US3547681A - Metallic composites - Google Patents

Description

United States Patent 3,547,681 METALLIC COMPOSITES Dean D. Lawthers, Commack, and George T. Pepino, Jr.,

Huntington Station, N.Y., assignors to Sylvania Electric Products Inc., a corporation of Delaware No Drawing. Filed Apr. 18, 1968, Ser. No. 722,213 Int. Cl. B44d N14 US. Cl. 11771 14 Claims ABSTRACT OF THE DISCLOSURE Heat-resistant articles are comprised of a tantalum-base substrate and a duplex metallic coating thereon that renders said substrate oxidation-resistant at both low and high temperatures. The duplex coating is obtained by first forming on the substrate, under heat and non-oxidizing conditions, a porous coating derived from HfB and a sintering aid, e.g., MoSi This porous coating is then infiltrated, under heat and non-oxidizing conditions, with a mixture or an alloy comprised of, by weight, at least 70% hafnium and a lesser but substantial amount of tantalum.

BACKGROUND OF THE INVENTION This invention relates broadly to metallic composites. More particularly it is directed to the art of protectively coating structures or bodies comprised or consisting essentially of tantalum or tantalum alloys as a substrate, whereby new and useful heat-resistant articles of manufacture are obtained. Still more particularly the invention is concerned with the formation on shaped bodies or structures of tantalum-base material (within which term is intended to be included both elementary Ta and alloys of Ta) a reliable, refractory coating that protects the metal substrate from oxidation in severe aerospace or propulsion environments. These environments range in temperature from less than 2000 F. up to 3400 F. or sometimes higher, and in pressure from 0.01 torr to several atmospheres.

The present invention is based on our discovery of a metallic composite that obviates the disadvantages of silicide and of aluminide coating systems on tantalum alloys, the latter systems providing adequate temperature resistance only up to approximately 3000 F. Above this temperature the volatility of the various coating components in the aforementioned systems and the lack of suflicient high-temperature strength to resist aerodynamic stresses become too severe for aerospace applications.

Silicide coatings on tantalum-base alloys cannot protect such metallic substrates much above 3200 F., although they are effective as protective coatings for tens of hours at much lower temperatures. Hafnium-tantalum coatings and/or claddings for equivalent thickness can protect a substrate of tantalum-base alloy for a very short period of time at a temperature above 3600 F. but their useful life is extremely short, e.g., from 1 to 2 hours, at any temperature.

OBJECTS AND SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide new and useful metallic composites.

Another object of the invention is to provide a hightemperature coating for substrates of tantalum or tantalum alloys that provides good oxidation resistance covering a broad range not only up to 2400-2500 F., but also 3,547,681 Patented Dec. 15, 1970 See up to about 3400 F. A more specific object within this broader object is to provide oxidation-resistant, high temperature coatings that can protect the aforementioned tantalum substrates for over 100 hours at 2500" F. and for one hour at a temperature of approximately 3400 F.

A further object of the invention is to provide articles of manufacture comprised of (1) a tantalum-base substrate and (2) a duplex metallic coating thereon that renders said substrate oxidation-resistant at both low and high temperatures.

Still another object of the invention is to provide a method of producing the metallic composites with which this invention is concerned.

Other objects of the invention will be apparent to those skilled in the art from the following more detailed description and from the appended claims.

Briefly described, the objects of the invention are attained by (a) first forming on a tantalum-base substrate, under heat and non-oxidizing conditions, a porous or skeletal metal coating from a pore-forming coating composition in which the essential ingredients in forming the said coating are hafnium diboride and a sintering aid for the same; and (b) infiltrating the 'voids of the said porous coating, also under heat and non-oxidizing conditions, with an impregnating composition in which the essential material is a mixture or an alloy comprising, by weight, at least hafnium and a lesser but substantial amount of tantalum. The amount of this impregnating composition that is employed with respect to the pore-forming composition of (a) is only sufficient to substantially completely impregnate the aforesaid porous metal coating but not appreciably in excess thereof.

In forming the porous metal coating of (a), the wet, applied pore-forming composition is dried, e.g., in air, after which the tantalum-base substrate thereon is heated under non-oxidizing conditions at a temperature and for a period of time sufficient to form in situ a porous, adhering metal coating on the said substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The initially-formed porous or skeletal coating is produced, for example, by applying to a tantalum-base substrate, such as an alloy of, by weight, 90% Ta and 10% W, a relatively thick layer of a slurry comprised of a convenient vehicle having suspended therein a mixture of powdered hafnium diboride (HfB and a sintering aid, e.g., a powdered molybdenum polysilicide, specifically molybdenum disilicide (MoSi These components are preferably present in the powdered mixture in an approximate weight ratio of, for example, from about to 95% HfB to from about 5% to 20% MoSior other sintering aid, and specifically about wt. percent HfB2 and about 10 wt. percent MoSi or its functional equivalent. This slurry or coating composition is applied by spraying, dipping or any other suitable means. It may be applied in any suitable thickness, e.g., such that the coating thickness after drying and firing is from about 10 to 20 mils.

