EP3374129B1 - Sintered, polycrystalline, flat, geometrically structured ceramic grinding element, method for the production thereof, and use thereof - Google Patents

Description

TECHNICAL AREA

The present invention relates to a sintered, polycrystalline, flat, geometrically structured ceramic grinding element for use in synthetic resin-bonded grinding wheels, in particular cutting wheels. The present invention also relates to a method for producing such a sintered, polycrystalline, flat, geometrically structured ceramic grinding element and its use.

STATE OF THE ART

A special form of synthetic resin-bonded grinding wheels are the synthetic resin-bonded cutting disks, which are used in the context of this application as examples of synthetic resin-bonded grinding wheels, but this does not mean that the invention is limited to cutting-off wheels. Rather, it has been found in the present work that the grinding elements according to the invention, which were originally designed for use in cutting wheels, are generally suitable for synthetic resin-bonded grinding wheels.

Cutting disks are flat circular disks that are mostly used to cut off sections of material. Different cutting discs are used for the different materials to be processed, such as metal, stainless steel, natural stone, concrete or asphalt, whereby the cutting discs can be divided into two main groups, namely synthetic resin-bonded cutting discs and diamond cutting discs. To produce synthetic resin-bonded cutting discs, abrasive grains such as corundum or silicon carbide are mixed together with fillers, powder resin and liquid resin to form a mass, which is then pressed into cutting discs of various thicknesses and diameters in special machines. In doing so, the abrasive is made into a fabric Glass fiber embedded in order to be able to withstand the enormous centrifugal forces that occur when using the cutting discs. With diamond cutting discs, which are used almost exclusively for use in natural stone, concrete or asphalt, diamond segments are applied to steel stems using various processes, such as sintering, soldering or laser welding.

The abrasives industry over the past few years has constantly sought ways to improve the performance of cut-off wheels, with a particular focus on the use of high quality abrasive grains. So describes the EP 1 007 599 B1 Cutting discs that have a mixture of different sol-gel corundums as abrasive grains. The EP 0 620 082 B1 describes cutting discs which, in addition to highly abrasive components such as cubic boron nitride or diamond, have microcrystalline filament-shaped aluminum oxide particles with a uniform orientation, the abrasives being in the form of segments that are applied to a metal blade.

Ceramic abrasive grains in the form of tetrahedra or pyramids obtained via the sol-gel process are produced according to FIG U.S. Patent Application No. 2013/0040537 A1 used in a mixture with other high-quality abrasive grains in synthetic resin-bonded cutting discs. Similar synthetic resin-bonded cutting discs are used in the U.S. Patent Application No. 2013/0203328 A1 described, wherein ceramic abrasive grains obtained via sol-gel processes in the form of triangular platelets, prisms or truncated conical pyramids are used in turn alongside other high-quality abrasive grains in a mixture with phenolic resins, grinding aids, fillers and other additives.

With the help of such abrasive grain mixtures, in which abrasive grains with defined shapes are used, astonishingly high increases in performance could be achieved not only in synthetic resin-bonded cutting discs, but also generally in synthetic resin-bonded grinding wheels, compared to grinding wheels with high-quality abrasive grains with undefined cutting edges.

DISCLOSURE OF THE INVENTION

Spurred on by such results, the abrasives industry continues to seek improvements in the performance of synthetic resin-bonded grinding wheels, especially cut-off wheels.

The present invention is therefore based on the object of offering abrasives for use in synthetic resin-bonded grinding wheels, in particular cutting wheels, which have advantages over the prior art.

The object is achieved by sintered, polycrystalline, flat, geometrically structured ceramic grinding elements according to claim 1, which are intended to be installed in synthetic resin-bonded grinding wheels, in particular cutting wheels, instead of grinding grains.

The object of the present invention is also to provide a method for producing sintered, polycrystalline, flat, geometrically structured ceramic grinding elements for use in synthetic resin-bonded grinding wheels.

