 chemical vapour deposition of nitrite coating. By this we mean mainly the deposition of nitride of transitional metals which are known for their hardness, anti welding property and also anti diffusion property. So, these materials are extremely suitable for those parts which are mechanically functional and if we consider the transitional elements of group 4 B, 5 B and 6 B, we can have a matrix here. This is titanium, zirconium, hafnium, this is 4 B, then comes 5 B. Here we have fanadium, niobium and tantalum and finally, in the group 6 B, we have here chromium, molybdenum and tungsten. Now, all those strategic materials which are used for those high end application requiring extremely high wear resistance, hardness, anti welding property, chemical stability. Now, their carbides, nitrides, borides and oxides can give us these type of properties and in addition to this 9 element, we can also add here aluminium and silicon because their carbide, boride or nitride can also give us such kind of property. Now, here we find that this titanium is so popular and so important in that this titanium carbide has a tremendous success in offering such kind of property particularly in all forming tools or tool of production just like a forming tool or cutting tool and this TIC at the top of the tool, at the top of the tool this works so successfully that it can raise the lifetime of the tool by few times or even the cutting speed can be increased quite substantially. Now, this TIC is indeed an effective solution to those problems where we have the wear on the tool that becomes a great problem and a challenge. Now, if we consider TIN against this TIC then we find that this has an improved property in comparison to TIC because of the simple reason this is more chemically stable than TIC, its anti welding property is better and diffusion of carbon say for example, in steel will be more than diffusion of nitrogen or we can say in this way that this titanium nitride is more stable against steel than titanium carbide and we can identify many more application where we can see even this TIN is found to be much more effective high performing than TIC. So, this TIN can be used in the form of a coating over this tool substrate to give this surface property so that we can get a high performing product. Now, this TIN how we can put it on the surface now here we can see. So, here we can see this is a substrate and say it is made of tungsten carbide and cobalt or it can be even HSS or say dye steel, tool steel, bearing steel and many more alloyed steel where we need to augment the surface property or to enhance the surface property. Now, by this what we can put here a TIN coating which can give us this kind of augmentation of the property. However, the question here it is just not the TIN alone this material alone. First of all we have to find out the process which is compatible to this substrate HSS or tungsten carbide number 1. Number 2 we can have proper nucleation growth rate and high nucleation density these are the essential requirements to be fulfilled in order to have one effective coating because it is just not the coating there are various issues are to be addressed. For example, smoothness of the coating density of the coating obviously, comes the hardness of the coating and last, but not the least the adhesion of the coating that means, this is going to be the interface and how this interface is strongly established between this coating and this substrate. Now, here we can mention that we have this 3 into 3 matrix and apart from that this aluminum nitride or silicon nitride these also appeared to be extremely successful in offering this surface property which are useful for application requiring high wear resistance and hardness and sometimes anti welding property. Now, here we see that nitride of transitional element nitride of aluminum and nitride of silicon their significance can be felt and understood by those who are using this mechanical component or mechanically functional component which are going to be used in those application. Now, it comes to the question of CVD chemical vapour deposition of nitride and here we focus our attention to titanium nitride because of the simple reason that the success story of TIC is well known and following the same path if we can also deposit effectively titanium nitride that will be 1 degree ahead in the in terms of coating technology and the coating material and that product will be much more useful and effective in comparison to TIC. Now, here comes the most important issue that if we have such a block which is made of tungsten carbide and cobalt it can be also tungsten carbide cobalt plus TIC plus TSC these are routinely used or on the one side we have all sort of steel families steel which includes HSS high speed steel, die steel then tool steel bearing steel and those are the few are the steel materials which are alloyed steel consisting of molybdenum, cobalt and similar materials chromium, vanadium to enhance the bulk property. Now, here comes the question the CVD coating already known for its versatility, trying power ability to coat a complex geometry that is well known, but how to put this TIN here the comes the question already it has been discussed that the substrate and the coating, so that is actually the zone of discontinuity. So, that is the zone of discontinuity we can see here this is the strong interface we need here, but at the same time one has to understand that is the zone of discontinuity this is the coating and that is the substrate. So, they are alpha value coefficient of thermal expansion that varies widely the value of e that means, this young modulus and then comes the deposition temperature that is the deposition temperature that is also very important. So, when it is used and I mean when it undergoes such kind of treatment requiring high temperature and then it has to be cooled down to the room temperature lot of stress develop over this