The Wet-coated substrate is dried to remove the more volatile organic matter contained in the coating, after which the coated part is fired under non-oxidizing conditions, e.g., under vacuum and/or in an atmosphere of an inert gas, at a temperature within the range of about 3000-4000 F. but, of course, not above the melting point of the tantalum substrate or of the HfB component of the coating composition. Typically the firing temperature is about 3200-3400 F., and the time at the maximum temperature within this range is from about 10 to 30 minutes.

A porous or permeable metal coating is formed on the tantalum-base substrate as a result of the above-described firing step. This coating is then infiltrated or filled with the aforementioned hafnium-tantalum combination. This may be done using either a physical admixture of powdered hafnium and powdered tantalum or a powdered, preformed alloy of hafnium and tantalum in the desired proportions. Typically there is used a powdered admixture or alloy of, by weight, at least 70% Hf and the remainder Ta and incidentally impurities. Advantageously the aforesaid remainder also includes from about 0.01 to about by weight of the admixture 0r alloy, of one or more elementary metals or metalloids that depress or lower the fusion or sintering temperature of the hafnium-tantalum combination, and hence may be designed as a depressant. Silicon is the preferred fusionor sintering-temperature depressant. In a more specific formulation wherein Si is a component there is employed a powdered admixture containing, by weight, about 15% to 30% (specifically about 20%) Ta, about 0.15% to about 0.5% (specifically about 0.25% Si, and the balance Hf.

Other depressants of the fusion or sintering temperature of the hafnium-tantalum combination may be employed. Their choice depends, for instance, upon the particular proportions of Hf and Ta used and the incidental impurities contained in these metals. Examples of other depressants that are contemplated are magnesium, aluminum, yttrium, and rare-earth elements of the lanthanide series (atomic numbers of 57 through 71).

The final combination coating is obtained by applying the second or impregnating coating to the porous or permeable coating on the substrate as heretofore described, followed by drying and firing in essentially the same manner described above with reference to the formation of the porous initial coating.

The combination coating thereby produced has improved oxidation resistance at elevated temperatures as compared with a coating of a mixture of 80% Hf and 20% Ta that has been initially applied as a slurry to a tantalum-alloy (e.g., 90% Tat-% W) substrate that has not been precoated. Such Hf-Ta coatings slurry-applied directly onto a tantalum-alloy substrate, such as that just mentioned by way of example, have oxidation-resistant properties at 3600 F. that nearly duplicate those of a hafnium-base alloy containing about 20 wt. percent Ta and the remainder Hf at the same temperature.

THE TANTALUM-BASE SUBSTRATE As indicated hereinbefore, the tantalum-base substrate may be either elementary Ta or an alloy of Ta. We prefer to use an alloy of Ta. The Ta alloy can be one wherein the Ta constitutes at least 45%, more particularly a major percentage (e.g., from 60% to 99% or more), by weight thereof. The other component(s) of the alloy may be one or more of such metals as, for example, tungsten, rhenium, osmium, molybdenum, niobium (columbium), iridium, ruthenium, boron, rhodium, chromium, zirconium, titanium, thorium, vanadium, platinum, hafnium and other metals and/ or metalloids having melting points of at least about 1700 C.

As desired or as may be required by the particular environmental conditions encountered in the particular service application of the metallic composite, the substrate may be a tantalum alloy which also includes a small amount, up to about 1%, of such other elements as iron, nickel or cobalt. Or, the tantalum-alloy substrate may be one that includes, in addition to such elements as those mentioned in this and in the preceding paragraph. a small amount of such elements as carbon (e.g., up to about 1",? oxygen (e.g., up to about 0.8%) and/or nitrogen (e.g., up to about 0.5%). However, it is preferred that the sum of any two or all three of these last-named elements, if present, does not exceed about 1.5%. All percentages mentioned above, as well as elsewhere in this specification, are percentages by weight unless otherwise specified.

Specific examples of tantalum-base alloys that can be used as the substrate in carrying this invention into effect include the following:

Trade identification: Composition T a-lOW Ta-1OW-2.5Mo Ta-7W-3Re Tl l1 Ta-8W-2Hf T-222 Ta-8.7W-3.15Hf-0.02C

R-18 Ta-6.5W-2.70Re-1.00Hf- 0.29Zr-0.017Y R-50 Ta-6.2W-2.95Re-1.1Hf-

The specific alloying component or components with tantalum used in the tantalum-base substrate, and the percentage proportions of tantalum and of the components alloyed therewith, is determined by such influencing factors as, for example, the particular properties desired in the tantalum-base substrate, the cost of the modifying component or components, and the ease or difficulty of fabricating the resulting alloy of tantalum with the particular modifying metal or metalloid that is chosen.

Typical of the properties of the tantalum-base substrate, e.g., in thin sheet form, are the following:

COATING PROCEDURES The general procedures employed in applying the first or permeable coating and the second or infiltrating coating, hereinbefore briefly described, on the tantalum-base substrate are essentially the same.

