alumina particles require much higher temperatures for sintering. Of course, such higher temperatures lead to crystal growth and other negative consequences.

Examples of solid microcrystalline seed materials are beta alumina, alpha iron (III) oxide, alpha alumina, gamma alumina, chromia and other fine crystal debris that provide a nucleation site for the alpha alumina, with alpha alumina being preferred. The seed additives may also be added in the form of a precursor such as iron (III) nitrate solution. In general the seed material with the alpha alumina should be structurally identical, have similar crystal lattice dimensions (within 15%) and at the temperatures at which the conversion to alpha alumina takes place (about 1000 ⁰ C to 1100 "C), may be present in the dried gel.

The fibers may have any convenient cross-section, such as. B. round, square, triangular or star-shaped. For most applications, a round cross section is preferred, but in some applications the purpose may be limited. U. better met by others, such as square or triangular cross-sections.

The green abrasive fibers can be produced from the gel by a variety of methods, such as extrusion or spinning. The extrusion is most suitable for the green fibers between about 0.25 mm and 1.5 mm in diameter which, after drying and firing in diameter, correspond approximately to the screen openings used for 100 and 24 grit abrasive grains, respectively. Spinning is most convenient for fired fibers having a diameter of less than about 100μιτι. According to the invention, burned fibers having a fineness up to 0.1 μm (0.0001 mm) have been produced by spinning. The green fibers shrink when burning. For example, at a solids content of 60%, they shrink about 40% in diameter from their extruded diameter.

Particularly suitable gels for extrusion should have a solids content of from about 30% to about 68%, and preferably from about 45% to about 64%. The optimum solids content changes directly in proportion to the diameter of the extruded fiber, with a solids content of about 60% being preferred for fibers whose diameter approximates after firing the sieve opening for a size 50 (about 0.28mm) cut abrasive grain. As indicated above, attempts to achieve too high a solids content in the gel by incorporation of solids usually have a very detrimental effect on the stability of the gel. The extrudate generally has a low green strength and often does not retain the fiber shape except at a diameter of about 2mm.

The spinning of the fibers in accordance with the solution of the invention can be carried out by applying a quantity of gel to a disc which rotates rapidly and discards green fibers which air dry almost immediately. Optionally, the gel may be introduced into a centrifuge drum having holes or slots drilled on its outer periphery. For example, the centrifuge drum may rotate at 5000 rpm to produce the fibers drawn in the desired "green" diameter process Other well known spinning techniques may be used to form the green fibers about 20% and 45%, with about 35% to 40% being preferred.

When the abrasive fibers are produced by spinning, it is beneficial to use about 1% to 5% of a non-glass forming spin aid, such as a spin finish. As polyethylene oxide, the sol from which the gel is formed to give the gel favorable viscoelastic properties for fiber formation. The optimum amount of spin aid changes in inverse proportion to the solids content of the gel. The spin aid is burned out of the fibers during calcination or firing. Since very little adjuvant needs to be added (generally none at all for extrusion), it has no significant influence on the properties of the fired fibers.

The extruded gel fibers can be given various desired shapes by extruding the gel through dies of the shape desired for the fiber cross section. When the gel fibers have a relatively large cross section or have been made from a gel with a high water content, it may be necessary or desirable to dry them at a temperature below 100 ° C for 24 to 72 hours before being subjected to heating above 100 ⁰ C. become. For gel fibers with a relatively thin cross-section, or if they consist of very high solids gels, drying may not be necessary.

It is of course also possible that the fibers of the invention are largely dried and fired as continuous fibers. You can then as reinforcing elements in a matrix z. As a metal or plastic, are used, with their toughness is of great advantage.

In preparation for use as abrasive fibers, the initially formed continuous fibers are preferably broken or cut to the maximum length desired for the intended abrasive application. In general, any forming or separating operation needed to convert the continuous fibers into discrete bodies to alter their shape is best carried out in the gel state or the dried state, since then it can be much less labor and costly. It is much more difficult to process the much harder and stronger fibers which are produced after the final firing according to the solution according to the invention. The continuous fibers can therefore be shortened immediately after emerging from the extruder die by any known method to fibers of the desired length, for example by means of a rotating wire cutter, which is mounted close to the front side in front of the pressing nozzle. Optionally, the dried fibers may be crushed or easily crushed and then classified into desired lengths.

After the gel fibers have been shaped and cut or shredded in a predetermined manner and dried as necessary, they are converted to fibers of the final form by controlled firing. The firing converts substantially all of the alumina content of the gel fibers to crystalline alpha alumina. However, it should be done at a high temperature, or too long, because excessive burning promotes undesirable grain or crystal growth, thereby producing a product that is less effective in use as an abrasive. In general, for the preferred seeded gels firing at a temperature between 1200 ° C and 1350 ⁰ C with a duration of 1 hour up to 5 min sufficiently; however, other temperatures and times may be used. For fibers thicker than 0.25 mm, the pre-baking of the dried material is preferably about 400-600 ⁰ C over a period of several hours or 10 minutes to remove the remaining volatiles and bound water which upon firing cracking could cause the fibers. The burning of unvaccinated gels generally requires higher temperatures and longer times, as disclosed in US-P 43,827. Especially for fibers of seeded gels, excessive burning rapidly results in larger grains absorbing most or all of the smaller grains in the environment, thereby reducing the uniformity of the product on a microstructural scale.

The fibrous abrasive particles according to the invention have an aspect ratio, that is the ratio between the longitudinal dimension and its diameter of at least 1: 1. If the cross section is not round, z. B. polygonal, the largest dimension is used perpendicular to the longitudinal direction in the determination of the longitudinal ratio. The aspect ratio is preferably between about 2: 1 and 8: 1, although longer fibers are also useful in many applications. The fibers of particular utility in the practice of the invention have a hardness of at least 16GPa and preferably at least 18GPa for most applications (Vickers indenter, load 500g) and density of preferably at least 90% and preferably 95% of the theoretical density. Pure dense alpha alumina has a hardness of about 20-21 GPa. At least in some cases, the abrasive fibers used may be longitudinally twisted or slightly curved or bent.

It is believed that curved or twisted abrasive fibers may be superior to their straight counterparts when used in bonded abrasive products because the curved or twisted configuration makes it more difficult for the heart to peel such shaped abrasive fibers from bonding. In addition, such coiled or twisted abrasive fibers make it easier to achieve desirable ranges of looser densities in a grinding wheel. The diameter of the abrasive fibers may be up to about 1.5 mm in a range up to about 2 mm. It has been found that with the abrasive fibers of the present invention, bonded abrasive abrasive articles can be produced which will far outnumber comparable products containing comminuted melt-formed and sintered abrasive grains having a cross-section (grain size) approximately equal to the diameter of the abrasive fiber. The fibrous abrasive particles of the invention can be used in abrasive articles such. Abrasive wheels, segment grinders, and whetstones composed of a binder and SoI-GeI sintered abrasive fibers. The amounts of binder and abrasive may vary by volume in% between 3% and 76% binder, 24% and 62% abrasive, and 0% and 73% pores. As can be seen from these proportions in% by volume, the fibrous abrasive permits the production of bonded abrasives having significantly higher microstructural characteristics in softer grades than heretofore possible with similar abrasive of conventional form. Conventional pore-forming agents, such. B. hollow glass beads, solid glass beads, hollow resin beads, resin beads, foam glass particles, bubble alumina u. a. However, they can be stored in the grinding wheels and thus offer even more flexibility with regard to changes in quality and structural parameters.

