WO2014118003A1

WO2014118003A1 - A fixed abrasive sawing wire with nickel oxide interfaces between nickel sub-layers

Info

Publication number: WO2014118003A1
Application number: PCT/EP2014/050715
Authority: WO
Inventors: Steven LANGEROCK; Stefanie Vangheluwe
Original assignee: Nv Bekaert Sa
Priority date: 2013-01-30
Filing date: 2014-01-15
Publication date: 2014-08-07
Also published as: CN104955601B; CN104955601A

Abstract

An inventive fixed abrasive sawing wire is described comprising a metallic core wire whereon abrasive particles are attached in an electroplated nickel layer. The electroplated nickel layer comprises two or more stacked sub-layers of which in between at least one pair of consecutive sub-layers an interface containing nickel oxide is present. A nickel oxide layer acts as a preferred fracture plane during sawing. As a result the abrasive particles are more easily uncovered during first use than in conventional fixed abrasive sawing wires of this type. This without leading to an increased overall wear of the wire. The inventive fixed abrasive sawing wire must therefore not be dressed prior to use. Furthermore, different exemplary methods are described to introduce a nickel oxide layer in between the nickel layers.

Description

A fixed abrasive sawing wire with nickel oxide interfaces between nickel sub-layers

Description

Technical Field

[1 ] The invention relates to a fixed abrasive sawing wire that can be used for cutting hard and brittle materials wherein the sawing wire has abrasive particles attached to a metallic substrate wire by means of an

electrolytically deposed nickel layer.

Background Art

[2] The predominant technology for cutting hard and brittle materials like

sapphire, silicon, quartz, silicon carbide, gallium arsenide, magnetic materials or other expensive materials has long been the use of internal diameter saw blades. In such saw the item to be cut was introduced in the central hole of a rotating, tensioned saw blade. The inner rim of the saw blade was coated with a diamond nickel composite. The technology has disadvantages in terms of kerf loss (the loss in material due to the width of the cut) and throughput (as it concerns a serial operation).

[3] A disrupting technology emerged with the development of multi-loop single wire saws generally known under the misnomer "multiwire saw". In such a "multiwire saw" a single, very fine steel wire is guided over a set of two or more grooved capstans in loops arranged parallel to one another thereby forming a wire web. The sawing action can either be obtained by the use of an abrasive slurry (usually silicon carbide suspended in a viscous medium like polyethylene glycol, loose abrasive sawing) or by the attachment of abrasives to the wire itself (fixed abrasive sawing). The wire is driven by the capstans either in a back-and-forth mode or a single direction movement. Multiwire saws allow for a very thin kerf (lower than 150 μιτι) and parallel processing of a single ingot (into sometimes 1000 plus wafers per load) leading to an increased throughput and large savings in material cost.

[4] While the use of an abrasive slurry is now widespread for slicing silicon wafers for use in the solar cell industry or the semiconductor industry, it does bring some pertinent drawbacks with it in terms of slurry

management leading to increased environmental cost (the slurry must be discarded in a controlled way), process control cost (as the slurry gets loaded with silicon and iron from the steel wire it must be renewed constantly) and consumable cost (the abrasive and the wire wear in the process).

[5] The use of a 'fixed abrasive sawing wire' wherein the abrasive is fixed to a substrate wire has these disadvantages to a lesser extent in that only a coolant must be used to flush away the cutting swarf. The coolant predominantly contains the material that has been cut which can therefore easily be recycled. However, the requirements to the wire itself are more stringent:

- The abrasive particles must be well attached to the substrate wire as the sawing action diminishes when abrasive is lost in action;

- The selection of abrasive particles is important in terms of size, shape, hardness and sharpness;

- The wire must not only be strong enough to sustain the cutting tension (usually about 25N) but must also perform dynamically well as it enters into the workpiece about hundred times more than with loose abrasive sawing.