Components such as HfB and a sintering aid, e.g., MoSi used in the preparation of the first-applied coating composition, and hafnium, tantalum and silicon (or an alloy thereof) normally used in the preparation of the second or infiltrating coating composition are pulverized to form powders if they are not initially in the form of finely divided powders. The particle sizes or ranges of particle sizes may be those that are commonly employed in powder metallurgy techniques, and are at least such that all will pass through a 325-mesh screen (US. Standard Sieve Series).

The elemental metals, alloys or compounds used in each of the individual coating compositions are thoroughly mixed together in a suitable blender or mixing device (e.g., a V Blender) until a substantially homogeneous composition has been obtained.

The hafnium diboride and a sintering aid or aids, e.g., MoSi used in the first-applied coating composition may be present therein in an amount by weight corresponding to from about to 98% (preferably from to of the former to from about 2% to about 25% (preferably from 5 %to 10%) of the latter.

The invention is not limited to the use of MoSi as a sintering or densification aid. Illustrative examples of other such aids that may be used are powdered mixtures or alloys of the following elements in the stated percentages, which are by weight:

(a) 97% Zr and 3% B (b) 96% Ni and 4% B (c) 80% Zr and 20% B (d) 78% Zr, 2.7% B and 19.3% Nb The following description is given with particular reference to the preparation and application of the aforementioned initial coating, but the general procedure is equally applicable to the preparation and application of the second or infiltrating coating.

The powdered mixture of hafnium diboride and sintering aid is converted into a liquid coating composition, adapted for application (e.g., by dipping, brushing, spraying or the like) to the tantalum-metal substrate or part, by suspending it in a suitable vehicle, e.g., a solvent solution of a natural or synthetic, thermoplastic, temporary or fugitive binder. The use of thermosetting binders is not precluded; but generally they are likely to be less satisfactory because of the greater difficulty in removing all of the carbonaceous residue during subsequent fusiontreatment of the coated part.

Examples of binders that may be employed are solvent solutions or dispersions of the various available synthetic polymers, among which may be mentioned polyacrylamide, polyvinyl acetate and the homopolymers and copolymers of the lower alkyl (e.g., C through C alkyl) acrylates with each other and with other compounds containing a monoethylenically unsaturated grouping. We prefer to employ an ordinary nitrocellulose (pyroxylin) lacquer wherein the solvent is, for example, amyl acetate.

The concentration of the powdered material in the vehicle and the amount of solvent in the same are varied as desired or as may be required, depending upon such influencing factors as, for instance, the particular method of applying (e.g., brushing, spraying or dipping) the coating composition, the desired thickness of the individual coating, the number of coatings to be applied, the viscosity of the vehicle, the desired covering or penetrating power of the individual first or second coating composition, and other influencing factors.

Typically, the hafnium diboride and MoSi (or other sintering aid or aids) are present in the first-applied coating composition in the proportions with respect to each other heretofore given and, also typically, in a combined amount of from about 300 grams thereof to about 100 grams of the liquid or flowable composition. In the case of the second or penetrating coating composition, the relative proportions of Hf, Ta, and Si or other fusion depressant are within the ranges previously mentioned, and they too are present in the coating composition (i.e., powdered metal plus vehicle) in, typically, a combined amount of from about 300 grams thereof to about 100 grams of the liquid or flowable composition.

The powders are mixed with the vehicle by mechanical stirring. Mixers of the type employed in mixing paints can be used for this purpose. Mixing is continued at any suitable temperature for a time sufiicient to form a substantially homogeneous composition.

The coating is applied to the cleaned surface of the tantalum-base substrate. The surface of the part may be cleaned, for example, by (l) abrasive blasting with iron or aluminum oxide grit; (2) acid pickling for one minute in a solution of 1 part concentrated HP, 1 part concentrated HNO and 1 part water; or (3) by heat-treating under vacuum. There is no preferred method of application insofar as wettability or protectiveness of the coating is concerned. Hence the selected method depends primarily upon such other factors as, for instance, convenience, availability of suitable equipment, accessibility of the surface to be coated, and personal experience. If all surfaces are readily accessible, spraying is the preferred method.

The applied slurry coating is controlled by recording the weight gain in milligrams per square centimeter of surface area. For example, 30 mg./cm. results in a l-mil coating, 300 mg./cm. =1O mils, 450 mg./cm. :l5 mils, and 600 mg./crn. =20 mils when an HfB -l0MoSi slurry coating has been applied in one step, and a dense final coating has been achieved by infiltrating with Hf-20Ta-0.25Si the permeable coating that results from firing in vacuo the air-dried initial coating.

Heavier coatings can be obtained, if desired, either by applying a thicker wet layer of the HfB -containing slurry composition initially, or by applying one or more fresh layers over the air-dried coating followed by air-drying after each successive application of the aforesaid coating.

After air-drying, the coated part is placed either on suitable heat-resistant pads, e.g., pads of ZrO ThO or HfO or is suspended by tantalum wires in a cold-wall vacuum furnace and fired up to and at a temperature which is near, at or slightly above the melting point of the inorganic material in the as-applied composition, but which is preferably above said melting point. In such a furnace, the coated part is supported or suspended inside the heating element wherein it is heated by radiation, and is therefore quickly heated to a uniform temperature.