The abrasive articles can be bonded either with a resinous or with a ceramic binder. The preferred resinous binders are based on phenol-formaldehyde resin, epoxy resin, polyurethane, polyester, shellac, polyamide, polybenzimidazole or mixtures of these substances. The binders may contain, in a manner known per se, 0 to 75 parts by volume in% of one or more fillers or grinding assistants. When the binder is of the resinous type, suitable fillers are u. a. Cryolite, iron sulfide, calcium fluoride, zinc fluoride, ammonium chloride, copolymers of vinyl chloride and vinylidene chloride, polytetrafluoroethylene, potassium fluoroborate, potassium sulfate, zinc chloride, kyanite, mullite, nepheline syenite, molybdenum disulfide, graphite, sodium chloride or mixtures of these various materials.

While the ceramic binders are suitable for compounding fillers, the number of useful fillers is somewhat low because of the relatively high temperatures required to burn such binders. However, fillers such as kyanite, muimt, nepheline syenite, graphite and molybdenum disulfide may be used depending on the baking temperature of a particular ceramic binder. The grinding wheels with ceramic binders can also be soaked with a grinding aid, such as. Molten sulfur, or they can be soaked with a binder such as epoxy to introduce a grinding aid into the pores of the disks.

In the manufacture of abrasive sheets using the abrasive fibers of the invention, they are fixed directly to a fiber end by a bonding layer or contact layer on a flexible backing. The fibers generally stand off the backing and are normally coated with a make coat to more firmly anchor them to the backing. This can be formed by a known per se used for abrasive backing, such. As woven or Nähwirkware, foil or paper. Depending on the application, a variety of per se known tissue or paper treatment methods and materials are used to prepare the backing, all of which can also be used in the production of abrasive sheets according to the invention. Accordingly, it is also possible to use any known contact layer used in the production of abrasive sheets.

If the mean aspect ratio of the abrasive fibers of the invention is e.g. 1.5: 1 to 6: 1 or even higher, then the make coat can be applied by normal roll coating methods. If the abrasive fibers have a greater aspect ratio, the coating is better done in other ways, e.g. B. by spray coating, whereby the fibers are not pressed down too much. For the binder layer, any known materials used in the production of abrasive sheets can also be used. It has been found that the abrasive sheets according to the invention have a much longer life than those containing comminuted abrasive grains in a manner known per se. Their cutting performance is usually less different, and they ensure a more consistent in the use of surface quality than the abrasive grinders known per se in the workpiece to be ground.

Surprisingly, it has been found that the use of abrasive sheets containing the abrasive fibers of the present invention are particularly effective in low sanding abrasive work.

A particular advantage of the solution according to the invention is that abrasive sheets can not be produced only with abrasive fibers of any length, but the size distribution for a particular application can be arbitrarily exact or diverse, which was previously unreachable with the crushed crushed abrasive grain. Another advantage is that in abrasive sheets containing the abrasive fibers of the invention, the fibers can be cut to a variety of lengths to simulate a normal sized CAMI fiber blend. Optionally, the abrasive sheets may contain a sized blend of coarse, control and thin abrasive fibers.

Another advantage of using abrasive fibers is that there is no waste as with conventional comminuted grains, so that the production process is more economical and effective.

The abrasive fibers of the present invention and the abrasive article containing the abrasive article and the abrasive article sheets are generally superior to the conventional abrasives as shown in the following examples. The abrasives are suitable for abrading all types of metal, for example, various steels such as stainless steel, cast steel, hardened tool steel cast iron such as ductile iron, malleable cast iron, ductile iron, ductile iron and nodular cast iron, and metals such as chromium, titanium and aluminum. Like all abrasive mills and abrasive articles or abrasive sheets containing them, the abrasive and abrasive articles of the invention are more effective at grinding certain metals than others and perform better in certain abrasive applications than others. Examples of preferred implementations

Example I

In this example, 196.4 kg of Pural ^e NG aluminum monohydroxide powder purchased from Condea Chemie GmbH, 38.2 kg of water, was mixed with 1.37 kg of alpha alumina seed and 28.8 kg of distilled water in a standard double-jacket V mixer mixed for five minutes to give a substantially uniform slurry. At this point, 16kg of nitric acid (70% concentration) diluted with 44.6kg of distilled water was added to the mixer with continued rotation of the mixer blades. After five minutes of further mixing, the sol was converted to a gel containing about 61% solids including the substantially evenly distributed seed. The inoculum in this example was prepared by charging a batch of distilled water in a Sweco mill, Model 45, with 88% alumina standard grade (12mm diameter χ 12mm length each) purchased from the Diamonite Products Company, Shreve, Ohio , was ground until the crystal nuclei in the water reached a specific surface area of at least 100m ² / g.

The Pural ^R NG powder used had a purity of about 99.6% with small additions of carbon, silica, magnesia and iron oxide.

The seeded gel was usually extruded through a smooth-walled die having multiple holes about 1.19 mm in diameter to produce continuous gel fibers. The gel fibers were then dried for 24-72 hours at a temperature of 75 to 80 ⁰ C and a relative humidity of 85%. After this drying step, the filaments were relatively brittle and could easily be crushed or broken into short lengths. For this example, fibrous bodies having an average length of 2 mm to 8 mm were produced from the continuous fibers. These short fibers were then converted by heating at an increase in temperature of 2 ° C / min to 800 ° C, with an increase of about 5 ° C / min from 800 ° C to 1370 ⁰ C in alpha alumina, in the latter temperature is about held for five minutes, after which they were cooled. Upon cooling, the fibers had a mean diameter of about 0.58mm and random lengths of about 1.5mm to 6mm, and consisted largely of pure alpha alumina with a mean crystallite size of 0.3pm and a Vickers hardness greater than about 1 , 6GPa. All crystallite sizes were measured by the line-cut method.

These last-described abrasive fibers were only slightly smaller in diameter than standard grain size 30 abrasive grain. These fibrous grains were processed in a manner known per se in US Pat. No. 4,543,107 into ceramic bonded abrasive wheels. Comparative abrasive wheels were prepared from 32a molten abrasive grains sold by the Norton Company, Worcester, Massachusetts. These test wheels were made with a diameter of 178 mm (7 "), a thickness of 12.7 mm (" "") and a bore of 31.75 mm (1 V * ") The total percentage of abrasive in% by volume in each wheel was kept constant at 48%, and the content of the ceramic binder of the composition A in volume percentages (see Table I) is kept constant at 7.21%.

