KR20110119705A - Fixed abrasive sawing wire - Google Patents

Abstract

According to the present invention, a fixed abrasive sawing wire 40 is disclosed comprising a steel wire and abrasive particles 42 fixed thereon. The steel wire has a high carbon steel high strength pearlite core 14 and a low carbon steel ferrite sheath 12. The particles are indented into the shell 12 and further retained in place by the bonding layer 18. Good sawing results are obtained that are presumed to be due to the use of low carbon steel (or even iron) sheath 12. The fixed abrasive sawing wire 40 may be conveniently used in existing wire saws for cutting silicon wafers (solar cells and semiconductors) or other expensive materials, but does not have the need for abrasive slurry.

Description

Fixed abrasive sawing wire {FIXED ABRASIVE SAWING WIRE}

The present invention relates to a sawing wire, and more particularly to a sawing wire in which fixed abrasive particles are fixed in the outer skin of a carbon steel wire and fixed thereto by a binder layer. Specifically, in the wire, low carbon content steel is used as the sheath on the high carbon content steel wire core. Such wires may be made of quartz (eg, for quartz oscillator or mask blanc), silicon (eg, for integrated circuit wafer or solar cell), gallium arsenide (eg, for high frequency circuit), It can be used to cut hard and brittle materials such as silicon carbide or sapphire (eg for blue LED substrates), rare earth magnetic alloys (eg for recording heads), or even natural or artificial stones. .

Wires for sawing large and small objects (hereinafter, workpieces) formed from hard and brittle materials have been known for centuries. In this field, different names are used to name members that are in contact with the workpiece and are sawing the workpiece. That is, the terms 'wire saw' (ie sawing devices using wires may also be indicated by this, but sawing wires), 'sawing wire' (wires used for sawing) and All of the 'saw wire' (the wire used as the cow) is visible. The term 'sawing wire' will then be used fixedly.

'Sawing rope' or 'sawing cable' is a rope comprising a rope formed of several filaments to which an abrasive bead is fixed, and these ropes are often also referred to as 'sawing wire' However, it does not fall within the scope of the present application.

Basically, the sawing wire serves as a carrier for the abrasive member to friction the material with the object to be cut. These abrasive members,

Can be separated from the carrier and injected by one or another means between the wire and the object to be sawed. This process is often referred to as 'third body abrasion' (third party is an abrasive member) or 'loose abrasive cutting'. A famous example is the cutting of silicon ingots by ordinary carbon steel wire which carries a slurry into the cuts. The slurry contains fine abrasive particles that are attached between the wire and the workpiece to rotate and squeeze the material locally, thereby deepening the cut.

-Can be attached to the carrier wire in the form of protruding teeth formed from the same material as the wire (as in a wood saw), or

It can be attached to the carrier wire in the form of abrasive particles of a different material than the wire. In the latter case, the particles must be hard and firmly attached to the carrier wire.

Of interest in this application is the last type of wire, referred to as 'fixed abrasive sawing wire'.

From the fixed abrasive sawing wire, the following is predicted. In other words,

-Short lengths require reciprocating motion, so the sawing wire must be supplied at a sufficiently long length (at least several kilometers). Reciprocating motion involves repeated accelerations and decelerations and hence loss of energy and time. The longer the wire, the less turnaround.

In addition, thro and fro movement tends to rock the abrasive particles from the wire, thereby causing premature wear of the wire due to the loss of the particles. Therefore, the abrasive particles are required to be well fixed into or onto the wire. Good fixation means that the abrasive particles must be held in position and their elastic motion relative to the wire is kept low.

-The thinner the wire, the better. Thinner wires mean less kerf loss. Cut loss is the amount of workpiece material that is lost due to friction. Less cutting loss implies better use of the material. For expensive materials (such as silicon, gallium arsenide or rare earth magnetic alloys), a small reduction in cutting loss results in large financial benefits. The criterion is set in the release abrasive cut where a wire of dimension 120 μm is customary and the test is performed with an 80 μm wire. This results in cutting losses of 130 to 140 μm and 90 to 100 μm as the abrasive particles in the slurry carrier also occupy some space between the wire and the workpiece.

The wire must be tensioned during the sawing process to press the abrasive particles into the object, so the wire must be able to maintain some tension. The higher tension that can be maintained results in a greater contact force on the abrasive particles and therefore (but limited to this) higher sawing speed. The tension is typically at least 20 N.

The demand for a small width of the fixed abrasive sawing wire associated with a low sawing loss, ie a high sawing speed requiring high tension, makes it necessary to use high tensile strength wires. The tensile strength of the wire is defined as (break load, ie the force at which the wire breaks) / (cross-sectional area of the wire), and is expressed in N / mm 2. For safety reasons, the minimum breaking load of the wire should be at least about twice the tension. For example in the case of a 200 μm fixed abrasive sawing wire, this results in a minimum tensile strength of 1,300 N / mm 2, and in the case of a 140 μm fixed abrasive sawing wire, this results in a minimum tensile strength of 2,600 N / mm 2.

Preferably, the sawing wire should not contaminate the workpiece with contaminants. For example, because diffusion copper in silicon is electronically active and also because disposal of copper containing effluents is more difficult, it is attempting to remove copper from sawing of the silicon wafer.

Fixed abrasive sawing wires are described in various patent applications and patents, among which are most relevant to the purpose of the present application. In other words,

EP 0 243 825 describes a method for producing a fixed abrasive sawing wire starting from a steel wire rod with a gap therebetween and a tube surrounding the rod. The gap is filled with a mixture of metal powder and abrasive particles. The ends are sealed and the rod is heat treated and cold drawn in repeated steps, thereby obtaining a fixed abrasive sawing wire after the outer tube is removed by etching. According to the drawback of this method, it is not possible to produce a fixed abrasive sawing wire of considerable length (greater than 100 m), as a result of which the tensile strength of the wire produced is relatively low (eg less than 1800 N / mm 2), The resulting wire has an excessive thickness (1 mm).

