WO2012055711A1

WO2012055711A1 - A fixed abrasive sawing wire and a method to produce such wire

Info

Publication number: WO2012055711A1
Application number: PCT/EP2011/067984
Authority: WO
Inventors: Henk Troost
Original assignee: Nv Bekaert Sa
Priority date: 2010-10-28
Filing date: 2011-10-14
Publication date: 2012-05-03
Also published as: CN202640582U; TWI533960B; TW201236786A

Abstract

A fixed abrasive sawing wire (100) suitable for sawing hard and brittle materials is disclosed. The sawing wire is of the type wherein hard particles - preferably diamond particles - are pushed into a soft indentation layer that covers a hard steel wire core (110). After indentation particles are fixed further by means of a binding layer. The sawing wire (100) is particular in that the indentation layer is build-up out of two metallic layers: a first metal layer (112) closet to the steel wire core on the top of which a second metal layer (114) is deposited. The first metal is softer than the second metal. The second metal layer (114) is thin in order not to limit the indentation capability of the first metal layer (112). It is conjectured that the second metal layer (114) is like a skin that improves the particle retention capabilities of the fixed abrasive sawing wire (100). Also described are a number of methods to produce such a kind of wire.

Description

A fixed abrasive sawing wire and a method to produce such wire

Description

Technical Field

[0001 ] The present invention relates to the field of fixed abrasive sawing wires for cutting hard and brittle materials, more specifically it relates to a

monofilament sawing wire where abrasive particles are indented and held in an outer layer of a monofilament.

Background Art

[0002] At present the predominant technology used to cut expensive hard and brittle materials like quartz (for e.g. quartz oscillators or mask blancs), silicon (for e.g. integrated circuit wafers or solar cells), gallium arsenide (for high frequency circuitry), silicon carbide or sapphire (e.g. for blue led substrates), rare earth magnetic alloys (e.g. for recording heads) or even natural or artificial stone is by means of very thin, plain carbon steel, high tensile sawing wires. Although the wire used is called a 'sawing wire' it are abrasive particles fed to the wire in a viscous slurry - usually a suspension of silicon carbide particles in polyethylene glycol - that abrade the material away and saw. The method is generally referred to as 'loose abrasive sawing' and is one kind of 'third body abrasion' (the third body being the abrasive).

[0003] However, this 'loose abrasive sawing' is more and more under pressure because of its high consumable cost (abrasive and wire), its high environmental cost (reworking or disposal of the slurry) and its high operating cost (slurry management). Therefore, increasingly the

technology is converting to 'fixed abrasive wire sawing' and in particularly to cut the hardest materials like sapphire.

[0004] In 'fixed abrasive wire sawing' the relative movement between the

abrasive and the wire has been set to zero: the abrasive is fixed to the wire. This brings some major advantages with it:

> The wire wears much less as there is no impact between particles and wire. Hence, a better use of consumables follows. > The whole impulse of the particle is transferred to the workpiece: there is no 'stick and roll' of the particles between wire and workpiece.

> There is no need anymore for a liquid carrier to feed the particles in between wire and workpiece: the slurry and its cumbersome management can be eliminated. However, a coolant remains necessary to keep the temperature of the workpiece and sawing wire in control and to wash away the swarf.

> With the used wire, the used particles are discarded. There is no need for a separation step to separate swarf and abrasive (for reuse).

[0005] Next to the desire to have the abrasive particles fixed to the wire substrate there is also the requirement that the overall wire diameter, inclusive the fixed abrasive particles, should be as thin as possible. As the materials that are sawn are generally expensive, a low kerf width is mandatory in order to reduce the loss of this expensive material. Fixed abrasive sawing wires should therefore be as thin as possible which limits the dimension of the wire substrate. The goal is to have a wire that is overall thinner than 150 micrometer.

[0006] On the other hand there is the requirement that fixed abrasive sawing wires must be able to sustain the tension forces that occur during the sawing process without breaking. The higher the wire tension is, the more lateral force can be transferred to the work piece whereby a higher cutting speed can be achieved. Operating wire tensions in current wire saws are at least 10 newton, typically 25 newton, sometimes 40 newton and more. Therefore very high tensile steel wires are used as the core of the wire as such wires are best in terms of modulus and ultimate tensile strength, which is needed in order to be able to make the wire thin.

