 sputter deposition of molybdenum disulphide coating. Now, this molybdenum disulphide or say tungsten disulphide these are the materials which we can put in the form of this formula M X 2 and where this M stands for one element of this transitional group and this sulfides are known to have very low coefficient of friction or in other words the super lubricity. So, this materials are known for their super lubricious property. Now, how this super lubricity is obtained? Now, to look into this issue what we can see this is actually MO S 2 that means, it is as if it is S, M, O and S. So, when we like to see its structural formation we can see that we have a hexagonal array of this sulfur and molybdenum. So, this is actually one plane containing the sulfur in an hexagonal array in an hexagonal arrangement and just with it adjacent to it we have another plane and this plane is actually the plane containing molybdenum this is also in hexagonal array. So, that means, here we can show these lines and finally, what we have here another plane that is containing sulfur. So, this is one set of MO S 2 and below that what we have further to this we have again here another plane containing sulfur and followed by molybdenum and sulfur. So, this is one set this sulfur molybdenum sulfur then also we have sulfur molybdenum sulfur. So, what we find normally that the bond between this molybdenum and sulfur that is a covalent bond that is very strong. However, the bond between this sulfur and this adjacent sulfur which is from another MO S 2 that set this bond is rather weak and also this distance this inter atomic distance that is more than the inter atomic distance here between two sulfur planes. So, this distance is actually high compared to this distance and this force between this planes that is a very weak force and that is actually van der Waals force of attraction and that is weak force. So, when a force is very much acting on this plane then this sulfur molybdenum bond that does not break, but the bond between these two being the weak. So, this plane can easily slide over this one and that ultimately results in a very very low value of friction and it is exactly what we know as super lubricious property of those materials like molybdenum disulphide or tungsten disulphide. Now, here what we can see two planes are there, Basel plane and H plane these are to be taken into consideration because the orientation of this plane is extremely important to have a very low value of friction, low value or high value good performance or poor performance these two will depend upon the very orientation of this Basel plane or this H plane. Now, let us look what is mean by this Basel plane and H plane and how they should be oriented. Now, what we see in actual field of working say this is one substrate it may be made of just a low carbon steel C 20 steel and we like to deposit a coating of molybdenum disulphide over this. So, it is just not a coating of MOS 2 it is just not MOS 2, but it will be MOS 2 with right orientation and if we cannot get this right orientation we may not get a very good value of coefficient of friction or a what we call super lubricious property. So, this is very important. So, let us look what is what does it mean if we put this hexagon like this, this is one way of showing this is one way of showing. So, that means, here this axis what we call C axis that is perpendicular C axis is perpendicular to the surface of the substrate. So, this is one situation. So, this C axis is perpendicular to the substrate or we call it Basel plane. So, here two thing we must take notice of those two things one is Basel plane is parallel to the substrate surface and obviously, C axis is perpendicular, but we can have another extremity. So, another extremity means this way. So, let us have a look quick look here that this is another situation ok. So, this is here what we see this Basel plane this plane. Now, this is no more parallel to the substrate surface, but it is perpendicular that means, the C axis C axis is now parallel to the substrate surface and this is actually the H plane and that is now parallel to the surface. So, this is one of the extreme worst situation under which condition what we can have we can have a very poor performance of this coating. Now, this can be so, here we can summarize one thing that even to have this coating whether it is PVD or any other process first and foremost thing is that whether this Basel plane is parallel to the substrate surface because only in this direction as it slides as it slides here in this direction then only we can expect good coefficient of friction and if it is perpendicular like this then this sliding will not be easy rather sliding will be difficult and with result of high value of friction, high sliding force and removal of the coating. Another thing also we can look here that this normal orientation say we have the substrate here and these are the this is just the arrays of coating. So, if this is the orientation and then these are the Basel plane is like this. So, then there is also attack from the atmospheric oxygen and that can also damage the coating. So, when this H plane that is parallel to this substrate surface in that case also we can have oxygen attack in this way that means, here the attack will be difficult, but