WO2012055712A1

WO2012055712A1 - A sawing wire with abrasive particles electrodeposited onto a substrate wire

Info

Publication number: WO2012055712A1
Application number: PCT/EP2011/067985
Authority: WO
Inventors: Luc Hofman; Stefanie Vangheluwe
Original assignee: Nv Bekaert Sa
Priority date: 2010-10-29
Filing date: 2011-10-14
Publication date: 2012-05-03
Also published as: CN103189158A; TW201235135A; CN103189158B; TWI552820B

Abstract

The invention relates to a fixed abrasive sawing wire (100) of the type wherein the abrasive particles (130) are held onto the wire (100) by means of an electrolytically deposited binder layer (140). The substrate wire differs from prior-art substrate wires in that it comprises a plain carbon steel core wire (110) covered with a metal sheath (120). The steel core wire (110) ensures that the wire has sufficient strength. The metal sheath (120) is made of a metal with a conductivity that is at least three times, preferably eight times, higher than that of the plain carbon steel. The sheath (120) thickness is at least 2% of the diameter of the sheath covered steel core wire (110). These conditions ensure that at least half of the electrical current that goes through the substrate wire goes through the sheath (120) i. e. where it is needed in the electrodeposition process. Such a substrate wire reduces the need to have multiple electrical contact points during electrodeposition of the abrasive particles (130).

Description

A sawing wire with abrasive particles electrodeposited onto a substrate wire

Description

Technical Field

[0001 ] The invention relates to a fixed abrasive sawing wire suitable for sawing hard and brittle materials wherein the abrasive particles are attached on top of a substrate wire by means of an electrolytically applied binding layer.

Background Art

[0002] Currently the predominant technology for sawing hard and brittle materials like silicon, quartz, gallium arsenide, silicon carbide, sapphire, magnetic materials or any other expensive material with a cutting length below an arm's length is by means of a multi-loop single wire saw. In such a saw slurry comprising abrasive particles (mostly silicon carbide) suspended in a carrier liquid (usually poly ethylene glycol) is dragged by a round, thin, high tensile steel wire into the cut of the work piece. The wire is guided by grooved capstans in loops arranged parallel to one another thus forming a wire web. The work piece is cut due to the roll-stick action of the abrasive particles rolling between the wire and the work piece. The process is delicate as the slurry composition changes during use, expensive as both the wire and abrasive gets worn and must be replenished constantly, dirty as the slurry is spilled around by the capstans and has a high

environmental cost as the used slurry (with work piece material debris and steel parts in it) and wire must be discarded in a controlled way.

[0003] Therefore the industry is looking for solutions to dispense with the slurry preparation, slurry control and discarding. One of the solutions proposed is to eliminate the use of a carrier liquid for the abrasive particles and to fix the abrasive particles directly on the wire. Only a coolant is then still needed to wash away the debris that may collect on the wire. This results - in a better sawing action as the complete impulse of the abrasive

particle is transferred to cut in the work piece (no rolling of particles anymore), - in a purer residual products as the coolant mainly contains work piece debris and

- in a better use of consumables as the abrasives are better used and the wire wear is much less (as the abrasive is immobile relative to the wire).

Wire sawing inherently results in a loss of - sometimes precious - work piece material. There is therefore also a constant strive to keep the 'kerf loss' (the amount of work piece material lost as sawing debris) as low as possible. This can be achieved by keeping the diameter of the wire as low as possible. For loose abrasive sawing the standard is 120 pm while it has been shown that 80 pm wires can be used to cut e.g. silicon ingots. The 120 pm gauge wire results in a kerf loss of about 135 pm as also the abrasive takes some width in the cut.

[0004] Furthermore in wire sawing a tension must be present on the wire. The longitudinally applied tension is transferred to a transversal force in the plane of the cut as the wire is pushed against the work piece. This transversal force - in combination with the wire longitudinal movement - makes the wire cut the work piece. The wire therefore takes the shape of a bow during sawing. The higher the tension, the smaller the bow, the faster the cut. In practise this tension force is about 25 newton in the web.

