 chemical vapor deposition of aluminium oxide. Let us look at this aluminium oxide and try to understand and appreciate its significance as engineering material. Now, this aluminium oxide is well known for its chemical inertness, high temperature thermal stability, retention of hot hardness and very low thermal conductivity. Now, these are the characteristics of aluminium oxide which has made this material an interesting and prospective candidate for many engineering use and one of those is that cutting tool material. Now, this cutting tool material requires certain properties which interestingly aluminium oxide does possess. However, when it is the question of tensile strength or toughness, then this aluminium oxide shows its weakness and that is why though this material has other high temperature properties, but this cannot become an universal tool for a general application. Now, we have already understood the very idea of coating the cutting tool so that the required properties can be given on the top surface which becomes the functional surface, but this functional surface having this coating can be also well supported by a core which is having certain other requirements namely tensile strength, bulk hardness, then retention of the geometry, chemical compatibility with the coating material these are the some required properties. Now, already TIC or titanium nitride and many more similar materials are already in use and these have augmented the cutting properties of a carbide tool or a high speed steel tool. However, when one compares those required properties which are essential at the top working surface of the tool definitely aluminium oxide can outperform this TIC or TIN or even titanium carbon nitride. That is why the whole idea here is that if we can have this aluminium oxide which is having all the required properties namely chemical, thermal then if we just put it over this substrate which is made of tungsten carbide cobalt or similar material in the form of a coating then the whole purpose is subbed and here we have one of the best candidate in the form of coating which can perform its task very efficiently that means, its use as a cutting tool. Now, application of aluminium oxide as hard coating that means, when we go for high speed operation, then actually this high speed is associated with high temperature and it is actually the wear on the tool which matters most rather than any other mode of failure and in this respect material which is at the top of the tool surface it needs some very special characteristics which can enhance its wear rate and that is exactly what we know as chemical stability. So, this chemical stability is very much associated with the free energy of formation of this particular compound and in this case aluminium oxide has a clear edge over TIC or TIN which are already two useful material as coating on the cutting tool. Now, this aluminium oxide need to be deposited on this substrate. Now, here what we can see that CVD can be a very good way of handling this coating technology and coating process. Now, in CVD we have a global reaction for deposition of aluminium oxide the global reaction is something like this A L C L 3 plus H 2 O plus C O 2 that should give A L 2 O 3 plus C O plus H C L. Now, here this is gas, this is also vapour, this is deposited as solid and these two should escape as the gaseous reaction product. Now, this is the overall global CVD reaction. Now, here the main problem is how to get this aluminium trichloride? Now, this aluminium trichloride can be commercially available so easily. So, to get this work done what is done that aluminium trichloride has to be generated during this CVD process. Now, this global reaction can be also splitted into two reaction steps that means, first of all what happens this H 2 plus C O 2 when admitted in the CVD reactor it will actually transformed into water vapour H 2 O and C O. Now, this H 2 and C L 3 plus this H 2 O so this aluminium trichloride will undergo hydrolysis and with this hydrolysis A L 2 O 3 will form and at the same time H C L will form. So, if we combine this is one global, this is one reaction step this is three. So, when we put two and three together then we get A L C L 3 plus H 2 plus C O 2 that actually that moves in the direction of A L 2 O 3 C O and H C L. So, these are the basics of this CVD principle for deposition of aluminium oxide. Now, here what we have mentioned that production of A L C L 3 is required very much required and to have that one has to go for this reaction that means, this is actually aluminium chip plus here one has to pass H C L vapour which is in the form of gas. So, this is solid and this will lead to A L C L 3 plus H 2 O H 2 hydrogen and to move this reaction in the forward direction one can have a look into this illing hum diagram. So, here we have this is delta G 0 T and this side is the temperature. So, here we have to see the equilibrium of A L C L 3 with H C L that means, if we find that A L C L 3 that is varying in this form and we have the delta G 0 T curve for H C L like this. This is for A L C L 3 and this is for H C L this becomes the threshold point or crossover point. So, on this side of this threshold temperature A L C L 3 is very stable and the reaction can move in this direction. Obviously, it is evident that temperature need not to be very high and as per the practice it is done say in the around of 200 2 and 250 degree that generation of A L C L 3 vapour in this CVD reactor this is done at the upstream side of the reactor. So, from this considering this vapour of A L C L 3 H C L one can regulate the vapour pressure of H C L and that of hydrogen to move the reaction in the