EP 0 982 094 describes a fixed abrasive sawing wire having a stainless steel core, an intermediate layer to prevent hydrogen embrittlement of the core wire and a bonding layer in which diamond particles are incorporated therein. The bonding layer in which the diamond is located is deposited by electroplating or electroless deposition from a deposition bath containing diamond. The given example describes nickel as both the intermediate layer and the bonding layer.

WO 99/46077 is a metal wire; And superabrasive grains attached to the wire through brazing or brazed metal bonds, the particles preferably being abrasive abrasive comprising superabrasive particles, disposed on a surface having a preselected surface distribution. The wire is described. This application mentions that wires may lose strength due to brazing and heat treatments requiring soldering. This is undesirable to meet tensile strength requirements.

EP 0 081 697 describes a method and apparatus for enclosing a wire with diamond particles. One embodiment starts from a wire coated with a layer of copper or nickel prior to envelopment of diamond particles between hardening wheels that rotate the wire about that axis through repeated axial movement of one or both of the wheels. Thereafter, the diamond is fixed in position by an overcoat applied by electrolysis.

The summary of JP 5016066 A2 describes the production of sawing wires having a high carbon steel core and a low carbon steel sheath through controlled decarburisation of the high carbon steel wire. However, the wire is intended to be used in release abrasive slurry processes. Therefore, the abrasive particles from the release abrasive slurry are not fixed in the carbon shell but are attached in a continuous manner and then released again. Furthermore, decarburization always results in loss of carbon and hence loss of wire strength.

It is a primary object of the present invention to provide a fixed abrasive sawing wire having improved properties. It is a further object of the present invention to provide a wire in which the abrasive particles are well fixed and exhibit less elastic movement during sawing. Another object is to provide a fixed abrasive sawing wire having a tensile strength high enough to allow low cutting losses associated with high sawing speed. A method for producing such a wire, for example longer than 1 km, is described, which is a further object of the present invention.

According to a first aspect of the invention, a product in the form of a fixed abrasive sawing wire is disclosed. The sawing wire comprises a center wire formed of steel. Since steel is an alloy of iron and carbon and other elements, steel always contains carbon in some amount. The outer periphery of the wire has a different composition than the inner core of the wire. This outer perimeter will then be referred to as the skin. In the shell, abrasive particles are fixed. A bonding layer is applied on the skin to better retain the particles in the skin.

The cross section of the wire may have any suitable shape. The shape is affected by the sawing method. For example, multi-wire sawing, where a single wire is threaded repeatedly in parallel over two or more guiding rolls, so the name given to the machine is slightly misleading since only one wire is used. In, the cross section is preferably round. Indeed, in this multi-wire, the wires tend to rotate due to the large number of bendings on the pulleys and guide rolls, so rotationally symmetrical wires or round wires are most suitable.

The overall diameter (including the abrasive particles) of such round fixed abrasive sawing wire may be from 80 μm to up to 300 μm, which diameter is again determined by the machine in which the wire is used. For example, in a sawing machine suitable for cutting square blocks (12.5 × 12.5 cm 2) from one polysilicon block (eg, 1 m × 1 m) as described in patent no. CH 692489, a large force of 1 m Since it is necessary to pull the wire over a long length, a wire of about 250 μm will be more appropriate. In the case of a laboratory sawing machine that cuts small samples with small force, an 80 μm wire can perform the operation. For slicing in multi-wires for the semiconductor industry, wires of 100 to 200 μm appear to be most suitable, where the user will of course prefer the thinnest wire.

In the alternative, the cross section may be oval or even rectangular in shape. For example, teardrop shapes such as those disclosed in US Pat. No. 5,438, 973 are best when using a frame saw (in a frame saw, the individual wires being stretched parallel to each other in a frame reciprocating over the workpiece). Do. Tear drop shapes make it possible to further reduce cutting loss without giving up strength. In addition, the high bending stiffness when bent in the long side plane allows for higher sawing pressure at the cut.

In another alternative, flat wire twisted about its own axis is used. Even star-shaped cross sections may be useful in some cases, for example in manually operated cattle.

Fixed abrasive steel wire differs from the prior art in that the core of the steel wire has a pearlite metal structure and the shell has a metal structure. Determination of the metal structure of steel is a standardization technique. That is, the wire is embedded in an epoxy block that is cut perpendicular to the axis of the wire and subsequently polished. The shiny surface of the cross section is then etched in a nital solution which is a mixture of about 3 volume% nitric acid (HNO ₃ ) and an alcohol such as ethanol (C ₂ H ₅ OH). Etching makes the crystal structure of the steel visible under a metallographic microscope at about 100 to 500 times magnification.

Thereafter, given the composition of the material in terms of its components in the fraction of its total weight, this composition will be expressed as 'wt%', unless it is clear that it is not 'percent by weight'. Should be understood as'.

Pearlite tissue (or 'pearlite' for short) exhibits a brown-grey pearly appearance (derived from it) under a microscope. Pure pearlite is formed after proper heat treatment of the steel (austenitization at temperatures above 723 ° C. and subsequent slow cooling). Pearlite is a mixture of 88% by weight ferrite (iron containing little carbon) and 12% by weight cementite (Fe ₃ C), resulting in a process concentration of 0.80% by weight carbon. When the carbon content of the steel is less than 0.80% by weight, such as 0.40% by weight, the steel is called a hypo-eutectoid and the formed pearlite is observable in the region surrounded by ferrite. When the carbon content is higher than 0.80% by weight, such as 1.2% by weight, the steel is referred to as hyper-eutectoid and contains microstructures containing pearlite and cementite ('grain boundary cementite'). Has Skilled analysts can assess carbon content (in weight percent) through metallographic images in steps of about 0.2 weight percent carbon.