here it will be attack will be rather easy and in this way the atmospheric oxygen or humid in humid atmosphere the moisture can get in and then it may cause deterioration of the quality of the coating. So, then under the action of this force this coating will immediately break and that will lead to high value of friction. So, this is what we call Basel plane and H plane. Now, we have type 1 and type 2 structure. So, type 1 structure what we can call we can refer to this previous figure that means, if we have the Basel plane parallel to the substrate surface then we call this is as type 2 structure type 2 and when the Basel plane is perpendicular to the substrate surface we call it type 1 structure. So, obviously, our choice will be type 2 structure considering that low value of friction is most desired and this is for the purpose of having the best performance of any component or where part. Now what we can see further to this sputtering of molybdenum disulphide. Now this MOS 2 what we have written MOS 2 this sputtering is possible by at least two processes because this is not a very good conductor of electricity that is why earlier this is sputtering that was not very popular and what we had it is mostly RF sputtering RF radio frequencies sputtering and we know that with this RF it is possible to use a nonconductor or a poor conductor as one of the target material. So, it was a RF, but RF has a complexity that is also noted that it is the complexity of matching the capacitance with an external capacitor and it is also the impedance matching. So, that is why later on came into being what we know as mid frequency pulse DC it is actually mid frequency pulse DC sputtering. So, both are very very useful in having a coating of molybdenum disulphide on a substrate and in this case it is mostly for all mechanical application it is going to be a steel substrate which can be coated with this molybdenum disulphide to have better performance in all tribological activities. Now here the sputtering of molybdenum disulphide we can take a solid target of MOS 2. However, the question is that during sputtering when this material is coming like a flux and intercepted on this side by the substrate. So, this is actually the substrate and this is target. So, the question is whether we get here also MOS 2 or it is MOS X where X is less than 2 and if it is less than 2 is there any problem associated with the value of friction. So, these are the few things can be also noted for this sputtering operation and at the same time there is another way of doing. So, this is applicable whether it is RF or mid frequency pulse DC sputtering and in this case definitely the parameters which are going to influence this value of X that is on one hand we have the cathode current. So, it is actually the cathode current that is the set value of cathode current this is number 1, number 2 is also the process pressure process pressure that is also another parameter and number 3 what also appears to be quite important and it can also influence that is substrate bias voltage substrate bias. So, these three this combination is very important their selection of this and their combined effect can lead to a particular value of X and not only that it can be also this formation of this basal plane or H plane. So, this is what we can mention here. So, when it is the basal plane so, from this XRD X-ray diffraction diagram we get a peak of 002 and that is a confirmation and that is a confirmation of the existence of this basal plane which is given by this 002 orientation and that is parallel to the substrate surface. It can be H plane and in that case the peaks comes from this 111 or 110 and for that we have definite angle to theta and from this from the standard XRD corresponding to this MOS2 we can find out the value of theta which corresponds to this 002 or 111 or 110 and from that concerning the quality of the coating we can have fair idea and whether it is the effect of high cathode current or wrong choice of process pressure or the substrate bias selection all this thing can be known just by looking at here that whether on the XRD diagram which peak is coming up and depending upon the sharpness of the peak or the little widening of the peak from there also we can have fair idea impression about the size of the grain fineness of the structure apart from scanning microscopy. So, these are the few things one has to look while going for this MOS2 coating deposition because ultimately it is going to be one aim that it is the best possible mechanically functional coating. So, from this XRD we can get a fast information and the very preliminary information, but it is very very useful in that that whether we are in the right track or right direction in the whole deposition process or deposition activity and whether the process parameters selections are proper which may finally lead to one of the best performing coating. So, this is one thing we one has to look, but apart from this it is also possible just instead of using a target which is made of MOS2 what we can also do it is also called reactive sputtering reactive sputtering. So, in that case we have one we have one molybdenum target and then we have one substrate facing this molybdenum target and here just like reactive deposition by sputtering say for example, titanium nitride deposition or aluminium nitride or chromium nitride or even it is molybdenum nitride, but in this case it is going to be molybdenum disulphide. So, MO flux sputter flux will come in this side it will on its flight towards the substrate and then we what we have we have here incoming instead of nitrogen it will be H2S that is the source of sulphur and then this MO from this target and this H2 from H2S from this external source that can also make this finally, MOS2 this is also possible and in that case perhaps this problem of less sulphur in the coating here. So, this is on the on the substrate that means, less sulphur deficiency in sulphur that can be eliminated by this reactive sputtering. So, that is also possible. Now comes coating architecture what we want we want a stable coating and this stable coating means it should be stable not only in a very restricted atmosphere that by that what we mean that it should be high vacuum this is a restricted atmosphere high vacuum it is a dry environment no moisture content and we know that from our experience that this high vacuum dry environment here MOS2 it is one of the best material for giving one of the lowest value of friction, but when it comes to the question of practical use normal use and field use where we cannot maintain this high low value of humidity it is normal atmosphere in that case this MOS2 can show its weakness that means, it will simply absorb moisture and then from that oxygen will be active and this sulphur will be attacked and it will be actually S and O2 it will lead to sulphur oxide. So, oxygen will convert this sulphur into sulphur oxide or it can go inside it can also make molybdenum oxide. So, there are lot of investigation there are lot of documentation in this particular subject whatever may be the thing ultimately it is the total failure of the coating. So, what we like to say here that just MOS2 alone that can be good candidate for such high vacuum or dry environment, but for mass scale use in numerous mechanical application this alone MOS2 cannot just serve as a good candidate and good coating material. So, that is why there has been lot of effort to formulate to have a new formulation of this MOS2 and that is done by doping with some other material that means, on one side we have MOS2, but another thing it can be say titanium chromium. So, these are the good candidates at for the literature research literature there had been also silver for example, then lead also used which may not be encouraged much then gold. So, these are the candidates which has been tested and here as per the literature search it is found that this titanium chromium and also silver they appear to be quite effective in that now this MOS2 with little addition of titanium chromium or silver become more stable in high humidity and also with little rise of temperature. We know that from research document that MOS2 that is in open atmosphere that is stable up to 400 degree centigrade. So, when it is the requirement is above 400 degree centigrade for any application of that component or part then we have seen that this frictional coefficient it rises quite fast and that means, that is the end of the service life of the coating. So, this coating is no more useful however, the whole idea here is also to have this titanium chromium or silver with MOS2 to increase its capability to work at a higher temperature. And in that in this respect we can mention there that we have on one side MOS2, but also we have WS2 that is a addition later addition tungsten disulfide. So, this has a capability of holding its stability it is chemically stable still up to 700 degree centigrade that can be also another good candidate, but it is to be also seen whether by adding such material in this side whether we can also augment its property for various field of application where still we have to work with a higher temperature or with a higher level of humidity. Whatever may be the case now this coatings routine practices to not just MOS2, but to have this doping of titanium with MOS2. So, the basic structure of the architecture of the coating which was we used with RF that was just MOS2 and it is mostly a columnar structure. So, it is the substrate this is the substrate and on that we have MOS2 coating and it is mostly RF sputtering with just conventional magnetron RF sputtering, but with the advent of this unbalanced UBM and CF UBM it has opened up new opportunities and new scope of working with this MOS2. And in this case we can have a better architecture and we can blend this MOS2 with one material say for example, titanium. So, with this addition the architecture looks like and here we have MOS2 plus titanium. So, this titanium addition increases the capability of this coating that has been also established that with this addition of titanium this coating can work in an environment with is rich in moisture, water content and the coefficient of friction is quite low or even if we increase the temperature of the coating still this one is better than this one. So, here also the pressure that means, the process pressure that can be brought down compared to what is needed here because it is now unbalanced magnetron or close fields unbalanced magnetron. So, with that we can bring high ion current on this side and that is actually the case where we can work with still low value of the pressure and according to this new structural diagram for this coating architecture we can see that even with low pressure and low temperature we can get a very good architecture of this coating and then we have widened the scope of activity of this coating. So, this is one of the structure, but when we like to use this thing we can also have the architecture like this, this is the substrate and then obviously, we have to have a sub layer of titanium which is a adhesion layer. So, this is of titanium and over that what we get we get actually this MOS 2 and titanium this is actually MOS 2 plus titanium. Now, during the sputtering what one has to do just to have a transition first of all the routine process route has to be followed that means, first of all cleaning the surface with titanium then this titanium is used as the ion for cleaning this one then changing the condition of sputtering this titanium now should not be just used as for ion etching, but now this titanium should get deposited over the surface. So, the first step is cleaning this substrate surface with titanium followed by deposition of titanium and process parameter has to be monitored properly and then according to the requirement when we get this titanium sub layer that means, that is called the adhesion layer then obviously, the target for MOS 2 that should be switched on that should be activated and here obviously, it is the titanium cathode and MOS 2 cathode these are the two cathode and for the reason. So, this is you have titanium cathode current and MOS 2 cathode current. So, this cathode current should be adjusted and this should be. So, we have proper intermixing between titanium and MOS 2 in this transition. So, it is gradually the titanium percentage is getting down and MOS 2 percentage is going up and finally, what is required in the coating that is actually MOS 2 plus titanium and that is the final content in titanium. So, once the transition is done then we have a steady state deposition and where keeping this titanium cathode current constant and MOS 2 cathode current constant this deposition can be continued for several hours to have a consistent bulk value of MOS 2 and the bulk value of T i content in this composite coating. So, this way one architecture can be built up we can have another architecture that is say for example, it is also known that if this is the substrate then over the substrate we can build a titanium nitride coating there is no difficulty in getting a titanium nitride coating. So, titanium nitride by reactive sputtering by using a DC or RF or pulse DC that can be very well used. However, the whole idea here is to improve the performance of MOS 2, we know that we call them superlubricious coating, but at the same time this MOS 2 or WS 2 we also call them soft coating. So, we have actually 2 extremities ultra hard coating hard coating and this is soft coating. So, that is a wide range on one side we have diamond CBN those are ultra hard material in between we have all hard material that means, conventional hard material say for example, all carbide nitride boride of transitional element we have aluminum silicon they are nitride or oxides that can be also mentioned. However, on this side on this extremity we have a soft coating of MOS 2 and tungsten disulphide. Now, this is going to be used on a steel whose surface property has to be improved a lot and this surface property means it is the tribological surface property. So, this obviously our choice will be a superlubricious coating which we have already mentioned. However, it is found that this coating can be also improved further what we mean its performance if we put a coating of titanium and if the deposition is done on titanium that means, this should be now the architecture. So, this is now the architecture this is MOS 2 plus titanium. So, it has been also seen that established from research document that a coating of MOS 2 plus titanium directly deposited on a steel substrate on a steel substrate and a coating deposited on an intermediate layer of TIN. So, this architecture is better than a architecture only containing MOS 2 and titanium. So, this way one can also design the architecture of the coating just to improve the performance and here it is said that the load bearing capability of this coating is better and higher when it is put over a hard coating. So, a soft coating is well supported well borne by one hard coating and here the resistance to deformation is more because when the load is applied on this side there will be a tendency of deformation since the substrates of lower hardness. So, naturally whole thing will get deformed along with MOS 2, but if we have a properly chosen architecture of a hard layer hard metal like TIN then it is expected that this coating will improve its performance. Now, this is one architecture, but it can be extended to this one say we have again a steel substrate. However, on this we can have titanium coating this is a titanium nitrite coating and on this what we have it is just not MOS 2, it is just not MOS 2, but here as we see here say for example, this situation here it is actually co-deposition. So, this we say we should say it is a composite it is a co-deposition co-deposited coating. Similarly, now what we can do instead of titanium we change this titanium to titanium nitride. So, this top layer now this top layer is going to be a MOS 2 plus titanium nitride. So, in this case what has been done it is in principle MOS 2 on which titanium is added to improve its property to stabilize this MOS 2 coating to stabilize 002 plane and so on. In this case what we like to have TIN that is one hard coating. So, on that we have put MOS 2 TIN coating. So, if we consider alone how will this performance will differ say this is just one TIN coating and substrate. So, we have now three architecture one just with TIN another with TIN on the top of that we have MOS 2 TIN coating and in another case we have TIN on the top of that we have MOS 2 plus TIN and what we have already discussed this is actually a coating which is having just what we have mentioned this is actually MOS 2 plus Ti. So, whole idea here to maintain this lubricity this remarkable lubricity of MOS 2 over a longer period of time and in any atmosphere in any practical environment. So, for that lot of architecture can be proposed and can be tested. Now, here that what is the difference between these. So, here it is just on the substrate. So, compared to this we have a better load bearing capacity here. However, TIN is a better choice when it is the wear resistance and hardness is concerned. So, just to improve its lubricity or say coefficient of friction in that case we are adding a top layer, but what is done here in this case it is actually a TIN coating plus MOS 2 plus TIN. Now, whether it is complementary to each other or supplementary to each other that is the big question here because it is something comes in support of the other whether it is MOS 2 that is going to help out TIN or it is rather it is the other way round. What we have to see here the whole idea here to have what we call a hard coating and also lubricant and also having this lubricious property. So, it is also hard coating and also it is serving like a lubricant of course, it is solid lubricant. Now, all this MOS 2 WS 2 these are going to be a substitute for all sort of grease oil type of lubricant and in most of the parts of machine heavy machine tools. The parts are going to be coated with this MOS 2 or similar material and we must have suitable process to coat large parts. So, that those machine can become also dry machine. So, dry machine means without having any application of oil or grease or any blend of that, but however it will be just a dry part. So, there is no question of oil drop or grease coating. So, it is a solid lubrication. So, here what we find that we want both hardness and lubricant because of the simple reason that when it is the question of wear resistance and with high sliding speed, high temperature and high load. In that case this coating from family of hard coating that means, the material from the family of hard coating they are found to be very effective in that the wear resistance is excellent and remarkable, but when it comes to the question of very stable friction coefficient even at a very very low speed and at a high load say for micro movements and other places, then it comes to the question of super lubricious material which can maintain that condition of very smooth movement without any jerk without any stick slip and for that this MOS 2 and similar materials are so useful and so effective. But here the idea would be the whole idea would be not to just keep this use restricted only to slides guides because mechanical component means it can be slider, mechanism, slides guides, screw nut, it can be cam and follower, it can be the bush and the bearing, it can be gear and pinion and so on. Wherever we find any sort of rubbing, it is only where we can pay attention just by putting this solid lubricant in the form of MOS 2 and similar material, but say when it is the tool of production say metal forming tool or metal cutting tools in those cases also this material having this super lubricious property that can be also have that can also have one use effective use and that can be also attractive for those and that is the idea behind here to have this blending of this hard coating and lubricious coating and that is why one can also call it as hard loop coating. So, it is actually hard coating and else the self lubricating coating. So, this coating can be used for all sort of application like machining as a cutting tool in all metal forming die, punches and similar areas this can find extensive use. Now, the question that these are the architecture. Now, how we can rank this coating architecture? Now, for ranking this coating architecture we have to find out some ways or means that means, this coating has to undergo some kind of characterization and it is going to be a mechanical sort of characterization that means, the coating whatever we have mentioned here that means, just molybdenum disulphide without any support of TIN. We have a hard coating a conventional hard coating TIN and we have MOS to with TIN support and we have also MOS to with a TIN support and this is a composite coating. So, with that what we can find here this hard loop coating for ranking what we have to see here this say hardness of the coating hardness of this coating then adhesion strength and then coefficient of friction ok. So, against this what we can see that when it is the hardness obviously, that hardness of titanium that will be the best out of that family. So, titanium will be the best and it is also our experience that this titanium nitride plus MOS to with a support of titanium nitride at the just beneath it. So, that beneath it that is also another thing. So, that is actually the support that is the second best. So, this is one thing now when it is the coefficient of friction and then what are the other coating what we have mentioned hardness wise this MOS to titanium or MOS to over this these are not showing the same characteristics as that of this titanium nitride and MOS to supported by this TIN, but when it is the coefficient of friction obviously, when it is the coefficient of friction obviously, a coating with MOS to plus titanium that will be the best one because it is having high percentage of MOS to and that gives the super lubricity and it is better than this than this titanium MOS to and titanium nitride is on the other side. However, if we like to consider both wear resistance and coefficient of friction then they blend of this one with right blending between titanium nitride and MOS to that would be one of the right choice in that direction and accordingly that coating can be ranked as number one in this family which can give a hard loop property and this is means the hardness of the titanium nitride and also the lubricity of molybdenum disulphide. Another thing is also found out that this particular MOS to this MOS to is also assisting in refining the this grain structure which is otherwise quite coarse and that grain structure can be refined by this MOS to and this way it is also playing another role that refinement of this titanium nitride structure and thereby improving its property. So, from this we can see that this titanium nitride and MOS to that can be one of the good candidate for all sort of application of coating which require both wear resistance and also lubricity and at the same time we have to also evaluate how good it is in terms of coefficient of friction or wear resistance or hardness in comparison to MOS to on one side and another side is that titanium nitride ok titanium nitride. So, these are the few things one has to look. So, one has to look to this and by this also what we can see that it is MOS to, but similar effort can be also made with other coating material say for example, W s 2 with some of the hard coating from this hard coating family and it can be titanium nitride or chromium nitride or some of this. So, that can be also useful to augment this property that means, it is the hardness at on one side and also the wear resistance on the other side. So, these are the coating architecture which are of immediate interest and immediate use and these are the materials also need proper evaluation that means, here comes the question of once we are resolved that it is actually titanium nitride and MOS 2 that is the combination then we have to also find out what will be the percentage of this titanium nitride and molybdenum disulphide to get the best value of hardness best value of grain size best value of critical load for adhesion and also the best value of coefficient of friction. Now, for this what we need to have here actually the process parameter for depositing MOS 2 and the process parameter for deposition of titanium nitride and for that what we need the cathode current, cathode current for titanium cathode current for MOS 2 then the substrate bias and process pressure keeping other thing constant process pressure keeping other thing constant means if it is a pulse DC then pulse frequency for the target and the substrate that is not altered. So, with this one can find out the best combination of these two thing to get one of the best value of hardness of the coating adhesion of the coating and low value of the coefficient of friction. So, with this we can make a summary of our today's discussion that MOS 2 and tungsten disulphide these are the materials which are called superlubricious material because of their architecture and this hexagonal array of this one set of molybdenum that is in between two layers of sulphur. Now, molybdenum sulphur has a good bonding, but between the sulphur of adjacent layer the bond is weak and that leads to a low value of friction because the van der Waals force of friction force of attraction is prevailing there. Now, what we have also seen that MOS 2 coating can be augmented its property can be improved not just by use it alone, but by blending with the material like titanium chromium and sometimes also it has been found that material like silver gold can also helps in getting low value of friction. The most important thing in the whole architecture is to have the Basel plane orientation that means, this 002 plane of this whole structure that should be parallel to the substrate surface to have the lowest value of friction. It has been also seen that instead of this MOS 2 or titanium there can be a good blending of one hard component and one soft component and with right blending and in the sputtering process by controlling the process parameter one can make a good composite coating giving one of the very best value of coefficient of friction and one of the highest wear resistance.