[0007] The abrasive particles must also be fixed well to the wire as sawing is normally performed in a thru and fro movement, a movement that tends to wiggle to particles out of their positions. Hence, a considerable part of the sawing wire cross sectional area is devoted to a layer that holds the abrasive particles, which makes the strength requirement even more difficult.

[0008] Finally, the wire must be deliverable in sufficiently long lengths as in a multi-loop wire saw quite some length of wire is already needed to thread the web: depending on the type of machine and the number of loops this varies from 500 to 1500 meter. In a multi-loop wire saw, a single wire is threaded over grooved capstans in loops, side by side. The corresponding surface of parallel arranged wire loops is called the web. The more generally used name is a multi-wire saw which is actually a misnomer as there is only one wire in the web. Wires must therefore be supplied in lengths of at least 10 kilometre in order to be useful on the machine.

[0009] At present basically three main routes are being pursued offering viable options for the production of a fixed abrasive sawing wire:

[0010] A first route was taken by the technologists that were familiar with the making of metallic based fixed abrasive tools such saw blades wherein abrasive particles - usually diamonds - are embedded in a nickel coating by electroplating or electroless deposition out of a nickel bath comprising diamond particles. An example is described in EP 0 982 094 wherein a metallic wire, the embodiment of a stainless steel wire is given, is coated with nickel with diamonds. However, the coating speed attained is slow.

[001 1 ] A second route was taken by chemically skilled persons that sought to apply their knowledge on organic binders for abrasives for making a fixed abrasive sawing wire. There are numerous examples of which US

6,070,570, EP 1 025 942 and WO 2005/01 1914 are three. US 2 793 478 is a particularly early example of this technology. Although this results in a cost effective and efficient way of fixing the abrasive on the -usually metallic - wire the fixation is not that strong and the resulting cutting speeds are lower than those obtained in the other routes mentioned.

[0012] EP 0 081 697 describes a method and an apparatus to incrust a wire with diamond particles. One departs from a wire that is coated with a copper or nickel layer prior to incrustation of diamond particles between hardened wheels that roll the wire around its axis through a repetitive axial movement of one or both of the wheels. Thereafter the diamonds are fixed in position by means of an electrolytically applied overcoat. This route offers advantages in terms of throughput and product quality and allows the production of reasonably long lengths.

[0013] Still other routes - such as described in WO 99/46077 or US 3 854 898 - are based on soldering or brazing the particles to the wire. However, these routes are less preferred in that they imply excessive heat flow to the wire, that may lead to a loss of strength of the wire due to the heat load. The heat load problem is even more severe for finer diameters as they have - over a unit length - a lot of surface but no mass to absorb the heat.

[0014] For all of the above mentioned techniques, it remains a challenge to fix the abrasive particles sufficiently well on the wire surface.

Disclosure of Invention

[0015] It is therefore an object of the invention to offer further improvements for abrasive particle retention on fixed abrasive wire saws. It is also an objective to offer a method to produce such wires.

[0016] According a first aspect of the invention a fixed abrasive sawing wire is claimed as a product. The fixed abrasive sawing wire comprises a steel core wire and an indentation layer covering said steel core wire. Abrasive particles are indented in said indentation layer and a binding layer covers the indentation layer and the abrasive particles. Specific about the wire is that the indention layer - additional to what was known from the prior-art - comprises a first metal layer covering the steel core and a second metal layer covering said first metal layer, wherein the first metal is softer than the second metal. Alternatively formulated: the indentation layer comprises two different layers: a soft under layer and a hard top layer.

[0017] The core wire of the fixed abrasive sawing wire is made of steel.

Preferably the core is made of a plain carbon steel although other kinds of steel such as stainless steels are not excluded. Steels are more preferred over other high tensile wires such as tungsten, titanium or other high strength alloys because it can be made in high tensile grades. This can be achieved by extensive cold forming of the wire through circular dies.

[0018] A typical composition of a plain carbon steel for the core of the fixed

abrasive sawing wire is as follows

- At least 0.70 wt% of carbon, the upper limit being dependent on the other alloying elements forming the wire (see below)

- A manganese content between 0.30 to 0.70 wt%. Manganese adds - like carbon - to the strain hardening of the wire and also acts as a deoxidiser in the manufacturing of the steel. - A silicon content between 0.15 to 0.30 wt%. Silicon is used to deoxidise the steel during manufacturing. Like carbon it helps to increase the strain hardening of the steel.

- Presence of elements like aluminium, sulphur (below 0.03%),

phosphorous (below 0.30%) should be kept to a minimum.

- The remainder of the steel is iron and other elements

The presence of chromium (0.005 to 0.30%wt), vanadium (0.005 to 0.30%wt), nickel (0.05-0.30%wt), molybdenum (0.05-0.25%wt) and boron traces may improve the formability of the wire. Such alloying enables carbon contents of 0.90 to 1 .20%wt, resulting in tensile strengths that can be higher as 4000 MPa in drawn wires. The diameter of the intermediate wire, i.e. the wire prior to drawing to the final size, must be chosen large enough in order to obtain such a high tensile strength. The metallographic structure obtained is a fine, far-drawn pearlitic structure.

[0019] Preferred stainless steels contain a minimum of 12%Cr and a substantial amount of nickel. More preferred stainless steel compositions are austenitic stainless steels as these can easily be drawn to fine diameters. The more preferred compositions are those known in the art as AISI 302 (particularly the 'Heading Quality' HQ), AISI 301 , AISI 304 and AISI 314. 'AISI' is the abbreviation of 'American Iron and Steel Institute'.

[0020] As the main purpose of the 'indentation layer' is indeed to indent abrasive particles in it, it must be suitable for indentation of the particles. It follows that the whole of the indentation layer must deform under the indentation action of the individual abrasive particles. The invention particularly resides in the fact that the indentation layer is made of two layers: a first metal layer that is in contact with the steel core and a second metal layer covering said first metal layer. It is imperative to the invention that the second layer is harder than the first layer. Whether the second metal layer is harder than the first metal layer, can easily be assessed by means of a standard micro-Vickers hardness. Reference is made to ISO 6507-3 'Metallic Hardness Test: Vickers Test less than HV 0.2'. The first metal layer acts as a cushion that plastically deforms under the action of the abrasive particle. The second metal layer acts as a skin that prevents the abrasive particle of sinking too deep into the soft first metal layer. [0021 ] Of course the first layer of the indentation layer must be sufficiently thick to allow plastic deformation while the second layer of the indention layer should be sufficiently thin in order to allow deformation of the first layer. On the other hand, the indentation layer as a whole should not be too thick as otherwise the overall strength of the wire diminishes because a lot of cross sectional area is taken up by the indentation layer which is not as strong as the steel core. Furthermore the total thickness of the indentation layer stands in relation with the abrasive particle size. If the particles are much larger than the indentation layer, they will not be properly held by the indentation layer, while when they are too small with respect to the indentation layer, too much overall strength of the wire is lost. The thickness of the indentation layer must be more than 3.5% of the diameter of the steel core wire inclusive the indentation layer in order to be able to accommodate the abrasive particles. Preferably the thickness of the indentation layer is 7% of the diameter of the steel core wire inclusive the indentation layer. When the thickness of the indentation layer is more than 10% of the diameter of the steel core wire inclusive the indentation layer, the cross sectional area taken up by the indentation layer is already 36 % of the total cross sectional areas which will lead to an unacceptable loss in overall strength of the wire.

[0022] In the indentation layer about 0.5 micrometer up to maximum 3 micron is taken up by the second metal layer. Most preferred is between 1 to 2 micrometer. For thin wires (120 pm) the thickness is preferred to be between 0.5 to 1 pm. The thickness of the second metal layer does not scale with the size of the wire as it only serves as a skin to the soft first metal layer.

[0023] Suitable metals or alloys for the first metal layer are softer metals and

alloys like copper, zinc, brass, bronze, tin, lead, aluminium. More preferred are zinc and copper, while copper is the most preferred.

[0024] The interface between the steel core and the first metal layer can exhibit a certain degree of roughness and can even be interlocking. The advantage of such an interface is that the indentation layer better adheres to the steel core wire. With 'interlocking' is meant that certain protrusions of the first metal layer hook-in into corresponding recesses of the steel core wire. The degree of roughness - for the purpose of this application - is expressed in terms of the arithmetical mean deviation roughness 'R_a' as determined on a metallographical cross section. The average 'R_a' must be larger than 0.50 micrometer, even more preferred is if it is above 0.70 micrometer.

[0025] The average 'R_a' is determined by taking separate pictures of different segments of the perimeter of the wire and determining the roughness 'R_a' for every segment and then calculating the average. At least half of the perimeter of the cross section must be measured in different segments in order to obtain a good coverage over the whole perimeter. A magnification of 500 to 1000 times should be used.

[0026] Suitable metals or alloys for the second metal layer are copper, brass, nickel, nickel-phosphorous, iron, zinc-aluminium, copper-nickel, copper- beryllium, chromium, cobalt, molybdenum or tungsten. Alloys - like brass - are more preferred as an alloy is generally always harder than its constituting pure metals. Also preferred are nickel and iron, but most preferred is nickel. In any case the requirement remains that the metal or alloy of the first metal layer must be softer than the metal or metal alloy of the second metal layer.

[0027] The abrasive particles can be superabrasive particles such as diamond (natural or artificial, the latter being more preferred because of their lower cost and their grain friability), cubic boron nitride or mixtures thereof. For less demanding applications particles such as tungsten carbide (WC), silicon carbide (SiC), aluminium oxide (AI2O3) or silicon nitride (S13N4) can be used: although they are softer, they are considerably cheaper than diamond. But artificial diamond remains most preferred.

[0028] The size of the abrasive particles must be chosen in function of the

thickness of the indentation layer (or vice versa). Determining the size and shape of the particles themselves is a technical field in its own right. As the particles have not - and should not have - a spherical shape, for the purpose of this application reference will be made to the 'size' of the particles rather than their 'diameter' (as a diameter implies a spherical shape). The size of a particle is a linear measure (expressed in

micrometer) determined by any measuring method known in the field and is always somewhere in between the length of the line connecting the two points on the particle surface farthest away from each other (through the bulk of the particle) and the length of the line connecting the two points on the particle surface closest to one another (through the bulk of the particle).

[0029] The size of particles envisaged for the fixed abrasive sawing wire fall into the category of 'microgrits'. The size of microgrits can not longer be determined by standard sieving techniques which are customary for macrogrits. In stead they must be determined by other techniques such as laser diffraction, direct microscopy, electrical resistance or

photosedimentation. The standard ANSI B74.20-2004 goes into more detail on these methods. For the purpose of this application when reference is made to a particle size, the particle size as determined by the laser diffraction method (or 'Low Angle Laser Light Scattering' as it is also called) is meant. The output of such a procedure is a cumulative or differential particle size distribution with a median size d50 (i.e. half of the particles are smaller than this size and half of the particles are larger than this size).

[0030] Superabrasives are normally identified in size ranges by this standard rather than by sieve number. E.g. particle distributions in the 20-30 micron class have 90% of the particles between 20 micrometer (i.e. 'd₅') and 30 micrometer (i.e. ₉₅') and less than in 1 in 1000 over 40 microns while the median size d₅₀ must be between 25.0 +/- 2.5 micron.

[0031 ] As a rule of thumb, the median size (i.e. that size of particles where half of the diameters have a smaller size and the other half a larger size), should be smaller than 1/12th of the circumference of the steel core wire, more preferably should be smaller than 1/18th the circumference of the steel core wire in order to accommodate the particles well in the skin. At the other extreme the particles can not be too small as then the material removal rate (i.e. the amount of material abraded away per time unit) becomes too low.

[0032] The overall geometrical constraints of the wire diameter, indentation layer, and abrasive particle sizes are summarised in Table I. The particularly preferred sizes for the wire are as follows: Indentation layer (in pm)

Preferred

Diameter of steel (minimum|preferred|maximum)

Median particle core wire (in pm) First metal Second metal

size d₅₀ (in pm) layer layer

120 4.0|8.5|14.0 0.5 to 1 pm 8 to 20

175 5.6|12.2|20.0 1 to 2 pm 20 to 40

250 9.4|18.3|28.3 1 to 3 m 40 to 60

Table I

[0033] As to how many particles must be present at the surface of the sawing wire, much depends on the type of material to be cut. A too high density will induce too low forces on the particles which will polish the particles decreasing their cutting ability. On the other hand a too low density may lead to particles being torn out of the skin as the forces become too large or to a too low cutting rate as not enough particles pass the material per unit time. The presence of particles can be quantified by the ratio of the area occupied by the particles to the total circumferential area of the wire: the 'coverage ratio'. This can be done in a Scanning Electron Microscope by selecting the particles with a typical composition out of the general picture and calculating the occupied area by the particles relative to the total area. Only the centre part of the wire picture should be used as the sides tend to overestimate the particle surface due to the turning away of the wire surface.

[0034] The target coverage ratio for the particles is function of the material one intends to cut, the cutting speed one wants to reach or the surface finish one wants to obtain. The inventors have found that in order to have the best sawing performance for the materials envisaged the ratio of particle area over total area should be between 1 and 50%, or between 2 to 20% or even between 2 and 10%.

[0035] The binding layer serves to hold the abrasive particles in the indentation layer. Two options exist for the binding layer:

Either the binding layer can be metallic in nature. In that case one applies - usually by deposition out of an electrolytic bath - a metallic layer on top of the abrasive particles and the sheath. The binder layer must be a relatively hard metal as it is subject to wear and tear during sawing. By preference a metal or alloy out of the group comprising iron, nickel, nickel- phosphorous, chromium, cobalt, molybdenum, tungsten, copper and brass is chosen. Here also alloys can be used as binding layer metals as they tend to be harder than there constituents. Also nickel is a preferred metal for the binding layer with or without addition of phosphorous. Phosphorous makes the nickel-phosphorous layer more ductile and wear resistant.

[0036] Alternatively the binding layer can be an organic binding layer. The

organic binding layer can be a thermosetting - also called

thermohardening - organic polymer compound. Alternatively the binding layer can be a thermoplastic polymer compound. As thermosetting polymers - once cured - do not soften when the temperature gets higher during use they are more preferred for this kind of application. Preferred thermosetting polymers are phenol formaldehyde, melamine phenol formaldehyde or acrylic based resin or amino based resins like melamine formaldehyde, urea formaldehyde, benzoguanamine formaldehyde, glycoluril formaldehyde or epoxy resin or epoxy amine.

Less preferred - but nevertheless still usuable - are polyester resin or epoxy polyester or vinyl ester or alkyd based resins.

Preferred thermoplastic polymers are: acrylic, polyurethane, polyurethane acrylate, polyamide, polyimide, epoxy. Less preferred - but nevertheless still useable are vinyl ester, alkyd resins, silicon based resins,

polycarbonates, poly ethylene terephtalate, poly butylene terephtalate, poly ether ether ketone, vinyl chloride polymers

The list is non-exhaustive and other suitable polymers can be identified. The indentation layer as well as the particles can be treated with an organic primer in order to improve the adhesion between the polymer binding layer and the particle.

[0037] The inventive fixed abrasive sawing wire can be clearly discriminated from prior-art fixed abrasive sawing wires in that, on a cross section, radially below the indented abrasive particles the second metal layer that has been pushed into the softer first metal layer is present. This feature can be found back even if both the second metal layer and the binding layer are of identical material.

According a second aspect of the invention, a method to produce a fixed abrasive sawing wire is provided. The method comprises the steps of: - Providing a steel core wire. The steel of this steel core wire should have a composition according the lines stipulated in paragraphs [0018] and [0019] of this application. The tensile strength of the steel core wire is chosen such that after all steps of the method have been completed, the final fixed abrasive sawing wire must have a breaking load high enough to survive the sawing process. As the indentation layer and binder layer in general do not add much to the strength of the wire, the steel core wire must in practice be at least as strong as the final fixed abrasive sawing wire. This means that for steel core wires the following minimal breaking loads must be met (Table II):

Table II

- In a second step the steel core wire is covered with a first metal layer which results in a first intermediate wire. For example, and by preference, this is performed by means of electrolytically coating the wire with e.g. one out of copper, zinc, brass, bronze, tin, lead or aluminium. The thickness of the layers is as per Table I

- In a third step this first intermediate wire is coated with a second metal layer which results in a second intermediate wire. Again this is by preference done by electrolytically coating the wire with e.g. one out of copper, brass, nickel, iron, zinc-aluminium copper-nickel, copper- beryllium, chromium, cobalt, molybdenum or tungsten. The metal of the first metal layer must be softer than that of the second metal layer. The thickness of the layer is between 0.5 to 3 pm and scales with the diameter of the steel core wire (smaller diameter preferably have thinner second metal layers).

- In a fourth step abrasive particles of the preferred size (see Table I) are indented into the second intermediate wire. As the indentation sheet is relatively soft compared to the abrasive particles this goes relatively easy. Indentation can by way of example be performed by feeding the wire downwardly in between two sheaves wherein a groove is made that tightly fits the wire profile. The abrasive particles are fed between the groove and the wire at the necessary feed rate, while the sheaves push the particles into the indentation layer. In this way a third intermediate wire is obtained.

- Finally this third intermediate wire can be coated with a binding layer that either can be metallic or organic in nature. When the binding layer is metallic, it is by way of example applied by means of electrolytic deposition of the preferred binding layer metal that can be one out of copper, brass, nickel, nickel-phosphorus, iron, zinc-aluminium copper- nickel, copper-beryllium, chromium, cobalt, molybdenum or tungsten. Most preferred is nickel or nickel-phosphorous. For nickel-phosphorous the electroless deposition route is preferred.

Alternatively the binding layer can be organic. In that case the second or third intermediate wire can be treated with an adhesion promoter adapted to increase adhesion between the organic coating and the intermediate wire surface. Organic coatings can be applied by powder coating, extrusion, dipping followed by die wiping or any other appropriate technique. Depending on the type of coating it must be followed by a hardening step which is either thermal in nature or through ultraviolet or infrared radiation. Appropriate coating materials are one out of the group comprising phenol formaldehyde, melamine phenol formaldehyde or acrylic based resin or amino based resins like melamine formaldehyde, urea formaldehyde, benzoguanamine

The process is finished by winding the wire on an appropriate carrier such as a spool. [0039] The process steps can be performed separated from one another or some of the process steps can be combined. For example the steps of indentation and providing a binding layer can easily be combined.

[0040] In a further preferred embodiment of the method, the first intermediate wire is subjected to a drawing operation prior to being covered with the second metal layer. The coating with the first metal layer can thus be performed on a thicker steel core wire, but must naturally be scaled relative the thickness to steel core wire one wants to obtain in the end. Of course one must take drawing losses of the first layer into account (i.e. one has to make the coating a little bit thicker than needed). Drawing loss is the coating material loss that occurs during wire drawing. Also the strain hardening of the steel must be carefully assessed such that the final steel core wire has the appropriate tensile strength. As a bonus, as the first metal layer is made of a soft metal that will be softer than steel, a rough, interlocking interface will form during drawing, provided that the metal of the first metal layer do not interdiffuse with the steel of the coure. Such a rough interface helps to improve the adhesion between the first metal layer and the steel core. The other method steps are performed in the same way as before.

[0041 ] As a second preferred embodiment of the invention, the drawing operation is performed on the second intermediate wire. Again the same

considerations apply as in the first preferred embodiment: the thickness of the layers must be scaled, drawing losses must be taken into account as well as the strain hardening of the wire. Again the interface between steel wire core and first metal layer will be rough.

[0042] As a further bonus, the drawing conditions can be chosen such that the metal of the second metal layer diffuses into the metal of the first metal layer. Hence, the drawing conditions must be such that locally sufficient heat is generated during drawing for the diffusion to take place. The alloy layer then becomes the second metal layer. A particularly preferred couple of metals is therefore to have copper as a first layer on top of which zinc is deposited. The zinc helps to improve drawability while, during drawing, it alloys with the copper to form a brass second metal layer. Alternatively, in stead of zinc, tin can be used which results in a bronze second metal layer.

Brief Description of Figures in the Drawings

[0043] FIGURE 1 a shows an overall cross section of the inventive wire, with in FIGURE 1 b an enlarged part where an abrasive particle was indented.

[0044] FIGURE 2 shows comparative sawing results obtained with the inventive sawing wire and a reference wire.

