WO1979000778A1 - Process for the manufacture of abrasive particles - Google Patents

Abstract

Procedure for the manufacture of abrasive particles (1) of abrasive grains (3) in particular grains of diamond or cubic boron nitride kept together by a metal phase (2) which may be used for instance in resin bonded grinding wheels. A blank is made of the abrasive grains and a bonding agent consisting of metal or a metal alloy whereby a continuous network is formed in the blank by addition of a pore forming material (4). The abrasive particles are then obtained by disintegration of the blank.

Description

Process for the manufacture of abrasive particles

Tools for grinding, sawing, filing and similar purposes can use in their active parts abrasive grains of very hard materials like diamond, cubic boron nitride, oxides, carbides, nitrides, borides, etc. These abrasive grains are arranged in a bond which is a continuous phase of metal, plastic, ceramic materials, glass etc. The simplest embodiment is here a uniform distribution of the abrasive grains in the active zone of the continuous phase.

There are, however, more complicated distribution patterns. The individual grains may thus be kept together in aggregates by means of a special aggregate bond. These aggregates are then arranged in a continuous bond which makes up the active zone of the tool. Different types of such aggregates with widely varying properties are described in the literature. It has thus been suggested (American patent 2216728) to build up aggregates containing fine diamond grains by means of a small quantity of a special binding material for the aggregates. These aggregates, it is hoped, should have the same grinding properties as individual abrasive grains of the same size as the aggregates. It has been suggested to use metals like nickel with embrittling additives (U.S. patent 2216728) for the bond of these aggregates. The structure of a resin bonded grinding wheel containing such aggregates may be described in this way: diamond grains in simultaneous direct contact with each other and the metallic aggregate bond in its turn surrounded by a resin phase.

Another distribution pattern, which has gained a large industrial acceptance, is the use of metal coated abrasive gains, in particular metal coated diamonds, particularly in plastic bonded grinding wheels. At a suitable thickness of the metal coating, life of these grinding wheels is much better than with resin bonded wheels with diamonds with no metal coating (Swedish patent 306271). When the thickness of the metal coating increases, however, life will become reduced, sometimes below figures which are reported for naked diamonds. The structure may in this case be described as one diamond grain contained in a metal phase in its turn contained in a resin phase.

According to a third invention (Swedish patent 326122) the ratio between metal and diamond can be increased further with an increased positive effect on a life as a result,by means of the feature that every particle of metal contains several abrasive grains which are separated from each other by a metal phase. The abrasive grains exert a strengthening and stiffening up action on the abrasive particle while retaining a certain elasticity of the same. This embodyment might at first be considered as a combination of the two earlier mentioned inventions. This is, however, not the case. Properties of the abrasive particles according to the Swedish patent 326122 cannot be derived from the properties of abrasive grains or abrasive particles according to the two earlier inventions. Also there is a marked difference between the structures of the abrasive particles according to these inventions.

The two most important differences between the aggregates according to the U.S. patent 2216728 and the Swedish patent 326122 is that the abrasive grains according to the American patent are in direct contact with each other at adjacent points with no intermediate binding material whereas the abrasive grains according to the Swedish patent are separated by a metal layer. Therefore the amount of binding material in the elastic aggregates according to the American patent is much smaller than the quantity which is used in the abrasive particles according to the Swedish patent. These differences are due to the main difference between the abrasive particles according to the two inventions. The object of the American patent is abrasive particles with the same properties as single abrasive grains of the same size as the aggregates whereas the object of the Swedish patent is elastic abrasive particles for increased life thanks to a better retention of the grains.

Abrasive particles according to the two patents may be produced by means of for instance powder metallurgical methods or by means of casting methods. Hereby (1) the quantity of metal (5-10 %, or alternatively more than 20 % counted on the quantity of abrasive grains), (2) the mechanical properties of the metal phase (brittle or alternatively good mechanical strength and elasticity) and (3) the surface condition of the abrasive grains (naked or alternatively metal coated) will determine whether the process of manufacture will give abrasive particles of the kind' which are aimed at in the American resp. in the Swedish patent.

The present invention refers to a particularly advantageous procedure for the manufacture of abrasive particles of the various kinds which have been described above. In the manufacture of these particles, which thu s contain two or more abrasive grains which are kept together by a binding material, most frequently sintering or casting processes are being used. Frequently one then first makes a kind of blank which is then crushed and ground down to desired particle size. The crushing and grinding moments are difficult operations particularly in the manufacture of particles according to the Swedish patent 326122 since here the metallic component is elastic and tough. Abrasive particles according to the American patent 2216728 can on the other hand be crushed down more easily because of the minimum quantity of binding material and because of the embrittling additions to the metallic binding material in this case.