forward direction. So, this is how this A L C L 3 is vapour is produced. So, reaction for generation of A L C L 3 that is already shown here. Now, what are those process parameters for this CVD of aluminium oxide? Now, when it is the CVD of A L 2 O 3. So, this side we want A L 2 O 3 that means, the yield of A L 2 O 3 that means, the growth in milligram per hour or micron per hour that is one of the criteria which is an index of production rate and this is the outcome with of course, the morphology of the coating that means, whether it is a coarse coating or a fine grained structure with a smooth surface or rough textured then the density of the coating these are the issues to be addressed and this is very much associated with this A L 2 O 3 coating, but on this side the process parameters one can look this is as the temperature then process pressure. So, these are the two thing one should look in however, in addition to that since we have already noticed that it is carbon dioxide CO 2 hydrogen these are admitted in the reactor maintaining certain temperature and pressure plus at the same time we have aluminium trichloride vapour which is continuously generated and admitted. So, these are all six are the process parameters which can have a dictating term in producing or in providing the final outcome that means, A L 2 O 3 coating yield effect of partial pressure of aluminium trichloride. Now, this can be represented by this graph this side we have partial pressure of A L C L 3 and on this side that is called R that is the growth rate in micron per hour. Now, here the nature will be more or less like this and this curve that is applicable or valid for a particular value of partial pressure of hydrogen and by changing this partial pressure of hydrogen we can have different location of this curve and it may also little bit changes its shape. For example, it can be something like this it can change its peak position and say these are 5 such condition and 5 such condition means pH 2 partial pressure of hydrogen is varied say for example, the total pressure if it is 100 tor for example, this pressure can be varied say between say 10 to 90 tor. Now, the question is which one will be applicable for 10 tor or 90 tor. One thing we can look here that this particular growth rate which is expressed in micron per hour that will be given at a particular value of partial pressure of aluminium trichloride and this is a very low value it may starts with say 0.5 tor it can be 1 it 2 and 3 something like that. So, it is 1.5. So, it is also an experience that at a particular value of PLC 3 this growth rate assumes very highest value and on the two sides we have a very low value. Now, this can be explained by this fact that on this low side concentration of aluminium trichloride is quite low which may lead to the low yield of aluminium oxide which is given by in by micron per hour and on this side what happens because of this reaction which already we have seen AlCl3 plus H2O plus CO2 which gives Al2O3 plus HCl. Now, if one can balance this equation one can look into this how the thing can be balanced. So, this is going to be 3. So, this is going to be 6 HCl and here we have CO also and this is actually 3 CO and here we have 3 CO. So, what we find that for 2 units of AlCl3 we get 6 unit of HCl. So, that means, the volume of the reaction product is increasing and that can push the reaction in this direction. So, if we increase AlCl3 on this side then it will become counter productive and there could be a fall in this yield of the coating. Now, one thing we can also look into that this graphs what we have mentioned this 1, 2, 3, 4, 5 graphs that we got for different values of partial pressure of hydrogen. Now, say if it is a 100 tor in that case for example, and if we keep the partial pressure during this CVD around 1 tor then obviously, considering this reaction that means, this CO2 plus H2O which gives H2O plus CO that means, here one thing is also extremely important that this production of water vapour. So, what we find that this growth or production of water vapour that would be highest when we take 1 is to 1 ratio of hydrogen to carbon dioxide CO2 and H2O 1 is to 1. So, naturally what happens if we keep the pressure around 50 tor around 50 tor then we can get this curve this curve we may expect it is not unreasonable to expect such curve around 50, 60. So, this can be also quite high which is around 60. However, when we go either on a very high value of pH 2 say for example, 90 in that case this curve this curve will shift its position and it can take up this position or when we take pH 2 as 10 then also we get a low value of this one because in this case the yield of H2O will be less because it is not 1 is to 1. So, if we increase pH 2 that means, keeping the same pressure CO2 PCO2 has to be automatically adjusted. So, whether it is pH 2 is high means PCO2 is less then this 1 is to 1 condition cannot be maintained. Again when we have partial pressure of H2 as 10 then PCO2 becomes quite high that can also cannot also give us 1 is to 1 ratio and those in those condition the process is not at all efficient and it leads to low yield. So, these are the two curves where the pH 2 value will be either too low or too high and this is more or less in the central position about 50 percent of the total pressure and it may be somewhere around 30 35 torque. So, this is a curve one would expect during CVD of aluminium oxide just growth rate of aluminium oxide versus partial pressure of aluminium oxide. Effect of pH 2 and that effect already we have shown here that this effect that means, this graph actually shows the combined effect of PalCl3 with that