TABLE I Melt oxide composition of binder A

SiO ₂ 47.61 Al ₂ O ₃ 16.65 Fe ₂ O ₃ 0.38 TiO ₂ 0.35 CaO 1.58 MgO 0.10 Na ₂ O 9.63 K ₂ O 2.86 Li ₂ O 1.77 B ₂ O ₃ 19,03 MnO ₂ 0.02 P ₂ O ₆ 0.22 100.00

An example of an optional ceramic binder has been disclosed in pending U.S. Patent Application Serial No. 07/236,586, filed August 25, 1988, assigned to the same assignee as the present invention. An example of such a binder is designated 3GF259A, designated and sold by the O. Hommel Company of Pittsburgh, Pennsylvania. This fritted binder consists of 63% silica, 12% alumina, 1.2% calcium oxide, 6.3% sodium oxide, 7.5% potassium oxide and 10% boron oxide, all in parts by weight in%. The mixture and the green abrasive wheels are manufactured in the usual manner and the latter fired at 900 ⁰ C to bake the binder, the firing cycle being from a heating section with 25 ° C / h temperature rise from room temperature to 900 ° C, an eight-hour soak at 900 ° C and cooled to room temperature with free cooling rate.

After mixing the abrasive grains with the glass binder ingredients, the test wheels in steel molds were compression molded to the desired porosity of 44.79%. The wheels were then heated for 43 hours at 900 ^c C, held at this temperature 16h, then allowed to cool to room temperature. The abraded wheels were dressed and sized to prepare for a 6,35 mm ( ¹ ") groove grinding test The grinding wheels containing the abrasive fibers of the invention were designated SN119 and the comparative wheels were designated 32A30, the per se known melt-formed abrasive was on Rc60 (Rockwell Hardness) hardened D3 tool steel, the length of the ground groove was 40.64cm (16.01 "). The tests were performed on a surface grinder from Brown and Sharpe, with the peripheral speed of the wheel at 30.48m / s (6000 feet / min) and the table speed was set to 0.254 m / s (50 feet / min). Tests were performed at three depth feeds: 0.025mm, 0.051mm and 0.076mm (1.2 and 3 mils) per double pass, all to a total depth of 1.524mm (60 mils). Grinding wheel wear, metal removal and performance were measured at each infeed speed. The term "G-ratio" as used in Table II and further refers to the number resulting from the division of the metal volume removed by the decrease in the volume of the grinding wheel due to wear in a given grinding operation, the higher the quotient, the higher is the quality of the grinding wheel

 The test results are shown in Table II.

Table II Results of the dry grooving test on D3 steel

abrasive disc feed G ratio Specif. energy (Type) No. (Mil) * (Steel / disk) PS / inch ³ Joules / min mm ³ molten 32A30 1 4.0 7.09 19.35 (Blocky) 2 4.25 9.02 24.62 3 Disc blocked sintered SN119 1 30.28 5.11 13.95 (extruded 2 21.31 4.91 13.40 fibers) 3 48.16 8.94 24.41 • mil = 0.001 "= 0.0254 mm

When dry-sanding D3 steel with a peripheral wheel speed of 30,48 m / s (6000 feet per minute), the abrasive wheels made with the abrasive wheels of the present invention had five to ten times the life and consumed less energy to remove one volume of steel than the best per se known roll-broken melt abrasive grains of similar cross-sectional diameter.

The advantage of the abrasive discs with elongated fibrous abrasive grains made according to the invention was particularly pronounced at high metal removal rates. For a given abrasive quality, the abrasive abrasive fiber discs show much better chip removal as shown by the lower energy values in Table II, and produce less heat, which in turn results in a brandless surface finish of the workpiece. Low heat and lack of branding are necessary to avoid metallurgical damage to the workpiece being machined, such as cutting tools.

Example Il

In this example, ceramically bonded segment abrasives having the same grain sizes as described in Example I were prepared. These segments were fabricated to fit in a 30.45 "(12") diameter CORTLAND chuck, each segment having a height of 12.7cm (5 ") and a cross section equal to the chord portion of a circle of 30, 48cm (12 ") at a chord length of 19.05cm (7.5"). The segments were shown according to the grinding wheels in Example I. A grinding test comparing the abrasive of the present invention with the best presently used molten abrasive was performed on 10.18 "square steel plates of 1018 steel with a BLANCHARD vertical grinder applied in water in the ratio 1:40.

Three depths of shear were tested:

0.406mm / min, 0.559mm / min and 0.711mm / min (0.016 "/ min, 0.022'Vmin and 0.028'Vmin), and in each case four passes were made, each with a total depth feed of 2.54mm (100 The disc wear, metal removal and performance were measured for each operation and the overall results are given in Table IH.

Table III Results of the Segment Surface Grinding Test on 1018 Steel

Abrasive Segment Feed Rate G Ratio Leistg.

(Type) No. (mil / min) * (mm / min) (steel / disc) (kW)

Melted 32A30S 16 0.406 7.44 8.4

(blocky) 22 0,559 5,75 12,0

28 0.711 4.48 12.0

Sintered SN119s 16 0.406 34.32 8.8

(extruded 22 0.559 12.64 9.2

Fibers) 28 0.711 12.64 9.6 ♦ 1 mil = 0.001 inches = 0.0254mm

As can be seen from the results shown in Table II, the segments made from the fibrous abrasives of the present invention outperformed the best currently used abrasive by 300 to 500% in the G-ratio while their power consumption was significantly lower at the higher feed speeds.

Example III

In this example, a batch of lower fiber diameter fibrous abrasive was prepared by mixing 3.2 kg of Pural® NG aluminum monohydroxide with 1.3 kg of water mixed with 22 g of alpha alumina seed material as in Example I. After five minutes of mixing, 200 g of 70% nitric acid were diluted with 750cm ³ of distilled water was added to produce a gel having 59% solids content, in which the inoculum was evenly distributed. The seeded gel was then extruded in a conventional manner through a smooth-walled die having a plurality of orifices each of 0.60 mm diameter. After drying, the extruded fibers were broken to lengths averaging 3 mm and then fired at 1320 ^° C. for 5 minutes. After firing, the degree of the cross section of the individual fibers corresponding to a standard abrasive grit 50. The firing temperature of 1320 ⁰ C with a burning time of 5 minutes was slightly less than in Example I. As in Example I the fibers were bent here and twisted. These fibers were made into test wheels according to the procedure of Example I except that the disk diameter was 127 mm (5 ") and comparative wheels were made with a seeded sol-gel alumina abrasive having the same composition as that of the fibrous abrasive but running through The abrasive discs with the fibrous abrasive of the present invention were designated X31-1 and the discs with the blocky sol-gel grain were designated SN5 by grooving hardened D3 steel as tested in Example I. The results are shown in Table IV.