Abrasive particles of aggregate type should gain extensive use in grinding technology since the aggregate represents a new dimension with respect to product adjustment compared to a single grain. Aggregates are, however, not much in use in spite of this feature which is due to the fact that one also has to consider the method to be used for the production of the aggregates in the specification of the aggregate bond which therefore cannot be optimized solely for its function as aggregate bond. The present invention solves in a suprisingly simple manner the problems which have been discussed above and the result is that the binding material for the aggregates can be formulated with the sole consideration of it's function in the finished abrasive particles. In the same time the manufacture of abrasive particles is simplified and becomes cheaper. A further advantage is that the invention makes it possible to control the structure of the abrasive particles in a simple and straightforward manner to give the particles a structure shape and size which is optimized for each particular application and each type of abrasive grain.

The characteristic feature of the present invention is that a continuous net-work of holes (cavities) is arranged in the blank from which the abrasive particles are to be made by disintegration of the blank. The holes in the blank which demarcate massive elements which correspond to the desired abrasive particles, are produced by means of pore forming materials added to the mixture of abrasive grains, metal and the other compo- nents from which the blank is made by sintering, by casting or by means of other known processes to make coherent materials. These pore forming materials should preferably be present in the solid state at the temperature at which bond of the blank is being formed since this gives good possibilities to control the pore structure of the blank. Pore forming materials in the liquid state can, however, in certain cases be of advantage, in particular when there is a possibility to govern the structure at sintering by means of a suitable shape and size of the particles of the powder mix. Liquid pore forming materials which leave as vapour during sintering can also produce a coherent net-work of holes in the blank because of the mobility of the liquid state when used in comparatively small quantities.

It is also possible at least in principle for the purpose of systematization to talk about performing materials which are present in a gaseous state already before the sintering operation. This corresponds to what in the practice is called loose sintering with no special spacers. The powder-mix is packed loosely in the sintering mould in order to achieve a large open porosity in the goods. This is, however, in general not a very suitable method since the strength of the abrasive particles will be fairly small because only moderate pressure can be applied for the compaction of the powder mix. It is thus not possible to control the structure by means of - if the nomenclature is permitted - gaseous spacers.

In the following description of the invention we shall restrict ourselves to pore forming materials which are present in the solid state at the temperature at which the forming of the blank is performed by pressing, rolling or by means of other known and suitable methods for compaction, including casting. Such pore forming materials are of two kinds. One kind of pore forming material disintegrates or evaporates at the sintering temperature and leaves the blank as a gas. Holes appear in the blank where the particles of the pore forming materials have been present and where the produced gas has taken its way out of the blank. The other kind of pore forming material remains in the blank during the process of it's formation and is then removed by dissolution or in a mechanical way.

Examples of the first kind of pore forming materials are ammoniumbicarbonate, urea, cellulose derivates, camphor, fine powders of organic materials like polyethen etc. This kind of pore forming materials should in general not be used with casting processes where at least one of the metallic components in the mix mel since the holes may then to a large extent become eliminated when the pore forming material has escaped out of the blank when the latter at least partly is present as a melt.

The second kind of pore forming materials can be used both with sintering processes as well as with casting processes etc. In the casting processes it is of advantage to use pore forming materials which are present in the solid state during the whole casting process.

In sintering processes on the other hand it may sometimes be of advantage to make use of pore forming materials which melt during the last phase of the sintering and which then again solidify during the cooling of the sintered blank.

It is often particularly advantageous to choose a pore forming material which itself is sintering together to a coherent structure which serve as a matrix for the sintering of the metallic components of the blank. The spacer may also simultaneously serve as a sintering activator for the metallic components. Additives may also be used for this purpose e.g. nickel chlorides for the sintering of nickel based aggregate bonds. Such pore forming materials should preferably be removed by leaching and should therefore be soluble preferably in water solutions. Examples of such suitable pore forming materials are among other the halides of the alkali- metals. These materials may also serve as a sintering activator thanks to pyrohydrolytic production of hydrogen chloride in reaction with present water vapour.

