CA2407736A1 - A12o3/sic nanocomposite abrasive grains, method for producing them and their use - Google Patents

Abstract

The invention relates to a method for producing A12O3/SiC nanocomposite abrasive grains. SiC nanoparticles are added to a sol containing aluminium oxide and said sol is then gelatinized, dried, calcinated and sintered. The invention also relates to A12O3/SiC nanocomposite abrasive grains.

Description

AI203/SiC Nanocomposite Abrasive Grains, Method for Producing Them, and Their Use The present invention relates to sintered A1203/SiC nanocomposite abrasive grains as defined in the preamble to Patent Claim 11, a method for producing these as set out in the preamble to Patent Claim 1, as well as the use thereof as grinding agents.
Because of their great hardness, chemical inertness, and resistance to high temperatures, large quantities of abrasive grains that are based on AI203 are processed industrially to form grinding abrasives. In addition to fused corundum, which can be manufactured in a relatively cost-effective manner in arc furnaces, more recently reinforced sintered corundums, which can be obtained by way of ceramic or chemical methods, have been used for specific applications. From the standpoint of abrasive engineering practice, the advantages of sintered corundums are determined by their microcrystalline structure that, in its turn leads, to a particular wear mechanism of the abrasive during the grinding process. Removal performance can be greatly enhanced by using sintered corundums, mainly in applications that require high contact pressures, for example , when machining special steels, hardened steels, or alloys that are difficult to machine.
The grain of sintered corundum, which is of a microcrystalline structure, is considerably more resistant to wear for these applications than the structure of fused corundum, which is macrocrystalline. In addition, there is the fact that during the grinding process, when using microcrystalline corundum, smaller areas break away from the grains so that new cutting edges are formed, and these-in their turn-take part in the grinding process. No such self-sharpening of the grains takes place in the case of macrocrystalline fused corundum because here the cracks that result during the grinding process because of the mechanical stresses on the grains can no longer be deflected, but continue along the crystal planes of the overall grains and thus lead to the destruction of the abrasive grain.
When microcrystalline sintered abrasive grains are used, in many applications it is possible to see that given comparable hardness and density, the finer the configuration of the structure, the better the abrasive grain will behave during the grinding process. Particularly fine structures can be obtained using the sol-gel method, in which, for example, finely dispersed aluminum oxide monohydrate of the boehmite type is used, this being processed to a gel once it has been colloidally dissolved and then further processed by drying, calcination, and sintering to form a compact and dense a-A1203 sintered body. This is subsequently processed to form an abrasive grain. The advantage of the sol-gel method for producing microcrystalline corundum lies in the fact that very finely divided and reactive starting substances can be used and the resulting green body can be consolidated at relatively low sintering temperatures, which facilitates the formation of a fine structure.
EP-B-0 152 768 describes microcrystalline corundums that are produced by the sol gel technique by the addition of specific crystallization nuclei at sintering temperatures of 1400°C and whose primary crystallites have diameters that are mostly or all smaller than 1 Nm.
The growth rate of the crystals during the sintering process can be greatly limited because of the low sintering temperatures and by the addition of crystallization nuclei. Even finer structures at higher densities and hardness are described in EP-B-0 408 771. According to EP-B-0 408 771, corundum abrasive grains with a mean crystallite size of < 0.2 Nm are obtained by using the sol-gel technique with the addition of very finely divided crystallization nuclei and whilst keeping to a special temperature and sintering program, in which the temperature range varies between the 900 and 1100°C in less than 90 seconds; the material can be brought to its maximal temperature, which should not exceed 1300°C, for a brief period of time and subsequently densely sintered beneath this maximal temperature in the range between 1000 and 1300 °C. The temperature program is selected so as to permit a high level of consolidation without the resulting sintered bodies or their precursors being exposed for too long to temperatures that would favor crystal growth.
If one wishes to obtain the finest possible crystalline structure, it is recommended that sintering additives be used in addition to using crystallization nuclei, since these additives inhibit crystal growth or accelerate the sintering process and thus indirectly inhibit the formation of larger crystals. The effect of individual additives on the sintering process and on crystal growth when sintering A1203are described in the Journal of the American Ceramic Society, Volume 39, No. 10, 1956. The following are examples of the numerous patents that describe the use of sintering additives or combinations of sintering additives with crystallization nuclei for the production of abrasive grains using the sol-gel method. EP-B-0 024 099 describes the addition of spinets or precursors there that are converted to spinets during production process. EP-B-0 200 487 describes the use of a-FE203 crystallization nuclei in combination with at least one of modifying component from the group comprising the oxides of magnesium, zinc, cobalt, nickel, zirconium, hafnium, chromium and/or titanium. EP-B-0 373 765 describes yttrium and neodymium compounds-also in combination with a-Fe203 nuclei-in addition to the above-quoted oxides as additional modifying components. When used for specific applications, the abrasive grains produced using the above method have advantages as compared to the prior art.
The large number of different AI203 sintered abrasive grains can be explained by the fact that grinding is itself an extremely varied process during which both the material that is being processed as well as working conditions such as contact pressure, cooling, and the like can be greatly varied. This means that the most varied materials (various types of steel, alloys, and metals, plastics, wood, stone, ceramic and the like) can be processed under the most varied conditions, depending on the objective that is to be achieved (surface quality, removal of material, and the like). The demands that are placed on the abrasive grains that is to be used are similarly varied, so that the usefulness and efficacy of an abrasive grain for a specific grinding process cannot be characterized by variables such as hardness, density, and crystallite structure alone. Other criteria, such as chemical inertness, thermal conductivity, resistance to oxidation and temperature, toughness, etc., will play a major role, depending on the particular application.