of hydrogen and this can be also drawn in another way that means, in this case what we can show here that means, this is the growth rate which is in micron per hour and this gives us partial pressure of pH 2 that is in torque. We can have another one side by side showing this growth rate of aluminium oxide coating which is given in micron per hour this abscise pH 2 and here also we can draw, but only the difference is that in this case this graph is drawn for a particular PalCl3 and this is for the temperature that means, in this case what we see if we have say for example, this is around one torque we can have variation which can be something like this, this is may be 0.5 torque of aluminium chloride then we can also have with 1.2 with 3 and with 1.5. So, what we can see here that this is with 1 torque of PalCl3 this is with 2 this is with 0.5 this one with 1.5 and this one with 3 that means, as we increase or decrease the partial pressure of PalCl3 definitely the peak point get shifted and it is also the same fact that highest yield can be obtained at an intermediate value and at the low end of this PalCl3 or the very high value of PalCl3 then we can have a low value of the yield. Now, for the temperature what we see here that with this pressure higher the temperature higher is the growth rate and this is actually the activation and with this activation it is a temperature dominated reaction. So, naturally higher the temperature the reaction rate increases and as a result the growth rate is also in that is also increasing. So, this way we can also show that growth rate versus temperature which is micron per hour and this can be shown something like that which goes exponentially and here this temperature is in the order of about 1000 degree plus may be around 1100 degree centigrade and in this case the rate of yield of this coating or the growth rate attains a very high rate and it goes at a very faster rate. Higher illustration can be also shown here micron per hour on one side and in this case this is the temperature this is actually 1 upon T 1 upon T and in this case what we see we can get a series of curve something like this and all these curves these are actually meant for a particular value of partial pressure of hydrogen. As we have seen here that a particular partial pressure of hydrogen which is around middle of the total pressure and there we see the highest possible production of water vapor which can be quite effective in pushing the reaction in the forward direction. So, this would be so for this particular situation if we consider one point here we can get in the vertical line 4 points showing the different growth rate. Naturally one can conclude that this is one of the pressure of partial pressure of hydrogen which facilitates high rate of production of H 2 O and in all other condition that is not that efficient. So, this pressure partial pressure of hydrogen it is situated somewhere about 50 percent of the total partial pressure if we consider the partial pressure of aluminium chloride quite negligible and insignificant. This is this can be concluded that the partial pressure of hydrogen should be around 50 percent of the total pressure. So, the remaining partial pressure that should be of that of CO 2. Now, here one thing is also important that aluminium oxide that is the CVD product here this coating is a CVD product. Now, in this case what happens that the fineness of the structure to get a finer structure of this is very important. Now, in this issue that generation or production of water vapor production of H 2 that has a decisive role production of H 2 O and that is given by what we called super saturation super saturation of H 2 O and this super saturation of H 2 O means that partial pressure of H 2 O which is the input partial pressure divided by P H 2 in the equilibrium. So, the objective of CVD should be to make this ratio which is the index of super saturation as high as possible in order that this reaction that means, A L C L 3 plus H 2 O which is nothing, but hydrolysis of A L C L 3 and that will go at a faster rate in this direction with the result of A L 2 O 3. So, if we move this reaction at a very fast rate then we can have rapid growth of this A L 2 O 3 and in that case nucleation rate that will be also quite high and as a result of that we may expect a finer grain size and a smoother coating on this carbide tool substrate. So, in principle one should attempt to make this super saturation of H 2 O as high as possible. Now, this is actually effect of P C A 2 and P H 2 ratio this is shown, but this can be also shown this way that this is the yield R which is in micron per hour and here we have this is P H 2 by P C O 2 and on this side we have say P C O 2 by P H 2 O. So, obviously, here we have when it is 1 is to 1 there we have the very high value and on both the sides we can have a decreasing trend on this growth rate. So, that we can see as the effect of this ratio. So, this ratio should be ideally 1 is to 1 partial pressure ratio to have highest production of H 2 O and this will lead to faster growth rate of A L 2 O 3. Effect of temperature. So, effect of temperature we have mentioned that this is actually activated by temperature naturally the growth rate will be quite high at higher temperature. However, one thing should be also taken into consideration that it is the limit of the CVD reactor and also the substrate of choice which should not be affected by this high temperature. This is effect of total process pressure. Effect of total process pressure say this side it is the growth rate which we can expressed as micron per hour or it can be just increase in weight of the substrate in terms of milligram per hour and this side that is the total pressure inside the CVD chamber the total pressure and here we may expect a curve something like this. Now, this curve we can interpret this curve in this form say this is a zone interesting zone for conducting the CVD operation. Now, why it is so? If we are on the higher side we can see growth rate is falling drastically and even with a low partial pressure total pressure growth rate is also not adequate. Now, when we are on the higher side what happens with this high rate of deposition high rate of total pressure we can have homogeneous nucleation. Homogeneous nucleation that means, this reaction is occurring in the gas phase. So, AlCl3 plus H2O and if we have a very high pressure then this Al2O3 that may form in the gas phase, but ideally it is supposed to form condense and synthesize on this carbide substrate, but this does not happen. So, the growth rate is obviously, it is falling in nature and when the pressure is too low in that case the residence time of this gas which are incoming gas on this substrate. So, this must not this is not getting enough time at the dual time to reside and to get adsorbed on this surface. So, this is also a function of this total pressure. So, when it is too low it is actually the problem with dual time or residence time and the problem associated with adsorption and when it is the high pressure then it is homogeneous nucleation that means, this formation of Al2O3 that is taking place in the gas phase and this substrate does not receive much of this vapor which could have condense and synthesize over this surface. Now, there how this CVD is conducted. So, here if we try to see this thing what we should have here actually we have what we call the Al2 generator. This is actually aluminum chip, aluminum chips are placed here that means, this vessel is filled with aluminum chip and from this side it is H2O plus HCl that mixture that is being fed and here we expect AlCl3 to form and this will be fed in the CVD reactor and this is actually the CVD reactor. Now, on this from this side also we have another stream which are all metered and this is actually CO2 plus H2. So, these should be mixed and there should be metered quantity and we have all the provisions for getting the right temperature you have the substrate here and on the downstream side we must have the pumping system which may lead to the vent here also we have vent before the gases are fed into the reactor. So, all these features must be provided in the CVD system. So, this is the actually the pumping system and here in the downstream side there is a throttle valve for controlling the system pressure. So, partial pressure of hydrogen, partial pressure of CO2, partial pressure of AlCl3 which is saturated with hydrogen then the temperature here inside the CVD reactor and the total pressure prevailing inside the chamber that is controlled by this throttle valve. So, this is in basic basically a CVD principle which is practiced in this particular CVD chamber. Now, comes very important issue that means, influence of the substrate. Now, we see that this aluminium oxide coating happens to be one of the very prospective candidate to be used with tungsten carbide substrate that means, what we call cemented carbide substrate and this is going to be straight in a cutting tool and which can be used with a higher process parameter that means, higher cutting velocity. So, that is the aim of the whole work to have this coating on cemented carbide substrate. Now, this cemented carbide substrate is a very successful substrate when it is TIC or TIN for that this is a very good candidate, but when it comes to deposition of aluminium oxide on this cemented carbide substrate we find lot of difficulties. Now, what are those difficulties let us look into detail. So, it is actually CVD of aluminium oxide on this cemented carbide substrate. Now, before we go to this intermediate layer let us try to understand the basic problem with this cemented carbide. It is actually a composite structure consists of cobalt and without this cobalt it will be extremely difficult to put to hold this carbide grains whether it is WC or TIC, TSC together. So, existence of cobalt that is a that is a must for this carbide to give the adequate toughness. However, when it comes the question of ALCL3 for HCL which are fed and which will be deposited which will adsorb on the surface there is every chance that this ALCL3 attacks the surface cobalt leading to cobalt chloride. In some extreme situation this HCL can also chemically dissolve cobalt leading to this cobalt chloride and when this whether it is a reaction product of CVD or the reactants of CVD if they really attack the substrate surface then the whole purpose is lost. So, the thing what one should go or do is how to suppress this cobalt and if one try to deposit aluminium oxide then what happens that we have a non-uniform coating the coating is not uniformly covered you have discrete some island which are certain favourable sites where the coating may grow, but it is unpredictable unreliable. So, one comes to an immediate conclusion that with such an existing substrate this deposition of aluminium oxide we have great difficulty apart from the mismatch with alpha value E value and other issues. So, it is just the interference of the chemical reactants with the substrate surface that creates that becomes the root of the problem. So, what is done in this case it is an intermediate layer of TIC that is put over the surface. So, that means, in this case this is the cemented carbide and over that one layer of TIC is grown it is also CVD TIC and this thickness is around 5 to 7 micron and this TIC can be deposited by this reaction TICL 4 CH 4 and with excess then you have the temperature then pressure excess H 2 and that will lead to TIC and that can go pretty well with this one and over that over this this AL CL 3 plus H 2 O that vapour that will come and if we follow the global reaction that means, all this thing AL CL 3 plus H 2 O plus CO 2 the whole mass that arrives here and that is on TIC coating. So, this AL 2 O 3 which is formed that is actually condensing. So, it is adsorption then this condensation of AL 2 O 3 nucleation of AL 2 O 3 and growth of AL 2 O 3. So, this way actually the substrate cobalt is suppressed in this case and we can expect a good coating with much better uniformity, better smoothness even improved adhesion and that will be reflected in its performance during actual machining task. So, this is one way of handling the problem just by depositing one layer of TIC it is by CVD. However, further investigation has also been made to look for something even better than TIC. So, this is actually an intermediate layer of titanium oxy carbide, titanium oxy carbide. So, this titanium oxy carbide if one puts on the surface this is cemented carbide and this is a layer of titanium oxy carbide and this x and y can vary one can vary this value of x y, but there is an optimum value of x and y which leads to one of the best deposition of aluminium oxide over this surface and it has to be it must be mentioned that this aluminium oxide that is just in the order of 1 to 2 micron and with this 1 to 2 micron the performance of the tool can be remarkably increased in comparison to a monolayer coating of TIC. So, if we have here TIC and if we have which is also say 5 to 7 micron TIC, but over that instead of that if one can put this titanium oxy carbide and that is the intermediate layer and over that if we have 1 to 2 micron thick aluminium oxide coating then this coating material it is now behaving like one ceramic tool. It is not exactly the ceramic tool where we have aluminium oxide as the solid piece, but here we have the tool basically it is cemented carbide, but over that we have the intermediate layer and followed by one aluminium oxide layer. So, this aluminium oxide ceramic has low toughness, but this carbide can compensate for that and it has this less tensile strength which can be also well taken care of by this carbide, but and at the top we have aluminium oxide. So, it becomes the functional surface behaves almost like the surface of a ceramic tool and that is why its capability is enhanced in with in a few order of magnitude and that is why this becomes one of the high performance cutting tool. So, this aluminium oxide coating can be put on this titanium oxy carbide. Now, how this titanium oxy carbide is given on the surface? Now, we can see that how titanium carbide is formed titanium carbide is formed by T i C l 4 and C h 4 that gives us T i C. Now, when we have T i C l 4 and this here to get this T i C l 4, if we have H 2 O that can give us T i O and this H 2 O is can be obtained from this reaction. So, this is 1 and this is 2, but to get this H 2 O one has to also conduct this reaction that means, CO 2 plus H 2 O that gives us H 2 O plus CO and this H 2 O can be used here, but this gives us T i O, but one is interested to have not T i O, but a oxy carbide. This gives 100 percent T i C and this gives 100 percent T i O through this hydrolysis, but if one can takes this CO here that means, T i C l 4 plus CO plus H 2 and if we have this kind of thing, one can also go for T i it is actually 0.5 and 0.5 and here we have 4 C l plus 2 and that can give us 4 H C l. So, instead of using this CO 2 if one uses CO in this reaction in that case this T i oxy carbide with 0.5 ratio fraction of that atomic percent of 0.5 to 0.5. So, this is 1 is to 1 that can be formed. So, it is a 50-50 oxy carbide and that can be put here over the surface over the surface as 0.5 O 0.5 this is an oxy carbide and on that a l 2 can be easily deposited and that is more important here to have high density of nucleation. So, with this subsurface or intermediate layer immediately one can get with ease it aluminum oxide coating with high nucleation density smoother coating and well adherent coating and this coating in comparison to a coating which is deposited on T i C they can show their difference in performance and it is mostly the resistance to wear that means, with a high degree of adhesion and with a high degree of density and with a high fineness of the coating all the required qualities of the coating can be obtained just by putting a intermediate layer of titanium oxy carbide. So, this is one of the step in synthesizing aluminum oxide on tungsten carbide cemented carbide because the deposition of aluminum oxide by CVD by CVD that interest mostly comes from the use as a hard coating and that is mostly that is that comes from the manufacturing industry. So, with that we can summarize the whole topic like this chemical vapour deposition of aluminum oxide is based on the principle of coupling reactions between aluminum trichloride hydrogen and carbon dioxide. However, to conduct the process in situ generation of aluminum trichloride is required this is followed by the coupled CVD reaction. The chemistry of the cemented carbide substrate is not that compatible and does not facilitate growth of aluminum oxide coating with favourable morphology and adhesion. The problem can be well addressed by providing an intermediate layer of titanium carbide. However, an intermediate layer of titanium oxy carbide is found to be more effective than titanium carbide in enhancing the performance of aluminum oxide coating.