Hence, the sawing wire must be able to at least sustain 1 .5 to 2 times this force as otherwise there is a risk of wire breakage. There is therefore a conflict between further reducing the kerf width, that scales with the diameter of the wire, and the strength of the wire that scales with the square of the diameter of the wire. This problem even becomes more prominent when fixing the abrasive particle to the wire in a fixed abrasive sawing wire as will be shown further.

[0005] Currently various techniques are explored in order to fix the abrasive

particles - that are usually diamond dust particles - to a carrier wire:

- Fixation can be done through a mechanical bond: by pressing diamond particles in a soft sheath high tensile wire such as e.g. described in application with application number EP2010/055678 of the current applicant. - A metallurgical bond has also been considered e.g. by brazing or soldering the particles to the surface of the wire like e.g. described in WO 99/46077. However, this is less preferred because the heat loading of the wire results in an unfavourable loss of strength of the wire. The heat loading becomes even more problematic with fine wires below 150 pm as the ratio of surface of the wire to volume of the wire becomes too high: the heat inflow increases, while the mass of the wire heated decreases, resulting in even higher heat loads.

- A resin bond such as e.g. described in US 6070570 has also been studied extensively. However, it turns out to be difficult to hold the particles in the resin during sawing.

- By electrolytic or electroless fixation of the diamond particles. This route has emerged from the superseded saw blade technology for cutting silicon ingots and is regularly used for other diamond cutting tools as well.

As in this application improvements are presented for the latter type of fixation technology it will be more discussed in detail. Basically there are two distinct ways to attach diamonds to a wire by electrochemical means: There is the electroless nickel (EN) process. In this process the wire is submerged in a bath of abrasive particles with an aqueous solution comprising diamonds and an electrolyte comprising metal ions (nickel), a reducing agent (e.g. sodium hypo phosphate), complexing agents (such as citric acid) and bath stabilisers. The bath is autocatalytic in that by submerging the wire into the bath a nickel coating will form on the wire. There is no current source needed to plate the wire (hence the term electroless). When now the diamond particles are covered with a conductive layer (preferably of nickel) the diamond particles will likewise be conformally covered with nickel, thereby fixing the diamond particles to the wire. The process has the drawbacks that it goes slow, the bath has a finite lifetime and needs stringent bath care. Also the process generates hydrogen at the surface which can be detrimental for the fatigue life of the steel. Usually the EN coating is enriched with phosphorous in order to make the coating harder. This hardening requires a heat treatment in excess of 400°C which is again not preferred on fine wires. [0007] US 2003/0140914 describes the use of an electroless bath to coat a steel wire of chromium-nickel steel with diamond particles with nickel. In order to prevent hydrogen embrittlement of the wire, the stainless steel wire was first coated with an intermediate layer (copper, nickel or brass) of thickness 1 to 10 pm in order to prevent the diffusion of hydrogen into the steel.

The use of stainless steels is problematic for strength: indeed it is more difficult to obtain a high tensile level as the austenitic or martensitic structure of stainless steels do not allow a high level of strain hardening. Also the oxide skin on a stainless steel wire will prevent the autocatalytic reaction to start on the surface which necessitates the pretreatment of the wire with e.g. a nickel strike.

When coating with an electroless bath, a batch process is normally used, an example of such a batch process being described in US 2004/0001922.

[0008] Next to that there is the electrolytic deposition of nickel and nickel coated abrasive particles. In this case an electrical current is made to flow between the wire substrate and the electrolyte bath which moves the cations (nickel ions) towards the substrate in the resulting electrical field in the bath. By bringing the diamond particles in electrical contact with the wire substrate, they are also covered with nickel and stick to the wire.

[0009] JP 2010 082773 describes a process wherein a plain carbon steel is first covered with a stainless steel coating in order to prevent the occurrence of hydrogen embrittlement. Thereafter diamond particles are fixed in the nickel coating by electrolytic deposition. The publication also mentions that one must limit the current density to below a threshold value in order to limit hydrogen evolution at the entry of the bath. Limiting the current density implies also increasing the bath length and/or reducing the coating speed which is not favoured.