Other variables that affect the grinding process are bonding and the specification of the grinding agent, which can also be further varied by the addition of additives such as secondary grinding agents, pore forming materials, and the like.
Thus, even in the case of abrasive grains produced by a the sol-gel method attempts were made in the past to increase performance by varying the degrees of fineness of the crystallite structure, and to obtain particularly favorable characteristic for a specific application by way of doping. EP-A-0 228 856 describes the addition of yttrium that is added to the sol-gel process of the a-aluminum monohydrate dispersion (for example, in the form of an yttrium salt) with a slightly volatile anion (nitrate, acetate, or the like), which reacts with the AI203 to form yttrium-aluminum garnet. This material has particular advantages when machining stainless steel, titanium, nickel alloys, aluminum and other alloys that are difficult to machine as well as when machining simple structural steel. Obviously, the incorporation of garnet crystals in the abrasive grains imparts a special wear resistance for these applications, and this is reflected in a high removal performance . In addition to Y203 or its precursors, the addition of crystallization nuclei and/or other sintering additives is also described. EP-B-0 293 164 describes the addition of rare earths from the groups that comprises praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium and/or combinations of several from this group.
Together with the AI203, the rare earths form hexagonal aluminates that obviously bring about an additional increase in the wear resistance of the abrasive grains when incorporated into the AI203 matrix. EP-B-0 368 837 describes the abrasive grains whose toughness is increased by the formation of whisker-like crystals that are obtained by adding cerium compounds.
Here, too, increased toughness is achieved by strengthening the structure.
By using the sol-gel method, one also obtains the composites that are described in DE-A-196 07 709, which differ from the compounds described above in that in addition to the A1203 matrix there are at least two additional discontinuous structural components that differ from each other with respect to average particle size by a factor of at least 10. EP-B-0 4 91 184 describes composites based on AI203 that has inclusions of isometric hard materials that are greater than the primary crystals of which the matrix is build up by a factor of at least 10.
All of the above methods and materials are based on the sol-gel technology with which it is possible, given the simultaneous use of sintering additives, to obtain a very fine, preferably sub-micron, crystal structure. In addition, there is the fact that the abrasive grains are frequently tailored and optimized for specific areas of application by additional doping.
Generally speaking, the grinding agents or abrasive grains can simply be divided into two major groups. In addition to SiC, corundum belongs to the so-called conventional grinding agents that have been known for a considerable time and which can be produced very economically and in large quantities. More recently, ever more frequent use has been made of the so-called super abrasives such as diamond and cubical boron nitride, the production costs for which are between 1000 and 10,000 times the production costs for conventional abrasive grains but which, because of their enhanced performance and the associated reduction in machine down time and lower use of the grinding agent itself, or because of the increase in the unit rate per unit time and grinding body offer an extremely favorable cost-performance ratio for many grinding operations.
The use of super abrasives requires special machinery which, in its turn , requires appropriate investment, which means that the range of applications for high-performance grinding agents is restricted even further.
For this reason, one of the main objectives foreseen for the development of new abrasive grains is to obtain abrasive grains that can be used on conventional machinery but which lie between conventional grinding agents and super abrasives as far as level of performance is concerned. This has been achieved, in part, with the above-quoted sol-gel corundums, which can be used for many grinding operations at a very favorable price/performance ratio. However, the sol-gel corundums are to be classified more as conventional types of the abrasive grains not only in view of their production costs but also in view of their performance, and for this reason are better suited to replace conventional corundurns for grinding operations that do not justify the use of super abrasives.
For this reason, it is the objective of the present invention to provide abrasive grains with even better performance potential as compared to the above described prior art, and a method for manufacturing these.
According to the present invention, this objective has been achieved with the features as set out in Patent Claim 11 or Patent Claim 1. The secondary claims refer to advantageous configurations of the present invention. Claim 20 applies to the use of the abrasive grains according to the present invention.
The expression nanocomposite, which has been in use for some ten years in the domain of ceramics, is used to describe systems that comprise at least two different solid phases, of which at least one phase has particle sizes in the manometer range.
A1203/SiC composites in which SiC particles have been incorporated in an AI203 matrix for reinforcement, are described in EP-B-0 311 289 and are intended, for example, as structural ceramic for use in engines and turbines.
The diameter of the SiC particles, which account for between 2 and 10 mol-% , should be less than 0.5 Nm, whereas the A1203 particles should not be larger than 5 pm. These materials, in which the SiC particles are dispersed among the A1203 particles, are distinguished by great toughness and extremely high transverse rupture strength, and because of their good high-temperature properties can be used as structural ceramic in engines.
Similar AI203/SiC nanocomposites, which differ from the known whisker, fibre, or platelet reinforced composite materials by their good high-temperature properties and their resistance to oxidation, are described by Niihara in the Journal of the Ceramic Society ofJapan, 99 (10) 974-982 (1991). The effect of finely divided SiC particles on the grain growth and sintering behavior of the AI203 matrix is described by Stearns, Zhao, and Harmer in the Journal of the European Ceramic Society, 10 (1992) 473-477.
The mechanical properties of AIz03/SiC nanocomposites are described by Zhao, Stearns, Harmer, Chan, Miller, and Cook in the Journal of the American Ceramic Society, 76, (2) 503-510, 1993. Nanocomposites produced by the sol-gel method are described by Xu, Nakahira, and Niihara in the Journal of the Ceramic Society of Japan, 1994, 102, 312-315.

