Title of Invention	AN IMPROVED PROCESS FOR THE PREPARATION OF BORON NITRIDE COATED CARBON FIBER
Abstract	In order to improve the oxidation resistance of carbon fiber, a boron nitride coating was developed by a non hazardous and cost effective dip coat technique. Dip coating was carried out in saturated boric acid solution followed by nitridation to produce BN coating. Before dip coating the carbon fibers were chemically activated. Due to interaction of activating chemical agents during the activation treatment, the surface properties e.g. surface area, surface microstructure of the carbon fibers were significantly varied. These improvements of surface properties subsequently reduce the born nitride coating formation temperature.

Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF BORON NITRIDE COATED CARBON FIBER

Abstract

In order to improve the oxidation resistance of carbon fiber, a boron nitride coating was developed by a non hazardous and cost effective dip coat technique. Dip coating was carried out in saturated boric acid solution followed by nitridation to produce BN coating. Before dip coating the carbon fibers were chemically activated. Due to interaction of activating chemical agents during the activation treatment, the surface properties e.g. surface area, surface microstructure of the carbon fibers were significantly varied. These improvements of surface properties subsequently reduce the born nitride coating formation temperature.

Full Text	Field of Invention The present invention relates to an improved process for the preparation of Boron nitride coated carbon fiber. Background of Invention and Description of Prior Art Carbon-carbon composites are important materials for aerospace applications because they retain high strength and toughness at high temperature. However, the use of carbon-carbon composite material is presently limited because of its susceptibility to oxidation. Oxidation protection may be provided by depositing a protective layer on the surface of the carbon-carbon composite which is resistant to oxidation. A preferred material for the aforesaid purpose is boron nitride. Boron nitride provides an intermediate phase with low shear strength, between the fiber and the matrix at which cracks propagating through the matrix are deflected. The crack deflection ability of the boron nitride coating, and its resistance to oxidation and reaction with various ceramics, provides a mechanism for toughening the composite and for preventing serious mechanical failures. Boron nitride coated carbon fiber carbon matrix composite is used in aerospace industry where numerous structures have required to retain high strength and toughness at high temperature, including rocket and space shuttle nose cones, re¬entry heat shields, rocket and jet engine nozzles, and leading edges of aircraft and spacecraft. Carbon fiber carbon matrix composite has widespread application in military, commercial air craft breaks which require BN coated carbon fiber for improvement of oxidation resistance of the composite system. Boron nitride is typically applied to fibers by chemical vapor deposition (CVD). Although CVD techniques have some disadvantages too. For instance, CVD process may require high vapor pressure of some hazardous and expensive chemical precursors, such as BCI3 and NH3, for boron nitride CVD coating and the process requires long application times. The synthesis of CVD BN was described by T. Matuda, N. Uno, H. Nakae, and T. Hirai, in J. Mater. Sci., 21 (1986), pp.649 -658. In another example Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride was described by M. Leparoux, L. Vandenbulcke, and C. Clinard, in J. Am. Ceram Soc, 82 (5) (1999), pp.1187-1195. To avoid the disadvantages experienced with the current CVD methods of boron nitride application, various alternatives to CVD applied boron nitride have been investigated. Most involve the use of "pre-ceramic" polymers containing boron and nitrogen and the steps of synthesizing these polymers, applying these polymers to substrates, evaporating the solvent and pyrolyzing the coatings to obtain boron nitride. Some of the polymers are based on borazine. Others use boron hydrides. These processes are cumbersome as they require a multiple step synthesis of the polymer and may leave a carbon residue from the polymer employed. In U.S Pat. No. 5,399,377 Economy et. al. described a process to form liquid borazine oligomers to create composites of fibers bonded to boron nitride as a matrix and also to coat or increase the density of various fiber composites. Draw backs of the hitherto known prior arts are summarized as follows: 1) Complex operational process like CVD 2) Use of extremely toxic chemicals 3) Non-uniform coating thickness and adherence 4) Processes are expensive and not complying environmental