Title of Invention	"AN IMPROVED PROCESS FOR THE MANUFACTURE OF THERMALLY STABLE NON-METALLIC COATING FOR METALLIC SUBSTRATES AND A PROCESS OF COATING THEREOF"
Abstract	In the present invention there is provided an improved process for the manufacture of thermally stable, high temperature resistant, non-metallic, glass ceramic coating material suitable for metallic substrates and a process of application of the coating on such substrates as nickel and cobalt based super alloys and stainless steel. In this improved process the partially crystallized coating composition comprises of a mixture, based on its oxide content, of barium oxide, calcium oxide, magnesium oxide, silicon di-oxide, boron tri-oxide and molybdenum oxide. The coating materials are further processed with a mixture of washed chromium oxide, precipitated silica, cobalt oxide and china clay in water and applied to pre-cleaned metal surface and fired to provide a glass-ceramic coating on the metal. The coating is defect free, thermal shock resistant, highly adherent and suitable for continuous use at temperatures in the range of 1000°C to 1050°C in oxidizing environment.

Title of Invention

"AN IMPROVED PROCESS FOR THE MANUFACTURE OF THERMALLY STABLE NON-METALLIC COATING FOR METALLIC SUBSTRATES AND A PROCESS OF COATING THEREOF"

Abstract

In the present invention there is provided an improved process for the manufacture of thermally stable, high temperature resistant, non-metallic, glass ceramic coating material suitable for metallic substrates and a process of application of the coating on such substrates as nickel and cobalt based super alloys and stainless steel. In this improved process the partially crystallized coating composition comprises of a mixture, based on its oxide content, of barium oxide, calcium oxide, magnesium oxide, silicon di-oxide, boron tri-oxide and molybdenum oxide. The coating materials are further processed with a mixture of washed chromium oxide, precipitated silica, cobalt oxide and china clay in water and applied to pre-cleaned metal surface and fired to provide a glass-ceramic coating on the metal. The coating is defect free, thermal shock resistant, highly adherent and suitable for continuous use at temperatures in the range of 1000°C to 1050°C in oxidizing environment.

