 chemical vapour deposition of chromium that means, it is CVD of chromium. Now, let us see what is the significance of chromium coating in engineering application? Now, this use of chromium is already well established in reducing oxidation or corrosion of various machine component and part. Say for example, the turbine blade and vanes of aircraft engine or turbine and also in tubes and fittings this is also used as a oxidation preventive layer or corrosion resistant layer. It is also used for graduating the property from the surface of the substrate to the top functional coating that means, the top functional coating has a different young modulus and coefficient of thermal expansion which is quite different from that of the substrate and here in particular a steel substrate. So, this chromium deposited on this steel substrate subs as a buffer in between the top functional layer and this substrate. It can have further to this one very important function it is mostly used for metallization of various ceramic material for enhancing its weightability towards some metal binder. Normally, this ceramic material are inert to metal binder. So, when we can deposit a thin layer of chromium on the surface of the ceramic then it attain certain weightability which means it can improve the bond with the metal matrix. So, these are actually the three areas where one can find the significant use of this chromium coating. Now, CVD of chromium has its origin in fact, CVD of chromium finds its principle from the very chromizing process or pack chromizing process. Now, in this pack chromizing process what happens it is the chromium powder plus one activator like NH4F or it can be NH4I which can be used as an activator plus powder of aluminum oxide which is a inert material. However, it provides the necessary porosity in that mixture of this mass of chromium powder, this ammonium fluoride or iodide and aluminum oxide plus it is the steel based specimen samples. Now, the whole thing is put inside a reactor and which must have a temperature 1000 degree plus greater than 1000 degree and during this process actually chromium transformed into some halide of this which can be fluorine or iodine or even chlorine and then it gets deposited over this iron sample to produce a layer of chromium and this is exactly what we know as pack chromizing process. However, CVD process that means chemical vapour deposition of chromium has a distinct advantage over this pack chromizing process. Number 1 very important point one should look in that means, here the chromium coating has a better uniformity than that can be achieved by this pack chromizing process number 1 and number 2 is also the advantage in terms of temperature. We can have a temperature ranging between 750 to 1000 to conduct this CVD of chromium this process. So, we can immediately recognize the two very advantage or strength of this CVD process. Now, here we understand this chromizing process it is actually chemical transport process and a chemical transport reaction that means, the material will be transported to the on to the surface of the substrate where the necessary reaction will take place releasing chromium vapour and which will condense in the form of a solid film. So, these are the advantages which is already mentioned and this uniformity is very much important when the geometry of the substrate is complex in nature. Now, difficulty of CVD of chromium. Now, one can look into all conventional CVD process what is very important to have the metal donor. Metal donor means here it is in most cases it is one halide of the metal say for example, we have tungsten hexafluoride or molybdenum pentachloride, tantalum pentachloride or even titanium tetrachloride. So, we find that these are all metal donors the advantage here is that these materials are available in the form of liquid commercially. So, to handle this liquid which is available commercially that gives a clear advantage in the use of this metal donor which can be evaporated which can be transported near or on the substrate surface to do the necessary CVD reaction. However, in case of chromium this is just this does not happen. So, what commercially what is commercially available we find that this is actually CRCL 2 6 H 2. So, this is a material which is available commercially. However, it needs complete drying number 1 and this material has to be fed in a metered quantity through this reactor near the substrate surface to have a close control on the growth and deposition rate. So, unless one has a sophisticated system for doing this thing this will not be very interesting way of conducting the CVD of chromium by using this commercial product. So, what is preferred it is in seto production of some chromium halide and which can be an iodide fluoride bromide or even chloride. So, the necessity of in seto production of this halide is an important step in the CVD of chromium. So, what exactly we need here just in the upstream side of the reactor we want we must have one chromium halide generator. So, this is actually this generator which will generate the vapor of chromium halide that is in simple term we understand. Now, if we have a close look in the Ellingham diagram we can immediately find this is the Ellingham diagram x axis is T and this is the 0 level and this is delta G 0 at certain temperature this side plus and this is minus. So, here one would like to represent the free energy change in a reaction. And if we like to see that chromium iodide or chromium bromide their location is somewhere here and these materials can be decomposed by thermal process thermal activation and through this thermal decomposition reaction one can have this chromium