Table IV Results of the dry grooving test on D3 steel

abrasive disc feed G ratio Specif. energy (Type) No. (Mil) · (Steel / disk) PS / inch ³ J / mm ³ min Sol-Gel SN5 0.5 24.3 23.0 62.8 (Blocky) 1.0 35.8 15.5 42.3 2.0 28.8 10.6 28.9 Sol-Gel X31-1 0.5 26.27 18.2 49.7 (extruded 1.0 48.58 12.9 35.2 fibers) 2.0 73.78 8.7 23,75

1 mil = 0.001 inches = 0.0254mm

These results clearly demonstrate the advantage of the sol-gel fibrous alumina abrasive over the block-shaped grain SoI-GeI alumina abrasive. At the highest feed rate, the grains of the invention had a 255% higher G ratio and 18% lower energy consumption.

Example IV

Four sets of hot-pressed phenol-formaldehyde resin standard hand sanding wheels were prepared in a manner known per se having a diameter of 15.24 (6 "), a thickness of 1.59 cm (0.625") and a bore of 1.59 cm (0.625 ") Pulley set contained the molten, block-shaped alumina zirconia abrasive (AZ) of USP 3891408; a second pulley set contained the blocky shaped sol-gel seeded alumina abrasive (SGB) according to USP 4,623,364 grade 16 (U.S. Standard Sieve Series); and a third pulley set containing the fibrous seeded sol-gel alumina abrasive (SGF) described in Example I above, but with a diameter of 1.5 mm (0.074 "). All wheels were essentially the same except for the abrasive type; they were relatively hard with percentages by volume of 48% abrasive, 48% binder and 4% pores. All grinding wheels were used in a grinding process that simulated bindings given when grinding railroad tracks. The results were as follows, with the abrasive wheels containing the per se known fused alumina zirconia (AZ) abrasive used as a benchmark.

Table V Rail Grinding Test - Relative Results in%

abrasive Const. disk- material kW G ratio variant Leistg. wear- material removal kW rate rate AZ 1.7 100.0 100.0 100.0 100.0 SGB 239.9 116.8 106.7 48.6 SGF 140.2 141.6 107.8 101.0 AZ 2.2 100.0 100.0 100.0 100.0 SGB 286.4 117.7 101.2 41.1 SGF 149.1 137.2 103.8 92.0 AZ 2.3 100.0 100.0 100.0 100.0 SGB 152.7 99.0 101.4 64.8 SGF 140.0 128.2 99.6 91.5 AZ 2.5 100.0 100.0 100.0 100.0 SGB 248.3 107.5 103.1 43.3 SGF 117.5 120.9 103.5 102.9

As from the G-ratios, d. H. From the material volume removal rate per unit of wheel wear, the overall quality of the currently used AZ abrasive was far superior to the blocky shaped sol-gel seeded abrasive and the fibrous seeded sol-gel abrasive described herein is equivalent to AZ only. When rail grinding, however, it is crucial that the railroad tracks for the necessary refurbishment of the rails by regrinding are as short as possible out of service. Thus, the speed with which a grinding wheel removes metal becomes the determining factor in assessing the quality of a rail grinding wheel. The metal removal rate of the abrasive wheels containing the fibrous seeded sol-gel abrasive was vastly superior to both the rate of the AZ abrasive and the blocky seeded sol-gel abrasive. In the various grinding passes, the fibrous abrasive was about 42%, 37%, 28%, and 21% better than AZ, respectively, and about 25, 20, 29, and 13% better than the block-shaped abrasive discs in terms of metal weight removal seeded sol-gel abrasives. Why the fibrous seeded sol-gel abrasive is superior even to its molded counterpart is not completely clear, but the difference was clear.

Example V

A series of commercially available phenol-formaldehyde resin-bonded cutting wheels was prepared by methods known per se. The dimensions of the abrasive wheels were 50.8 cm χ 0.33 cm χ 2.54 cm (20 inches χ 0.130 inches χ 1 inch), and the disks were laterally reinforced by a glass fiber fabric disk having a radius approximately half the radius of the cutting wheel, i.e., the glass washers. the reinforcing fabrics had a diameter of about 25.4 cm (10 inches). The cut-off wheels were made in one-third with block-shaped crushed 24 grade fused alumina (based on the U.S. Standard Sieve Series) sold by Norton Company under the name ALUNDUM (57A), ALUNDUM being a registered trademark of Norton Company. A second third of the slices contained the granular 24 block-shaped seeded sol-gel abrasive described in the aforementioned USP 4,623,364 (SGB). The final one-third of the abrasive discs contained the inventive sol-gel seeded fibrous sol gel (SGF) having a cross section approximating the diameter of the equiaxial 57A and the block-shaped, seeded, sol gel 24 abrasive, i. about 0.74 mm. On a volume basis, all wheels contained 48% abrasive, 46% binder and 6% pores.

The wheels were tested by dry-cutting 3.81 cm (1.5 ") thick C1018 steel and 3.81 cm (1.5") 304 stainless steel. The wheels were tested in a Stone M 150 cut-off machine at a peripheral speed of 60.96 m / s (12,000 feet per minute), with each wheel grinding 30 cuts at 2.5 and 4 seconds per cut on the C1018 steel ingots and on the 304 stainless steel ingot. The results of the comparative test when separating C1018 steel and 304 stainless steel are shown in Tables VI and VII, respectively.

Table VI Material separation - C1018 steel

disc grinding Time/ MA * ¹ SV * ¹ G-encryption kW Relat. No. medium cut Inches / 3 / Inches / 3 / relationship G-Ratio. Type S min min % 1 57 A 2.5 5.47 0.82 6.67 14,26 100 2 57A 2.5 5.43 0.81 6.67 13.97 100 3 57 A 4.0 3.45 0.75 5.58 9.27 100 4 SGB 2.5 5.47 0.51 10.79 12.67 193.4 5 SGB 2.5 5.51 0.51 10.79 13,20 193.4 6 SGB 4.0 3.42 0.40 8.65 8.79 155.0 7 SGF 2.5 5.51 0.32 17.24 11.90 258.4 8th SGF 2.5 5.39 0.25 21.54 11,95 323.4 9 SGF 4.0 3.37 0.16 21.54 8.04 470.3

* MA = material removal SV = disc wear

When cutting C1018 steel, the cutting wheels containing the fibrous seeded sol gel alumina abrasive (SGF) were in the overall grade and G ratio of the cutting wheels using the fused aluminum abrasive 57 A and the block-shaped abrasive SGB, respectively Contain counterpart to the SGF material, absolutely superior. At the split time of 2.5 seconds, the G ratios of the SGF cutting wheels were 158.5 percentage points higher than the corresponding values of the 57 Α wheels, and at the 4 second separation time, they were 370.3 percentage points higher. While the superiority of the SGF over the SGB material was not as great as the 57A, it was still very large, 93.4 percentage points at the 2.5 second separation time and 55 percentage points at the 4 second separation time. It should also be noted that in addition to the much higher grinding quality (G-ratio), the SGF cutting discs had a significantly lower power consumption in kilowatts (kW) than the discs 57 A or SGB abrasives. Total power was 31.89kW for all three SGF disks tested, 34.66kW for the three SGB disks and 37.55kW for the three 57A disks. The SGF abrasive resulted in energy savings of 15.1% compared to the 57A-containing wheels and 7.9% savings over wheels containing the SGB abrasive.

