 coating of monolayer abrasive grain by wetting. Now, exactly what we mean that a surface which can be a metal or alloy or even a ceramic that can be coated with one abrasive grain having a monolayer configuration. This can be illustrated by this sketch if this is the substrate on which the abrasive grains can rest they can be anchored or fixed or bonded in the form of a monolayer. Now, here comes the question how we are fixing this grains on this substrate and that is the bonding technique and the bonding material which are being used and at the same time this is not a continuous layer of abrasive that means, there is some gap in between and it is intentionally done to have some of the desired effect from this particular product and this is exactly what we mean as a monolayer configuration and this monolayer configuration that is being supported by one substrate. So, these are actually abrasive particle and this is the substrate. Now, where we can use such a product that will be of immediate interest. Now, here we see that application of abrasive it has a wide spread application in the area of manufacturing. So, this abrasive can be used in a grinding wheel it can be used as loose abrasive in the lapping process it can be even honing process or it can be what we know as coated abrasive that means, in a belt grinder one endless belt that is actually coated a medium that means, the medium of the belt that is coated with a layer of abrasive and in this case it is actually glued and this is also a single layer formation, but what we mean as we have presented here this is actually the bonding of this abrasives in a monolayer configuration in particular it is going to be a metal or one alloy substrate material. Now, bonded abrasive and conventional bonding material. Now, this is actually what we mean it is actually the grinding wheel or the abrasive tool and as we know that there are basically three types of bond material one is resin the second one is metal and the third one is vitreous bond and this we call conventional bond that means, this is a conventional way of working with this bond. So, that this bond and the abrasive together can make an abrasive mass which can be of end use just like one abrasive tool or one grinding wheel and here this abrasive can be conventional abrasive or it can be super abrasive. Conventional abrasive means aluminium oxide or silicon carbide and whereas, super abrasive means it is diamond or cubic boron nitride. Now, what we find here the requirement on the abrasive tool the requirement of the abrasive tool this is actually what we find in fact, if we see this is just a circular wheel which is rotary in nature and here we have the grit materials. Now, these grit materials are bonded and this is one ideal situation that means, we have some spacing then the protrusion of the grit tip above the bond. So, these are some of the specification of the wheel and this thing cannot be independently chosen what we mean here this protrusion of the tip above the bond and this spacing they have some kind of relation with the steeps chip storage volume that means, this grit protrusion and the spacing they when we put them together they make actually the steep chip storage volume that means, the chip space for easy disposal and evacuation of the chip. However, what is mean by chip volume that chip volume also depends upon the thickness of the chip which is varying from 0 and which attains its highest value once the grit is about to leave the work piece. So, it starts with 0 it assumes the maximum value here and if we can consider this arc of contact and here this is the wheel depth of cut. The wheel depth of cut is quite small compared to the diameter of the wheel and in that for in that practical purpose what we find the volume of the material removed by each grit that means, this grit starts its action from this point and that is the end of this grinding action. It repeats itself however, there will be series of this grits arranged over the entire periphery and each has its own cutting action and the volume of the chip that will be given by half a m into l. This is one thing then this a m value it is also possible to find out and this is given by V w by V c into p into root over small d by capital D. So, V w that is one operational parameter V c that is the peripheral speed of the wheel V w that is the work table speed small d that is the wheel depth of cut and capital D that is the diameter of the wheel. So, from there we can determine a m and what is l that is the length of the contact and this can be very well approximated by root over capital D into small d. So, from all this we are in a position to determine what will be the volume of the chip. Now, the problem begins from that point when we like to use this wheel just not as a grinding wheel, but as a wheel for super abrasive machining at is high speed using one of the best grit material from the diamond family or from the CBN family. So, it is not the problem with the grit material neither it is the problem alone of the bond material, but it is the question of what is the protrusion of this grit which we can maintain above the bond and how with what uniformity these grits can be placed over the entire periphery and this is actually one plane of the wheel and this has a back engagement that means, normal to this picture we have what we called width of cut. So, what we see here that it is mostly the chip storage problem that becomes the major issue rather than the cutting capability of the grit or the strength of the bond. So, the problem here is in summary it is the chip disposal problem or