The object is achieved by the method of claim 8.

Another object of the present invention is to provide improved synthetic resin-bonded grinding wheels, in particular cutting wheels.

This object is achieved by using sintered, polycrystalline, flat, geometrically structured ceramic grinding elements as a replacement of abrasive grains in synthetic resin-bonded grinding wheels, in particular cutting wheels, according to claim 10 or claim 11.

Said sintered, polycrystalline, flat, geometrically structured grinding elements are sintered molded bodies with a homogeneous microstructure, a chemical composition that is uniform over the entire area of the grinding element and a uniform geometric structure. The sintered body has a first surface and a second surface which is opposite and parallel to the first surface. Both surfaces are separated from one another by a side wall with a thickness (t) between 50 µm and 2000 µm. The ratio of diameter to thickness of the grinding element is greater than 30, preferably greater than 50. The mean diameter of the crystals forming the homogeneous microstructure is less than 10 μm, preferably less than 5 μm.

The chemical composition of the sintered, polycrystalline, flat, geometrically structured ceramic grinding elements is preferably based on aluminum oxide and / or other chemical compounds selected from the group consisting of carbides, oxides, nitrides, oxy-carbides, oxy-nitrides and at least carbides one of the elements selected from the group consisting of Al, B, Si, Ti and Zr.

The sintered polycrystalline, flat, geometrically structured grinding elements preferably have a Vickers hardness Hv of at least 15 GPa, particularly preferably at least 18 GPa.

In a preferred embodiment of the present invention, the density of the sintered, polycrystalline, flat, geometrically structured ceramic abrasive elements is at least 95% of the theoretical density, preferably at least 97.5% of the theoretical density.

The grinding elements are preferably circular disks or segments of a circle, the diameter and thickness of which are adapted to the cutting disks to be formed therefrom.

In a preferred embodiment, the grinding elements according to the invention are designed as perforated ceramic bodies provided with recesses. The perforation or the recesses of the ceramic body advantageously have a homogeneous geometric structure with geometrically shaped openings or recesses. The volume ratio of the openings to the massive proportions of the grinding elements is preferably constant over the entire usable diameter of the grinding elements, the usable diameter being understood to mean the area of the grinding element that is used when working with the grinding element.

A further advantageous embodiment of the present invention provides that the sintered, polycrystalline, flat, geometrically structured ceramic grinding elements are porous ceramic bodies which either per se have sufficient porosity to guarantee the porosity required for the grinding wheels, or additionally likewise are perforated or have recesses, the perforation or the recesses, however, then being less pronounced. Porous ceramic bodies in the context of the present invention are to be understood as those ceramic bodies which are interspersed with more or less small pores, while the above-mentioned perforations and recesses are large-volume and preferably geometrically structured.

In a preferred embodiment of the present invention, the basis for the chemical composition of the abrasive elements is aluminum oxide, the chemical composition preferably at least 50% by weight aluminum oxide and optionally one or more of the oxides selected from the group consisting of SiO ₂ , MgO, TiO ₂ , Cr ₂ O ₃ , MnO ₂ , Co ₂ O ₃ , Fe ₂ O ₃ , NiO, Cu ₂ O, ZnO, ZrO ₂ and the rare earth oxides. In addition, other chemical compounds based on oxides, carbides, nitrides, oxy-carbides, oxy-nitrides and carbo-nitrides, selected from the group consisting of the elements, are also suitable from Al, B, Si, Ti and Zr, suitable materials for producing the ceramic abrasive elements according to the invention.

The grinding elements according to the invention can be produced by different processes, in which case a moldable ceramic mass is first produced from which flat, geometrically structured precursors for ceramic grinding elements are formed, which are sintered to polycrystalline, flat geometrically structured ceramic grinding elements.