[0010] During the electrolytic deposition of the nickel and nickel coated diamond, a current is sent through the substrate wire, and hence a contact must be established between the wire and the source. As the purpose is to make the sawing wire in long lengths (at least tens of kilometres) such an electrical contact must allow movement between a moving wire and the current source in order to allow for a reel-to-reel production process. While this can easily be established by means of a contact roll when the surface of the wire is smooth, this is not longer possible as soon as abrasive particles stick to the surface. Indeed the contact surface between roll and wire will greatly diminish due to the presence of the particles and local high current densities will emerge (sparks) with an irregular coating as result.

[001 1 ] There are some possibilities to provide a moveable electrical contact

between a current source and a moving wire with an irregular coating such as a fixed abrasive sawing wire. There is for example the possibility to use an electrolyte as an anodic contact to the wire. This is e.g. described in WO 2007/147818 of the current applicant. However, this contact has the drawback that part of the previously applied coating is dissolved again which would loosen the abrasive particles only partly attached to the wire.

[0012] Another solution is to use an electrical conducting fibre covered wheel such as described in JP 2004 082253. The fibres e.g. stainless steel fibres, are arranged like a brush on the contact wheel and ensure good electrical contact even in the presence of diamond particles. However, the drawback is that the fibres also tend to be coated with nickel which necessitates regular replacement.

[0013] There is therefore a need to overcome this 'electrical contact' problem

when producing fixed abrasive sawing wire according the electrolytic route.

Disclosure of Invention

[0014] The inventors have therefore come up with a solution wherein aspects of both the product and the process are combined in order to overcome the problems associated with this electrical contact problem. Moreover, the suggestion they make also overcomes the problems that emerge when trying to produce small gauge sawing wires in an electrolytic coating process. With 'small gauge' is meant: with a substrate wire that is thinner than 150 pm. Moreover, the solution they offer has some other side- advantages such as a sufficient breaking load, an increased production speed and an increased shielding from hydrogen.

[0015] According a first aspect of the invention, a fixed abrasive sawing wire is claimed comprising a plain carbon steel core wire having a diameter 'd' and a metal sheath covering the steel core wire. With diameter 'd' is meant the diameter of a circle having the same area as the steel area of a cross section of the wire through a plane perpendicular to the axis of the wire. The total diameter of the plain steel core inclusive the sheath cover is Ό'. The thickness of the sheath is thus (D-d)/2. Abrasive particles are fixed onto said metal sheath by means of a metallic binder layer. The difference with the prior-art is that the electrical conductivity of the metal of the metal sheath is at least three times higher than the electrical conductivity of the plain carbon steel of said core wire and that the diameter 'd' of the core wire is less than 96 percent of the diameter Ό' of said sheath covered steel core wire. The latter requirement can also be formulated in that the thickness of the sheath must be at least 2 percent of the overall diameter Ό'.

[0016] By preference the diameter of the plain carbon steel core wire is smaller than 150 pm, preferably smaller than 140 or even 130 pm for example 120 pm, although diameters as low as 100 pm are being explored now.

[0017] The ever decreasing diameter strive presents serious problems for small gauge wires with diameters less than 150 m as then the electrical resistance per meter increases strongly and becomes a limiting factor in the electrolytic coating process. This is illustrated in Table I where for a number of steel types the resistance per meter is calculated for two extreme wire sizes: 250 pm and 120 pm (no coating).

Table I The increased resistance per meter for small gauge wires results in a longitudinal voltage drop in the plating bath leading to an increased plating current density on the wire surface at the contact side of the bath but a decreasing current density further away. With 'contact side' is meant that side of the plating bath where the electrical current feed is and hence the current enters the bath. The local high current density leads to increased hydrogen evolution - due to depletion of the metal ions - at the entrance of the bath and a lack of coating further down the wire as the voltage drops exponentially.

The table makes also clear that wire resistance is not an issue for thicker wires.

[0018] The choice of a plain carbon steel core is to be preferred above that of a stainless steel wire as its conductivity is already markedly higher. Also plain carbon steels can be cold drawn to true elongations of 3 and higher which results in favourable tensile properties.

[0019] Practical plain carbon steel compositions do not only comprise iron and carbon but a lot of other alloy and trace elements, some of which have a profound influence on the properties of the steel in terms of strength, ductility, formability, corrosion resistance and so on. As for this application strength is of the essence, the following elemental composition is preferred for the plain carbon steel core wire:

- 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 0.03wt%),

phosphorous (below 0.30wt%), copper (below 0.60wt%) should be kept to a minimum.

