Claims:1. A method of preparing a metal matrix composite, said method comprising the steps of:
introducing a pre-ceramic polymer within a metal plate at a groove on the metal plate;
dispersing the pre-ceramic polymer into a metal matrix of the metal plate by friction stir process; and
pyrolyzing the dispersed pre-ceramic polymer in the metal matrix to a ceramic phase to obtain a metal matrix composite,
wherein the pre-ceramic polymer is selected from poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) for metal plate of copper and aluminium respectively.
2. The method as claimed in claim 1, wherein the pyrolysis is an in-situ process achieved during friction stir process.
3. The method as claimed in claim 1, wherein pyrolysis is achieved by supplying heat externally.
4. The method as claimed in claim 1, wherein the metal plate along with the dispersed pre-ceramic polymer is heated at any of 800 °C for three hours and 500 °C for ten hours to pyrolyze the pre-ceramic polymer.
5. The method as claimed in claim 1, wherein the poly (urea methyl vinyl) silazane is cross-linked by heating it to 375K for five hours in an inert atmosphere, and wherein obtained cross-linked poly (urea methyl vinyl) silazane is powdered before being introduced into the copper metal plate.
6. The method as claimed in claim 1, wherein the poly (methyl hydro siloxane) is cross-linked by adding a predetermined quantity of 1,4-Diazabicyclo [2.2.2] octane (DABCO) to the poly (methyl hydro siloxane) at room temperature whereupon cross-linked poly (methyl hydro siloxane) is obtained after eight hours, and wherein the obtained cross-linked poly (methyl hydro siloxane) is powdered before being introduced into the aluminium metal plate.
7. The method as claimed in claim 1, wherein one or more additional passes of friction stir process is carried out after said pyrolysis for even distribution of said ceramic phase in said metal matrix.
8. The method as claimed in claim 1, wherein the metal matrix composite is annealed in a temperature range of 500 – 900 °C.
9. A metal matrix composite comprising:
a dispersed ceramic phase in a metal matrix,
wherein the dispersed ceramic phase is obtained by dispersing a pre-ceramic polymer into the metal matrix of a metal plate by friction stir process and pyrolyzing the pre-ceramic polymer to a ceramic phase to obtain the metal matrix composite, and
wherein the pre-ceramic polymer is selected from poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) for metal plate of copper and aluminium respectively.
10. The metal matrix composite as claimed in claim 9, wherein grain size of the metal matrix composite is in the range of 1 – 6µm.
, Description:TECHNICAL FIELD
[1] The present disclosure relates generally to the field of composites. In particular, the present disclosure relates to preparation of in-situ polymer derived metal matrix composites using friction stir processing.

BACKGROUND
[2] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[3] Metal matrix composites (MMCs) are made by dispersing a reinforcing material into a metal matrix. The matrix into which the reinforcement is embedded is a monolithic material and completely continuous, unlike other composites in which two materials are sandwiched together. The reinforcement, however, does not always serve a purely structural task of reinforcing the compound, but is also used to modulate mechanical properties such as tensile strength, ductility, hardness, wear resistance, friction coefficient, or thermal conductivity, to name a few.
[4] Manufacture of a metal matrix composite primarily involves dispersion of reinforcing material within the matrix of metal that is being reinforced in a desired pattern and can be done in either liquid phase or in solid state. Some processes do so in vapour phase or semi solid state also. Examples of liquid phase processing include,
• squeeze casting or squeeze infiltration where molten metal is injected into a mould/form with fibres pre-placed inside it;
• spray deposition in which molten metal is sprayed onto a continuous fibre substrate; electroplating and electroforming in which a solution containing metal ions loaded with reinforcing particles is co-deposited forming a composite material;
• stir casting in which discontinuous reinforcement is stirred into molten metal, which is allowed to solidify; and
• reactive processing in which a chemical reaction occurs, with one of the reactants forming the matrix and the other the reinforcement.
[5] Examples of traditional solid-state methods include,
• powder blending and consolidation (powder metallurgy), in which powdered metal and discontinuous reinforcement are mixed and then bonded through a process of compaction, degassing, and thermo-mechanical treatment; and
• foil diffusion bonding in which layers of metal foil are sandwiched with long fibres, and then pressed through to form a matrix.
[6] Further, methods such as semi-solid powder processing are employed, in which powder mixture is heated up to semi-solid state and pressure is applied to form the composites.
