Title of Invention

AN IMPROVED PROCESS OF MAKING GLASS-CERAMIC COATINGS ON METALLIC SUBSTRATES MADE THEREBY.

Abstract

The present invention describes a simple and cost-effective process of making glass-ceramic coating on metallic substrate, wherein heat treatment of the glass coated metallic substrate is effected by controlled application of microwave energy. The novelty of this process is appropriate control of kinetics of nucleation and growth of ceramic crystallites in a glassy matrix by suitable application of microwave energy such as microwave heating, to obtain a glass-ceramic coating with uniform microstructure, lower crystallite size, lower surface roughness, glazed surface texture, superior microhardness and no surface microcracks which is not obtainable by conventional processing. Further, microstructure and properties of the glass-ceramic coating can be easily tailored by suitably controlled microwave heat treatment. The present invention may have significant potential for applications in automobile and power generation industries wherein oxidation, corrosion, abrasion and thermal resistance of the metallic components can be improved by using suitable glass-ceramic coating.

Full Text

The present invention relates to an improved process of making glass-ceramic coatings on metallic substrates and glass-ceramic coatings on metallic substrates made thereby. The present invention has main usages in industries such as automobile and power generation. In these industries applications require the metallic components to have improved thermal resistance and also improved resistance to oxidation, corrosion and abrasion. The improvement in such properties of metallic components can be achieved by the improved process of the present invention for making glass-ceramic coating on metallic substrate. High temperature materials have great demand for use in hot zone components of gas turbine engines and heat exchangers. Nickel, cobalt or iron based super alloys are widely used as high temperature materials. Even the superalloys tend to deteriorate by oxidation during its continuous operation in an oxidizing atmosphere at high temperature (>1000°C). Therefore, effective surface coating is required to protect the superalloys under extreme hostile conditions. Glass-ceramic coating can provide a solution to this problem. It protects the superalloys from oxidation, corrosion, wear and thermal degradation. Hence, these coatings extend service life of the expensive and strategic materials under stringent operational conditions. Dramatic improvements in the properties of the glass-ceramic coating can be achieved by tailoring microstructure of the coating. Reference may be made to United States Patent No. 5,298,332, Andrus, L. Ronald, MacDowell, F. John, in Glass-ceramic coatings for titanium-based metal surfaces, wherein alkaline earth silicate glass with suitable additives is applied to titanium, titanium alloy or titanium aluminide surface in the form of fine glass powders by conventional method. The glass powder coated substrate is then heated to produce a continuous glass coating. Subsequently, the glassy coating is converted to crystalline glass-ceramic coating by heat treatment. However, small seed, fine surface crack and pinhole defects as well as minor edge spalling are observed in some of the coatings. Fine tuning of the processing is necessary to avoid the defect formation and spalling in the resultant glass-ceramic coating. Further, use of lots of additives makes this process less cost-effective. Another reference may be made to United States Patent No. 4,385,127, Chyung and Kenneth, in Glass-ceramic coatings for use on metal substrates, wherein low carbon steels and titanium-stabilized stainless steels are coated with alkaline earth silicate glass. Then, the glass-coated substrate is fired by a critically controlled heat treatment to fuse the frit particles together into integral, non-porous coating and to result extensive crystallization in situ therein simultaneously. As a result, glass-ceramic coatings are formed on metal substrates. The drawback of the referred process is that it produces glass-ceramic coatings suitable for use only at lower temperatures on metal substrates and requires stringent control of heat treatment schedule. Reference may also be made to United States Patent No. 5,250,360, Andrus, L. Ronald, MacDowell, F. John, in Coated metal article, wherein the surface of a preformed superalloy body is coated with barium silicate and strontium silicate based powdered glass containing suitable fillers and additives in conventional manner. The glass powder coated metal body is then heated to form continuous glass coating. The glass-coated body is then heated to develop crystal phase. Thus, protective glass-ceramic coating is formed onto