For example, the ceramic mass or the ceramic precursor material can be added by wet grinding α-aluminum oxide with an average particle size of preferably less than 1 μm in a ball mill in the presence of a dispersant and subsequent addition of an organic binder and optionally a plasticizer and / or an antifoam agent the dispersion can be obtained. The dispersion is mixed for several hours until a stable colloidal dispersion has formed, which is processed into a layer with a layer thickness of up to 3 mm using film casting. The film-cast layer is dried and precursors of the flat, geometrically structured grinding elements are cut out, which are then calcined and sintered.

In addition, all methods are suitable in which malleable ceramic masses are obtained, from which the corresponding grinding elements can then be shaped and then sintered.

For example, sol-gel processes are also very suitable for producing a moldable ceramic mass, the sol-gel compositions comprising a liquid carrier in which the ceramic precursor material is converted into a ceramic material, such as, for example, α-aluminum oxide , Silicon oxide, titanium oxide, zirconium oxide or mixtures thereof, can be converted, dissolved or dispersed. Many of these for the production of ceramics based on Sols suitable for aluminum oxide are commercially available as boehmite sols under the brand names “Dispal”, “Disperal”, “Pural” or “Catapal”.

The sol-gel compositions can comprise modifying additives or precursors to modifying additives. The function of these additives is to improve the desired properties of the sintered, flat, geometrically structured ceramic abrasive elements. Typical modifying additives or precursors of modifying additives are oxides, carbides, nitrides, oxy-carbides, oxy-nitrides, carbon-nitrides or water-soluble salts of magnesium, zinc, iron, silicon, cobalt, nickel, zirconium, hafnium and rare earths.

Additionally or alternatively, the sol-gel composition can contain crystallization nuclei in order to accelerate the conversion of hydrogenated or calcined aluminum oxide into α-aluminum oxide and thus to limit crystal growth. Suitable crystallization nuclei for this include fine particles of α-aluminum oxide, finely divided α-iron oxide or its precursors, titanium oxide and titanates, chromium oxide or other compounds which are able to promote the conversion to α-aluminum oxide.

The particular advantage of the sol-gel process is that grinding elements with a particularly fine crystalline structure, high hardness and extraordinary toughness can be obtained in this way. In the sol-gel process too, layers are formed which are then dried. The precursors of the flat, geometrically structured grinding elements are cut out of the dried layers and then sintered. Alternatively, the gels obtained in the sol-gel process can also be placed directly in a corresponding mold, then dried and then sintered.

Further suitable processes for the production of flat, geometrically structured ceramic grinding elements are injection molding, pressing, roll forming and rapid prototype development or additive manufacturing, such as for example 3D printing, stereolithography and the LOM process (Laminated Object Manufacturing).

BRIEF DESCRIPTION OF THE FIGURES

die Figuren 1 bis 8

zweidimensionale Draufsichten auf unterschiedlich geometrisch strukturierte Schleifelemente;

die Figur 9

eine Übersicht mit unterschiedlichen geometrischen Aussparungen und

die Figuren 10a - 10c

schematische Darstellungen unterschiedlicher Spanwinkel.

The present invention is additionally explained with reference to figures. Show it

Figures 1 to 8

two-dimensional top views of different geometrically structured grinding elements;

Figure 9

an overview with different geometric cutouts and

Figures 10a-10c

schematic representations of different rake angles.

No restriction is to be seen in the selection of the geometric structures shown in the above-mentioned figures. In addition to the structures shown, a large number of other structures are possible and useful in order to achieve the object according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The Figure 1 shows a plan view of a radially designed round grinding element, in the center of which a circular recess 1 can be seen, which corresponds to the receptacle of the grinding wheel in which the grinding element is to be installed. The body 2 of the grinding element is star-shaped, the ends of the rays 3 being perpendicular to the circular recess 1 and forming a circle whose diameter corresponds to the diameter of the grinding wheel for which the grinding element is intended. Recesses 4 can be seen between the rays 3, which are suitable for providing the grinding wheel with the required porosity. The recesses 4 are like this dimensioned so that the volume ratio of recesses 4 to the solid areas of the grinding element is constant over the diameter of the grinding element used in the grinding process. This ensures that if the grinding wheel is radially worn, the porosity of the grinding wheel and thus the grinding conditions remain unchanged over the entire grinding process. In the Figure 1 this relationship is illustrated by the ratio of the distances A / B and A '/ B' relating to the circumference U or U 'for a specific pulley diameter.

The Figures 2 and 3rd also show top views of radially designed grinding elements, the rays 3 in FIG Figure 2 Form an angle to the circular recess 1. In the Figure 3 the rays 3 are additionally curved. Here, too, the recesses 4 are again dimensioned in such a way that the volume ratio of the recesses to the solid areas of the grinding element is constant over the diameter of the grinding element used in the grinding process, which is again determined by the ratio of the distances A / B and A '/ relating to the circumference. B 'is clarified.

Another feature for characterizing the flat, geometrically structured ceramic grinding elements is the rake angle γ, which corresponds to the inclination of the chip surface (contact surface) to the reference surface, which is arranged perpendicular to the tangent of the disk. There are three different types of rake angle possible: positive, negative and exactly zero. A positive rake angle γ helps to reduce the cutting force and thus the energy requirement during cutting, whereas a negative rake angle Y increases the edge strength and the service life of the grinding element or grinding wheel. The rake angle Y is also based on the Figures 3, 4 , 8th , 10a, 10b and 10c explained.

The grinding element according to Figure 3 has a positive rake angle Y of 18 °. During the grinding process, the rake angle Y falls back to zero with increasing wear (decreasing radius) of the grinding wheel.

The Figure 4 shows a circular disk-shaped grinding element, the body 2 of which has a circular recess 1 corresponding to the receptacle of the grinding disk. The porosity of the grinding wheel is ensured in the present case with round holes 4, which become larger with increasing radius of the wheel, so that here too the volume ratio of recesses 4 to the solid areas of the grinding element is constant over the diameter of the grinding element used in the grinding process, which is again illustrated by the ratio of the distances A / B and A '/ B' relating to the circumference. The rake angle Y of the grinding element begins with + 29 ° and changes with decreasing grinding wheel radius after passing the zero in the negative range down to -90 °. In the next row of round holes 4, the rake angle starts with + 90 °, falls back to zero and then changes to the negative range down to -90 °. This process then repeats itself with each beginning row of holes.

The Figures 5 to 8 also show circular disk-shaped grinding elements that have perforations 4 in other geometric shapes. In the Figure 5 are trapezoidal holes 4 in which Figure 6 diamond-shaped holes 4 in which Figure 7 hexagonal, honeycomb-shaped holes 4 and in the Figure 8 triangular holes 4 can be seen. In all cases, the volume ratio of recesses 4 to the solid areas of the grinding element is constant over the diameter of the grinding element used in the grinding process, which is again illustrated by the ratio of the distances A / B and A '/ B' relating to the circumference. The rake angle γ of the grinding element according to Figure 8 is 32 ° and remains constant during the entire grinding process.

The rake angle γ is generally based on the Figures 10a to 10c explained, where Figure 10a shows a positive rake angle Y, the rake angle γ according to Figure 10b is zero and Figure 10c shows a negative rake angle γ- During removal, the grinding element 7 produces a chip 6 on the workpiece 5, with a positive rake angle Y contributing to reducing the cutting force and thus the energy requirement during cutting, while a negative rake angle Y the edge strength and the service life of the Increased sanding element 7.

As already mentioned at the beginning, the in the Figures 1 to 8 shown embodiments of the abrasive elements by an arbitrary selection, in which no restriction is to be seen. In the Figure 9 are examples of other geometric surfaces that show possible shapes for recesses or holes. There is no restriction to be seen in this summary either.

In addition to the Figures 1 to 8 It is of course also possible to produce and use circular segments with the same geometric structures. The advantage of the circular segments is that they are easier to manufacture and handle and the risk of grinding elements breaking during processing is lower. Fractions with half, a third, a quarter and an eighth of a complete circular grinding element are particularly suitable as practical circular segments.

Ultimately, the geometric design of the grinding elements essentially depends on the field of application of the grinding wheel, with the person skilled in the art choosing the geometric shape with which the desired grinding conditions can best be set and which is also the easiest to manufacture.

Examples

An 80% α-aluminum oxide suspension with an average particle size D ₅₀ of 0.144 μm was obtained by wet grinding an α-aluminum oxide starting powder with an average particle size of less than 1 μm. The suspension was stabilized by adding 0.75% by weight of a polymethacrylate (KV5182, Zschimmer & Schwarz). A latex binder (B-1000, Dow Chemicals) was then added to the stabilized suspension.

Then, to increase the viscosity of the liquid suspension, 5% by weight of an aqueous 1.25% cellulose solution (Methocel K15M) was added to water admitted. With the ceramic precursor produced in this way, which had an aluminum oxide content of 72.6% by weight and a viscosity of 1300 mPa * s, foils with different thicknesses between 200 and 500 μm were cast, from which precursors of the flat, geometrically structured ones were then cast ceramic abrasive elements according to the Figures 1 to 8 were punched.

The precursors of the abrasive elements were dried, whereby due to the high aluminum oxide content only a slight contraction in volume and no cracking could be seen. The dried precursors were heated to 600 ° C. at a heating rate of 1 ° C./min to remove the binder, and then sintered at a heating rate of 5 ° C./min up to a maximum temperature of 1600 ° C. The holding time at 1600 ° C. was 30 minutes. The flat, geometrically structured grinding elements obtained in this way have a density of 3.94 g / cm ³ (98.3% of the theoretical density), a Vickers hardness Hv of 18.4 GPa and a crystallite size of less than 2 μm.

Separation test

To produce a synthetic resin-bonded cutting disc with a diameter of 125 mm, a star-shaped, flat, geometrically structured grinding element according to FIG Figure 1 used with a thickness of 300 µm. To ensure the stability of the disc, corundum was added to the resin as a filler. A comparison disk with a single crystal corundum (TSCTSK, Imerys Fused Minerals) with the grain size F46 / 60 was used as the standard.

Cr-Ni stainless steel round bars with a diameter of 20 mm were used as workpieces and machined at a cutting speed of 6000 μm / sec at a disc speed of 8800 revolutions per minute. For this purpose, 3 pre-cuts and 12 further cuts were made. The disk loss was then determined from the decrease in the diameter of the disks. The G-ratio was then determined from the quotient of material removal and disk loss.

The results are summarized in the following table 1: Table 1 example G ratio cm ² / cm ² Grinding performance (%) According to Figure 1 300 µm 3.41 112 Standard (comparison) TSCTSK 46/60 3.04 100

The example given above illustrates the potential of the abrasive elements of the invention. By varying the geometric structure, the thickness and the inherent porosity of the grinding elements, tailor-made grinding elements can be made available for a wide variety of applications. Grinding elements with high inherent porosity are, for example, porous oxide ceramics, the porosity of which can be set between 10% and 90% pore volume with the aid of known ceramic technologies.

A further potential for optimization results from the use of several grinding elements that can be used parallel to each other in a grinding wheel, with the hole patterns of the grinding elements also advantageously being offset from one another so that the porosity has an optimal homogeneous distribution over the width of the grinding wheel. An example of such a disk is a double-layer staggered cutting disk which has two flat, geometrically structured grinding elements, each of which is 150 μm thick.