- The remainder of the steel is iron and other elements With this kind of plain carbon steels, tensile strengths in excess of 3500 N/mm² are easily reached, while - with a proper selection of material and processing - strengths in excess of 3750 N/mm² and even 3900 N/mm² can be obtained. These levels are obtained on the steel core wire (not taking account of the strength of the sheath).

[0020] 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 reduce the formation of grain boundary cementite for carbon contents above the eutectoid composition (0.80%wt C) and thereby 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 on steel core wire level. Such steels are also preferred and are presented in US 2005/0087270.

[0021 ] In general the tensile strength TS' of the steel core wire - expressed in N/mm² - must be above:

TS > 4700 - 7.4 x d wherein 'd' is the diameter of the core wire expressed in micrometer.

[0022] However, even the use of the more conductive plain carbon steel core wires does not suffice to eliminate the resistance problem as experienced by the inventors.

[0023] The presence of a highly conductive metal sheath of a certain thickness at the circumference of the steel core wire enables a decreased electrical resistance per meter of the wire. As most of the cross sectional area of a round wire is just below its circumference, it is best to make the sheath the more conductive part. In addition it is on the sheath that the current is consumed by the metal ions in the bath, not in the core. The fraction 'B_COre' of the total electrical current that goes through the core is then given by the formula: wherein a=d/D and

wherein Ysheath and Y_COre are the

conductivities (in Siemens per metre, S/m) of sheath and core. The fraction of electrical current that goes through the sheath is then B_Sheath=1 -

Bcore-

[0024] In any case the fraction of electrical current that goes through the sheath must be higher than the fraction that goes through the core i.e. B_COre must be smaller than 0.5 in order to have most current there where it is most needed: at the surface of the wire. This can be achieved by imposing two restrictions:

- The higher the conductivity of the sheath metal is, the higher the

fraction of the current through the sheath will be and the thinner the sheath can be. By preference the conductivity of the sheath metal must at least be three times that of the steel core wire i.e. A > 3. In that case the core can be larger than 0.866xD i.e. the core still has sufficient strength. Suitable sheath metals are then silver (conductivity is 62.1 MS/m, mega Siemens per meter), copper (58.8 MS/m), gold (41 .7 MS/m), aluminium (37.0 MS/m), cobalt (17.2 MS/m), zinc (16.8 MS/m),. Alloys are less preferred as alloys generally have a lower conductivity than their constituting metals.

- Even more preferred is if the conductivity of the sheath metal is at least eight times that of the steel core wire i.e. A > 8. Suitable metals that remain are only copper and silver of which copper is more preferred due to its availability and relatively low cost. For copper the optimal diameter of the core is between 0.88 to 0.95 times the diameter Ό' of the sheathed steel core wire which is equivalent to a sheath diameter between 2.5 and 6 % of the total sheathed steel core wire.

- The diameter of the core can not be larger than 96 percent of Ό' the diameter of the sheath covered steel core wire (i.e. Ό' inclusive the outer sheath). A larger core can not lead to a further decrease in resistance per meter.

[0025] An additional advantage of the wire is that a closed metallic sheath

prevents hydrogen embrittlement. Hydrogen embrittlement is a particularly severe problem for plain high carbon steel, high tensile wires. [0026] A further advantage of the reduced resistance per meter of the wire is that less resistance heat is generated in the wire per unit length. This heat can be problematic in the process as a small temperature increase on a long line can already result in a length increase that makes the wire slack between different process steps.

[0027] 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 sheath layer better adheres to the steel core wire. With 'interlocking' is meant that certain protrusions of the metal sheath 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.

[0028] 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.

[0029] The abrasive particles can be superabrasive particles such as diamond (natural or artificial, the latter being somewhat 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. Most preferred is artificial diamond.

[0030] The abrasive particles are at least partially coated with a conductive

coating as otherwise they will not be electronically covered with the metallic binder layer. Exemplary coatings are titanium, titanium carbide, silicon, zirconium, palladium, tungsten, tungsten carbide, chromium, iron, copper or nickel coatings. Copper and nickel coatings are most preferred as they are readily available. The metal coating weight ratio should be less than 50% of the total weight (diamonds and coating), preferably less than 30%.