the metal surface. In this invention, use of additives permits to form a continuous well-flowed glass coating before sufficient crystallization occurs to impede flow. This is very much essential to avoid the formation of porous, cracked coating. The drawback of the process is that blister, pinhole defects, slight spalling and/or porosity were observed with some coatings. Hence, precision processing control is required in order to achieve a defect free glass-ceramic coating. Moreover, it is an expensive process as it involves the use of fillers and additives. The main object of the present invention is to provide an improved process of making glass-ceramic coatings on metallic substrates and glass-ceramic coatings on metallic substrates made thereby, which obviates the drawbacks of the hitherto known prior art methods as detailed herein above. Another object of the present invention is to obtain, a glass-ceramic coating with uniform microstructure onto metallic substrate. Still another object of the present invention is to provide a glass-ceramic coating with lower crystallite size on metallic substrate. Yet another object of the present invention is to obtain a glass-ceramic coating with lower surface roughness on metallic substrate. Another object of the present invention is to provide a glass-ceramic coating with glazed surface texture onto metallic substrate. Still another object of the present invention is to obtain a glass-ceramic coating with superior hardness on metallic substrate. Yet another object of the present invention is to provide a simple and cost-effective process for the formation of glass-ceramic coating on metallic substrate. Another object of the present invention is to provide a means for tailoring the microstructure and hence properties of glass-ceramic coating formed on metallic substrate. The present invention describes a simple and cost-effective process of making glass-ceramic coating on metallic substrate, wherein heat treatment of the glass coated metallic substrate is effected by controlled application of microwave energy. The novelty of this process is appropriate control of kinetics of nucleation and growth of ceramic crystallites in a glassy matrix to obtain a glass-ceramic coating with uniform microstructure, lower crystallite size, lower surface roughness, glazed surface texture, superior microhardness and no surface microcracks which is not obtainable by conventional processing. This has been made possible by the non-obvious inventive step of suitable application of microwave energy such as microwave heat treatment of the glass coated metallic substrate. Further, microstructure and properties of the glass-ceramic coating can be easily tailored by suitably controlled microwave heat treatment. The present invention may have significant potential for applications in automobile and power generation industries wherein oxidation, corrosion, abrasion and thermal resistance of the metallic components can be improved by using suitable glass-ceramic coating. In the present invention use of microwave energy in heat treatment of the glass coated metallic substrate has resulted in improved properties. Microwave energy is delivered to the material through molecular interaction with the electromagnetic field. During interaction, energy in the form of heat is generated in the material due to transformation of electromagnetic energy to thermal energy. As microwave radiation penetrates and propagates through the material, transfer of electromagnetic energy takes place throughout the whole volume of material resulting in volumetric heating. Hence, microwave heating ensures rapid and uniform heating of the material. That results in uniform and refined microstructure. In contrast, the conventional electrical heating causes the surface to get heated first, which then propagates the thermal energy to the bulk of the material. As a result of this thermal gradient and time consuming electrical heating process the microstructure is non uniform and much coarser. Thus, microwave processing gives rise to better mechanical properties compared to the conventional processing. In addition, rapid heating rates lead to lower processing times. Consequently, lower processing cost is anticipated. Dielectric materials with intermediate loss tangents can absorb microwave energy. The loss tangent is a measure of the absorption of microwaves by the material. The microwave absorption varies linearly with the relative dielectric constant and the loss tangent of the material. Since the values of the relative dielectric constant and the loss tangent increase with increasing temperature, microwave absorption also increases at higher temperatures. Accordingly the present invention provides an improved process of making glass-ceramic coatings on metallic substrates, which comprises melting a glass-forming batch of a predetermined composition followed by fritting the glass; crushing the glass frit and dry milling the resultant glass particles to obtain uniform particle size; wet milling the fine glass particles with some mill additions for about 40 to 48 hours to obtain particles of 3 to 5 micron size; preparing a liquid slurry of the milled glass particles; cleaning the metal surface to be coated; applying the said liquid slurry onto the cleaned metal surface by conventional spraying technique; drying the glass powder coated metallic substrate initially at ambient condition and then at a temperature in the range of 110°C to120°C in air for a period in the range of 30 to 45 minutes to obtain a dried sample; firing the said dried sample at a predetermined temperature in air for a definite period to obtain an integral, impervious and well-formed continuous glass coating; characterized in that the said glass coated metallic substrate being subjected to heat treatment in air by controlled application of microwave energy. In an embodiment of the present invention, the glass frit composition is such as having thermal expansion coefficient matching with the metallic substrate to be coated. In another embodiment of the present invention, the metallic substrate to be coated is such as of any nimonic alloy. In yet another embodiment of the present invention, the cleaning of the metallic substrate to be coated is done by thermal degreasing followed by sand blasting and finally ultrasonic cleaning with acetone for a period of 10 to 15 minutes. In still another embodiment of the present invention, the heat treatment of the glass coated metallic substrate is effected in a microwave furnace in air following a heat treatment schedule of initial soaking at nucleation temperature followed by soaking at growth temperature. In a further embodiment of the present invention, the heat treatment of a MgO-AI2O3-TiO2 based glass-ceramic coating on a nimonic alloy (AE-435) substrate is effected in a 2 .00 KW, 2.45 GHz microwave furnace following a heat treatment schedule of initial soaking at nucleation temperature of the order of 875 °C for a period of 30 to 120 minutes followed by soaking at growth temperature of the order of 1000 °C for a period of 30 to 120 minutes. Accordingly the present invention provides glass-ceramic coatings on metallic substrates made by the improved process of the present invention for making glass-ceramic coatings on metallic substrates, as herein described. The present invention provides MgO-AbOa-TiO^ based glass-ceramic coating onto the nimonic alloy substrate. Conventional enamelling technique has been used to form the glass coating on metallic substrate. Generally, glass coated metallic substrate is heat treated by any conventional means (electrical energy converted to thermal energy) to develop crystal phases in the glassy matrix. In the present work microwave heating has been utilized to convert glassy coating to glass-ceramic coating. Conventional heat treatment has been also carried out to compare the results obtained with the microwave treatment. The glassy coating is heated by the microwave energy by conversion of the energy absorbed from oscillating electrorrfagnetic field into thermal energy of lattice. Initially, glass coating cannot absorb microwave energy at low temperature. It needs to be heated by the thermal energy radiated from a microwave susceptor material like silicon carbide or graphite. Glass coating can absorb and couple with microwave energy in an enhanced rate at higher temperatures above a critical temperature (Tcrit.) because of large values of the loss tangent and the relative dielectric constant. However, energy loss mechanisms are not yet clear and may be linked to ionic migration, ionic vibration and electronic polarization. The surfaces of the processed materials have been evaluated through XRD, SEM, microhardness and surface roughness measurement (Ra). X-ray diffraction analysis identifies magnesium-aluminium-titanate, magnesium-silicate and aluminium-titanate crystalline phases in the resultant glass-ceramic coatings produced by microwave and conventional heat treatment. Scanning electron microscopy clearly reveals that microwave processing generates finer crystals that are dispersed much more uniformly in the glassy matrix as compared to those obtained under identical condition through conventional processing. SEM micrographs also show that glazed surface texture appears in case of microwave heat treated samples. On the other hand, surface of the glass ceramic coating obtained by conventional processing is thoroughly rough and full of microcracks. Further, Vickers microhardness and surface roughness measurement show that microwave heat treatment increases the