The time and temperature of firing the air-dried HfB containing coating in all cases are sufficient to form a porous or permeable, tenaciously adhering coating on the substrate. Generally this heat or diffusion-treatment is for from about 10 minutes to 1 hour at a maximum temperature within the range of from about 325035 00 F., more particularly for 15 minutes at about 3310 F., under non-oxidizing (substantially non-oxidizing) conditions. Thus, this heat-treatment can be carried out in an atmosphere of an inert gas, e.g., helium, argon, krypton, xenon or other members of Group 0 of the Periodic Table of the Elements. Advantageously the non-oxidizing conditions for the heat-treatment are obtained by heating the coated part under a high vacuum, e.g., at less than 0.001 or 10*" torr, and preferably at less than 0.0001 or 1O- torr. The temperature and time of firing depend upon such influencing factors as, for example, the particular coating composition and tantalum-base (i.e., tantalum metal including tantalum alloy) substrate that is employed. If desired, heating can be carried out initially under partial vacuum and completed in an atmosphere of an inert gas.

To obtain a porous or permeable coating of the desired thickness on the tantalum substrate, the porous coating initially formed by the firing step can be built up to provide a thicker coating, if desired, by repeating the application of a wet coating one or more times, followed each time by air-drying and firing as has just been described with reference to the firing of the air-dried coating applied in one or more steps.

The procedure for applying, air-drying and firing the I-lf-Ta impregnating slurry composition in order to impregnate the porous coating first formed on the substrate is essentially the same as that described above with referenct to the HfB -containing slurry composition used to create the aforesaid porous coating. However, after this impregnant has been applied, the air-dried coated substrate is then usually fired at a temperature somewhat lower (e.g., 40-50 F. lower) than that used in the prior firing step. For example, when the maximum firing temperature for the first stage is 3310 F., the firing temperature in the second stage advantageously is about 3270 F.

The thickness of the final or duplex coating (i.e., impregnated, porous coating) on the substrate may range for example, from about 5 to about 25 mils or even mils, but generally is within the range of from about 10 to about 20 mils. The unit increase in Weight of the finished duplex coating may range, for instance, from about 150 to about 750 mg./cm. more particularly from about 300 to about 600 mg./cm.

Using the preferred compositions and procedures of this invention, optimum results are obtained when the HfB containing material (specifically, 90% HfB -10% MoSi and the Hf-Ta impregnating composition (specifically, 79.75% Hf20 Ta0.25% Si, which sometimes is later designated herein as R515) are present in such proportions With respect to each other in the initial as-applied compositions that the latter (i.e., the R515) constitutes between about 40% and by weight of the former. In general, the use of a weight excess of R515 is to be avoided in order to obviate or minimize the possibility that a reaction between excess R515 with the HfB -Mosi and the tantalum-alloy (specifically Ta-l0% W) substrate will cause or tend to cause dissolution of said 7 substrate. If not enough R515 is used with respect to the HfB -MoSi coating, the final duplex coating may have a degree of porosity that shows up in oxidation tests as being detrimental to the coating. Substantially complete impregnation of the porous coating with the impregnant is, therefore, important.

The metal coatings with which this invention is concerned can be applied selectively to portions of parts or assemblies. Also, modifications of the primary compositions (or even basically different compositions if believed to be necessary) can be applied to specific areas of the same part or assembly where experience indicates that different compositions would be more suited to the particular environmental conditions.

In order that those skilled in the art may better understand how the present invention can be carried into effect, the following examples are given by way of illustration and not by way of limitation. All parts and percentages are by weight unless otherwise specified. All the metal and metallic compound powders (200 mesh HfB others, 325 mesh) employed are technical grades. (When available, 325 mesh HfB is preferred.)

EXAMPLE 1 (A) Formation of the porous coating HfBg and MoSi in powdered form are weighed out in the proportion of 90% HfB l% MoSi charged into a V-blender, and blended for 2 hours. The blended powder mixture, which also may be designated as a coating mixture, is then added to a lacquer of high-purity, lowresidue nitrocellulose in a solution of amyl acetate solvent in the approximate proportion of 1 to 1 by volume, and is thoroughly mixed or sheared with a mechanical stirring or shearing device for about 10 minutes. The mixing or shearing step may be effected in two stages, e.g., first at 125 F. for 5 minutes, cooled and resheared for 5 minutes. The nitrocellulose lacquer employed may be one such as type L-18 manufactured by Rafii and Swanson, Wilmington, Mass. To minimize rapid gravity separation, there may also be incorporated in the slurry a small amount of, for example, toluene. The amount of toluene or its equivalent may be, for instance, from about to 20% by weight of the nitrocellulose lacquer component of the slurry.

The resulting slurry or coating composition is poured into the jar or reservoir of a conventional-spray gun. This receptacle is preferably provided with means for continuous, mechanical stirring of the slurry in order to prevent settling of the metal compounds.