[7] All these traditional processes have a number of drawbacks such as uneven distribution of reinforcing material due to agglomeration, difficulty and risk in handling materials such as molten metals, time consuming as cycle of melting, mixing and solidification is long, presence of imperfections such as cracks and porosity etc. There is also disadvantage of high energy demand and corresponding carbon footprint and pollution.
[8] Friction Stir Processing (FSP) has been used to disperse reinforcing material in the matrix of a metal to improve its properties. Friction Stir Processing (FSP) is defined as a severe plastic deformation technique used to modify microstructural and mechanical properties. The process was first used for joining/welding purposes, as Friction Stir Welding (FSW), where a rotating tool with a protruding pin is plunged into the material to cause plastic flow of the material. Heat generated due to frictional forces during the process causes material in processing zone to turn into plastic state, helping flow and rearrangement of material.
[9] FSP has been used to produce micro and nano composite layers on surface/upper layer of metal alloys. Considering the promise that FSP holds, especially in view of its clean and eco-friendly nature, fast and easy processing and low energy requirement, it is worthwhile to explore possibility of using this process for preparing various metal matrix composites. The disadvantage of mixing hard ceramic particles in the solid state is that these particles act as abrasives and wear away the Friction Stir Welding tool. To avoid this, tools that are resistant to wear in the harsh environments that exist in the weld nugget are required to be used, resulting in substantial increase in cost of the process if nano particles are used to mix using FSP, the costs are high as nano particles are generally very expensive.
[10] Polymer-Derived ceramics (PDCs) are a new class of ceramics that combine the functional properties of polymers with the mechanical and chemical durability of ceramics. The processing of ceramic materials via polymer precursors involves the synthesis of pre-ceramic oligomers or polymers from monomer units by cross-linking and setting to form an unmeltable pre-ceramic network. Transformation of the cross-linked precursors into ceramic is achieved by heat treatment (pyrolysis).Recently, a method was proposed where a polymeric precursor was dispersed in a metal matrix by FSP and pyrolyzed to obtain a composite with five times the hardness of the base metal. During the dispersion of the polymer, the particles refine to the nanoscale. After the dispersion of the polymer into the matrix, the composite is heated to a temperature below its melting point to obtain ceramic phases. Further processing is done to disperse the particles, consolidate the material and to refine the grain structure.
[11] However, fine-grained structures are not very stable at high temperatures as the grains tend to minimize boundary energy by increasing in size. If a considerable variation in grain size exits in a sample, large grains grow at the expense of smaller grains resulting in an abnormal grain structure. Abnormal Grain Growth (AGG) is initiated in grains, which have a distinct advantage to grain growth over their neighbours. Further, the AGG is also influenced by the second phase particles, surface effect and texture. Although by FSP, fine-grained structure is obtained in the nugget region, they are prone to abnormal grain growth at high temperatures.
[12] The present invention endeavours to provide a method for preparing an in-situ polymer derived metal matrix composite using friction stir processing that has improved thermal stability of grain structure.
[13] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[14] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[15] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[16] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[17] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

OBJECTS OF THE INVENTION
[18] A general object of the present invention is to provide a method for preparing a metal matrix composite that uses friction stir processing.
[19] Another object of the present invention is to provide a method for preparing metal matrix composite using friction stir processing that has polymer derived ceramic as reinforcing material.
[20] An object of the present invention is to provide a method for preparing polymer derived ceramic metal matrix composite using friction stir processing that uses in situ conversion of polymer to ceramic phase.
[21] Another object of the present invention is to provide an in-situ polymer derived ceramic reinforced metal matrix composite of copper and aluminium using friction stir processing that has markedly enhanced thermal stability of grain structure.

SUMMARY
[22] The present disclosure relates generally to the field of composites. In particular, the present disclosure relates to preparation of in-situ polymer derived metal matrix composites using friction stir processing.
[23] In an aspect, the present disclosure provides a method of preparing a metal matrix composite, said method including the steps of: introducing a pre-ceramic polymer within a metal plate at a groove on the metal plate; dispersing the pre-ceramic polymer into a metal matrix of the metal plate by friction stir process; and pyrolyzing the dispersed pre-ceramic polymer in the metal matrix to a ceramic phase to obtain a metal matrix composite, wherein the pre-ceramic polymer is selected from poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) for metal plate of copper and aluminium respectively.
[24] In an embodiment, the pyrolysis can be an in-situ process achieved during friction stir process.