Since pure pearlite tissue with exactly 0.80% by weight of carbon is fictitious, the main claim is expressed as 'substantially the metallic structure of pearlite'. For the purposes of the present application, a steel that has been treated correctly to obtain such a structure having a carbon content in excess of 0.40% by weight is considered to represent 'substantially the metallic structure of pearlite'.

The sheath of the fixed abrasive sawing wire substantially represents the metal structure of the ferrite, or in short 'ferrite'. Ferrites appear much brighter and have no color, so they are clearly identifiable in metallographic images. For the purposes of the present application, ferrite is formed in steel with a carbon content of 0.04% to 0.20% by weight.

Actual steel compositions include iron and carbon and also large amounts of other alloys and trace elements, some of which have a profound effect on the properties of the steel in terms of strength, ductility, formability, corrosion resistance, and the like. In connection with the present application, since the strength is absolutely necessary, the following elemental composition is preferred for the core of the steel wire. In other words,

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

Manganese content from 0.30 to 0.70% by weight. Manganese, like carbon, increases strain hardening of wires and also acts as a deoxidizer in the manufacture of steel.

Silicon content of 0.15 to 0.30% by weight. Silicon is used to deoxidize steel during manufacture. Like carbon, silicon helps to increase strain hardening of steel.

-The presence of elements such as aluminum, sulfur (less than 0.03%) and phosphorus (less than 0.30%) should be kept to a minimum.

The rest of the river is iron and other elements.

The presence of chromium (0.005 to 0.30% by weight), vanadium (0.005 to 0.30% by weight), nickel (0.05 to 0.30% by weight), molybdenum (0.05 to 0.25% by weight) and boron (trace) is determined by vacancy composition (0.80% by weight C). It is possible to reduce the formation of grain boundary cementite for carbon contents higher than), thereby improving the formability of the wire. Such alloys allow a carbon content of 0.90 to 1.20% by weight, resulting in a tensile strength that can be as high as 4000 MPa in the wire drawn. Such steels are more preferred and are described in US 2005/0087270. Hereinafter, expressed as 'high carbon amount' or 'high carbon steel', it should be understood as the carbon content of the core of the steel wire.

The steel composition of the outer sheath of the wire is less important because most of the iron with some carbon (0.04% to 0.20% by weight) and other trace elements is in it. If later referred to as 'low carbon amount' or 'low carbon steel', it should be understood as the carbon content or steel of the sheath of the steel wire.

All the aforementioned steel compositions have the properties of 'usually carbon steel' because the main alloy component is carbon. As such, the core of the wire must be able to withstand all forces, so steel that enables high strength is most preferred, and the skin is formed of low strength low carbon steel with much lower strength due to the presence of abrasive particles. Moreover, as in the circular cross section where most of the area is at the periphery of the circle, most of the area is low carbon steel and thus does not contribute to the total breaking load of the fixed abrasive sawing wire. This makes a fine diameter fixed abrasive sawing wire of sufficient strength an uncertain challenge.

Fixed abrasive sawing wires according to the invention are typically greater than 2000 N / mm 2 for diameters smaller than 250 μm, greater than 2250 N / mm 2 for diameters less than 150 μm and greater than 2500 N / mm 2 for diameters less than 120 μm. Tensile strength. Tensile strength is defined as [break load of a fixed abrasive sawing wire] / [the metal surface thereof (except the area occupied by the abrasive particles)]. The metal surface is determined at the cross section of the wire as used in metallographic crystallization. Any metal layer is considered for the surface.

'는 '0'으로부터 'R'까지 Γ(r)을 적분함으로써 구해질 수 있다. 즉,The local carbon content, as measured radially from the core to the sheath, will represent a reduction function. This is schematically illustrated in FIGS. 2A and 2B in which the radial local carbon distribution Γ (r) in weight percent is shown as a function of the distance 'r' from the center. The steel wire has a radius 'R' and thus a diameter of 2R. Average carbon content ''

'Can be obtained by integrating Γ (r) from' 0 'to' R '. In other words,

'이다. 탄소 함량은 이들의 계면을 갖지 않도록 입자 및 고정 층의 제거 후에 결정되어야 한다. 평균 탄소 함량은 적어도 0.40 중량%이어야 한다. 0.55 중량%를 초과하면 더 바람직하고, 0.60 중량% 이상이면 훨씬 더 바람직하다.Experimentally, the easiest to determine, of course, is the average carbon content, e.g. by the LECO CS230 carbon and sulfur testers,

'to be. The carbon content should be determined after removal of the particles and the fixed layer so as not to have their interface. The average carbon content should be at least 0.40 wt%. It is more preferable if it exceeds 0.55 weight%, and even more preferable if it is 0.60 weight% or more.

Measurement of Γ (r) is difficult, but can be performed in a variety of ways. In other words,

There is a method for assessing metal structure. As noted above, a skilled analyst can assess carbon content by fractions separated by about 0.2% carbon by weight. In either case, the presence of ferrite and pearlite is the standard procedure for the determination.

The most precise method, of course, is to obtain microanalysis of the carbon content along the radius of the wire locally. This analysis is now possible with scanning electron microscope equipped with wavelength dispersive spectrometers (SEM-WDS). This is the ultimate reference method.

Indirect measurement of relative carbon distribution can be obtained via Vickers micro-hardness measurement. In this preferred known method (ISO 6507-3 'Metal Hardness Test: Vickers Test <HV 0.2'), a square diamond pyramid has a microindentor with a face angle of 136 ° specified in the material. Pressed with a specified force (0.9807 N, longitude symbol HV 0.1) for a time (10 seconds). Thereafter, the geometric shape of the indentation portion is measured and from this measurement the local micro-hardness (unit: N / mm 2) can be calculated. To increase the spatial resolution of this method, the wire is surrounded by epoxy resin, cut under a certain angle with respect to the axis of the wire, and polished. The hardness at regular points along the major axis of the formed ellipse is measured and the exact radial position with respect to the axis of the wire is calculated. The hardness measured is a function of the steel metal structure, the amount of strain hardening given to the wire (same on the cross section of the wire) and the carbon content. Vickers micro-hardness measurements are particularly important because they provide a measure of the extent to which abrasive particles can easily be pressed into the shell.