In this and in other examples that follow, the substrate or part to be coated is 0.03-in. thick tantalum-tungsten alloy sheet wherein the percentage of tungsten is either about 9.5% or about 10.1%. The 9.5% W alloy was stabilized grain-size material with an yttrium addition (about 10 to 20 ppm.) while the latter was a commercial grade of Ta-lOW alloy. Both types contained, as incidental impurities, oxygen, nitrogen and carbon, and metals and metalloids of various kinds, none of which exceeded about 105 parts per million. The sheet to be coated can be of any size or shape.

The Ta-W part is previously cleaned by immersion for 30 seconds in a solution of equal proportions of concentrated HF, concentrated HNO and water followed by a water rinse, and finally by air-drying.

The parts are sprayed with the slurry of HfB -Mosi on both sides using normal spray-painting techniques, airdried for from 10 to minutes, resprayed, etc., until the applied layer has a green (unfired) weight of approximately 250 mg./cm.

The air-dried parts are than placed across small ceramic (ZrO combustion boats which in turn, are placed in a vacuum furnace. The furnace is sealed and pumped to a vacuum of less than 10 torr at which point heating is begun. The heat is initially applied slowly until at some 8 low temperature (less than 500 F.) it is apparent (evidenced by the sudden increase in pressure within the furnace) that the organic solvent is volatilizing from the vehicle component of the applied coating composition. At this point heating is temporarily interrupted until it is observed that the pressure in the furnace is again at a level of 10' torr or lower. Thereafter, heating is continued to a furnace temperature of 3310" F., held there for 15 minutes after which the furnace heat is turned off. When the furnace has cooled to a temperature sufficiently low to prevent damage to the internal parts of the furnace, air is admitted thereto, and the furnace is opened and the specimens removed. The resultant porous coating on the specimens has a thickness of about 20 mils on each side.

The furnace employed in firing the coated specimens of this example is a cold-wall, metallic-resistance-element furnace, the heat-up time being about 10 minutes and the cool-down time about 30 minutes in carrying out the desired firing step. In larger vacuum furnaces of similar or different design and/or with larger work loads, longer heat-up and cool-down times are necessary. However, these factors are not critical in the chemistry involved in the firing step nor in the performance characteristics of the finished coated part.

The porous coating resulting from firing the green coating comprised of HfB and sintering aid, specifically MoSi forms a tightly adhering bond with the Ta-lOW sheet and with almost no reaction with the substrate when the air-dried coating is fired for 15 minutes under vacuum at 33l0 F. or thereabouts. In marked contrast HfB powder alone will not bond to the same T a-10W alloy sheet at any temperatures up to 3600 F. With the addition of other additives to HfB e.g., 0.5% Si, the applied coating actually peels away from the substrate during vacuum firing.

(B) Formation of the final coating The final coating is prepared following the general procedure previously described in the portion of this specification prior to the examples. Additional details follow.

The porous-coated substrate resulting from Step A, supra, is infiltrated with R515 using the following formulation in making the impregnating composition:

Grams Hafnium 480 Tantalum Silicon 1.5 L-18 Lacquer (high-purity, low-residue nitrocellulose dissolved in amyl acetate) 100 MPA 60 (toluene) 15 The hafnium powder contains about 4.8% Zr, about 0.4% O, as well as small amounts (not exceeding about 1100 ppm. for any one of them) of N, C, and various metals and metalloids as incidental impurities.

The slurry mix is obtained by shearing the above ingredients together for 5 minutes at about F., cooling, and reshearing again for 5 minutes, after which it is ready for use.

The R515 impregnating composition is applied to the porous-coated Ta-W substrate from Step A in essentially the same manner therein described with reference to the application of the HfB -MoSi coating composition to the uncoated substrate except that in this step vacuumfiring of the air-dried coating is carried out at a maximum temperature of 3270 F. instead of 3310 F. as in Step A.

The amount of the R515 slurry mix that is applied is sufiicient to completely impregnate the initial or porous coating. It is generally between about 40% and about 55% of the weight of the HfB -10MoSi coating.

(outings on substrates that have been prepared by a two-step process in accordance with this invention, and as illustrated by the foregoing example, are some 1 1 strate for either 1 hour (for the R515-coated coupons) or for 3 and also 24 hours for the duplex-coated specimens.

The results are given in Table IV which follows:

No effect of air pressure on oxidation rate could be detected between the R515 single coatings and the duplex coatings of HfB -MoSi under the described testing conditions. Photomicrographs of various coated specimens also did not reveal any effect of higher atmospheric pressure.

EXAMPLE 3 In this example tests were made on /2-inch discs of 30 mil Ta-lOW alloy that were either slurry-coated with 30 mils per side of R515, or duplex-coated with HfB MoSi infiltrated with R515. The effort here was to evaluate the relative merits of the coatings at low air pressures and at temperatures near 3100" F. and 3500 F.

The coated specimens were prepared in essentially the same manner described in Examples 1 and 2.