Full Text	The present invention relates to an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates and a process of coating thereof. The present invention particularly relates to an improved process for the manufacture of thermally stable non-metallic coating, for metallic substrates such as super alloy components, starting from from a novel synergistic composition as described and claimed in our co-pending patent application no. NF-31 / 03 and a process of coating metallic substrates with the novel thermally stable non-metallic coating. The present invention more particularly relates to an improved process for the manufacture of thermally stable non-metallic coating comprising a glass-ceramic selected from the barium magnesium silicate system by a novel processing technique. The thermally stable non-metallic coating of the present invention over the surface of the metal or alloy substrate serves as an oxidation resistant coating to prevent oxygen attack of the metal at elevated temperatures and also as a thermal barrier to the metal surface to prevent rapid heat up of the metal surface. There is a well recognized need, in hot zone components of turbine engines and heat exchangers, for example, for materials capable of withstanding continuous operating temperatures in excess of 1000°C. While a variety of high temperature materials are known, it is common practice to use high temperature stable alloys known as super alloys in the more severe applications. Characteristically, these alloys are rich in nickel, cobalt, chromium, or iron, with nickel or cobalt being the principal metal bases. These super alloys have been distinguished from other high temperature resistant materials on the basis of being sufficiently resistant to oxidation to permit operation in an oxidizing atmosphere without a protective coating. Nevertheless, under extreme hostile conditions, such as encountered by hot zone components in gas turbine engines, even the super alloys tend to deteriorate unless protected by a oxygen barrier coating. A very well known method of protecting metal components from oxidation at elevated temperatures in a hostile environment is to apply a continuous monolithic glass coating on the exposed surface. This coating completely encapsulates and isolates the metallic surface from the surrounding oxygen containing atmosphere. However, viscous flow of the glass coating may occur when large surface stresses develop during high temperature use, resulting in thin spots and catastrophic coating failure. Another method of achieving higher viscosity is by mixing crystalline materials as second phase with the glass frits prior to application of the coating. However, these glass-crystalline mixtures sinter rather non-uniformly, the crystal size and homogeneity of the resultant microstructure being very difficult to control. Certain portions of the substrate, therefore, tend to be entirely free from crystals, whereas other portions have too many (or too large) crystals to sinter well. A void free coating with this heterogeneous glass-crystal mixture is thus difficult to obtain. References may be made to the following U. S. Patents: No. 3,397,076 (Little et al.) describes fused crystallizable ground and cover coats for high temperature alloys in which the major elements are cobalt, nickel, chromium, iron or mixtures. The ground coat is lithium free and contains 35-65% Si02 and12-45% BaO. Examples also contain substantial amounts of R20, B2C3 and /or TiO2. The drawbacks are use of two separate coating systems, one ground coat and other cover coat. The coating composition contains substantial amount of alkali oxides which is normally unsuitable for continuous use at high operating temperature. No. 3,467,534 (MacDowell) discloses glass-ceramic articles consisting essentially of 20-70% BaO and 30-80% SiO2 and having a barium silicate principal crystal phase. A preferred example is described as considered for coating metals. This article is related to glass-ceramic objects not applicable to coatings. No. 3,531,303 (Bahat) discloses glass-ceramic articles in the alkaline earth aluminosilicate field wherein hexagonal alkaline earth feldspar or a triclinic form is the principal crystal phase. The materials are highly refractory with service temperatures up to 1700°C and consist essentially of 12-53% SiO2, 17-55% RO where RO is 17-50% SrO and 25-50% BaO, 10-58% AI2O3 and a nucleating agent. The drawback of the process is same as above, that is it relates to glassceramic objects and is not applicable to coatings. No. 3,578,470 (Bahat) discloses glass-ceramic materials in the BaO-AI2O3-Si02 composition field nucleated with Ta205 and/or Nb205 that are especially suited to sealing with tungsten or molybdenum and their alloys. This method describes a sealing material and is not applicable to coatings. No. 3,837,978 (Busdiecker) discloses barium aluminosilicate glass-ceramics nucleated by tin oxide, having a hexacelsian primary crystal phase, and having a coefficient of thermal expansion in the range of 50-170X10"7/°C. Nos. 4,256,796; 4,358,541 and 4,385,127 disclose mixed barium magnesium and calcium magnesium silicate coatings, fluxed with B2Os. The drawback in the high temperature resistance of these coatings is the use of B2O3. A high B2O3 residual glass (or even a borate crystal) tends to allow the microstructure to move and flow at temperatures much below the solidus of the primary refractory silicate phases. Gas turbine engines and heat exchangers operate in a hostile environment at operating temperatures in excess of 1000°C. There is a considerable demand for materials capable of withstanding those severe working conditions. While a variety of high temperature materials are known, it is a common practice to use certain" nickel or cobalt based super alloys in the more severe applications. These super alloys have been distinguished from other high temperature materials on the basis of being considerably resistant to oxidation to permit continuous operation in an oxidizing atmosphere without a protective coating. Nevertheless, under more severe conditions, such as encountered by hot zone components in gas turbine engines in an advanced aircraft, even the super alloys tend to deteriorate very fast unless protected by a suitable coating. A well known technique of protecting the super alloys from oxidation at elevated temperatures is to apply an impervious stable continuous monolithic glassy coating. This completely encapsulates and isolates the material from the .surrounding hostile environment. However, viscous flow of the glassy coating material may occur when large surface stresses develop during high temperature use. In some cases generation of hot spots occurs which led to premature catastrophic failure of the coating as well as the component. The viscosity being a structural property of the system, can be easily tailored by mixing certain refractory materials with the glassy coating materials (frits) during processing (milling), before application of the coating on metal surface.. However, these glass- crystalline mixtures are very difficult to fuse at the required firing temperature and sinter rather non-uniformly. The microstructure and homogeneity of the coating is very difficult to control. Certain portion of the resultant coating generates hot spots which may cause premature failure. A void free coating with this heterogeneous coating system is thus very difficult to obtain. From the above referred hitherto known prior art disclosures it is clear that there is a definite need for providing an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates such as nickel or cobalt base super alloy and stainless steel components that are required to operate at temperatures above 1000°C. The main object of the present invention is to provide an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates and a process of coating thereof, which obviates the drawbacks as detailed above. Another object of the present invention is to provide an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates such as super alloy components and a process of coating thereof. Still another object of the present invention is to provide an improved process for obtaining a coating material with superior set of oxidation and hot corrosion resistance properties by following a novel composition and novel processing techniques. Yet another object of the present invention is to provide an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates starting from a novel synergistic composition as described and claimed in our copending patent application no. NF-31 / 03. Still yet another object of the present invention is to provide an improved process for obtaining a coating material by using a combination of alkaline earth metal oxides such as BaO, CaO and MgO instead of BaO alone. A further object of the present invention is to provide an improved process for obtaining a coating material by using a bonding oxide (MoOa) to promote adherence of the coating to the base metal substrate. A still further object of the present invention is to provide an improved process for obtaining a coating containing BaMgSi04 crystal phase as the major crystalline constituent of the coating. Another object of the present invention is to provide an improved process for obtaining a coating material which can be melted in a ceramic crucible at a temperature in the range of 1400 to 1450 degree C in an ambient atmosphere. Yet another object of the present invention is to provide an improved process of coating metallic substrates such as super alloy components which obviates the drawbacks as detailed above. Still another object of the present invention is to provide an improved process of coating metallic substrates such as super alloy components that is more effective, easier to apply, than previously known coatings of the prior art. Still yet another object of the present invention is to provide an improved process for the manufacture of thermally stable non-metallic coating for obtaining a reliable and reproducible high temperature protective coating for nickel or cobalt base super alloy and stainless steel substrates that are required to operate at temperatures above 1000°C. A further object of the present invention is to provide an improved process for obtaining a high temperature protective coating that adheres strongly and resists spelling during thermal cycling. A still further object of the present invention is to provide an improved process for obtaining a high temperature protective coating technique that exhibits the excellent flow characteristics of a glass coating as it is fired in one temperature range, and becomes resistant to flow due to subsequent crystallization as it is heated in the required operating temperature range. Another object of the present invention is to provide an improved process for obtaining a coating that provides a useful degree of thermal insulation to the base metal surface. In the present invention there is provided an improved process for the manufacture of thermally stable, high temperature resistant, non-metallic, glass ceramic coating material suitable for metallic substrates and a process of application of the coating on such substrates as nickel and cobalt based super alloys and stainless steel. In this improved process the partially crystallized coating composition comprises of a mixture, based on its oxide content, of barium oxide, calcium oxide, magnesium oxide, silicon di-oxide, boron tri-oxide and molybdenum oxide. The coating materials are further processed with a mixture of washed chromium oxide, precipitated silica, cobalt oxide and china clay in water and applied to pre-cleaned metal surface and fired to provide a glass-ceramic coating on the metal. The coating is defect free, thermal shock resistant, highly adherent and suitable for continuous use at temperatures in the range of 1000°C to 1050°C in oxidizing environment. The method for preparing and applying the novel inventive coatings of the present invention involves five fundamental steps: First, a glass-forming batch of a predetermined formulation is melted; Second, the melt is cooled to a glass and the glass comminuted to a fine powder or frit; Third, a liquid slurry is prepared of the frit; Fourth, the slurry is applied as a coating onto a surface of a desired substrate; and