Whereas the above cited references relate mostly to composites with SiC
fractions of > 2 mol-%, the mechanical properties of hot-pressed A1203/SiC
composites with smaller fractions of SiC are described in an article by Wilhelm and Wruss in cfi/Ber. DGK 75, 40-44 (1988). AI203/SiC
nanocornposites are described in numerous other publications in addition to the above cited references, and these have been combined in a survey by Stirnizke in the Journal of the European Ceramic Society, 17 (1997), 1061-1082. This article expresses the idea that, as abrasive grains, AI203/SiC
nanocomposites could fill the gap between conventional abrasive grains and super abrasives. In contrast to this conjecture, however, almost all the publications referred to in the article, and the material properties quoted from this, relate expressly to use as structural ceramic. Thus, the microstructures, thermodynamic stability, density, hardness, breaking strength, fracture tougness, and creep rate are discussed. Certainly, all of these variables play an important role in the grinding process without, however, permitting a valid statement with respect to the usability of a material as abrasive grain. For example, hardness is certainly a basic requirement for using a material as an abrasive grain. However, as is shown by the example of B4C, which is frequently referred to in professional circles, and which despite its great hardness, has never been used to any noteworthy extent as a grinding agent because of it deficient chemical and thermal resistance and its great brittleness, the sum of a number of properties must be considered in order to identify suitability as a grinding agent. Other hard materials that have hardness values that range between conventional grinding agents and super abrasives have not found acceptance as abrasive grains because they lack additional properties such as toughness, thermal and chemical stability, or other prerequisites that are important for the grinding process. The nanocomposite materials that are described in the literature, and which have properties that are essential for the grinding process, have not up to now been used successfully as abrasive grains. They behave more like the cutting ceramics based on AI203 that have been used with great success for milling or turning operations but which, once processed to form grit, display only an unsatisfactory removal performance which is at the level of conventional smelted corundums or even below this during the grinding process.
In practice, it is been found extremely difficult to characterize the usability or the grinding behavior that can be expected from abrasive grains simply on the basis of specific material properties which are known to have a favorable effect on the grinding process. Up to now , theories covering the mechanisms that actually take place during the removal of material by a grinding tool could be developed only subsequently to the process itself, on the basis of the work piece that is machined and on the basis of changes that take place in the grinding tool. Naturally, in addition to all the material properties of the abrasive grains, the composition of the grinding tool (bonding, porosity, additives, and the like) and the work piece itself affect grinding behavior, so that it is frequently very difficult to look back and correlate specific grinding results that have been achieved with specific material properties of the abrasive grains. One can arrive at a firm conclusion only by way of application tests of abrasive grints or even by way of practical and field testing, which entail considerable expenditures of time and money.
For this reason it is worthwhile attempting to find an independent measuring method and measurement factors that make a direct statement with regard to the usefulness of a material as an abrasive grain. In practice, in recent years increasing use has been made of the so-called individual grit test (Figure 1: Individual grit scratch test), in which an individual abrasive grain is examined under conditions that as far as possible are realistic and modeled on the grinding process. The test apparatus is a converted surface grinding machine in which a scratch disk is mounted on the grinding spindle in place of a grinding disk. The scratch disc that, for practical reasons, is manufactured from a relatively light material that is easy to machine (e.g., aluminum) has on its periphery a holder into which a carrier, which has a grain of abrasive grains soldered onto it, is inserted. During the scratch process itself, the table with the work piece clamped to it is moved in the x-direction against the direction of rotation beneath the rotating scratch disk.
Because of a predetermined setting in the y-direction the abrasive grit, which extends beyond the periphery of the disc, leaves a scratch track in the work piece during each rotation. As the length of the scratch or scratch time increases, the depth of the scratch and the cross section of the scratch decrease because of wear on the grain, and this continues until such time as the tip of the grain is worn down by the amount of the setting in the y-direction and it no longer leaves a track. These scratch tracks can be sampled with a surface measuring apparatus and then evaluated. The underlying principle for these measurements in shown in Figure 1 and Figure 2, and is described in detail below using the relevant reference numbers.