chip storage problem and as a consequence of this could be that this space cannot accommodate this volume of the chip which is being produced and the entire space will be filled in and then that will be squeezed between this job work piece on one side and that is the bond material on the other side and the two grits one is following the other. So, this is actually the confined volume where if it is not adequate if it is not adequate then the whole chip will be squeezed over this surface and the constraints are given here either by this volume of the chip or by length of the chip. Length of the chip also very much comes in picture because of the simple reason this length should be adequate to accommodate the length of the chip. It is of course, uncut length which will be reduced little bit, but one thing should be also kept in mind that when we have larger arc of contact particularly with a large diameter wheel and also large depth of cut. In that case the length will be sizably large it will be sufficiently large and if we do not provide adequate length in between the two crystals there will be folding back it will be like a coiling and this coil also can have some squeezing effect in this volume available between the bond and this arc of contact and these two grits which are one is following the other. So, this is one of the major issue as we consider grinding and transformation of that grinding to abrasive machining involving high speed and also high material removal rate. There comes the question of the chip disposal or chip evacuation. Now, considering this aspect we see that there are some limitations serious limitations with this present state of the art. Apart from this chip storage problem what we can see the complexity of the manufacturing route we start this is when we use these abrasive grit with either resin metal or vitreous bond then it is mostly the conventional way of working that means, we need a mould without this mould we cannot give the shape of the wheel. So, it can be cylindrical it can be cup shape it can have some beveling it can have a v form it can be a small wheel very large wheel whatever may be the case we need a mould and this mould shape is important in that that will give us the final form or geometrical form on the wheel and when it is a complicated form then it will become rather difficult to transfer this geometry from that mould to that actual piece and also the flexibility of making the wheel of any shape that also becomes a questionable issue that how flexible the process is when the demand for variety of geometry is so high. Now, as a solution to this problem we have another kind of wheel it is a different concept it is no more a solid mass as we see in a conventional wheel it is a solid mass which is moulded in a cavity and then it has to go through all the process. So, it is a mould it is a grain moulding then sintering or firing then follow up process before grinding we must go through this follow up process which is string dressing and grit conditioning. So, these three processes the wheel must go through before it become one of the efficient grinding wheel for this particular grinding action. Now, this truing dressing or grit conditioning it may be not that difficult task when it is a conventional abrasive what we mean aluminum oxide or silicon carbide then this truing dressing and grit conditioning can be well handled and managed, but when it becomes diamond or CBN then this follow up process after this firing or sintering this is not so easy it is a very delicate and difficult task to carry forward this truing dressing and grit conditioning. So, considering two issues or problem one is chip storage problem another is the flexibility in manufacturing the wheel which can be used with a particular shape where the manufacturing itself is a very flexible process plus the material plus the profile of the grit or profile of the grinding wheel is prepared in that it can take up heavy grinding that means, high remove material removal red grinding or what we call stock removal grinding and for that we have a new concept and that is exactly what we call monolayer abrasive tool and it is going to offer a solution to this this aforesaid problems. So, monolayer abrasive tool basically what we see here that as we have mentioned at the very beginning of this discussion that means, on a substrate it can be say for example, this is metal substrate and over that we have the grits. So, this is just like a one layer formation it is one way it is very similar to a coated tool where on a high speed steel or a carbide tool we put a coating and the life service life of the tool depends upon the survival of this coating similarly. So, it is just not a tool with composite structure that means, regrinding or resharpening of this tool is not just possible once the coating fails we also can say that leads that is the failure of the tool similarly here this one layer of grit that is supposed to work for a reasonable period of time and it is not even a composite structure in a composite structure of conventional wheel when the first top layer become dull or become flatten obviously, the truing or dressing operation comes in picture and that removes the dull layer. So, that one layer from behind that can appear and that becomes the active layer of the abrasive, but when we set one layer of grit then this whole performance of this wheel that depends upon this retention of this one layer on this surface. So, here of course, we must have some kind of bonding agent some kind of bonding agent and also the technique of bonding. So, this is the substrate