Fifth, the coated substrate is fired to temperatures of at least 1200°C to fuse the frit particles together into an integral, essentially non-porous, vitreous coating and subsequently heat treated to cause crystallization in situ to take place therein. In general, to insure adequate flow of the glass during sintering, thereby providing an essentially "pinhole-free" coating, and to develop a high volume of crystallinity, the frit will desirably be powdered to particles passing a 325 United States Standard Sieve (44 microns). Aqueous slurry can be employed for coating. The initial step in the preparation of the porcelains of this invention is to prepare a glass frit having the desired composition. The raw materials which are used in the preparation of the glass frit can be of commercial glass-making quality. The raw materials should be free of any impurities which will adversely affect the quality of the final porcelain. Particular attention should be directed to the amount, if any, of alkaline impurities in the raw materials. The raw materials can be the specific oxide required or a material which upon heating to the melting temperature employed in the glass making process, which is of the order of 1400 to 1500 degree C, will be converted to the desired oxide. Examples of the latter material include magnesium carbonate (extra light grade) and barium carbonate (extra light grade). The raw materials are weighed out based on the oxide content and the components are blended together. In order to facilitate further processing steps the mixture of materials should also be heat treated to remove any moisture. The mixture is then melted using conventional glass melting techniques. The raw materials are gradually heated to about 1400 to 1450 degree C. and then the resultant molten mass are maintained at this temperature with occasional stirring until a homogeneous melt is obtained. Typically, it has been found that a melt time of about one hour is sufficient depending upon the equipment and amount of material employed. The next step in the process is to convert the molten glass mass into a glass frit. Various well known methods can be employed for this process. It has been found that for the purposes of this invention it is preferable to pour the molten stream of glass over a water chilled stainless steel tray to result in broken pieces of solidified glass. The solidified glass frit is then crushed and the resulting particles are placed in a ball mill and dry milled with a grinding media until the particles are substantially uniform in size. Then the dried milled glass particles are subjected to a second grinding. This time along with certain mill additives which include precipitated silica, washed chromium oxide and cobalt oxide, water is added to the ball mill in an amount sufficient to form slurry with the glass particles. The slurry is tumbled in the ball mill with a grinding media for about 10 to 48 hours until the particle size of the glass is reduced to between 3 and 5 microns. The slurry is removed from the ball mill and additional water is added to dilute the slurry from between 40 and 60 weight percent of glass frit. This suspension is the stock solution (slip) used in subsequent coating steps. The glass frit of this invention can advantageously be used to form glass-ceramic coatings on various types of metal structures. The glass-ceramic coating of this invention is especially useful as coatings for high temperature protection of super alloy components which require a high degree of performance and reliability under adverse conditions. In practicing the invention, the surface of a super alloy component to be coated, must be clean, free from any impurity and oxide, may be coated with the slip in any conventional manner. The method we prefer is conventional vitreous enameling technique. In working with the barium silicate system coatings, it was observed that good adherence was obtained when the coating was fired in ambient atmosphere at a temperature of 1200°C. The glass powder coated metal body is then heated to a temperature of 1000°C to 1200°C. This softens the glass particles to a lower viscosity which spreads uniformly over the entire metal surface, reacts with it and produces a dense, smooth, impervious, well-formed continuous glass coating that is essentially amorphous. The glass-coated component is then heated to a somewhat lower temperature in the range of 820 to 840°C. This effects development of a glassceramic which forms a dense, adherent, strong, refractory, crystalline coating. A key feature of this process is the ability to control the heat treatment schedule, and thus the reproducibility of the coating process. In our co-pending patent application no. NF-31 / 03, we have described and claimed a synergistic composition for making thermally stable non-metallic coating on metals like cobalt or nickel base super alloys, which comprises 32.2 to 38.0 % (w/w) SiO2; 43.8 to 48.8 % (w/w) BaCO3l BaNO3 or mixture thereof; 0 to 6.8 % (w/w) CaCO3; 0 to 8.8 % (w/w) MgCO3; 0 to 4.7 % (w/w) ZnO; 0 to 5.1 % (w/w) bonding agent such as (NH4)3MoO4, MoO3and 0 to 10.1 % (w/w) H3B03. The SiOa used may be taken as quartz powder, quartz sand powder (-100 mesh). Accordingly, the present invention provides an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates which comprises: weighing, mixing and sieving ingredients in the range of 32.2 to 38.0 % (w/w) Si02; 43.8 to 48.8 % (w/w) BaCO3, BaNO3 or mixture thereof; 0 to 6.8 % (w/w) CaCO3; 0 to 8.8 % (w/w) MgC03; 0 to 4.7 % (w/w) ZnO; 0 to 5.1 % (w/w) bonding agent such as (NH4)3Mo04, Mo03and 0 to 10.1 % (w/w) H3B03\|to obtain a glass forming batch, melting under stirring the said glass forming batch at a temperature in the range of 1400°C to 1450°C for a period in the range of 2 to 3 hours to obtain molten glass, quenching the molten glass and comminuting to obtain glass frit powder, mixing the glass frit with mill additions essentially consisting of washed chromium oxide in the range of 5.0 to 45.0% w/w, precipitated silica in the range of 0 to 5.0% w/w, china clay in the range of 0 to 10.0% w/w and cobalt oxide in the range of 0 to 1.0% w/w in an aqueous medium to obtain a non-metallic coating slurry. In an embodiment of the present invention quenched molten glass is comminuted by known methods to obtain glass frit powder of the order of 44 microns. In another embodiment of the present invention the mixing of the glass frit with mill additions in an aqueous medium to obtain a slurry is carried out in a ball mill with grinding media for about 10 to 48 hours to obtain particle size in the range of 3 to 5 microns. In a further embodiment of the present invention there is provided a process of coating metallic substrates with the non-metallic coating slurry herein above obtained, which comprises: diluting the coating slurry by adding water in the range of 40 to 60 weight percent of glass frit, applying the liquid coating slurry onto a pre-cleaned surface of a desired metal substrate to obtain a coated substrate, firing the said coated substrate at a temperature in the range of 1000°C to 1200°C over a period in the range of 5 to 30 minutes to obtain a coated fired body, heat treating the coated fired body at a temperature in the range of 820°C to 840°C for a period in the range of 1.0 to 8.0 hours. In a still further embodiment of the present invention the pre-cleaning of the metal substrate is done by known metal cleaning process to remove impurities such as oxides, oil or grease, dirt, and other contaminants. In a yet further embodiment of the present invention the application of the liquid coating slurry onto a pre-cleaned surface of a desired metal substrate is carried out by any conventional method, preferably conventional vitreous enameling technique. In another embodiment of the present invention the metal substrate is such as stainless steel, nickel base super alloys, cobalt base super alloys, Inconel, and Nimonic alloys. The novelty of the present invention resides in providing an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates, wherein the coating is defect free, thermal shock resistant, highly adherent and suitable for continuous use at temperatures in the range of 1000°C to 1050°C in oxidizing environment. The present invention provides a coating for metallic substrates which is a very effective oxygen barrier over the surface of a nickel or cobalt base super alloy or stainless steel substrate. The barrier is in the form of a coating comprising a barium magnesium silicate based glass-ceramic material. The present coating is continuous, impervious, and free from all types of enameling defects such as holes, fish scales, hairlines, blisters, cracks or thin spots and resists spelling or chipping during thermal cycling. The novelty has been achieved in the present invention by the non-obvious inventive steps of providing high temperature viscosity to the glass-ceramic coating by mixing refractory material, a novel blend of alkaline earth metal oxides such as herein described, in the amorphous form which increases high temperature viscosity of the resultant glass-ceramic coating. In this improved process the partially crystallized coating composition comprises of a mixture, based on its oxide content, of barium oxide, calcium oxide, magnesium oxide, silicon di-oxide, boron tri-oxide and molybdenum oxide. In another inventive step the coating materials are further processed with mill additions which are essentially a mixture of washed chromium oxide, precipitated silica, cobalt oxide and china clay in water and applied to pre-cleaned metal surface. Yet another inventive step resides in the key feature of this process which is the ability to control the heat treatment schedule, and thus the reproducibility of the coating process. In this inventive step the glass powder coated metal body is fired at a temperature of 1000°C to 1200°C to provide a glass-ceramic coating on the metal. This softens the glass particles to a lower viscosity which spreads uniformly over the entire metal surface, reacts with it and produces a dense, smooth, impervious, well-formed continuous glass coating that is essentially amorphous. The glass-coated component is then heated to a somewhat lower temperature in the range of 820 to 840°C. This effects development of a glassceramic which forms a dense, adherent, strong, refractory, crystalline coating. The resultant coating fuses smoothly in the required firing temperature and produces a uniform microstructure and eliminates possibility of generating hot spots during application. In particular, these amorphous additives permit the glass to soften and flow smoothly over the metal surface into a continuous, glassy coating before viscosity of the system increases considerably due to refractory material addition. This inventive finding is a key point to producing a high temperature protective coating on super alloy substrates. Thus in the present invention the novel blend of alkaline earth metal oxides produces more thermally stable glassy coating on the super alloy surface and at the same time provides a novel glass ceramic coating having different set of crystalline phases in the resultant coating microstructure. The present glassceramic contains barium magnesium silicate as the principal phase along with barium silicate as a minor phase. The resultant coating is defect free, thermal shock resistant, highly adherent and suitable for continuous use at temperatures in the range of 1000°C to 1050°C in oxidizing environment. Stepwise details of the improved process of the present invention for the manufacture of thermally stable non-metallic coating for metallic substrates are given as below: 1. A glass forming batch of pre-determined composition which comprises of 32.2 to 38.0 % (w/w) SiO2, 43.8 to 48.8 % (w/w) BaCO3, 0 to 6.8 % (w/w) CaCO3, 0 to 8.8 % (w/w) MgCO3, 0 to 4.7 % (w/w) ZnO, 0 to 5.1 % (w/w) (NH4)3Mo04 and 0 to 10.1 % (w/w) H3BO3is weighed, mixed and sieved. 2. The above said glass-forming batch is melted under stirring at a temperature in the range of 1400°C to 1450°C for a period in the range of 2 to 3 hours to obtain molten glass. 