Title of Invention

"A PROCESS OF MAKING ALUMINIUM OXIDE COATED ALUMINIUM"

Abstract

The present invention describes a process of making aluminium oxide coated aluminium. The novelty of this process lies in the fact that coating thickness can be easily tailored by minor adjustments of the processing parameters. In addition to this, the method is advantageous in terms of simplicity, cost-effectiveness and varied coating microstructure. Various potential applications of the present invention include aerospace, automotive, marine, electrical and construction industries wherein corrosion, oxidation, electrical or thermal resistance of the aluminium components may be improved by using aluminium oxide coating.

Full Text

The present invention relates to a process of making aluminium oxide coated aluminium. The present invention particularly relates to a process for providing aluminium oxide coating on aluminium and aluminium alloy objects and components. Aluminium and aluminium alloys are widely used as components in automotive, marine, aerospace, electrical, construction industries and other engineering fields because they are inexpensive and have high specific strength and low densities. 8ut aluminium is soft and chemically active. Most of its applications involve wear, corrosion, oxidation or thermal degradation. Aluminium components which are widely used in aerospace, automotive, marine, electrical and construction industries are subjected to corrosion, oxidation, electrical or thermal resistance and as such the characteristics of the aluminium components need to be improved. These improvements can be brought about by using aluminium oxide coating. Therefore, the process of the present invention for making aluminium oxide coated aluminium will find wide industrial utility. One simple way to protect the surface of the metal is to coat the metal with a suitable ceramic coating. Oxide coating specially aluminium oxide coating provides high oxidation, corrosion, wear and thermal resistance to metal. There are many existing techniques such as thermal spraying, sol-gel and vapor deposition, which are well practiced for forming ceramic coating on metal. But these are complicated and cost-ineffective. Anodizing is a well-known technology to develop protective oxide coating on aluminium and aluminium alloys. Reference may be made to United States Patents No. 6,258,463, Corridan; Michael Kevin, in Anodized cryogenically treated aluminium, wherein aluminium and aluminium alloy surfaces have been coated with oxide coatings by cryogenically treating the aluminium and thereafter anodizing. The drawback of this process is that maximum coating thickness is limited to 500 |jm. Also, cryogenic treatment makes this process cost-ineffective. Micro-arc can be used for the formation of oxide coating on aluminium. Reference may be made to United States Patent No. 6,197,178, Patel; Jerry L; Saka; Nannaji, in Method for forming ceramic coatings by micro-arc oxidation of reactive metals, wherein oxide coatings have been formed on aluminium and aluminium alloys by micro-arc oxidation. In this method, at least two aluminium metal bodies are suspended in an electrolytic bath. The metal bodies are connected to electrodes connected to a multiphase AC circuit. A multiphase power potential is imposed between the bodies. When bodies are moved into correct positions relative to each other, micro-arcs occur on the surfaces of the bodies. These electrical discharges cause oxidation of the bodies. Consequently, an oxide coating develops. The drawbacks are that it is complicated and requires precision control. Another reference may be made to United States Patent No. 4,938,993, Bennett; Michael J., in Protective coatings for alloys, wherein a protective surface coating of alumina has been formed on aluminium containing alloy. In this method, the alloy is coated with amorphous silica via plasma assisted vapor deposition and subsequently heat-treated in such conditions that a coherent, protective alumina coating can form. The drawbacks of the referred method are that it is complicated and costly. Moreover, this process provides a coating substantially of alumina. From the above referred prior art it is clear that there is a definite need for development of a process of making aluminium oxide coated aluminium. The main object of the present invention is to provide a process of making aluminium oxide coated aluminium, which obviates the drawbacks as detailed above. Another object of the present invention is to provide a simpler process of making aluminium oxide coated aluminium. Still another object of the present invention is to provide a cost-effective process for the development of aluminium oxide coating on aluminium. Yet another object of the present invention is to provide enhanced coating thickness enabling a thick aluminium oxide coating on aluminium for improved characteristics. A further object of the present invention is to provide a process