Table VII Material separation - 304 stainless steel

disc grinding Time/ MA * ' SV " G-Ver kW Relat. No. medium cut Inch ³ / Inch ³ / relationship G-Ratio. Type S min min % 10 57 A 2.5 5.51 1.08 5.11 12.96 100 11 57 A 2.5 5.39 0.92 5.85 12.06 100 12 57 A 4.0 3.45 0.48 7.22 8.94 100 13 57 A 4.0 3.42 0.39 8.66 9.12 100 14 SGB 2.5 5.64 0.52 10.79 12.43 211.2 15 SGB 2.5 5.51 0.51 10.85 12.34 185.5 16 SGB 4.0 3.50 0.20 17.24 9.09 238.9 17 SGB 4.0 3.45 0.20 17.24 8.61 200.5 18 SGF 2.5 5.34 0.37 14.43 11.81 282.4 19 SGF 2.5 5.30 0.37 14.43 12.48 246.7 20 SGF 4.0 3.39 0.16 21.54 8.82 298.3 21 SGF 4.0 3.31 0.15 21.54 8.43 248.7

* MA = material removal SV = disc wear

As with C1018 steel cutting, the SGF-containing wheels were vastly superior to the cutting wheels containing the normally-used crushed fused corundum abrasive 57A and significantly better than the wheels containing the SGB abrasive. At 2.5 seconds per cut, the G ratios of the SGF slices were 182.4 and 146.7 percentage points higher, respectively, than those of the 57A slices, and at 4 seconds per slice, the same differences were 198.3 and 148, respectively , 7 percentage points in favor of the SGF discs. Compared to the SGB-containing wheels, the quality advantage of the SGF wheels at the 2.5 sec cutting time was 71.2 and 61.2 percentage points, respectively, and 59.4 and 48.2 percentage points, respectively, in lengthening the cutting time to 4 seconds. In terms of power consumption, the SGF-containing wafers largely resulted in energy savings compared to the 57 A or SGB-containing wafers, however, the savings were relatively small.

Example Vl

Four sets of commercially available phenol-formaldehyde resin-bonded cutting wheels measuring 50.8 cm χ 0.22 cm χ 2.5 cm (20 "χ 0.130" x 1 ") and laterally reinforced by glass fiber tissue discs having half the radius of the blade were prepared in the usual manner. The cutting wheels had a composition in percent by volume of 50% abrasive, 32% binder and 18% pores The first blade set contained a crushed blocky shaped fused alumina abrasive called 53 ALUNDUM (53 A), where ALUNDUM is a registered trademark of Norton Company, Worcester, Massachusetts; the abrasive had a grain size of 50, based on the U.S. Standard Sieve Series The second pulley set contained a block-shaped sintered seeded Ge-GeI abrasive (SGB) according to U.S. Patent No. 4,623,364; also of the grain size 50. The third and the fourth cutting disk set contained the fiber described in Example I. shaped sintered seeded sol-gel abrasive, but with a cross-section that is approximately

the grain size of the 53-axis co-axial abrasive and the 50-gauge block-shaped seeded sol-gel abrasive. All inoculated sol-gel abrasives had submicron crystallite sizes. The abrasive in the latter two sets of discs had a grain diameter of about 0.118 "(0.011"), but in discs 26 and 27 the average aspect ratio was 9, in discs 28 and 29 the average aspect ratio was 6; in Table VIII below with SGF (a) and SGF (b) respectively.

A 414-inch diameter 4340 steel roller was used with a Campbell # 406 reciprocating grinder, which was separated by wet-grinding with water at 4.12 cm (1.62 ") at 57 cycles per minute and Separation times of 1 or 2 minutes. The separation was done at a peripheral wheel speed of 50.14 m / s (9870 feet / min). The results were:

Table VIII Material Separation - 4340 stainless steel

disc Schleifmit Time / Cut 60 Mittl.relat. Mittl.relat. No. teltyp S 60 G ratio power 22 53 a 60 100 100 24 SGB 60 113 97 60 26 SGF (a) 60 319 101 60 28 SGF (a) 120 335 102 120 23 53 a 120 100 100 25 SGB 120 99 84 27 SGF (a) 120 350 103 120 29 SGF (b) 401 102

G-ratio = volume ratio of material removal to disc wear.

At a time of 60 seconds per cut, the cutoff wheels containing the fibrous sintered seeded sol gel abrasive SGF (a) and SGF (b), respectively, outperformed the wheels with the conventional 53A fused alumina abrasive and the related block molded inoculated sol-gel abrasive SGB. The disk containing the SGB abrasive had a G-ratio 13 percent higher than the 53 Α disk, while the SGF (a) and SGF (b) disks increased the standard 53 Α disks 219 and 219, respectively . 235 percentage points were superior. When extending the cutting time for the 10.2 cm (4 ") diameter to 120 seconds, the quality of the 53 A and SGB disks was about the same, while the quality of the separation disks containing the fibrous sintered seeded sol gel solders SGF ( a) and SGF (b) were 3.5 and 4 times higher, respectively, than the 53 A and SGB disks, and there was no significant difference in power consumption between the two inventive SGF abrasives and the SGB and öSA However, even a 25-30% lower power consumption would be of minor importance to the cutting wheels containing the SGB and 53A abrasives, considering the superiority of 219 to 301 percentage points of the fibrous sintered seeded sol-gel abrasive ,

Another benefit of the seeded gel fibrous abrasive was found in a coated abrasive where it demonstrated the unexpected ability to maintain high chip removal power longer than the roll-broken SG abrasive.

Example VII

In this example, the chip performance of seeded alumina (SG) fibrous abrasive particles and the surface finish they produce are compared to the values of SG abrasive grains obtained by conventional roll breaking. The fibrous abrasive particles had a mean diameter of 0.33 mm (0.013 "), which corresponded to approximately 50 grit abrasive grains (0.01369"), and a randomly distributed aspect ratio ranging from about 2: 1 to 8: 1. Nevertheless, the batch classification resulted in some very long fibers and a disproportionately large amount of undersize, as shown below. The composition of abrasive fibers and roll-cut 50 gauge SG control grains was the same.

