Title of Invention

"A SYNERGISTIC COMPOSITION USEFUL FOR MAKING THERMALLY STABLE NON-METALLIC COATING ON METALS"

Abstract

The present invention discloses and describes a synergistic composition useful for making thermally stable non-metallic coating on metals which obviates the drawbacks of the hitherto known prior art as it is capable of providing a very effective oxygen barrier over the surface of a nickel or cobalt base super alloy substrate. The synergistic composition of the present invention has the novel features of usefulness as a well bonded thermally stable non-metallic coating on metals and for providing a very effective oxygen barrier over the surface of a nickel or cobalt base super alloy substrate for high temperature protection. The novelty is achieved by the non-obvious inventive steps of providing a blend of alkaline earth metal oxides and bonding agent which results in the synergistic composition of the present invention.

Full Text

The present invention relates to a synergistic composition useful for making thermally stable non-metallic coating on metals. The present invention particularly relates to a synergistic composition for making thermally stable nonmetallic coating on metallic substrates such as super alloy components. This invention more particularly relates to a synergistic composition from which a thermally stable non-metallic coating comprising a glass-ceramic selected from the barium magnesium silicate system can be made for coating metallic substrates such as super alloy components. 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% Si02and12-45% BaO. Examples also contain substantial amounts of R2O, B203 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% Si02 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-SiO2 composition field nucleated with Ta2O5 and/or Nb2O5 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 B2Os. 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 advanced aircraft, even the super alloys tend to deteriorate very fast unless protected by a suitable coating. A well known prior art 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 thermally stable non-metallic coating on metals The main objective of the present invention is to provide a synergistic composition useful for making thermally stable non-metallic coating on metals which obviates the drawbacks of the hitherto known prior art as detailed above. Another object of the present invention is to provide a synergistic composition useful for obtaining a coating material with superior set of oxidation and hot corrosion resistance properties. Still another object of the present invention is to provide a synergistic composition consisting of a combination of alkaline earth metal oxides such as BaO, CaO and MgO, instead of BaO alone and the said synergistic composition is useful for making a thermally stable non-metallic coating material for metals. Yet another object of the present invention is to provide a synergistic composition useful as a thermally stable non-metallic coating material by incorporating a bonding agent such as molybdenum tri oxide (MoOa) to promote adherence of the coating to the base metal substrate. Still yet another objective of the present invention is to provide a synergistic composition useful as a reliable and reproducible high temperature protective coating for nickel or cobalt base super alloy substrates that are required to operate at temperatures above 1000°C. The present invention discloses and describes a synergistic composition useful for making thermally stable non-metallic coating on metals which obviates the drawbacks of the hitherto known prior art as detailed above as it is capable of providing a very effective oxygen barrier over the surface of a nickel or cobalt base super alloy substrate. The present invention describes a synergistic composition which has the novel features of usefulness as a well bonded thermally stable non-metallic coating on metals and for providing a very effective oxygen barrier over the surface of a nickel or cobalt base super alloy substrate. The novelty is achieved by the nonobvious inventive steps of providing a blend of alkaline earth metal oxides and bonding agent which results in the synergistic composition of the present invention. Accordingly the present invention provides a synergistic composition useful for making thermally stable non-metallic coating on metals which comprises: 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 and 0 to 10.1 % (w/w) H3BO3. In an embodiment of the present invention the SiO2 is such as quartz powder, quartz sand powder of -100 mesh. In another embodiment of the present invention the bonding agent is such as ammonium molybdate (NH4)3MoO4), molybdenum tri oxide (MoO3). In yet another embodiment of the present invention the raw materials are of commercial glass-making quality. In still another embodiment of the present invention the raw materials should be free of impurities such as alkaline impurities. In still yet another embodiment of the present invention the raw materials should be free of moisture. The composition of the present invention is not a mere mechanical admixture of the ingredients but is a synergistic mixture having properties which are distinct and different from the mere agglomeration of the properties of the individual components. The synergistic composition of the present invention is useful for producing thermally stable non-metallic coating on metals and can be advantageously used to form coatings of glass-ceramic on various types of metal structures, especially for high temperature protection of super alloy components which require a high degree of performance and reliability under adverse conditions. In a co-pending patent application no NF - 31/03, we have described and claimed an improved process for the manufacture of thermally stable non-metallic coating for metallic substrates and a process of coating thereof. In the said co-pending patent application no NF - 31/03, the starting material is the novel synergistic composition of the present invention. The following examples are given by way of illustration of the practical utility and effectiveness of the synergistic composition of the present invention which is useful for making and providing thermally stable non-metallic coating on metals. The following examples therefore 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: (Table 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 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: (Table 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. 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 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 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: (Table 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 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. 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 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. (Table 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 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. 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 present invention are: 1. The synergistic composition is useful for providing thermally stable nonmetallic coating on metals. 2. Can be advantageously used to form coatings of glass-ceramic on various types of metal structures, especially for high temperature protection of super alloy components which require a high degree of performance and reliability under adverse conditions. 3. The synergistic composition results in a more adherent coating. We claim: 1. A synergistic composition useful for making thermally stable non-metallic coating on metals which comprises: 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, ammonium molybdate (NH4)3MoO4) or molybdenum tri oxide (M0O3) and 0 to 10.1 % (w/w) H3BO3. 2. A synergistic composition as claimed in claim 1, wherein, the SiO2 is quartz powder, quartz sand powder of -100 mesh. 3. A synergistic composition useful for making thermally stable non-metallic coating on metals substantially as herein described with reference to the examples.

Documents:

98-DEL-2003-Abstract-(27-10-2008).pdf

98-del-2003-abstract.pdf

98-DEL-2003-Claims-(27-10-2008).pdf

98-del-2003-claims.pdf

98-DEL-2003-Correspondence-Others-(27-10-2008).pdf

98-del-2003-correspondence-others.pdf

98-del-2003-correspondence-po.pdf

98-DEL-2003-Description (Complete)-(27-10-2008).pdf

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

98-del-2003-form-1.pdf

98-del-2003-form-18.pdf

98-DEL-2003-Form-2-(27-10-2008).pdf

98-del-2003-form-2.pdf

98-DEL-2003-Form-3-(27-10-2008).pdf

98-del-2003-form-3.pdf