Abstract:
The present invention provides a method for making a high temperature coating comprising applying to a surface a precursor polymer which decomposes to a substantially pure product selected from the group consisting of a refractory metal carbide and a refractory metal boride, and exposing the precursor polymer to conditions effective to decompose the precursor polymer to said substantially pure product.

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods for making high temperature coatings comprising a substantially pure refractory metal carbide and/or a substantially pure refractory metal boride. 
     BACKGROUND OF THE INVENTION 
     Components used in high stress, high temperature applications (&#34;high intensity&#34; components) typically are provided with protective coatings to prevent material oxidation and hot corrosion during service. One type of protective coating for high intensity components, such as gas turbines, is an overlay coating. A popular overlay coating has a chemical composition of &#34;MCrAlY&#34;--where &#34;M&#34; is nickel, cobalt, or both, Cr is chromium, Al is aluminum, and Y is yttrium. 
     Certain types of components are subject to particularly high stress and high temperature conditions during use (hereinafter called &#34;super high intensity&#34; components). Examples of super high intensity components are jet engine parts and turbo-superchargers. In order to withstand the extreme service conditions, super high intensity components typically are made of a base material known as a &#34;superalloy.&#34; Superalloys exhibit high temperature mechanical integrity with an unusual degree of oxidation and creep resistance. Popular protective coatings for super high intensity superalloy components are thermal barrier coatings (TBCs). TBCs maintain the temperature of the superalloy substrate at an acceptable operating level during service. 
     The temperatures at which components are expected to operate continue to increase. New coatings are needed which will protect high intensity and super high intensity components at ever higher temperatures. 
     Certain metal carbides and metal borides have extremely high melting temperatures. For example, hafnium carbide has a melting temperature of 3890° C. and tantalum carbide has a melting temperature of 3880° C. Metal carbides also exhibit desirable brittle to ductile transition temperatures in the range of 1725-1980° C. 
     A high temperature coating made of a refractory metal carbide and/or metal boride and comprising between about 20-30% particulate silicon carbide theoretically should provide excellent high temperature stability. In fact, the United States Air Force recently initiated a new program--Integrated High Pay-Off Rocket Propulsion Technology (IHPRPT)--to incorporate such advanced materials into rocket and space propulsion systems. Methods are needed for preparing high temperature coatings comprising refractory metal carbides and/or refractory metal borides. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for making a high temperature coating comprising, applying to a surface a precursor polymer which decomposes to a substantially pure product selected from the group consisting of a refractory metal carbide and a refractory metal boride, and exposing the precursor polymer to conditions effective to decompose the precursor polymer to said substantially pure product. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Synthetic inorganic and organometallic chemistry has been used to produce a variety of metal-containing polymer species which, upon pyrolysis or other energetic treatment, decompose to yield substantially pure metal carbides and/or metal borides. Two different approaches were used to obtain such organometallic precursor polymers. 
     Polymerization of Unsaturated Precursors 
     In a first embodiment, a transition metal salt is mixed with one or more organometallic(s) containing at least one unsaturated carbon-carbon bond to form organo-transition metal complexes, which are polymerized to form the precursor polymer. This embodiment has the advantage of guaranteeing that each unit of monomer will contain a metal atom. One disadvantage of this embodiment is that it does not produce high molecular weight precursor polymers. 
     High molecular weight precursor polymers are advantageous for use in polymer infiltration pyrolysis (PIP--used to prepare ceramic matrix composites) and could prove to be advantageous for making high temperature coatings. Unfortunately, the viscosity of a polymer also increases with the molecular weight of the polymer. Precursor polymers with lower viscosity are preferred for an ideal PIP process, and may be preferred for making high temperature coatings. This inherent conflict may be resolved by using high molecular weight precursor polymers having relatively low viscosity, preferably a viscosity similar to a warm honey-like consistency. In order to produce such precursor polymers, the organo-transition metal complexes described above are polymerized with other comonomers which have low tendency to increase viscosity, as described in more detail below. 
     Preferred organometallics for use in this first embodiment include, but are not necessarily limited to metal coordinated substituted and unsubstituted allyl and vinyl organometallics comprising in the range of from about 2 to about 8 carbon atoms, preferably in the range of from about 2 to about 4 carbon atoms. Suitable allyl organometallics include, but are not necessarily limited to 1-methyl-2-propenyl, magnesium chloride, 1-methyl-2-propenyl-magnesium bromide, 2-methyl-1-propenyl magnesium chloride, 1-methyl-2-propenyl-magnesium bromide, allyl magnesium chloride, allyl magnesium bromide. Suitable vinyl organometallics include, but are not necessarily limited to substituted and unsubstituted: vinyl lithium chlorides; vinyl magnesium chlorides; vinyl magnesium bromides; and similar compounds. Such compounds are available from Aldrich Chemical Co. A preferred organometallic is allyl magnesium bromide. 
     The organometallic should be reacted with a salt of a transition metal, defined herein as a transition metal selected from the group consisting of hafnium, tantalum, zirconium, titanium, vanadium, niobium, chromium, molybdenum, and tungsten. Preferred transition metals are selected from the group consisting of tantalum, hafnium, and zirconium. Such salts include but are not necessarily limited to metal halides, metal nitrates, metal sulfates, and metal acetates, with preferred salts being hafnium and tantalum chloride. Hafnium and tantalum chloride, and other metal halides, are available from a number of chemical sources. For example, hafnium chloride is available from Advance Research Chemicals, Inc., Catoosa, Okla., and Teledyne Wah Cheng, Albany, Oreg. Hafnium boride is available from Noah Chemical, Div. Noah Technologies Corp., San Antonio, Tex. Hafnium bromide and tantalum bromide are available from Wilshire Chemical Co., Inc., Gardena, Calif. Tantalum chloride is available from several sources, including Aithaca Chemical Corp., Uniondale, N.Y. and Trinitech International, Inc., Twinsberg, Ohio. 
     In a preferred embodiment, hafnium or tantalum chloride is suspended in a suitable organic solvent, preferably dry ether, and chilled to a temperature in the range of from about -70° C. to about -90° C., preferably about -78° C. An excess of organometallic is added over a period of about 5 minutes. The excess preferably should be a slightly more than a ratio based on the number of halogen atoms in the transition metal salt. For example, if the transition metal salt contains four chloride atoms, then an excess of organometallic of just over about 4:1 is preferred. If the salt contains five chloride atoms, than an excess of organometallic of just over about 5:1 is preferred, etc. 
     The resulting solution, which typically will have an orange (hafnium) or green (tantalum) color, is stirred under an inert gas besides nitrogen, preferably argon, for a time in the range of from about 10 to about 20 hours, preferably about 16 hours, and the product is isolated by filtration through a suitable medium, such as filter paper. In a preferred embodiment, using allyl magnesium bromide, the product is allyl hafnium or allyl tantalum, which converts to the respective carbide in substantially pure form upon pyrolysis. 
     If it is necessary to increase the molecular weight of the precursor polymer, comonomers may be copolymerized with the foregoing organo-transition metal complexes during the same reaction. Suitable comonomers include, but are not necessarily limited to styrene, vinyl and divinyl benzene, and alkadienes having a number of carbon atoms in the range of from about 4 to about 14. 
     In an alternate reaction, the &#34;organic compounds&#34; are boranes and carboranes, preferably ortho-deca-carboranes (C 2  B 10  H 12 ). The carboranes are polymerized by reaction with organometallic halides to form what is believed to be the following: ##STR1## To prepare these precursor polymers, ortho-carborane, which may be obtained from Aldrich Chemical Co., should be lithiated, preferably by reaction with butyl lithium at about -78° C. for a time in the range of from about 1 to about 3 hours. About 0.5 equivalents of a suitable organometallic halide, preferably bis(pentamethyl cyclopentadienyl) hafnium dichloride, should be added to the above prepared solution of the lithiated ortho-carborane at about -78° C. and the solution should be slowly warmed to room temperature. 
     Other suitable commercially available boranes and carboranes include, but are not necessarily limited to meta-deca-carborane and closo-, nido-, arachno-, hypho-, and conjuncto-boranes, which could be deprotonated to the sodium, lithium, or potassium salt using techniques well known to persons of ordinary skill in the art. Boranes are widely commercially available, for example, from Aldrich Chemical Co. and from Fluka Chemical Co. Ortho-deca-carborane and meta-deca-carborane are commercially available from Aldrich Chemical Co. 
     Suitable ligands for the metal halide include, but are not necessarily limited to phosphines, amines, imines, sulfur-containing ligands, and cycloalkenyl groups. Preferred metal halides, which minimize the chance of adding impurity to the resulting borides, have the metal bound to at least one, preferably bound to two cycloalkenyl groups. Preferred cycloalkenyl groups are pentamethyl cyclopentadienyl groups. 
     Introduction of Organometallic Moieties into Preformed Polymers 
     In a second embodiment, organometallic moieties are immobilized on (or &#34;grafted to&#34;) functional groups in a preformed backbone polymer. Suitable backbone polymers for use in this embodiment have repeated double bonds, and include, but are not necessarily limited to olefins, nitrites, acids, and ketones. In this embodiment, the molecular weight of the resulting precursor polymer is dictated by the molecular weight of the backbone polymer. 
     Suitable backbone polymers include a broad range of molecular weights, preferably in the range of from about 1500 to about 7000. In a preferred embodiment, the backbone polymers include, but are not necessarily limited to heteroatom free polyalkadienes, heteroatom free polystyrene polyalkadiene block copolymers (PSPB&#39;s), and isoprene polymers. Suitable PSPB&#39;s and polyalkadienes are widely commercially available. Suitable commercially available isoprene polymers include, but are not necessarily limited to polybutadiene-isoprene, poly(isoprene), and poly(isoprene-styrene), which are available from Polysciences, Inc. A preferred backbone polymer is polybutadiene, available from Phillips Chemical Co., Div. of Phillips Petroleum Co., Borger, Tex., and from E. L. Puskas Co., Akron, Ohio. 
     The backbone polymers possess alkene groups which are reactive to certain organometallic compounds, such as those containing metal hydride (M--H) functions. The alkene bond will react with a metal hydride, incorporating the metal into the backbone polymer. Substantially any hydride comprising a transition metal selected from the group consisting of hafnium, tantalum, zirconium, titanium, vanadium, niobium, chromium, molybdenum, and tungsten should work in the invention. Preferred hydrides comprise a metal selected from the group consisting of hafnium, tantalum, and zirconium. 
     Preferred organometallic hydrides for use in this embodiment are bis(pentamethylcyclo-pentadienyl) hafnium dihydride, which may be obtained from Strem Chemicals, Inc. and dicyclopentadienyl tantalum trihydride. Organo-hafnium and tantalum dihydrides may be made from the respective commercially available chlorinated compounds as illustrated in the following equations wherein &#34;Cp*&#34; stands for a pentamethyl cyclopentiadienyl compound: 
     
         Cp*.sub.2 HfCl.sub.2 +2BuLi→Cp*.sub.2 Hf(Bu).sub.2 
    
     
         Cp*.sub.2 Hf(Bu).sub.2 +H.sub.2 →Cp*.sub.2 HfH.sub.2 
    
     The Cp* hafnium dichloride and the lithiated butyl compound preferably should be mixed at a pressure of about 101.325 kPa (1 atm) and at a temperature in the range of from about -50° C. to about -90° C., preferably about -78° C. and stirred for a period of time in the range of from about 15 minutes to about 3 hours, preferably for about 2 hours. The chloride atoms in the Cp* hafnium dichloride will be substituted by the butyl groups from the lithiated butyl compounds, resulting in dibutyl Cp* hafnium. The butyl Cp* hafnium then should be mixed with hydrogen gas at ambient temperature (typically in the range of from about 20 to about 25° C.) and at ambient pressure (typically about 101.325 kPa or 1 atm) for a time period in the range of from about 10 hours to about 20 hours. The butyl groups will be substituted by hydrogen atoms to form dicyclopentadienyl hafnium dihydride. 
     To manufacture dicyclopentadienyl tantalum trihydride, tantalum (V) chloride is reacted with sodium Cp and sodium borohydride as follows: 
     
         TaCl.sub.5 +2NaCp+NaBH.sub.4 →Cp.sub.2 TaH3 
    
     Pure Cp 2  TaH 3 , which can be obtained by sublimation, may be reacted with the backbone polymer, such as polybutadiene, under refluxing conditions or under high temperature (about 200° C.) and high pressure (120 psi). 
     The foregoing reactions produce yellow or off-white precursor polymers, which are believed to have the following structure: ##STR2## Pyrolysis of the off-white or yellow precursor polymers at between about 1200-1400° C. produces the respective metal carbide. 
     Polymer immobilized metal hydrides are sensitive to air and moisture and should be stored in an inert atmosphere, such as a dry-box, and transferred under a blanket of inert gas besides nitrogen, preferably argon. 
     Pyrolysis or Other Energy Treatment 
     The precursor polymers may be directly converted into high temperature coatings. The precursor polymer should be applied to a desired surface. Suitable surfaces comprise a wide variety of materials including, but not necessarily limited to, metals, alloys, intermetallics, ceramic matrix composites, metal matrix composites, polymer matrix composites, and polymers. The precursor polymer may be applied to the surface using any suitable method including, but not necessarily limited to, painting, spraying, and dipping. If needed, the precursor polymer may be mixed with a filler including, but not necessarily limited to one or more powder selected from the group consisting of ceramics, metals, and alloys. Once coated with the precursor polymer, the surface should be treated with an energy source sufficient to convert the precursor polymer to a refractory carbide or boride. Suitable energy sources include, but are not necessarily limited to pyrolysis and microwave treatment. In a preferred embodiment, the coated surface is fired to a temperature in the range of from about 800° C. to about 1600° C., preferably to about 1400° C., for about two hours, or for a time sufficient to form a coating of refractory carbide or boride. Surfaces bearing such coatings exhibit high temperature oxidation resistance. 
     The foregoing synthesis procedures are sufficiently flexible to provide a range of precursor polymers having a range of viscosities. The refractory metal carbides and metal borides should provide increased oxidation resistance. 
     The invention will be better understood with reference to the following examples, which are illustrative only, and should not be construed as limiting the present invention: 
    
    
     EXAMPLE I 
     Hafnium chloride (6.6 g, 20.6 mmol) was suspended in 500 mL of dry ether and chilled to -78° C. 84 mL (84 mmol) of 1M allyl magnesium bromide was added dropwise over a period of 5 minutes. The orange solution was stirred under argon overnight and then filtered. The solvent was removed under vacuum and 0.98 g of the residue was fired at 1400° C. with an argon purge. The resulting char weighed 0.19 g (19% ceramic yield). The powder x-ray diffraction (XRD) trace indicated that the char was mostly hafnium carbide with only a trace impurity of magnesium oxide. 
     EXAMPLE II 
     Tantalum chloride (5.0 g, 14 mmol) was suspended in 100 mL of ether chilled to -78° C. and 70 mL of 1M allyl magnesium bromide was added over a period of 5 minutes. The dark green solution was stirred at -78° C. under an argon blanket overnight and then filtered. The solvent was removed by vacuum and 0.99 g of the residue fired at 1400° C. with an argon purge. The resulting char weighed 0.19 g (19% ceramic yield). The powder XRD trace of the char indicated that the product was purely tantalum carbide. 
     EXAMPLE III 
     Bis(pentamethyl cyclopentadienyl) hafnium dihydride was prepared as described in D. M. Roddick, et al, Organometallics 4 (1985) 97-104, incorporated herein by reference. 0.69 g of the bis(pentamethyl cyclopentadienyl) hafnium dihydride was dissolved in 20 mL of dry toluene and 0.1 g of polybutadiene (MW=3,000) was added. The solution was stirred for 24 hours at room temperature under an argon atmosphere. The solvent was removed by vacuum leaving an orange solid. A 0.63 g sample of the residue was fired at 1400° C., yielding 0.25 g of a black char (39.7% ceramic yield). Powder XRD analysis of the char indicated a mixture of hafnium carbide and a small impurity of various hafnium oxide phases. 
     Bis(pentamethyl cyclopentadienyl) hafnium dihydride was dissolved in 20 mL of dry toluene and 0.12 g of polybutadiene (MW=1800) was added. The solution was stirred overnight under argon atmosphere. The solvent was removed by vacuum and 0.51 g of the residue was fired at 1400° C., yielding 0.13 g of char (25.5% ceramic yield). The XRD trace of the gray char material indicated largely hafnium carbide with a significant hafnium oxide phase. 
     EXAMPLE IV 
     1.0 g of Cp 2  TaH 3  was dissolved in 35 ml of 1,2-dimethoxyethane. 15 ml of polybutadiene (MW 3000) was added. The mixture was refluxed for 16 hours. The mixture was cooled to room temperature and the solvent was evaporated under vacuum to produce a yellow polymer. The polymer was heated to 1400° C. for 1/2 hour to obtain pure TaC at a 25% ceramic yield. 
     EXAMPLE V 
     14 mmoles of ortho-carborane was dissolved in 120 mL of dry ether which was cooled to -78° C., to which 14 mmoles of methyllithium was added. The mixture was stirred at -78° C. for two hours and thereafter warmed to room temperature. A solution of 7 mmoles of CP* 2  HfCl 2  in 50 mL of toluene was added. The mixture was stirred at room temperature for 40 hours. After filtration, the solvent was evaporated, producing a yellow tacky polymer. When heated to 1400° C., the polymer produced pure hafnium boride in a 38% ceramic yield. 
     Persons of ordinary skill in the art will recognize that many modifications may be made to the present invention without departing from the spirit and scope of the present invention. The embodiment described herein is meant to be illustrative only and should not be taken as limiting the invention, which is defined in the following claims.