that means, the basic material and there we have this bonding agent it can be various ways it can be done. So, this is the bonding agent now question is whether with such a bonding agent and a bonding technique we can really bring up a tool ideal tool which can be given any shape according to the need of the grinding and at the same time we can also provide adequate space in between the crystal and the space above the bond. So, that adequate space is available for easy clearance chip removal and at the same time this wheel can be used with high material removal rate even under dry cutting without any support of lubricant or some coolant and at the same time most important thing is that this grit should not pull out or get dislodged then that is one of the issue that means, what will be the level of coverage if this is the grit. So, one would be interested to know that if we consider just one single grit being resting on a metal support and to what extent we have to put this layer whether this is as low as 10 20 percent or it should be as high as 70 percent that is the issue and on that the success of this bonding material and the bonding technique will depend because ultimately the whole aim of this work is to bring up a tool or an abrading surface which provides adequate clearance space. Obviously, that means, low bond coverage, but at the same time without the risk of fallout of the grit. So, these are to be met by using a particular bonding technique and a bonding material. Now, comes the question which has been raised how to manufacture mono layer abrasive tool, there are actually two technology for making such thing one we call electroplating technology another is active brazing technology. So, these are the two available processes which can be used for making such configuration that means, the abrasive grit with mono layer configuration. Now, here what we see that this is just a schematic representation of this wheel where it can be CBN or diamond. Now, why do we prefer in single layer configuration CBN or diamond the reason is obvious the choice is obvious that means, here if this is the metal support which is the base for holding the grit and these are the grit these are the grits and there is a bonding material. So, that is the substrate this is the bonding material and these are the abrasive grit. Now, the whole life of the tool depends also on the quality of abrasive or wear resistance of the abrasive that means, its retention of the hardness resistance to plastic deformation then thermal stability chemical stability all this issue should be taken into consideration and these wheel wear out with passage of time that means, if we consider that this is the limiting bond level this is the limiting bond level we cannot go below that point that is the critical thickness of the bonding layer then from this point to this point that is actually the zone over which this grit is expected to work efficiently. So, this is actually the rate of wear which will determine the service life of this grit. Now, wear from this point to the very close to this bond level and in that we know that aluminum oxide or silicon carbide they are not that wear resisting in comparison to CBN or diamond. So, where we normally use silicon carbide as conventional abrasive. Now, if it will be it has to be replaced by a super abrasive then the choice is diamond similarly, in most of the cases if aluminum oxide being the conventional abrasive is to be substituted for by CBN substituted for by super abrasive then it becomes CBN. So, CBN and diamond one layer that is that can be equivalent to few hundreds of layer of grains of aluminum oxide or silicon carbide. So, grinding ratio if we consider the grinding ratio of CBN to AL to O 3 it can be few hundred the ratio that means, wear volume of aluminum oxide will be few hundred times compared to the wear volume of CBN. So, this from there we come to immediate conclusion that it should be CBN or diamond those are the best candidate for this single layer tool. So, here what we see that with this galvanic tool though it is better much better compared to the conventional wheel that means, where we need this molding technology. However, the question is that to hold this grit in place that fallout should not be allowed dislodgement. So, it is also our common experience that for holding this grit in place with this layer that this is the electro plated layer or galvanic layer and which is in most of the cases nickel or in electro less splitting we also use nickel phosphide and that has to be raised at least two third or seventy percent for holding this. So, though we get thirty percent, but still considering the requirement of grinding and lot of high expectation still this does not give us the best possible solution for solving this problem of grit holding capability of the bond or creation of the chip space. So, if we consider these issues then we must look for a better process though we understand that this is a wheel which can show mart improvement over the conventional wheel when it is the question of high material removal rate number one and number two when we need the wheel of different geometry in short notice the idea is that in this case we what we need we just need a preform a preform it can be any shape. So, it can be machined with by precision machining and over that this grit can be anchored by this electro plating technique. So, only the preform is necessary and we can do away with this molding or the requirement of the molding. So, that is one of the greatest advantage of this technology. Now, this limitation of galvanically bonded