In the experimental apparatus employed, samples were positioned horizontally within the work coil of a kw. induction furnace, and were held at the outer edges by a three-point contact alumina support system. Temperatures were read at 0.65 micron with a Leeds and Northrop optical pyrometer sighting on the upper flat surface of the sample. For measurements in air at ambient pressure, the apparatus was left open to the atmosphere. For measurements at lower pressure, the apparatus was initially evacuated and then filled with air to the desired pressure level. Arcing of the induction unit at the desired 10 torr and 1 torr pressure levels was eliminated by a combination of proper grounding of a rotary feed-through unit of the apparatus and alteration of the coil design of the said unit to a two-turn pancake configuration. By these modifications samples could be heated to 3600 F. in air at total pressures of 10 torr and below without addition of inert gas.

The results obtained on the R515 and the HfB -Mo'Si duplex coatings under similar conditions of temperature and pressure are compared in Table V. The coated discs were tested under relatively static atmospheric condi tions. Photographs of the different samples at the end of the testing period indicate a lower overall recession for the duplex-coated specimens. Unlike the typical silicide coatings, neither the R515 coating nor the duplex coating of this invention is subject to a low-pressure failure. In fact, for both types of coatings the oxidation resistance improves as the pressure is reduced to near both 3100 F. and 3500 F. For the R515 coating, the weight gain per unit area increases approximately as the pressure is raised to the 0.3 power (P between pressures of l and 760 torr. For the duplex coating the same pressure dependence holds for the data at 1 and 10 torr, the re- 12 sults again being independent of temperature. Table V follows:

TABLE V [Comparison of low-pressure oxidation results with R515- and duplex coated samples] B rightness Weight temp., Time ain, Air pressure torr F. Sample mins. mg. cm.

3135 R515 (P9) 30 14.1 3120 Duplex (P23) 30 6. 5 3090 515 (P-10 30 37. 9 3090 Duplex (P-24) 30 12. 7 3490 5 (P- 15 13. 6 3450 Duplex (P-25) 30 7. 2 3540 5 (P11) 15 32. 5 3450 Duplex (P-Zti; 30 12. 5 3450 Duplex (P27 120 15. 1

EXAMPLE 4 The procedure of Example 1 is repeated in duplex coating Ta-W sheet material of the kind used in that example with the exception that other sintering aids (sometimes designated as densification aids) are employed instead of MoSi as a modifier of the HfB Formulations of some of the powdered mixes that showed adaptability for use as a substitute for MoSi as evidenced by bonding of the slurry coating to the Ta-W substrate, are given below. The specified numerals before the symbol for the individual element or compound refer to its weight percent in the total mix or in the mixture of elements constituting the sintering aid.

Formulation 95HfB -5 (97Zr-3 B) 95 HfB -5 (96Ni-4B 95HfB -5 Zr-20B) HfB -l0(10Zr-3B) 90HfB -10(78Zr-2.7B-19.3Nb)

90Hfb -10Mo Si None of these sintering or densification aids was as effective for the intended purpose in a formulation with HfB as was a mixture of HfB and M081 specifically about 90% HfB and about 10% MoSi EXAMPLE 5 Example 1 is repeated using, as the substrate, elementary Ta wire having a diameter of about 30 mils and containing only incidental impurities. A good duplex coating is obtained.

The present invention provides metallic composites comprising a refractory substrate, more particularly a tantalum-base substrate, and an oxidation-resistant duplex coating thereon that increases the oxidation lifetimes of the said composites at temperatures below 3100 F. without loss of behavior at temperatures as high as about 3400 F. Specifically a duplex coating produced by a two-step coating system, 90% Hfb -l0% MoSi vacuum-fired 15 minutes at 3310 F. and thin infiltrated with 40% R515 (i.e., 20% Ta, 0.25% Si, and the ba ance times designated herein for purpose of brevity as a duplex coating or sample.

Five duplex samples of this example were cyclic oxidation tested at 2500 F. The samples went directly from room temperature into a hot horizontal furnace. After one hour the samples were automatically removed to cool for 5 minutes, and then automatically reinserted into the hot furnace. This constitutes one hourly cycle. One sample was stopped after 50 hours for metallographic examination. The other four samples went 102 hours at 2500 F. without failure. The weight gains and substrate hardness values are given in Table I. Excellent reproducibility of results was obtained. The substrate alloy hardens slightly as a function of time. Metallographic examination indicates that this is caused by the porosity of the coating.

Table I follows:

TABLE I [2500" F. cyclic oxidation results on IItBHOMoSig plus R515 (Duplex) coated 'iu -l\V alloy samples] Weight Vickcrs Hourly gain, hardness of cycles rug/cm substrate Sample No.

-3 i i U 31 TABLE II [Flamt typc oxidation tests on duplexcoated 'Iu-lfiW alloy samples] Brightness temperature. F.

Back side at number of cycles Time Flame Cycles (minutes) side 0 (i ll 24 35 1 No failure.

Although the temperature of the back or air side decreases as a function of time with a constant flame-side temperature, this effect is not as extreme as is found upon photomicrographic examination of samples coated with the same thickness of R515 alone.