In much the same way as the carbon content, the weighted average Vickers micro-hardness 'HV _mean ' can be calculated by replacing 'Γ (r)' by the local Vickers micro-hardness as a function of the radial distance 'μ (r)'. have. In other words,

In the experimental results, the integration can be conveniently approximated by taking the average of the under and upper sum of the weighted discrete measurement points with the annular surface between the points.

Preferably, this average hardness is higher than 500 N / mm 2 or more preferably 550 to 650 N / mm 2. Excessively low average hardness will not allow sufficient strength, and excessively high average hardness will not allow proper indentation of the particles. Reference is made to the schematic diagrams of FIGS. 2A and 2B. The sheath can be defined as part of a wire having a hardness below average and the core can be defined as part of a wire having a hardness above average. The sheath and core meet at the border. At the boundary, the local Vickers micro-hardness crosses the weighted average micro-hardness. This boundary lies approximately at radius 'b'. The thickness 'Δ' of the skin can then be conveniently defined as the radial distance between the wire boundary and the outer periphery or 'Δ = R-b'.

The sheath should prevent micro-crack damage by indentation of abrasive particles in the core of the steel wire. Indeed, steel wires are more susceptible to surface damage with increasing tensile strength. This is expressed as loss of fatigue strength and / or loss of strength (as damage is the start of cracking). The sheath must also retain the particles in position. Therefore, the indentation depth of the particles should not be greater than the shell thickness 'Δ'.

According to a first preferred embodiment, the transition from the high carbon core to the low carbon sheath may be abrupt as shown in FIG. 2A. There will be carbon exchange between the core and the sheath at the nm level, but no metallographic mixed phase is discernible at the μm level.

According to a second preferred embodiment, the transition from the high carbon core to the low carbon sheath is smooth and includes a mixed metallographic phase that exhibits increased ferrite presence and reduced pearlite presence when viewed from the core to the sheath. The transition portion is smoothed due to the processing to be described later. Then, the carbon content distribution is as shown in Fig. 2b. The width 'δ' of the transition portion may be defined as the distance at which μ (r) varies between 350 and 650 N / mm 2. These values are consistent with what can be expected from hard drawing low carbon steels having C less than 0.2 wt% and hard drawing high carbon steels having more than 0.40 wt% C. The transition has the advantage that when the abrasive particles are pressed into the sheath and reach the core of the wire, they encounter a continuous increasing push force instead of a sudden change. The transition also has the advantage that the skin is diffusion bonded to the core and that loss of adhesion between the skin and the core is substantially impossible. Therefore, the transition layer width 'δ' should be at least 5 μm and preferably more than 10 μm to have good bonding.

In contrast, an excessively thick transition region will result in an unacceptable surface area loss for the high strength core, so the wire will lose excessively overall strength. With only minor modifications needed, this transition area will result in excessive hard shell. Therefore, the width of the transition should be kept below 40 μm, more preferably below 30 μm and even more preferably below 20 μm.

Compared to the conventional copper clad type fixed abrasive sawing wire as disclosed in EP 0 081 697, the use of a low carbon coating has the further advantage that the low carbon coating further increases the overall strength of the wire. This is because low carbon has a higher cold deformation hardenability than copper. In this way, finer fixed abrasive sawing wires can be produced having the same strength as conventional sawing wires. This can reduce the cutting loss during sawing.

The abrasive particles may be superabrasive particles such as diamond (natural or artificial, the latter being slightly more preferred due to its lower cost and its particle firmness), cubic boron nitride or mixtures thereof. In less demanding applications, particles such as tungsten carbide (WC), silicon carbide (SiC), aluminum oxide (Al ₂ O ₃ ) or silicon nitride (Si ₃ N ₄ ) may be used. That is, they are softer, but they are considerably cheaper than diamonds. Diamond is most preferred.

The size of the abrasive particles is slightly adjusted depending on the diameter of the wire. Determining the size and shape of the particles themselves is an independent technical field in itself. Since the particle should or should not have a spherical shape, for the purposes of the present application it will be expressed as its 'size' instead of the 'diameter' of the particle (since its diameter implies a spherical shape). The particle size is a linear measurement (in micrometers) determined by any measurement method known in the art, and is always the length of the line connecting the two points on the farthest particle surface and the 2 on the particle surface closest to each other. Somewhere between the lengths of the lines connecting the two points.

The particle size expected for a fixed abrasive sawing wire belongs to the category of 'microgrit'. The size of the fine particles can no longer be determined by standard sieving techniques customary to macroparticles. Instead, they must be determined by other techniques such as laser diffraction, direct microscopy, electrical resistance or photosedimentation. The standard ANSI B74.20-2004 deals in more detail with these methods. For the purposes of the present application, when expressed as particle size, it means the particle size as determined by the laser diffraction method (or also referred to as 'low angle laser light scattering'). The result of this procedure is a median d ₅₀ size (i.e. half of the particles are smaller than this size and half of the particles are larger than this size) or generally 'd _p ' [wherein 'P' of the particles Cumulative or differential particle size distribution with% less than 'd _p ' and the remainder (100-P)% greater than this 'd _p '.

Superabrasive particles are typically identified by size ranges by this standard instead of sieve numbers. For example, in a particle distribution in the 20-30 μm class, 90% of the particles have 20 μm (ie 'd ₅ ') to 30 μm (ie 'd ₉₅ ') and less than 0.1% greater than 40 μm Meanwhile, the median size d ₅₀ should be between 25.0 +/− 2.5 μm.

Empirically, the median size (ie, the size of a particle whose half is smaller in size and the other half is larger in size) is equal to 1 / 12th of the circumference of the steel wire to accommodate the particles well in the sheath. ^th and more preferably less than 1/18 ^{th of the} circumference of the steel wire. In contrast, the particles may not be excessively small such that the material removal rate (ie, the amount of material being rubbed / unit time) is excessively low.