microhardness and lowers the surface roughness of the coatings than those obtained by the conventional process. The earlier works of the hitherto known prior art related to the present work had the drawback of incorporation of lots of fillers and additives with the glass frit material making the processes cost-ineffective. Additionally, precision processing control is necessary to get the defect free glass-ceramic coating. Another earlier work referred above requires stringent control of heat treatment schedule to produce glass-ceramic coatings on metallic substrates. In the present invention, these drawbacks have been obviated through the provision of a simple and economical process for the formation of glass-ceramic coating onto the metallic substrate. Thus, the novelty of this process is appropriate control of kinetics of nucleation and growth of ceramic crystallites in a glassy matrix by suitable application of microwave energy such as to obtain a glass-ceramic coating with uniform microstructure, lower crystallite size, lower surface roughness, glazed surface texture, superior microhardness and no surface microcracks which was not obtainable by conventional processing. Such coatings may have useful applications in aeronautical, aerospace, automobile and power generation industries. In addition, microstructure and properties of the glass-ceramic coating can be easily tailored by controlled microwave heat treatment. The inventive step in the process described lies in heat treatment of the glass coated metallic substrate by suitably controlled application of microwave energy. The detailed steps of the improved process of the present invention for making glass-ceramic coatings on metallic substrates are as follows: 1. A glass-forming batch of a predetermined composition is weighed, mixed and sieved. 2. The prepared glass batch is melted using conventional glass melting techniques. 3. The melt is quenched to yield glass frit. 4. The glass frit is crushed and the resultant particles are dry milled in a porcelain ball mill to obtain uniform glass particles. 5. The dry milled glass particles are wet milled with some mill additions in a porcelain ball mill for about 40 to 48 h to obtain fine glass particles of 3 to 5 micron size. 6. A liquid slurry is prepared of the milled glass particles. 7. The surface of metallic substrate is prepared by thermal degreasing followed by sand blasting. 8. The metal surface is cleaned ultrasonically with acetone for 10 to15 min. 9. The coating material slurry is applied onto the cleaned metallic substrate by conventional spraying technique. 10. The glass powder coated metallic substrate is dried initially at ambient condition and then at a temperature in the range of 110°C to 120°C in air for a period in the range of 30 to 45 min. 11. The dried sample is fired at a predetermined temperature in air for a definite period to obtain an integral, dense and well-formed continuous coating. 12. The glass-coated sample is then heat treated in a microwave furnace in air following suitable heat treatment. The following examples are given by way of illustration of the present invention in actual practice and therefore should not be construed to limit the scope of the present invention in any way. Example -1 A glass forming batch that comprises of SiO2 - 33.13; AI203 - 22.41; B203 -9.73; MgO - 13.77; TiO2- 13.78; CaO - 0.65; Na2O - 1.76; K2O - 4.77 in wt% was weighed, mixed and sieved. The coating material was prepared by melting the prepared glass-forming batch at 1400°C for 3 h. Subsequently the molten glass was fritted. The glass frit was crushed and then dry milled in a porcelain ball mill. Again, the glass particles were wet milled with the mill addition of 5 wt% clay in a porcelain ball mill for about 40 h. Coating material slurry is prepared of the milled glass particles. Nimonic alloy (AE-435) was used as metal substrate. The metal surface was prepared by thermal degreasing followed by sand blasting. Thereafter the metal surface was cleaned ultrasonically with acetone for 15 min. The coating material slurry was applied onto the cleaned metal substrates by conventional spraying technique. The glass powder coated samples were dried initially at ambient condition and then at 120°C for 30 min. Then the dried samples were fired at 1160°C for 6 min in air in a muffle furnace. Next, the glass-coated samples (five nos.) were heat treated in air in a microwave furnace (Multilab 2.0 KW, 2.45 GHz, Linn High Therm GmbH, Germany) at the nucleation temperature of 875°C for 30 min and growth temperature of 1000°C for 30 min. A specially designed applicator with in-built insulation of alumina fibre boards was fabricated in the laboratory and utilized for the microwave heat treatment. The sample was kept in a silicon carbide crucible placed inside the insulation arrangement. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 131.26± 21.0 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 6.86 ±0.14 GPa. The average surface roughness of the coatings was 0.35±0.06 micron. The coatings showed glazed surface texture. The same glass-coated samples (five nos.) were also heat treated in air in a muffle furnace at the nucleation temperature of 875°C for 30 min and growth temperature of 1000°C for 30 min, for the purpose of comparison only. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 137.341 23.35 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 6.55±0.13 GPa. The average surface roughness of the coatings was 0.45±0.09 micron. The coating surfaces appeared to be rough and full of microcracks. Example-2 A glass forming batch that comprises of Si02 - 33.13; AI2O3 - 22.41; B203 -9.73; MgO - 13.77; TiO2-13.78; CaO - 0.65; Na2O -1.76; K2O - 4.77 in wt% was weighed, mixed and sieved. The coating material was prepared by melting the prepared glass-forming batch at 1400°C for 3 h. Subsequently the molten glass was fritted. The glass frit was crushed and then dry milled in a porcelain ball mill. Again, the glass particles were wet milled with the mill addition of 5 wt% clay in a porcelain ball mill for about 45 h. Coating material slurry is prepared of the milled glass particles. Nimonic alloy (AE-435) was used as metal substrate. The metal surface was prepared by thermal degreasing followed by sand blasting. Thereafter the metal surface was cleaned ultrasonically with acetone for 10 min. The coating material slurry was applied onto the cleaned metal substrates by conventional spraying technique. The glass powder coated samples were dried initially at ambient condition and then at 110°C for 45 min. Then the dried samples were fired at 1160°C for 6 min in air in a muffle furnace. Next, the glass-coated samples (five nos.) were heat treated in air in a microwave furnace (Multilab 2.0 KW, 2.45 GHz, Linn High Therm GmbH, Germany) at the nucleation temperature of 875°C for 45 min and growth temperature of 1000°C for 45 min. A specially designed applicator with in-built insulation of alumina fibre boards was fabricated in the laboratory and utilized for the microwave heat treatment. The sample was kept in a silicon carbide crucible placed inside the insulation arrangement. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 122.76± 18.42 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 7.54 ±0.15 GPa. The average surface roughness of the coatings was 0.32± 0.04 micron. The coatings showed glazed surface texture. The same glass-coated samples (five nos.) were also heat treated in air in a muffle furnace at the nucleation temperature of 875°C for 45 min and growth temperature of 1000°C for 45 min, for the purpose of comparison only. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 175.89± 26.38 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 7.00±0.21 GPa. The average surface roughness of the coatings was 0.51±0.11 micron. The coating surfaces appeared to be rough and full of microcracks. Example-3 A glass forming batch that comprises of Si02 - 33.13; AI2O3 - 22.41; B2O3 -9.73; MgO -13.77; Ti02-13.78; CaO - 0.65; Na2O -1.76; K2O - 4.77 in wt% was weighed, mixed and sieved. The coating material was prepared by melting the prepared glass-forming batch at 1400°C for 3 h. Subsequently the molten glass was fritted. The glass frit was crushed and then dry milled in a porcelain ball mill. Again, the glass particles were wet milled with the mill addition of 5 wt% clay in a porcelain ball mill for about 48 h. Coating material slurry is prepared of the milled glass particles. Nimonic alloy (AE-435) was used as metal substrate. The metal surface was prepared by thermal degreasing followed by sand blasting. Thereafter the metal surface was cleaned ultrasonically with acetone for 15 min. The coating material slurry was applied onto the cleaned metal substrates by conventional spraying technique. The glass powder coated samples were dried initially at ambient condition and then at 120°C for 30 min. Then the dried samples were fired at 1160°C for 6 min in air in a muffle furnace. Next, the glass-coated samples (five nos.) were heat treated in air in a microwave furnace (Multilab 2.0 KW, 2.45 GHz, Linn High Therm GmbH, Germany) at the nucleation temperature of 875°C for 60 min and growth temperature of 1000°C for 60 min. A specially designed applicator with in-built insulation of alumina fibre boards was fabricated in the laboratory and utilized for the microwave heat treatment. The sample was kept in a silicon carbide crucible placed inside the insulation arrangement. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 105.29± 15.79 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at load of 100 g was 7.74± 0.23 GPa. The average surface roughness of the coatings was 0.31 ±0.04 micron. The coatings showed glazed surface texture. The same glass-coated samples (five nos.) were also heat treated in air in a muffle furnace at the nucleation temperature of 875°C for 60 min and growth temperature of 1000°C for 60 min, for the purpose of comparison only. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 188.43± 30.15 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 7.35±0.29 GPa. The average surface roughness of the coatings was 0.62±0.14 micron. The coating surfaces appeared to be rough and full of microcracks. Example-4 A glass forming batch that comprises of SiO2 - 33.13; AI203 - 22.41; B20s -9.73; MgO - 13.77; Ti02-13.78; CaO - 0.65; Na2O - 1.76; K2O - 4.77 in wt% was weighed, mixed and sieved. The coating material was prepared by melting the prepared glass-forming batch at 1400°C for 3 h. Subsequently the molten glass was fritted. The glass frit was crushed and then dry milled in a porcelain ball mill. Again, the glass particles were wet milled with the mill addition of 5 wt% clay in a porcelain ball mill for about 40 h. Coating material slurry is prepared of the milled glass particles. Nimonic alloy (AE-435) was used as metal substrate. The metal surface was prepared by thermal degreasing followed by sand blasting. Thereafter the metal surface was cleaned ultrasonically with acetone for 15 min. The coating material slurry was applied onto the cleaned metal substrates by conventional spraying technique. The glass powder coated samples were dried initially at ambient condition and then at 110°C for 45 min. Then the dried samples were fired at 1160°C for 6 min in air in a muffle furnace. Next, the glass-coated samples (five nos.) were heat treated in air in a microwave furnace (Multilab 2.0 KW, 2.45 GHz, Linn High Therm GmbH, Germany) at the nucleation temperature of 875°C for 80 min and growth temperature of 1000°C for 80 min. A specially designed applicator with in-built insulation of alumina fibre boards was fabricated in the laboratory and utilized for the microwave heat treatment. The sample was kept in a silicon carbide crucible placed inside the insulation arrangement. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 97.85± 11.74 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 7.80 ±0.31 GPa. The average surface roughness of the coatings was 0.30± 0.05 micron. The coatings showed glazed surface texture. The same glass-coated samples (five nos.) were also heat treated in air in a muffle furnace at the nucleation temperature of 875°C for 80 min and growth temperature of 1000°C for 80 min, for the purpose of comparison only. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 209.13± 35.55 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 7.05±0.42 GPa. The average surface roughness of the coatings was 0.74± 0.17 micron. The coating surfaces appeared to be rough and full of microcracks. Example - 5 A glass forming batch that comprises of SiO2 - 33.13; AI2O3 - 22.41; B203 -9.73; MgO - 13.77; TiO2 -13.78; CaO - 0.65; Na2O - 1.76; K2O - 4.77 in wt% was weighed, mixed and sieved. The coating material was prepared by melting the prepared glass-forming batch at 1400°C for 3 h. Subsequently the molten glass was fritted. The glass frit was crushed and then dry milled in a porcelain ball mill. Again, the glass particles were wet milled with the mill addition of 5 wt% clay in a porcelain ball mill for about 48 h. Coating material slurry is prepared of the milled glass particles. Nimonic alloy (AE-435) was used as metal substrate. The metal surface was prepared by thermal degreasing followed by sand blasting. Thereafter the metal surface was cleaned ultrasonically with acetone for 10 min. The coating material slurry was applied onto the cleaned metal substrates by conventional spraying technique. The glass powder coated samples were dried initially at ambient condition and then at 120°C for 30 min. Then the dried samples were fired at 1160°C for 6 min in air in a muffle furnace. Next, the glass-coated samples (five nos.) were heat treated in air in a microwave furnace (Multilab 2.0 KW, 2.45 GHz, Linn High Therm GmbH, Germany) at the nucleation temperature of 875°C for 90 min and growth temperature of 1000°C for 90 min. A specially designed applicator with in-built insulation of alumina fibre boards was fabricated in the laboratory and utilized for the microwave heat treatment. The sample was kept in a silicon carbide crucible placed inside the insulation arrangement. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 90.55± 9.96 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 7.89 ±0.39 GPa. The average surface roughness of the coatings was 0.29± 0.03 micron. The coatings showed glazed surface texture. The same glass-coated samples (five nos.) were also heat treated in air in a muffle furnace at the nucleation temperature of 875°C for 90 min and growth temperature of 1000°C for 90 min, for the purpose of comparison only. The average crystallite size of main crystalline phase (magnesium aluminium titanate) was 223.48± 40.23 nm. The average value of Vickers microhardness of resultant glass-ceramic coatings measured at a load of 100 g was 6.83± 0.41 GPa. The average surface roughness of the coatings was 0.83±0.13 micron. The coating surfaces appeared to be very rough and full of microcracks. Based on the above examples, we can conclude that glass-ceramic coating of unique properties can be formed on metallic substrate by properly controlled microwave heat treatment. The main advantages of the improved process of the present invention for making glass-ceramic coatings on metallic substrates and glass-ceramic coatings on metallic substrates made thereby, are: 1. Forms glass-ceramic coatings with uniform microstructure on metallic substrates. 2. Provides a glass-ceramic coating with lower crystallite size on metallic substrate. 3. Provides a glass-ceramic coating with lower surface roughness onto metallic substrate. 4. Provides a glass-ceramic coating with glazed surface texture on metallic substrate. 5. Provides a glass-ceramic coating with superior hardness on metallic substrate. 6. The microstructure and properties of the glass-ceramic coating can be easily tailored for specific applications. 7. A simple and cost-effective process for the formation of glass-ceramic coatings on metallic substrates. We claim: 1. An improved process of making glass-ceramic coatings on metallic substrates, which comprises melting a glass-forming batch of a predetermined composition followed by fritting the glass; crushing the glass frit and dry milling the resultant glass particles to obtain uniform particle size; wet milling the fine glass particles with some mill additions for about 40 to 48 hours to obtain particles of 3 to 5 micron size; preparing a liquid slurry of the milled glass particles; cleaning the metal surface to be coated; applying the said liquid slurry onto the cleaned metal surface by conventional spraying technique; drying the glass powder coated metallic substrate initially at ambient condition and then at a temperature in the range of 110°C to120°C in air for a period in the range of 30 to 45 minutes to obtain a dried sample; firing the said dried sample at a predetermined temperature in air for a definite period to obtain an integral, impervious and well-formed continuous glass coating; characterized in that the said glass coated metallic substrate being subjected to heat treatment in air by controlled application of microwave energy. 2. An improved process as claimed in claim 1, wherein the glass frit composition is such as having thermal expansion coefficient matching with the metallic substrate to be coated. 3. An improved process as claimed in claim 1-2, wherein the metallic substrate to be coated is such as of any nimonic alloy. 4. An improved process as claimed in claim 1-3, wherein the cleaning of the metallic substrate to be coated is done by thermal degreasing followed by sand blasting and finally ultrasonic cleaning with acetone for a period of 10 to 15 minutes. 5. An improved process as claimed in claim 1-4, wherein the the heat treatment of the glass coated metallic substrate is effected in a microwave furnace in air following a heat treatment schedule of initial soaking at nucleation temperature followed by soaking at growth temperature. 6. An improved process as claimed in claim 1-5, wherein the heat treatment of a MgO-AbOa-TiCb based glass-ceramic coating on a nimonic alloy (AE- 435) substrate is effected in a 2 .00 KW, 2.45 GHz microwave furnace following a heat treatment schedule of initial soaking at nucleation temperature of the order of 875 °C for a period of 30 to 120 minutes followed by soaking at growth temperature of the order of 1000 °C for a period of 30 to 120 minutes. 7. Glass-ceramic coatings on metallic substrates made by the improved process as claimed in claim 1-6. 8. An improved process of making glass-ceramic coatings on metallic substrates, substantially as herein described with reference to the examples. 9. Glass-ceramic coatings on metallic substrates made by the improve* process of making glass-ceramic coatings on metallic substrates substantially as herein described with reference to the examples.

Documents:

883-del-2006-abstract.pdf

883-del-2006-Claims-(03-07-2013).pdf

883-del-2006-claims.pdf

883-del-2006-Correspondence-Others-(03-07-2013).pdf

883-del-2006-correspondence-others.pdf

883-del-2006-description (complete).pdf

883-del-2006-form-1.pdf

883-del-2006-form-2.pdf

883-del-2006-form-3.pdf

883-del-2006-form-5.pdf