vapor which can be condensed on the surface of the substrate. That means, here these are the points which suggest that this is the threshold temperature for a halide to have decomposition into its metal and that halogen. So, if one can attain this temperature and this side is this on the minus side where this halide is very stable, but what we can understand from this Ellingham diagram that this iodide and bromide of chromium are not so stable like fluoride or chloride of chromium. So, the advantage which can be immediately exploited is that that chromium iodide which is in this form if it is heated we can get. So, this is actually gas or vapor. So, we get chromium as a film solid and leaving this iodine. Similarly, we can also have for bromine also these are less stable halide. Now, how to put this thing in practice? So, to do this thing one can have either a horizontal type reactor or a vertical type reactor. In principle, this thing can be put in a practice say this is one tower of a CVD reactor this is the base and here we have one flux that means, the cover. So, it is supported by this O-ring and just outside that we have a shell like furnace like this which is like a shell. So, what we can do here? This top part that means, this upper stage of this vertical reactor here we can put chromium powder or chromium particle chromium nugget, chromium chip and in the downstream side for example, this is a stage where the substrates can be held placed. Now, this is actually the flow line for this halogen it can be iodine or it can be a bromine. So, this will pass through this stage the top most stage where we have chromium nuggets or chromium chips and it is placed this temperature controlling of this temperature is very important. So, this is the temperature and here also you have another temperature. So, this temperature gradient that helps to take the reaction in the forward direction that means, first of all here this part we call the generator that chromium iodide generator. So, here actually this chromium iodide is formed and then this chromium iodide is transported you can have also hydrogen as the transporter. However, this is called a closed tube process we can use it in a closed loop form closed tube. It is a closed tube means these are the substrate and because of this temperature differential what we can find that on this side if we look at this diagram this chromium iodide will form on this side where chromium iodide is more stable. However, on this side which is above the 0 line where obviously, chromium iodide since it is delta G T 0 that is in the standard state which is plus it cannot be stable. So, it will decompose into chromium and iodine. So, to have this CVD one has to take the temperature above this little greater than that and which is shown here. So, these two temperature will be somehow different and then leaving this chromium coating and release of iodine. Now, this iodine can be recycled and this becomes a closed loop system. So, there can be some arrangement for circulating this iodine. So, this iodine will be again fed back over this chromium granules making chromium iodide and the process will keep on repeating itself. So, what are those two important thing to move the thing in the forward direction what we can find here that delta G T at a particular pressure that we can write as delta G 0 T plus R T into ln. If this reaction goes in this forward direction we can write this is actually ratio of partial pressure of iodine divided by the partial pressure of chromium iodide. Now, from this is one of the governing equation. So, we can fix the temperature we can also adjust the partial pressure of this one quick evacuation of iodine and with that we can take the reaction in the forward direction. That means, here it should be we can write this step that at this stage in the generator it should be chromium plus iodine that should lead to chromium iodide. However, in the downstream side it should be chromium iodide to chromium plus iodine. So, this way a process can be conducted and this can be well used for CVD of chromium. Now, here also we can use chromium chloride as the source of chromium and in this case what happens that this chromium chloride has to be also produced in the similar way as it is the case with chromium iodide formation. So, here it is not a closed tube process, but we can call it one open tube process, open tube process. So, here also we have a tower and this tower is encased in a flux and at this top stage here we have the chromium granules and in this case what is passed here it is actually HCl vapour this is HCl vapour and we can also have H2 or argon also. So, these are the some of those gases which can be passed through this chromium granules and later in this area where the substrates are placed on that the necessary CVD reaction can take place. Now, this generation of chromium chloride or chromium trichloride that can be possible by various ways let us have a closer look this chromium and HCl that can give chromium chloride and hydrogen. We can still further we use titanium trichloride for generation of chromium chloride this is also possible. So, here this titanium trichloride instead of HCl vapour we can also have a provision for passing TICL 4. So, this will be the passage for passing TICL 4 and as a result we have chromium chloride plus TICL 2. So, this is another way of getting this thing it is further possible by directly use chlorine in this case we have this chromium CRCl 3 and with further addition of chromium with CRCl 3 it becomes CRCl 2. So, at least we find 4 ways 4 chemical reactions leading to formation of chromium chloride. Now, in this case what happens now comes how to reduce it. So, this is actually formation of chromium chloride and which will now travel this distance. So, this is actually vapour of chromium chloride. So, this is the chromium chloride generator. Now, in here what is to be done here actually this chromium chloride has to be reduced to film chromium and a byproduct. So, it can be possible if it is a still substrate say for example, in a still substrate and here we can have the byproduct which we must show. So, this is upstream side and that is the downstream side it is an open tube process. So, in chromium chloride it will adsorb on still substrate giving such kind of possibility it is condensed chromium. However, F e transform to F e Cl 2 this is one way it can be possible further to this just by hydrogen reduction. So, this is actually chromium plus 2 H Cl it is also possible to have this C R Cl 2 as C R C R plus C R Cl 3 of course, everywhere we have temperature pressure combination and in certain case we have the reducer. However, this temperature and pressure they these are the governing parameters directing a giving the clear idea in which direction the reaction should move. Now, if we look into this first chemical reaction by following which we get chromium coating we can have another step where this F e Cl 2 can be reduced by hydrogen releasing iron and H Cl. Now, this is the basic principle of chromium deposition by hydrogen reduction. Now, here one thing we can look in this Ellingham diagram. Ellingham diagram it is one which can guide us say this is the free energy variation with temperature for chromium chloride and we can also find this is actually the free energy change for H Cl. We are following this equation that means, reduction of chromium chloride by hydrogen which happens to be the most interesting in this case. One thing if we look into this chromium chloride deposition is possible. However, deposition of some compound of titanium cannot be ruled out because of the very presence of titanium chloride. So, this graph is important in that that the location of this temperature location of this temperature that is the threshold point and what we can see if we go on the left hand side C R Cl 2 is more stable than H Cl, but when you go on the right hand side we find H Cl is more stable than C R Cl 2 and this is the point where C R Cl 2 is no more stable and it may split up into chromium and chlorine by just thermal decomposition, but we are interested in this curve. So, naturally for the equation for the chemical reaction that means, here chromium plus 2 H Cl which will give us C R Cl 2 plus H 2. So, this one we can take the help of this diagram here actually we have formation of C R Cl 2 and that will be transported here and in the process we must choose a temperature which favours C R Cl 2 formation that means, in this case we can write for this reaction to happen this is given by this delta H 0 T minus T into delta S 0. Now, this we actually this is actually the free energy change of the reaction. So, we can put this symbol for this reaction at that particular temperature in question. So, this is actually the reaction. So, this way we can find out where it is negative that means, the reaction will be spontaneous. However, to aid the reaction we can write further to this as R T into L N and from this we can write it is in terms of partial pressure C R Cl 2 into partial pressure of hydrogen divided by this P H Cl to the power 2. So, this part which is with a plus sign. So, here one can regulate or manipulate the respective partial pressure of C R Cl 2 H 2 H Cl to move the reaction in the forward direction and that would can be a driving force. Now, when it is this so, this is generation of C R Cl 2. So, this is on this side. So, we can shift this point and get a very stable value of delta G T which is highly negative. So, that the reaction move in the forward direction. Now, when it comes to reduction of this C R Cl 2 and that happens with H 2 that means, now the reaction should be reversed. So, this will give us C R which is solid film and H Cl. So, we can see that this is just reversed. So, how to make it happen? So, to do this thing again the same equation we have to write, but in a different form. So, in this case delta G T is equal to delta H 0 T minus T delta S O and one can find out that both the reactions are actually exothermic because of their negative delta H. However, what we can find that when we increase the temperature on this side in that case H Cl become more stable that means, C R Cl 2 will be less stable and H Cl will be more stable that means, it promotes formation of or reduction of metallic chromium and for this we can write delta G T is equal to delta G O T plus R T L N. However, in this case we have to write P H Cl which will be now in the numerator divided by P C R Cl 2 into P H 2. So, we can compare these two and accordingly in the CVD reactor that condition must prevail that means, number 1 that though this thing can be also handled at the same temperature, but one would expect that if we have a furnace with a hot wall furnace then we can control this temperature in this zone and we have various zones in the furnace and this is another zone. So, we can control this temperature of these two thereby we can work for generation of C R Cl 2 we can work in this zone. However, when we are interested in deposition of chromium which is shown by this reverse direction then we must have a temperature which favours the reaction to proceed in this direction. Similarly, one can also have an interesting look here that means, here P H Cl 2 that has to be increased, but in this case what we have to see that here P H 2 that means, hydrogen has to be increased that means, to generate chromium chloride we must have less concentration of hydrogen here, but more of H Cl, but in this zone what we have higher concentration of hydrogen and less H Cl. So, in principle one would expect to have a better result or a driving force by increasing the quantity of hydrogen on the downstream side near the substrate. So, if some arrangement can be made to have some kind of arrangement and source so, that would be an hydrogen which will have a higher concentration near the substrate then possibly the reaction can be moved also in the forward direction. So, these are the some specialties of CVD of chromium and here we are using mostly the reduction reaction and in this reduction reaction we are using H Cl for generation of chromium chloride vapour. However, in the downstream side what we are using we are using excess hydrogen so, that the reduction can take place and this increase of hydrogen acts like a driving force. So, here one would be interested in the growth rate of the coating as we have said one of the important role of this chromium coating would be to graduate the property of the coating from the steel substrate to the top functional coating. So, here we find that this chromium when deposited say for example, AISI 440 C martensitic steel in this case on the substrate when chromium gets deposited chromium actually becomes partially converted into a carbide and here we have high amount of carbon. So, this actually becomes one complex carbide in this form so, it is actually a complex carbide of chromium and iron with 7 suffix and C 3. So, this is actually one interlayer that forms immediately on the substrate and if one is interested to see the growth rate say this is micron per hour and this is time in hour. So, what we can find it is something like that this growth is quite fast and then it slows down. So, this fast growth rate actually it is the it can be explained by the rapid pickup of this carbon from the substrate and thereby this chromium becomes a part of this complex carbide and later on what we can see. So, here chromium and carbon from the substrate they are participating to make a chromium carbide along with iron, but later on this hydrogen become very active in the later part of the process that means, CRCl 2 plus hydrogen that gives chromium plus HCl and here this ratio of CRCl 2 by hydrogen that indeed matters to have nucleation and growth rate of the coating on a substrate on a given substrate. If we fix the chemistry of the substrate then obviously, this CRCl 2 by hydrogen that partial pressure ratio, concentration ratio or flow ratio that will definitely matter in the growth nucleation of the coating over that substrate. So, it can be even one is to one or it can be as high as one is to 100. Now, here we understand the role of hydrogen it is only to move the reaction in the forward direction and not to have too much of HCl which can also push the reaction in this direction. So, this is one is to 100 and if we now see the influence of both one can find that one is to 100 that gives a higher value whereas, one is to one that gives a lower value of this coating growth. So, this is actually one is to 100 CRCl 2 by hydrogen one is to 200 and this is something like one is to one. Here also we can have a smoother coating as it has been seen through some investigation carried out that with this high flow ratio of hydrogen in relation to chromium chloride. One can get a smoother coating that means, we have a higher nucleation density on the substrate surface than what we can see with a ratio of one is to one. Now, this thickness of the coating for this steel substrate where graduating the layer thickness is very important there at least the coating thickness kept is it is kept in the order of 6 to 10 micron that is the order of the thickness. So, this is the thickness which is necessary to graduate the property of the coating. Now, this coating nucleation and further growth that is also affected by the chemistry of the substrate surface. If we have higher percentage of carbon in the substrate it is a source of carbon then immediately the coating grows at a faster rate because of the affinity of chromium towards carbon and it is a very good receptor surface. So, chromium carbide grows at a very faster rate. However, if we have a substrate where the chemistry is such that chromium cannot very easily participate in any reaction at the interface then this nucleation growth that will be having some difficulty and some limitations too. Say for example, we have seen that this chromium is used say for example, for metallization of ceramic and it has been investigated that when it is a graphite or diamond chromium can grow very easily over the surface it is because of the simple reason that chromium carbide has a higher chemical stability than that of carbon and as a result immediately chromium carbide forms. However, when it comes to this deposition of chromium in the range of 1000 degree centigrade that is a range which is of technical interest for any industrial CVD then we found that this chromium could not have a uniform coating here we had a uniform coating, but here we did not have any uniform coating few global like thing in the form of some island which shows typical of a non-weighting substrate on this surface and this is not a very good sign of chromium coating on CBN. Now, this can be explained by the fact that in this case this chromium though deposited, but the substrate participation was not that encouraging because it is BN and when it comes chromium which is deposited one would expect that it should result in either chromium boride or chromium nitride these are the two thing should form. However, what we find from the thermodynamic data that this boron nitride is much more chemically stable than that of chromium boride or chromium nitride and as a result this 1000 degree was not enough under that condition to have a continuous coating of this chromium on this boron nitride substrate. Here also one thing one should also look in that this deposition of chromium is necessary on this graphite or diamond as we have mentioned at the beginning of this lecture to enhance the weightability of a metal. Say for example, nickel weighting of nickel on graphite or diamond that would have been extremely difficult, but when we have a chromium deposition then this chromium which is either in the form of chromium or chromium carbide that will be weighted immediately by this nickel. However, here one point should be looked into that this temperature of the deposition if it is conducted at 1000 degree crack in the coating is not unexpected. However, because of this driving force of carbon which facilitates deposition of this chromium at even 900 degree centigrade. So, at 900 we could reduce chromium chloride by hydrogen and releasing chromium and this chromium could have immediately chemisoption it can be said chemisoption on this surface because even at 900 degrees chromium could form a carbide of chromium over diamond surface with a good adhesion. But if we go to very high temperature then and if it is a thicker coating then what is going to happen that because of large variation in thermal coefficient of expansion or young modulus there is a risk of coating cracking of the coating and the whole purpose may be seriously affected and the whole purpose may be lost. So, the question is that a temperature as low as possible as we have said that this deposition can be done between 750 to 1000 degree centigrade. So, one can look into this temperature low end of the temperature which will be quite suitable for deposition of this chromium over this graphite or diamond. Now, when it comes to the question of oxidation resistance or corrosion resistance it is chromium which readily gets oxidized and form a fine layer of chromium oxide and which can serves as a buffer layer or a passivating layer which prevents further oxidation or corrosion. Now, comes the question of graduating the layer. Now, this is particularly important when one like to coat the ball of a steel ball of a ball bearing or the roller or the inner rays of a anti-friction bearing. Now, the top most outer most layer is actually TIC outer most layer is TIC and the core area that is the material which is AISI 440C steel and in this case if one deposit TIC on this deposition because of the high carbon content perhaps deposition of TIC is not a difficult task, but what would be important here to have a good adhesion of the coating and also proper matching of young modulus and coefficient of thermal expansion. That means, mechanical continuity, physical continuity and thermal continuity all those things should be properly maintained and that is why we need here this chromium coating and this chromium coating will form a chromium like carbide on this steel and over that a coating of TIC can be deposited. So, this way one can deposit chromium carbide over this surface and can have one of the end product which is high performing. Now, concerning the chromium coating in particular what we have mentioned in case of weightability enhancement in this case what we have seen that the chromium coating for passivation and chromium coating for graduating the surface layer from the core area to this top layer and chromium coating for oxidation prevention requirements are not same requirements are not same. In one case, we want a inert surface that means, no further oxidation or attack of the substrate is possible that is totally ruled out. So, that is the role of this chromium while providing this passivating layer or anti oxidation or anti corrosion. Now, when it is the top TIC coating on the steel ball this is the ball of a bearing in that case it is not directly functioning, but it is actually a buffer layer and the top layer is TIC, but when it is the question of weightability it is just changing the surface chemistry of the ceramic which is otherwise inert towards say liquid metal. So, when it is the question of weightability one to micron thick layer is sufficient, but when it is the question of passivation it is a pore free dense coating is necessary and when it is a buffer layer which graduates the property it should come like a bridge between the core area and the top. So, the three functions which can be done by this chromium coating, but the layer thickness then the chemical or physical characteristics, mechanical adhesion characteristics these requirements may not be the same. So, with this we come to we have discussed all those issues. Now, we come to this summary on this chemical vapour deposition of chromium which takes the principle from packing process of chromizing. CVD process is known for better uniformity of chromium coating and relatively low processing temperature. However, the useful halide of chromium is not available easily thus in situ generation of some halide of chromium is preferred. This can be followed by CVD reaction either by thermal decomposition or reduction of the halide of chromium. The chemistry of the substrate plays an important role in growth and structure of the coating. The CVD chromium can be used as oxidation or corrosion resistant layer. It is also used as an intermediate layer to graduate the property from the substrate to the top functional coating. It can also find use in metallizing ceramic for increasing its weightability.