Figure 1 shows the principles underlying the construction of the test apparatus, with the scratch disc (1) and the scratch grain (2), with the axes (3 ,4, 5) in the x, y, z directions, and with the work piece (6), the machine table (7) and the grinding spindle head piece (8). In order to make these measurements, it is necessary to define standard conditions for the cut rate v~, the work piece speed vW, the setting ae, all of which should as far as possible be matched to the grinding operation for which the abrasive grain will subsequently be used. In addition, the work piece material and the use of cooling lubricants (9) must be defined.
The evaluation principle can be seen from the curves for different types of abrasive grains (Figure 2) that are shown here as examples, in which the change in the scratch cross section AR~/ARO is plotted against the length IR
of the scratch. ARO is the scratch cross section during the first test, and AR"
is the cross section of a scratch that is n mm long.
The performance factor LF25 for the individual grains is located at the intersection point of the characteristic curve for the individual grains types with the ordinates after a scratch length of 25 mm, and corresponds to the change of the scratch cross section ARO/AR25. The performance factor is expressed as a percentage relative to the theoretical case that there is no wear on the grain and ARZS = ARO. The evaluation after a scratch that is 25 mm long is selected because, in the case of the typical curve , the decisive first, steep area of the curve, when the grains that is under the greatest load, is closed. This area, which is a relatively close approximation to the actual grinding process relative to the setting ae,makes it possible to arrive at the very good assessment of the capability of an abrasive grain. As they continue, the curves flatten out since the grains are not as greatly stressed because of the reduced setting, and they also wear at a slower rate. In order to arrive at a representative result for an abrasive grains, at least twenty grains of one type should be measured and the wear curve should be formed from the average value of the individual measurement points.
In agreement with the results achieved in practice, the single grain scratch test permits an assessment of the suitability of abrasive grains in which all the variables in the grinding process, such as hardness, toughness, density, strength, creep rate, thermal and chemical resistance, crystallite structure, and the like all contribute indirectly to the overall sum, without specific properties or combinations of properties having to be known or identified explicitly, and accordingly taken into account. Nevertheless, specific minimum prerequisites must be satisfied for all the properties in order that a material can be considered as an abrasive grain at all. Thus, for example, a material which is of a hardness that is clearly below the usual hardness for grinding agents will never be suitable for the grinding process, even if all its other properties are outstanding.
Most surprisingly, using the method described above, it has been possible to find performance factors that are clearly superior to the performance factors found up to now for AIZ03/SiC nanocomposites with SiC fractions below 5 mol-%, which have been produced directly by the sol-gel method with the addition of crystallization nuclei. The performance factors for the nanocomposites according to the present invention lie above the values for the known pure or doped sol-gel corundums and lie in the desired range between conventional abrasive grains and the super abrasives.
In contrast to known AI203/SiC nanocomposites that are manufactured by powder technology, by mixing the starting substances, compressing (for example, by hot pressing, non-pressure sintering, or hot isostatic pressing) and sintering, the abrasive grains according to the present invention are produced by wet-chemical methods by a direct sol-gel method using crystallization nuclei. Xu, Nakahira, and Niihara in an article that appeared in the Journal of the Ceramic Society of Japan, 1994, 102, 312-315 describe the use of the sol-gel technique for manufacturing AI203/SiC
nanocomposites. However, they only use this technique in order to arrive at the most homogenous possible mixture of the nano-powder by way of a preceding colloidal solution of the particles. This sol is subsequently processed to form an homogenous mixture of ultra-fine AI203 and SiC
powders by drying and calcining, and this is then hot pressed in a nitrogen atmosphere at a pressure of 30 MPa, at a temperature of 1600°C, in the same way as when using conventional powder technology.
Specific advantages of the sol-gel method that are important for manufacturing abrasive grains are lost by isolating the powder as an intermediate product, with subsequent conventional powder technique processing. The grinding properties of the composite produced by the method described heretofore corresponds to those of the previously discussed nanocomposite. In addition, there are economic aspects, since cost effective large-scale mass production of abrasive grains is not possible using a hot press method.
In contrast to this, in the case of the direct sol-gel method according to the present invention, which is used to produce a AI203/SiC nanocomposite, AI203 sol is first produced in the usual manner. When this is done, very finely dispersed aluminum oxide monohydrate of the Boehmite type, aluminum alkoxides, aluminum halogenides and/or aluminum nitrate can be used as the solid component that contains the aluminum oxide. These are dispersed with a dispergator, a powerful agitator, or by using ultrasound. It is preferred that the solids content of the suspension lie between 5 and 60%-wt. Then, nano-scale SiC, preferably in the form of a suspension-so as to arrive at the most homogenous possible distribution, between 0.1 and < 5 mol-%, preferably in the range between the 0.3 and 2.5 mol-% relative to the aluminum content of the mixture, calculated as AI203 -is added to this suspension. It is, of course, also possible to stir SiC into the suspension as a solid. As the examples set out in Table 3 show, particularly good results are achieved with comparatively small quantities of SiC. Finely ground SiC
powder obtained by the Acheson process, or even nano powder manufactured in the gas phase by thermal or laser assisted gas phase reactions or by various plasma and methods, can be used as the SiC basis.
Additional sintering additives in the form of crystallization nuclei, crystal growth inhibitors, and/or other modifying components can be added, preferably before gelling, in order to enhance the subsequent sintering process. All known sintering additives for AIz03, for example, spinet-forming oxides of Co, Mg, Ni, and Zn, the oxides of Ce, Cu, B, Ba, Hf, K, t_i, Nb, Si, Sr, Ti, Y, Zr or the rare earths or their precursors, and the oxides with a corundum-like structure such as Fez03, Cr203, AI203, and others, that act as crystallization nuclei. Combinations of these can be used in order to impart specific properties to the abrasive grain.
It is preferred that an aqueous suspension of very finely ground AI20s be added to the AI203 sot prior to the gelatinization of the SiC. The maximum size of the «-A1203 particles, which serve as a crystallization nuclei, is less than 1 pm, and preferably less than 0.2 arm. The quantity of nucleation material that is to be used depends on the particle size, and lies between 0.5 and 10%-wt, relative to the AI203 of the end product. Since the number of nuclei is important in addition to fineness, at very high levels of fineness very small quantities of nuclei by weight are sufficient to further the sintering process.
The prepared suspension is then heated to boiling point and gelled by the addition of acid. Here, too, it is again possible to use all of the known types of gelatinization (aging, addition of electrolyte, temperature increase, and reduction of the suspension, amongst others). The gel is dried (after cooling) in a temperature range between 50°C and 120°C.
Calcining then takes place in the temperature range between 500°C and 800°C, in order to vaporize residual water and the acid. After calcining, the composites are in the form of green bodies with diameters of up to several millimeters, and these are then sintered. The advantages of direct densification lie in the particularly high sinter activity of the dried and calcined green bodies, in which the starting materials are already bonded to each of chemically, so that densification and consolidation to form a solid composite proceed far more effectively and efficiently.
The process, and thus the quality of the product, can be further improved by the additional use of sintering additives or crystallization nuclei. It is preferred that the calcined gel be sintered at temperatures between 1300°C
and 1600°C, preferably under inert conditions (e.g., in an nitrogen atmosphere) and, particularly advantageously, in a gas tight rotary tube furnace in order that the product is heated as rapidly as possible and the sintering time is kept as short as possible, since this has a particularly favorable effect on the structure and thus on the efficacy of the abrasive grains. Alternatively, any other known type of furnace can be used, WO 01/21547 PtrT/EP00/09230 providing it permits a rapid heating rate and high temperatures. Since the sintering takes place very quickly, it is possible for processing to take place in a vacuum or in an oxidizing atmosphere, since the greatest part of the SiC nano particles are imbedded in the matrix and thus protected against oxidation.
Reduction to the desired grains size can take place either before or after sintering, using the usual reducing apparatuses. It is advantageous to prepare the calcined gels in the green state, since once sintering has taken place, considerably more energy has to be expended for reduction of the then dense and hard composite material.
During sintering, the nano-scale SiC acts as a crystal growth inhibitor for the AI203 matrix and, at the same time, delays densification of the green body so that, comparatively speaking, higher sintering temperatures have to be used than is the case of sol-gel material that is based on pure aluminum oxide, in order to achieve sufficient densification of the material, when not inconsiderable crystal growth can take place. Even at 1400°C, much larger crystallites occur more frequently. This phenomenon has already been described in U.S. Patent 4,623,364 wherein the undesirable occurrence of coarse crystals in an otherwise fine matrix is attributed to impurities. In this specification, an attempt is made to find the finest crystalline matrix possible, with the fewest possible coarse fractions, as have been described in the patents cited in the introduction hereto, and which correspond to the prior art.
Most surprisingly, it has now been found that the abrasive performance of the nanocomposite abrasive grains according to the present invention is particularly high if a specific fraction of coarse crystals with lengths of up to 20 Nm and with an average diameter of greater than 2 pm, preferably greater than 5pm, is present in the matrix. In this case, the removal performance is clearly above that of the finely structured pure sol gel AI203 grinding grains, the average crystal size of which is usually 0.2-0.3 pm and in which all crystals are in the sub-micron range, preferably in the range below 0.4 Nm. This is all the more surprising since it is generally known in professional circles that the abrasive performance of sintered corundums is increase dramatically as the structures become finer, particularly in the d5o range beneath 0.5 pm.

As is shown in Examples 1-6 and in the comparative Examples 7-11, which describe the effect of sintering conditions on the structure and the performance strength of sinter corundums, the performance curve for the AI203/SiC nanocomposite is a non-linear curve with a maximum at the sintering temperature between 1400-1450°C. The first coarse crystalline and columnar crystals appear within the matrix in this temperature range, at a holding time of 30 minutes. It is preferred that the coarse AI203 crystals be of an elongated form with a length to width ratio between 2:1 and 10:1, especially between 4:1 and 6:1. Typical images of the matrix with the coarse crystalline incorporations are shown as electron and optical microscope images in Figures 3 and 4 on Page 19. Below 1400°C, a purely sub-micron structure occurs, in which all the particles are in the range of <

pm, preferably < 0.5 Nm. The efficiency of these materials also lies above the efficiency of pure sol-gel corundums as based on the prior art, but surprisingly lies beneath that of materials with coarse crystallite incorporations obtained in the above-quoted temperature range. At even higher sintering temperatures, at which there is an increased occurrence of coarse crystals, the performance curve falls off once again.

However, even at sintering temperatures of 1500°C with high proportions of quartz crystals, abrasive performances that are at the level of the best pure sol-gel corundums obtained in the above-quoted temperature range are achieved. In contrast to this, in the case of the pure sol-gel corundums , one can see an almost linear curve of the performance potential with the fineness of the structure, and good performance is achieved first in the sub-micron range at an average crystal size d5o < 0.4 Nm.
Obviously, in the case of the nanocomposite, the coarse crystallites act as a kind of structural reinforcement that has a positive effect on the grinding behaviour of the grit and compensate not only for the reduction in performance, anticipated because of the grains growth; in combination with the incorporated nano-SiC particles, they help the abrasive grains to achieve a clear increase in performance.
From the examples shown in Table 4 it can be seen that improvement of the product by the incorporation of SiC particles is confined not only to nano-SiC
powder; outstanding abrasive performance can also be achieved with grains that have relatively coarse SiC incorporations. However, it is quite clear to see that the finer the SiC powders that are used, the better the abrasive performance will be. For commercial reasons, and for reasons of availability, initially only the powders in the examples were used to produce the abrasive grains according to the present invention; these were obtained by fine grinding industrial SiC that had been obtained by using the Acheson method.
One can, however, proceed in that the above-quoted trend is continued by using even finer powders.
In the nanocomposites according to the present invention, the SiC particles can be both intragranular, within the AI203 matrix particles, as well as inter-granular, on the grain borders between the AIz03 particles; it should be observed that the smaller particles are preferably incorporated so as to be intragranular. The effect that the type of incorporation of the Sic particles has on abrasive performance is the subject of ongoing investigations and, for the time being, can only be regarded on a speculative basis.
Some theories have been discussed in the publications referred to above;
however, these discuss only the individual properties of composite materials and do not discuss the effect of the totality of the properties that are decisive for abrasive performance. At all events, Examples 14-17 clearly illustrate the trend that abrasive performance is enhanced with decreasing particle sizes of the incorporations. From this, it can be concluded that it is mainly intergranular SiC that is responsible for the improvement of abrasive performance .
The present invention creates nanocomposite abrasive grains that are based on AIzO3 and which have predominantly intragranular SiC nanoparticles incorporated in them. They have a hardness (HVo,2) that is greater than 18 GPa, a density that is 95% of the theoretical density, and a performance factor LF25 °> 75% (° = measured as the mean value of 20 individual measurements on 100Cr6 material (HRH = 62) with a cutting speed of 30 m/s, a feed rate of 20 pm, a work piece speed of 0.5 mm/s, and using a 3-emulsion as a cooling agent.
The present invention will be described below on the basis of the examples that follow, without necessarily being restricted thereto.
Examples 1 - 6 Using a Megatron MT 1-90 dispergator (Kinematica), 10 kg pseudo-boehmite (Disperal, Condea) is dispersed in SO liters of distilled water, the pH value of which had been adjusted to 2.4 by the addition of approximately 300 ml of concentrated nitric acid. Approximately 300 ml of 50% nucleation slurry that contains a-AI203 within maximum particle size of dmaX = 0.4 Nm and which was obtained by wet grinding and subsequent centrifuging of a fine particle a-A1203 powder (CS40M, Martinswerk) was then added to the dispersion, also by using a dispergator. After the addition of the nucleation slurry, approximately 2%-wt of AI203 crystallization nuclei was present in the sol.
Suspension B (SiC suspension) 1.5 g of a 50-% aqueous polyethyleneimine suspension (Fluka) was added to 600 ml of distilled water during vigorous stirring. Next, 30 g nano-scale SiC
(UF 45, H.C. Starck) was stirred into the diluted suspension.
Suspension B was added to the boehmite sol (Suspension A) during stirring and the pH value of the mixture was adjusted to 1.8 using nitric acid. Next, the mixture was heated to 95°C whilst being constantly stirred, and gelling was initiated by drop-by-drop addition of additional nitric acid. After cooling, the gel was dried in a drying cabinet at 85°C. The dry gel is reduced to a particle size of less than 5 mm and then calcined at 500°C.