monolayer abrasive wheel which has been just now highlighted because it is at least 70 percent coverage that can be reduced to 20 to 30 percent coverage by this brazing technique. So, what we are doing here we are not changing neither this material this core area of the wheel which is a steel a special steel according to the need of grinding also the same grit material it remains as it is, but what we are changing here it is mostly the bonding material and the bonding technique. So, one can easily compare this bond level which is quite high and that can be reduced here and interesting part is that this is done by this reactive wetting. So, this term is very important reactive wetting of a braze alloy or the bonding alloy and this because of this wetting if we can achieve good wetting we get a concave surface whereas, in the previous case it was like a convex. So, at the root here we have less coverage and in between we have high rise of the bond we can compare this here we have larger thickness and because it is non-wetable wetting is not that good and so here it is like a convex very typical of not wetting character, but here it is wetting character. So, what we need in between we need low bond coverage that means high clearance and along this side of this grit what we need that material should be pulled up by lowering this interfacial tension. So, that spontaneously there will be the material will climb up and giving this necessary support on the two sides of the wheel two sides of the grid and as this one each grid is participating there will be grinding force and in that case this chip which is coming out it has larger space for easy accommodation. There will be less chances of wheel loading or squeezing and the wheel offers free cutting even under dry condition not only that because of this large grid protrusion from this point to this point the service life of this cutting point will be longer in comparison to what we find in this case in this case this will be only the protrusion available. So, what is important in this case very important issue that as we have mentioned that this is reactive wetting it is not no more non-reactive wetting. So, here we must have a reaction layer and that has been shown by this schematic figure that say this is one abrasive grain maybe diamond or CBN and this is a low carbon steel substrate and this grain is held over this steel substrate. However, in between this is the active brazing alloy which can go to a chemical reaction with this abrasive material and the result would be the formation of a reaction layer at this point. Now, if it is CBN that means boron and nitrogen very much there and if we use say for example, one alloy this bonding alloy which is silver copper with addition of titanium as an reactive material then what is going to happen this BN plus titanium that will lead to TiN and TiB2 that means the surface of the CBN which if it is a smooth or featureless surface after this reaction if one removes this whole thing this area by proper etching chemical etching then the surface will reveal itself but it will not be any more that CBN, but there will be a reaction layer and it is obvious that this thickness of the reaction layer that can influence the quality of this brass mint or brass joint. We have already discussed this issue that this thickness of the reaction layer whether it is grit material or a bulk ceramic that there must be a reaction but this reaction should be confined to a very narrow zone. So, a reaction should occur but thickness of the reaction layer should be as small as possible. So, this point should be also considered in this particular issue. Now, requirement of the bonding alloy now what are the requirement of the bonding alloy? Obviously, it should be wetting number one the necessary condition that is the wetting the material must be also ductile this ductility is also one important term in that unless the material is ductile there will be tremendous amount of thermal stress at this brass mint it is because of the simple reason that ceramic though it is very small it is CBN and on that other side we have steel and in between the bonding alloy which is the brass alloy. So, alpha E this alpha E and alpha E that means thermal expansion coefficient and young modulus these are varying widely. So, one should be flexible enough to adjust itself so that this difference can be adjusted through this deformation of this brass alloy otherwise CBN is cannot be deformed steel is also not that easy. So, we must have a soft alloy that means having the ductility and it is also one of the important factor then basic strength of the material it should also have certain strength so that under the action of the force the material should not undergo plastic deformation if it undergoes plastic deformation then the whole purpose will be lost purpose of brazing because there will be some grinding force and because of the grinding if there be some rise of temperature then at that temperature material strength should be sufficient not to allow allow large deformation. So, that should be within the control limit so it is also the strength holding that is also one of the important issue and another thing so strength comes and more importantly wear resistance. If we get back to this figure quickly here what we can see that this chip is sliding and this will obviously it is expected normally it will slide over the surface now this surface should have some kind of tribological quality that means either the friction or wear resistance it must have a good value of this friction coefficient or wear resistance so that it should