[25] In another embodiment, pyrolysis can be achieved by supplying heat externally.
[26] In another embodiment, the metal plate along with the dispersed pre-ceramic polymer is heated at any of 800°C for three hours and 500°C for ten hours to pyrolyze the pre-ceramic polymer.
[27] In another embodiment, the poly (urea methyl vinyl) silazane can be cross-linked by heating it to 375K for five hours in an inert atmosphere, and wherein obtained cross-linked poly (urea methyl vinyl) silazane can be powdered before being introduced into the copper metal plate.
[28] In another embodiment, the poly (methyl hydro siloxane) can cross-linked by adding a predetermined quantity of 1,4-Diazabicyclo [2.2.2] octane (DABCO) to the poly (methyl hydro siloxane) at room temperature whereupon cross-linked poly (methyl hydro siloxane) is obtained after eight hours, and wherein the obtained cross-linked poly (methyl hydro siloxane) can be powdered before being introduced into the aluminium metal plate.
[29] In another embodiment, one or more additional passes of friction stir process can be carried out after said pyrolysis for even distribution of said ceramic phase in said metal matrix.
[30] In another embodiment, the metal matrix composite is annealed in a temperature range of 500 – 900 °C depending on the melting temperature of the matrix material.
[31] In an aspect, the present disclosure provides a metal matrix composite, which includes: a dispersed ceramic phase in a metal matrix, wherein the dispersed ceramic phase is obtained by dispersing a pre-ceramic polymer into the metal matrix of a metal plate by friction stir process and pyrolyzing the pre-ceramic polymer to a ceramic phase to obtain the metal matrix composite, and wherein the pre-ceramic polymer is selected from poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) for metal plate of copper and aluminium respectively.
[32] In an embodiment, grain size of the metal matrix composite is in the range of 1 – 6µm.
[33] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[34] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[35] FIG. 1 illustrates an exemplary pictorial representation of friction stir process, as known in the art
[36] FIG. 2 illustrates an exemplary process flow chart for preparing polymer derived ceramic metal matrix composite, in accordance with an embodiment of the present disclosure.
[37] FIG. 3 illustrates an exemplary representation of the metal plate 114 for introduction of polymer in metal matrix, in accordance with an embodiment of the present disclosure.
[38] FIGs. 4A – 4F illustrate exemplary scanning electron microscope (SEM) images of copper PDC composite and after heat treatments at 600oC, 700oC, 800oC, 900oC and 950oC respectively, in accordance with an embodiment of the present disclosure.
[39] FIG. 5 illustrates an exemplary graph of variation of grain size of the copper PDC composite with heat treatment temperature, in accordance with an embodiment of the present disclosure.
[40] FIG. 6 illustrates an exemplary graph of variation of micro-hardness of the copper PDC composite with heat treatment temperature, in accordance with an embodiment of the present disclosure.
[41] FIGs. 7A and 7B illustrate exemplary inverse pole figures (IPF) of the aluminium PDC composite, as processed and after heat treatment at 500°C respectively, in accordance with an embodiment of the present disclosure.
[42] FIG. 8 illustrates an exemplary representation of distribution of grain size in the aluminium PDC composite before and after heat treatment at 500°C, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[43] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[44] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.
[45] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[46] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. These exemplary embodiments are provided only for illustrative purposes and so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. The invention disclosed may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Various modifications will be readily apparent to persons skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure). Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.
[47] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non – claimed element essential to the practice of the invention.
[48] In an aspect, the present disclosure provides a method of preparing a metal matrix composite, said method including the steps of: introducing a pre-ceramic polymer within a metal plate at a groove on the metal plate; dispersing the pre-ceramic polymer into a metal matrix of the metal plate by friction stir process; and pyrolyzing the dispersed pre-ceramic polymer in the metal matrix to a ceramic phase to obtain a metal matrix composite, wherein the pre-ceramic polymer is selected from poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) for metal plate of copper and aluminium respectively.
[49] In an embodiment, a method for preparing a metal matrix composite of copper and aluminium is provided, in which metal matrix is reinforced by nano sized ceramic particles that are obtained by phase change of a polymer within the matrix during friction stirring process. In an aspect, polymer is converted into particles of an amorphous ceramic phase within the metal matrix. In another aspect, temperature required for conversion of polymer to ceramic phase is reached during friction stir processing itself and or can be supplied externally.