Table IX Classification results of roll-cut grains compared to abrasive fibers

grain type Serial no. granulation class LPD " Walzengebr. Grain abrasive fibers 08 D 163.3 08 E 168.7 50 50 +3.2 + 1.9-2.5 + 30.9 180.7 209.1

* LPD = Dulling test at low grinding pressure

As can be seen from the above table, there was a considerable difference in the classification of the two types of abrasive grains. The indication "+3.2 + 1.9" means that the sample of roll-cut abrasive grain when tested using the CAMI sizing system had 3.2% too much excess and 1.9% too much undersize, which is within the allowable tolerance range On the other hand, the indication "-2.5 + 30.9" for the abrasive fiber sample indicates that the sample had 2.5% too little oversize and 30.9% too much undersize, a disproportionately high value , The high value for the undersize portion is due to the fact that the abrasive fibers ("grains") all have the same shape The lower the number, the larger the approximate diameter which is smaller than the mesh size of the control screen for the grain size 50.

Fiber abrasive sheets were made in the usual way. The coating weights were approximate: adhesive layer lbs / ream *, topcoat 23 lbs./crisit, abrasive fibers 65g, roll breaking grains 52g.

The produced fiber abrasive sheets were first evaluated after the usual bending test in a blunting test at low grinding pressure. The results for the comparison of the chip performance and the surface quality produced are shown in Table X below

* 1 lb = 0.453kg

Table X Comparison of chip removal rates and surface qualities on a flat 3 "blank sheet between a fibrous abrasive and a broken abrasive grain

removal Kumul. Abtr g Fiber shape Material Korng 50 Cumul Abtr. G Surface quality pc Fiber form. Material Korng. 50 Time period SG-Vergl - Material Korng 50 Div. / Sec. G SG-Vergl.- Matenal Korng 50 Surface quality Div. / Sec Surface quality Ra Rtm Pc No Ra Rtm

29

55

94

119

136

146

20 37 68 92

112 128 143 154

164 105 80 48 37 34

1005 736 625 414 331 282

227

265 163 115 103 96

125

103

109

53

33

28

28

40

995 885 757 386 268 222 193 297

Ra = deviation from the center line in microns. Rtm = average roughness in microns Pc = number of increases

The surface finish of a metallic workpiece is normally measured by the values Ra and Rt taken from sections at various locations (e.g., in the center and at the right and left edges) along the finished workpiece. The importance of these statistical parameters is well known to those skilled in the art. They are clearly defined in a publication titled "Introduction to Surface Texture and Part Geometry" by Industrial Metal Products Incorporated (IMPCO), the full disclosure of which is referred to here. "In general, Ra is a measure of mean surface roughness This number is usually supplemented by other parameters derived from the same surface, but in the case of fine machining of metals, the Rt value is often used as a complement to the Ra measurement, and the value of Rt is a Measure the depth of marks or scratches that may remain on the workpiece surface after finishing. Pc is a number that generally indicates the scratch frequency.

As can be seen from the above data, the abrasive sheet with the abrasive fibers outperformed the abrasive sheets with the conventional roll-cut abrasive grain in the cutting performance. During the first time periods, the abrading rate of the abrasive sheet with the abrasive fibers was initially somewhat lower, but then remained for a much longer time. The surface roughness produced by the abrasive fibers or roll-broken grains appears to be approximately equal. It should be noted that the batch classification of the fibrous abrasive particles as described above gave a disproportionately large undersize fraction and some very long fibers. A more controlled "classification" would probably result in a different surface quality.

Another test (112 Dsl) was performed to compare the chip removal and the surface roughness generated between the abrasive fibers and 50/36 grit abrasive grit. The results are shown in the following Table XI. The specimen has a frontal area of 25.4mm (1 ") diameter.

Table Xl Comparison of the chip performances and surface finishes on the steel test piece with a 1 'diameter face between abrasive fibers and roll-cut abrasive grain

removal # 952 * SGV * 44 surface finish rtm pc # 952 * rtm pc SG-Comp-Mat. rtm pc SGV * Kg. Kg. 256 SG-Comp-Mat. 1053 214 Körng. 1455 149 Körng. 1 181 163 Kg. 50 36 526 Körng. 852 195 50 1279 173 36 1079 224 50 removal 779 50 839 233 1019 179 840 117 Kumulat. 1 001 1018 243 714 159 711 284 ing 31 1 181 599 353 730 196 surface finish 820 145 1 41 189 1334 surface finish 420 320 581 230 Ra 526 179 5 215 419 1437 Ra 254 100 surface finish 423 152 226 534 181 10 438 640 1506 152 - - Ra 477 139 205 394 168 15 627 869 1541 154 - - 207 431 200 135 359 181 20 779 1060 - 119 _ _ 196 489 96 128 503 115 25 880 1228 136 - - 158 347 90 129 - - 30 926 1350 63 136 75 35 - 1444 41 124 73 40 - 1496 42 92 54 44 - 1532 - 80 70 48 - - 87 80 - 48 - - 73 65

* SGV = SG comparative material / * 952 = abrasive fibers

Ra = deviation from the center line in microns.

Rtm = mean roughness in microns.

Pc = number of increases.

As can be seen from the above results, the 50 grit abrasive fibers not only outperformed the 50 grit broken abrasive grains in the cut, but the abrasion was equal to that of the 36 grit comparison material. Again, the abrasive fibers initially had a slightly lower stock removal rate than the roll broke grains, However, these remained for a long time. In this test on steel with a face of 25.4 mm diameter, the fibrous abrasive particles produced a similar surface finish to the 36 grade comparison material.

Example VIII

This example illustrates the effect of in-situ generation of the alpha alumina in the form of abrasive fibers as opposed to the extrusion of alpha alumina particles in a suitable medium followed by sintering for

Formation of a coherent structure.

The product of the present invention was prepared by mixing boehmite ("Disperal" from Condea) with water and 1 wt% in% based on boehmite, alpha alumina of submicron grain size in a V-Mixer for two minutes, then adding a nitric acid solution of 18%. % By weight giving 7 parts by weight in% nitric acid relative to the boehmite and mixing was continued for a further five (5) minutes to obtain a boehmite gel

produce.

A series of products were then made for comparison purposes which corresponded to the above product, except that more alpha alumina (of the type used above as inoculum) was added such that the total mixture contained much higher proportions by weight of alumina. The boehmite was retained to make the mixture extrudable. The approaches are described in the following Table XII.

Table XII

approach

variant

% Solid

Comparative Approach A Comparative B * Comparative Approach D Comparative Approach E Comparative Approach F Example 1 Example 2 Example 3

30% alpha alumina 30% alpha alumina 90% alpha alumina / 10% gel 60% alpha alumina / 40% gel 60% alpha alumina / 40% gel 1% alpha alumina (seed additive) 1% alpha alumina (seed additive) 1% alpha alumina (seed additive)

70% 70%

62% 58% 59%

* Additional ultrasonic mixing of the slurry was applied.

These materials were then extruded to form fibers which were dried and sintered under the conditions described below. Higher temperatures were required for sintering the high alpha alumina content control approaches than for the seeded SoI-GeI approaches. Samples of fibers were then tested for strength using a simple three-point method using an Instron testing machine with a crosshead speed of 2.0 mm / min. The abrasive fiber lay on two edges 10mm apart (9mm in the case of comparative approaches C, D and E). A downward pressure acted in the middle between these support points over a knife edge. The pressure was gradually increased until the fiber broke, and this pressure divided by the cross-sectional area of the fiber is given in the following Table XIII as breaking strength.

charge burning burning thread breaking strength Upper W. temperature duration diameter Mean. ⁰ C min mm kp / cm ² 7.465 Vergleichsans.A 1500 30 0.32 6,831 6,268 Vergleichsans.B 1550 30 0.3175 6,162 6,646 Compare Sans. C 1450 60 1.00 5,424 4,036 Compare Sans. D 1300 6 0.88 3,430 2,436 Compare Sans. e 1350 6 0.87 2,378 13.239 Ex. 1 1370 4 0.054 11,197 15.986 Ex. 2 1350 30 0.043 14.366 17.112 1350 5 0.046 14.154 16.549 1325 30 0.046 14.296 14.859 1350 30 0.053 10.281 18.169 Ex. 3 1350 30 0,020 16,000

The fibers of the comparative batches were much thicker, as it was more difficult to extrude thinner fibers which had uniform dimensions after extrusion and before firing. It was found that higher alpha alumina levels significantly aggravated this problem.

From a comparison of the above data, it can be seen that the comparative fibers had significantly lower breaking strengths, and it is believed that this is due to the weaker sintering bonds that developed as a result of the sintering process between the alpha alumina crystals. Therefore, the preferred fibers of the invention preferably have a breaking strength of at least 8000 and preferably at least 10000kp per square centimeter in cross-section as measured by the test described in Example VIII. This is in contrast to products made by sintering preformed alpha alumina, with much lower strengths being achieved.

Example IX

This example illustrates the preparation of fibers from a seeded SoI gel according to the invention.

Twenty grams of polyethylene oxide in 200 ml of distilled water was mixed with 28 ml of water which had been ground in a Sweco mill with alpha alumina media until the water contained 5% alpha alumina solid. This mixture was gently mixed for 16 hours. Then it was added to 200 g of alumina monohydrate (Condea NG) and mixed for 5 minutes in a Hobart mixer. Eighty milliliters of 20% by volume in% nitric acid were then added and the whole was mixed for an additional 5 minutes.

The resulting mixture, a viscoelastic gel, was placed in a 152.4 mm (6 ") diameter drum with plexiglass plate clamped in. The drum was mounted on a vertical motor shaft and housed in a 0.557 m ² {6 foot ² ) housing The drum was rotated at 2,500-3,500 rpm and, by forcing the gel outward and upward, caused the plexiglass to shift and open a nip, the gel was pressed outwardly and drawn out into fibers The very small diameter of the fibers dried them very quickly and the fibers were collected on the screen of an exit port and then fired at 1350 ° C for 15 minutes, with a linear temperature rise of 15 ° C per minute.

The collected fibers were of high purity and contained 98% very fine alpha alumina crystallites.

Example X

This example illustrates the production of fibrous materials other than the straight cylindrical shape by extrusion of a seeded sol-gel alumina material having different cross-sectional shapes. As indicated in the description, the largest dimension of the fibrous material is also referred to herein as "grain size." The products are summarized in Table XIV.

Table XIV

granulation 110 24 24 28 24 50 Shape (cross section) square Tube (large opening) Tube (small opening) triangle round round Firing., ° C 1270 1270 1270 1270 1250 1250 Burning time, min 5 5 5 5 18 18 Hardness (GPa) 19.6 20.9 20.3 18.7 18.9 20.1 Dia. (Mm) 0.14 0.89 0.89 0.71 0.91 0.33 L / D 5.8 Density * (g / cm ³ ) 3.84 3.89 3.90 3.91 3.88 3.86 Crystal size (цт) 0.14 0.19 0.18 0.17 0.16 0.18

* Evaluated with a helium pycnometer.

All grain shows excellent properties when used as abrasive grain.

Example XI

This example illustrates the effect of crystal size on the grinding performance of the inventive abrasives.

The batches of abrasive grain were made by a method using a seeded sol gel except for a batch ("G" where the larger crystal size was easily achieved by omitting the seed).

The characteristics of the abrasive grain were:

Table XV

granulation Watertight Crystal size * Sandstrahleindring.- # (g / cm ³ ) (Μηη) depth (mm) A 3.94 1.16 3.91 B 3.93 0.65 3.84 C 3.89 0.54 3.83 D 3.92 0.42 4.14 e 3.90 0.39 4.16 F 3.98 0.26 3.92 G* 3.95 2.54 2.99

* Crystal sizes were all measured by the line-cut method.

The diameter of the circular cross-section particles corresponded to a grain size of 50. There was a certain spread of aspect ratios in the samples used to make a 127mm x 12.7mm χ 31.75mm abrasive disk using the same ceramic binder to make the disks was used. Each disc was dressed to a 6.4mm wide rectangular edge and subjected to a "dry" or "wet" sanding test. For "dry" grinding, a D-3 steel plate Rc60 (Rockwell hardness) of about 100 mm χ 400 mm was used and the peripheral speed of the grinding wheel was 33.02 m / s (500 feet / min).

"Wet" grinding used a 4340mm hardened steel plate of 100mm χ 400mm and a White and BagleyE55 coolant in tap water at a ratio of 1:40 with the cooling liquid applied to a flexible 25mm D die was 43.18 m / s (8500 feet / min).

The following parameters were used in the process:

1. Table speed 15.24m / min.

2. depth feed values 0.5; 1.0 and 1.5 for dry grinding and 0.5; 1.0 when wet grinding. Total depth feed of 2.54mm (100 mils *).

3. To measure were the grinding wheel wear (SV), the metal removal rate (MAR), surface finish, power and force after 100 mils (2.54mm) except at the 1.5 mil (0.038 mm) depth feed, where after 100 To measure 5 mils).

4. The grinding wheels were to be dressed with a pointed diamond at a depth feed of 1 mil (0.0254 mm) and a face advance of 250 mm / min.

The data obtained are shown in Tables XVI and XII below.

The comparative data refer to commercially available conventional grade 54 sol-gel material processed with the same binder.

Table XVI Dry grinding

Identification Depth feed (mil) * Avg. Peak power (W) Customs duties MAR ** SV ** G ratio Surface quality Comparison: 0.5 1.0 1.5 940 960 1120 0.2470 0.5942 0.8839 0.0051 0.0096 0.0178 58.1 62.0 49.8 о о о to со о G 0.5 1.0 1.5 400 500 640 0.1035 0.1939 0.2910 0.1652 0.3127 0.4882 0.6 0.6 0.6 240 320 300 A 0.5 1.0 1.5 720 850 1000 0.2364 0.0992 0.7182 0.0430 0.0690 0.0892 5.5 7.1 8.1 170 200 280 B 0.5 1.0 1.5 800 1000 1120 0.2631 0.5196 0.7916 0.0301 0.0514 0.0515 9.7 10.1 15.4 120 120 260

1 mil = 0.001 inches = 0.0254mm.

MAR = material removal rate SV = disk closure

Table XVI (continued)

Distinguishing deep Mittl Spit Inch ³ / inch SV ** G-encryption SURFACE voltage feedrate zenleistung 0.0238 haltnis chengute (Mil) * (W) MAR ** 0.0312 0.5 640 0.2625 0.0458 11.0 110 C 1.0 960 .5532 0.0262 17.7 150 1.5 1040 .8239 0.0321 18.0 170 0.5 640 .2736 0.0317 10.5 190 D 1.0 920 .5650 0.0247 17.6 180 1.5 1 120 .8543 0.0333 26.9 200 0.5 480 .2613 .0471 10.6 190 e 1.0 690 .5550 0.0079 16.7 180 1.5 920 .8284 0.0156 17.6 200 0.5 680 .2915 0.0176 37.1 170 F 1.0 880 .5838 37.3 200 1.5 1040 .8796 44.8 200

• 1 mil = 0.001 inches = 0 0254mm

* · MAR = material removal rate SV = disk wear

Table XVII Wet grinding

Identification Depth feed (mil) * Middle peak power (W) Customs duties MAR ** SV ** G-ratio Surface good comparison 0.5 1.0 1560 1760 0.2470 0.5942 0.0051 0.0096 58.1 62.0 60 80 G 0.5 1.0 960 960 0.0741 0.1416 0.2006 0.3962 0.4 0.4 230 200 A 0.5 1.0 880 1040 0.1422 0.3060 0.1193 0.1958 1.2 1.6 120 120 B 0.5 1.0 960 1120 0.2016 0.4236 0.0453 0.0760 4.8 5.6 180 110 C 0.5 1.0 1200 1360 0,2439 0,4524 0.0191 0.0661 12.7 6.8 140 110 D 0.5 1.0 1440 1520 0,2885 0,5202 0,0100 0,0169 29.0 30.7 120 200 e 0.5 1.0 1440 1760 0,2883 0,5658 0.0092 0.0198 31.2 28.6 100 130 F 0.5 1.0 1360 1480 0.2961 0.5892 0.0043 0.0105 69.0 59.1 120 120

* 1 mil = 0.001 inches = 0.0254mm

** MAR = material removal rate SV = disk wear

From the above data, it can be seen that the conductor line improves significantly with decreasing crystal size. In addition, the disk worked better when dry sanding, the stronger the attacking force was (greater depth feed) This is highly unexpected. The general experience is that the G-ratio decreases with the force of attack, as the grains begin to polish and their effectiveness as cutting edges decreases. For the most part, the abrasive particles of the invention have been getting better and better, with the additional wafer wear remaining low

Claims (16)

1. fiber based on sintered sol-gel alumina, characterized in that it consists of sintered alpha alumina crystals with a grain size of at most 2 μιη, preferably of 1 μηι, wherein the fiber has a substantially uniform cross section, a diameter of at most 2 mm, an average aspect ratio of at least 1: 1 and a hardness of at least 16GPa. 2. Fiber according to claim 1, characterized by an aspect ratio of 1.5: 1 to 25: 1. 3. Fiber according to claim 1 or 2, characterized in that the fiber is a particle having a length of less than 10 mm and / or a diameter of 0.0001 mm to 2 mm. 4. Fiber according to one or more of the preceding claims, characterized in that the fiber consists of at least 80% of generally equiaxed crystals having a dimension of at most 0,4μηη. 5. A fiber according to one or more of the preceding claims, characterized by a density of at least 95% of the theoretical density, wherein to 1 to 50 parts by weight in% an oxide selected from the group of zirconia, titania, magnesia, hafnia, ceria, spinel , Mullite, manganese dioxide, precursors and mixtures of these oxides. 6. Fiber according to one or more of the preceding claims, characterized in that the alpha alumina crystals are vaccinated alpha alumina crystals having a dimension of less than 1 μιτι. 7. Fiber according to one or more of the preceding claims, characterized in that the fiber is curved along its longitudinal axis. 8. Fiber according to one or more of the preceding claims, characterized in that the fiber is twisted along its longitudinal axis. 9. Fiber according to one or more of the preceding claims, characterized by a substantial freedom from glassy constituents and by a breaking strength of at least 8000kp / cm ² . 10. A process for producing a sintered sol-gel alumina-based fiber, characterized in that a gelled dispersion of hydrated aluminum oxide particles in the submicron range is prepared and the gelled dispersion is formed into fibers which are dried and thereafter at a temperature between 1090 ⁰ C and 1500 ° C, are burned during a burning period, in which the fibers are converted into a ceramic body, which is formed predominantly of alpha alumina crystals with a size of 2 μιη or less. A method according to claim 10, characterized in that the gelled dispersion contains an effective amount of sub-micron seed particles to facilitate rapid conversion of the hydrated alumina particles to alpha alumina upon sintering to form alpha alumina crystals having a crystal size of about 1μιη or below , 12. The method according to claim 10 or 11, characterized in that the alpha alumina crystals have a crystal size of about 0.4 μιη or below. 13. The method according to any one of claims 10 to 12, characterized in that the seed particles are selected from a group consisting of alpha alumina, alpha iron (lll) oxide, gamma alumina, chromium oxide, precursors of alpha alumina, gamma alumina, chromia or alpha iron (lll ) oxide and mixtures of these substances, and that the seed particles are used in an amount of about 0.5 to about 10 parts by weight in%. 14. The method according to any one of claims 10 to 13, characterized in that the fibers are formed from the gelled dispersion by extrusion or spinning. 15. The method according to any one of claims 10 to 14, characterized in that a fiber which consists of sintered Alphaaluminiumoxidkristallen with a grain size of at most 2 μιη, preferably of 1 μιη and below and a substantially uniform cross section, a diameter of at most 2 mm, has a mean aspect ratio of at least 1: 1 and a hardness of at least 16GPa. 16. Use of the fiber based on sintered sol-gel alumina according to one of claims 1 to 9 and prepared by the method according to any one of claims 10 to 15, characterized in that the fiber as a) an abrasive fiber in a bonded abrasives abrasive article, especially in the form of abrasive wheels, segment abrasives, or abrasive stones, and an abrasive sheet material, especially in the form of round abrasive sheets or abrasive belts b) a reinforcing fiber is used in a metallic and / or ceramic composite material.