Abrasive particles will be best retained when indented into the shell by more than half of its median size. Therefore, the indentation depth should be at least greater than half the median size of the particles. Since the sheath thickness should be thicker than the maximum indentation depth, the sheath thickness should be at least greater than half the median particle size in order to retain the particles well.

It is even more desirable if the shell thickness is thicker than 'd ₉₀ ' (90% of the particles have a size smaller than d ₉₀ ). Therefore, the chance for micro-crack damage to the core is very small, thereby avoiding breaks in use. In practice, the sheath thickness should be about 10% of the diameter of the steel wire and should be at least 5% or at least 7% of the diameter. Thus, 8 to 15 mm for very thin wires of 80 to 150 μm and about 20 mm for 210 μm wires. It should be noted that with the sheath thickness of 10% of the diameter, already 36% of the cross-sectional area of the wire is occupied by the low tensile skin material.

Whether the particles are properly indented can be readily assessed for the cross section of the wire where the abrasive particles are removed and thus crevices remain in the sheath. At the indentation, this cross section will expose two extreme points at the outer periphery of the wire which can be connected by a line. The distance between the points will be referred to as the indentation width 'W _indentation '. The maximum distance taken at right angles to the line reaching the steel wire will be referred to as indentation depth 'D _indentation '. In order to ensure a good fixation, twice the indentation depth must be greater than the indentation width. When a large number of crevises are measured, good indentation is characterized by an average of a ratio greater than one (2 × D _indentation / W _indentation ). At least 20 crevises should be measured.

The number of particles that must be present on the surface of the sawing wire depends greatly on the kind of material to be cut. An excessively high density will induce an excessively low force on the particles to polish the particles, thereby causing a decrease in their cutting capacity. On the other hand, excessively low densities cause the force to be excessively large, causing particles to rupture from the sheath or excessively low cutting speeds such that sufficient particles per unit time cannot pass through the material. 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, ie the 'coverage ratio'. This can be done by SEM by selecting particles having a typical composition from a general picture and calculating the area occupied by the particles over the total area. Since the sides tend to overestimate the particle surface due to the turning away of the wire surface, only the central portion of the wire picture should be used. An example is given in FIG.

The target coverage for the particles depends on the material to be cut, the cutting speed to be reached or the surface finish to be obtained. In order to have the best sawing performance for the expected material, it has been found that the ratio of particle area to total area should be 1 to 50%, 2 to 20% or even 2 to 10%.

The bonding layer is applied onto the outer surface of the wire and helps to bind the particles into the sheath, in other words keeping the particles fixed in the sheath. Preferably, the bonding layer is a metal layer. Particularly preferred metals are nickel and iron. Preferred metals of the alternative are chromium, cobalt, molybdenum, tungsten, zinc and alloys thereof. The thickness of this layer is preferably 1 to 5 μm.

As mentioned above, it is clear that the fixed abrasive sawing wire of the present invention is substantially free of copper. No intentionally added copper is present in the wire or the coating. Therefore, contamination of the silicon workpiece with copper is avoided during drawing. Copper atoms diffused into the silicon form electron active defects within the energy interval of the silicon. In addition, removal of copper from the effluent stream (eg, resulting from cleaning of the refrigerant or finished wafer) can be avoided in this manner.

According to a second aspect of the invention, a method of manufacturing a wire is disclosed. In general, this method involves three main steps. In other words,

In the first step, a steel wire of sufficiently fine diameter with a high carbon core and a low carbon sheath should be provided.

In the second step, abrasive particles of the desired size and type are pressed into the shell.

In the third step, the abrasive particles are fixed to the bonding layer.

Preferably, the second and third steps may be carried out in line concept where the wire is continuously fed from one process step to the next fixing step. However, the separation of these steps is not excluded by the fact that a batch process is possible, for example as described in EP 1375043 for the third step. There are various ways in which the starting steel wire of the first stage can be produced.

In a first embodiment of the first method step, high carbon steel wire is coated with pure iron from an electrolytic bath (eg, US5014760). Some alternative approaches are possible.

According to a first alternative, the final diameter steel wire is coated with iron. The transition between the iron shell layer and the high carbon core is sharp and no mixed phase will form between the core and the shell. Then, an embodiment as described in Fig. 2A is obtained. According to the advantages of this method, relatively little iron has to be laid on the wire to reach an appropriate layer thickness (greater than 7% of the steel wire diameter).

According to a second alternative, the steel wire may be coated with iron to an appropriate intermediate diameter prior to further wet wire drawing. By median diameter is meant the diameter between the wire rod diameter and the final diameter of the wire (the median diameter is typically 2.70 to 0.90 mm). Heat generated during drawing will result in the formation of small transition regions of about 5 μm or more due to the diffusion of carbon into iron.

In a third alternative, the wire can be coated with iron to a medium diameter level, possibly repeatedly patterned and drawn. In this case, the transition zone is larger due to the single heat treatment of the skin causing the further diffusion of carbon into iron. Then, the transition region is 5 to 30 mu m.

According to a second preferred embodiment of the first method step, the high carbon steel core is surrounded by a low carbon steel strip or iron foil which is closed by welding and forms a sheath. Again, alternatives are possible, as in the first preferred embodiment.

In a first alternative, a medium steel wire of 2.40 to 0.90 mm diameter is surrounded by iron foil or low carbon strip before further wet wire drawing. Again, the heat generated during drawing will result in the formation of small transition regions of about 5 μm due to the diffusion of carbon into iron.

In a second alternative, the high carbon steel cores are surrounded-coated with low carbon or iron streams or foils to intermediate diameter levels, and perhaps repeatedly patterned and drawn to the final diameter. Again, the transition region is slightly wider due to one or possibly two or three heat treatment (s) of the wire leading to further diffusion of carbon into the shell. Then, the transition region is 5 to 30 mu m. The transition region is increased with the number of patenting steps.