[50] In another embodiment, a two-step process is adopted for preparation of the composite: After the first step of mixing the polymer into the metal using multi pass friction stir processing and pyrolysis in the metal to form a ceramic, the reinforcing ceramic particles are redistributed in the matrix by second step of friction stir processing. The second stage processing also helps in removing the porosity that is formed during the pyrolysis of the polymers.
[51] In another embodiment, the disclosed method does not involve a reaction with environment, and, therefore, differs from methods that rely on internal oxidation. In an aspect, all constituents of the ceramic phase can be contained within the organic polymer.
[52] In an aspect, the present disclosure provides a metal matrix composite, which includes: a dispersed ceramic phase in a metal matrix, wherein the dispersed ceramic phase is obtained by dispersing a pre-ceramic polymer into the metal matrix of a metal plate by friction stir process and pyrolyzing the pre-ceramic polymer to a ceramic phase to obtain the metal matrix composite, and wherein the pre-ceramic polymer is selected from poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) for metal plate of copper and aluminium respectively.
[53] FIG. 1 illustrates an exemplary pictorial representation 100 of friction stir process, as known in the art. Depicted therein is a non-consumable friction stir tool 102 that can be cylindrical and stepped comprising of pin 104 having lower diameter and shoulder 106 with larger diameter. The stirring tool 102 can be of suitable material that can withstand high temperature and is wear resistant.
[54] The stirring tool 102 can be configured on a machine (not shown) that can provide rotational motion 108, vertical motion 110 and lateral motion 112 relative to work piece 114. Lateral motion can be in two perpendicular directions to enable the tool 102 to cover any point on surface of the work piece 114. Tool 102 can additionally have an ability to tilt (not shown) in one or more planes so as to approach work piece 114 from different angles/along different axes. The machine can have adequate power to overcome resistive forces as tool 102 is plunged down 110 in work piece 114 or moved laterally 112.
[55] In application, rotating tool 102 can be lowered till bottom face of pin 104 touches top surface of work piece 114 and moved further down slowly to plunge pin 104 into the work piece 114, which can be a metal to be friction stirred. As the pin 104 plunges into the metal work piece 114, frictional force between metal work piece 114 and pin 104 shall generate heat, raising temperature of metal work piece 114 around pin 104. Combined effect of raised temperature and pressure from tool 102 makes material under bottom face of pin 104 to plastically flow and relocate above the top face of work piece 114 in form of a bulge. Relocation of material from space occupied by pin 104 to top surface can continue till bottom face of shoulder 106 comes in contact with bulged material, which in similar action, can remove bulged material. Thereafter, bottom face of shoulder 106 shall come in contact with top face of work piece 114. In an alternate embodiment, a hole of suitable diameter and depth can be pre-drilled at starting point to save time.
[56] After bottom face of shoulder 106 has come in contact with top face of work piece 114, downward plunge of tool 102 can stop, and it can now be moved laterally, for example, in direction 122, along which a set of grooves 120 (described in succeeding paragraphs) can be configured to place material that needs to be mixed with matrix of work piece 114. During lateral movement of tool 102, leading face of pin 104 can relocate material from front by plastic deformation, shifting it to space behind its trailing face. Bottom face of shoulder 106 that is now in contact with top face of work piece can prevent bulging out of relocated material leaving a level top surface 116 on work piece.
[57] FIG. 1 also depicts a typical cross section of friction stirred material where 118 is the deformed region, also known as nugget zone. Localized plastic deformation in nugget zone mixes the material without changing the phase that generally takes place during melting, and creates a microstructure with fine, equiaxed grains.
[58] In state of art, friction stir processing has also been used to distribute reinforcing material within a metal matrix, further improving its mechanical properties. In an embodiment, the present disclosure provides a method to prepare ceramic reinforced metal matrix composite of metals such as copper, and aluminium friction stirring process. In another embodiment, the work piece 114 is a metallic plate of material such as copper and aluminium, and the metallic plate can have dimensions of about 200mm length, 70mm width and thickness of 6mm. For dispersion of reinforcing material within the matrix of metal, a groove of appropriate size such as of 4mm depth, 160mm length and 3mm width can be made on top surface of the metallic plate to be converted to metal matrix composite.
[59] It should be appreciated that above described method of introducing reinforcing material during FSP is only exemplary. Any other method with variations to suit machine parameters, tool configuration, size/area of work piece and other such parameters can be adopted by a person skilled in art and all such methods are well within the scope of the present disclosure.
[60] In another embodiment, ceramic reinforcement of metal matrix is nano size particulate of polymer derived ceramic, and in an aspect, conversion of polymer to ceramic phase takes place within the metal matrix without any interaction with environment and all constituents of the ceramic phase are contained within the organic polymer. Powder organic polymermixed with suitable media such as ethanol, that is evaporated leaving the organic polymer, can be filled in the groove and holes, and dispersed in metal matrix by friction stir processing. The polymer powder is malleable and can disintegrate into nano size fragments during the friction stir processing. Nano size fragments of polymer get converted to ceramic phase at elevated temperature attained during friction stirring.
[61] To achieve thermal stability in grains the nanoscale particles at the grain boundaries should be present to increase the grain boundary drag. For this, a PDC is an ideal reinforcement as it can be refined to nano-scale and converted to ceramic phase in-situ. Hence polymers are selected so as to have a pyrolysis temperature above processing temperature during FSP and below the melting point of the metallic matrix. Such polymers are dispersed into the metallic matrix and then pyrolyzed at a certain temperature and duration. The pyrolyzed composites are further processed to obtain uniformly distribute matrix with a fine-grained structure.
[62] In another embodiment polymers used in the current process can be poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) used for synthesis of copper and aluminium composites respectively. The polymers are liquid at room temperature, which are cross-linked to form a solid polymer which can be dispersed into the matrix. The poly (urea methyl vinyl) silazane is thermally cross-linked at about 375K for five hours in an inert environment to obtain the solid polymer. The poly (methyl hydro siloxane) is cross-linked at room temperature using a catalyst such as 1,4-Diazabicyclo [2.2.2] octane (DABCO). Five percent by weight of DABCO is added to the poly (methyl hydro siloxane) and cross-linking is completed in about 8 hrs to yield a solid polymer. The solid polymers are ground into fine powder using any suitable milling process. Upon pyrolysis, poly (urea methyl vinyl) silazane yields SiCN and poly (methyl hydro siloxane) yields SiCO.
[63] FIG. 2 illustrates an exemplary process flow chart 200 for preparing polymer derived ceramic metal matrix composite (PDC-MMC), in accordance with an embodiment of the present disclosure. At step 202, polymer precursors that are liquid at ambient temperature can be cross-linked to convert them into solid polymers. In an aspect, polymers used in the process can be poly (urea methyl vinyl) silazane and poly (methyl hydro siloxane) used for synthesis of copper and aluminium composites respectively.
[64] At step 204, the hard organic can be milled into a powder that can be introduced in metal matrix. At step 206, the work piece 114 (hereinafter, also referred to as “metal plate”) can be prepared to provide appropriate means such as grooves, for introducing polymer powder in the metal matrix.
[65] FIG. 3 illustrates an exemplary representation of the metal plate 114 for introduction of polymer in metal matrix, in accordance with an embodiment of the present disclosure. A groove 302 of appropriate size such as of 4 mm depth, 160 mm length and 3mm width, can be made on top surface of the metal plate 114. It should be appreciated that above described configuration of holes and groves is only exemplary and any other method with variations to suit machine parameters, tool configuration, size/area of work piece and other such parameters can be adopted by a person skilled in art and all such methods are well within the scope of the present disclosure.
[66] Referring to FIG.2, at step 208, the groove can be filled with the organic polymer powder. The poly (urea methyl vinyl) silazane is introduced in the copper metal plate and the poly (methyl hydro siloxane) is introduced in the aluminium metal plate.
[67] At step 210, the metal plate can be subjected to friction stir process at optimized process parameters such as tool RPM of 1000-1500 rpm, traverse speed of 25-40 mm/min, tool tilt angle of 2o-3o, plunge rate of 1 mm/min and initial heating time of 10-20 sec such that organic particles are fragmented to nano size and dispersed within metal matrix. The metal plates can be subjected to multiple passes of friction stirring to ensure proper mixing and dispersal of reinforcing material in metal matrix. It should be appreciated that above process parameters for friction stirring are exemplary and can be changed to suit machine, tool, work piece and other such variables and all other possible combinations are well within the scope of the present disclosure.