of tailoring the thickness of aluminium oxide coating for specific applications. In the present invention there is provided a process of making aluminium oxide coated aluminium wherein a partially oxidized aluminium sample is microwaved, causing the oxide layer thickness to increase with the increase in microwaving time. In this way, a thick oxide coating can be developed on aluminium which has various potential applications such as in automotive, aerospace, marine, electrical and construction industries wherein corrosion, oxidation, electrical or thermal resistance of the aluminium components need to be improved. The novelty of the process of the present invention lies in the fact that aluminium oxide coating thickness can be easily tailored for specific applications by minor adjustments of the processing parameters within the specified ranges. In addition to this, the method is advantageous in terms of simplicity, cost-effectiveness and varied coating microstructure. An oxide coating can be developed on aluminium by known method such as heating in a conventional air furnace. Beyond a critical time of heating coating thickness becomes almost standstill. No f r heating can increase the coating thickness notably. So, thicker coating cannot be generated by normal heating method in spite of heating for many hours. But when a partially oxidized aluminium sample is microwaved, as in the process of the present invention, oxide layer increases with the increase in microwaving time. In this way, a thick oxide coating can be developed on aluminium, thereby providing improved characteristics and making the material suitable for potential applications such as in automotive, aerospace, marine, electrical and construction industries. Accordingly the present invention provides a process of making aluminium oxide coated aluminium which comprises polishing aluminium to be coated by known methods followed by cleaning and drying; subjecting the cleaned and dried aluminium to partial oxidation at a temperature in the range of 550°C to 615°C in air for a period in the range of 0.5 to 200 hours to obtain partially oxidized aluminium; exposing the partially oxidized aluminium in the presence of air to microwave energy of power in the range of 0.8 to 5 KW for a period in the range of 35 to 90 minutes to obtain aluminium oxide coated aluminium. In an embodiment of the present invention the aluminium to be coated is such as objects or components of various geometrical shapes such as solid cylinder, hollow cylinder, solid cube and thin block. In another embodiment of the present invention the cleaning and drying of the polished aluminium is carried out by thorough rinsing with acetone for a period in the range of 5 to 15 minutes to obtain a clean sample, followed by air drying at a temperature in the range of 125°C to 150°C for a period in the range of 15 to 30 minutes. In yet another embodiment of the present invention the partial oxidation is effected by subjecting to continuous heating or alternate heating and cooling at a temperature in the range of 550°C to 615°C in air for a period in the range of 0.5 to 200 hours. In still another embodiment of the present invention the thickness of the oxide coating obtained varies with initial oxide coating thickness prior to microwave treatment and subsequent microwave exposure time. In the process of the present invention the aluminium is partially oxidized in order to form an oxide coating on its surface which is necessary for the subsequent process step of exposure to microwaves. This is so because metals are known to reflect out microwave energy. The loss tangent is a measure of the absorption of microwaves by the material. Materials have extremely high loss tangents lie. metals reflect microwaves. Polar ceramic materials with intermediate loss tangents can absorb microwave energy. A polar ceramic is microwave heated by conversion of the energy absorbed from oscillating field into thermal energy of lattice. However, energy loss mechanisms are not yet clear and may be linked to ionic migration, ionic vibration and electronic polarization. For this reason, aluminium oxide coating absorbs microwave radiation and becomes volumetrically heated. This generated heat is conducted to the aluminium metal, which increases the surface temperature of aluminium locally. When surface temperature of aluminium reaches the oxidation temperature, oxidation occurs resulting in increment of oxide coating thickness. The microwave absorption varies linearly with the relative dielectric constant and the loss tangent of a dielectric material. Both these properties increase with an increase in temperature. The loss tangent is more affected by temperature in comparison to the relative dielectric constant. In the present case, aluminium oxide coating is not able to absorb microwave energy at ambient temperature because the loss factor is low. But as soon as it is heated above a critical temperature (Tcrjt.), it starts