The first and second preferred embodiments of the first method step result in a hardness profile having a sharp shape at the surface of the skin instead of showing a continuous decrease.

According to a third preferred embodiment of the first method step, the shell is formed by decarburization of the high carbon steel wire. An example of decarburization is described in US 5014760. The outer layer of the steel wire then loses much of its carbon and forms a low carbon sheath, while the core retains most of the carbon. Decarburization requires passing the wire at high temperatures of 700 to 1000 ° C. in an oxidizing atmosphere furnace, which will result in an unacceptable loss of strength and therefore it is not possible to decarburize the final diameter wire.

Therefore, the decarburization treatment is preferably carried out on medium diameter wires larger than about 0.90 mm. The decarburization treatment step may be performed at the rod diameter level, followed by one or two regular patenting treatment steps (ie, under reducing atmosphere) between or after the wire drawing machining operations. In the alternative, the decarburization step may be the last heat treatment before the final wire drawing. The latter is slightly preferred because subsequent regular patenting treatment (under a reducing atmosphere) results in redistribution of carbon in the wire. This redistribution results in expansion of the transition region.

In general, decarburization of high carbon steel wires inevitably results in loss of carbon, i.e. loss of overall strength, which is disadvantageous to the requirements of the need for strength. In order to reach the required overall strength for the final product, the carbon content of the high carbon starting wire must be enormously high. In addition, the external diffusion of carbon results in a carbon profile that can result in excessive soft outer layers. Therefore, the hardness profile will be reduced to steady state and will not show a sharp shape in the skin. Carbon profiles are also difficult to control because the shell thickness is affected by the diffusion law.

During the second method step, the sheath of the wire is pressed into the abrasive particles. This can be conveniently done by temporarily fixing the abrasive particles to the wire before rolling the abrasive particles into the shell by the roll. An example of how this is done is disclosed in EP 008169. According to an improvement on the technique, the particles are temporarily fixed, for example by adding a viscous material to which the particles can be attached and subsequently washed (preferably in water). According to a further refinement, the rolling is carried out between hardening rolls with mating semicircular grooves in which the wire is guided. According to another improvement, different pairs of rolls under different angles can alternately follow.

In a third process step, the particles are fixed by a fixed layer, which is preferably metal in nature. Application of the fixation layer should be performed under low temperature conditions (below about 200 ° C.) to avoid lowering the tensile strength of the wire. Therefore, the most preferred method is to use an electrodeposition technique that deposits metal ions from the metal salt electrolyte onto a wire that is maintained at a negative potential with respect to the electrolyte. Even so, care must be taken not to cause excessive resistive heating of the steel wire as the steel is a less good electrical conductor and the wire is fine. In addition, the presence of particles makes the electrical connection to the wire difficult because the particles are insulators in nature, and a simple rolling contact will result in sparking. Therefore, a non-contact method such as that described in WO 2007/147818, for example, in which contact with the wire is formed through a second electrolyte in an electrolytic cell, which is separated from the metal electrodeposition electrolytic cell, is preferred.

1A and 1B are different metallographic cross-sectional photographs of the same wire with indented particles removed from the indentation.
FIG. 2A schematically shows the weight-based radial echo concentration of the carbon content Γ (r) or local microscope Vickers hardness μ (r) in the case of steel wire with a sharp transition from a high carbon core to a low carbon sheath.
FIG. 2B schematically illustrates the weight-based radial echo concentration of the carbon content Γ (r) or local microscope Vickers hardness μ (r) in the case of a steel wire with a smooth transition from a high carbon core to a low carbon sheath.
3A shows the actual measurement of the local microscope Vickers hardness of the first example.
3B shows the actual measurement of the local microscope Vickers hardness of the second example.
4 shows how the coverage of abrasive particles can be determined.
5 shows how the indentation width and depth of the particles can be measured.

According to an example of the invention, a high carbon wire rod (nominal diameter: 5.5) having a carbon content of 0.8247% by weight, a manganese content of 0.53% by weight, a silicon content of 0.20% by weight and an Al, P and S content of less than 0.01% by weight Mm) is chemically descaled according to methods known in the art.

Subsequently, the wire is surrounded by a low carbon strip having a thickness of 0.03% by weight carbon and 0.60 mm. The seam is welded. As such, the total diameter of the enclosed wire is about 6.7 mm. The strip thickness is 8.96% of the total wire thickness.

These composite wires are dry drawn in a manner known in the art up to a total diameter of 2.40 mm (ie core wire + strip enclosure). The material is separated into two separate batches.

The first group of materials (example 1) are further dry drawn to a total diameter of 1.20 mm. Thereby, the thickness of the low carbon strip is reduced to 105 μm (ie 8.75% of the total wire thickness). This material is then patterned in the conventional manner (lead patenting). After patenting, there is already a clear indication of carburisation of the low carbon strip, and the strip is fully fused to the core. Thereafter, another dry drawing step up to a total diameter of 0.90 mm is performed. Subsequently, this wire is wet wire drawn to a total diameter of 210 μm. Due to the patenting treatment, the low carbon strip is carburized and the transition from the core to the shell is no longer clearly identifiable. This sample experiences only one patenting process.

First, a second group of materials (example 2) are patterned in lead, subsequently dry drawn to a diameter of 0.90 mm, and again patterned in lead. Thereafter, this wire is again wet wire drawn to 210 mu m. This sample experiences two patenting operations.

A comparison between both samples is given in Table 1:

Property Example 1 Example 2 Breaking load (N) 78 73 Tensile Strength (N / ㎠) 2284 2102 Sheath μHV (N / ㎠) 274 283 Number of patenting steps One 2 Total decrease since last patenting process 1.20 mm to 210 μm 0.90 mm to 210 μm

'Envelope μHV' represents the measured Vickers micro-hardness as measured for the drawn wire.

The initial Vickers micro-hardness of the low carbon steel strip is 143 N / mm 2. This seems to have increased considerably due to the following. In other words,

-High cold forming during drawing known to result in harder materials.