In addition, the physical properties of the grinding elements can be changed by doping them. For example, the toughness and breaking strength of the grinding elements can be improved by adding zirconium oxide. The choice of the starting materials and the production process offers further possibilities for variation and optimization approaches for the invention Grinding elements. For example, the sol-gel process can be used to produce particularly fine-crystalline grinding elements with known technologies, which have an average crystallite size in the range of 100 nm. Ceramic materials of this type have extraordinary toughness and hardness and are particularly suitable for machining high-alloy steels.

A particularly interesting field of application for the grinding elements according to the invention are thin synthetic resin-bonded disks with a thickness between 100 μm and 200 μm and a small diameter between 1 cm and 4 cm, as used in the dental field.

Claims (11)

1. Sintered polycrystalline flat-shaped geometrically structured ceramic abrasive element consisting of a sintered shaped body having

   - a homogeneous microstructure,

   - a chemical composition consistent across the whole abrasive element, and

   - a uniform geometrical structure,

   wherein the sintered body has a first surface and a second surface opposite the first surface and parallel to it, wherein both surfaces are separated by a sidewall having a thickness between 50 µm and 2000 µm, and the diameter-to-thickness ratio of the abrasive element is greater than 30,
   characterised in that
   the average diameter of the crystals forming the homogeneous microstructure of the sintered body is less than 10 µm, and that
   the abrasive element is a perforated ceramic body.
2. Abrasive element according to claim 1,
   characterised in that
   the chemical composition of the abrasive element is based on aluminium oxide and/or other chemical compounds selected from the group consisting of carbides, oxides, nitrides, oxy-carbides, oxy-nitrides and carbo-nitrides of at least one of the elements selected from the group consisting of Al, B, Si, Zr and Ti.
3. Abrasive element according to claim 1 or 2,
   characterised in that
   the abrasive element is a circular disk or a segment of a circle.
4. Abrasive element according to claim 3,
   characterised in that
   the perforation of the ceramic body features a homogeneous geometrical structure with geometrically shaped openings.
5. Abrasive element according to any of claims 1 to 4,
   characterised in that
   the abrasive element is a porous ceramic body.
6. Abrasive element according to any of claims 1 to 5,
   characterised in that
   the volume ratio of the openings to the massive portions of the abrasive element is constant across the whole usable diameter of the abrasive element.
7. The abrasive element according to any of claims 1 to 6,
   characterised in that
   the chemical composition of the abrasive element comprises at least 50 wt.-% alumina and optionally one or more oxides selected from the group consisting of SiO₂, MgO, TiO₂, Cr₂O₃, MnO₂, Co₂O₃, Fe₂O₃, NiO, Cu₂O, ZnO, ZrO₂, and rare earth oxides.
8. Method of manufacturing a flat-shaped geometrically structured ceramic abrasive element of any of claims 1 to 6, comprising the steps of:

   - preparing a ductile mass of a ceramic precursor material;

   - forming a precursor of a flat-shaped geometrically structured abrasive element from said ductile mass; and

   - calcining and sintering the said precursor to obtain flat-shaped geometrically structured ceramic abrasive element.
9. Method according to claim 8,
   further characterised by the steps

   - preparing a dispersion of α-alumina in water by wet-milling α-alumina having an average particle size of less than 1 µm in the presence of a dispersant;

   - adding an organic binder and optionally a plasticiser and/or an antifoaming agent to the dispersion;

   - mixing the dispersion for several hours to obtain a stable colloidal dispersion;

   - tape casting the stable colloidal dispersion to a film having a thickness up to 3 mm;

   - drying the tape cast film;

   - cutting out precursors of a flat-shaped geometrically structured ceramic abrasive element; and

   - calcining and sintering the precursors to obtain flat-shaped, geometrically structured ceramic abrasive elements.
10. Use of flat-shaped geometrically structured ceramic abrasive elements of any of claims 1 to 6 for the manufacture of synthetic resin-bonded grinding wheels.
11. Cut-off wheels comprising flat-shaped geometrically structured ceramic abrasive elements of any of claims 1 to 6.

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