safety regulations. Boron nitride has unique chemical and physical properties, such as low density, high melting point, high thermal conductivity, superior chemical inertness and high electrical resistivity. Accordingly the material is widely used both as bulk materials and as in other forms such as thin films, fibers and coatings in electronic and ceramic composite applications. Boron Nitride has several kinds of polymorphs: hexagonal (h-BN), rhombohedral (r-BN), amorphous (a-BN), turbostratic (t-BN), zinc blend type cubic (c-BN), and wurtzite type hexagonal (w-BN). The c-BN and w-BN are hard and dense high pressure stable phases. In case where boron nitride is synthesized at relatively low temperature, for example at 900°C or below, a-BN is generated. It is known that if boron nitride synthesis is carried out at elevated temperature or if a-BN is heat treated at elevated temperature, h-BN is formed. Carbon fiber carbon matrix (C/C) composite has widespread application in military and commercial air craft brakes due to their unique combination of thermal, mechanical and wear properties. However, their inherent susceptibility to oxidation at 500°C limits their potential applications. Efforts have been made to improve the oxidation resistance of carbon fiber carbon matrix (C/C) composite with an interfacial coating of BN. BN has been proposed because of its graphite like structure and it's better oxidation resistance. Several researchers found that carbon fiber /boron nitride matrix (C/BN) or BN interfacial coating in fiber reinforced ceramic-matrix-composite (CMCs) have better oxidation resistance and superior mechanical properties. Higher oxidation resistance is due to the fact that at higher temperature, BN forms a protective B2O3 layer that does not readily volatilize below 1000°C. Another important application of thin BN coating is an interfacial layer for controlling the bonding in fiber-reinforced ceramic matrix composites (CMCs). The mechanical properties of CMCs are largely dependent on the fiber-matrix bonding, which must be weak enough to allow crack deflection along the interface, yet strong enough to retain load transfer from the matrix to the fibers. Boron nitride is typically applied to fibers by chemical vapor deposition (CVD). Although CVD techniques have some disadvantages too. For instance, CVD process may require high vapor pressure of some hazardous and expensive chemical precursors, such as BCI3 and NH3, for boron nitride CVD coating and the process requires long application times. To avoid the disadvantages experienced with the current CVD methods of boron nitride application, various alternatives to CVD applied boron nitride have been investigated. Most involve the use of "pre-ceramic" polymers containing boron and nitrogen and the steps of synthesizing these polymers, applying these polymers to substrates, evaporating the solvent and pyrolyzing the coatings to obtain boron nitride. Some of the polymers are based on borazine. Others use boron hydrides. These processes are cumbersome as they require a multiple step synthesis of the polymer and may leave a carbon residue from the polymer employed. In this invention, a dip-coating method for BN coating on carbon fiber was performed in nitrogen at 1200°C. In this study carbon fibers were chemically activated to improve its surface area, adsorption capacity and thus a high degree of surface reactivity. BN coating on carbon fiber were characterized and mechanical properties of the coated fibers were evaluated. The advantage of this method is that it is simple, using much cheaper and less toxic chemical precursor i.e. H3BO3 and N2/ NH3. Objects of the Invention The main object of the present invention is to provide a process of making Boron nitride coated carbon fiber which obvious the drawbacks of the hither to known prior arts as detailed above. Another objective of the present invention is to provide a process requiring lower nitridation temperature. Still another object of the present invention is to provide a process that eliminates the use of toxic chemical. Summary of the Invention In the present invention a dip coating method for BN coating on carbon fiber was performed in nitrogen at 1200°C. In this study carbon fiber were chemically activated to improve its surface area, absorption capacity and thus a high degree of surface reactivity. BN coating on carbon fiber were characterized and mechanical properties of the coated fibers were evaluated. The advantage of this method is that it is simple, using much cheaper and less toxic chemical precursor i.e. H3BO3 and Nitrogen. In the conventional method dip coating is applied just after sizing removal of the fiber. Though there is a structural compatibility between boron nitride and graphite, as both are hexagonal but the adequate bonding between BN formed during the reaction with carbon atom of the carbon fiber is essential for stronger adherence. This problem was not adequately addressed in prior art. The present inventers have found that if the carbon fiber after sizing is properly chemically activated prior to the application of BN coating not only the coating thickness remains uniforms but also the nitridation temperature is reduced by 200°C in addition to substantially improving oxidation resistance of the resultant composite. This, according to the inventors of the present invention, is due to formation of graded composition from carbon surface to BN surface form oxygen rich layer to nitrogen rich layer. The oxygen rich layer at the inner surface on carbon fiber may put a diffusion barrier for diffusion of further oxygen from the environment to inner carbon fiber. This phenomenon is responsible for strong adherence of coating layer over the substrate as well as imparting good oxidation resistance to the resultant coated fiber. The chemical activation of the carbon fiber induced partial oxidation of surface carbon which intern makes the fiber chemically more active as well as providing controlled oxygen required for interface bonding. The chemical activation is done by nitric acid in such a way that control oxidation was possible. Thus the novelty of the process is providing a BN coated carbon fiber material with better oxidation resistance and all other related properties with the inventive steps involving sizing removal the fibers followed by chemical activation by HN03. Accordingly the present invention provides an improved process for the preparation of Boron nitride coated carbon fiber, wherein the steps comprise: [a] sizing removal of carbon fiber at 700 to 800 degree C for 20 to 21 hrs in N2 atmosphere to obtain clean carbon fiber; [b] placing the clean carbon fiber as obtained in step [a] in concentrated HNO3 followed by heating at a temperature in the range of 80 to 90 degree C for a period of 1 to 2 hrs; [c] rinsing the heated carbon fiber as obtained in step [b] with distilled water to remove nitrates and drying the rinsed fiber at a temperature in the range of 100 to 115 degree C to obtain chemically treated dry carbon fiber; [d] treating the chemically treated dry carbon fiber as obtained in step [c] with saturated boric acid solution at a temperature of 25 to 30 degree C for 10 to 20 min followed by drying at a temperature in the range of 100 to 110 degree C to obtain dry boric acid coated carbon fiber; [e] heating the dry boric acid coated carbon fiber as obtained in step [d] at a temperature in the range of 450 to 500 degree C for a period of 5 to 10 min under N2 atmosphere to obtain a prehetreated boric acid coated carbon fiber followed by further heating at a temperature in the range of 1150 to 1200 degree C with a soaking period in the range of 2 to 3 hrs keeping N2 atmosphere in the system to obtain Boron nitride coated carbon fiber. Detailed Description of the Invention The process of the present invention recites the preparation of Boron nitride coated carbon fiber which comprises sizing removal of carbon fiber at 700°C for 20 hrs in N2 atmosphere to obtain clean carbon fiber, placing the clean carbon fiber in a beaker containing concentrated HNO3, heating on at a temperature in the range of 80 - 85°C for a period in the range of 1 - 2 hrs, rinsing the treated carbon fiber by distilled water to remove nitrates, drying the rinsed fiber at a temperature in the range of 100 - 110°C to obtain chemically treated dry carbon fiber, treating chemically treated dry carbon fiber with saturated boric acid solution at room temperature, drying boric acid treated fiber at a temperature in the range of 100 -110°C to obtain dry boric acid coated carbon fiber, heating dry boric acid coated carbon fiber at a temperature in the range of 450 - 500°C for a period range of 5 -10 mins under N2 atmosphere to obtain a prehetreated boric acid coated carbon fiber, prehetreated boric acid coated carbon fiber is further heat treated at a temperature in the range of 1150 - 1200°C with a soaking period in the range of 2 - 3 hrs keeping N2 atmosphere in the system to obtain BN coated carbon fiber. In an embodiment of the present invention rinsing of the chemically treated fiber may be continued till last traces of nitrates are removed In another embodiment of the present invention clean carbon fiber may be treated with saturated boric acid solution under vacuum in a vacuum cell. In still another embodiment of the present invention prehetreated boric acid coated carbon fiber may be retreated with boric acid solution repeatedly to increase the coating thickness by completing the cycles. The entire process is described in the following steps: Step 1: Sizing removal of carbon fiber at 700°C for 20 hrs in N2 atmosphere. Step 2: After sizing removal the fibers were placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80 - 85°C for 1- 2 hrs. After chemical treatment the fibers were rinsed in distilled water several times to remove all the nitrates. The rinsed fibers were dried at a temperature in the range of 100 - 110°C to obtain chemically treated dry carbon fiber. Step 3: A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The reduced pressure in the chamber induced the release of gases absorbed on the nanoporous carbon surface, so that better infiltration could be achieved. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 100 - 110°C. Step 4: The dry boric acid coated fibers were heated in the range of 450 - 500°C in N2 atmosphere for a range of 5 - 10 min and repeat the step 3 and followed by step 4 for three to five times to increase the coating thickness. Step 5: Nitridation was carried out in the range of 1150 - 1200°C in N2 atmosphere for 2 - 3 hrs. The following examples are given by way of illustration of the working of the invention in actual practice and should not be construed to limit the scope of the present invention in any way. Example 1 Carbon fibers were placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80°C for 1 hr. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes followed by drying in oven at 110°C. The dry boric acid coated fibers were fired at 1150°C in N2 atmosphere for 2 hrs. Example 2 Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber (0.1ml Hg pressure) was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 450°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for three times. These three time coated fibers were nitrided at 1150°C in N2 atmosphere for 3 hrs. Example 3 Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were nitrided at 1200°C in N2 atmosphere for 2 hrs. Example 4 Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HN03 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 500°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for two times. Nitridation was carried out at 1200°C in N2 atmosphere for 3 hrs. Example 5 Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HN03 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 500°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for three times. Nitridation was carried out at 1200°C in N2 atmosphere for 3 hrs. Example 6 Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 500°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for five times. Nitridation was carried out at 1200°C in N2 atmosphere for 3 hrs. The main advantages of the present invention are: 1) Very simple and cost effective process 2) Use of extremely non-toxic chemicals 3) Uniform coating thickness and adherence 4) Process requiring lower nitridation temperature 5) Better oxidation resistance We claim: 1. An improved process for the preparation of Boron nitride coated carbon fiber, wherein the steps comprise: [a] sizing removal of carbon fiber at 700 to 800 degree C for 20 to 21 hrs in N2 atmosphere to obtain clean carbon fiber; [b] placing the clean carbon fiber as obtained in step [a] in concentrated HN03 followed by heating at a temperature in the range of 80 to 90 degree C for a period of 1 to 2 hrs; [c] rinsing the heated carbon fiber as obtained in step [b] with distilled water to remove nitrates and drying the rinsed fiber at a temperature in the range of 100 to 115 degree C to obtain chemically treated dry carbon fiber; [d] treating the chemically treated dry carbon fiber as obtained in step [c] with saturated boric acid solution at a temperature of 25 to 30 degree C for 10 to 20 min followed by drying at a temperature in the range of 100 to 110 degree C to obtain dry boric acid coated carbon fiber; [e] heating the dry boric acid coated carbon fiber as obtained in step [d] at a temperature in the range of 450 to 500 degree C for a period of 5 to 10 min under N2 atmosphere to obtain a prehetreated boric acid coated carbon fiber followed by further heating at a temperature in the range of 1150 to 1200 degree C with a soaking period in the range of 2 to 3 hrs keeping N2 atmosphere in the system to obtain Boron nitride coated carbon fiber. 2. A process as claimed in claiml, wherein rinsing of the chemically treated fiber is continued till last traces of nitrates are removed. 3. A process as claimed in claiml, wherein clean carbon fiber is treated with saturated boric acid solution under vacuum in a vacuum cell. 4. A process as claimed in claiml, wherein prehetreated boric acid coated carbon fiber is retreated with boric acid solution repeatedly to increase the coating thickness by completing the cycles. 5. An improved process for the preparation of Boron nitride coated carbon fiber substantially as herein described with reference to the foregoing examples.