In the Examples 1-6, only the sintering temperature is varied. In Table 1, the measured hardness values, performance factors, and crystal structures are set out as a function of the sintering conditions.
Table 1, Examples 1-6 Example Sintering Program Hardness Crystallite LFZSoo (HVo,z) Structure (dso) 1 1300/N2/60/30 11.3 GPa < 0.4 ~.m 23 2 1350/N2/60/30 13.3 GPa < 0.4 ~cm 29 3 1380/N2/60/30 19.8 GPa < 0.4 wm 73 4 1400/N2/60/30 22.9 GPa l~.m 85 1450/N2/60/30 20.7 GPa 5 - 10 ~,m 83 6 1500/N2/60/30 20.1 GPa 10 - 20 ~m 70 Sintering program =
Sintering temperature (°C)/ furnace atmosphere/ heating rate (°C/minute)/holding time (min).
Comparative examples 7-il (without SiC incorporations) Using a Megatron MT 1-90 dispergator (Kinematics), 10 kg pseudo-boehmite (Disperal, Condea) is dispersed in 50 liters of distilled water, the pH value of which had been adjusted to 2.4 by the addition of approximately 300 ml of concentrated nitric acid. Approximately 300 ml of 50% nucleation slurry that contains a-AI203 within maximum particle size of dmaX = 0.4 Nm and which was obtained by wet grinding and subsequent centrifuging of a fine particle «-AI203 powder (CS400M, Martinswerk) was then added to the dispersion, also by using a dispergator. After the addition of the nucleation slurry, approximately 2%-wt of AI203 crystallization nuclei was present in the sol.
The pH value of the mixture was adjusted to 1.8 using nitric acid. Next, the mixture was heated to 95°C while being constantly stirred and gelling was initiated by drop-by-drop addition of additional nitric acid. After cooling, the gel was dried in a drying cabinet at 85°C. The dry gel is reduced to a particle size of less than 5 mm and then calcined at 500°C.
In the Examples 7-11, only the sintering temperature is varied. In Table 2, the measured hardness values, performance factors, and crystal structures are set out as a function of the sintering conditions.
Table 2: Comparative Examples 7-11 Example Sintering Program Hardness Crystallite LFzsaa (HVo,2) Structure (d5o) 7 1240/NZ/60/30 19.7 GPa 0.2-0.3 ~,m 75 8 1300/N2/60/30 22.4 GPa 1 ~m 63 9 1350/N2/60/30 23.1 GPa 1 - 5 ~.m 60 1400/Nz/60/30 21.6 GPa 3 - 7 ~,m 49 11 1450/N2/60/30 20.6 GPa S - 10 ~m 40 *Sintering program = Sintering temperature (°C)/ furnace atmosphere/
heating rate (°C/minute)/holding time (min).

Example 12 Production of Example 12 is effected analogously to the Examples 1-6.
However, 75 g of a nano-scale SiC UF45 was used.
Example 13 Production is effected as for Example 12. 150 g of nano-scale SiC UF45 was used instead of 75 g. Table 3 shows the performance factors as function of the SiC concentration.
Table 3: Examples 4, 12, 13 Example Sintering Program Hardness Crystallite LFZS~,o (HVo.2) Structure (dso) 4 1400/Nz/60/30 22.9 GPa 1.0 ~,m 85 12 1400/Nz/60/30 22.4 GPa 2.5 ~cm 59 14 1400/N2/60/30 23.1 GPa 5.0 ~m 37 ~

Example 14 The production of Example 14 was effected analogously to Example 4. The somewhat coarser SiC OF 25 (H.C. Starck) was used in place of the SiC
UF45. Sintering was carried out at a temperature of 1400°C in a nitrogen atmosphere. The heating rate was 60°C per minute and the holding time was 30 minutes.
Example 15 The production of Example 15 was effected analogously to Example 14. The somewhat coarser SiC OF 15 (H.C. Starck) was used in place of the SiC
UF25.
Comparitive Example 16 The production of Example 16 was effected analogously to Example 15. SiC
P1000 (Elektroschmelzwerk Kempten) was used in place of the SiC UF15.
Com~aritive Example 17 The production of Example 17 was effected analogously to Example 16. SiC
P600 (Elektroschmelzwerk Kempten) was used in place of the SiC P1000.
Table 4 shows the performance factor of the nanocomposite as a function of the particle size of the incorporated SiC's .
Table 4: Examples 4, 15-18 Example SiC Average particleHardness LF25 (%) size d5o (HVo,z) 4 UF45 300 nm 19.7 GPa 85 14 UF25 500 nm 22.4 GPa 82 15 UF15 600 nm 23.1 GPa 77 16 P1000 18 ~,m 21.6 GPa 73 17 P600 26 ~,m 23.3 Gpa 58 Grinding tests In addition to the scratch test, several selected examples were subjected to a grinding test in grinding belts. The results of the test are set out in Table 5.
Table 5 Grinding test (belt grinding) Steel Types Abasive grainTurbine Steel Titanium alloy Removal (g) Performance Removal (g) Performance (%) (%) Example 4 1096 145 127 176 Example 5 994 131 109 151 Example 14 1023 135 112 155 Example 15 843 111 85 118 Example 7 781 103 68 94 Commercial 757 100 72 100 sol-gel corundum Fused corundum320 42 23 32