surface and even if we have some chemical bonding that advantage will be offset because of this wide variation in value of E and L which actually leads to residual thermal stress and here we have breakage because of this stress development. So, at any cost this has to be stopped, so that is why when it is a severely coating though we know that TIN in every respect it is better than TIC, but we have no other choice, but also to bring TIC as the buffer as the buffer between the coating and the substrate and thereby we get this kind of grading graduation of the property. So, for all practical purpose what we have to have here this is the substrate and then what we have here we have here the TIC coating this is not exactly the way we mean it is not exactly the top functional coating in classical term, but it is it helps in graduating the property and on the top we have say this TIN coating, but before that what has to be attained gradually that means here you have exactly a transition layer this one this is in fact, TICN with value of X and N. So, this TIC has a very good compatibility with cemented carbide substrate which is O or plus TIC plus TAC this is a formulation and with that TIC has a good compatibility and then to bring that compatibility over TIC one has to go for titanium carbon nitride having carbon and nitrogen in that lattice and varying in the value of X and Y. So, X plus Y is equal to 1, so X and Y can be varied and when it comes to the top of this transition there the value of Y may be quite high and X may be small and at the beginning X value is high compared to the value of N. So, this transition keep on going and finally, what we get at the top we have this TIN and this TIN is actually the functional coating. So, though it is TIN coating CVD, CVD of TIN coating, but to have one of the effective high performing coating one has to do this preparation starting from substrate getting TIC then transitional coating and finally, this TIN coating. So, this way one can get a well adherent integral coating with this substrate surface. Now, this TIN coating can be carried out by following a CVD reaction skim of CVD. Now, here what we have we have here titanium tetrachloride plus nitrogen plus hydrogen which will reasonably give TIN which comes as the coating that means, it is solid plus HCl and here what we get something in this form for HCl and which is gas which should escape this CVD reactor or CVD system. Now, here what are those thing necessary as we find CVD process variables. Now, this CVD process variables are actually partial pressure of TICL 4, partial pressure of nitrogen, partial pressure of hydrogen these are on the input side these are on the input and as a result of this CVD what we get in the form of gas here it is solid which comes like a coating this is the coating. So, here also in the output we get pHCl. Now, these are the operational parameters that means, the concentration of various reactants and this is automatically the concentration of the reaction product and here what we have to have we have to have the process temperature and the system pressure that means, inside the CVD reactor one has to maintain this temperature and pressure and to have the reaction goes in the forward direction that means, what we like to have here it is actually a product favored reaction and this reaction should take place under a condition of temperature and pressure with certain parametric combination. So, it can move in this direction. Now, here one can see that to make it happen this is delta G 0 T this is plus this is minus and that is the temperature and here according to this it is just just like showing Ellingham diagram and we may have a plot like this and that is the crossover point. So, this is this line is showing actually the change in free energy of the reaction which just now we have stated here this is the CVD reaction and the change of free energy of this reaction. Now, here what we can find that this is the crossover point that means, here this reactants and the products are in equilibrium and at that temperature it is a proper balance between the reactants and products. Now, this temperature we can reasonably find just by putting this way delta G 0 T is equal to delta H minus T delta O S. So, these are in the standard state naturally this delta G 0 T and delta H that we can find from for titanium tetrachloride nitrogen and hydrogen. So, these are on the product side and then sorry these are on the reactant side and this is on the product side. So, here we can have this delta H 0 means actually this is delta H 0 of the product minus delta H 0 of reactants. Similarly, we can find this one as a product minus sigma S O of reactants. So, with this values we can get from thermodynamic table. So, from there one can find out just by putting the value of this delta G 0 T corresponding to this point one can be able to find out the value of T. However, it is also of our interest to shift this curve say for example, like this if it is possible say what we have shown by this dotted line and to see that is the net shift that means, this is also a value of delta G 0 T and which will be quite negative not 0 that means, here we can write further to this delta G T is equal to delta G 0 O T plus R T into ln k which is actually the equilibrium constant and this equilibrium constant we can find out from this relation that k is equal to from there we can write that P H C L to the power 4 minus divided by P T I C L 4 into partial pressure of nitrogen that is to the power half and P H 2. So, this is another relation we get and it is obvious that if we follow this relation to our idea is to make this more negative and to make this more negative what we can do we can check the partial pressure of hydrogen chloride which comes like a value to the power 4 and at the which is in the numerator and in the denominator it is T I C L 4 it is nitrogen to the partial pressure to the power half and partial pressure of hydrogen. So, the goal should be to make this as negative as possible so that this this part in the in this relation on the right hand side second part in the right hand side becomes highly negative. So, that we can get a net negative value here and that means, that this is actually a driving potential for this reaction to push or to drive in the product direction at or the reaction becomes product favored. So, this is one way one can look into this problem and determining the respective concentration of respective elements and the reactants and also the system pressure and also the temperature. Now, from the experience it is known that this CVD temperature for deposition of T I N that ranges around 850 to 1000 degree centigrade it can be done pretty well and industrial CVD in many application it is around 900 degree centigrade which is carried out in the commercial scale. Now, what are the outcome of CVD? Now, here we can have CVD process variables we have discussed. Now, what is the CVD scheme of CVD? That means, what are the component or the element the system should have for successful operation of this chemical vapour deposition of titanium nitride basically what we need? We need one reactor. So, this is one reactor and then within this we have one tower and there we have the different stages. So, that the substrate can be kept over this these are the substrates which we can put over this and externally it is a hot wall reactor. Obviously, there must be a heating element it looks like a resistance heater and this is placed schematically we can see it. So, this is actually the furnace. Now, there must be two lines one is the exhaust another is incoming. So, incoming line it is like this it is the incoming line and through this what gets in T I C L 4 plus N 2 plus hydrogen and here we have the storage for this T I C L 4 it is a flux hydrogen should be bubbled through this. So, this is one such thing and here we have metals for hydrogen that goes inside. So, it is something in this form. So, this is hydrogen and then we have one for nitrogen. So, this is the basic scheme of course, here we have the mass flow controller. So, these are the M F C plus we have many peripherals like the pressure gauge this valves and other thing here also we have I mean inlet valve and this side of course, we have two pumps one is and this goes to the ventilation. So, this is actually water ring pump and this one and this one that is the vane pump. So, these are the basically the CVD setup what we can see here and this is T I C L 4. So, these are admitted and this will go over the surface and it will be showered and as the outcome we have H C L. So, during the CVD operation this vane pump does not work it is actually the water ring pump where this H C L is neutralized and any residual gas will escape through this ventilation. So, this water ring pump actually or the liquid ring pump it contains sodium hydroxide which is used for neutralizing this H C L. So, here the system pressure here we have a valve with this valve by throttling we control the pressure that means, the process pressure which we have already mentioned and through this furnace we meant we control the temperature at different zone if it is so necessary. So, this P and T plus here we have hydrogen and this is a saturated with T I C L 4. So, at a particular temperature if we can maintain a particular temperature. So, at the temperature hydrogen will be saturated and one can also determine what is the percentage of T I C L 4 which saturates hydrogen. So, these amounts are known pressure temperature known naturally from this temperature one should be able to find out the rate of reaction or the conversion and finally, one would be able to determine that growth rate or the thickness of the coating which will grow per hour and that is also one of the outcome of this whole operation. Now, come so this is basically a summary of the CVD system we have the reactor hot wall reactor the furnace the gas dispensing system and also the gas disposal. So, these are the few things just we have summarized and now comes the CVD variables we have already mentioned it is the process pressure process temperature concentration of T I C L 4 concentration of nitrogen and concentration of hydrogen. Now comes the outcome of this CVD the outcome of the CVD is outcome of the CVD one outcome is the deposition parameter that means, the deposition rate. So, deposition rate one would be interested to know what is the deposition rate. So, if this is the deposition rate and this is the concentration or partial pressure of T I C L 4. So, it comes like this and it falls drastically the deposition rate with for a for a what does it mean that means, for a given condition if one keep on increasing titanium tetrachloride then it is found that deposition rate starts falling and almost comes to 0 this is because of the reason that one can look into this reaction that means, here T I C L 4 plus half N 2 plus H 2 O. So, for one molecule of T I N what do we get we get actually 4 H C L. So, if without increase of H 2 or other parameter if we keep on increasing this P T I C L 4 partial pressure of titanium tetrachloride then what is going to happen that there will be too much of formation of H C L and it will revert the reaction in this direction and this reaction in the forward direction it will be seriously affected. So, this P T I C L 4 if it is increased without touching hydrogen or temperature on any other parameter then we can see such a trend. Similarly, what we find this is also the case with nitrogen also it has a falling curve that P N 2. However, this is also a falling curve however if we increase hydrogen under the same condition then what we can find out this growth rate which is may be. So, many milligrams per hour per unit area of the substrate or simply micron per hour that is the thickness of the coating. So, if we specify the growth rate. So, this way we can see the nature of the curve and here we can see it is a rising trend it is simply because of the reason that here it has a better reducing effect and if we have excess hydrogen then this reaction is going to move in the forward direction and as a result we have a rising tendency. Now, comes the lattice parameter. Now, this lattice parameter that is influenced by these parameters and here what we find that this lattice parameter that is the function of temperature. So, when temperature is increased and this what we mean that the temperature of interest is around 800 to 1100 and in this zone it is found that this lattice parameter keeps on falling with this value of nitrogen with the value of temperature and with increase of nitrogen it also keeps on increasing. So, this is actually the partial pressure of N 2. So, it goes like this. So, this is a lattice parameter which is an index of the stress inside the unit cell and by refining this we can also refine the structure of the coating. Now comes the grain size. Now, this grain size is one of the parameter of interest because one would like to have finer grain size for various reason. Number one the toughness of the coating at the edge can be improved a lot. Number one then if we have a fineness of the grain then roughness of the coating can be also significantly reduced and these are the two things at least one should have an attention at this point. So, here if we see the grain size. So, it comes like this by increasing the temperature we can see fineness of the grain. Now this happens because of the simple reason that when the molecules are formed on the surface because of the low mobility of the add atom the there is less diffusion on the surface. So, because of this less mobility of the add atom they cannot join together and become an integral unit and there remains lot of void space porosity and gap and this can be full I mean filled in if we have better mobility of this add atom and this can be enhanced by simply increasing the temperature. So, within this condition of experiment that means, within this 800 to 1100 when this temperature is increased there is a marked improvement in the grain size of the coating and one can note that out of these three parameters that means, where we are considering this partial pressure of hydrogen partial pressure of nitrogen and temperature it is mostly the temperature that has a very strong influence on the final grain size and here we see that by increasing the temperature we can have a finer structure and it is because of the simple reason that this porosity or the void can be significantly reduced and we can have a dense structure and thereby we can increase the density of the coating. Surface morphology in fact, what we can see here that surface morphology this is one of the criteria that means, what is the structure whether it is just like a broomstick or equiax that means, it is spreading in every direction or it is just like a dome like top or it is have a particular crystallographic plane just like a 1 1 plane preferential 1 1 plane and it becomes just like one face of a pyramid. So, these are the few things one has to look in. So, it is also one observation that when the temperature is less there is less amount of mobility of this adder term and they cannot move a lateral shift and thereby we can get a structure which is not. So, dense and it is porous, but when we have a high temperature then there is diffusion across this atoms which are which arrived on the surface of the substrate and which makes this T i n and by this rise of temperature we get a favorable morphology which is finer in size and also there is less amount of dome like structure, less amount of porosity and we have more of a densified structure. So, this amount of that means, the temperature has one effect and if we increase the temperature definitely a compact structure can be expected. Now comes the coating hardness. Now, this coating hardness coating hardness means here resistance to penetration that means, this structure should not have much of residual stress that means, which is very highly compressive stress and the coating becomes very very brittle, but this is this hardly be the case with a CVD coating where the coating is mostly having a stress which is tensile in nature. So, it is a tensile stress. So, the coating is not may not be that brittle. However, what one can see that if the coating is porous and having voids imperfection in that case the indenter can have an easy penetration indicating less hardness. So, in that case what is understandable that when we have a finer grain that means, finer grain means obviously, a denser structure. So, with the same volume if we have larger grain and if we have finer grain obviously, with a finer grain we get a denser densified structure and which is much more compact in nature. So, with this compact structure one would expect increase in hardness. So, as a corollary we can say that if we increase the temperature the hardness of the coating is also increasing. However, what happens with flow of hydrogen and nitrogen that one has to look in this is partial pressure of N 2 and this is partial pressure of H 2. It can be also expressed in terms of flow rate of the reactive gases and here one can find that this is having a trend like this and this one is also having a trend like this and this happens one would expect here that if we have less amount of nitrogen what we get we get a porous structure and here similarly is the case with less amount of hydrogen, but when we have increased quantity of nitrogen or hydrogen which favours a reaction of titanium nitride then we get a densification of the coating. But if we increase this value of nitrogen or hydrogen excessively then we can also have a shoot like structure which is like a shoot and which has less compaction and less hardness. So, in that case we have what we called it is like a dust formation and it is homogeneous reaction which can takes place in the gaseous phase and as a result of that what is going to happen this is not exactly happening on the substrate surface and as a result of this the structure is not so, densified and this will be translated into a low value of hardness. So, we can summarize that with excessive nitrogen and excessive hydrogen there could be a fall of the hardness. Now comes coating adhesion now adhesion of the coating say this is L C and this is temperature this is also critical load P N 2 this is L C and P H 2. Now what is what exactly happens at the interface we can try to follow this way this is the substrate and T i C L 4 plus half N 2 plus 2 H 2 and as a result of that we like to have T i N and that should grow on this surface and this definitely this will be promoted by temperature because it is the ok. This is actually the interface where one would like to have the reaction on the substrate and not in the space between the shower and the substrate surface. So, what we will like to have here that this reaction should take place on this surface and for that definitely temperature will be a promoter of this reaction. So, this reaction and the right temperature on this substrate then this reaction will take place on this surface and then this T i N will have a good attachment with T i C N and there will be a perfect transition between this carbonitride and nitride. Now here temperature has a very big role to play. Now what is going to happen we can see that this curve is actually going like this and then falls. This is also a curve what we see on this side and also with p hydrogen that means, for all the cases what is sin that this critical load this critical load means if it comes like this, this is the coating and there is one the scratching tool which moves on this side and it has a penetration and it is actually the load at which the first detachment is detected and for that we have the dedicated instrument that adhesion scratch tester. So, this adhesion scratch tester detects the point corresponding to a normal load at which this first separation takes place and that is known as the critical load which is the normal load here. So, this critical load. So, this critical load has a rise because of this reaction very strong adhesion chemical attachment, but because of this high temperature finally, that is going to be offset or neutralized by the residual stress residual stress at this point. So, this one has to also look in while setting the temperature similarly p n 2 and p h 2 these are also the partial pressure and with this partial pressure exactly what happens the reaction we want to have the reaction here, but because of the high pressure of nitrogen or hydrogen this can be promoted in the gas phase before the reactance reaches the surface. So, if that be the case then we can have some flake formation, some void may form, some dust may form which is less adherent and as a result this kind of fall of the critical load may be observed. So, it is actually CVD of titanium nitride using ammonia and what we have seen at the very beginning that if it is the delta G 0 T. So, this is the temperature and this is minus plus. So, when it is the reaction using T i C l 4 hydrogen and nitrogen then we have a temperature here which is around 900 degree centigrade for this reaction to occur, but if one is interested in having a low temperature CVD. So, this is in the order of at least 900 degree centigrade temperature, but if one is interested in the CVD around 550 to 600 then some atom can also be made with instead of nitrogen using ammonia and in this case it is something like that NH 3 plus H 2 that gives T i N plus H C l and this can happen at this temperature of 500 degree to 600 degree centigrade. However, here we have this break up. So, that means, it is actually NH 3 plus N. So, this gives actually N 2 plus H 2. So, that will be splitted here. So, this is also another reaction which may occur and when we put this T i C l 4 plus this N 2 at this no reaction takes place. So, one has to also look into this type of reaction that this dissociation does not occur. So, in that case this N 2 cannot come in as an effective means for converting this T i C l 4 into T i N. However, with this use of ammonia and hydrogen and taking appropriate measure if one can make this ammonia to react with hydrogen and then this reaction can move in the right direction and in that case we get a T i N at low temperature and this T i N can be used for those application where this high temperature can be little delicate and where this high temperature can also damage the metallurgical property or the mechanical characteristics of the material. So, with this we can summarize the thing that means, chemical vapour deposition of titanium can be done using titanium tetrachloride as the resource material and this is actually the donor of titanium and what we can see further to this as a source for nitrogen we have two options either nitrogen or ammonia, nitrogen with hydrogen as the carrier gas can be rather conveniently used and this particular reaction requires at least a temperature above 800 or 850 degree to happen and this reaction can be quite useful for those substrates which are unaffected say for example, cemented titanium carbide which can be one of the best candidate for such thing. However, in this case we need to have a transition from T i C to titanium carbon nitride and followed by T i N. However, for H SS and other materials if we like to carry out thisreaction at a high temperature a special heat treatment after coating is necessary. Alternatively, ammonia and hydrogen can be used and in this case the reaction can be reaction can may be made to happen around a temperature of 550 to 600, but there one have to be careful that ammonia and hydrogen can split into nitrogen and hydrogen and in this case this nitrogen will not be effective for conducting the CVD at this 500 to 600 degree centigrade.