Full Text

Field of Invention
The present invention relates to an improved process for the preparation of Boron nitride coated carbon fiber.
Background of Invention and Description of Prior Art
Carbon-carbon composites are important materials for aerospace applications because they retain high strength and toughness at high temperature. However, the use of carbon-carbon composite material is presently limited because of its susceptibility to oxidation. Oxidation protection may be provided by depositing a protective layer on the surface of the carbon-carbon composite which is resistant to oxidation. A preferred material for the aforesaid purpose is boron nitride.
Boron nitride provides an intermediate phase with low shear strength, between the fiber and the matrix at which cracks propagating through the matrix are deflected. The crack deflection ability of the boron nitride coating, and its resistance to oxidation and reaction with various ceramics, provides a mechanism for toughening the composite and for preventing serious mechanical failures.
Boron nitride coated carbon fiber carbon matrix composite is used in aerospace industry where numerous structures have required to retain high strength and toughness at high temperature, including rocket and space shuttle nose cones, re¬entry heat shields, rocket and jet engine nozzles, and leading edges of aircraft and spacecraft.

Carbon fiber carbon matrix composite has widespread application in military, commercial air craft breaks which require BN coated carbon fiber for improvement of oxidation resistance of the composite system.
Boron nitride is typically applied to fibers by chemical vapor deposition (CVD). Although CVD techniques have some disadvantages too. For instance, CVD process may require high vapor pressure of some hazardous and expensive chemical precursors, such as BCI3 and NH3, for boron nitride CVD coating and the process requires long application times. The synthesis of CVD BN was described by T. Matuda, N. Uno, H. Nakae, and T. Hirai, in J. Mater. Sci., 21 (1986), pp.649 -658. In another example Influence of Isothermal Chemical Vapor Deposition and Chemical Vapor Infiltration Conditions on the Deposition Kinetics and Structure of Boron Nitride was described by M. Leparoux, L. Vandenbulcke, and C. Clinard, in J. Am. Ceram Soc, 82 (5) (1999), pp.1187-1195.
To avoid the disadvantages experienced with the current CVD methods of boron nitride application, various alternatives to CVD applied boron nitride have been investigated. Most involve the use of "pre-ceramic" polymers containing boron and nitrogen and the steps of synthesizing these polymers, applying these polymers to substrates, evaporating the solvent and pyrolyzing the coatings to obtain boron nitride. Some of the polymers are based on borazine. Others use boron hydrides. These processes are cumbersome as they require a multiple step synthesis of the polymer and may leave a carbon residue from the polymer employed.

In U.S Pat. No. 5,399,377 Economy et. al. described a process to form liquid borazine oligomers to create composites of fibers bonded to boron nitride as a matrix and also to coat or increase the density of various fiber composites.
Draw backs of the hitherto known prior arts are summarized as follows:
1) Complex operational process like CVD
2) Use of extremely toxic chemicals
3) Non-uniform coating thickness and adherence
4) Processes are expensive and not complying environmental safety regulations.
Boron nitride has unique chemical and physical properties, such as low density, high melting point, high thermal conductivity, superior chemical inertness and high electrical resistivity. Accordingly the material is widely used both as bulk materials and as in other forms such as thin films, fibers and coatings in electronic and ceramic composite applications.
Boron Nitride has several kinds of polymorphs: hexagonal (h-BN), rhombohedral (r-BN), amorphous (a-BN), turbostratic (t-BN), zinc blend type cubic (c-BN), and wurtzite type hexagonal (w-BN). The c-BN and w-BN are hard and dense high pressure stable phases. In case where boron nitride is synthesized at relatively low temperature, for example at 900°C or below, a-BN is generated. It is known that if boron nitride synthesis is carried out at elevated temperature or if a-BN is heat treated at elevated temperature, h-BN is formed.

Carbon fiber carbon matrix (C/C) composite has widespread application in military and commercial air craft brakes due to their unique combination of thermal, mechanical and wear properties. However, their inherent susceptibility to oxidation at 500°C limits their potential applications. Efforts have been made to improve the oxidation resistance of carbon fiber carbon matrix (C/C) composite with an interfacial coating of BN. BN has been proposed because of its graphite like structure and it's better oxidation resistance. Several researchers found that carbon fiber /boron nitride matrix (C/BN) or BN interfacial coating in fiber reinforced ceramic-matrix-composite (CMCs) have better oxidation resistance and superior mechanical properties. Higher oxidation resistance is due to the fact that at higher temperature, BN forms a protective B2O3 layer that does not readily volatilize below 1000°C. Another important application of thin BN coating is an interfacial layer for controlling the bonding in fiber-reinforced ceramic matrix composites (CMCs). The mechanical properties of CMCs are largely dependent on the fiber-matrix bonding, which must be weak enough to allow crack deflection along the interface, yet strong enough to retain load transfer from the matrix to the fibers.
Boron nitride is typically applied to fibers by chemical vapor deposition (CVD). Although CVD techniques have some disadvantages too. For instance, CVD process may require high vapor pressure of some hazardous and expensive chemical precursors, such as BCI3 and NH3, for boron nitride CVD coating and the process requires long application times.

To avoid the disadvantages experienced with the current CVD methods of boron nitride application, various alternatives to CVD applied boron nitride have been investigated. Most involve the use of "pre-ceramic" polymers containing boron and nitrogen and the steps of synthesizing these polymers, applying these polymers to substrates, evaporating the solvent and pyrolyzing the coatings to obtain boron nitride. Some of the polymers are based on borazine. Others use boron hydrides. These processes are cumbersome as they require a multiple step synthesis of the polymer and may leave a carbon residue from the polymer employed.
In this invention, a dip-coating method for BN coating on carbon fiber was performed in nitrogen at 1200°C. In this study carbon fibers were chemically activated to improve its surface area, adsorption capacity and thus a high degree of surface reactivity. BN coating on carbon fiber were characterized and mechanical properties of the coated fibers were evaluated. The advantage of this method is that it is simple, using much cheaper and less toxic chemical precursor i.e. H3BO3 and N2/ NH3.
Objects of the Invention
The main object of the present invention is to provide a process of making Boron nitride coated carbon fiber which obvious the drawbacks of the hither to known prior arts as detailed above.
Another objective of the present invention is to provide a process requiring lower nitridation temperature.

Still another object of the present invention is to provide a process that eliminates the use of toxic chemical.
Summary of the Invention
In the present invention a dip coating method for BN coating on carbon fiber was performed in nitrogen at 1200°C. In this study carbon fiber were chemically activated to improve its surface area, absorption capacity and thus a high degree of surface reactivity. BN coating on carbon fiber were characterized and mechanical properties of the coated fibers were evaluated. The advantage of this method is that it is simple, using much cheaper and less toxic chemical precursor i.e. H3BO3 and Nitrogen.
In the conventional method dip coating is applied just after sizing removal of the fiber. Though there is a structural compatibility between boron nitride and graphite, as both are hexagonal but the adequate bonding between BN formed during the reaction with carbon atom of the carbon fiber is essential for stronger adherence. This problem was not adequately addressed in prior art. The present inventers have found that if the carbon fiber after sizing is properly chemically activated prior to the application of BN coating not only the coating thickness remains uniforms but also the nitridation temperature is reduced by 200°C in addition to substantially improving oxidation resistance of the resultant composite. This, according to the inventors of the present invention, is due to formation of graded composition from carbon surface to BN surface form oxygen rich layer to nitrogen rich layer. The

oxygen rich layer at the inner surface on carbon fiber may put a diffusion barrier for diffusion of further oxygen from the environment to inner carbon fiber. This phenomenon is responsible for strong adherence of coating layer over the substrate as well as imparting good oxidation resistance to the resultant coated fiber. The chemical activation of the carbon fiber induced partial oxidation of surface carbon which intern makes the fiber chemically more active as well as providing controlled oxygen required for interface bonding. The chemical activation is done by nitric acid in such a way that control oxidation was possible.
Thus the novelty of the process is providing a BN coated carbon fiber material with better oxidation resistance and all other related properties with the inventive steps involving sizing removal the fibers followed by chemical activation by HN03.
Accordingly the present invention provides an improved process for the preparation of Boron nitride coated carbon fiber, wherein the steps comprise:
[a] sizing removal of carbon fiber at 700 to 800 degree C for 20 to 21 hrs in N2 atmosphere to obtain clean carbon fiber;
[b] placing the clean carbon fiber as obtained in step [a] in concentrated HNO3 followed by heating at a temperature in the range of 80 to 90 degree C for a period of 1 to 2 hrs;
[c] rinsing the heated carbon fiber as obtained in step [b] with distilled water to remove nitrates and drying the rinsed fiber at a temperature

in the range of 100 to 115 degree C to obtain chemically treated dry carbon fiber;
[d] treating the chemically treated dry carbon fiber as obtained in step [c] with saturated boric acid solution at a temperature of 25 to 30 degree C for 10 to 20 min followed by drying at a temperature in the range of 100 to 110 degree C to obtain dry boric acid coated carbon fiber;
[e] heating the dry boric acid coated carbon fiber as obtained in step [d] at a temperature in the range of 450 to 500 degree C for a period of 5 to 10 min under N2 atmosphere to obtain a prehetreated boric acid coated carbon fiber followed by further heating at a temperature in the range of 1150 to 1200 degree C with a soaking period in the range of 2 to 3 hrs keeping N2 atmosphere in the system to obtain Boron nitride coated carbon fiber.
Detailed Description of the Invention
The process of the present invention recites the preparation of Boron nitride coated carbon fiber which comprises sizing removal of carbon fiber at 700°C for 20 hrs in N2 atmosphere to obtain clean carbon fiber, placing the clean carbon fiber in a beaker containing concentrated HNO3, heating on at a temperature in the range of 80 - 85°C for a period in the range of 1 - 2 hrs, rinsing the treated carbon fiber by distilled water to remove nitrates, drying the rinsed fiber at a temperature in the range of 100 - 110°C to obtain chemically treated dry carbon fiber, treating chemically treated dry carbon fiber with saturated boric acid solution at room

temperature, drying boric acid treated fiber at a temperature in the range of 100 -110°C to obtain dry boric acid coated carbon fiber, heating dry boric acid coated carbon fiber at a temperature in the range of 450 - 500°C for a period range of 5 -10 mins under N2 atmosphere to obtain a prehetreated boric acid coated carbon fiber, prehetreated boric acid coated carbon fiber is further heat treated at a temperature in the range of 1150 - 1200°C with a soaking period in the range of 2 - 3 hrs keeping N2 atmosphere in the system to obtain BN coated carbon fiber.
In an embodiment of the present invention rinsing of the chemically treated fiber may be continued till last traces of nitrates are removed
In another embodiment of the present invention clean carbon fiber may be treated with saturated boric acid solution under vacuum in a vacuum cell.
In still another embodiment of the present invention prehetreated boric acid coated carbon fiber may be retreated with boric acid solution repeatedly to increase the coating thickness by completing the cycles.
The entire process is described in the following steps:
Step 1: Sizing removal of carbon fiber at 700°C for 20 hrs in N2 atmosphere.
Step 2: After sizing removal the fibers were placed in a beaker containing
concentrated HNO3 and were heated on water bath maintaining temperature of
reactant at 80 - 85°C for 1- 2 hrs. After chemical treatment the fibers were rinsed in

distilled water several times to remove all the nitrates. The rinsed fibers were dried
at a temperature in the range of 100 - 110°C to obtain chemically treated dry
carbon fiber.
Step 3: A saturated boric acid solution was prepared. A vacuum chamber was used
to infiltrate saturated H3BO3 solution into the carbon surface. The reduced pressure
in the chamber induced the release of gases absorbed on the nanoporous carbon
surface, so that better infiltration could be achieved. The fibers were kept in the
solution for 10 minutes and followed by drying in oven at 100 - 110°C.
Step 4: The dry boric acid coated fibers were heated in the range of 450 - 500°C in
N2 atmosphere for a range of 5 - 10 min and repeat the step 3 and followed by step
4 for three to five times to increase the coating thickness.
Step 5: Nitridation was carried out in the range of 1150 - 1200°C in N2 atmosphere
for 2 - 3 hrs.
The following examples are given by way of illustration of the working of the invention in actual practice and should not be construed to limit the scope of the present invention in any way.
Example 1 Carbon fibers were placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80°C for 1 hr. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution

was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes followed by drying in oven at 110°C. The dry boric acid coated fibers were fired at 1150°C in N2 atmosphere for 2 hrs.
Example 2
Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber (0.1ml Hg pressure) was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 450°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for three times. These three time coated fibers were nitrided at 1150°C in N2 atmosphere for 3 hrs.
Example 3
Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 80°C for 2

hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were nitrided at 1200°C in N2 atmosphere for 2 hrs.
Example 4
Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HN03 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 500°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for two times. Nitridation was carried out at 1200°C in N2 atmosphere for 3 hrs.
Example 5
Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HN03

and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 500°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for three times. Nitridation was carried out at 1200°C in N2 atmosphere for 3 hrs.
Example 6
Carbon fiber was placed in a furnace and heated at 700°C for 20 hrs in N2 atmosphere. The fibers were then placed in a beaker containing concentrated HNO3 and were heated on water bath maintaining temperature of reactant at 85°C for 2 hrs. After chemical treatment the fibers were rinsed in distilled water for five times. The rinsed fibers were dried at a temperature of 110°C. A saturated boric acid solution was prepared. A vacuum chamber was used to infiltrate saturated H3BO3 solution into the carbon surface. The fibers were kept in the solution for 10 minutes and followed by drying in oven at 110°C. The dry boric acid coated fibers were heated at 500°C in N2 atmosphere for 10 min. The above dipping in H3BO3 and subsequently firing at 450°C in N2 atmosphere for 10 min was carried out for five times. Nitridation was carried out at 1200°C in N2 atmosphere for 3 hrs.

The main advantages of the present invention are:
1) Very simple and cost effective process
2) Use of extremely non-toxic chemicals
3) Uniform coating thickness and adherence
4) Process requiring lower nitridation temperature
5) Better oxidation resistance

We claim:
1. An improved process for the preparation of Boron nitride coated carbon fiber, wherein the steps comprise:
[a] sizing removal of carbon fiber at 700 to 800 degree C for 20 to 21 hrs in N2 atmosphere to obtain clean carbon fiber;
[b] placing the clean carbon fiber as obtained in step [a] in concentrated HN03 followed by heating at a temperature in the range of 80 to 90 degree C for a period of 1 to 2 hrs;
[c] rinsing the heated carbon fiber as obtained in step [b] with distilled water to remove nitrates and drying the rinsed fiber at a temperature in the range of 100 to 115 degree C to obtain chemically treated dry carbon fiber;
[d] treating the chemically treated dry carbon fiber as obtained in step [c] with saturated boric acid solution at a temperature of 25 to 30 degree C for 10 to 20 min followed by drying at a temperature in the range of 100 to 110 degree C to obtain dry boric acid coated carbon fiber;
[e] heating the dry boric acid coated carbon fiber as obtained in step [d] at
a temperature in the range of 450 to 500 degree C for a period of 5 to
10 min under N2 atmosphere to obtain a prehetreated boric acid
coated carbon fiber followed by further heating at a temperature in the
range of 1150 to 1200 degree C with a soaking period in the range of
2 to 3 hrs keeping N2 atmosphere in the system to obtain Boron nitride
coated carbon fiber.

2. A process as claimed in claiml, wherein rinsing of the chemically treated fiber is continued till last traces of nitrates are removed.
3. A process as claimed in claiml, wherein clean carbon fiber is treated with saturated boric acid solution under vacuum in a vacuum cell.
4. A process as claimed in claiml, wherein prehetreated boric acid coated carbon fiber is retreated with boric acid solution repeatedly to increase the coating thickness by completing the cycles.
5. An improved process for the preparation of Boron nitride coated carbon fiber substantially as herein described with reference to the foregoing examples.

Documents:

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Patent Number

270978

Indian Patent Application Number

1238/DEL/2008

PG Journal Number

05/2016

Publication Date

29-Jan-2016

Grant Date

28-Jan-2016

Date of Filing

19-May-2008

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI-110001,INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	GHATAK SANKAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O.JADAVPUR UNIVERSITY, KOLKATA 700 032
2	BASU ASIS KUMAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O.JADAVPUR UNIVERSITY, KOLKATA 700 032
3	DAS MITUN	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O.JADAVPUR UNIVERSITY, KOLKATA 700 032

PCT International Classification Number

C23C

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA