ABRASIVE SEGMENTS

BACKGROUND OF THE INVENTION

This invention relates to abrasive segments. In particular, this invention relates to abrasive segments having regions of differing properties

Diamond containing saw blades are used extensively for cutting stone in construction work and the like. The blade comprises a metal, generally steel, disc and a plurality of saw segments brazed or otherwise bonded to the periphery of the disc. Each saw segment comprises a metal base or support and a working portion which comprises diamond particles uniformly distributed in a matrix which may be metal or ceramic. Other segments comprising a base and a working portion are used in other cutting/abrasive tools. As such, the term ' segment' is considered to include drill bit inserts, blades, wire drawing dies and other wear parts.

Saw segments are also known in which the working portion comprises diamond- bearing layers separated by layers of non-diamond-beaπng metal sheet All the layers are parallel to the plane of the segment motion, i e at right angles to the working surface of the segment These layers are joined to each other in at least one distinct step using conventional sintering techniques such as hot pressing This can be very labour intensive and does not lend itself to automation Also,

joining between the layers is not of consistently good quality which compromises the strength of the segment itself.

In addition, the need for distinct bonding steps limits the complexity of the segment shapes that can be constructed, mainly for economic reasons.

It is considered desirable that segments used for the frame sawing of rocks comprise portions or regions with at least two different wear resistances (i.e higher and lower wear resistant portions) This allows the lower wear-resistant portions to wear away faster, exposing the higher wear resistant portion thereby reducing the overall cutting/abrasive forces required to remove rock. Reduction of cutting/abrasive forces on the blade is desirable since this reduces the tendency of the blades to deviate from the cutting plane during the sawing process.

A need therefore exists for a commercially viable abrasive segment including at least two regions which differ from each other.

SUMMARY OF THE INVENTION

According to a first aspect to the present invention there is provided a segment including a working portion comprising a bonding matrix including at least two regions which differ from each other in abrasive particle content, size, type or distribution wherein the regions are derived from layered, columnar or cuboid shaped components which differ from each other in abrasive particle content, size, type or distribution and which are manufactured by slip casting, high shear compaction, molding and/or casting

The regions may contain differing concentrations of discrete abrasive particles in the bonding matrix, for example, or the regions may differ in that at least one region contains no abrasive particles Preferably the at least two regions are

electro discharge sintered to each other The segment may also include a support onto which the working portion is bonded

The bond matrix may be homogeneous or heterogeneous. Term 'homogeneous' is intended to mean that the working portion is comprised of essentially the same bond matrix material

The regions can take any suitable form or shape such as a layer or strip When in the form of layers, the layers can be parallel to a working surface of the working portion or be transverse thereto. The regions can also be randomly or otherwise distributed through the working portion

One or more of the regions can protrude beyond the working surface of the working portion. For example the working portion may comprise a plurality of alternating layers perpendicular to the working surface, one set of layers having an abrasive content higher than the other set and some of the first set of layers protruding beyond the general working surface. The protruding layers provide the working portion with protruding "teeth" which give the working portion a ciaw type cutting action

The regions may be arranged to provide a functionally graded working portion where the wear resistance increases from the inside of the working portion towards the working surface thereby reducing the tendency for chipping to occur

A region of fine abrasive particles may be provided at opposed side surfaces of the working portion to reduce the amount of polishing required In other words the two outer sides of the working portions of the segments (not the working surface) which would be rubbing against the walls inside the cut could contain wheel grit sized diamond or other abrasive particle, thereby grinding these surfaces to some extent and reducing the number of steps needed to achieve a final polish of the product.

In a preferred embodiment of the present invention, the regions may be strips of bonding matrix (bond material) in powder form, either in the green state containing polymer binder or pre-sintered but not completely sintered (<90% deπsified).