-Carburizing of the shell material. That is, higher carbon contents are known to result in harder materials.

Carburizing seems to play a more dominant role in the hardness of the shell, as Example 1 experiences a greater reduction but only undergoes one patenting step, while Example 2 obtains a lower final decrease with two patenting steps. .

The hardness profile of the wire of Example 1 is measured and shown in FIG. 3A. Hardness is measured at the elliptical cross section so that each indentation (marked '◆') is far enough apart. The outer point ('■') is the point measured on the skin (Table I). The dashed-dotted line marked 'HV _mean ' represents the surface-weighted average micro-hardness, in this case 597 N / mm 2 (lower sum: 586, upper sum: 606). The thickness 'Δ' of the shell is the distance from the outer circumference to a position whose hardness is equal to the average Vickers micro-hardness. In this case, the boundary between the sheath and the core is between 80 and 84 μm radius, so the sheath thickness 'Δ' is about 21 to 25 μm. As such, the sheath is about 8.5-12% of the steel wire diameter. The transition region 'δ' has a thickness of about 17 μm.

The hardness profile of the wire of Example 2 is shown in FIG. 2B. Different symbols ('◆' and '▲') indicate repeated measurements. Average weighted micro-hardness is 577 N / mm 2 ('▲', represented by a dashed-dotted line; lower sum: 559, upper sum: 595) and 589 N / mm 2 ('◆', represented by a dashed dashed line; lower sum: 571, upper sum: 607). The outer thickness 'Δ' is about 22 μm, while the transition region 'δ' is wider, ie 23 μm.

According to the comparison of the hardness curves of Examples 1 and 2, the hardness at the surface does not differ significantly, but the double patenting treatment results in a harder overall shell hardness. Obviously, carbon proceeds from the high carbon core to the low carbon shell.

Example 2 is selected for further processing. The steel wire of Example 1 is press fitted with diamond particles of median size 'd ₅₀ ' 25.3 μm (d ₁₀ = 15.1 μm, d ₉₀ = 40.6 μm). This is accomplished by dip-coating the wire with a water soluble adhesive (available from the trade name Aquabond). Immediately thereafter, the wire with the particles is guided between two pairs of rolls with mating semicircular grooves of radius 109 μm. The two pairs have their axes perpendicular to each other.

In subsequent depositions, the wire is coated with a nickel bonding layer after washing the adhesive in hot water. This is done in an apparatus as described in WO 2007/147818. The thickness of the layer is about 3 μm.

The degree of coverage of the wire is about 5-8% and is determined in the SEM in the backscattered electron detection mode. 4 shows the resulting photograph in which diamond particles on the surface of the fixed abrasive sawing wire 40 are displayed as black areas 42 on a gray background 44. By the photo reading software, the ratio of the black area to the black and gray areas or the degree of covering can be easily evaluated.

Metallographic cross-sectional photographs, which are cross sections at different locations of the same wire 10, are shown in FIGS. 1A and 1B. When viewed through a binocular microscope, it is clear that the core 14 of the wire 10 exhibits a different tissue than the shell 12. Core 14 represents a high carbon drawing pearlite metal structure, while shell 12 represents a substantially ferrite structure, ie, a structure having a low carbon content. The original circular cross section of the wire is pressed into the particles, which are subsequently drawn out during polishing of the sample. These particles leave the indentation 16. Since no nickel layer is observable inside the crevis left by the diamond, it is clear that the indentation takes place prior to coating with the nickel bonding layer 18. The crevis has a depth of about 20 μm (as measured from the outer nickel surface) and does not enter the core.

It is confirmed in the breaking load test of the finished wire that the indentation of particles does not adversely affect the strength of the sawing wire. That is, no significant loss of breaking load is observed in comparison with the results obtained with steel wires (Table I). Therefore, indentation does not damage high strength steel wire.

The quality of the indentation can be evaluated by comparing its width to the depth of the crevis. The way this is done is shown in Fig. 5, in which a cross section 50 as in Fig. 1A or 1B is reproduced. When connecting the external points 'A' and 'B' of the crevis 52, the width 'W _indentation ' can be determined. Similarly, the depth 'D _indentation ' is determined by measuring the maximum depth perpendicular to the line AB. The measurement (2 × D _indentation / W _indentation ) does not depend on the exact location where the cross section is taken.

For this purpose, the twenty indentation widths and depths are measured at the cross section perpendicular to the wire axis (although the longitudinal direction cross section is equally well suited). The minimum ratio (2 × D _indentation / W _indentation ) is 0.45, the maximum ratio is 2.57, and the average is 1.17 which is greater than 1, which means that the indentation is sufficient.

The performance of the fixed abrasive sawing wire is confirmed on the Diamond Wire Technology CT800 reciprocal lab saw machine. Half of the single crystal silicon semi-square, 10 cm wide and 5 cm high, is cut several times by the wire of the present invention. The machine is operated in a 'constant bow mode' set to 3 °, the wire tension remains constant at about 15 N, and the 30 m wire is periodically moved (in the through and forward directions) within 7 seconds. Thereby providing an average velocity of about 8.6 m / s (2 × 30/7 =). Water with additives is used as the refrigerant.

In Table 2, a comparison of the sawing speed obtained by the sample of the present invention with the sawing speed as disclosed in the prior art (only silicon samples considered) is given.

Conventional technology Sample size (unit: mm × mm) Sawing speed (unit: mm2 / min) WO 99/046077 ■ 25 × 25 37.5 EP 0982094 A2 ■ 10 × 10 16 to 7 EP 0243825
The present invention
Prior art Φ50
158
37 This application ■ 100 × 50 148

In another series of experiments, the gain in strength obtained by using a low carbon clad wire instead of another cladding material such as copper is evaluated. Starting from 0.80% carbon steel, two 0.30 mm wire samples, one with a low carbon coating and one with an electrolytic copper coating, are prepared. The resulting wires are characterized as having the data in Table 3.