[0031 ] The binder layer is important as it is this layer that will attach the abrasive particles to the sheathed steel core wire. The abrasive particles are attached onto the metal sheath by means of electrolytic deposition. The methodology by which abrasive particles are indented in the sheath layer is herewith explicitly not considered. Hence the sheath layer of the inventive sawing wire is substantially free of indentations by the abrasive particles, i.e. the abrasive particles are only retained by means of the metallic binder layer and not by means of the sheath layer.

[0032] Preferred metals for the metal in the metallic binder layer are iron, nickel, chromium, cobalt, molybdenum, tungsten, copper, zinc, tin, and alloys thereof. By far the most preferred is nickel as the other metals imply either environmental or health restrictions or are not compatible in their use with the work piece material such as e.g. semiconductor materials notably silicon. The layer according the invention is solely applied by means of an electrolytic coating process (not an electroless coating process).

Consequently the binder layer shows a crystal structure with grains.

Electroless nickel shows an amorphous structure.

[0033] The thickness of the metallic binder layer is between 2 to 10 micrometer preferably between 3 to 6 micrometer with a preferred thickness of about 4 micrometer.

[0034] The size of the abrasive particles must be chosen in function of the

thickness of the metallic binder 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).

[0035] 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. Instead 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).

[0036] 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.

[0037] The abrasive particles median size d₅₀ is by preference between 1 and 5 times the thickness of the metallic binder layer. Smaller particles disappear into the coating, larger particles can not be adequately held by the binding layer. More preferred is if the median particle size is between 1 and 3 times the metallic binder layer thickness, most preferred is if the particles are 1 to 2 times the metallic binder layer thickness.

[0038] The amount of abrasive particles present depends on the material of the work piece to be cut. On the one hand the density should not be too high in order to prevent loading of the sawing wire, on the other hand it should not be too low as otherwise the wire does not cut. In general an area coverage of between 1 and 50%, or between 2 to 20% or even between 2 and 10% is preferred. In this way sufficient cutting performance can be obtained while the loading of the wire during use does not occur. The area coverage can be estimated by visual inspection. [0039] According a second aspect of the invention a method is offered to produce the fixed abrasive sawing wire according the invention. The method comprises the steps of:

a. Selecting a plain carbon steel core wire of diameter 'd'. By preference this wire has a diameter of less than 150 pm and has a composition in accordance with that of paragraphs [0019] or [0020]. The tensile strength in N/mm² of the steel core wire must be above 4700 - 7.4xd ('d' in pm).

b. The steel core is coated with a coating metal that has an electrical conductivity that is at least three times higher than the electrical conductivity of the plain carbon steel of the steel core wire. The coating is thickened until the steel core wire inclusive the coating has an overall diameter Ό' of at least d/0.96.

c. The sheathed steel core wire is electrolytically coated with a metallic binder layer by guiding the wire through an electrolyte bath comprising binder metal ion species and abrasive particles. This step is performed in a continuous process wherein the sheathed steel core wire is unwound from a pay-off reel, coated with the binder layer and the abrasive particles and wound onto a take-up spool. The contact point is with the sheathed wire, and not with the abrasive coated wire - and all current is supplied at that contact point, trough the wire and through the bath. The contact point is therefore only at the wire entrance side of the bath.

[0040] In step 'a' possible plain carbon steel core wires are for example sawing wires as they are used for loose abrasive sawing.

[0041 ] In step 'b' the coating can be laid down electrolytically from an electrolytic bath that contains the preferred metal species as mentioned in [0024] in the required thickness.

[0042] In step 'c' the abrasive particles are preferably at least partly coated with a conductive layer in order to have a good binder metal growth on the abrasive particles. Preferably the abrasive particles are brought in close contact with the wire by guiding the wire through a mixture of abrasive particles and binder metal containing electrolyte. [0043] In a further preferred embodiment, the step 'c' is followed by further thickening the metallic binder layer by electrolytic coating in an electrolyte bath comprising the metal binder metal by drawing electrical current from the same electrical contact point as in step 'c'. By preference the electrolyte bath is the same is for step 'c' except that no abrasive particles are present in the bath.