3. The melt is cooled rapidly in air and the solid formed is comminuted to a fine powder or frit; 4. A liquid slurry is prepared by milling the frit with mill additions which comprises washed chromium oxide in the range of 5.0 to 45.0% w/w, precipitated silica in the range of 0 to 5.0% w/w, china clay in the range of 0 to 10.0% w/w and cobalt oxide in the range of 0 to 1.0% w/w. in water. The slurry is tumbled in the ball mill with a grinding media for about 10 to 48 hours until the particle size of the glass is reduced to between 3 and 5 microns. 5. Diluting the coating slurry by adding water in the range of 40 to 60 weight percent of glass frit. The slurry is applied as a coating onto a surface of a precleaned metal substrate. 6. The coated substrate is fired for 5 to 30 minutes at temperatures in the range of 1000°C to 1200°C to fuse the frit particles together into an integral, essentially non-porous, vitreous coating and subsequently heat treated in the temperature range of 820°C to 840°C for a period in the range of 1.0 to 8.0 hours to cause crystallization in situ to take place therein. The following examples are given by way of illustration of the actual practice of the improved process of the present invention for the manufacture of thermally stable non-metallic coating for metallic substrates. Therefore, these illustrative examples should not be construed to limit the scope of the present invention. EXAMPLE-1 Raw batch materials were mixed in amounts calculated to provide a glass having a weight percent analysis of: Raw materials Si02 BaC03 CaC03 MgC03 ZnO (NH4)3MoO4 H3B03 Parts by weight 32.25 48.77 6.69 2.27 0.00 0.00 10.02 The resulting batch was melted and then quenched in air to produce a frit. The ground frit (-100 mesh) was milled for a period of 10 hours in water with appropriate mill additions as detailed below, including precipitated silica, washed chromium oxide, china clay and cobalt oxide to keep the glass particles in suspension and to form an enamel slip. Mill additions Frit Washed chromium oxide Precipitated silica China clay Cobalt oxide Water Parts by weight 100 5.0 0.0 5.0 0.0 50.0 The slip was spray applied to a nickel base super alloy substrate, the surface of which had been previously blasted with abrasive particles and degreased. The dried article was then heated to the fusion temperature of the coating material, about 1000°C thereby forming a glossy green amorphous coating. The coating was finally heat treated at a temperature of 820°C for 8 hours whereby the colour becomes somewhat dull green and abrasion resistance of the resultant coating increases by a factor of 2. EXAMPLE-2 Raw materials were mixed in amounts calculated to provide a glass having a weight percent analysis of: Raw materials (Figure removed) The resulting batch was melted and then quenched in air to produce a frit. The ground frit (-100 mesh) was milled for a period of 40 hours in water with appropriate mill additions as detailed below, including precipitated silica, washed chromium oxide, china clay and cobalt oxide to keep the glass particles in suspension and to form an enamel slip. Mill additions Frit Washed chromium oxide Precipitated silica China clay Cobalt oxide Water Parts by weight (Figure removed) The slip was spray applied to a nickel base super alloy substrate, the surface of which had been previously blasted with abrasive particles and degreased. The dried article was then heated to the fusion temperature of the coating material, about 1160°C, thereby forming an amorphous glossy green coating and subsequently heat treated at a temperature of about 820°C for one hour, the resultant coating become partially crystallized and losses some of its gloss and become more abrasion resistant. EXAMPLE-3 Raw materials were mixed in amounts calculated to provide a glass having a weight percent analysis of: Raw materials (Figure removed) The resulting batch was melted and then quenched in air to produce a frit. The ground frit (-100 mesh) was milled for a period of 48 hours in water with appropriate mill additions, including precipitated silica, washed chromium oxide, china clay and cobalt oxide to keep the glass particles in suspension and to form an enamel slip. Mill additions Frit Washed chromium oxide Precipitated silica China clay Cobalt oxide Water Parts by weight 100 5.0 3.0 0.0 0.5 50.0 The slip was spray applied to a nickel base super alloy substrate, the surface of which had been previously blasted with abrasive particles and degreased. The dried article was then heated to the fusion temperature of the coating material, about 1200°C, thereby forming an amorphous glossy green coating and subsequently heat treated at a temperature of about 820°C for 8 hours whereby the coating become a glass-ceramic and its colour become somewhat dull and abrasion resistance of the resultant coating increases by a factor of 2. EXAMPLE-4 Raw materials were mixed in amounts calculated to provide a glass having a weight percent analysis of: Raw materials (Figure removed) The resulting batch was melted and then quenched in air to produce a frit. The ground frit (-100 mesh) was milled in water for a period of 45 hours with appropriate mill additions, including precipitated silica, washed chromium oxide, china clay and cobalt oxide to keep the glass particles in suspension and to form an enamel slip. Mill additions Frit Washed chromium oxide Precipitated silica China clay Cobalt oxide Water Parts by weight The slip was spray applied to a nickel base super alloy substrate, the surface of which had been previously blasted with abrasive particles and degreased. The dried article was then heated to the fusion temperature of the coating material, about 1160°C, thereby forming an amorphous green, glossy coating and subsequently heat treated at a temperature of about 820°C for 8 hours whereby the colour become somewhat dull and abrasion resistance of the resultant coating improves by a factor of 2. EXAMPLE-5 Raw materials were mixed in amounts calculated to provide a glass having a weight percent analysis of: (Figure removed) The resulting batch was melted and then quenched in air to produce a frit. The ground frit (-100 mesh) was milled in water for a period of 48 hours with appropriate mill additions, including precipitated silica, washed chromium oxide, china clay and cobalt oxide to keep the glass particles in suspension and to form an enamel slip. Mill additions Frit Washed chromium oxide Precipitated silica China clay Cobalt oxide Water Parts by weight The slip was spray applied to a nickel base super alloy substrate, the surface of which had been previously blasted with abrasive particles and degreased. The dried article was then heated to the fusion temperature of the coating material, about 1200°C, thereby forming an amorphous glossy green coating and subsequently heat treated at a temperature of about 820°C for 2 hours whereby the colour become somewhat dull and abrasion resistance of the resultant coating remains almost unaffected. The resultant glass-ceramic coating retains all the functional properties on prolonged exposure to high temperature. EXAMPLE-6 Raw batch materials were mixed in amounts calculated to provide a glass having a weight percent analysis of: Raw materials (Figure removed) The resulting batch was melted and then quenched in air to produce a frit. The ground frit (-100 mesh) was milled in water for a period of 15 hours in water with appropriate mill additions as detailed below, including precipitated silica, washed chromium oxide, china clay and cobalt oxide to keep the glass particles in suspension and to form an enamel slip. Mill additions Frit Washed chromium oxide Precipitated silica China clay Cobalt oxide Water Parts by weight 100 5.0 5.0 0.0 1.0 50.0 The slip was spray applied to a nickel base super alloy substrate, the surface of which had been previously blasted with abrasive particles and degreased. The dried article was then heated to the fusion temperature of the coating material, about 1200°C thereby forming an amorphous coating. The coating was finally heat treated at a temperature of 840°C for 8 hours whereby the colour becomes somewhat dull and abrasion resistance of the resultant coating increases significantly. The major crystalline phase formed in the above coatings is barium magnesium silicate, along with barium silicate and chromium oxide as minor phases. The abrasion resistance of the coatings expressed in terms of loss in weight value is in the range of 2.0 to 6.0 mg/cm2 per 50,000 cycles when tested in a P. E. I. abrasion tester. The main advantages of the improved process of present invention for the manufacture of thermally stable non-metallic coating for metallic substrates and a process of coating thereof are: 1. Preparation of a new high temperature resistant coating material with superior set of oxidation and hot corrosion resistance. 2. The present process results in a more adherent coating. 3. The present process controls the high temperature viscosity of the molten coating material by using a novel set of mill additions obtaining a defect free impervious continuous glass-ceramic coating. 4. The present process results in a coating having superior thermal endurance at temperature of the order of 1000°C. 5. The present process results in a coating that provides a useful degree of thermal insulation to the base metal substrate. We claim: 1. An improved process for the manufacture of thermally stable non-metallic coating for metallic substrates which comprises: weighing, mixing and sieving ingredients in the range of 32.2 to 38.0 % (w/w) SiO2; 43.8 to 48.8 % (w/w) BaCO3, BaNO3 or mixture thereof; 0 to 6.8 % (w/w) CaCO3; 0 to 8.8 % (w/w) MgCO3; 0 to 4.7 % (w/w) ZnO; 0 to 5.1 % (w/w) bonding agent such as (NH4)3MoO4, MoO3and 0 to 10.1 % (w/w) H3B03,to obtain a glass forming batch, melting under stirring the said glass forming batch at a temperature in the range of 1400°C to 1450°C for a period in the range of 2 to 3 hours to obtain molten glass, quenching the molten glass and comminuting to obtain glass frit powder, mixing the glass frit with mill additions essentially consisting of washed chromium oxide in the range of 5.0 to 45.0% w/w, precipitated silica in the range of 0 to 5.0% w/w, china clay in the range of 0 to10.0% w/w and cobalt oxide in the range of 0 to 1.0% w/w in an aqueous medium to obtain a non-metallic coating slurry. 2. An improved process as claimed in claim 1, wherein the quenched molten glass is comminuted by known methods to obtain glass frit powder of the order of 44 microns. 3. An improved process as claimed in claim 1-2, wherein the mixing of the glass frit with mill additions in an aqueous medium to obtain a slurry is carried out in a ball mill with grinding media for about 10 to 48 hours to obtain particle size in the range of 3 to 5 microns. 4. A process of coating metallic substrates with the non-metallic coating slurry as prepared by the improved process as claimed in claim 1-3, which comprises: diluting the coating slurry by adding water in the range of 40 to 60 weight percent of glass frit, applying the liquid coating slurry onto a pre-cleaned surface of a desired metal substrate to obtain a coated substrate, firing the said coated substrate at a temperature in the range of 1000°C to 1200°C over a period in the range of 5 to 30 minutes to obtain a coated fired body, heat treating the coated fired body at a temperature in the range of 820°C to 840°C for a period in the range of 1 .0 to 8.0 hours. 5. A process of coating metallic substrates as claimed in claim 4, wherein the pre-cleaning of the metal substrate is done by known metal cleaning process to remove impurities such as oxides, oil or grease, dirt, and other contaminants. 6. A process of coating metallic substrates as claimed in claim 4-5, wherein the application of the liquid coating slurry onto a pre-cleaned surface of a desired metal substrate is carried out by any conventional method, preferably conventional vitreous enameling technique. 7. A process of coating metallic substrates as claimed in claim 4-6, wherein the metal substrate is such as stainless steel, nickel base super alloys, cobalt base super alloys, Inconel, and Nimonic alloys. 8. An improved process for the manufacture of thermally stable non-metallic coating for metallic substrates, substantially as herein described with reference to the examples. 9. A process of coating metallic substrates with the non-metallic coating, substantially as herein described with reference to the examples.