Low carbon sheath Copper cladding One Full wire diameter 300 μm 300 μm 2 Core wire diameter 250 μm 244 μm 3 Jacket thickness 25 μm 28 μm 5 Core tensile strength 2984 N / ㎡ 2720 N / ㎡ 6 Core breaking load 146 N 128 N 7 Total breaking load 188 N 145 N

The difference in core tensile strength and steel cross-sectional area may account for the observed difference of 18 N (= 146-128 N) at a breaking load of 43 N (188-145 N). The remaining difference of 25 N (= 43-18 N) can only be attributed to the difference of low carbon to copper of the material. Replacing copper with low carbon in the fixed abrasive sawing wire while keeping all others the same can result in an increase of 17% of the breaking load. As such, it is possible to further reduce the diameter of the fixed abrasive wire and thus the cutting loss while maintaining the same breaking load.

The low carbon sheathed wire is further indented with diamond particles, which are fixed by a nickel layer. Two different degrees of coverage, i.e. having coverage of about 0.60% and having coverage of about 2%, are formed. The sample is tested on a piece of monocrystalline silicon according to the same protocol as described above (paragraph [0088]) but with varying tension. For a coverage rate of 2%, at 18 N tension, a sawing speed of 133 mm 2 / min is obtained, which is increased to 164 mm 2 / min under a tension of 27 N. A coverage sample of 0.60% shows poor cutting results.

Thus, surprisingly, the use of low carbon steel on the sheath of the fixed abrasive sawing wire proved to be a very good choice for retaining abrasive particles. On the other hand, while other metals in the prior art are used on top of the steel core as sheaths (such as copper and nickel), the use of low carbon steel as sheath material has proved to have many advantages that are assumed to be related to the sheath material. In other words,

The modulus of elasticity of iron is 224,000 MPa compared to 124,000 MPa for copper and 196,000 MPa for nickel. Therefore, when the abrasive particles are rocked in the penetrating and advancing directions in the sawing process, iron will provide the particles with stronger bearing capacity than, for example, copper.

The shell material adheres very well to the core material. When the low carbon steel is placed on the high carbon steel, the materials are exchangeable and thus better adhere to each other.

Moreover, when partial diffusion of carbon can occur between the high carbon core and the shell, the shell and the core are like welded to each other.

When the wire is repeatedly mechanically loaded (by bending or tensioning) in a corrosion environment in which hydrogen is formed, the wire breaks prematurely, which is known as hydrogen induced corrosion cracking. This happens because hydrogen enters the steel structure and makes the river vulnerable (hydrogen embrittlement). For example, fixed abrasive sawing wires that are continuously immersed in refrigerant liquid are vulnerable to this corrosion. The closed ferrite layer is known to reduce hydrogen organic corrosion cracking of the wire as described in US 5 014 760.

If the particles are diamond particles, it is further speculated that the carbon of the diamond will diffuse at least to some extent into the iron, thereby curing the indented low carbon steel embedded with diamonds. Furthermore, this diffusion can lead to better adhesion of the diamond in the skin.

These assumptions were made after the described tests, are an attempt to explain prominent results, and are only assumptions made according to the fact and should not be used to limit the invention.

Claims (15)

A sawing wire comprising a steel wire having a core and a sheath, abrasive particles fixed in the sheath, and a bonding layer on the sheath that bonds the particles within the sheath,
The sheath has a substantially ferrite metal structure that allows for indentation of the particles, and the core has a substantially pearlite metal structure that provides strength to the sawing wire,
A sawing wire, characterized in that. The steel wire of claim 1 wherein the steel wire has a weighted average Vickers micro-hardness, the skin has a lower than average local Vickers micro-hardness, the core has a local Vickers micro-hardness higher than the average, and the core and the skin at the boundary. Met, sawing wire. The sawing wire of claim 2, wherein the weighted average Vickers micro-hardness is at least 500 N / mm 2. The sawing wire according to any one of claims 1 to 3, wherein the transition from the core to the shell is sharp at the boundary and no mixed phase is metallographically discernible. The sawing wire according to any one of claims 1 to 3, wherein the transition from the core to the sheath at the boundary is smooth and comprises a mixed metallographic phase of ferrite and pearlite. The sawing wire of claim 5, wherein the width of the transition is greater than 5 μm and the width is defined as a radial distance in which the local Vickers micro-hardness varies between 400 and 650 N / mm 2. The sawing wire according to claim 1, wherein the steel wire has an average carbon content of at least 0.40% by weight. The sawing wire of claim 7, wherein the total tensile strength of the wire is greater than 2,000 N / mm 2. The shell of claim 2, wherein the shell has a shell thickness defined as a radial distance measured from the outer perimeter towards the boundary, and the particles have an indentation depth to the outer periphery of the wire, the shell thickness. The sawing wire is at least the maximum indentation depth to avoid damage to the core. The method of claim 9, wherein the shell thickness d is at least ₉₀ for improved fixing of the particles in the sheath, d ₉₀ of 90 dogs of the 100 particle d ₉₀ A sawing wire, the size of the particles having a size less than. The sawing wire of claim 10, wherein the shell thickness is at least one half the median size of the particles. The sawing wire according to any one of claims 9 to 11, wherein the steel wire has a predetermined diameter and the sheath thickness is greater than 7% of the predetermined diameter. The sawing wire according to any one of claims 1 to 12, wherein the fraction of (area covered by particles) / (total wire area) is 1 to 50%. The method of claim 1, wherein the bonding layer is a metal bonding layer and the metal is selected from the group comprising iron, nickel, chromium, cobalt, molybdenum, tungsten, copper, zinc and alloys thereof. Sawing wire. The sawing wire of claim 1, wherein the abrasive particles are selected from the group comprising diamond, cubic boron nitride, silicon carbide, aluminum oxide, silicon nitride, tungsten carbide, or mixtures thereof.

Images