to absorb and couple more efficiently with microwave energy due to higher power dissipation. This leads to very rapid increase in local surface temperature of aluminium. Hence, rate of oxidation increases significantly which in turn increases the coating thickness. For this reason, oxidecoating thickness becomes significantly high in case of higher microwave exposure time. Through microwave treatment, it is possible to coat a partially oxidized aluminium sample with aluminium oxide coating to a desired thickness in a few minutes. In the hitherto known prior art, the drawbacks encountered are such as complicated process steps, cost-ineffectiveness, cumbersome operations and stringent requirement of precision control. In the present invention, these drawbacks have been obviated through the provision of a simpler and costeffective process for the development of aluminium oxide coated aluminium. Thus, the novelty of the process of the present invention resides in obtaining aluminium with a coherent and thicker oxide coating over its surface which has special applications in aerospace, automotive, marine, electrical and construction industries. Thickness of the oxide coating can be tailored by varying initial oxide coating thickness prior to microwave treatment and microwave exposure time. The inventive steps in the process of the present invention which enables the novelty resides in partial oxidation of the aluminium object or component followed by exposure to microwaves to obtain a coherent and thicker oxide coating over the partially oxidized aluminium. The process steps of the present invention are: 1. Aluminium objects or components of various geometrical shapes such as solid cylinder, hollow cylinder, solid cube and thin block are polished and rinsed thoroughly with acetone for a time period of 5 to 15 minutes. 2. Rinsed samples are dried in air at a temperature in the range of 125°C to 150°C for a time period of 15 to 30 minutes. 3. Dried samples are partially oxidized in the temperature range of 550°C to 615°C in air for 0.5 to 200 hours either continuously or by alternate heating and cooling. 4. Thereafter the samples are placed inside a microwave oven and microwaved in air with power in the range of 0.8 to 5 KW for a time period in the range of 35 to 90 minutes. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example -1 Commercially pure aluminium (99%) was used. Sample shape was hollow cylindrical (outer diameter- 18.4 mm, inner diameter- 16.7 mm and length- 15.5 mm). The sample was conventionally polished and rinsed with acetone for 10 minutes. It was dried in air at 150°C for 15 minutes. Then it was partially oxidized at 615°C for 4 hours continuously in air using a muffle furnace. The coating had a thickness of * 0.81±0.19jj,m. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 35 minutes. The coating thickness was « 0.98±0.15uim. Example - 2 Commercially pure aluminium (99%) was used. Sample shape was hollow cylindrical (outer diameter- 18.4 mm, inner diameter- 16.7 mm and length- 15.5 mm). The sample was polished conventionally and rinsed with acetone for 5 minutes. It was dried at 140°C for 20 minutes. Then it was partially oxidized at 615°C for 4 hours continuously in air using a muffle furnace. The coating had a thickness of ~ 0.81±0.19(im. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 45 minutes. The coating thickness was * 6.12±0.95|^m. Example - 3 Commercially pure aluminium (99%) was used. Sample shape was solid cube of 8 mm side length. The sample was polished and rinsed with acetone for 10 minutes. It was dried at 130°C for 25 minutes. Then it was partially oxidized at 615°C for 4 hours continuously in air using a muffle furnace. The coating had a thickness of « 4.77±0.79jj.m. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 35 minutes. The coating thickness was * 5.23±0.75 |im. Example - 4 Commercially pure aluminium (99%) was used. Sample shape was solid cube of 8 mm side length. The sample was polished and rinsed with acetone for 15 minutes. It was dried at 150°C for 15 minutes. Then it was partially oxidized at 615°C for 4 hours continuously in air using a muffle furnace. The coating had a thickness of « 4.77±0.79(im. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 70 minutes. The coating thickness was » 6.29±0.91nm. Example - 5 Commercially pure aluminium (99%) was used. Sample shape was solid cube of 8 mm side length. The sample was polished and rinsed with acetone for 10 minutes. It was dried at 125°C for 30 minutes. Then it was partially oxidized at 615°C for 4 hours continuously in air using a muffle furnace. The coating had a thickness of « 4.77±0.79nm. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 90 minutes. The coating thickness was « 9.12±1.15nm. Example - 6 Commercially pure aluminium (99%) was used. Sample shape was thin block (length- 8.4 mm, breadth- 8.6 mm and width- 0.7 mm). The sample was polished and rinsed with acetone for 5 minutes. It was dried at 150°C for 15 minutes. Then it was partially oxidized at 615°C for 4 hrs continuously in air using a muffle furnace. The coating had a thickness of * 0.74±0.17nm. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 35 minutes. The coating thickness was « 1.71 ±0.28 Example - 7 Commercially pure aluminium (99%) was used. Sample shape was thin block (length- 8.4 mm, breadth- 8.6 mm and width- 0.7 mm). The sample was polished and rinsed with acetone for 10 minutes. It was dried at 130°C for 25 minutes. Then it was partially oxidized at 615°C for 4 hrs continuously in air using a muffle furnace. The coating had a thickness of * 0.74±0.17 \an. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 55 minutes. The coating thickness was » 2.53±0.39nm. Example - 8 Commercially pure aluminium (99%) was used. Sample shape was thin block (length- 8.4 mm, breadth- 8.6 mm and width- 0.7 mm). The sample was polished and rinsed with acetone for 15 minutes. It was dried at 150°C for 15 minutes. Then it was partially oxidized at 615°C for 4 hrs continuously in air using a muffle furnace. The coating had a thickness of » 0.74±0.17Lim. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 90 minutes. The coating thickness was * 3.47±0.51 tun. Example - 9 Commercially pure aluminium (99%) was used. Sample shape was solid cylindrical (diameter- 18 mm and length- 10 mm). The sample was polished and rinsed with acetone for 10 minutes. It was dried at 140°C for 20 minutes. Then it was partially oxidized at 600°C for 200 hrs by alternate heating and cooling in air using a muffle furnace. The coating had a thickness of » 16.94±2.08^im. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 60 minutes. The coating thickness was « 40.26±4.78^im. Example- 10 Commercially pure aluminium (99%) was used. Sample shape was solid cylindrical (diameter- 18 mm and length-10 mm). The sample was polished and rinsed with acetone for 5 minutes. It was dried at 150°C for 15 minutes. Then it was partially oxidized at 600°C for 200 hrs by alternate heating and cooling in air using a muffle furnace. The coating had a thickness of « 16.94±2.08(im. Next, the sample was microwaved in a domestic microwave oven (800W) in air for 90 minutes. The coating thickness was * 661.72±65.24(am. Based on the above examples which illustrate the process of the present invention, we can conclude that aluminium oxide coating of different thickness can be developed on aluminium of various geometrical shapes as a function of initial oxide coating thickness prior to microwave treatment and microwave exposure time. The main advantages of the present invention are: 1. Simple process provides a means to develop aluminium oxide coating on aluminium. 2. Cost effective process process for the formation of aluminium oxide coating on aluminium. 3. Capable of providing tailored thickness of aluminium oxide coating for specific applications. 4. Capable of providing enhanced coating thickness thus enabling a thick aluminium oxide coating on aluminium for improved characteristics. We claim: 1. A process of making aluminium oxide coated aluminium which comprises polishing aluminium by known method and cleaning it with acetone for a period in the range of 5 to 15 minutes, followed by air drying at a temperature in the range of 125°C to 150°C for a period in the range of 15 to 30 minutes, subjecting the cleaned and dried aluminium to partial oxidation at a temperature in the range of 550°C to 615°C in air for a period in the range of 0.5 to 200 hours to obtain partially oxidized aluminium; exposing the partially oxidized aluminium in the presence of air to microwave energy of power in the range of 0.8 to 5 KW for a period in the range of 35 to 90 minutes to obtain aluminium oxide coated aluminium. 2. A process as claimed in claim 1 wherein the the aluminium used is geometrical shape selected from the group consisting of solid cylinder, hollow cylinder, solid cube and thin block. 3. A process as claimed in claims 1 & 2 wherein the partial oxidation is effected by subjecting to continuous heating or alternate heating and cooling at a temperature in the range of 550°C to 615°C in air for a period in the range of 0.5 to 200 hours. 4. A process of making aluminium oxide coated aluminium, substantially as herein described with reference to the examples accompanying this specification.

Documents:

57-del-2003-abstract.pdf

57-DEL-2003-Claims-(14-07-2008).pdf

57-del-2003-claims.pdf

57-DEL-2003-Correspondence-Others-(14-07-2008).pdf

57-DEL-2003-Correspondence-Others-(24-07-2008).pdf

57-del-2003-correspondence-others.pdf

57-del-2003-correspondence-po.pdf

57-del-2003-description (complete)-14-07-2008.pdf

57-del-2003-description (complete).pdf

57-del-2003-form-1.pdf

57-del-2003-form-18.pdf

57-DEL-2003-Form-2-(14-07-2008).pdf

57-del-2003-form-2.pdf

57-DEL-2003-Form-3-(14-07-2008).pdf

57-DEL-2003-Form-3-(24-07-2008).pdf

57-del-2003-form-3.pdf

57-DEL-2003-Petition-138-(24-07-2008).pdf