[0044] In another preferred embodiment of the process, a plain carbon steel wire is selected of diameter 'd₀' and subsequently coated with a metal sheath until an overall wire diameter of Ό₀' is obtained. Ό₀' is at least d₀/0.96. The metal sheath is of the preferred metal species as mentioned in [0024]. This wire is now reduced in diameter by drawing it through subsequently smaller dies until a final diameter D is obtained. The method is preferred because:

- Either it allows the use of sheath metal strips to cover the steel core wire at large diameter (a method that can not be applied on wires with diameter below 150 pm), which is more economical:

- Or, it allows higher current densities to electrolytically put down the sheath metal coating on the thicker wire. Indeed for the thicker wire, the resistance per meter of the wire is low enough in order not to provoke problems.

In addition the interface between the steel core wire and the metal sheath of the drawn wire shows a rough interface on a metallograpic cross section. Such rough interface is beneficial to improve the adhesion between the steel core wire and the metal sheath.

Brief Description of Figures in the Drawings

[0045] FIGURE 1 depicts an embodiment according the inventive product.

[0046] FIGURE 2 shows an installation whereon the inventive product can be made.

Mode(s) for Carrying Out the Invention

[0047] FIGURE 1 schematically shows a cross section of the fixed abrasive

sawing wire 100 according the invention. There is the plain carbon steel core wire 1 10 which has a diameter 'd' and the metal sheath 120 that covers the steel core wire. The sheath covered steel core wire has a diameter Ό'. Abrasive particles 130, 130', 130" are attached onto the metal sheath by means of a metallic binder layer 140. When the interface between the steel core and the sheath is rough - as depicted here - one must take the average diameter of the steel core wire for 'd'.

[0048] In a first series of tests, a plain carbon steel wire rod (nominal diameter 5.5) was used with a high carbon steel with a nominal carbon content of 0.925 wt% and a composition in line with that of paragraph [0020]. The wire was descaled, dry drawn to 3.05 mm, patented and further dry drawn to a diameter of 0.89 mm again followed by patenting. The material was split into two parts. The first part was sequentially plated with copper and zinc followed by diffusion. The coating amount was about 5 to 6 g/kg and the copper content of the coating was 67%Cu. This wire was further drawn on a wet wire drawing bench to 120 pm wire diameter in total. The brass coating then has a thickness of about 0.15 pm. Brass has a conductivity of about 13 MS/m. The breaking load of the drawn wire was 45.3 N.

[0049] This wire was used as the metal sheathed core wire for electrolytic coating with abrasive particles on a reel-to-reel installation 200 as shown in FIGURE 2. The installation has a pay-off stand for unwinding a feed spool 210 of sheathed core wire, a series of run-over trays I, II, III, IV wherein separate electrolyte solutions are continuously circulated and a take-up unit with a take-up spool 260. Each of the run-over trays has an anode 242, 242', 242", 242"' that is connected to respective controlled current sources 240, 240', 240", 240"'. All sources are connected to a common conductor 250 that connects to the single, electrical contact roll 230. No other contact rolls were present. It will be clear from the electric schema that all current 'i_tot' from each of the coating baths , i₂, 13 and i₄ has to pass the wire 220 i.e. i_tot = i-i + 12 + 13 + -

[0050] The first tray I contains a mixture of nickel coated diamonds and

electrolyte over the immersion length of 70 cm. The diamonds have a median size of 9 pm with a range from 6 to 12 pm (5 % and 95% limits) and are nickel coated (30 % of nickel on the total weight). The electrolyte that flows through tray I has the following composition, acidity and temperature:

Nikkelsulfamate electrolyte Amount (unit)

Ni sulfamate

(Ni(S0₃NH₂)₂-4H₂0) 440 g/l

NiCI₂-6H₂0 20 g/l

PH 3,2 - 3,80

Temperature 45°C

Baths II, III, IV are of identical make and have an immersion length of 25 cm. They serve to 'thicken-up' the nickel binder layer up to a total thickness of 4 pm of nickel and do not contain abrasive particles.

[0051 ] The first bath was held at a current density of 15 A/dm² at a voltage of 10.7 volt. The current density in the other baths was held at 20 A/dm² with a voltage from 10V up to 19V for the last tray. The total current i_tot sent through the wire was 0.96 ampere resulting in a heat dissipation of 14.7 watt per meter. Hydrogen evolution was noticeable at the entrance of the bath and the abrasive particles were poorly held by the coating. The wire was also brittle during handling which is an indication for hydrogen embrittlement. No sawing test could be performed with it. The breaking load dropped to 41 .4 N (8.6% drop).

[0052] When now calculating the fraction of the current that goes through the core and through the sheath, one obtains that only 1 .4 % of all current goes through the sheath. The ratio of conductivities 'A' is only 2.9. The wire has a resistance per meter of 15.9 Ω/m which is about that of a plain carbon steel wire without coating. The inventors noticed that the current was not flowing where it should be flowing.

[0053] In a subsequent test the part of 0.89 patented wire that was set aside was coated with an electrolytic copper coating of about 370 g/kg of pure copper. Pure copper has as conductivity of 59 MS/m. This wire was subsequently wet wire drawn to a total diameter of 137.6 pm. The steel core diameter was 123 pm resulting in a coating thickness of 7.3 pm. The breaking load was 48.2 N which results in a tensile strength of sheathed core wire of 3313 N/mm². The steel core wire tensile strength was estimated to be about 4200 N/mm². The interface clearly showed a degree of roughness and even interlocking. The resistance per meter was 4.1 Ω/m.

[0054] Exactly the same coating process as for the first sample was performed on this sheathed core wire. Again, the first bath was held at a current density of 15 A/dm² but now only a voltage of 4.7 volt was needed. The current density in the other baths was held at 20 A/dm² with a voltage that was at the most 7V. The total current i_tot sent through the wire was 1 .10 ampere resulting in a heat dissipation of 4.8 watt per meter. No hydrogen evolution was noticeable at the entrance of the bath and the abrasive particles were well held by the coating. The final strength of the wire was 48.1 N in a tensile test (a negligible loss of 0.2%).

[0055] The conductivity of the copper is 10.6 times the conductivity of the plain carbon steel core. The diameter ratio of steel core wire diameter to sheathed steel core wire diameter is 89%. The fraction of the current that goes through the sheath is 73% i.e. larger than 50%. Only 27 % goes through the steel core wire. The inventors concluded that most of the current is running where it is needed: at the surface.

[0056] Validation sawing tests were performed with this wire on a DWT RTS-480 single wire, reel-to-reel saw. A 25x125 mm² block of mono crystalline silicon was sawn from the shorter side of the block i.e. the contact length between wire and block is 25 mm. The wire was reciprocated at an average wire speed of 450 m/min over a length of 180 m, the wire tension was held constant at 12 N and the sawing speed was 4.5 mm/min. The bow formed during the cut was measured as a function of time. A large bow implies low cutting ability, a small bow high cutting ability. By repeated sawing with the same wire, the wear of the wire can be assessed. The inventive wire showed at the end of the first cut a bow of 8 mm, at the end of the second cut 1 1 mm, and at the end of the third cut 16 mm.

[0057] In the same manner (with the same raw materials) samples were prepared with a different copper coating thickness:

- A sample with a 1 pm thick coating having a total diameter of 122 pm.

Before plating, the wire had a breaking load of 44 N. The ratio of diameters is 0.984, and the current fraction through the core is 74% i.e. only 26% of the current goes through the coating. The resistance per meter is 1 1 .7 Ω/m. During coating hydrogen evolution was observed. The breaking load after coating dropped to 42.6 N (a loss of 3.2%). - A sample with a 4 pm thick coating having a total diameter of 127 pm.

Before plating, the wire had a breaking load of 46.1 N. The ratio of diameters is 0.937, and the current fraction through the core is 40% i.e. 60% of the current goes through the coating. The resistance per meter is 6.5 Ω/m. During coating no hydrogen evolution was observed. After coating the wire showed a breaking load of 47.6 N (a gain of 3 %)). The results illustrate that a minimum sheath thickness with a minimum conductivity for the sheath metal is required in order to be able to perform the electrolytic coating with abrasive particles in a good manner on fine wires.

Claims

1 . A fixed abrasive sawing wire comprising

a plain carbon steel core wire having a diameter 'd';

a metal sheath covering said steel core wire,

the sheath covered steel core wire having a diameter Ό', and

abrasive particles fixed onto said metal sheath by means of a metallic binder layer,

characterised in that

the electrical conductivity of the metal of said metal sheath is at least three times higher than the electrical conductivity of said plain carbon steel of said core wire and

said diameter 'd' of said core wire is less than 96 percent of said diameter Ό' of said sheath covered steel core wire.

2. The fixed abrasive sawing wire according to claim 1 wherein said electrical conductivity of said metal of said metal sheath is at least eight times higher than the electrical conductivity of said plain carbon steel of said core wire.

3. The fixed abrasive sawing wire according to claim 1 or 2 wherein said metal sheath is one out of the group comprising copper, silver, aluminium, gold, zinc, cobalt.

4. The fixed abrasive sawing wire according to claim 3 wherein between said

metal sheath and said 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 pm.

5. The fixed abrasive sawing wire according to any one of claims 1 to 4 wherein said plain carbon steel comprises at least 0.70 wt% of carbon, a manganese content between 0.30 to 0.70 wt%, a silicon content between 0.15 to 0.30 wt%, an aluminium content of less than 0.03 wt%, a sulphur content of less than 0.03 wt%, a phosphorous content of less than 0.30 wt%, a copper content of less than 0.60 wt%, the remainder being iron.

6. The fixed abrasive sawing wire according to claim 5 wherein said plain carbon steel further comprises a chromium content between 0.005 wt% to 0.30 %wt, a vanadium content between 0.005 wt% to 0.30 wt%, a nickel content between 0.05 wt% to 0.30 wt%, a molybdenum content between 0.05 wt% to 0.25 wt%.

7. The fixed abrasive sawing wire according to any one of claims 1 to 6 wherein said plain carbon steel core wire diameter 'd' is less than 150 micron.

8. The fixed abrasive sawing wire according to any one of claims 1 to 7 wherein said plain carbon steel core wire has a tensile strength of at least 3500 N/mm²

9. The fixed abrasive sawing wire according to any one of claims 1 to 8 wherein said abrasive particles are selected out of the group comprising diamond, cubic boron nitride, silicon carbide, aluminium oxide, silicon nitride, tungsten carbide or mixtures thereof.

10. The fixed abrasive sawing wire according to claim 9 wherein said abrasive particles are coated with a coating out of the group comprising titanium, titanium carbide, silicon, zirconium, palladium, tungsten, tungsten carbide, chromium, iron, copper or nickel.

1 1 . The fixed abrasive sawing wire according to any one of claims 1 to 10 wherein the metal in said metallic binder layer is one out of the group comprising iron, nickel, chromium, cobalt, molybdenum, tungsten, copper, zinc, tin, and alloys thereof.

12. The fixed abrasive sawing wire according to any one of claims 1 to 1 1 wherein the thickness of said metallic binder layer is between 2 and 10 micrometer.

13. The fixed abrasive sawing wire according to claim 12 wherein the median size of said abrasive particles is between 1 and 5 times the thickness of said metallic binder layer. .

14. A method to produce a fixed abrasive sawing wire according to any one of the claims 1 to 13 comprising the steps of:

- selecting a plain carbon steel core wire of diameter 'd';

- coating said steel core wire with a metal sheath until a diameter Ό' is obtained, said diameter D being larger than d/0.95, the metal of said metal sheath having an electrical conductivity that is at least three times higher than the electrical conductivity of said plain carbon steel;

- fixing abrasive particles in a metallic binder layer onto said metal

sheath by feeding an electrical current at a contact point through said sheathed wire and an electrolyte bath comprising the binder metal and abrasive particles in a continuous reel-to-reel process;

15. The method of claim 14 followed by further electrolytically thickening said metallic binder layer in an electrolyte bath comprising the binder metal by applying electrical current through said same contact point.

16. The method of claim 14 wherein after the step of coating said steel core wire with a metal sheath, a drawing step is introduced to reduce the diameter of the sheathed steel core wire to a finer diameter prior to coating the wire with said metallic binder layer comprising abrasive particles.