EXAMPLE 2 This example illustrates the results obtained on both (a) R515 alone as the coating material and (b) duplexcoated (i.e., as in Example 1) samples of Ta-W alloy sheet, such as that used in Example 1, at l and torr flowing air pressure at 2500 F. and 2800 F. Th coated coupons were subjected to the re-entry simulation life test hereafter described.

The R515 slurry composition is described in Example 1. It is sprayed on the substrate, air-dried, fired under non-oxidizing conditions at about 3270 F., resprayed, and again air-dried followed by refiring. The thickness of the final coating is about 20 mils. The duplex-coated (i.e., HfB -M0Si first, followed by R515) coupons are prepared as described in Example 1. The total thickness of coating on the R515 (alone)-coated and duplex-coated specimens are substantially the same.

Re-entry simulation life test This test is carried out in an automatic, programmed, recycling, pressure-temperature-profile simulator. It consists of a Globar-heated, mullite tube furnace and controls, appropriate vacuum seals, vacuum-pumping system, automatic traversing mechanism, and programmed pres sure-control instrumentation. Temperature variations are obtained by varying the position of the boat containing the sample relative to the hot zone of the furnace. A temperature-controller programmer permits the attainment of a temperature profile of any desired shape. The pressurecontrol system consists of an absolute pressure transducer, a programmer, a pneumatic-pressurer controller, and a diaphragm-driven, needle-bleed valve. The system is capable of controlling pressure in the range of from 0.100 torr to 10 torrs accurately and reproducibly. Th specimen temperature is monitored by a sheathed platinum/ platinum-rhodium alloy thermocouple, which is inserted through a vacuum seal through a stoking tube, the bead of which is adjacent to and rides with the test specimens as it is stoked in and out of the hot zone of the furnace.

The results of the test are given in Table III which follows:

TABLE III [Low-pressure oxidation behavior of R515 and duplex-coated samples] Pres- Weight Vickers Time, sure, gain, hardness Sample hours Cycles torr. rug/cm. substrate All R515 samples at 2. 500 F.

l Hole failure. 2 Edge failure.

It will be noted that the duplex-coated samples show greater oxidation resistance at a temperature of 2500" F. and a pressure of 10 torr than do the R515-coated specimens as evidenced by the lower weight gain in mg./ cm. at the end of longer periods of time.

R5l5- and duplex-coated Ta-W substrates prepared as described in this example also were subjected to a highpressure oxidation test at 2000 F. with flowing air at one and at four pounds pressure passing over the coated sub- Hf) followed by vacuum-firing at 3270" F., gives improved oxidation resistance to a tantalum-base substrate subjected to temperatures below an optical 3300 F. This duplex coating also will withstand three hours at 3300 F. in cyclic flame tests and 100 hours with hourly cycles at 25 F. both at 760 and torr air pressure. Furthermore, oxidation at four atmospheres air pressure at 2000 F. does not seem to greatly increase the oxidation rate.

One of the advantages of the coatings with which this invention is concerned is that they are substantially uniform in thickness. This is due mainly to the preferred technique of applying them to the tantalum-base substrate. In this respect, they differ markedly from coatings applied by a pack-cementation process wherein the coated part is embedded in an insulating powder pack inside a retort and is heated by conduction. By this latter technique the parts nearest the walls of the retort are heated up much more rapidly than those at the center; hence, the resulting coatings are not uniform either in composition or in thickness.

The articles (metallic composites) of this invention are useful, for example, as part of a space re-entry vehicle such as a leading edge of a wing or rudder; and in the fabrication of hypersonic aircraft. Such aircraft operate at very high altitudes so that high-temperature, low-pressure oxidation resistance is required in this application as well as for reusable re-entry vehicles. Thus, the leading forward edges or components of the fore part of a hypersonic aircraft advantageously may be constructed at least in part of a metallic composite of the present invention.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

We claim:

1. An article comprising a tantalum-base substrate and a duplex metallic coating thereon having a thickness of from about 5 to about 50 mils, said duplex metallic coating comprising a porous portion and a permeable portion in the voids of said porous portion,

said porous portion comprising from about 75% to about 98% by weight of hafnium diboride and from about 2% to about 25% by weight of a sintering aid selected from the group consisting of molybdenum polysilicide and a mixture of boron and at least one metal selected from the group consisting of zirconium, nickel and niobium, and

said permeable portion comprising at least 70% by weight of hafnium and the remainder comprising tantalum and from 0 to 5% of at least one metal from the group consisting of silicon, magnesium, aluminum, yttrium and rare earths of the lanthanide series.

2. An article as in claim 1 wherein the tantalum-base substrate is a tantalum-alloy substrate.

3. An article as in claim 1 wherein said permeable portion is a mixture comprising, by weight, about to 30% Ta, about 0.15% to 0.5% Si, and the balance Hf.

4. An article as in claim 1 wherein said permeable portion is a mixture consisting essentially of, by weight, about Ta, about 0.25% Si, and the balance Hf.

5. An article as in claim 1 wherein the sintering aid is a molybdenum polysilicide.

6. An article as in claim 5 wherein the molybdenum polysilicide is molybdenum disilicide.

7. An article as in claim 1 wherein the tantalum-base substrate is a substrate of an alloy of at least 45% by weight of tantalum with one or more other metals or metalloids having a melting point of at least about 1700 C.; and the sintering aid for the hafnium diboride in the pore-forming coating composition is molybdenum disilicide in an amount corresponding to from about 2% to about 25 by weight of the total amount of hafnium diboride and molybdenum disilicide.

8. An article as in claim 7 wherein the substrate is a tantalum-tungsten alloy containing at least 60% tantalum and the amount of molybdenum disilicide in the pore-forming coating composition corresponds to from about 5% to about 20% of the total amount of hafnium diboride and molybdenum disilicide.

9. An article as in claim 8 wherein the substrate is an alloy of, by weight, about tantalum and about 10% tungsten.

10. An article as in claim 9 wherein the amount of said permeable portion is from about 40% to about 55% of the weight of said porous portion.

11. The method of producing an article of manufacture which comprises:

(A) applying to a clean surface of a tantalum-base substrate that is to be protectively coated a layer of a pore-forming coating composition comprised of (a) a powdered mixture of hafnium diboride and a sintering aid selected from the group consisting of molybdenum polysilicide and a mixture of boron and at least one metal selected from the group consisting of zirconium, nickel and niobium for the same, (b) a fugitive organic binder and (c) a volatile solvent for said binder;

(B) drying the applied coating;

(C) heating the tantalum-base substrate with its dried coating thereon under non-oxidizing conditions at a temperature and for a period of time sufilcient to form in situ a porous, adhering metal coating on the said substrate; and

(D) infiltrating the voids of the porous metal coating on the tantalum-base substrate, also under heat and non-oxidizing conditions, with an impregnating composition comprising, by weight, at least 70% hafnium and up to 30% tantalum, the amount of the said impregnating composition that is employed with respect to the pore-forming coating composition being only sufficient to substantially completely impregnate the aforesaid porous metal coating.

12. The method as in claim 11 wherein the tantalumbase substrate is a tantalum-alloy substrate; and the nonoxidizing conditions under which the coated substrate is heated in each of steps C and D are attained by means which include the use of vacuum.

13. The method as in claim 11 wherein the tantalumbase substrate is an alloy of at least 45 by weight of tantalum with one or more other metals or metalloids having a melting point of at least about 1700 C.; the sintering aid for the hafnium diboride in the pore-forming coating composition is molybdenum disilicide in an amount corresponding to from about 2% to about 25 by weight of the total amount of hafnium diboride and molybdenum disilicide; the impregnating composition is a mixture or an alloy comprising, by weight, at least 70% by weight of hafnium, and the remainder comprising tantalum and from 0 to 5% of at least one metal from the group consisting of silicon, magnesium, aluminum, yttrium and rare earths of the lanthanide series; and the coated substrate is heated under non-oxidizing conditions in each of steps C and D at a temperature within the range of from about 3000 F. to about 4000 F., but not above the melting points of the tantalum substrate or the HfB component of the coating composition used in step C, and at a pressure of less than 10- torr, thereby to form the porous metal coating in step C and to infiltrate the said coating in step D with the above-defined material of the impregnating composition.

14. The method as in claim 11 wherein the tantalumbase substrate is a tantalum-tungsten alloy containing at least 60% tantalum; the sintering aid for the hafnium diboride in the pore-forming coating composition is molybdenum disilicide wherein the latter constitutes from about to about by weight of the total amount of the said ingredients; the material in the impregnating composition is a mixture consisting essentially of, by weight, about tantalum, about 0.25% silicon and the balance hafnium; the amount of the material in the aforesaid impregnating composition is from about to about of the weight of the porous metal coating; and the coated substrate is heated in step C at a temperature of about 3310 F. for about 15 minutes, in step D at a temperature of about 3270 F. also for about 15 minutes, and in both of said steps it is heated at a pressure of less than 10- torr.

References Cited UNITED STATES PATENTS 3,069,288 12/1962 OXX, Jr. et a1. 1l7-71(M) 3,189,477 6/1965 Shaffer 117169X 3,303,559 2/1967 Holtzclaw 29182.1X 3,449,120 6/1969 Zdanuk et a1 29-182.1X

ALFRED L. LEAVITT, Primary Examiner 10 C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R.

*zgx gg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,547,681 Dated December 15, 1970 Inventor) Dean D. Lawthers and George T. Pepino, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

r- Column 11, TABLE IV, of the spgcification, under the heading Weight gain in mg. /cm. "29.2" should be--29.4-

"33.6" should be--33.2--, "3.4" should be--3.6--, "5.8" should be--5.0--, "5.0" should be--5.8--, "6.8" should be--6.6--, and "6.6" should be--6.8--

Column 12, TABLE V, the fifth heading eight gain, mg./cm. should read--Weight gain, mg. /cm.

Column 12, line 74 "thin" should read-then--.

Signed and sealed this 22nd day of June 1 971 (SEAL) Attest:

EDWARD M.FLE'I'CHER,JR. WILLIAM E. SCHUYLER, J Attesting Officer Commissioner of Patent