Full Text

The present invention relates to an improved process for the manufacture of
thermally stable non-metallic coating for metallic substrates and a process of
coating thereof. The present invention particularly relates to an improved process
for the manufacture of thermally stable non-metallic coating, for metallic
substrates such as super alloy components, starting from from a novel
synergistic composition as described and claimed in our co-pending patent
application no. NF-31 / 03 and a process of coating metallic substrates with the
novel thermally stable non-metallic coating. The present invention more
particularly relates to an improved process for the manufacture of thermally
stable non-metallic coating comprising a glass-ceramic selected from the barium
magnesium silicate system by a novel processing technique.
The thermally stable non-metallic coating of the present invention over the
surface of the metal or alloy substrate serves as an oxidation resistant coating to
prevent oxygen attack of the metal at elevated temperatures and also as a
thermal barrier to the metal surface to prevent rapid heat up of the metal surface.
There is a well recognized need, in hot zone components of turbine engines and
heat exchangers, for example, for materials capable of withstanding continuous
operating temperatures in excess of 1000°C. While a variety of high temperature
materials are known, it is common practice to use high temperature stable alloys
known as super alloys in the more severe applications. Characteristically, these
alloys are rich in nickel, cobalt, chromium, or iron, with nickel or cobalt being the
principal metal bases. These super alloys have been distinguished from other
high temperature resistant materials on the basis of being sufficiently resistant to
oxidation to permit operation in an oxidizing atmosphere without a protective
coating. Nevertheless, under extreme hostile conditions, such as encountered by
hot zone components in gas turbine engines, even the super alloys tend to
deteriorate unless protected by a oxygen barrier coating.
A very well known method of protecting metal components from oxidation at
elevated temperatures in a hostile environment is to apply a continuous
monolithic glass coating on the exposed surface. This coating completely
encapsulates and isolates the metallic surface from the surrounding oxygen
containing atmosphere. However, viscous flow of the glass coating may occur
when large surface stresses develop during high temperature use, resulting in
thin spots and catastrophic coating failure.
Another method of achieving higher viscosity is by mixing crystalline materials as
second phase with the glass frits prior to application of the coating. However,
these glass-crystalline mixtures sinter rather non-uniformly, the crystal size and
homogeneity of the resultant microstructure being very difficult to control. Certain
portions of the substrate, therefore, tend to be entirely free from crystals,
whereas other portions have too many (or too large) crystals to sinter well. A void
free coating with this heterogeneous glass-crystal mixture is thus difficult to
obtain.
References may be made to the following U. S. Patents:
No. 3,397,076 (Little et al.) describes fused crystallizable ground and cover coats
for high temperature alloys in which the major elements are cobalt, nickel,
chromium, iron or mixtures. The ground coat is lithium free and contains 35-65%
Si02 and12-45% BaO. Examples also contain substantial amounts of R20, B2C3
and /or TiO2. The drawbacks are use of two separate coating systems, one
ground coat and other cover coat. The coating composition contains substantial
amount of alkali oxides which is normally unsuitable for continuous use at high
operating temperature.
No. 3,467,534 (MacDowell) discloses glass-ceramic articles consisting
essentially of 20-70% BaO and 30-80% SiO2 and having a barium silicate
principal crystal phase. A preferred example is described as considered for
coating metals. This article is related to glass-ceramic objects not applicable to
coatings.
No. 3,531,303 (Bahat) discloses glass-ceramic articles in the alkaline earth
aluminosilicate field wherein hexagonal alkaline earth feldspar or a triclinic form
is the principal crystal phase. The materials are highly refractory with service
temperatures up to 1700°C and consist essentially of 12-53% SiO2, 17-55% RO
where RO is 17-50% SrO and 25-50% BaO, 10-58% AI2O3 and a nucleating
agent. The drawback of the process is same as above, that is it relates to glassceramic
objects and is not applicable to coatings.
No. 3,578,470 (Bahat) discloses glass-ceramic materials in the BaO-AI2O3-Si02
composition field nucleated with Ta205 and/or Nb205 that are especially suited to
sealing with tungsten or molybdenum and their alloys. This method describes a
sealing material and is not applicable to coatings.
No. 3,837,978 (Busdiecker) discloses barium aluminosilicate glass-ceramics
nucleated by tin oxide, having a hexacelsian primary crystal phase, and having a
coefficient of thermal expansion in the range of 50-170X10"7/°C.
Nos. 4,256,796; 4,358,541 and 4,385,127 disclose mixed barium magnesium
and calcium magnesium silicate coatings, fluxed with B2Os. The drawback in the
high temperature resistance of these coatings is the use of B2O3. A high B2O3
residual glass (or even a borate crystal) tends to allow the microstructure to
move and flow at temperatures much below the solidus of the primary refractory
silicate phases.
Gas turbine engines and heat exchangers operate in a hostile environment at
operating temperatures in excess of 1000°C. There is a considerable demand for
materials capable of withstanding those severe working conditions. While a
variety of high temperature materials are known, it is a common practice to use
certain" nickel or cobalt based super alloys in the more severe applications.
These super alloys have been distinguished from other high temperature
materials on the basis of being considerably resistant to oxidation to permit
continuous operation in an oxidizing atmosphere without a protective coating.
Nevertheless, under more severe conditions, such as encountered by hot zone
components in gas turbine engines in an advanced aircraft, even the super alloys
tend to deteriorate very fast unless protected by a suitable coating.
A well known technique of protecting the super alloys from oxidation at elevated
temperatures is to apply an impervious stable continuous monolithic glassy
coating. This completely encapsulates and isolates the material from the
.surrounding hostile environment. However, viscous flow of the glassy coating
material may occur when large surface stresses develop during high temperature
use. In some cases generation of hot spots occurs which led to premature
catastrophic failure of the coating as well as the component.
The viscosity being a structural property of the system, can be easily tailored by
mixing certain refractory materials with the glassy coating materials (frits) during
processing (milling), before application of the coating on metal surface..
However, these glass- crystalline mixtures are very difficult to fuse at the required
firing temperature and sinter rather non-uniformly. The microstructure and
homogeneity of the coating is very difficult to control. Certain portion of the
resultant coating generates hot spots which may cause premature failure. A void
free coating with this heterogeneous coating system is thus very difficult to
obtain.
From the above referred hitherto known prior art disclosures it is clear that there
is a definite need for providing an improved process for the manufacture of
thermally stable non-metallic coating for metallic substrates such as nickel or
cobalt base super alloy and stainless steel components that are required to
operate at temperatures above 1000°C.
The main object of the present invention is to provide an improved process for
the manufacture of thermally stable non-metallic coating for metallic substrates
and a process of coating thereof, which obviates the drawbacks as detailed
above.
Another object of the present invention is to provide an improved process for the
manufacture of thermally stable non-metallic coating for metallic substrates such
as super alloy components and a process of coating thereof.
Still another object of the present invention is to provide an improved process for
obtaining a coating material with superior set of oxidation and hot corrosion
resistance properties by following a novel composition and novel processing
techniques.
Yet another object of the present invention is to provide an improved process for
the manufacture of thermally stable non-metallic coating for metallic substrates
starting from a novel synergistic composition as described and claimed in our copending
patent application no. NF-31 / 03.
Still yet another object of the present invention is to provide an improved process
for obtaining a coating material by using a combination of alkaline earth metal
oxides such as BaO, CaO and MgO instead of BaO alone.
A further object of the present invention is to provide an improved process for
obtaining a coating material by using a bonding oxide (MoOa) to promote
adherence of the coating to the base metal substrate.
A still further object of the present invention is to provide an improved process for
obtaining a coating containing BaMgSi04 crystal phase as the major crystalline
constituent of the coating.
Another object of the present invention is to provide an improved process for
obtaining a coating material which can be melted in a ceramic crucible at a
temperature in the range of 1400 to 1450 degree C in an ambient atmosphere.
Yet another object of the present invention is to provide an improved process of
coating metallic substrates such as super alloy components which obviates the
drawbacks as detailed above.
Still another object of the present invention is to provide an improved process of
coating metallic substrates such as super alloy components that is more
effective, easier to apply, than previously known coatings of the prior art.
Still yet another object of the present invention is to provide an improved process
for the manufacture of thermally stable non-metallic coating for obtaining a
reliable and reproducible high temperature protective coating for nickel or cobalt
base super alloy and stainless steel substrates that are required to operate at
temperatures above 1000°C.
A further object of the present invention is to provide an improved process for
obtaining a high temperature protective coating that adheres strongly and resists
spelling during thermal cycling.
A still further object of the present invention is to provide an improved process for
obtaining a high temperature protective coating technique that exhibits the
excellent flow characteristics of a glass coating as it is fired in one temperature
range, and becomes resistant to flow due to subsequent crystallization as it is
heated in the required operating temperature range.
Another object of the present invention is to provide an improved process for
obtaining a coating that provides a useful degree of thermal insulation to the
base metal surface.
In the present invention there is provided an improved process for the
manufacture of thermally stable, high temperature resistant, non-metallic, glass
ceramic coating material suitable for metallic substrates and a process of
application of the coating on such substrates as nickel and cobalt based super
alloys and stainless steel. In this improved process the partially crystallized
coating composition comprises of a mixture, based on its oxide content, of
barium oxide, calcium oxide, magnesium oxide, silicon di-oxide, boron tri-oxide
and molybdenum oxide. The coating materials are further processed with a
mixture of washed chromium oxide, precipitated silica, cobalt oxide and china
clay in water and applied to pre-cleaned metal surface and fired to provide a
glass-ceramic coating on the metal. The coating is defect free, thermal shock
resistant, highly adherent and suitable for continuous use at temperatures in the
range of 1000°C to 1050°C in oxidizing environment.
The method for preparing and applying the novel inventive coatings of the
present invention involves five fundamental steps:
First, a glass-forming batch of a predetermined formulation is melted;
Second, the melt is cooled to a glass and the glass comminuted to a fine powder
or frit;
Third, a liquid slurry is prepared of the frit;
Fourth, the slurry is applied as a coating onto a surface of a desired substrate;
and
Fifth, the coated substrate is fired to temperatures of at least 1200°C to fuse the
frit particles together into an integral, essentially non-porous, vitreous coating and
subsequently heat treated to cause crystallization in situ to take place therein.
In general, to insure adequate flow of the glass during sintering, thereby
providing an essentially "pinhole-free" coating, and to develop a high volume of
crystallinity, the frit will desirably be powdered to particles passing a 325 United
States Standard Sieve (44 microns). Aqueous slurry can be employed for
coating.
The initial step in the preparation of the porcelains of this invention is to prepare
a glass frit having the desired composition. The raw materials which are used in
the preparation of the glass frit can be of commercial glass-making quality. The
raw materials should be free of any impurities which will adversely affect the
quality of the final porcelain. Particular attention should be directed to the
amount, if any, of alkaline impurities in the raw materials. The raw materials can
be the specific oxide required or a material which upon heating to the melting
temperature employed in the glass making process, which is of the order of 1400
to 1500 degree C, will be converted to the desired oxide. Examples of the latter
material include magnesium carbonate (extra light grade) and barium carbonate
(extra light grade). The raw materials are weighed out based on the oxide
content and the components are blended together. In order to facilitate further
processing steps the mixture of materials should also be heat treated to remove
any moisture.
The mixture is then melted using conventional glass melting techniques. The raw
materials are gradually heated to about 1400 to 1450 degree C. and then the
resultant molten mass are maintained at this temperature with occasional stirring
until a homogeneous melt is obtained. Typically, it has been found that a melt
time of about one hour is sufficient depending upon the equipment and amount of
material employed.
The next step in the process is to convert the molten glass mass into a glass frit.
Various well known methods can be employed for this process. It has been found
that for the purposes of this invention it is preferable to pour the molten stream of
glass over a water chilled stainless steel tray to result in broken pieces of
solidified glass. The solidified glass frit is then crushed and the resulting particles
are placed in a ball mill and dry milled with a grinding media until the particles are
substantially uniform in size.
Then the dried milled glass particles are subjected to a second grinding. This
time along with certain mill additives which include precipitated silica, washed
chromium oxide and cobalt oxide, water is added to the ball mill in an amount
sufficient to form slurry with the glass particles. The slurry is tumbled in the ball
mill with a grinding media for about 10 to 48 hours until the particle size of the
glass is reduced to between 3 and 5 microns. The slurry is removed from the ball
mill and additional water is added to dilute the slurry from between 40 and 60
weight percent of glass frit. This suspension is the stock solution (slip) used in
subsequent coating steps.
The glass frit of this invention can advantageously be used to form glass-ceramic
coatings on various types of metal structures. The glass-ceramic coating of this
invention is especially useful as coatings for high temperature protection of super
alloy components which require a high degree of performance and reliability
under adverse conditions.
In practicing the invention, the surface of a super alloy component to be coated,
must be clean, free from any impurity and oxide, may be coated with the slip in
any conventional manner. The method we prefer is conventional vitreous
enameling technique.
In working with the barium silicate system coatings, it was observed that good
adherence was obtained when the coating was fired in ambient atmosphere at a
temperature of 1200°C.
The glass powder coated metal body is then heated to a temperature of 1000°C
to 1200°C. This softens the glass particles to a lower viscosity which spreads
uniformly over the entire metal surface, reacts with it and produces a dense,
smooth, impervious, well-formed continuous glass coating that is essentially
amorphous. The glass-coated component is then heated to a somewhat lower
temperature in the range of 820 to 840°C. This effects development of a glassceramic
which forms a dense, adherent, strong, refractory, crystalline coating. A
key feature of this process is the ability to control the heat treatment schedule,
and thus the reproducibility of the coating process.
In our co-pending patent application no. NF-31 / 03, we have described and
claimed a synergistic composition for making thermally stable non-metallic
coating on metals like cobalt or nickel base super alloys, which comprises 32.2 to
38.0 % (w/w) SiO2; 43.8 to 48.8 % (w/w) BaCO3l BaNO3 or mixture thereof;
0 to 6.8 % (w/w) CaCO3; 0 to 8.8 % (w/w) MgCO3; 0 to 4.7 % (w/w) ZnO; 0 to 5.1
% (w/w) bonding agent such as (NH4)3MoO4, MoO3and 0 to 10.1 % (w/w) H3B03.
The SiOa used may be taken as quartz powder, quartz sand powder (-100
mesh).
Accordingly, the present invention provides an improved process for the
manufacture of thermally stable non-metallic coating for metallic substrates which
comprises: weighing, mixing and sieving ingredients in the range of 32.2 to 38.0
% (w/w) Si02; 43.8 to 48.8 % (w/w) BaCO3, BaNO3 or mixture thereof; 0 to
6.8 % (w/w) CaCO3; 0 to 8.8 % (w/w) MgC03; 0 to 4.7 % (w/w) ZnO; 0 to 5.1 %
(w/w) bonding agent such as (NH4)3Mo04, Mo03and 0 to 10.1 % (w/w) H3B03|to
obtain a glass forming batch, melting under stirring the said glass forming batch
at a temperature in the range of 1400°C to 1450°C for a period in the range of 2
to 3 hours to obtain molten glass, quenching the molten glass and comminuting
to obtain glass frit powder, mixing the glass frit with mill additions essentially
consisting of washed chromium oxide in the range of 5.0 to 45.0% w/w,
precipitated silica in the range of 0 to 5.0% w/w, china clay in the range of 0 to
10.0% w/w and cobalt oxide in the range of 0 to 1.0% w/w in an aqueous
medium to obtain a non-metallic coating slurry.
In an embodiment of the present invention quenched molten glass is
comminuted by known methods to obtain glass frit powder of the order of 44
microns.
In another embodiment of the present invention the mixing of the glass frit with
mill additions in an aqueous medium to obtain a slurry is carried out in a ball mill
with grinding media for about 10 to 48 hours to obtain particle size in the range of
3 to 5 microns.
In a further embodiment of the present invention there is provided a process of
coating metallic substrates with the non-metallic coating slurry herein above
obtained, which comprises: diluting the coating slurry by adding water in the
range of 40 to 60 weight percent of glass frit, applying the liquid coating slurry
onto a pre-cleaned surface of a desired metal substrate to obtain a coated
substrate, firing the said coated substrate at a temperature in the range of
1000°C to 1200°C over a period in the range of 5 to 30 minutes to obtain a
coated fired body, heat treating the coated fired body at a temperature in the
range of 820°C to 840°C for a period in the range of 1.0 to 8.0 hours.
In a still further embodiment of the present invention the pre-cleaning of the metal
substrate is done by known metal cleaning process to remove impurities such as
oxides, oil or grease, dirt, and other contaminants.
In a yet further embodiment of the present invention the application of the liquid
coating slurry onto a pre-cleaned surface of a desired metal substrate is carried
out by any conventional method, preferably conventional vitreous enameling
technique.
In another embodiment of the present invention the metal substrate is such as
stainless steel, nickel base super alloys, cobalt base super alloys, Inconel, and
Nimonic alloys.
The novelty of the present invention resides in providing an improved process for
the manufacture of thermally stable non-metallic coating for metallic substrates,
wherein the coating is defect free, thermal shock resistant, highly adherent and
suitable for continuous use at temperatures in the range of 1000°C to 1050°C in
oxidizing environment. The present invention provides a coating for metallic
substrates which is a very effective oxygen barrier over the surface of a nickel or
cobalt base super alloy or stainless steel substrate. The barrier is in the form of a
coating comprising a barium magnesium silicate based glass-ceramic material.
The present coating is continuous, impervious, and free from all types of
enameling defects such as holes, fish scales, hairlines, blisters, cracks or thin
spots and resists spelling or chipping during thermal cycling.
The novelty has been achieved in the present invention by the non-obvious
inventive steps of providing high temperature viscosity to the glass-ceramic
coating by mixing refractory material, a novel blend of alkaline earth metal oxides
such as herein described, in the amorphous form which increases high
temperature viscosity of the resultant glass-ceramic coating. In this improved
process the partially crystallized coating composition comprises of a mixture,
based on its oxide content, of barium oxide, calcium oxide, magnesium oxide,
silicon di-oxide, boron tri-oxide and molybdenum oxide. In another inventive step
the coating materials are further processed with mill additions which are
essentially a mixture of washed chromium oxide, precipitated silica, cobalt oxide
and china clay in water and applied to pre-cleaned metal surface. Yet another
inventive step resides in the key feature of this process which is the ability to
control the heat treatment schedule, and thus the reproducibility of the coating
process. In this inventive step the glass powder coated metal body is fired at a
temperature of 1000°C to 1200°C to provide a glass-ceramic coating on the
metal. This softens the glass particles to a lower viscosity which spreads
uniformly over the entire metal surface, reacts with it and produces a dense,
smooth, impervious, well-formed continuous glass coating that is essentially
amorphous. The glass-coated component is then heated to a somewhat lower
temperature in the range of 820 to 840°C. This effects development of a glassceramic
which forms a dense, adherent, strong, refractory, crystalline coating.
The resultant coating fuses smoothly in the required firing temperature and
produces a uniform microstructure and eliminates possibility of generating hot
spots during application. In particular, these amorphous additives permit the
glass to soften and flow smoothly over the metal surface into a continuous,
glassy coating before viscosity of the system increases considerably due to
refractory material addition. This inventive finding is a key point to producing a
high temperature protective coating on super alloy substrates.
Thus in the present invention the novel blend of alkaline earth metal oxides
produces more thermally stable glassy coating on the super alloy surface and at
the same time provides a novel glass ceramic coating having different set of
crystalline phases in the resultant coating microstructure. The present glassceramic
contains barium magnesium silicate as the principal phase along with
barium silicate as a minor phase. The resultant coating is defect free, thermal
shock resistant, highly adherent and suitable for continuous use at temperatures
in the range of 1000°C to 1050°C in oxidizing environment.
Stepwise details of the improved process of the present invention for the
manufacture of thermally stable non-metallic coating for metallic substrates are
given as below:
1. A glass forming batch of pre-determined composition which comprises of 32.2
to 38.0 % (w/w) SiO2, 43.8 to 48.8 % (w/w) BaCO3, 0 to 6.8 % (w/w) CaCO3, 0 to
8.8 % (w/w) MgCO3, 0 to 4.7 % (w/w) ZnO, 0 to 5.1 % (w/w) (NH4)3Mo04 and 0 to
10.1 % (w/w) H3BO3is weighed, mixed and sieved.
2. The above said glass-forming batch is melted under stirring at a temperature
in the range of 1400°C to 1450°C for a period in the range of 2 to 3 hours to
obtain molten glass.
3. The melt is cooled rapidly in air and the solid formed is comminuted to a fine
powder or frit;
4. A liquid slurry is prepared by milling the frit with mill additions which comprises
washed chromium oxide in the range of 5.0 to 45.0% w/w, precipitated silica in
the range of 0 to 5.0% w/w, china clay in the range of 0 to 10.0% w/w and cobalt
oxide in the range of 0 to 1.0% w/w. in water. The slurry is tumbled in the ball mill
with a grinding media for about 10 to 48 hours until the particle size of the glass
is reduced to between 3 and 5 microns.
5. Diluting the coating slurry by adding water in the range of 40 to 60 weight
percent of glass frit. The slurry is applied as a coating onto a surface of a precleaned
metal substrate.
6. The coated substrate is fired for 5 to 30 minutes at temperatures in the range
of 1000°C to 1200°C to fuse the frit particles together into an integral, essentially
non-porous, vitreous coating and subsequently heat treated in the temperature
range of 820°C to 840°C for a period in the range of 1.0 to 8.0 hours to cause
crystallization in situ to take place therein.
The following examples are given by way of illustration of the actual practice of
the improved process of the present invention for the manufacture of thermally
stable non-metallic coating for metallic substrates. Therefore, these illustrative
examples should not be construed to limit the scope of the present invention.
EXAMPLE-1
Raw batch materials were mixed in amounts calculated to provide a glass having
a weight percent analysis of:
Raw materials
Si02
BaC03
CaC03
MgC03
ZnO
(NH4)3MoO4
H3B03
Parts by weight
32.25
48.77
6.69
2.27
0.00
0.00
10.02
The resulting batch was melted and then quenched in air to produce a frit. The
ground frit (-100 mesh) was milled for a period of 10 hours in water with
appropriate mill additions as detailed below, including precipitated silica, washed
chromium oxide, china clay and cobalt oxide to keep the glass particles in
suspension and to form an enamel slip.
Mill additions
Frit
Washed chromium oxide
Precipitated silica
China clay
Cobalt oxide
Water
Parts by weight
100
5.0
0.0
5.0
0.0
50.0
The slip was spray applied to a nickel base super alloy substrate, the surface of
which had been previously blasted with abrasive particles and degreased. The
dried article was then heated to the fusion temperature of the coating material,
about 1000°C thereby forming a glossy green amorphous coating. The coating
was finally heat treated at a temperature of 820°C for 8 hours whereby the colour
becomes somewhat dull green and abrasion resistance of the resultant coating
increases by a factor of 2.
EXAMPLE-2
Raw materials were mixed in amounts calculated to provide a glass having a
weight percent analysis of:
Raw materials
(Figure removed)
The resulting batch was melted and then quenched in air to produce a frit. The
ground frit (-100 mesh) was milled for a period of 40 hours in water with
appropriate mill additions as detailed below, including precipitated silica, washed
chromium oxide, china clay and cobalt oxide to keep the glass particles in
suspension and to form an enamel slip.
Mill additions
Frit
Washed chromium oxide
Precipitated silica
China clay
Cobalt oxide
Water
Parts by weight
(Figure removed)
The slip was spray applied to a nickel base super alloy substrate, the surface of
which had been previously blasted with abrasive particles and degreased. The
dried article was then heated to the fusion temperature of the coating material,
about 1160°C, thereby forming an amorphous glossy green coating and
subsequently heat treated at a temperature of about 820°C for one hour, the
resultant coating become partially crystallized and losses some of its gloss and
become more abrasion resistant.
EXAMPLE-3
Raw materials were mixed in amounts calculated to provide a glass having a
weight percent analysis of:
Raw materials
(Figure removed)
The resulting batch was melted and then quenched in air to produce a frit. The
ground frit (-100 mesh) was milled for a period of 48 hours in water with
appropriate mill additions, including precipitated silica, washed chromium oxide,
china clay and cobalt oxide to keep the glass particles in suspension and to form
an enamel slip.
Mill additions
Frit
Washed chromium oxide
Precipitated silica
China clay
Cobalt oxide
Water
Parts by weight
100
5.0
3.0
0.0
0.5
50.0
The slip was spray applied to a nickel base super alloy substrate, the surface of
which had been previously blasted with abrasive particles and degreased. The
dried article was then heated to the fusion temperature of the coating material,
about 1200°C, thereby forming an amorphous glossy green coating and
subsequently heat treated at a temperature of about 820°C for 8 hours whereby
the coating become a glass-ceramic and its colour become somewhat dull and
abrasion resistance of the resultant coating increases by a factor of 2.
EXAMPLE-4
Raw materials were mixed in amounts calculated to provide a glass having a
weight percent analysis of:
Raw materials
(Figure removed)
The resulting batch was melted and then quenched in air to produce a frit. The
ground frit (-100 mesh) was milled in water for a period of 45 hours with
appropriate mill additions, including precipitated silica, washed chromium oxide,
china clay and cobalt oxide to keep the glass particles in suspension and to form
an enamel slip.
Mill additions
Frit
Washed chromium oxide
Precipitated silica
China clay
Cobalt oxide
Water
Parts by weight
The slip was spray applied to a nickel base super alloy substrate, the surface of
which had been previously blasted with abrasive particles and degreased. The
dried article was then heated to the fusion temperature of the coating material,
about 1160°C, thereby forming an amorphous green, glossy coating and
subsequently heat treated at a temperature of about 820°C for 8 hours whereby
the colour become somewhat dull and abrasion resistance of the resultant
coating improves by a factor of 2.
EXAMPLE-5
Raw materials were mixed in amounts calculated to provide a glass having a
weight percent analysis of:
(Figure removed)
The resulting batch was melted and then quenched in air to produce a frit. The
ground frit (-100 mesh) was milled in water for a period of 48 hours with
appropriate mill additions, including precipitated silica, washed chromium oxide,
china clay and cobalt oxide to keep the glass particles in suspension and to form
an enamel slip.
Mill additions
Frit
Washed chromium oxide
Precipitated silica
China clay
Cobalt oxide
Water
Parts by weight
The slip was spray applied to a nickel base super alloy substrate, the surface of
which had been previously blasted with abrasive particles and degreased. The
dried article was then heated to the fusion temperature of the coating material,
about 1200°C, thereby forming an amorphous glossy green coating and
subsequently heat treated at a temperature of about 820°C for 2 hours whereby
the colour become somewhat dull and abrasion resistance of the resultant
coating remains almost unaffected. The resultant glass-ceramic coating retains
all the functional properties on prolonged exposure to high temperature.
EXAMPLE-6
Raw batch materials were mixed in amounts calculated to provide a glass having
a weight percent analysis of:
Raw materials
(Figure removed)
The resulting batch was melted and then quenched in air to produce a frit. The
ground frit (-100 mesh) was milled in water for a period of 15 hours in water with
appropriate mill additions as detailed below, including precipitated silica, washed
chromium oxide, china clay and cobalt oxide to keep the glass particles in
suspension and to form an enamel slip.
Mill additions
Frit
Washed chromium oxide
Precipitated silica
China clay
Cobalt oxide
Water
Parts by weight
100
5.0
5.0
0.0
1.0
50.0
The slip was spray applied to a nickel base super alloy substrate, the surface of
which had been previously blasted with abrasive particles and degreased. The
dried article was then heated to the fusion temperature of the coating material,
about 1200°C thereby forming an amorphous coating. The coating was finally
heat treated at a temperature of 840°C for 8 hours whereby the colour becomes
somewhat dull and abrasion resistance of the resultant coating increases
significantly.
The major crystalline phase formed in the above coatings is barium magnesium
silicate, along with barium silicate and chromium oxide as minor phases. The
abrasion resistance of the coatings expressed in terms of loss in weight value is
in the range of 2.0 to 6.0 mg/cm2 per 50,000 cycles when tested in a P. E. I.
abrasion tester.
The main advantages of the improved process of present invention for the
manufacture of thermally stable non-metallic coating for metallic substrates and a
process of coating thereof are:
1. Preparation of a new high temperature resistant coating material with superior
set of oxidation and hot corrosion resistance.
2. The present process results in a more adherent coating.
3. The present process controls the high temperature viscosity of the molten
coating material by using a novel set of mill additions obtaining a defect free
impervious continuous glass-ceramic coating.
4. The present process results in a coating having superior thermal endurance at
temperature of the order of 1000°C.
5. The present process results in a coating that provides a useful degree of
thermal insulation to the base metal substrate.