Mode(s) for Carrying Out the Invention

[0045] According to a first example of the invention, a high carbon, chromium doped wire rod (nominal diameter 5.5 mm) with a nominal carbon content of 0.925 wt% with a composition in line with that of paragraph [0018] was chemically descaled according to the methods known in the art. The wire was dry drawn to 3.05 mm, patented and again dry drawn to an

intermediate diameter of 0.87 mm. This intermediate wire will become the steel core wire. A copper coating of about 341 gram per kilogram of steel wire was electroplated on this intermediate diameter, yielding an overall diameter of 0.99 mm.

[0046] In a wet wire drawing operation, this intermediate wire was sequentially drawn through successively smaller dies, till a total diameter (steel core wire and first metal layer) of 137 micron was obtained: the steel core wire has a diameter of about 1 19 micron, while the first metal layer has a thickness of about 9 micron. This is the first intermediate wire in the language of the method claims. The copper coating is the first metal layer in the language of the product claims. Part of this first intermediate wire was set aside for making the inventive wire, another part was used to make a conventional wire.

[0047] The conventional wire was made by indenting diamond particles into the copper layer by mechanically indenting diamond particles into it. The diamond particles had a median particle size of 10 pm. The particles were indented into the copper layer by guiding the wire through a tightly fitting set of grooved sheaves where in between diamond was fed. The particles were subsequently fixed by electrodepositing a binding layer of nickel onto it about 4 to 5 m thick in a reel-to-reel installation according the principles described in WO 2007/147818 of the current applicant. This is considered as the reference wire.

[0048] The inventive wire was first covered with a second metal layer before

indenting particles into it. A nickel layer - the second metal layer - was therefore deposited on the copper layer - the first metal layer - by means of electroplating, in the manner as known from WO 2007/147818. The nickel layer is harder than the copper layer. The copper layer showed a Vickers hardness of about 88 N/mm² (at a load of 0.098 N for 10 seconds). This nickel layer was about 1 to 2 m thick. Nickel coatings generally have a micro hardness in excess of 850 N/mm². This is the second intermediate wire in the language of the method claims.

[0049] The same type and size of abrasive particles were indented in exactly the same matter as for reference wire i.e. diamond particles with a median particle size of 10 pm. Again the particles were fixed by means of binding layer of nickel, with the difference that the nickel coating was somewhat reduced: about 4 pm thick.

[0050] The resulting wire had a remaining strength of 43 N. Its cross section is shown in FIGURE 1 . FIGURE 1 a shows a cross section 100 of the complete wire. The steel core wire 1 10 is clearly discernable, as well as the first layer of the indentation layer 1 12 i.e. the copper layer. As the second metal layer of the indentation layer and the binding layer are identical 1 14 they appear homogeneous when no indentations are present. However, when looking more in detail FIGURE 1 b, it is clearly visible that radially under the abrasive particle, a thin nickel layer is present (between the lines 120 and 122). Out of the deformation of the first metal layer, it can be deduced that the second metal layer must have been present prior to indentation of the particles.

[0051 ] The cross section also clearly shows a rough interface (indicated 124 in the detail) between steel core wire 1 10 and first metal (copper) layer 1 12. This rough interface is a consequence of the drawing of the copper clad wire. The average roughness of the interface R_a is about 1 .37 pm. It is believed that such a rough interface helps the adherence of the first layer to the steel core wire. [0052] Both reference and inventive wires were tested in a single wire saw machine of type RTS-480 obtained from DVVT. A block of mono-crystalline silicon of size 25x125 mm² was cut through from the smaller side i.e. the sawing wire contacts the silicon over a length of 25 mm. The machine was operated in a constant table speed mode of 4.5 mm/min at a wire tension of 12 N with an average wire speed of 450 m/min. The saw is reciprocal and about 180 m of wire passes the block on each half cycle. The bow height of the wire was monitored during sawing and is considered a measure for the loss of cutting ability. If the bow increases too fast during cutting, the wire looses its cutting ability quickly which may be due to a loss of diamonds (other reasons - such as diamond polishing or wire loading - can not be excluded per se). The best wire is the wire that shows a low bow increase during sawing.

[0053] FIGURE 2 shows the results of these tests. In abscissa the time of sawing is plotted and in ordinate the resulting bow during sawing. In the first five minutes of cutting the bow builds up. After that the bow stabilises and a constant working regime occurs. There is no difference in the first 5 minutes between the different wires which is an indication that their initial sawing performance does not differ greatly. In the stabilised regime, a difference is noticeable between the reference and the inventive fixed abrasive sawing wire in that the slope of the curves are different. When drawing a least square line through the points after the initial phase the following results are obtained (Table III):

Table III The end bow is the bow observed at the end of the cut. It is a measure for the cutting ability at the end of the cut: the higher the bow the lower the cutting ability.

It is the hypothesis of the inventor - although this hypothesis should not be used to limit the invention in any way - that due to the presence of the thin, second, harder metal layer prior to indentation of the particles, the particles are better held, as - during indentation - the second metal layer 'encases', 'envelopes' the particle when it is being pushed into the soft first metal layer. As a result the inventor believes that the particles are better held.

Claims

1 . A fixed abrasive sawing wire comprising a steel core wire, and an indentation layer covering said steel core, abrasive particles indented in said indentation layer and a binding layer covering said indentation layer and said abrasive particles

characterised in that

said indentation layer comprises a first metal layer covering said steel core and a second metal layer covering said first metal layer, wherein said first metal is softer than said second metal.

2. The fixed abrasive sawing wire according to claim 1 wherein said second metal layer has a thickness of between 0.5 and 3 pm.

3. The fixed abrasive sawing wire according any one of claims 1 to 2 wherein the first metal layer comprises a metal or metal alloy selected from the group of copper, zinc, brass, bronze, tin, lead, aluminium.

4. The fixed abrasive sawing wire according to claim 3 wherein between said first metal layer and said steel core wire a rough interface is discernable in a metallographic cross section in a plane perpendicular to said wire and wherein the arithmetical mean deviation roughness R_a of said rough interface is on the average higher than 0.50 micrometer.

5. The fixed abrasive sawing wire according to any one of claims 1 to 4 wherein said second metal layer comprises a metal or metal alloy selected from the group of copper, brass, nickel, iron, zinc-aluminium copper-nickel, copper- beryllium, chromium, cobalt, molybdenum, tungsten.

6. The fixed abrasive sawing wire according to any one of claims 1 to 5 wherein the material of said binding layer is a metal selected from the group of copper, brass, nickel, nickel-phosphorus, iron, zinc-aluminium copper-nickel, copper- beryllium, chromium, cobalt, molybdenum, tungsten.

7. The fixed abrasive sawing wire according to claim 6 wherein the material of said binding layer is the metal of said second metal layer.

8. The fixed abrasive sawing wire according to any one of claims 1 to 5 wherein the material of said binding layer is an organic binding material.

9. The fixed abrasive sawing wire according to claim 8, wherein said organic

binding layer comprises one out of the group comprising phenol formaldehyde, melamine phenol formaldehyde or acrylic based resin or amino based resins like melamine formaldehyde, urea formaldehyde, benzoguanamine

formaldehyde, glycoluril formaldehyde or epoxy resin or epoxy amine.

10. The fixed abrasive sawing wire according any one of the claims 1 to 9, wherein said abrasive particles are one out of the group comprising artificial diamond, natural diamond, cubic boron nitride, tungsten carbide, silicon carbide, aluminium oxide, silicon nitride.

1 1 . A method to produce a fixed abrasive sawing wire according to any one of claims 1 to 10 comprising the following steps:

- Providing a steel core wire;

- Covering said core wire with a layer of a first metal resulting in a first

intermediate wire;

- Covering said first intermediate wire with a second metal layer resulting in a second intermediate wire;

- Indenting abrasive particles into said second intermediate wire resulting in a third intermediate wire;

- Covering said third intermediate wire with a binding layer resulting in said fixed abrasive sawing wire;

characterised in that

said first metal layer is softer than said second metal layer.

12. The method according to claim 1 1 wherein said first intermediate wire is

subjected to a drawing operation prior to being covered with said second metal layer

13. The method according to claim 1 1 wherein said second intermediate wire is subjected to a drawing operation prior to being indented with abrasive particles.

14. The method according to claim 13 wherein, during drawing, an alloy layer

forms of the metals of the first and second metal layer, this alloy layer becoming the second metal layer.