[6] Different ways for attaching the abrasive particles to the substrate wire have been explored of which the most prominent ones are attachment of abrasives by means of a bonding or soldering metal, by means of a resin, by means of mechanical indentation or by means of incorporation in an electrolytic metallic holding layer. Although the winning technology is not clear yet, the latter method is certainly a good candidate. Abrasives are held in an electrolytic nickel coating that offers good abrasive retention as well as good wear resistance. Pertinent patent applications describing this technology are US7704127, EP1886753 and, EP2277660.

[7] Predominantly manmade diamond is used for the abrasive particles. In order to hold the diamond particles very well, the diamond particles are plated with a semiconducting (such titanium carbide TiC, silicon carbide SiC cfr. US 7704127) or conducting layer (nickel, nickel-phosphorous Ni-P cfr EP2277660). In this way also the diamond particles are overcoated with a metallic layer thereby being further held in the coating.

[8] However, this overcoating of the diamond particles with nickel must first be removed from the diamond particle before it can actually start cutting. Starting the sawing process while the overcoat is still present leads to an initially increased saw bow. After the nickel overcoat has been removed from the tips of the diamond particles, the particles can start to cut efficiently. One sometimes refers to this as the 'dressing of the wire' i.e. the accommodation of the wire for sawing.

[9] Different ideas have been suggested to prepare the wire 'off-line' for

service such as leading the wire through a 'dressing stone' (JP 2006 181 701 ), or through 'inverse electrolysis' (JP 201 1042561 ). Alternatively, a specific crossed-threading of the wire on the sawing machine have been suggested so that the wire 'self-dresses' prior to sawing: see JP 201 1 079073. However either these procedures involve a separate treatment of the wire, an adaptation of the machine or simply do not work.

Disclosure of Invention

[10] The primary object of the invention is to improve on the existing fixed

abrasive sawing wire. Primarily the invention is concerned by making the dressing step easier. An improved product and a method to make this product is described.

[1 1 ] According a first aspect of the invention a fixed abrasive sawing wire is claimed. The inventive product relates to the combination of features as described in claim 1 . Specific features for preferred embodiments of the invention are set in the dependent claims.

[12] In a first preferred embodiment of the invention, the fixed abrasive sawing wire comprises a metallic core wire and abrasive particles attached thereto in an electroplated nickel layer. The electroplated nickel layer comprises two or more stacked sub-layers of nickel. In between at least one pair of consecutive sub-layers an interface containing nickel oxide is present. Alternatively but equivalently formulated: the electroplated nickel layer comprises at least one internal interface layer containing nickel oxide. [13] As metallic core wire a round wire of size 50 μιτι up to 300 μιτι can be used. Typically the wire diameter will be chosen in function of the material to be cut. For example for cutting mono-crystalline silicon into wafers, small sized diameters such as from 80 to 150 μιτι are favoured. For cutting sapphire, larger sizes of 150 μιτι to 250μηη for example 175 μιτι are preferred. For ingot cutting (cropping e.g.) even larger diameters are used: more than 250 μιτι for example 300 μιτι.

[14] The core wire can be made of any metal having sufficient tensile strength.

Particularly preferred are metals that also display a relatively high electrical conductivity as in the coating process the electrolytic coating current goes through the core wire (plus any possibly already deposited layers). Particularly preferred in this respect are plain carbon steels with at least 0.70 wt% of carbon as they combine a good tensile strength (above 3000 MPa) with a relatively good electrical conductance (above 4.10⁶ S/m). With increasing carbon content the conductivity decreases while the strength increases. Copper wires - although having good electrical conductivity - barely can achieve tensile strengths of above 1000 MPa. Tungsten wire - that can resist heat very well - can be drawn to high tensile strength but then lack the sufficiently high conductivity.

[15] 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% ;

- A silicon content between 0.15 to 0.30 wt%;

- 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 [16] 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.

[17] 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 is expressed in micro meter.

[18] The metallic core wire can also be coated with a suitable metallic coating making it more adapted for its purpose. For example the wire can be coated with an electrically well conducting coating made from copper, silver, zinc or cobalt or any alloy of these metals. When sufficiently thick - for example 2% of the total diameter of the wire - this coating helps to reduce the in-line resistivity of the metallic core wire. See WO 2012 2055712. Also the wire can be coated with a metallic coating of copper, zinc or tin or alloys thereof such as brass or bronze to improve the processability of the wire. In this respect brass coated plain carbon steel wires as used for loose abrasive sawing wire have been found to be well suited.

[19] To this metallic core wire abrasive particles are attached by means of an electroplated nickel layer. The electroplated nickel layer may comprise traces of other elements such as sulphur, phosphorous, boron, cobalt, iron, copper, zinc, or other elements that may contaminate the layer during formation (for example due to their presence in the electrolyte) apart from the intentional presence of oxides.

[20] The electroplated nickel layer is build up out of at least two sub-layers stacked upon one another. This is due to the way the layers are grown: one on top of the other. The layers are discernible in a metallographic cross section after etching

• first with a solution of 38 ml nitric acid 65% (HNO3), 100 ml of glacial acetic acid (CH3COOH) and 10 ml of distilled water for about 10 seconds;

• and then in a solution of 50 ml nitric acid 65% (HNO3) in 50 ml glacial acetic acid (CH3COOH) for another 5 seconds.

[21 ] Characteristic about the present inventive sawing wire is now that in

between at least one pair of consecutive sub-layers, an interface

containing nickel oxide is present.

[22] Prior art fixed abrasive sawing wires have a consolidated nickel layer on top of the abrasive particle without nickel oxide layers in between. This implies that before the abrasive particle is available for sawing the nickel layer (nickel is on itself quite hard and wear resistant) on top of the abrasive particle must be removed i.e. the wire must 'dressed' which leads to increased handling and/or loss of sawing efficiency.

[23] The inventors found that the presence of the nickel oxide interfaces brings some unexpected advantages with it. The nickel oxide interfaces acts as a preferred fracture plane during sawing. As the contact pressure is highest at the top of the abrasive particle due to their small radius of curvature, the nickel sub-layers are more easily worn there due to the presence of the nickel oxide interface layer compared to the existing fixed abrasive sawing wire. Although the wear resistance of the electroplated nickel layer in between the particles is also diminished this does not lead to an increased wear of the wire as a whole. Indeed the radius of curvature in between the abrasive particles - which is about the radius of the wire itself - is much higher and hence the contact pressure is lower there. Also the layer in between the abrasive particles is less contacted by the work piece as the wire bears on the abrasive particles.

[24] The thickness of the interface layer containing nickel oxide is thinner than 50 nm, preferably lower than 20 nm or even better less than 10 nm. Too thick interface layers will result in nickel sub-layers that exfoliate when twisted or bend. In order to be discernible the nickel oxide interface must at least be 0.5 nm. Already at this thickness the desired effect of easier wear at the tops of the abrasive particles is expected.

[25] The presence of oxygen in the interface layer can best be deternnined with Scanning Transmission Electron Microscopy (STEM) in combination with Energy Dispersive X-ray spectroscopy (EDX). The EDX device is to confirm the presence of oxygen in the interface. The nickel oxide interface can generally not be confirmed in a Scanning Electron Microscope.

[26] With the 'thickness of the interface' is meant the 'full width at half height' of the oxygen K-line peak in the EDX spectrum. The height is taken relative to the oxygen K-line background signal of the adjacent nickel sub-layers that unintentionally contain some oxygen.

[27] The thickness of the sub-layers themselves may vary between 0.05 μιτι and 20 μιτι. Preferably the thickness of the sub-layers is between 0.1 μιτι and 10 μιτι, or 0.2 μιτι to 5 μιτι or 0.2 μιτι to 1 μιτι. The thickness of the layers directly relates to the amount of atoms deposited during deposition of a single sub-layer that, as the sub-layers are deposited electrolytically, directly relates to the amount of charge delivered during deposition (by Faraday's law). This amount of charge delivered will depend on current density, ion charge, and immersion time.

[28] In a second preferred embodiment, the interfaces that contain nickel oxide are to be found radially outward of the abrasive particles. With 'radially outward' is meant relative to the axis of the fixed abrasive sawing wire. Hence the interfaces containing nickel oxide are present at least at the outer side of the abrasive particles. Hence, the abrasive particles are then coated with at least two sub-layers of nickel separated with at least one interface containing nickel oxide. The number of sub-layers radially outward of the abrasive particles may be large. For example five to ten or even more. As many as sixty layers can be present radially outward of the abrasive particles.

[29] The summed thickness of all the sub-layers radially outward of the

abrasive particles is between 2 and 20 μιτι, preferably between 3 and 10 μιτι. A thicker top layer helps to hold the particles better. However, a too thick top layer tends to bury the abrasive particles in the electroplated nickel layer making them inactive for sawing.

[30] In a third preferred embodiment, the number of sub-layers present

between the metallic core wire and the abrasive particles is limited between one and ten, or even more preferred between one and three or even limited to one layer. These sub-layers are or this sub-layer is deposited on the metallic core wire prior to or during the deposition of the abrasive particles.

[31 ] The summed thickness of the sub-layers between the metallic core wire and the abrasive particles is between 0.05 μιτι and 15 μιτι, preferably between 0.1 and 5 μιτι for example 0.3 to 1 .0 μιτι. The presence of at least one nickel sub-layer between the metallic core and the abrasive particles helps to increase the consolidation of the abrasive particle in the electroplated nickel layer.

[32] The total thickness of the electroplated nickel layer is between 1 and 40 μιτι. With 'total thickness' is meant the summed thickness of all sub-layers in a region free of abrasive particles. The total thickness of the

electroplated nickel layer is preferably between one third and two thirds of the median size of the abrasive particles. For example, particles having an median grain size of 30 μιτι are best held in an electroplated nickel with total thickness between 10 and 20 μιτι. In case of sapphire sawing larger abrasive particles are preferred up to as much as 60 μιτι and hence the total thickness of the electroplated nickel layer amounts then to 20 to 40 μιτι. A more typical measure for abrasive particles in sapphire sawing is 30 μιτι with a total thickness of the nickel layer between 10 and 20 μιτι.

Oppositely, for the cutting of mono crystalline silicon smaller abrasive particles of less than 20 μιτι median size are preferred. A typical median size is in that use 1 1 μιτι. The total thickness of the electroplated nickel layer than is between 3 and 8 μιτι. The median grain size of the abrasive particles is preferably determined by means of the 'laser diffraction method' (or 'Low Angle Laser Light Scattering') according ANSI B74.20- 2004. [33] The abrasive particles are preferably super abrasive particles such as diamond or cubic boron nitride or mixtures thereof. Although other hard materials like tungsten carbide, silicon carbide, aluminium oxide or silicon nitride may also be considered, they are rarely used for fixed abrasive sawing wire. Most preferred for both its low cost and friability is man-made diamond that is crushed and sieved to size.

[34] Advantageously, the abrasive particles have at least a partial pre-coating with a conductive compound as otherwise they will not or hardly be electrolytically covered with the nickel sub-layers. With 'partial pre-coating' is meant that at least a part of the surface of the abrasive particle shows a coating prior to being incorporated into the electroplated nickel layer.

[35] Exemplary pre-coatings are any one out of the group comprising nickel, nickel-phosphor, nickel-boron, titanium, titanium carbide, zirconium, zirconium carbide, tungsten, tungsten carbide, vanadium, vanadium carbide, niobium, niobium carbide, molybdenum, molybdenum carbide, chromium, chromium carbide, silicon, silicon carbide. Most preferred is nickel phosphorous as it can be applied to the abrasive particle in an electroless way. The pre-coating weight should be less than 60% of the total weight (abrasive particle plus coating), preferably less than 35%. A too thick pre-coating also has to be abraded away before the abrasive particle can do its work i.e. aggravates the dressing problem. A particle with a pre-coating weight of less than 5% of the total weight is more difficult to embed in the electroplated nickel layer.

[36] In a further preferred embodiment of the fixed abrasive sawing wire the interface between the pre-coating of the abrasive particles and the nickel sub-layer first deposited thereon and hence adjacent thereto is

substantially free of oxygen. With 'substantially free of oxygen' is meant that the difference between the oxygen counts in a STEM - EDX profile at the interface and adjacent the interface is within the measuring limit. It is preferred that at least the layer first deposited on the abrasive particle adheres well to the abrasive particle to have a good retention.

[37] According a second aspect of the invention, a method to continuously produce a fixed abrasive sawing wire is disclosed. The method results in the fixed abrasive sawing wire as defined in the product claims. The method comprises the steps of:

[38] (A) Unwinding a metallic core wire such as described in the

paragraphs [0013] and [0018] from a carrier;

[39] (B) Guiding the wire through cleaning baths. Preferably an alkaline or neutral bath is used to clean the wire from organic residues followed by an acid bath to remove oxides and inorganic contaminants.

[40] (C) Then the wire is electroplated by repeatedly immersing the wire in electrolytic nickel plating baths. In each of the electrolytic nickel plating baths a sub-layer of nickel is deposited. At least one of the electrolytic plating baths comprises abrasive particles. In this bath the abrasive particles are incorporated into the electroplated nickel layer.

[41 ] (D) After the customary rinsing and drying steps the fixed abrasive sawing wire is wound on a carrier,

The terms 'before' and 'after' refer to the temporal sequence of events that the wire undergoes during processing.

[42] The method differs from known methods in that in between at least two subsequent nickel plating bath immersions of the wire a nickel oxide layer is formed by oxidation of the then outer nickel layer.

[43] Nickel plating baths are known in the field. There are Watts nickel plating solutions (nickel sulphate, nickel chloride, boric acid), nickel sulphamate based solutions (nickel sulphamate, nickel chloride, boric acid), all chloride solutions (nickel chloride, boric acid), sulphate-chloride solutions (nickel sulphate, nickel chloride, boric acid, but in different concentrations than a Watt's bath), fluoroborate solutions (nickel fluoroborate, nickel chloride, boric acid) and many other. The nickel sulphamate bath is most preferred for its high deposition rate.

[44] The following refinements can be made to the method:

Primo. The oxidation of the then outer nickel layer can be performed after the at least one bath comprising abrasive particles. In that case an interface containing nickel oxide will be present above the abrasive particles. By preference interfaces containing nickel oxide are only present radially outward of the abrasive particles and are not present between the metal core wire and the abrasive particles thereby not a priori excluding the possibility.

[45] Secundo. By preference the abrasive particles have a conductive pre- coating as explained in paragraph [0034] to [0035]. Advisable is that care is taken to prevent formation of an oxide skin that naturally forms on the conductive pre-coating. Thereby the presence of a nickel oxide containing interface between the pre-coating and the adjacent nickel sub-layer is avoided.

[46] Tertio. Basically the oxidation of the then outer nickel layer can be

performed in three different ways:

• By exposure to the oxygen present in the air, possibly improved by addition of heat. To this end the wire must be dried in order to expose the then outer nickel sub-layer to atmosphere;

• By electrochemical oxidation in an aqueous electrolyte medium. The wire is biased as an anode whereby oxidation reaction (oxygen formation) takes place.

• By immersion in an alkaline bath that leads to the oxidation of the then outer nickel layer;

[47] Practical examples and explanations are further disclosed in the next sections.

Brief Description of Figures in the Drawings

[48] Figure 1 describes schematically a first preferred embodiment according the invention.

[49] Figure 2 describes a second preferred embodiment of the invention in a schematic way.

[50] Figure 3a shows a STEM picture of the layered structure of the

electroplated nickel layer of a fixed abrasive sawing wire according the invention, while in Figure 3b the associated EDX spectrum of oxygen is shown.

[51 ] Figure 4a and 4b is a STEM picture and EDX spectrum obtained on

another sample.

[52] Figure 5 shows schematically an installation on which the inventive wire can be produced. [53] Figure 6 illustrates the sawing behaviour of a conventional sawing wire and an inventive sawing wire at first cut.

Mode(s) for Carrying Out the Invention

[54] Fixed abrasive sawing wires can be produced as explained in the

schematic diagram of the coating installation shown in Figure 5. Ancillary equipment such as pumps, wipers, drives, filters etc.. that are not needed to understand the invention are not shown in this schematic but are known to the skilled person.

[55] In this installation 501 a plain carbon steel wire 500 with a brass coating (65 wt% Cu, 3g/kg of coating weight) of diameter 120 μιτι is continuously unwound from a spool 502.

[56] In tray 504 an alkaline cleaning agent is used to remove organic residues of the wire such as lubricant remaining from the preceding drawing step. In tray 506 an acidic cleaning removes all other contaminants from the wire surface and activates the surface for plating.

[57] A contact roll 510 ensures continuous electrical current supply from source 512 to the wire 500 thereby making the wire 500 the cathode in the subsequently following first nickel plating bath 508. A first nickel layer is deposited. At the exit of the first nickel plating path 508 a drying station 522 is present that can be switched on or off at will. In the drying station 522, the wire is heated by air blowing, thereby forming an oxide interface on the then outer nickel layer. Alternatively the wire can be heated by resistive heating. More identical nickel plating stages can be introduced to thicken the layer up.

[58] The plating baths contain a nickel sulphamate plating electrolyte of

following composition:

Nickelsulphamate electrolyte Amount (unit)

Ni sulphamate (Ni(SO₃NH2)2-4H₂O) 440 g/l

NiCI₂-6H₂O 20 g/l

pH 3,2 - 3,80

Temperature 45°C

Table I [59] The subsequent nickel plating bath 514 comprises a nickel plating electrolyte wherein abrasive particles are held in suspension by means of mechanical agitation. The wire is again electrically contacted by roll 510' and fed by current source 512'. The abrasive particles are in this case crushed diamonds with a median size of 12 m pre-coated with nickel phosphorous (about 12 to 13 wt% of phosphorous) in an amount of about 16 wt% of nickel phosphorous per weight of diamond whereby care has been taken to prevent the formation of a native nickel oxide layer. The amount of diamonds deposited on the wire depends on the intended application of the wire and may vary between 0.10 to 2.5 grams per km of wire. For thin wires (120 μιτι) used for sawing silicon ingots, a value of 0.15 grams per km is aimed at. A thin nickel layer suffices to keep the diamond particles before going to the next station. A bath 520 containing sodium hydroxide provides an oxidation source leading to the formation of an nickel oxide containing interface on the then outer layer.

[60] In the multi-loop stage 519 following the wire is repeatedly guided over two grooved wire rolls 516, 516' into multiple loops parallel to one another. The wire enters the same nickel plating bath 509 repeatedly. Now the wire is contacted to the current source 512" through an electrolyte 518 e.g.

through an acid such as sulphuric acid. At the negative electrode in the electrolyte 518 reduction will take place, while at the wire surface oxidation will occur of the then outer nickel layer. The oxide skin can be tuned by either adjusting the current density at the wire in the electrolyte 518 (e.g. by changing its immersion length) or by dissolving the nickel oxide layer formed in a subsequent acid bath (not shown). Finally the wire is wound on a take-up spool 524.

[61 ] In this way the interface structure of the different sub-layers of nickel can be tuned at will. Figure 1 shows a schematic build-up of the different layers around an abrasive particle of an exemplary fixed abrasive sawing wire according the invention. A cleaned wire substrate 102 is first coated with a first nickel layer 104 that is subsequently oxidised where after a nickel-oxide layer 106 forms (the presence of a nickel oxide interface is indicated in the graph with a hatched line). This is subsequently coated with a second nickel sub-layer 108 on top of which again an nickel oxide interface 1 10 forms. The third 1 12 and fourth 1 16 sub-layers of nickel are formed with an interface 1 14 free of nickel oxide (indicated in the graph with a full line). During the growth of the fourth sub-layer an abrasive diamond particle 130 was incorporated into the layer. The particle has nickel phosphorous pre-coating layer 132. Care is taken that the outer surface of the pre-coating layer is free of nickel oxide to prevent weak adherence to the third and fourth nickel sub-layer. After deposition of the fourth layer a nickel oxide interface 1 18 is grown followed by deposition of the fifth 120 and sixth 124 nickel sub-layer with nickel oxide containing interface 122 in between. On the outer layer an atmospheric nickel oxide layer 126 will grow over time.

[62] An alternative practical example 200 is shown schematically in Figure 2.

Like tens and unit numbers indicate identical features as in Figure 1 unless indicated otherwise hereafter. The same number and arrangement of layers is present as in the example of Figure 1 . Here only nickel oxide interface layers 214, 218, 222 are present radially outward of the abrasive particle which have been deposited in the multi-loop arrangement 519 of Figure 5. The other nickel oxide formation means 522 and 520 have been switched off and hence the interface layers 206, 210 are free of nickel oxide.

[63] The presence of a nickel oxide interface layer can be ascertained by

means of a Scanning Transmission Electron Microscope (STEM) in combination with an Energy Dispersive X-ray detection unit (EDX). Figures 3 and 4 show a picture of a real life stack of layers obtained from a fixed abrasive sawing wire according the invention. Samples were polished to 25 μιτι overall thickness and further prepared by ion beam milling with a Baltec RES101 machine. The high resolution pictures as well as the EDX spectra were obtained on a Tecnai G2 microscope operated at 200 kV. The detector used for the pictures was a high angle annular dark field detector (HAADF detector) that detects scattered off-beam electrons.

[64] In Figure 3a a HAADF picture of a sample of the coating taken through a plane normal to the outer surface of the fixed abrasive sawing wire is shown. The layered structure is clearly visible. The layers have a thickness of 300 nm. Different interface layers are present and the EDX spectrum (Figure 3b) - set at the oxygen, K-line - along the indicated trace (Figure 3a in white) shows clearly the presence of oxygen at the four interfaces crossed. As no other metals are present only a nickel oxide interface can form. The thickness of the interface layer comprising a nickel oxide is estimated to be 6 +/- 4 nm. Another cross section of the same sample is shown in Figures 4a (HAADF - STEM picture) and 4b (EDX spectrum).

[65] Another inventive sample intended for use as a sapphire sawing wire was made on a plain carbon, brass coated steel wire with tensile strength 3410 N/mm². A precoating of 5 μιτι of nickel was applied on the wire off-line. No additional precoating for nickel was given i.e. the nickel bath 508 Figure 5 was not used and the wire immediately went into the nickel sulphamate diamond suspension. Pre-coated diamonds with a median size 25-35 μιτι with 30 wt% Ni-P on diamond weight were deposited. The first deposited layer had a thickness of 0.1 μιτι and between the pre-coating of the diamonds and the nickel sub-layer deposited thereon no oxygen was present. In a multiloop system 56 sub-layers of nickel were deposited with inclusion of an oxygen interlayer by guiding them 56 times over a multi loop system. The electroplated nickel layer thickness was about 10 μιτι, the thickness of a sub-layer being 0.179 μιτι. An anodic liquid contact system was used. The inventive sample 'IS' was compared with a commercially available fixed abrasive sawing wire with the same metallic core but that was free of nickel oxide interface layers (the conventional sample 'CS').

[66] Figure 6 shows the first cut sawing result of a single wire saw test with both samples. The single wire saw was type RTS-480 obtained from DWT. A 2" (50.8 mm) circular sapphire ingot was cut. The machine was operated at a table speed of 0.42 mm/min at a wire tension of 22 N with an average wire speed of 400 m/min in reciprocal mode. During cutting the force 'F(in N)' exerted on the ingot holder is monitored over time 't (in seconds)' and is a measure for the sawing effort needed. Also when the cutting force increases, a larger bow is observed. Figure 6 shows that both wires show an increased effort at the start of the first cut. This is indicative that the diamonds have not yet obtained their full cutting performance and that they are still being 'dressed' during sawing. Remarkably, the extra effort diminishes after about 2000 seconds in case of the inventive sample 'IS' while for the conventional sample 'CS' this continues for about 3400 seconds. Further, compared to the stable force plateau following, the initial cutting effort is about 35% above the plateau in case of the conventional sample, while it is only 17% higher than the corresponding plateau of the inventive sample. So the inventive wire dresses faster and at less effort than the conventional wire. The inventors attribute this as a surprising advantage of the presence of nickel oxide interfaces between the nickel sub-layers.

Claims

1 . A fixed abrasive sawing wire comprising a metallic core wire and abrasive particles attached thereto in an electroplated nickel layer

characterised in that

the electroplated nickel layer comprises two or more stacked sub-layers of nickel of which in between at least one pair of consecutive sub-layers an interface containing nickel oxide is present.

2. The fixed abrasive sawing wire according to claim 1 wherein at least one interface containing nickel oxide is present radially outward of the abrasive particles relative to the center of said metallic core wire..

3. The fixed abrasive sawing wire according to any one of claims 1 to 2 wherein said interface containing nickel oxide is thinner than 500 nm.

4. The fixed abrasive sawing wire according to any one of claims 1 to 3 wherein the thickness of each nickel sub-layer is between 0.05 and 20 micrometer.

5. The fixed abrasive sawing wire according to claim 4 wherein the nickel sublayers radially outward of the abrasive particles are thinner than 10 μιτι.

6. The fixed abrasive sawing wire according to any one of claims 1 to 5 wherein the abrasive particles are diamond particles.

7. The fixed abrasive sawing wire according to claim 6 wherein the diamond particles have a pre-coating with any one out of the group comprising nickel, nickel-phosphor, nickel-boron, titanium, titanium carbide, zirconium, zirconium carbide, tungsten, vanadium, vanadium carbide, niobium, niobium carbide, tungsten carbide, molybdenum, molybdenum carbide, chromium, chromium carbide, silicon, silicon carbide.

8. The fixed abrasive sawing wire according to claim 7 wherein the interface between the precoating and the adjacent nickel sub-layer is substantially free of nickel oxide.

9. A method to continuously produce a fixed abrasive sawing wire comprising the steps of:

-unwinding a metallic core wire from a carrier;

-guiding said wire through cleaning baths;

-electroplating said wire with nickel by repeatedly immersing said wire in nickel plating baths

wherein at least one of said baths is a plating path comprising abrasive particles;

-winding the obtained fixed abrasive sawing wire on a carrier;

characterised in that

in between at least two nickel plating bath immersions a nickel oxide layer is formed by oxidation of the then outer nickel layer.

10. The method according to claim 9 wherein the oxidation of the then outer nickel layer is performed after said at least one bath comprising abrasive particles.

1 1 .The method according to any one of claim 9 to 10 wherein said oxidation of the then outer nickel layer is performed by exposure to air.

12. The method according to any one of claims 9 to 10 wherein said oxidation of the then outer nickel layer is performed by electrochemical oxidation in acqueous medium.

13. The method according to any one of claims 9 to 10 wherein said oxidation of the then outer nickel layer is performed by immersion through an alkaline bath.

14. The method according to any one of claims 9 to 13 wherein said repeated

immersions of said wire in said nickel plating baths is performed by guiding said wire over multiple loops through at least one same bath.