Some pore forming materials remain inert during the sintering of the blank for instance refractory oxides like MgO, and do not sinter or melt together to a coherent structure during the sintering process. These spacers cannot in general be removed by leaching but have to be removed by mechanical means or by sedimentation in liquids, flotation, sifting or by means of other procedures after the blank has been crushed up into the abrasive particles.

Irrespectively of which method is used to remove the pore forming material it is often of advantage to have the pore forming material present in the blank during the mechanical disintegration. This facilitates the disintegration and helps to disintegrate the blank in the desired way with only a minimum production of abrasive particles out of specification.

In certain cases it is possible to control the composition of the blank to the effect that it disintegrates by itself into abrasive particles after the bonding of the metallic phase. This can be done by means of shrinkage flaws generated in the weak parts between the abrasive particles to be or by using a large quantity of a non-sintering pore forming material with large particles so that the abrasive particles to be are completely separated from each other in the blank. After this description of the spirit of the invention we shall now in some detail describe some particularly important applications of the invention. Manufacture of abrasive particles in principle according to the Swedish patent 326122 is such a particularly important field of application. Abrasive particles of this kind frequently contain very expensive and qualified abrasive grains like diamond or cubic boron nitride. Long life of tools with such abrasive grains is therefore a necessity. The invention is, however, not limited to abrasive particles with the structure covered by the Swedish patent 326122. Abrasive particles for instance according to the American patent 2216728 may also be manufactured according to the present invention. Other binding materials than metal, for instance glass or ceramic materials may also be used as a binding material There are no limitations regarding the chemical composition of the abrasive grains. Furthermore the abrasive particle can be utilized in many different kinds of tools. For simplicity we shall, however, exemplify our description by means of resin bonded wheels with metal bonded abrasive particles containing diamond or cubic boron nitride.

In this example the pore forming material is composed of particles of sodium chloride which after crushing the blank can easily be removed by leaching with water. The structure of the blank, which can be shaped as a circular disk, a band, balls or be of other shape, is shown schematically in Figure 1. The circled parts (1) constitute the elements which after the disintegration of the blank and the removal of the pore forming material will constitute the abrasive particles. Such an element contains for instance nickel-coated diamonds whereby the thickness of the metal-coating may be for instance between 0.0001-0.02 mm and where the diamonds have a size of for instance about 0.07 mm. The coherent metallic phase (2),which in this case can be made by powder of carbonylnickel with a particle size of a couple of μm has been sintered together with the nickel-coating of the diamonds (3). The particles of the pore forming materials (4) form a continuous net-work which partly embrace the abrasive particles to be (l).The latter ones are, however, connected to each other by means fo bridges of sintered nickel powder (5).When the blank is crushed or subjected to a similar treatment these bridges will break up so that the abrasive particles to be become free from each other. After the removal of the pore forming material by mechanical means, leaching or in other ways the abrasive particles are ready for the brushing treatment.

The structure is in this case governed by the amount and size of the pore forming particles. When the particle size of the pore forming material is about 0.2 mm, the volume fraction amounts to 50-70 percent by volume calculated on the total dilatometric volume of the components, every abrasive to be will contain on the average about five abrasive grains.

The powder-mix is compacted to a green blank at a

2 pressure of 1.5 ton/cm . The blank is sintered in hydrogen 45 minutes at 780 C. After cooling in the protective gas the blank is crushed and ground for instance in a ball mill so that the abrasive particles become free. After leaching, screening and for instance after- sinteringgat 900 C in a protective.atmosphere in a fluidized bed the abrasive particles can be used for the manufacture of a grinding wheel for instance according to the Swedish patent 326122. It is sometimes of advantage to use bronze bond containing about 20 % tin for the aggregate. In this case the blank may first be sintered at 230 °C with after-sintering at 430 °C for another hour. If the fraction of pore forming materials in these examples is increased to about 90-95 % by volume or more the abrasive particles to be will become almost completely separated from each other in the blank and can therefore be very easily separated from each other by leaching. With non-leachable pore forming material like refractory oxides the abrasive particles can easily be separated from the spacer by means of mechanical or methods of sepation.

The characteristic feature of this invention is, as has been pointed out above, the holes which are generated in the blank. It is an easy task for the expert to find a most suitable method for generation of these holes in the blank from case to case depending on the size, shape and kind of abrasive grain and the size, shape and concentration of the abrasive grain and binding material in the abrasive particles. As a general rule special pore forming materials are used for this purpose. There are several alternative routes also when the specification of the abrasive particles is quite firm. It is therefore not possible and also not necessary to report specific rules for the choice of the pore forming material. The suggestions which have been given above in the description should be sufficient for the expert to find a suitable pore forming material in each separate case with no difficulty. The volume fraction of the pore forming material in the blank -can also be varied between wide limits among other things depending on the size and the shape of the pore forming particles. A sufficient amount of holes can be generated already at such low concentrations as 3-10 % by volume (dilatometiric volume) particularly at low pressures of compactions. An upper limit can also hardly be specified when thinking of the possibility, with this invention, to manufacture abrasive particles which are completely separated from each other in the blank as has been described above. It is, however, not economic to work with larger amounts of pore forming materials than about 95-98 % by volume. Very frequently a good working region is, with pore forming materials which leave as gas, 10-40 % by volume and, with pore forming materials, which are leached away, 40-80 % by volume and, with pore forming materials which are removed by mechanical means, 80-95 % by volume.

The size and shape of the holes or in other words the structure of the net-work which the holes produce in the blank is, with solid pore forming materials, to a large extent determined by the shape of the pore forming particles and their size relative to the other structure forming components which are present in the blank during its formation. The holes have two functions, they define the abrasive particles to be and they contribute to an efficient and defined disintegration of the blank.

In the formation of the blank there are three different kinds of components present namely, abrasive grains, binding material and pore forming materials. The particles of the binding material are in general about the same size or smaller than the abrasive grains. In certain cases when the ratio between the volumes of the binding material and the volume of the abrasive grains is large, say 10-1 or even 20-1 the particles of the binding material may be much larger than the abrasive grains, however. The volume of a particle of the binding material can in such cases be as much as 10 times larger than the volume of one abrasive grain. It should, however, in this connection be mentioned that formation by means of casting is a special case since the particles of the binding materials in this case are present. in the molten state during the formation apparently, are of atomic dimensions.

It is of course as difficult to define the structure of a hole as to define the structure of massive element in a porous body. Quite generally it can, however, be said that the holes in hte blank should frequently be of at least the same size as the abrasive particles to be. If a hole is thought to be produced at the site of particle of a pore forming material the conclusion must be drawn that the pore forming particles on the average should be larger than the average size of the largest one of the other components which are present in the blank during its formation. The pore forming particles may, however, in their turn often be aggregates of much smaller particles. This size ratio, measured as a volume ratio, should exceed 2 to 1. A figure which is frequent of advantage is between 5 and 20 to 1. In the case of abrasive particles containing many abrasive grains, say 50 to 100, and particles of binding material which are smaller than the abrasive grains it can, however, be necessary to use such large particles of the pore forming material that the ratio mentioned above is as large as between 100 and 500 to 1.

In the case of most common abrasive particles the pore forming particles will be smaller than 1 mm in the longest tip-to-tip direction. A frequently suitable region is 0,1-0,5 mm. There is seldom reason to go below about 0,05 mm. (An exception to this, however, is additions of smaller fractions of the pore forming materials which can frequently be of advantage since these small particles may serve as bridges of connection between the larger particles and thereby contributes to the creation of coherent net-work.) As mentioned above the particles of the pore forming material may be composed of small particles which may be as small as about 1 μ or below. Also the particles of the binding material may be present as aggregates. particles of carbonylnickel powder may thus be considered as aggregates of very small crystallites.

The desired shape of the pore forming particles is governed by the desired shape of the abrasive particles. It is frequently desired that the latter one should have a fairly symmetrical structure. From this follows that the pore forming particles in this case should also have a symmetrical structure, that is have about the same measures in all directions. A certain addition of dendritric character to the structure is sometime to advantage since this helps the development of connections between the larger holes. Abrasive particles of another shape, for instance strongly dendritic structures, needles or flakes necessarily require corresponding special forms of the pore forming particles.

Claims

Claim 1

Procedure for the manufacture of abrasive particles containing two or more abrasive grains kept together by a binding material whereby said abrasive particles are produced by the disintegration of blank characterized thereby that a continuous network of holes is arranged in said blank which holes demarcate elements of the blank which elements after the disintegration of the blank constitute the abrasive particles.

Claim 2

Procedure according to Claim 1 whereby the binding material is a metal or a metal alloy.

Cla.im 3 Procedure according to Claim 1 whereby diamonds are used as abrasive grains.

Claim 4

Procedure according to Claim 1 whereby grains of cubic boron nitride are used as abrasive grains.

Claim 5

Procedure according to Claim 1 whereby a water soluble salt is used as the pore forming material.