[68] At step 212, the composite obtained after the friction stir process are pyrolyzed at about 800oC for three hours and at about 500oC for ten hours to convert the polymer to ceramic phase to get PDC-MMC of metal being processed. Pyrolysis can be performed by optimising the FSP parameters so as to obtain an operating temperature for pyrolysis, or the obtained composite can be externally heated to pyrolysis temperature.
[69] At step 214, one more FSP pass can be done to redistribute ceramic particles uniformly and reduce porosity.
[70] In an embodiment, the proposed method followed for preparing polymer derived ceramic reinforced metal matrix composite of copper and aluminium, and resultant composite exhibits higher thermal stability of grain structure at elevated temperatures.
[71] FIGs. 4A – 4F illustrate exemplary scanning electron microscope (SEM) images of copper PDC composite and after heat treatments at 600oC, 700oC, 800oC, 900oC and 950oC respectively, in accordance with an embodiment of the present disclosure. It can be observed that the composite has a fine-grained microstructure even after heat treatment of 900oC for about an hour. It can be appreciated that 900oC is about 0.9 times the melting point of copper.
[72] FIG. 5 illustrates an exemplary graph of variation of grain size of the copper PDC composite with heat treatment temperature, in accordance with an embodiment of the present disclosure. It can be seen that, with heat treatment, there is minimal increase in grain size from the initial grain size of about 5.25µm and, specifically, at a heat treatment temperature of 800oC, the grain size is about 5.8µm. Here, pinning of ceramic particles on the grain boundary doesn't allow grains to grow. At temperatures beyond 800oC, the grain size is comparably larger but is still in the order of a few microns.
[73] FIG. 6 illustrates an exemplary graph of variation of micro-hardness of the copper PDC composite with heat treatment temperature, in accordance with an embodiment of the present disclosure. It can be observed that the micro-hardness of the composite is slightly decreased after heat treatment in the temperature range of 600 – 800oC.
[74] Although microscopy studies do not reveal noticeable grain growth in the composite in this temperature range, the softening can be attributed to reduction of dislocation density due to recovery and relaxation of internal stresses from rearrangement and annihilation of dislocations. At higher temperatures, beyond 800°C, there is a further decrease in hardness of the copper PDC composite, which can be due to an increment in the grain size.
[75] FIGs. 7A and 7B illustrate exemplary inverse pole figures (IPF) of the aluminium PDC composite, as processed and after heat treatment at 500°C respectively, in accordance with an embodiment of the present disclosure.
[76] Typically, after friction stir process, the grain boundary area increases due to grain refinement, thus increasing the energy stored in the material. When such materials are subjected to high temperatures, due to significant diffusion they tend to reduce the stored energy by grain growth. With increasing grain growth, the mechanical properties considerably deteriorate. Hence, the thermal stability of the fine-grained materials is an important property if they are to be used at high temperatures.
[77] In FIG. 7A, it can be seen that the as processed aluminium PDC composite has a fine-grained structure with random grain orientation.
[78] In FIG. 7B, it can be seen that, after heat treatment of 500 °C for one hour, the aluminium PDC composite does not show abnormal grain growth or grain coarsening.
[79] FIG. 8 illustrates an exemplary representation of distribution of grain size in the aluminium PDC composite before and after heat treatment at 500°C, in accordance with an embodiment of the present disclosure. The grain size of the aluminium PDC composite before heat treatment (curve 802) is about 3.05 ±1.15 µm, and the grain size of the aluminium PDC composite after heat treatment at 500°C for one hour (curve 804) is about 3.63 ±1.44 µm. further, it can be observed that the grain size distribution before and after heat treatment is similar.
[80] Thus, the present disclosure provides a method to synthesise PDC MMC materials with grain structure that is stable at high temperatures, even as high as 0.9 times the melting point of the metal. The thermal stability can be attributed to the presence of nano-sized particles, which serve to pin grain boundaries.
[81] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive patient matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “includes” and “including” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C ….and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practised with modification within the spirit and scope of the appended claims.
[82] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE INVENTION
[83] The present disclosure provides a method for preparing metal matrix composites that uses friction stir process.
[84] The present disclosure provides a method for preparing polymer derived ceramic metal matrix composite using friction stir process that uses in situ conversion of polymer to ceramic phase.
[85] The present disclosure provides a method for preparing metal matrix composite using friction stir process that has polymer derived ceramic as reinforcing material.
[86] The present disclosure provides a ceramic reinforced metal matrix composite of copper and aluminium using friction stir processing that has markedly enhanced thermal stability of grain structure.