not also wear out too fast and if it wears out then there will be fallout of this material. So, these four property this bonding alloy should have but at the same time this wetting what is this wetting what does it mean suppose we have this grit material and this is the bond material the steel substrate so the braze alloy should have a balanced wetting towards the steel and also towards the grit. If there is not this balance then the interfacial tension if there is too much difference in interfacial tension if interfacial tension between this braze alloy and this steel is high and with the braze alloy and the grit is low then the whole material will be pulled and you can have accumulation of the material which will be pulled from the place around this grit and the bond height will be excessively large so the whole purpose of brazing will be also again lost. So, but if we have adequate wetting on steel then perhaps we can expect to have a profile something of this form so here we can have something of this form so here we have good wetability of this alloy towards the steel that means the base and this is the abrasive and that is the braze alloy that means the bonding alloy so it will have very good wetability and it can give a supporting wall over the entire surface of the grit and thereby reducing also the bond level in the space between the crystal. So, good wetability towards the steel and also good wetability towards the grit both may both characteristics this bonding alloy should show. Now, requirement of the substrate material now requirement of the substrate material obviously the substrate steel if it is steel it should be such that as we have already mentioned that braze alloy should have good wetting over this and another requirement of the substrate is that it must also have adequate strength rigidity and since in the brazing we have little high temperature so the distortion or change in shape because of this rise and fall of the temperature at certain level that should be taken care of the material so that the it does not crosses its limit of distortion and otherwise this brazing cannot be accepted as a process for making grinding wheel. Now comes the brazing parameters now what are those brazing parameter number 1 it is actually the brazing cycle number 1. So, brazing cycle we can consider this is the time and that is the temperature so here the rate of heating rate of heating then soaking and then it can go like this and this is the holding at that brazing point and then it comes like this so the entire thing and it comes almost to room temperature so this whole thing we call a brazing cycle now here this rate of heating then soaking at a temperature just below the liquidus temperature of the material that is very important to have uniformity. Now when it is geometrically symmetrical in that case problem is not that complicated but if we have a complex geometry then rate of heating in steps and holding in various steps to have proper soaking that means to make the temperature throughout the entire mass uniform that is also one of the very important consideration and this way the temperature can be brought very close to the liquidus temperature and holding there so that the uniformity of the temperature over the entire mass can be ensured and then thereby quickly raising this point to that brazing temperature and holding there for a very short period of time that is also important. Now here this temperature of brazing time at temperature these are the two very important parameter apart from that what we also see that environment whether it is inert atmosphere or whether it is a reducing atmosphere or high vacuum atmosphere that also to be taken into consideration. Now one thing should be also mentioned here that it is reactive brazing. Now reactive brazing means one of the reactive material should be present in that metal or alloy so it is a metal or alloy or it is a metal and alloy reactive metal and alloy and this reactive metal what should be the amount of this one that is also very important because what we want here we want a wetting only to have coverage over this wall of the grid. This is ideally the brazing so to have this what is important to know the fluidity of the brazing material or the bonding alloy and that depends upon the basic constituents whether it is metal or alloy plus the amount of this reactive element which is present that should be also taken into consideration. If we are interested in diamond what is our experience that chromium can be used as one reactive element in nickel phosphorous or nickel boron silicon system so this is a quite effective alloy to get this kind of profile with diamond the reason is that this chromium can react with the diamond surface making a chromium carbide. So, this carbon and chromium that goes in this direction may be CR7C3 one of the carbide and here this delta G is negative so reaction proceeds in this way and this nickel will wet so we can get such a profile with such kind of alloy and the desired profile we can have, but when it comes to CBN so in that case we have to write this one with chromium, but this reaction does not move in this direction and we hardly we can achieve such profile that means this kind of alloy is not at all wetting the surface of boron nitride. Now, here one should look for how this chromium nitride or chromium boride can form now we have to look in the change in free energy now for brazing we must have a temperature limit of technical interest and if it is within a 1000 degrees lower it is always better, but if we consider a limit of 1000 degree it is shown that at 1000 even at 1000 this wetting of this alloy either chromium nickel phosphorous or chromium nickel boron silicon that alloy this wetting is not possible in this case so here we see that the free energy of boron nitride is higher than that of chromium nitride or chromium boride, but when it comes to when we take a alloy which is readily available titanium silver copper there can be many version or many formulation this is one of the basic this is one of the basic and here we can see that this carbon of diamond and titanium go straight to TIC formation this is wetting is perfect with this alloy also with boron nitride plus titanium we can also have TIB2 and TIN of course here hypostoichiometric layer should form on the outer surface, but the basic thing is that this TIB2 and TIN which are more stable than boron nitride so here we have a delta G minus this one also delta G minus so this way one can look into that the severity of the reaction the degree of reaction so this amount of reactive element and also it has been mentioned that the existence of the ternary element reducing the solubility of this reactive element thereby increasing its activity reducing the concentration requirement of this reactive element all these issues are taken into consideration so that finally what we can achieve this one this one we get back to this slide that means this thickness of this reaction layer that can be properly controlled so in summary what we can say that this thickness of the reaction layer should be controlled by this brazing temperature number one and number two by the amount of this reactive element and the presence of one of the ternary element which can control in a very favorable manner the activity of this reactive element now here we see the different level of bonding so if it is very high temperature the grids are totally covered and this is not a very desirable situation and in one case we can have also the brazing quite satisfactory brazing at a temperature as low as 720 holding these grids and in another case this has to be elevated to 800 degree centigrade and in all cases what we find that it is actually the amount of this reactive element and also the melting point or liquidous point of the brazing alloy or the presence or absence of this ternary element which regulate the activity of this reactive element in the alloy and all this will make this difference which are illustrated by this and obviously if it is one passive alloy then there is no chance of wetting and this passive alloy what can be interesting that this passive alloy it cannot wet the surface of the steel either so the existence of titanium or presence of very titanium is so useful effective also to have wet ability over the steel so here we see that this is the very strong reaction showing this needle like thing titanium diborite and it is a very strong reaction with a alloy having almost 10 percent titanium and temperature 920 degree whereas when it is at 720 we have this reaction but it is a mild reaction thickness is also very low and this is good enough to have a very good wetting but at the same time having a adequate bond which is guaranteed by this thinness of the reaction layer. Now this is the outcome of this brazing technique where the grits are implanted or bonded to this surface and it is actually the coating that means it is a coverage of the grit on this wheel surface in a mono layer formation and each grit can be located at a fixed distance thereby we can also maintain the gap between the grits this is required this is desirable from the requirement of grinding. So this is the actual profile of the grit which are bonded and this is the coverage of this grit over the surface now in this brazing technique what is in essence the effectiveness that the concentration of the grit can be adjusted according to the need of the grinding if it is large material removal rate we can increase the inter grit spacing but if it is more wear resistance precision grinding high surface finish we can also reduce the spacing and it can be done by this screen printing technique that means temporary fixation of the grit and then by this law of wetting a surface of this nature can be immediately realized. So this summary what is also important here to know that though this grit is brazed and apparently there is effective wetting satisfactory wetting and thinness of the reaction layer that is also ensured here. However what we can see here that because of this contraction at this point there may be a crack formation and this contraction depends upon the relative young modulus coefficient of thermal expansion and also the liquidus temperature which is also very important parameter which can affect this crack formation so their adequate attention has to be paid otherwise what will happen even after getting adequate bonding good chemical attachment because of this generation of the thermal stress this grit can have a breakage at this bond level. So this is the coverage of the bond and because of this crack formation this part can be removed just because of this thermal cracking. So with all this thing taken into consideration we can summarize the discussion of today's that high performance abrasive tool can be manufactured by brazing super abrasive grit on a metal substrate in mono layer configuration. Effectiveness of brazing depends on wetability of the bonding alloy towards the abrasive as well as the substrate. Precise regulation on the brazing cycle is also an important step in that direction. The brazing process of course facilitates control of grit spacing and grit protrusion on the tool surface.