| Title of Invention | A MEHTOD OF FORMING A CERAMIC COMPOSITE BODY GONTAINING AN AXISYMMETRIC HOLE OF VARYING GEOMETRY INSIDE A HOLLOW SUBSTRATE |
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| Abstract | The present invention discloses a method wherein a ceramic composite body containing an axisymmetric hole of varying geometry is formed inside a hollow substrate by introducing shaped refractory cores, having the geometric shapes of the desired axisymmetric hole, at the ends of the hollow substrate. The shaped refractory cores are inserted at the respective ends of the hollow substrate such that the shaped cores seal the ends of the hollow substrate. The hollow substrate is filled with a powdery thermit mixture such that an axial hole is formed in the powder mass. The hollow substrate, along with all the contents, is housed in a hollow body and fitted to a rotary fixture, such as mounted on the output shaft of a prime mover, such as an electric motor and rotated in order to keep the thermit mixture pressed against the inner surface of hollow substrate. The thermit mixture is ignited. The reaction propagates through the remaining powdery mixture resulting in molten products which collect on the inner surface of the hollow substrate under the influence of centrifugal force. The rotation is continued to allow the reaction products to solidify and cool to form the first cylindrical layer of the ceramic composite body on the inner surface of the hollow substrate If the desired geometry is not fully formed, the subsequent cylindrical layers of the ceramic composite body are formed on the inner surface of the first layer by following the preceding steps till the ceramic composite body containing the required axisymmetric hole of varying geometry is formed inside the hollow substrate. The ceramic composite body containing an axisymmetric hole of varying geometry, such as a nozzle, may be formed inside a hollow substrate, This will find usage wherein ceramic coatings are applied to metal surfaces to protect them from corrosion, wear and heat. Metallic nozzles, used in applications such as a gas turbine, are invariably provided with coatings of suitable ceramic materials. |
| Full Text | The present invention relates to a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry inside a hollow substrate. The present invention particularly relates to a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry, such as a nozzle, inside a hollow substrate. Ceramic coatings are applied to metal surfaces to protect them from corrosion, wear and heat. Metallic nozzles, used in applications such as a gas turbine, are invariably provided with coatings of suitable ceramic materials. These ceramic coatings are formed by many vapour and spray deposition methods. When such a nozzle is exposed to more aggressive environment, it is necessary to provide the nozzle with ceramic coating of a large thickness. It is difficult to provide a metallic nozzle with thick ceramic coating by these methods. In such cases, it is preferable to make the nozzle, or part of the nozzle, from a suitable ceramic and integrate the nozzle with a hollow metal part to meet the requirements. The ceramic nozzles are manufactured by powder metallurgy techniques. It can be integrated with a hollow metal part by inserting the ceramic nozzle in the metal part and filling the annular space between the ceramic nozzle and the metal part with a molten filler material having low melting point. However, complete filling of the narrow annular space with the filler material is difficult to ensure and may result in poor adhesion of the ceramic nozzle to the metal part. Also, the powder metallurgy techniques to fabricate ceramic nozzles and the method of integrating them with the hollow metal parts have other disadvantages like the requirement of sophisticated equipment's, high consumption of electric power and involvement of multiple steps. Reference may be made to (UK Patent No. GB 1497025 titled: Method of producing cast refractory inorganic materials, wherein a method has been developed to synthesize molten refractory inorganic materials, namely, carbides, borides, nitrides and silicides of metals as well as hard alloys, and produce their castings using thermit type reactions. Cast inorganic materials of high melting point are produced from at least one of the oxides of the metals of groups IV, V and VI of the Period Table, a metallic reducing agent and a non-metal or an oxide thereof. A small zone of the surface of the mixture of the starting materials is caused to ignite which leads to the burning zone spreading across the starting mixture. The burning is carried out under a pressure of the gaseous medium of 1-5000 atmospheres. The materials obtained, namely carbides, borides, silicides and nitrides as well as alloys can be used for the fabrication of components and equipment in machine tool construction. Reference may also be made to US Patent N0. 4,363.832 titled: Method for providing ceramic lining to a hollow body by thermit reaction, wherein a method has been described for providing ceramic lining to a hollow body by thermit reaction and centrifugal force. The invention provides a novel method for forming a ceramic lining layer on the inward surface of a hollow body such as a pipe. According to the invention, a thermit mixture, for example, composed of aluminum powder and an iron oxide is placed in the hollow space of the hollow body, which is rotated at a high speed so as that the thermit mixture is pressed against the wall of the hollow body by the centrifugal force and the thermit mixture is ignited, for example, by contacting with an acetylene flame. The thermit reaction of the thermit mixture propagates under the influence of the centrifugal force so that the molten metal formed from the reducible metal oxide and the ceramic oxide formed from the strongly reductive element are separated into stratified layers by virtue of their density difference with the ceramic oxide forming the innermost layer and the metal forming the intermediate layer between the ceramic oxide layer and the wall of the hollow body with strong bonding of the ceramic oxide layer upon solidification by cooling. The above method was also used by O. Odawara and J. Ikeuchi (Journal of American. Ceramic. Society., 69 (4), PP:80-81, 1986) to provide ceramic-ceramic composite lining to a metal pipe. Method described by the inventor is able to produce only uniform cylindrical linings. Although, the above method is capable of providing large thickness of ceramic composite lining to a hollow body economically, it does not generate the required axisymmetric hole of varying geometry inside the ceramic composite lining which is required as in the case of convergent-divergent nozzle of gas turbines and the like. Thus, these methods are neither intended nor useful to form a ceramic composite body containing an axisymmetric hole of varying geometry, such as a nozzle, inside a hollow substrate. From the details and drawbacks of hitherto known prior art methods for providing ceramic composite lining inside a hollow substrate, it is clear that there is a definite need and scope for providing a method of forming a ceramic composite body containing an pxisymmetric hole of varying geometry inside a hollow substrate. The main objective of the present invention is to provide a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry inside a hollow substrate, which obviates the drawbacks of the above referred hitherto known prior art. Another objective of the present invention is to provide a simple, convenient and inexpensive method of forming a ceramic composite body inside a hollow substrate useful for applications requiring corrosion, wear and heat resistance. Yet another objective of the present invention is to provide a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry inside a hollow substrate by single layer or multi layers of the ceramic composite. Still yet another objective of the present invention is to provide a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry inside a hollow substrate, particularly hollow metal substrate. Another objective of the present invention is to provide a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry, such as a nozzle, inside a hollow substrate. The present invention discloses a method of forming a ceramic composite body containing an axisymmetric hole of varying geometry inside a hollow substrate. A ceramic composite body containing an axisymmetric hole of varying geometry is formed inside a hollow substrate by introducing shaped refractory cores, having the geometric shapes of the desired axisymmetric hole, at the ends of the hollow substrate. The shaped refractory cores are inserted at the respective ends of the hollow substrate such that the shaped cores seal the ends of the hollow substrate. The hollow substrate is filled with a powdery thermit mixture such that an axial hole is formed in the powder mass. The hollow substrate, along with all the contents, is housed in a hollow body and fitted to a rotary fixture with the help of fasteners. The whole unit is mounted on the output shaft of a prime mover, such as an electric motor and rotated in order to keep the thermit mixture pressed against the inner surface of hollow substrate. The thermit mixture is ignited by an electric arc, struck between electrodes, at least at one point on its free inward surface so as to initiate the thermit reaction. The reaction then propagates through the remaining powdery mixture resulting in molten products which collect on the inner surface of the hollow substrate under the influence of centrifugal force. The rotation is continued for 5 to 30 minutes to allow the reaction products to solidify and cool to form the first cylindrical layer of the ceramic composite body on the inner surface of the hollow substrate. If the desired geometry is not fully formed, the subsequent cylindrical layers of the ceramic composite body are formed on the inner surface of the first layer by following the preceding steps till the ceramic composite body containing the required axisymmetric hole of varying geometry is formed inside the hollow substrate. The ceramic composite body containing an axisymmetric hole of varying geometry, such as a nozzle, may be formed inside a hollow substrate. This will find usage wherein ceramic coatings are applied to metal surfaces to protect them from corrosion, wear and heat. Metallic nozzles, used in applications such as a gas turbine, are invariably provided with coatings of suitable ceramic materials. The above stated objectives of the present invention have been achieved by the non-obvious inventive steps of introducing shaped refractory cores of desired section inside the hollow substrate, filling with powdery thermit mixture, rotating to bring about centrifugal force, initiating thermit reaction under rotation to form the axisymmetric hole of desired geometry inside the hollow substrate. The novelty of the present invention lies in producing ceramic composite body containing axisymmetric hole of varying geometry inside the hollow substrate. The present invention is illustrated in figurel of the drawing accompanying this specification which is a schematic illustration of a longitudinal section of a hollow substrate with ceramic composite body containing an axisymmetric hole of varying geometry viz. conical+cylindrical+conical. The various parts shown in the figure 1 of a longitudinal section of a hollow substrate with ceramic composite body containing an axisymmetric hole of varying geometry viz. conical + cylindrical + conical are as follows: (1) Multi-layer ceramic composite body formed on the inside surface of the hollow substrate. (2) Cylindrical hollow body. (3) Shaped refractory cores. (4) Conical feeder integrated with shaped refractory Core. Accordingly, the present invention provides a method of forming a ceramic composite oody containing an axisymmetnc hole of varying geometry such as nozzle inside a hollow substrate, which comprises a) Characterised in that introducing pre-shaped refractory cores(3,4) having the geometric shapes such as nozzle having axisymmetnc hole at the ends of a hollow substrate(2) such that the ends of the hollow substrate are sealed by the said shaped cores (3,4), b) filling the annular space between the hollow substrate (2)and the shaped cores (3,4) with a powdery thermit mixture such as herein described in a manner such that a hole, coaxial with the hollow substrate(2) is formed, c) subjecting the hollow substrate (2)along with its contents to centrifugal force by rotation on a rotary fixture capable of rotating the said hollow substrate along with its contents, d) initiating a thermit reaction by heating the powdery mixture at least at one point on its free surface, inside the hole of the powdery mixture, e) continuing the rotation of the hollow substrate along with its contents to allow the molten deposited ceramic composite layer to solidify and cool, f) stopping the rotation and removing the refractory cores to verify whether the desired geometry(1) such as a nozzle is formed inside the hollow substrate(2) g) repeating the steps (a) to (f), if the desired geometry(1) is not formed, to form the subsequent layers till the ceramic composite body of desired inner geometry(1) such as nozzle is formed inside the hollow substrate(2), and h) removing the refractory cores(3,4) to get the desired ceramic composite body(1) containing the required axisymmetnc hole of varying geometry such as a nozzle, formed inside the hollow substrate(2) In an embodiment of the present invention, the method is applicable to any hollow substrate capable of withstanding thermal shock of exothermic reaction heat and centrifugal force In another embodiment of the present invention, the hollow space of the substrate is of axisymmetnc truncated conical geometry In still another embodiment of the present invention, the shaped core is a hollow body of surface thickness in the range of 0.5 to 2 mm, capable of withstanding thermal shock of exothermic reaction heat and centrifugal force. In yet another embodiment of the present invention, the shaped core material is inert and non-wetting with respect to the products of the reaction being carried out. In still yet another embodiment of the present invention, the shaped core is such as graphite material. In another embodiment of the present invention, the shaped cores employed have axial holes so as to allow insertion of an igniter to initiate the thermit based reaction and to allow the gases, evolved during the reaction, to escape to the ambient. In another embodiment of the present invention, shaped cores are provided with conical feeders. In still yet another embodiment of the present invention, the shaped core is extended in stages to form the desired axisymmetric hole of varying geometry inside the ceramic composite body. In a further embodiment of the present invention, the shaped refractory cores are fitted to the ends of the hollow substrate such that the axial gap between the ends of the refractory shaped cores and the hollow substrate is sufficient to facilitate easy filling of thermit mixture. In a yet further embodiment of the present invention, the ends of the refractory cores, preferably flanged, are flush with the ends of the hollow substrate. In a still further embodiment of the present invention, the annular space between the hollow substrate and the shaped cores is filled with thermit or thermit based reaction mixture such that a hole intended for ignition of thermit mixutre, preferably coaxial with the hollow substrate, is formed. In yet further embodiment of the present invention, the diameter of the coaxial hole intended for ignition of thermit mixutre is at least 1mm. In another embodiment of the present invention, the thermit or thermit based reaction mixture used is of composition desired for the ceramic composite layer to be produced. In yet another embodiment of the present invention, the thermit mixture contains a strongly reductive element and strongly reducible metal oxide. In still another embodiment of the present invention, the strongly reductive element is selected from aluminum, magnesium, zirconium or silicon. In a further embodiment of the present invention, the strongly reducible metal oxide is selected from oxides of iron, nickel, copper, tungsten, titanium, molybdenum, vanadium, chromium, niobium, zinc or manganese. In a still further embodiment of the present invention, the thermit mixture contains aluminum and ferric oxide as strongly reductive element and strongly reducible metal oxide respectively. In yet further embodiment of the present invention, the aluminum and ferric oxide in the thermit mixture are in stoichiometric amounts. In another embodiment of the present invention, the aluminum and ferric oxide is of particle size of at least -170 mesh. In still another embodiment of the present invention, the hollow substrate and the shaped refractory cores are fitted together firmly in a rotary fixture in such a way that the axes of the refractory cores and the hollow substrate coincide. In yet another embodiment of the present invention, the hollow substrate along with its contents is subjected to centrifugal force by rotation on the shaft of a rotary fixture, such as a prime mover shaft, preferably a variable speed electric motor. In still yet another embodiment of present the invention, the rotation of the hollow substrate along with its contents is continued for a time period in the range of 5 to 30 minutes. In a further embodiment of the present invention, the hollow substrate along with its contents is rotated about its axis at a speed to generate a centrifugal force in the range of 10 to 1000 G. In a still further embodiment of the present invention, the narrow cylindrical region of the axisymmetric hole of varying geometry is formed at a speed of rotation which generates a centrifugal force of about 450 G. In a yet further embodiment of the present invention, the reaction method is carried out in inert atmosphere by employing inert gases like argon at one atmospheric pressure. In another embodiment of the present invention, the thermit mixture is diluted with the ceramic product of the thermit reaction. In still another embodiment of the present invention, the method is repeated, if required, for inner geometry formation inside the hollow substrate till the ceramic composite body containing the required axisymmetric hole of varying geometry is formed inside the hollow substrate by the multi-layers of the solidified reaction products. The present inventive and novel method is applicable to any hollow substrate and is most applicable to a substrate having a hole of axisymmetric geometry, such as a cylindrical or truncated conical hole. Any highly exothermic reaction, capable of self-propagating after ignition and releasing sufficient heat to result in molten product or products, is useful in the present method. Many thermit reactions, which readily meet these requirements, are more preferred owing to the easy availability of the starting materials at lower cost and easiness in controlling the reaction. A thermit mixture, required to carry out such a reaction, is a mixture of powders of a strongly reductive element and a reducible metal oxide. When a thermit reaction is carried out using such a thermit mixture, the oxide of the strongly reductive element and the metal of the reducible metal oxide are produced as products to form a ceramic-metal composite. The strongly reductive element can be aluminum, zirconium, magnesium or silicon to get the respective oxide as the ceramic product. The strongly reducible metal oxide can be chosen from the oxides of iron, nickel, copper, vanadium, titanium, manganese etc. to get the respective metal product. In addition, the thermit mixture may contain other components, either an element such as carbon, boron or silicon or a compound such as boric oxide, to form carbide, boride or silicide in place of pure metal product produced in a conventional thermit reaction. Thus, thermit based reactions can also be used in the present method to produce ceramic-ceramic composites. A few thermit and thermit based reactions useful for the present method and their estimated heats of reaction (AH) and adiabatic temperatures (Tad) (i.e. the temperature to which the products of the thermit reaction are raised) are shown in Table I. The values of AH and Tad were estimated using thermodynamic values available in hand books and assuming that the thermit reactions take place under adiabatic conditions. TABLE. 1 (Table Removed) A powdery mixture of a thermit or thermit based reaction containing non stoichiometric amounts of the reactants can also be used in the present invention if the reaction self propagate after ignition. Additionally, one of the products of a thermit or thermit based reaction can be added as an inert diluent to lower the adiabatic temperature of the reaction while maintaining the self propagating nature of the reaction. When the ceramic product of such a reaction is used as inert diluent, the yield of that product can be increased using the same amount of the stoichiometric mixture of the thermit or thermit based reaction. When a thermit or thermit based reaction is not self propagating, prior heating of the thermit mixture to a suitable temperature is the primary requisite. If the adiabatic temperature of a reaction is higher than the boiling or sublimation temperature of any of the reactants, the present method may be carried out under inert gas pressure. A homogeneous powdery mixture is prepared by uniformly blending the powders of the reactants, in stoichiometric amounts, of a thermit or thermit based reaction in a mechanical mixer or blender, preferably sealed, for 12 to 24 hours. The powders used are preferably of 200-mesh size and baked at about 120° C for 12 to 24 hours before mixing. In the next step, the shaped refractory cores are fitted to the ends of the hollow substrate such that the shapes of the cores can be imparted to the inward surface of the hollow ceramic composite body being formed. This arrangement is useful when the axial gap between the ends of the two refractory shaped cores is sufficient to facilitate easy filling of thermit mixture manually. The hollow substrate and the shaped refractory cores are fitted together firmly in a rotary fixture in such a way that the axes of the refractory cores and the hollow substrate coincide. The ends of the refractory cores, preferably flanged, should be flush with the ends of the hollow substrate, to prevent the products of the reaction from escaping. The material and thickness of the hollow substrate should be such that it does not get destroyed either by the heat it receives during the exothermic reaction, particularly while forming the first layer of ceramic composite directly on the inward surface of the substrate, or by the centrifugal force it experiences while carrying out the method. The hollow substrate can be made of a metal such as iron, nickel, titanium or their alloys, or a nonmetal, such as graphite or a ceramic. When the thickness and material of the metallic hollow substrate do not satisfactorily resist such destruction, thin metal strips can be wrapped one above the other over the hollow substrate. They can be held together and against the substrate using clamps, such as hose clips, over the outermost strip. The total thickness of the metal strips should be sufficient to support the hollow substrate adequately against the centrifugal force and serve as a heat sink to extract heat from the hollow substrate quickly. Alternatively, the hollow substrate, along with the shaped refractory cores, can be housed closely in a supporting hollow body. The hollow body, shaped refractory cores and the hollow substrate can be held together using a suitable rotary fixture. The supporting hollow body should be able to resist the centrifugal force it experiences to support the hollow substrate adequately and serve as a heat sink to extract heat from the hollow substrate quickly. The metal strips or supporting hollow body can be made of metals, such as copper, nickel, iron or their alloys, having good properties such as strength, ductility, machinability and thermal conductivity. The supporting hollow body can also be made from a nonmetal such as graphite. In such a case, It is preferable to fix a flexible clamp, such as a hose clip, over the periphery of the hollow body to prevent its destruction. The shaped cores employed should have axial holes so as to allow insertion of an igniter to initiate the thermit or thermit based reaction and to allow the gases, evolved during the reaction, to escape to the ambient. Also, it is preferable to use thin walled hollow cores to minimize the heat absorption by the cores. However, the wall thickness of such a hollow core should be adequate to withstand the centrifugal force it experiences while carrying out the method. The shaped core may be of hollow body of surface thickness in the range of 0.5 to 2mm. The shaped cores can be made of a material capable of resisting high temperatures with good thermal shock resistance. Also, it is preferable that the core material is inert and non-wetting with respect to the products of the reaction being carried out. Although many metals and ceramics are useful for making shaped cores, graphite is adequate in most cases owing to its easy availability at low cost and good properties, such as high temperature capability, tolerable inertness to many materials at high temperatures, machinability, good thermal shock resistance and strength at elevated temperatures. In the next step, the annular space between the hollow substrate and the shaped cores is filled with the prepared thermit or thermit based reaction mixture such that a hole, preferably coaxial with the hollow substrate, is formed. Depending on the amount of reaction mixture used, the diameter of this coaxial hole can be about 1 mm or more. The reaction mixture can be filled in the annular space and pressed manually. When the axial gap between the shaped cores is narrow it will be difficult to fill the annular space with the reaction mixture. In such a case, it is preferable to place one of the shaped cores at one end of the hollow substrate and fill the hollow space inside the substrate with the reaction mixture. The mixture at the other end is then scraped to accommodate the other shaped core. Alternatively, metal plungers, having shapes similar to those of the shaped cores, can be employed in a mechanical press to press the reaction mixture, placed inside the substrate, prior to fixing the cores to the ends of the hollow substrate. The shaped cores of short lengths, possessing part of the required axisymmetric geometry, can be employed when it is difficult to fill or accommodate the required amount of reaction mixture in the annular space between the full sized shaped cores and the substrate or the previous layer. Subsequently, the shaped cores can be extended in stages along the axis, as shown by dotted lines in figure 1 of the drawing, to get the desired complete geometry of the axisymmetric hole. In the next step, the rotary fixture, along with the hollow substrate, shaped cores and mixture of reactants, is rotated about the axis of the hollow substrate by any conventional means. The speed of rotation should be so selected that the mixture of reactants experiences the centrifugal force to yield products of the reaction with the desired characteristics. When the products of the reaction are immiscible and their densities differ significantly, the centrifugal force helps in separating them faster and they are cast as separate layers. If their densities are comparable, higher magnitude of centrifugal force will be required to aid such separation. In both the cases, the products will be cast together, either graded or non-separated, if the magnitude of the centrifugal force is inadequate. In all the cases, the centrifugal force helps to distribute the molten reaction products uniformly over the inward surface of the hollow substrate. Also, it helps in the formation of a cast cylindrical layer of a ceramic composite of uniform thickness inside an axisymmetric hollow substrate. In the next step, the thermit or thermit based reaction is initiated on the inward surface of the powdery mixture by heating it, at least locally on its free surface inside the axial hole, to or above its ignition temperature. This may be carried out, for example, by bringing an electric arc, struck between two electrodes, in contact with the free inward surface of the powdery mixture. The reaction, thus initiated, rapidly propagates through the remaining powdery mixture in the hollow substrate to convert all the reactants into molten products. Under the influence of the centrifugal force, the molten products distribute uniformly over the inward surface of the hollow substrate and remain pressed against it and the shaped cores. It is desirable to keep the products in molten condition for longer duration while in rotation so that the products spread uniformly, particularly near the shaped cores, and the gases, contained in the powdery mixture and those produced by the reaction, escape to the ambient due to the pressure difference over the thickness of the molten products. Partial evacuation would help to accelerate this degassing. Also, the method can be carried out inside an enclosure after evacuating the enclosure for long time to remove the gases trapped in the powdery mixture and back filling it with an inert gas, such as argon, to improve the quality of the solidified products. This approach also helps in overcoming the problem of ignition of some thermit or thermit based reactions in air when such reactions are employed in the present method. The formation of the axisymmetric hole inside the ceramic composite body is started. The rotation of the assembly is continued for 5 to 30 minutes to allow the molten products to solidify and cool by losing heat to the surroundings. This results in the formation of a cylindrical layer of the ceramic composite. The profiles of the shaped cores are imparted to the ends of the cylindrical layer to form the first segment of the axisymmetric hole. The rotation of the assembly is stopped. If the ceramic composite body of desired inner geometry is not formed inside the hollow substrate, the above said procedure is repeated till the ceramic composite body containing the required axisymmetric hole of varying geometry is formed inside the hollow substrate by the multilayers of the solidified reaction products. If a different thermit or thermit based reaction mixture is used, the ceramic composite layer produced will be of different composition. When the required quantity of the powdery mixture cannot be accommodated in the hollow space of the substrate, it is necessary to incorporate feeders at the ends of the shaped cores. These feeders, preferably conical, are filled with the extra quantity of the powdery mixture. Under the effect of centrifugal force, the feeders direct the molten products of the reaction towards the circumferential opening between the cores. These feeders are particularly useful to form the narrow cylindrical throat region of a nozzle. In some cases, it is adequate to provide such a feeder to one of the shaped cores, as shown in figure 1 of the drawings. In the next step, the hollow substrate, having the ceramic composite body containing the required axisymmetric hole of varying geometry, is separated from the rotary fixture. If the ceramic body does not adhere to the hollow substrate, it can be separated from the substrate. The novelty of the present invention lies in producing ceramic composite body containing axisymmetric hole of varying geometry inside a hollow substrate. The above said novelty has been achieved by the non-obvious inventive steps of introducing shaped refractory cores of desired section inside the hollow substrate, filling with powdery thermit mixture, rotating to bring about centrifugal force, and initiating thermit reaction under rotation to form the axisymmetric hole of desired geometry inside the hollow substrate. The following examples are given by way of illustration of the method of the present invention in actual practice and should not be construed to limit the scope of the present invention in any way. Example -1 Ferric oxide and aluminium powders, having particles of size -325 mesh, were dried in an electric oven at 120° C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 100g of ferric oxide and 33.8g of aluminium in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. A mild steel pipe of 21mm length, 43mm inside diameter and 6mm wall thickness was used as a hollow substrate. It was required to form an alumina based ceramic composite body, containing an axisymmetric hole of truncated conical geometry, having big end diameter of 37mm and apex angle of 60°, at one end, inside this pipe using the prepared thermit mixture. The axisymmetric hole had The remaining hole, connected to the smallend of the conical geometry, was cylindrical of 30mm diameter. A tapered core of graphite, having dimensions of the truncated conical geometry, was prepared by machining. It was provided with an integral flange of 49mm diameter and 4mm thickness at its bigend and an axial hole of 6mm diameter. This core was held in position inside the pipe such that its flange is flush with one end of the pipe. The pipe was filled with 20g of the prepared thermit mixture upto the other end of the pipe such that an axial hole of 6mm diameter was formed in the filled thermit mixture. A circular graphite flange, having 4mm thickness, 49mm diameter and a central hole of 10mm diameter, was provided to the other end of the pipe. The pipe, having thermit mixture, core and flange, was fitted firmly to a rotary fixture, comprising of flanges, by steel fastners outside the pipe. The rotary fixture was mounted on the shaft of an electric motor and rotated about the axis of the pipe at 3400rpm. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, as shown in Table. 1, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1.5mm radial thickness, comprising of stratified layers of iron and aluminum oxide, was formed on the inner surface of the pipe. The above procedure was repeated with 20g of thermit mixture inside the hollow substrate, containing the first layer of the ceramic composite, to form the second cylindrical layer of the ceramic composite of 2mm radial thickness on the inner surface of the first one. The profile of the shaped core was slightly formed at the end of the second layer where it was in contact with the shaped core. The third layer of the ceramic composite of 3mm radial thickness was formed on the inner surface of the second layer by repeating the procedure again with 30g of thermit mixture inside the hollow substrate containing the two layers. Thus, an alumina based ceramic composite body, containing an axisymmetric hole of truncated conical geometry, having big end diameter of 37mm and apex angle of 60°, at one end The remaining hole, connected to the small end of the conical geometry, was cylindrical of 30mm diameter. The edges of the cylindrical hole were deburred by grinding. Example - 2 Ferric oxide and aluminium powders, having particles of size -325 mesh, were dried in an electric oven at 120°C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 325g of ferric oxide and 109.8g of aluminum in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. A stainless steel pipe of 66mm length, 57mm inside diameter and 3mm wall thickness was used as a hollow substrate. It was required to form a convergent-divergent nozzle from an alumina based ceramic composite inside this pipe using the prepared thermit mixture. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30°, respectively, and big end diameters of 50mm. The cylindrical throat had 39mm diameter. Four mild steel strips, each 60mm wide and 1mm thick, were wrapped, one over the other, on the outside surface of the pipe to serve as a support to the pipe and heat sink. The strips were held against the pipe by external hose clips. This arrangement was made to prevent the pipe from bulging, due to the heat and centrifugal force it experiences, during the formation of the nozzle inside the pipe. Two tapered cores of. graphite, having dimensions of the desired convergent and divergent sections, were prepared by machining. They were provided with coaxial holes of 10mm diameter at their small ends and integral flanges of 63mm diameter and 4mm thickness at their big ends. The cores were made hollow such that the portions of the cores protruding inside the hollow substrate had 1.5mm wall thickness. The core, corresponding to the convergent section, was held in position inside the pipe such that its flange was flush with one end of the pipe. The pipe was filled with 43g of the prepared thermit mixture upto the level of the other end of the pipe such that an axial hole was formed in the filled thermit mixture. A circular flange of graphite, having 4mm thickness, 63mm outside diameter and a central hole of 10mm, was provided to the other end of the pipe. The pipe, having thermit mixture, core and flange, was fitted firmly to a rotary fixture, comprising of flanges, by steel fastners outside the pipe. The rotary fixture was mounted on the shaft of an electric motor and rotated about the axis of the pipe at 2800rpm. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, as shown in Table. 1, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1mm radial thickness, comprising of stratified layers of iron and alumina based ceramic, was formed on the inner surface of the pipe. The above procedure was repeated with 59g of thermit mixture inside the hollow substrate, containing the first layer of the ceramic composite, to form the second layer of the ceramic composite of 1.4mm radial thickness on the inward surface of the first layer. The substrate and its associated components were separated from the rotary fixture and the graphite flange was removed. The hollow substrate, having the core of the convergent section and two layers of the ceramic composite, was again filled with the thermit mixture upto the other end of the pipe such that an axial hole is formed in the mixture. The mixture at this end was scraped with a metal template such that the truncated conical geometry, corresponding to the divergent section, was formed. After scraping, the quantity of the thermit mixture held inside the substrate was 38g. The tapered graphite core, having apex angle of 30° and corresponding to the divergent section of the nozzle, was introduced at this end such that its flange was flush with the end of the pipe. The pipe was rotated about its axis at 2800rpm with the help of the rotary fixture and the thermit reaction was carried out, in the same manner as done previously, to form the third cylindrical layer of the ceramic composite of 1.6mm radial thickness on the inward surface of the second layer. The convergent and divergent sections were slightly formed at the ends of the third cylindrical layer where the layer was in contact with the shaped cores. The subsequent cylindrical layers of the ceramic composite, having 1.2, 0.9, 0.8, 0.7, 0.7 and 0.7 radial thickness, were formed on the inward surfaces of the corresponding previous layers, in the same manner as done previously, using 36, 33, 28, 26, 22, and 16g of the thermit mixture respectively. The innermost layer formed the cylindrical throat of 39mm diameter. The edges of the throat were deburred by grinding. Thus a convergent-divergent nozzle from an alumina based ceramic composite was formed inside the pipe. The convergent and divergent sections at the ends of the axisymmetric hole of the nozzle had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30°, respectively, and big end diameters of 50mm. The cylindrical throat had 39mm diameter. Example - 3 Ferric oxide and aluminium powders, having particles of size -325 mesh, were dried in an electric oven at 120° C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 100g of ferric oxide and 33.8g of aluminium in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. A mild steel pipe of 21mm length, 43mm inside diameter and 3mm wall thickness was used as a hollow substrate. It was required to form an alumina based ceramic composite body containing an axisymmetric hole of varying geometry inside this pipe using the prepared thermit mixture. The axisymmetric hole consisted of a truncated conical geometry, having big end diameter of 37mm and apex angle of 60°, at one end and the small end of this geometry was connected to the remaining cylindrical hole of 30mm diameter. A tapered core of graphite, having dimensions of the truncated conical geometry, was prepared by machining. It was provided with an axial hole of 6mm diameter at its small end and an integral flange of 49mm diameter and 4mm thickness at its big end. The core was made hollow such that the portion of the core protruding inside the hollow substrate had 1.5mm wall thickness. A graphite housing, having inner diameter of 49mm, wall thickness of 8mm and length of 24mm, was prepared by machining. It was provided with an integral flat endplate of 10mm thickness at one end. The endplate was provided with a central hole of 30mm diameter and 6 numbers of radial vent holes of 5mm diameter, open to the central hole, at the mid thickness of the plate. The core was held in position inside the pipe such that its flange is flush with one end of the pipe. The pipe was filled with 20g of the prepared thermit mixture upto the other end of the pipe such that an axial hole was formed in the filled thermit mixture. A 4mm thick circular graphite flange, having 49mm diameter and a central hole of 10mm diameter, was provided to the other end of the pipe. The pipe, having thermit mixture, core and flange, was inserted in the prepared graphite housing such that the flange of the core was placed between the end of the pipe and the endplate of the housing. The housing, along with its contents, was fitted firmly to a rotary fixture, comprising of flanges, by steel fasteners outside the housing. The rotary fixture was mounted on the shaft provided inside an enclosure of a centrifuge. The enclosure was evacuated continuously for three hours to maintain a vacuum in the enclosure at about 0.01mm of Hg. the vacuum pump was isolated and the enclosure was back filled with argon gas at 1 atmosphere. The rotary fixture was rotated about the axis of the pipe at 3400rpm inside the enclosure. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, as shown in Table. 1, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1.5mm radial thickness, comprising of stratified layers of iron and dense alumina based ceramic, was formed on the inner surface of the pipe. The above said procedure was repeated with 20g of thermit mixture inside the hollow substrate, containing the first layer of the ceramic composite, to form the second cylindrical layer of the ceramic composite of 2mm radial thickness on the inner surface of the first one. The profile of the shaped core was slightly formed at the end of the second cylindrical layer where it was in contact with the shaped core. The third cylindrical layer of the ceramic composite of 3mm radial thickness was formed on the inner surface of the second layer by repeating the above said procedure with 30g of thermit mixture inside the hollow substrate containing the two layers. The edges of the cylindrical hole were deburred by grinding. Thus, a dense alumina based ceramic composite body, containing an axisymmetric hole of varying geometry, was formed inside the pipe. The hole had truncated conical geometry, having big end diameter of 37mm and apex angle of 60°, at one end. The remaining hole, connected to the smallend of the conical geometry, was cylindrical of 30mm diameter. Example - 4 Ferric oxide and aluminum powders, having particles of size - 325 mesh and -170+325 mesh respectively, were dried in an electric oven at 120° C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 100g of ferric oxide and 33.8g of aluminum in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. A mild steel pipe of 21mm length, 43mm inside diameter and 3mm wall thickness was used as a hollow substrate. It was required to form a convergent-divergent nozzle from alumina based ceramic composite inside this pipe using the prepared thermit mixture. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30° and big end diameters of 37mm and 35.4mm respectively. The cylindrical throat was 30mm in diameter and 2mm in width. Two tapered cores of graphite, one having dimensions of the convergent section and another having dimensions of half the length of the divergent section consisting of bigend, were prepared by machining. Another two tapered cores of graphite, having dimensions of the convergent and divergent sections and provided with integral conical feeders at their smallends, were similarly prepared. Each core was provided with an integral flange of 49mm diameter and 4mm thickness at its big end. It was machined such that the surface facing the thermit mixture had thickness of 1.5mm and an axial hole of 10mm was formed at the small end of the core or its conical feeder. A graphite housing, having inner diameter of 49mm, wall thickness of 8mm and length of 24mm, was prepared by machining. It was provided with an integral circular flat plate of 10mm thickness at one end. The plate was provided with a central hole of 30mm diameter and 6 numbers of radial vent holes of 5mm diameter, open to the central hole, at mid thickness of the plate. The core, corresponding to the convergent section, was held in position inside the pipe such that its flange was flush with one end of the pipe. The pipe was filled with 20g of the prepared thermit mixture upto the other end of the pipe such that an axial hole was formed in the filled thermit mixture. A 4mm thick circular graphite flange, having 49mm diameter and a central hole of 10mm diameter, was provided to the other end of the pipe. The pipe, having thermit mixture, core and flange, was inserted in the prepared graphite housing such that the flange of the core was placed between the end of the pipe and the endplate of the housing. The housing, along with its contents, was fitted firmly to a rotary fixture, comprising of flanges, by steel fasteners outside the housing. The rotary fixture was mounted on the shaft provided inside an enclosure of a centrifuge. The enclosure was evacuated continuously for three hours to maintain a vacuum in the enclosure at about 0.01mm of Hg. the vacuum pump was isolated and the enclosure was back filled with argon gas at 1 atmosphere. The rotary fixture was rotated about the axis of the pipe at 2800rpm. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the hermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, as shown in Table. 1, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1.5mm radial thickness, comprising of stratified layers of dense iron and alumina based ceramic, was formed on the inner surface of the pipe. In the next step, the tapered core of graphite, having dimensions of half the length of the divergent section and containing bigend, was used in place of the flange at the other end of the pipe. Before fixing this core, the pipe, containing the first layer, was filled with the prepared thermit mixture upto the other end of the pipe such that an axial hole was formed in the filled mixture. The mixture at the other end was scraped using a template to accommodate the above core of the divergent section. After scraping, the weight of the thermit mixture inside the pipe was 14g. The above core of the divergent section was then fitted to the pipe. The pipe, having thermit mixture and cores, was inserted in the prepared graphite housing and rotated about its axis at 3400rpm, using the rotary fixture, inside the enclosure of the centrifuge. The thermit reaction was carried out under argon atmosphere, in the same manner as done previously, to form the second cylindrical layer of ceramic composite on the inner surface of the first layer. The second layer, thus formed, had radial thickness of 2mm and the profiles of convergent and divergent sections had slightly formed at the ends of the second layer where it was in contact with the shaped cores. The existing cores were replaced by the cores of convergent and divergent sections having conical feeders. The third cylindrical layer of the ceramic composite of 3.5mm radial thickness was formed on the inner surface of the second layer by repeating the above said procedure with 20g of thermit mixture and by rotating the pipe about its axis at 4500rpm. The edges of the cylindrical throat were deburred and the throat diameter of 30mm was finished by grinding. Thus a convergent-divergent nozzle from an alumina based ceramic composite was formed inside the pipe. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30° and bigend diameters of 37mm and 35.4mm, respectively. The cylindrical throat had 30mm diameter and 2mm width. Example - 5 Ferric oxide and aluminum powders, having particles of size -325 mesh and -170+325 mesh respectively, were dried in an electric oven at 120° C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 100g of ferric oxide and 33.8g of aluminum in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. A mild steel pipe of 21mm length, 43mm inside diameter and 3mm wall thickness was used as a hollow substrate. It was required to form a convergent-divergent nozzle from alumina based ceramic composite inside this pipe using the prepared thermit mixture. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30° and bigend diameters of 37mm and 35.4mm respectively. The cylindrical throat was 30mm in diameter and 2mm in width. Two tapered cores of graphite, one having dimensions of the convergent section and another having dimensions of half the length of the divergent section consisting of bigend, were prepared by machining. Another tapered core of graphite, having dimensions of divergent section and provided with an integral conical feeder, was similarly prepared. Each core was provided with an integral flange of 49mm diameter and 4mm thickness at its big end. It was machined such that the surface facing the thermit mixture had thickness of 1.5mm and an axial hole of 10mm was formed at the small end of the core or its conical feeder. A stainless steel housing, having inner diameter of 49mm, wall thickness of 8mm and length of 24mm, was prepared by machining. It was provided with an integral circular flat plate of 10mm thickness at one end. The plate was provided with a central hole of 30mm diameter and 6 numbers of radial vent holes of 5mm diameter, open to the central hole, at midthickness of the plate. The core, corresponding to the convergent section, was held in position inside the pipe such that its flange was flush with one end of the pipe. The pipe was filled with 20g of the prepared thermit mixture upto the other end of the pipe such that an axial hole was formed in the filled thermit mixture. A 4mm thick circular graphite flange, having 49mm diameter and a central hole of 10mm diameter, was provided to the other end of the pipe. The pipe, having thermit mixture, core and flange, was inserted in the matching stainless steel housing such that the flange of the core was placed between the end of the pipe and the end plate of the housing. The housing, along with its contents, was fitted firmly to a rotary fixture, comprising of flanges, by steel fasteners outside the housing. The rotary fixture was mounted on the shaft of an electric motor and rotated about the axis of the pipe at 2800rpm. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, as shown in Table. 1, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1.5mm radial thickness, comprising of stratified layers of iron and alumina based ceramic, was formed on the inner surface of the pipe. In the next step, the tapered core of graphite, having dimensions of half the length of the divergent section and containing bigend, was used in place of the flange at the other end of the pipe. Before fixing this core, the pipe, containing the first layer, was filled with the prepared thermit mixture upto the other end of the pipe such that an axial hole was formed in the filled mixture. The mixture at the other end was scraped using a template to accommodate the above core of the divergent section. After scraping, the weight of the thermit mixture inside the pipe was 14g. The above core of the divergent section was then fitted to the pipe. The pipe, having thermit mixture and cores, was inserted in the matching stainless steel housing and rotated about its axis at 3400rpm using the rotary fixture. The thermit reaction was carried out, in the same manner as done previously, to form the second cylindrical layer of ceramic composite on the inner surface of the first layer. The second layer, thus formed, was of 2mm radial thickness and the profiles of convergent and divergent sections had slightly formed at the ends of the second layer where it was in contact with the shaped cores. The existing divergent core was replaced by the core of divergent section having conical feeder. The third cylindrical layer of the ceramic composite of 3.5mm radial thickness was formed on the inner surface of the second layer by repeating the above said procedure with 20g of thermit mixture, inside the pipe and the conical feeder, and by rotating the pipe about its axis at 4500rpm. The edges of the cylindrical throat were debarred and the throat diameter of 30mm was finished by grinding. Thus, a convergent-divergent nozzle from an alumina based ceramic composite was formed inside the pipe. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the smalt ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30° and bigend diameters of 36mm and 35.4mm respectively. The cylindrical throat had 30mm diameter and 2mm width. Example - 6 Ferric oxide and aluminium powders, having particles of size - 325 mesh, were dried in an electric oven at 120° C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 300g of ferric oxide and 101.4g of aluminium in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. A stainless steel pipe of 66mm length, 57mm-inside diameter and 3mm-wall thickness was used as a hollow substrate. It was required to form a convergent-divergent nozzle from an alumina based ceramic composite inside this pipe using the prepared thermit mixture. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30°, respectively, and bigend diameters of 50mm. The cylindrical throat had 30.6mm diameter. Four numbers of 60mm wide and 1mm thick mild steel strips were wrapped, one above the other, over the outside surface of the pipe to serve as a support and heat sink to prevent the pipe from bulging, due to the heat and centrifugal force it experiences, during the formation of the nozzle. The strips were held against the pipe by external hose clips. A tapered core, having dimensions of the convergent section, and three tapered cores, having dimensions of the divergent section and smallend diameters of 40, 35, and 30mm, were prepared from graphite by machining. Each core was provided with an integral flange of 5mm thickness and 63mm diameter at its bigend and an axial hole of 10mm diameter at its smallend. They were machined such that their surfaces, facing the thermit mixture, were 2mm thick. The core, corresponding to the convergent section, was held in position inside the pipe such that its flange was flush with one end of the pipe. The pipe was filled with 43g of the prepared thermit mixture upto the level of the other end of the pipe such that an axial hole was formed in the filled thermit mixture. A circular flange of graphite, having 5mm thickness, 63mm outside diameter and an axial hole of 12mm diameter, was provided to the other end of the pipe. The pipe, having thermit mixture, core and flange, was fitted firmly to a rotary fixture, comprising of flanges, by steel fastners outside the pipe. The rotary fixture was mounted on the shaft of an electric motor and rotated about the axis of the pipe at 2800rpm. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, as shown in Table. 1, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1mm radial thickness, comprising of stratified layers of iron and alumina based ceramic, was formed on the inner surface of the pipe. The above said procedure was repeated with 59g of thermit mixture inside the hollow substrate containing the first layer of the ceramic composite. The second cylindrical layer of the ceramic composite of 1.4mm radial thickness was thus formed on the inward surface of the first layer. The substrate and its. associated components were separated from the rotary fixture and the graphite flange was removed. The hollow substrate was again filled with the thermit mixture upto the other end of the pipe such that an axial hole is formed in the mixture. The mixture at this end was scraped with a metal template to form a truncated conical hole so as to accommodate the core of the divergent section having smallend diameter of 40mm. After scraping, the quantity of the thermit mixture held inside the substrate was 38g. The tapered core of the divergent section, having smallend diameter of 40mm, was introduced at this end such that its flange was flush with the end of the pipe. The pipe was rotated about its axis at 2800rpm with the help of a rotary fixture and the thermit reaction was carried out, in the same manner as done previously, to form the third cylindrical layer of the ceramic composite of 1.6mm radial thickness on the inward surface of the second layer. The profiles of the shaped cores were slightly formed at the ends of the third layer where it was in contact with the shaped cores. The subsequent cylindrical layers of the ceramic composite, having 1.2, 0.9, 0.8 and 0.7 radial thickness, were formed on the inward surfaces of the corresponding previous layers, in the same manner as done previously, using 36, 33, 28 and 26g of the thermit mixture respectively. The innermost cylindrical layer had an internal diameter of about 41.8mm. The existing core of the divergent section was replaced by the similar core having smallend diameter of 35mm. The subsequent cylindrical layers of the ceramic composite, having 0.7, 0.9, and 1.0mm radial thickness, were formed on the inward surfaces of the corresponding previous layers, in the same manner as done previously, using 22, 16, and 14g of the thermit mixture respectively. The innermost cylindrical layer had an internal diameter of about 36.6mm. The existing core of the divergent section was replaced by the similar core having smallend diameter of 30mm. The subsequent cylindrical layers of the ceramic composite, each having 1.0mm radial thickness, were formed on the inward surfaces of the corresponding previous layers, in the same manner as done previously, using 14, 14, and 12g of the thermit mixture respectively. The innermost cylindrical layer had an internal diameter of about 30.6mm. The edges of the cylindrical throat were deburred by grinding. Thus, a convergent-divergent nozzle was formed from alumina based ceramic composite inside the pipe. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The convergent and divergent sections had apex angles of 60° and 30°, respectively, and bigend diameters of 50mm. The cylindrical throat had 30.6mm diameter. Example - 7 Ferric oxide and aluminium powders, having particles of size -325 mesh, were dried in an electric oven at 120°C for 12 hours. A thermit mixture was prepared from these powders by intimately mixing 350g of ferric oxide and 118.3g of aluminium in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric. The mixture was stored in a dessicator prior to use. An axisymmetric truncated conical pipe of stainless steel was used as the hollow substrate. It had uniform wall thickness of 4mm and an axial length of 66mm. The tapered hole in the substrate had 57mm diameter at the big end and an apex angle of 15°. It was required to form a convergent-divergent nozzle from an alumina based ceramic composite inside this pipe using the prepared thermit mixture. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry and the convergent section was located at the bigend of the pipe. The convergent and divergent sections had apex angles of 60° and 30° and bigend diameters of 51mm and 36mm, respectively. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The cylindrical throat had 22mm diameter. Three tapered cores, having smallend diameters of 30, 25, and 20mm and other dimensions of the convergent section, and the other three tapered cores, having smallend diameters of 30, 25, and 20mm and other dimensions of the divergent section, were prepared from graphite by machining. Each core of the convergent section was provided with an integral flange of 5mm thickness and 63mm diameter at its bigend. Similarly, each core of the divergent section was provided with an integral flange of 5mm thickness and 53mm diameter at its bigend. The cores, having smallend diameters of 20mm, were provided with integral conical feeders. Also, each core was machined such that its surface, facing the thermit mixture, was 1.5mm thick and an axial hole of 10mm diameter was formed at the smallend of the core or the conical feeder. The core of the convergent section, having smallend diameter of 30mm, was held in position inside the pipe such that its flange was flush with the bigend of the pipe. The pipe was filled with 20g of the prepared thermit mixture upto the smallend of the pipe such that an axial hole was formed in the filled thermit mixture. A circular flange of graphite, having 5mm thickness, 53mm outside diameter and an axial hole of 12mm diameter, was provided to the smallend of the pipe. The pipe, having thermit mixture, core and flange, was fitted firmly to a rotary fixture, comprising of flanges, by steel fastners outside the pipe. The rotary fixture was mounted on the shaft of an electric motor and rotated about the axis of the pipe at 2800rpm. While the pipe was rotating, the thermit reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit mixture. The reaction then propagated rapidly through the remaining thermit mixture in the pipe and was completed quickly. The molten products of the thermit reaction got separated, due to their differing densities, and collected at the bigend of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A wedge shaped first ceramic composite layer, comprising of stratified layers of iron and alumina based ceramic, was formed on the inner surface of the pipe. A segment of the convergent section was formed at the end of the first layer where the layer was in contact with the core. The inward surface of the inner ceramic layer was cylindrical. The above said procedure was repeated with 30g of thermit mixture inside the hollow substrate, containing the first layer of the ceramic composite. The second cylindrical layer of the ceramic composite was formed on the inward surface of the first layer. Another segment of the convergent section was formed at the end of the second layer, where the layer was in contact with the core. This segment continued the formation of the convergent section of the nozzle. The next four cylindrical layers of the ceramic composite were formed on the inward surfaces of the corresponding previous layers, in the same manner as done previously, using 30g of the thermit mixture to form each layer. The segments, formed by these layers, continued the formation of the convergent section of the nozzle. The innermost cylindrical layer had an internal diameter of about 39mm. The substrate, along with its associated components, was separated from the rotary fixture and the graphite flange was removed. The hollow substrate, containing the layers of the ceramic composite, was again filled with the thermit mixture upto the smallend of the pipe such that an axial hole is formed in the mixture. The mixture at this end was scraped with a metal template to form a truncated conical hole so as to accommodate the core of the divergent section having smallend diameter of 30mm. After scraping, the weight of the thermit mixture was 26g. The tapered core of the divergent section, having smallend diameter of 30mm, was then introduced at this end such that its flange was flush with the smallend of the pipe. The pipe was rotated about its axis at 3430rpm with the help of a rotary fixture and the thermit reaction was carried out, in the same manner as done previously, to form a cylindrical layer of the ceramic composite on the inward surface of the layer having inner diameter of 39mm. A segment of the divergent section was slightly formed at the other end of the this layer, where it was in contact with the shaped core of the divergent section, and the formation of the convergent section was further continued. The next four layers of the ceramic composite were formed using 20, 18, 16, and 15g of the thermit mixture. Each layer was formed on the inner surface of the preceding layer at 3430rpm, in the same manner as done previously, till the inner diameter of the last layer was about 30mm. Thus, a few more segments of the convergent and divergent sections were formed. The existing cores of the convergent and divergent sections were replaced by similar cores having smallend diameters of 25mm. The subsequent three layers of the ceramic composite were formed using 12, 11, and 8g of the thermit mixture. Each layer was formed on the inner surface of the preceding layer at 3430rpm, in the same manner as done previously, till the inner diameter of the last layer was about 25mm. Thus, a few more segments of the convergent and divergent sections were formed. The existing cores of the convergent and divergent sections were replaced by similar cores having smallend diameters of 20mm and integral conical feeders. The subsequent two layers of the ceramic composite were formed using 13 and 10g of the thermit mixture. Each layer is formed on the inner surface of the preceding layer at 3890rpm, in the same manner as done previously, till the inner diameter of the last layer was 22mm and the convergent and divergent sections were fully formed. The pipe, containing the previous layers, and the conical feeders were filled with the thermit mixture prior to the formation of each layer. The edges of the cylindrical throat were deburred by grinding. Thus, a convergent-divergent nozzle was formed from alumina based ceramic composite inside the conical pipe. The convergent and divergent sections, at the ends of the axisymmetric hole of the nozzle, had truncated conical geometry and the convergent section was located at the bigend of the pipe. The convergent and divergent sections had apex angles of 60° and 30° and bigend diameters of 51mm and 36mm, respectively. A cylindrical hole, connecting the small ends of the convergent and divergent sections, formed the throat of the nozzle. The cylindrical throat had 22mm diameter. Example - 8 Ferric oxide and aluminium powders, having particles of size -325 mesh, were selected to prepare a thermit mixture. Aluminum oxide powder, having particle size of 1 micron, was selected as an inert diluent. As aluminum oxide is one of the products of the thermit reaction, it is assumed that it does not take part in the reaction. It only reduces the adiabatic temperature of the thermit reaction by absorbing the heat released by the reaction. The selected powders were dried in an electric oven at 120° C for 12 hours. A thermit based mixture was prepared from these powders by intimately mixing 100g of ferric oxide, 33.8g of aluminium and 14g of aluminium oxide in a sealed double cone blender for 12 hours. The mixing ratio was nearly stoichiometric and the estimated adiabatic temperature of the thermit based reaction is 3000K. The mixture was stored in a dessicator prior to use. A mild steel pipe of 21mm length, 43mm-inside diameter and 6mm wall thickness was used as a hollow substrate. It was required to form an alumina based ceramic composite body, containing an axisymmetric hole of varying geometry, inside this pipe using the prepared thermit mixture. The hole had truncated conical geometry, having big end diameter of 37mm and apex angle of 60°, at one end. The remaining hole, connected to the smallend of the conical geometry, was cylindrical of 30mm diameter. A tapered core of graphite, having dimensions of the truncated conical geometry, was prepared by machining. It was provided with an integral flange of 49mm diameter and 4mm thickness at its bigend and an axial hole of 6mm diameter. This core was held in position inside the pipe such that its flange is flush with one end of the pipe. The pipe was filled with 19g of the prepared thermit based mixture upto the other end of the pipe such that an axial hole of 6mm diameter was formed in the filled mixture. A circular graphite flange, having 4mm thickness, 49mm diameter and a central hole of 10mm diameter, was provided to the other end of the pipe. The pipe, having thermit based mixture, core and flange, was fitted firmly to a rotary fixture, comprising of flanges, by steel fasteners outside the pipe. The rotary fixture was mounted on the shaft of an electric motor and rotated about the axis of the pipe at 3400rpm. While the pipe was rotating, the thermit based reaction was initiated by bringing an electric arc, struck between two mild steel electrodes, in contact with the inward surface of the thermit based mixture. The reaction then propagated rapidly through the remaining mixture in the pipe and was completed quickly. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, the products were produced in molten state. The molten products got separated, due to their differing densities, and collected uniformly over the inward surface of the pipe by the effect of centrifugal force. The rotation of the pipe was continued for about 15 minutes to allow the products to solidify and cool. A cylindrical ceramic composite layer of 1.5mm radial thickness, comprising of stratified layers of iron and aluminum oxide, was formed on the inner surface of the pipe. The above said procedure was repeated with 19g of thermit based mixture inside the hollow substrate, containing the first layer of the ceramic composite, to form the second cylindrical layer of the ceramic composite of 2mm radial thickness on the inner surface of the first one. The profile of the shaped core was slightly formed at the end of the second layer where it was in contact with the shaped core. The third cylindrical layer of the ceramic composite of 3mm radial thickness was formed on the inner surface of the second layer by repeating the procedure again with 28g of thermit based mixture inside the hollow substrate containing the two layers. Thus, an alumina based ceramic composite body, containing an axisymmetric hole of varying geometry, was formed inside the pipe. The hole had truncated conical geometry, having big end diameter of 37mm and apex angle of 60°, at one end. The remaining hole, connected to the smallend of the conical geometry, was cylindrical of 30mm diameter. The edges of the cylindrical throat were deburred by grinding. The present inventive method of forming a ceramic composite body, containing an axisymmetric hole of varying geometry, inside a hollow substrate has been demonstrated by these examples. The ceramic composite body was made up of cylindrical layers of the ceramic-metal composite. Each layer comprised of stratified layers of metal and ceramic and was produced from the products of a thermit reaction under the effect of centrifugal force. As the adiabatic temperature of the thermit reaction was higher than the melting points of the products, the products were produced in molten state. The magnitude of the centrifugal force was sufficient to cause separation of the products of the thermit reaction. The present method is useful for producing such ceramic composite bodies from other ceramic-metal and ceramic-ceramic layers, not covered in these examples, by using suitable strong exothermic reactions. The method is useful for forming a ceramic composite body containing an axisymmetric hole of any geometry other than cylindrical, such as, conical, parabolic and elliptical, inside a hollow substrate. However, it is most useful for forming a ceramic composite body, containing an axisymmetric hole of such geometry with a cylindrical hole at the small end, inside a hollow substrate. In general, the method is most useful for forming a ceramic composite body containing an axisymmetric hole of varying geometry, wherein the cylindrical hole connects the smallends of the holes of other geometry, inside a hollow substrate. The main advantages of the method of the present invention are: 1. The method requires no furnace to produce the required material in molten state for forming a ceramic composite body. 2. The power consumption is minimal. 3. The method is simple, easy and inexpensive as no sophisticated equipments are involved. 4. The method is useful for forming a ceramic composite body containing an axisymmetric hole of any geometry, such as, conical, parabolic and elliptical, inside a hollow substrate. 5. The method is most useful for forming a ceramic composite body containing a cylindrical hole at the small end of geometry, like conical+cylindrical, parabolic+cylindrical and elliptical+cylindrical holes, inside a hollow substrate. 6. As the cylindrical hole in the ceramic composite body is formed by the centrifugal force itself, no shaped core is required to form it. 7. The time required to form thick nozzles from a ceramic composite is short. We Claim: 1. A method of forming a ceramic composite body containing an axisymmetric hole of varying geometry such as nozzle inside a hollow substrate, which comprises: a) Characterised in that introducing pre-shaped refractory cores(3,4) having the geometric shapes such as nozzle having axisymmetric hole at the ends of a hollow substrate(2) such that the ends of the hollow substrate are sealed by the said shaped cores (3,4); b) filling the annular space between the hollow substrate (2)and the shaped cores (3,4) with a powdery thermit mixture such as herein described in a manner such that a hole, coaxial with the hollow substrate(2) is formed; c) subjecting the hollow substrate (2)along with its contents to centrifugal force by rotation on a rotary fixture capable of rotating the said hollow substrate along with its contents; d) initiating a thermit reaction by heating the powdery mixture at least at one point on its free surface, inside the hole of the powdery mixture; e) continuing the rotation of the hollow substrate along with its contents to allow the molten deposited ceramic composite layer to solidify and cool; f) stopping the rotation and removing the refractory cores to verify whether the desired geometry(1) such as a nozzle is formed inside the hollow substrate(2) g) repeating the steps (a) to (f), if the desired geometry(1) is not formed, to form the subsequent layers till the ceramic composite body of desired inner geometry(1) such as nozzle is formed inside the hollow substrate(2); and h) removing the refractory cores(3,4) to get the desired ceramic composite body(1) containing the required axisymmetric hole of varying geometry such as a nozzle, formed inside the hollow substrate(2). 2. A method as claimed in claim 1 wherein the hollow space of the substrate is of axisymmetric truncated conical geometry. 3. A method as claimed in claim 1-2, wherein the core is a hollow body of surface thickness in the range of 0.5 to 2 mm, capable of withstanding thermal shock of exothermic reaction heat and centrifugal force. 4. A method as claimed in claim 1-3, wherein the pre-shaped core material is inert and non-wetting with respect to the products of the reaction being carried out. 5. A method as claimed in claim 1-4, wherein the shaped core is such as of graphite material. 6. A method as claimed in claim 1-5, wherein the shaped cores employed have axial holes so as to allow insertion of an igniter to initiate the thermit based reaction and to allow the gases, evolved during the reaction, to escape to the ambient. 7. A method as claimed in claim 1-6, wherein the shaped cores are provided with conical feeders. 8. A method as claimed in claim 1-7, wherein the shaped core is extended in stages to form the desired axisymmetric hole of varying geometry inside the ceramic composite body. 9. A method as claimed in claim 1-8, wherein the shaped refractory cores are fitted to the ends of the hollow substrate such that the axial gap between the ends of the refractory shaped cores and the hollow substrate is sufficient to facilitate easy filling of thermit mixture. 10. A method as claimed in claim 1-9, wherein the ends of the refractory cores, preferably flanged, are flush with the ends of the hollow substrate. 11. A method as claimed in claim 1-10, wherein the annular space between the hollow substrate and the shaped cores is filled with thermit or thermit based reaction mixture such that a hole intended for ignition of thermit mixutre, preferably coaxial with the hollow substrate, is formed. 12. A method as claimed in claim 1-11, wherein the diameter of the coaxial hole intended for ignition of thermit mixture is at least 1mm. 13. A method as claimed in claim 1-12, wherein the thermit or thermit based reaction mixture used is of composition desired for the ceramic composite layer to be produced. 14. A method as claimed in claim 1-13, wherein the thermit mixture contains a strongly reductive element and strongly reducible metal oxide. 15. A method as claimed in claim 1-14, wherein the strongly reductive element is selected from aluminum, magnesium, zirconium or silicon. 16. A method as claimed in claim 1-15, wherein the strongly reducible metal oxide is selected from oxides of iron, nickel, copper, tungsten, titanium, molybdenum, vanadium, chromium, niobium, zinc or manganese. 17. A method as claimed in claim 1-16, wherein the thermit mixture contains aluminum and ferric oxide as strongly reductive element and strongly reducible metal oxide respectively. 18. A method as claimed in claim 1-17, wherein the aluminum and ferric oxide in the thermit mixture are in stoichiometric amounts. 19. A method as claimed in claim 1-18, wherein the aluminum and ferric oxide in the thermit mixture is of particle size of at least -170 mesh. 20. A method as claimed in claim 1-19, wherein the hollow substrate and the shaped refractory cores are fitted together firmly in a rotary fixture in such a way that the axes of the refractory cores and the hollow substrate coincide. 21. A method as claimed in claim 1-20, wherein the hollow substrate along with its contents is subjected to centrifugal force by rotation on the shaft of a rotary fixture, such as a prime mover shaft, preferably a variable speed electric motor. 22. A method as claimed in claim 1-21, wherein the rotation of the hollow substrate along with its contents is continued for a time period in the range of 5 to 30 minutes. 23. A method as claimed in claim 1-22, wherein the hollow substrate along with its contents is rotated about its axis at a speed to generate a centrifugal force in the range of 10 to 1000G. 24. A method as claimed in claim 1-23, the narrow cylindrical region of the axisymmetric hole of varying geometry is formed at a speed of rotation which generates a centrifugal force of 450 G. 25. A method as claimed in claim 1-24, wherein the reaction method is carried out in inert atmosphere by employing inert gases like argon at one atmospheric pressure. 26. A method as claimed in claim 1-25, wherein the method is repeated, if required, for inner geometry formation inside the hollow substrate till the ceramic composite body containing the required axisymmetric hole of varying geometry is formed inside the hollow substrate by the multi-layers of the solidified reaction products. 27. A method of forming a ceramic composite body containing an axisymmetric hole of varying geometry inside a hollow substrate, substantially as herein described with reference to the examples and drawing accompanying this specification. |
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1024-del-2004-complete specification (granted).pdf
1024-del-2004-correspondence-others.pdf
1024-del-2004-correspondence-po.pdf
1024-del-2004-decription (complete.pdf
| Patent Number | 242321 | ||||||||
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| Indian Patent Application Number | 1024/DEL/2004 | ||||||||
| PG Journal Number | 35/2010 | ||||||||
| Publication Date | 27-Aug-2010 | ||||||||
| Grant Date | 23-Aug-2010 | ||||||||
| Date of Filing | 04-Jun-2004 | ||||||||
| Name of Patentee | COUNCILOF SCIENTIFIC AND INDUSTRIAL RESEARCH | ||||||||
| Applicant Address | RAFI MARG NEW DELHI-110001, INDIA | ||||||||
Inventors:
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| PCT International Classification Number | C04B 38/00 | ||||||||
| PCT International Application Number | N/A | ||||||||
| PCT International Filing date | |||||||||
PCT Conventions:
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