According to a second aspect to the present invention there is provided a method for the manufacture of a segment including a working portion comprising at least two regions which differ from each other in abrasive particle content, size, type or distribution, the method comprising the steps of assembling a near net shape working portion from at least two layered, columnar or cuboid shaped components which differ from each other in abrasive particle content, size, type or distribution and bonding the at least two components to each other wherein the components are manufactured by slip casting high shear compaction, molding and/or casting

The components are preferably derived from liquid based pre-forms and may be heat treated prior to assembling the near net shape working portion. In a preferred embodiment the components are a self supporting, non-sintered malleable pre-form The term 'liquid based pre-forms' includes slurries comprising particles within aqueous or non-aqueous media.

The components may be sintered, semi-sintered or green state Preferably the working portion consists essentially of layered, columnar or cuboid shaped components

The components may be bonded together by efectro discharge sintering, hot pressing, brazing, infiltration, free sintering, hot isostatic pressing (hipping), Field Assisted Sintering Technology (FAST), laser sintenng and/or High Pressure High Temperature techniques.

According to a third aspect to the present invention there is provided a segment manufactured by a method as hereinbefore described.

By 'green state' it is intended to mean unsintered yet self supporting.

The components may have the same or different matrices. In other words the regions may differ from each other in nature of the bonding matrix or in other ways. Thus, working portions and hence segments or tool inserts can be produced with complex configurations designed to achieve a particular cutting or abrasive action.

DETAILED DESCRIPTION OF THE INVENTION

The segment of the invention will generally be a saw blade segment, a drill bit including a core drilling bit, a segment for a wire saw or gang saw, a drill bit including a twist drill bit, a wire drawing die or a wear part.

Additional applications in which the present invention has utility include:

« Band saw blades,

• Hacksaws,

• Frame saws,

• Concrete polishing,

• Core drill bits

• Wire beads,

• Impregnated bits,

• Roller cone bits,

• Twist drills,

• Grinding wheels,

• Grinding tips,

• Rotary dressers,

• Dresser logs for single and multiple log dressers,

• Profile dressers,

• Straight and profiled routers,

• Polishing cups,

• Single point tools,

• Calibration rollers,

• Single point turning tools,

• Gauge materials, and

• Hard facing.

The abrasive may be any known in the art such as diamond (including natural, High Pressure High Temperature and/or Chemical Vapour Deposition produced diamond), cubic boron nitride, a carbide, oxide or suicide, SJsN ₄ , SiC, AI ₂ O ₃ , AIC and/or Siθ ₂ - In addition, diamond composites such as a diamond/SiC composite are also contemplated.

The bonding matrix will generally be metal, but may also contain a ceramic additive, for example WC, SiC and/or AI ₂ Oa.

Metallic (bond matrix) materials are typically used in tool manufacture to physically hold diamond particles to allow them to cut through stone, etc. Generally, the metallic material begins as a powder to which diamond is added and this mixture is then sintered to a solid shape The sintering can be hot pressing in graphite, free sintering in a controlled atmosphere or even filling the voids in a tool with a liquid infiltraπt.

The typical materials used for free sintering and hot pressing are powders of cobalt, cobait mixed with copper or bronze, or mixtures of iron, copper and cobalt, in recent years there has been a growth in the use of pre-alloyed powders from manufacturers such as Umicore (HDR, CNF, Cobalite 601 , etc.) and Eurotungestene (Nexti 00, 900, etc.). Instead of a mixture of metal powders, such products are produced (chemically) to have an extremely fine mixture of the

constituents. In terms of particle size, finer powders sinter more easily, i.e at lower temperatures and pressures The overall properties of a resulting tool are also more homogeneous when using pre-alloyed powders. Typical powder sizes are in the 1 μm to 10μm diameter range.

A hard phase, such as tungsten carbide, can be added to the above bond materials to improve the wear resistance of the bond and hence the tool Again, the particle size is typically in the 1 μm to 10μm diameter range.

Infiltration is where a liquid infiltrant (typically a copper based bronze) flows through a porous powder containing diamond (or other abrasive particles) in a mould When cooled, the infiltrant fills the porosity and bonds the powder and abrasive particles together to form a tool Powders used for this process can be similar to those above but are more typically tungsten/molybdenum or iron. In this process, the powders can be quite coarse (10μm to 250μm) as they do not sinter but are bonded together by the infiltrant. That said, fine powders may also be used (10μm). In this process, there is scope to use large particles (up to 500μm) of hard media (tungsten carbide or any other hard ceramic materia!) to produce a very wear resistant bond

Abrasive particle bearing metallic segments may also be made using the electro discharge sintering (EDS) technique. This is achieved by filling a mould with the segment precursor material layers and subjecting the material to the EDS process

EDS is defined as sintering achieved by passing one or more pulses of high current through a porous metallic based medium while also under compressive force.

Details of EDS are as follows: Force applied (range):

Preferably greater than δOOkg/cm ² , more preferably greater than 600 kg/cm ² more preferably greater than 800 kg/cm ² , more preferably greater than 1000 kg/cm ² , more preferably greater than 1500 kg/cm ² , most preferably greater than 2000 kg/cm ² . Preferably less than 6000kg/cm ² , more preferably less than 5500kg/cm ² , more preferably less than 5000kg/cm ² , more preferably [ess than 4500kg/cm ² , most preferably less than 4000kg/cm ² ,

Amperage (range):

Preferably greater than 0.5kA, more preferably greater than 1 ,OkA, more preferably greater than 5.OkA. more preferably greater than 10.OkA ₁ more preferably greater than 20,OkA, more preferably greater than 50.OkA, most preferably greater than 80.OkA. Preferably less than 100OkA ₁ more preferably less than 90OkA, more preferably less than 80OkA, more preferably less than 70OkA, more preferably less than 60OkA ₁ more preferably less than 55OkA and most preferably less than 50OkA ₁

Voltage (range):

Preferably greater than 1V, more preferably greater than 2V ₁ more preferably greater than 3V, more preferably greater than 5V, more preferably greater than 8V ₁ more preferably greater than 10V, most preferably greater than 20V. and preferably less than 10OV ₁ more preferably less than 90V, more preferably less than 80V, more preferably less than 70V, more preferably less than 60V. more preferably less than 55V and most preferably less than 50V.

No. of pulses (range);

Preferably greater than 1 , for example 2. Preferably less than 10, more preferably less than 9, more preferably less than 8, more preferably less than 7, more preferably less than 6. more preferably less than 5. most preferably less than 4.

Duration each pulse (range):

Preferabiy greater than 0.1 ms, for example 0.2ms. Preferably less than 10ms, more preferably less than 9ms, more preferably less than 8ms, more preferably less than 7ms, more preferably less than 6ms. more preferably less than 5ms, most preferably less than 4ms

Alternatively tape casting can be used to manufacture strip from which segments or portions thereof (components) can be manufactured by punching, cutting or the (ike from the strip. For tape casting it is necessary to use a slurry consisting of binder matrix materials, the abrasive material as well as several additives like solvent, dispersing agents, binder and softener. The additives may be organic and/or aqueous. These are mixed together in a predetermined proportion and subsequently cast by means of a tape casting facility. During the process, the slurry of materials flows from a storage container onto a support, for example, a plastic foil , which is continuously dragged with a controlled velocity under the container. On the plastic foil a slurry layer is formed. The height of the slurry is controlled by a doctor blade, which determines the final layer thickness and which creates a smooth and even foil. After casting the slurry is dried, for example by passing through a drying chamber, in which the resulting foil may be purposefully dried at variable temperatures. Thus a self-carrying flexible foil or strip is manufactured, which can be cut, sawed or punched and which may be sintered at temperatures > 1000°C and coated, if desired, afterwards. The advantage of the procedure in relation to other methods, e.g. pressing, are a high manufacturing capacity, an economical production, the high reproductibility and the possibility of the industrial practicability of the production of strips.

The columns, cubes or other component shapes can then be cut, punched or sliced from the strip These component shapes can then be assembled into the segment according to the invention

As such, the present invention comprises a method for making abrasive particle containing segments using pieces of strip (components), the matrix binder and

abrasive particle composition, the dimensions of which can differ according to the design of the segment The pieces of strip are assembled (as components of the segment) and sintered together by means of any of the known sintering methods (e g EDS, FAST, hot pressing, free sintering, infiltration sintering, laser sintering, hipping). The component pieces can be obtained from strip comprising the desired metallic binder powder, the desired diamond (or other abrasive) content (if any) and possibly other additives. The strip can be manufactured using known slip-casting methods (see for example WO2004074534) The diamond may be incorporated into the strip as coated or uncoated particles, or as encapsulated particles.

In a preferred embodiment of the present invention the following method of producing an elongate, thin , coherent and self-supporting body is used The method comprises a mass of discrete abrasive particles uniformly dispersed and held in a support matrix and includes the steps of providing a mixture of the abrasive particles and the support matrix in particulate form, causing a thin layer of this mixture to be deposited onto a support surface, compacting the layer and heat treating the compacted layer under conditions which will not lead to degradation of the abrasive particles to produce the body

The method uses broadly the techniques and methods described in British patent No. 1 ,212,681 to produce abrasive particle-containing bodies. The bodies will be elongate, thin, coherent and self-supporting and will typically take the form of a strip, sheet or the like The strips may be produced with a certain degree of flexibility or ductility

The body will be thin and will generally have a thickness which does not exceed 1 mm. Typically, the thickness of the body will be in the range 0,2 to 0,7mm preferably in the range 0,2 to 0,5mm.

The bodies produced by this method of the invention may contain 50% or less by volume of a mass of discrete abrasive particles Generally, the abrasive particle content will be in the range of 20 to 40% by volume of the body but it will be appreciated that content will generally vary from 2.5% to 50% by volume. Examples of suitable abrasive particles are diamond, cubic boron nitride, silicon carbide, tungsten carbide and chromium boπde. The particles will generally have an average size of less than 500 microns, preferably less than 100 microns.

The support matrix may be metallic or resinous in nature, but is preferably metallic in nature. When the matrix is metallic, it is preferably an iron-containing alloy such as a stainless steel. Examples of other suitable metallic support matrices are nickel and cobalt based alloys. The alloys may be treated by nitriding or ion implantation to improve their abrasion resistance.

The compaction of the thin layer which is deposited on the support surface may be achieved by passing that layer through rollers. The pressure applied to achieve compaction will vary according to the nature of the support matrix, but will typically not exceed 60 tons (54.43 tonnes) Standard and well known lubricants may be used to ensure that the layer passes through the rollers smoothly

The heat treatment conditions will vary according to the nature of the support matrix and the abrasive used. When the support matrix is metallic the heat treatment is preferably earned out at a temperature below the melting point of the metal. Typically the metal will have a melting point above 1500 deg C and heat treatment will be carried out at a temperature in the range 600 to 1000 deg C for a period of 1 to 20 minutes.

The heat treatment should ideally take place under conditions which will not lead to degradation of the abrasive particle. For diamond particles the conditions must

be such as not to lead to any substantial formation of graphite. For cubic boron nitride particles, the conditions must be such as not to lead to any substantial formation of hexagonal boron nitride For these two abrasive particles it is thus preferable for the heat treatment to take place in a non-oxidising, reducing or inert atmosphere. Examples of such atmospheres are hydrogen, hydrogen/nitrogen and hydrogen/argon.

The particular mixture will generally have a suitable binder added to it prior to passing it to the compaction step In this regard, the particulate mixture may, for example, be slurried with a film-forming binder material in water, the slurry deposited on the support surface and a major part of the water removed, e.g. by heating from the slurry prior to the compaction step. The binder material may be dissolved or dispersed in water. The binder is preferably one which decomposes or volatilises at a temperature of 300 deg C or higher which enables it to be removed from the particulate mixture during the heat treatment step The binder is typically a cellulose binder such as methyl cellulose.

The body which is produced after the heat treatment step is coherent and self- supporting. When the body has a metal matrix, it may thereafter be subjected to further compaction and heat treatment steps or a combination of these steps to modify the properties of the body. The compaction step or steps will be as described above Similarly the subsequent heat treatment or treatments, which have the effect of annealing the metal matrix, will be as described above.

The components as described may be made near net shape by disposing the slurry into molds, for example, by spreading the slurry across a surface with indentations having the form of desired component shapes.

A particular advantage of the present invention is that the position of diamond (or other abrasive particle) layers within a generic segment can be controlled by using a starting material of layers/strips/other component shapes such as cubes

(e.g. a single grit particle encapsulated in a cube form) of bond material containing diamond or other abrasive particles. These can be shaped to suit the segment shape. Strips/layers/cubes containing different size or strength of diamond or other abrasive particles can be mixed to produce enhanced areas

These strips are preferably partially sintered and, in some cases, are preferably manufactured into a useable product using EDS as a sintering technique This results in a dense, sintered compact of diamond-bearing metal with diamond enhanced areas. The significant advantage associated with EDS is that the thermal degradation of the diamond is limited by the short temperature spiking

The present invention enables complex segment forms to be made in a single step, it also has the benefit that the segments can be more reliably and reproducibly formed.

More complex shaped segments are desirable in order to better control the required forces and the cutting/abrasive behaviour of the segment. For example, it is desirable in certain applications to have portions of the segment protruding, in a controlled way, from the working portion of the segment, other than as strips or parallel, curved ridges parallel to the segment motion, as is currently the art For example, it may be desirable to have protruding "teeth" of diamond-bearing metal portions at an active edge of the working portion

Enhancements in cutting performance can be achieved by adjusting the diamond or other abrasive particle content, strength or size in a particular region in comparison to another, thereby resulting in differing wear rates Adjustments in the bond materials can also be incorporated.

These differing regions could be in sheets (layers) parallel to the segment direction of motion, for example, or in columns in the direction of the segment height or lines in the direction of the segment direction or individual areas

distributed within the matrix of the working portion. According to the present invention, a functionally graded segment can be produced where wear resistance is increased from the inside of the segment, for example, towards the edge resulting in a tool which helps prevent edge chipping. This can be achieved by changing the size, type and/or strength of the diamond or other abrasive particle in individual layers/strips/cube components, or the type of bond material

Alternatively, a fine size of diamond or other abrasive particle of less than 200 microns, preferably less than 180 microns, more preferably less than 160 microns, more preferably less than 140 microns, more preferably less than 120 microns and most preferably less than 100 microns, for example grinding size, can be manufactured on the outside of the segment which can grind while cutting to reduce the numbers of steps after sectioning before polishing Alternatively, different components can be used which contain diamond or other abrasive particle of different concentration , size, strength etc to achieve the desired goal of improved cutting/abrasive performance.

With careful design of the sheets, columns or diamond or other abrasive particle bearing components, diamond or other abrasive particle positioning within the segment can be controlled in 1 , 2 and 3 dimensions.

The present invention thereby allows the production of diamond or other abrasive particle enhanced areas within segments which results in more aggressive cutting/abrasive capabilities. The teachings of the prior art do not allow this to be done. Where it can be done by a standard techniques, EDS does allow it to be done faster, cheaper and with a better product, in some cases

The sawing of rock, especially softer rocks such as marble, is especially benefited by the teachings of the present invention

The contents of applicant's co-pending application entitled Electro Discharge Sintering Manufacturing claiming priority from ZA 2007/01267 are hereby incoporated by reference.

The invention will now be described with reference to the following non-limiting illustrations in which:

Figure 1 shows how a strip of material containing diamond can be put together in layers to form a 1-D, 2-D or 3-D segment: and

Figure 2 shows differing wear patterns for such precoπstructed segments.

In Figure 1 , the strips for a 1-D example can be built up in the vertical direction as well as the horizontal direction, depending on the direction of sintering. If diamond particles in the strip component were arranged in an ordered fashion, maximum displacement between diamond particles could be achieved. Along with all of this, the same diamond (in terms of size, strength, concentration) need not be used in all the pieces of strip component. For example, stronger (or more) diamond particles can be used in the outer strip components so that these will wear at a slower rate making the edges of the segment higher than the centre. This gives a much better edge quality when cutting tiles.

The same can be done if a component column (or row) is used, which allows a more accurate placement of diamond. Finally, mixing cube components containing diamond with components that do not contain diamond and placing the cubes in desired positions allows very accurate placement of the diamond particles.

Segments can be manufactured from strip or columns or cubes (geneπcally called components) where ¹ -

1 The bond material can differ from component to component.

2. The diamond size, quality or concentration can differ from component to component

3. Both 1. and 2. above can differ from component to component.

In a 1 dimensional case where alternating layers of strip component containing diamond and not containing diamond are sintered together to form a segment, the positioning of the diamond can be determined along one dimension of the segment. Because of the different wear characteristics of the two strip components, a profiled segment results where ridges appear proud of the segment surface where the strip component containing diamond is and a valley where the strip component not containing diamond is. Along each of these ridges, the diamond is randomly (but most likely very evenly) distributed throughout the ridge, see Figure 2. This protruding ridge acts as an aggressive cutter allowing higher cut rates and reducing cutting forces.

The strip component not containing diamond may also be made from a different bond material. Either strip component could also contain abrasive particles to enhance the wear properties of the strip components. Instead of using strip component which does not contain diamond, a strip component with a different size, strength or concentration of diamond could be used to achieve the same protruding πdge effect.

In a 2 dimensional case, column components vertical in the segment (or alternatively rows horizontal in the segment) can be alternated between column components which do and do not contain diamond. In this way the diamond positioning can be determined along 2 dimensions in the segment The minimum distance between diamond particles can be determined along 2 dimensions. Because of the difference in abrasion rates between column components with and without diamond this allows the diamond containing areas to protrude more significantly and act as aggressive teeth cutting more quickly through the stone being cut.

The column components not containing diamond may also be made from a different bond material Either column component could also contain abrasive particles to enhance the wear properties of the strip components, instead of using column components which do not contain diamond, a column component with a different size, strength or concentration of diamond could be used to achieve the same aggressive teeth effect The same advantages as detailed in the 2 dimensional case are also possible here.

In the 3 dimensional case cube components vertical and horizontal in the segment can be alternated between cube components which do and do not contain diamond. In this way the diamond positioning can be determined along three dimensions in the segment.

In all cases, by preventing diamond particles from touching, the unnecessary loss of particles is reduced. Also, separating diamond particles and not allowing them to cluster together in a small group provides for a greater use of all particles.

'Near net shape ^' means that the initial production of the segment is very close to the final (net) shape such that the overall shape is similar and the dimensions of the initial shape are preferably no more than 40% different (on average) from that of the final net shape, more preferably no more than 30% different from that of the final net shape, more preferably no more than 20% different from that of the final net shape, most preferably no more than 10% different from that of the final net shape,

^' Layer' means a 3 dimensional component wherein a z dimension of the component is significantly less than x and y dimensions of the component. Preferably the smaller of the x and y dimensions is at least 3 times greater than the z dimension, preferably at least 5 times greater than the z dimension, more

preferably at least 10 times greater than the z dimension, most preferably at least 15 times greater than the z dimension.

'Columnar' means a 3 dimensional component wherein a z dimension of the component is significantly greater than x and y dimensions of the component. Preferably the z dimension is at least 3 times greater than the smaller of the x or y dimension, preferably at least 5 times greater than the x or y dimension, more preferably at least 10 times greater than the x or y dimension, most preferably at least 15 times greater than the x or y dimension.

'Cuboid' means a 3 dimensional component where x, y and z dimensions of the component are substantially similar. Preferably any two of the x, y and z dimensions do not differ by more than 50% on average, more preferably by not more than 25%, most preferably by not more than 10%,