Claims (20)

Claims

1. Method for producing AL2O3-SiC nanocomposite abrasive grains, characterised in that an aluminum oxide-type sol is mixed with sintering additives and SiC nano particles, gelled, dried, calcined, and sintered, the sintering being effected in a temperature range between 1300°C and 1600°C.

2. Method as defined in Claim 1, characterised in that the sol that contains aluminum oxide contains finely dispersed aluminum oxide monohydrate of the boehmite type, aluminum alkoxides, aluminum halogenides, and/or aluminum nitrate as the solid component.

3. Method as defined in one of the Claims 1 or 2, characterized in that the addition of the SiC nanoparticles is effected in a quantity between 0.1 and < 5 mol-%, preferably in the range from 0.3 and 2.5 mol-%, relative to the aluminum content of the mixture, calculated as Al2O3.

4. Method as defined in one or more of the Claims 1 to 3, characterized in that sintering additives in the form of crystallization nuclei, crystal growth inhibitors and/or other modifying components that affect the sintering process are added prior to gelling.

5. Method as defined in Claim 4, characterised in that finely divided .alpha.-aluminum oxide is used as crystallization nuclei.

6. Method as defined in one or more of the Claims 1 to 5, characterised in that gelling of the suspension is effected by increasing or lowering the pH value, ageing, or adding to the electrolyte, increasing the temperature, and/or concentrating the solution.

7. Method as defined in one or more of the Claims 1 to 6, characterised in that the gel is dried in a temperature range between 50°C and 120°C, subsequent calcining is effected between 500°C and 800°C, and sintering is carried out in a temperature range between 1300°C and 1600°C.

8. Method as defined in Claim 7, characterised in that sintering is carried out in a temperature range between 1380°C and 1500°C.

9. Method as defined in Claim 7, the characterised in that sintering is carried out under inert conditions.

10. Method as defined in one or more of the Claims 1 to 9, characterised in that reduction to the desired grain size is carried out before or after sintering.

11. Al2O3/SiC nanocomposite abrasive grain with hardness >16 GPa, density that is > 95% of the theoretical, and an SiC fraction between 0.1 and < 5 mol-% relative to the Al2O3 matrix, characterised in that the SiC particles within the Al2O3 matrix are in both intergranular and intragranular form, and the abrasive grains display a performance factor LF25 >75% in a single grain scratch test.

12. Al2O3/S SiC nanocomposite abrasive grain as defined in Claim 11, characterised in that the SiC fraction is preferably between 0.3 and <
2.5 mol-%, relative to the Al2O3 matrix.

13. Al2O3/S SiC nanocomposite abrasive grain as defined in one of the Claims 11 or 12, characterised in that the SiC particles are present in the Al2O3 matrix predominantly as intragranular particles.

14. Al2O3/Sic nanocomposite abrasive grain as defined in one or more of the Claims 11 to 13, characterized in that the Al2O3 crystals in the matrix are of an average diameter that is between 0.2 µm and 20 µm.

15. Al2O3/Sic nanocomposite abrasive grain as defined in one or more of the Claims 11 to 13, characterized in that the Al2O3 matrix is structured so as to be sub-micron and has an average particle size of < 1 µm, preferably < 0.5 µm.

16. Al2O3/Sic nanocomposite abrasive grain as defined in Claim 15, characterized in that coarse Al2O3 crystals are formed in the sub-micron Al2O3 matrix.

17. Al2O3/Sic nanocomposite abrasive grain as defined in Claim 16, characterized in that the coarse Al2O3 crystals are of an average diameter of > 2 µm, preferably > 5 µm.

18. Al2O3/Sic nanocomposite abrasive grain as defined in one of the Claims 16 or 17, characterized in that the coarse Al2O3 crystals are of an elongated shape.

19. Al2O3/Sic nanocomposite abrasive grain as defined in one or more of the Claims 16 to 18, characterized in that the length to width ratio of the coarse Al2O3 crystals is between 2:1 and 10:1, and preferably between 4:1 and 6:1.

20. Use of Al2O3/SiC nanocomposite abrasive grains as defined in one or more of the Claims 11-19 to produce grinding belts and grinding disks.