We claim:
1. An improved process for the manufacture of thermally stable non-metallic
coating for metallic substrates which comprises: weighing, mixing and sieving
ingredients in the range of 32.2 to 38.0 % (w/w) SiO2; 43.8 to 48.8 % (w/w)
BaCO3, BaNO3 or mixture thereof; 0 to 6.8 % (w/w) CaCO3; 0 to 8.8 % (w/w)
MgCO3; 0 to 4.7 % (w/w) ZnO; 0 to 5.1 % (w/w) bonding agent such as
(NH4)3MoO4, MoO3and 0 to 10.1 % (w/w) H3B03,to obtain a glass forming batch,
melting under stirring the said glass forming batch at a temperature in the range
of 1400°C to 1450°C for a period in the range of 2 to 3 hours to obtain molten
glass, quenching the molten glass and comminuting to obtain glass frit powder,
mixing the glass frit with mill additions essentially consisting of washed chromium
oxide in the range of 5.0 to 45.0% w/w, precipitated silica in the range of 0 to
5.0% w/w, china clay in the range of 0 to10.0% w/w and cobalt oxide in the range
of 0 to 1.0% w/w in an aqueous medium to obtain a non-metallic coating slurry.
2. An improved process as claimed in claim 1, wherein the quenched molten
glass is comminuted by known methods to obtain glass frit powder of the order of
44 microns.
3. An improved process as claimed in claim 1-2, wherein the mixing of the glass
frit with mill additions in an aqueous medium to obtain a slurry is carried out in a
ball mill with grinding media for about 10 to 48 hours to obtain particle size in the
range of 3 to 5 microns.
4. A process of coating metallic substrates with the non-metallic coating slurry as
prepared by the improved process as claimed in claim 1-3, which comprises:
diluting the coating slurry by adding water in the range of 40 to 60 weight percent
of glass frit, applying the liquid coating slurry onto a pre-cleaned surface of a
desired metal substrate to obtain a coated substrate, firing the said coated
substrate at a temperature in the range of 1000°C to 1200°C over a period in the
range of 5 to 30 minutes to obtain a coated fired body, heat treating the coated
fired body at a temperature in the range of 820°C to 840°C for a period in the
range of 1 .0 to 8.0 hours.
5. A process of coating metallic substrates as claimed in claim 4, wherein the
pre-cleaning of the metal substrate is done by known metal cleaning process to
remove impurities such as oxides, oil or grease, dirt, and other contaminants.
6. A process of coating metallic substrates as claimed in claim 4-5, wherein the
application of the liquid coating slurry onto a pre-cleaned surface of a desired
metal substrate is carried out by any conventional method, preferably
conventional vitreous enameling technique.
7. A process of coating metallic substrates as claimed in claim 4-6, wherein the
metal substrate is such as stainless steel, nickel base super alloys, cobalt base
super alloys, Inconel, and Nimonic alloys.
8. An improved process for the manufacture of thermally stable non-metallic
coating for metallic substrates, substantially as herein described with reference to
the examples.
9. A process of coating metallic substrates with the non-metallic coating,
substantially as herein described with reference to the examples.

Documents:

97-DEL-2003-Abstract-(12-02-2008).pdf

97-del-2003-abstract.pdf

97-DEL-2003-Claims-(12-02-2008).pdf

97-del-2003-claims.pdf

97-DEL-2003-Correspondence-Others-(12-02-2008).pdf

97-del-2003-correspondence-others.pdf

97-del-2003-correspondence-po.pdf

97-DEL-2003-Description (Complete)-(12-02-2008).pdf

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

97-del-2003-form-1.pdf

97-del-2003-form-18.pdf

97-del-2003-form-2.pdf

97-DEL-2003-Form-3-(12-02-2008).pdf

97-del-2003-form-3.pdf

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Patent Number

215650

Indian Patent Application Number

97/DEL/2003

PG Journal Number

12/2008

Publication Date

21-Mar-2008

Grant Date

29-Feb-2008

Date of Filing

06-Feb-2003

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	SOMESWAR DATTA	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032.
2	SUMANA DAS	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, KOLKATA 700 032,

PCT International Classification Number

C09D 5/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA