US3257230A - Diffusion coating for metals - Google Patents

Description

United States Patent Office 3,257,230 Patented June 21, 1966 3,257,230 DIFFUSION COATING FOR METALS Richard L. Wachtell, Scarsdale, and Richard P. Seehg, Hartsdale, N.Y., assignors to Chromalloy American Corporation, a corporation of New York No Drawing. Filed Mar. 24, 1964, Ser. No. 354,440 14 Claims. (Cl. 117107.2)

This application is a continuation-in-part of co-pending application Serial No. 807,025, filed April 17, 1959, and now abandoned.

- This invention relates to the diffusion coating of metal articles for the production thereon of an outer coating or layer of enhanced oxidation and corrosion or erosion and thermal shock resistance at high temperatures, as well as other enhanced surface characteristics, in which the article to be coated is heated as embedded in or otherwise in surface contact with a powdered pack or mixture including a metallic coating material, and, more particularly, to the production of such diffusion coatings where the coating pack or mixture includes at least one ingredient other than the primary coating material for controlling the rate at which the coating material is presented to the surface of the article to be coated for diffusion thereinto.

As will be understood, there is now known a wide variety of different processes and techniques for producing a diffusion coating or layer into or on the surface of metal articles. Some of these well-known techniques (and those generally of the character to which this invention particularly relates) involve embedding the article to be coated (or otherwise covering the surface thereof)in powdered coating pack including a powdered source of the coating material (with or without admixture with powdered inert filler) and .a vaporizable carrier ingredient '(such as a heat-volatile halide),'and heating the thus embedded article and pack in a sealed retort (or other controlled and generally non-oxidizing atmosphereyto an elevated temperature at which the carrier material will vaporize and/or otherwise react with or function as a carrier for transporting the coating material from or through the powdered pack to the surface of the article to be coated for diffusion or other reaction thereat. I Generally speaking, the various chemical reactions involved (e.g.', between the carrier and the coating material, between the coating material -and the metal or other components of the article to be coated, among whatever ingredients are in the pack, between coating material and whatever inter-metallics or alloys may have already been formed at or in the surface of the article being coated, etc.) occur more or less simultaneously during the heating treatment and are mostly of a reversible nature, so that the net result of the coating step and the chemical reactions therein involved may depend predominantly upon the various equilibria achieved. That is, as will be understood, under certain temperature conditions and with certain reactive carrier materials, ingredients in the coating pack may be inclined to combine with each other .at the same time (and, perhaps, even at the same rate) as one or another thereof may diffuse into the surface of the metal article; while (if the equilibrium conditions are appropriate) some portion of the metal from the article itself, or one or another component thereof, may also diffuse out of the article and into the pack ingredients.

Although it has been possible in the past to select certain specific ingredients or operating conditions, on a more or less empiric-a1 basis, for obtaining some commercial effectiveness of such pack coating techniques, it

is to be understood that a variety of limiting factors may be present in commercial operations with different coating materials and different metal articles to be coated which are actually inconsistent or incompatible with the factors or operating conditions which may maximize or control the equilibria or reaction rates most favorable to achieving the desired efficient coating or recovery of a particular coating material into a particular metal article.

For example, as will be understood, the chemistry within the coating pack may determine, for a specific temperature, the rate at which coating material is delivered to the surface of a coated article for diffusion thereinto, yet this delivery rate may not necessarily be directly related to the rate of diffusion of the same coating material into the article after having penetrated the surface thereof. Under such circumstances, the situation may arise where a particular high temperature is adequate for forcing a particular coating material to begin to diffuse into the surface of the article, but yet so high, with respect to the'diffusion rate of the material inside the article, that the coating material will be driven on into the center of the article and away from the surface thereof under prolonged exposure to the same high temperature. Such a result, of course, may not produce the desired surface coating, and may even result in a situation where too little concentration of the coating at the surface is obtained because temperatures high enough to decompose the pack ingredients for diffusion in the first place may be sufficiently high to drive the diffused coating material too far into the interior of the article being coated.

Conversely, as will be understood, preliminary diffusion of some coating material into the article surface may so inhibit the penetration of additional coating material sufiicient to form thedesired thickness of coating as to be incompatible with a treating temperature consistent with forcing the reversible reactions in the coating pack toward the desired equilibrium result. As another possibility, the composition ofv the article being coated may be subject to crystallographic or metallog-raphi-c changes at different-temperatures under circumstances which are completely unrelated to the reaction rates of the various reactions in the coating pack itself so that treating temperatures sufficient to induce diffusion of the coating material toward or into the surface of the article being coated are too low to achieve the desired temperature or metallographic thermal condition of the article being coated to receive the coating material in the desired manner. Indeed, with some low melting articles, and utilizing treating techniques in which the retort itself is sealed by preliminary melting of the fusible seal and/or where a certain threshold temperature is necessary for breakdown of the coating pack, the article to be coated may appr-oximate-a molten condition before thermal conditions can be established promoting the desired diffusion from the coating materials in the pack.

As merely an illustrative example of such difficulties as may be encountered under commercial operations, one may note such situations as where it is desired to diffusion coat aluminum into the surface of a metal article including a substantial proportion of, for example, copper to form only a protective surface coating or layer, which layer, for ultimate protection, it is desired to have formed as a particular one of a variety of possible copper aluminides. It has been found that aluminum is'readily and rapidly diffused into such copper-containing articles at a relatively moderate treating temperature, with the diffusion rate increasing rapidly as the temperature is increased, while the particular copper aluminide formed in the metal article also depends upon the treating temperature. Under these circumstances, if the treating temperature is controlled to no more than will force diffusion of aluminum at a fairly moderate rate (so as to avoid deep diffusion of the aluminum and retain a surface coating or layer), it may be found that the particularcopper aluminide formed in the coating is not the one desired; whereas, on the other hand, if the treating temperature is raised to the point Where the desired particular copper aluminide is formed in the surface of the article, such higher temperature may produce an aluminum diffusion rate greatly in excess of that desired and/ or so as to produce too deep penetration of the aluminum into the article being coated.

Whereas the foregoing example is advanced here merelyas illustrating a type of problem encountered in the commercial utilization of the diffusion coating techniques, it is one frequently encountered in the field of diffusion coating of so-called superalloy materials (i.e., high temperature resistant alloys having substantialproportions of nickel, cobalt, chromium, etc), in which field diffusion coatings become increasingly important for the oxidation protection of such superalloys as they become increasingly necessary for use in jet engine, rocket mechanisms, and similar high temperature applications.

Generally, such problems of too rapid transfer rate of the coating material from the pack ingredients to the surface of the article to be coated may be frequently encountered in treating high temperature or complex alloys, on the one hand, or articles with which the coating material and article may-form low-melting complexes or systems, on the other hand, and particularly in situations Where it is desired to utilize a more or less standardized-commercial operating technique for the application of various different coating materials to a wide variety of metal articles of greatly different chemical and metallurgical characteristics. In such situations, for example, the particular temperature levels which must be obtained to instigate and maintain the initial reactions between the particular carrier material and the source of coating material in the pack and/or the thermal conditions and temperature levels which must be maintained in order to drive such reactions to completion within any commercially tolerable length of time may have no direct relation at all (indeed, they usually do not have) with the chemical characteristics and/ or diffusion rates of the coating material at or into the surface of different articles being coated.

Similarly, as will be understood, the complexity of the alloy or inter-metallic surface of the article being coated (either originally or after a portion of the coating material has been diffused thereinto) adds a further complication or additional factor, at least from the standpoint of predicting a set of operating conditions which will in all cases produce a satisfactory diffusion coating and/or permitting conditions which will drive one of the various possible reactions desirably toward the particular result desired. Especially is this true when (as in most cases) there is a variety of different intermetallic compounds which may result from the combination of a particular coating material with a par-' ticular ingredient in the article and under circumstances where a variety of such different compounds may be produced in the same article at different temperature levels and/or depending upon other operating conditions which may rarely be so compatible as to permit maximizing the operating conditions for one set of reactions without upsetting diffusion rate and temperature conditions within the article which might be desired to produce a particular intermetallic resultant.

According to this invention, however, a variety of techniques and compositions is provided for ready application to or inclusion in the diffusion coating pack itself for controlling or inhibiting the rate of transfer of the coating material (as accomplished by a volatile carrier material) from the pack ingredients to the surface of the article being coated for correlating such rate of transfer, even independently of the inherent characteristics of the coating material, so that suchtransfer rate or presentation rate is predictably correlated with or adjusted to the article being coated and the particular compositions and ingredients thereof so as to assure that,

at commercially effective temperature levels, the particular intermetallic resultant desired Will be formed in the article surface, to the exclusion of other possible resultants, and/or that presentation of the coating material to the article surface in view of the diffusion rate thereof into the article neither inhibits nor excessively accelerates diffusion or reactions within the article inimical to or inconsistent with the formation at the surface thereof of a diffusion coating layer of the desired thickness, composition, and protective characteristics.

To accomplish such ends in accordance herewith, one or more extra ingredients are added to or included in the coating pack for combination or interaction with the material to be coated and/ or the carrier component of the pack for forming a rate-controlling intermetallic or other compound with the coating material or otherwise altering the normal transfer rate of such material through the pack at the particular temperature being used, with such extra ingredients usually being metallic (by which term is meant to include elements such as silicon or boron as well as elements usually considered as metals) and even may itself not be diffused into'the metal article to form a significant or contributing part of the ultimately desired coating thereon. Furthermore, the selection of the particular extra ingredient hereof, with due regard to the particular car- 'er component being utilized, is specifically correlated to the metallic or chemical characteristics of the article being coated and the diffusion or reaction rates of the coating material therein and the variety of possible intermetallic compounds which may be formed between the coating material and components of the coated article of to the end of regulating or controlling the transfer rate of the coating material from the-pack to the surface of the article being coated to conform to predetermined temperature and other operating conditions so as to provide in the finished coating the particular intermetallic or alloy or other composition desired notwithstanding the fact that the normal or inherent forming of such desired composition may be quite inimical to or incompatible with the predetermined temperature or other operating conditions desired and/ or, indeed, impossible of formation under any operating conditions without the provision of the added rate-controlling ingredients.

With the foregoing and additional objects in view, this invention will now be described in more detail, and other objects and advantages thereof will be apparent from the following description and the appended claims.

Although, as noted above, this invention and the teachings thereof are applicable to a wide variety of different coating situations utilizing different coating materials and different compositions of articles to be coated, it may generally be convenient initially to describe techniques and compositions embodying and for practicing this invention as particularly, although merely illustratively, applied to forming an essentially aluminide coating on high-nickel and high-cobalt superalloys and utilizing a coating pack containing, in addition to the aluminum to be coated, an inert filler, a volatilizable halide material such as ammonium fluoride as the carrier component, and metallic chromium as the extra rate-controlling ingredient.

As will be understood, although such superalloys inherently exhibit physical properties within a desired range, particularly when subjected in use to extremely high temperatures, the oxidation resistance and/or corrosion resistance of the surface of such alloys, particularly when subjected for prolonged times to such high temperatures, maybe less than desired for prolonged or severe use. If it is attempted to increase the oxidation and corrosion resistance of the surface of such high-nickel or high-cobalt alloys by diffusion coating thereinto of a metal such as chromium according to contions to which the article may be subjected in use.

,ventional diffusion coating techniques, an enhancing of the oxidation and corrosion resistance may be experienced, but, less than the optimum which may be desired. Alternatively, attempting to enhance the corrosion resistance of such alloys by applying thereto a surface coating of aluminum as by an aluminum dip or similar conventional technique, the oxidation-resistant aluminum coating may be found to have less than optimum per forming qualities, particularly under severe thermal shock and thermal deformation to which such articles may be subjected in use.

Nevertheless, a diffusion coating of aluminum produces, at the surface of ,such alloy articles, a layer of substantially enhanced resistance to oxidation and erosion, as well as a casing of good and uniform adherence to the article, even during thermal shocks and deforma- This latter characteristic is ofparticular importance in the production of oxidation-resistant coatings on high temperatu're alloys subjected to severe thermal conditions because, if the supposedly oxidation-resistant coating or outer casing is not maintained uniformly continuous and firmly adhered to the article during thermal deformations thereof, fissures or other discontinuities may occur in the oxidation-resistant coating as a result of thermal shock, which fissures, once having occurred, readily present easy access to the base metalof the article for oxidation corrosion or erosion thereof.

Considering the aluminum coatings of this particular illustrative example, especially satisfactory results are achieved with base metal alloys containing a substantial or preponderant proportion of nickel or cobalt and chromiu'me.g., such alloys as are particularly formulated for high temperature use and having physical properties and a useful life as desired when subjected for prolonged duration to both very high temperatures 7 and to severe thermal shocks and rapid changes of temperature over wide ranges. Whereas such superalloys, as well understood, may have small amounts (e.-g., usually less than 10%) of iron, they are composed primarily of a substantial proportion of chromium (e.g., about 10%- 20%), with at least about 50% or more of the composition being made up of nickel or cobalt and/or mixtures thereof.

Merely as illustrative of the types of high temperature alloys for which this invention is particularly adapted, one may note a commercial nickel-base alloy. sold by Utica Metals Division of Kelsey-Hayes Corporation under the designation Udimet 500 and a high cobalt-containing alloy commercially manufactured and sold by the Haynes-Stellite Division of Union Carbide under the designation X-40. Such alloys have approximately the following compositions (according to the respective manufacturers specifications) in weight percent:

High nickel alloy: Percent Carbon 0.12 Chromium 19 Cobalt 19 Iron 1 Molybdenum 4 Aluminum 3 Titanium 3 Nickel balance High cobalt alloy: Percent Carbon 0.50 Chromium 24.5 Nickel 10.5 .Tungsten 7.4

Iron 1 Cobalt balance Also illustrative of the type of superalloy materials with which satisfactory results are achieved with this invention are the alloy high temperature steels and alloys having about equal proportions of nickel, cobalt and iron.

Also as further illustrative of the procedures and coatbe noted that articles of such high temperature alloys for which this invention is particularly adapted are satisfactorily coated by procedures including embedding the article to be coated in a dry powder pack including an inert mineral material, a source of the metallic elements to be diffusion coated, and a source of a vaporizable or diffusible halogen. As embedded in such a pack (preferably contained within a metal container the seams of which are sealed by a fusible material such as a low-melting silicate to prevent excessive escape of the diffusing materials during heating and also excessive introduction of air into the pack during cooling), the articles are heated to a substantial temperature for a number of hours to cause diffusion coating of the desired metals, in conjunction with the elemental halogen, into the surface of the articles being treated.

Satisfactory results have been achieved according to this invention in so coating articles formed from any of the illustrative alloys mentioned above with the use of a coating pack comprising, as illustrative of this invention,

.approximately 69% alumina as the inert mineral, 22%

chromium metal and 8% aluminum metal as the metals to be diffusion coated, and 1% elemental iodine as the vaporizable halogen, the foregoing percentages being by weight. With such a pack, enclosed in a container in known manner for producing diffusion coating of various materials on various metallic alloys, satisfactory results have been achieved according to this invention by heating the articles of high-nickel or high-cobalt content in the pack for from 4 to 20 hours at temperatures of from about 1800 F. to 2100" F.

In connection with the foregoing ranges, it should be noted that, if thinner coating layers or cases are desired (e.g., where severe thermal shock and deformation of the articles are anticipated), the diffusion coating is carried out, preferably, at the lower temperatures and/or for shorter times within the foregoing ranges, and, where thicker cases may be desired (e.g., where oxidation and erosion resistance is of more importance than resistance to the possible disruption of the coating layer or casing by thermal shock), the diffusion coating step is conducted at higher temperatures and/or for longer times, thereby appropriately controlling the thickness of the diffused coating layer or casing produced in accordance with this invention.

As will be understood by men skilled in the art of the diffusion coating of metals, the proportions of materials, and the materials themselves, suggested in the above mentioned pack may be varied over fairly wide ranges. Thus, the proportion of inert filler is not critical and, although alumina is a preferred filler material for such a pack,

other inert fillers are satisfactory. Similarly, other halo gen sources than the elemental iodine mentioned above give satisfactory results provided they are capable of producing a vaporized halogen at the temperature and under the conditions of operation. In addition to elemental iodine, ammonium iodide and ammonium fluoride are' particularly satisfactory. Also as noted, the time and temperature conditions of the diffusion coating step may, to some extent, be varied depending upon the thickness of the coating desired. Satisfactory results have been achieved with coatings of the order of .001 to .002 inch,

although even thinner coatings give substantially enhanced oxidation and other protection to the base alloy,

and, for some applications, thicker coatings may be} sisting thermal shock and thermally induced dimensional variations to which the finished article will be subjected in use. Also the aluminum coatings, according to this invention, in addition to being oxidation-resistant, also exhibit good resistance to chemical corrosion attack, other than oxidation, at high temperature, as well as resistance to thermally induced metallurgical changes even after prolonged exposure to very high temperatures. Such coatings also exhibit good continued adherence to the coated article despite rapid and severe temperature changes to which the article may be subjected in use.

Purely as illustrative of some of the foregoing advantages, certain comparative test data may be noted. For example, an article composed of one of the highcobalt alloys to which this invention relates exhibits a satisfactory life of no more than about hours when continuously exposed to a temperature of about 2000 F., and the same alloy with a protective coating of aluminum alone exhibits a satisfactory life of only about 20 to 30 hours under exposure to the same high temperature, with a coating of chromium alone affording less protection than the aluminum coating. By contrast, the same highcobalt alloy having a diffusion coating of aluminum according to this invention exhibits a satisfactory life of 150 hours or more under continuous exposure to a temperature of 2000 F. One of the high-nickel alloys, without a protective coating of either chromium or aluminum, may exhibit a satisfactory life of up to 100 hours at 2000 F., with little or no extension of such life when the alloy is provided with a coating of aluminum alone or chromium alone. By contrast, however, satisfactory life of 150 hours or more at 2000 F. is achieved by such a high-nickel alloy when provided with an aluminum diffusion coating according to this invention.

Similar surprisingly enhanced results may also be noted with regard to thermal shock tests, and this characteristic of coatings embodying this invention may, for many purposes, be at least as significant as resistance to continued high temperature exposure. That is, considering, for example, parts of gas turbine airplane engines which operate at extremely high temperatures, but not necessarily for long periods of continuous operation, the internal parts of such gas turbine engines subjected to high operating temperatures must also be able to withstand the thermal shock incident to a sudden increase in temperature in but a few minutes all the way from the ambient temperature outdoors in the winter to the extremely high operating temperature of the engine upon starting of the engine, and, shortly thereafter, upon stopping, a severe fast cooling from the operating temperature back down to the ambient temperature. One of the high-nickel alloys was observed to Withstand about 130 rapidly repeated cycles of heating from room temperature up to 1850 F. to 2000 F. and cooling back down to room' temperature without failure, while a similar article composed of a high-cobalt alloy withstood about 260 cycles of the same testing.- Neither of these articles was notably improved in this respect by a coating of aluminum alone, and only slightly improved by a coating of chromium alone. By contrast, however, when coated according to this invention with a chromiumaluminum coating, both the high-nickel and high-cobalt parts withstood at least 1000 of such heating and cooling cycles satisfactorily and without failure.

Satisfactory results are achieved in accordance herewith utilizing widely varying proportions of the materials in the coating pack. For example, it is to be recognized that the rate controlling or inhibiting mechanism of a material such as chromium in an aluminizing pack may be related to the preliminary formation within the pack of a chromium aluminide at temperatures perhaps below the ultimate treating temperature. Although both chromium and aluminum may be initially present in the pack in elemental form, chromium aluminides may be formed during heating the pack and preferentially or prior to transfer or diffusion of the aluminum component into the surface of the article being coated. Thus, if a relatively stablechromium aluminide be formed in the pack prior to transfer of aluminum to the surfaceof the article being coated, the medium from which aluminum is to be transferred to-the article being coated is not metallic or elemental aluminum, but the chromium aluminide, which may require decomposition in order to diffuse aluminum into the surface of the article, especially if the diffusion rates (or even the possibility of diffusion) of the chromium aluminide is not thermodynamically effective or achievable at the particular treating temperature.

It has been found, as noted above, that aluminum diffusion into the surface of a predominantly nickel-containing article may be productive of a variety of different nickel aluminides, with the particular one formed being perhaps a function of the proportion of aluminum carried to the article surface or diffused therein at the particular operating temperature. If the particular aluminide desired is one containing less than the maximum amount of aluminum, formation thereof may not occur if aluminum from the pack be too rapidly presented to or available in the article surface. If the treatment is maintained at a sufficiently high temperature and prolonged to achieve a desired thickness of coating layer, too rapid transfer of aluminum from the pack to the surface of the article (or diffusion from the surface on inwardly of the article) may occur to form a lower melting high-aluminum aluminide rather than the one desired. By including a portion of a material, such as chromium, in the pack to form with the aluminum therein a preliminary intermetallic the diffusion of which into the surface of the article can only occur at a diminished rate (or, perhaps, cannot occur at all) at the desired treating temperatures, the availability or transfer of the aluminum component in the pack for diffusion into the surface of the article is readily inhibited or controlled so that the temperature levels or other thermodynamic conditions necessary to break down the preliminary chromium aluminide sufficiently for aluminum to be diffused will produce the desired conditions for the formation of the particular nickel aluminide desired in the surface of the article.

Utilizing an inhibiting or rate controlling component such as chromium in an aluminum pack-for preliminary formation therein of a chromium aluminide has been found to produce satisfactory results in that temperatures high enough to break down the chromium aluminide for diffusion coating of aluminum are at levels where the desired particular nickel aluminide will be formed in the surface of a nickel-containing alloy (and substantially the same considerations have been found to apply to highcobalt alloys). In fact, once a chromium aluminide is formed in the pack ingredients, it may be necessary, in addition to temperature control, to utilize an extremely aggressive carrier material (e.g., a fluoride instead of an elemental halogen or a chloride or iodide) for sufficiently aggressive attack to break up a preliminarily formed intermetallic (somewhat along the lines as disclosed in Patent No. 3,096,205). As will be understood, of course, the inhibiting chromium aluminide may be formed preliminarily to the actual coating operation and the pack composed of such intermetallic, or elemental chromium and aluminum may be added to the pack originally with the article therein, since the preliminary formation of the inhibiting aluminide occurs during heating of the pack and, generally, prior to any significant diffusion of aluminum (or simultaneously with preliminary diffusion) in accordance with the various inherent equilibria and thermodynamic conditions of the pack ingredients.

Generally, in these particular illustrative examples in accordance herewith, satisfactory results are achieved in the aluminizing of such high-nickel and high-cobalt superalloys utilizing a variety of different chromium-aluminum ratios or proportionings in the pack ingredients and with or without an inert powdered filler material (such as alumina, kaolin, and similar refractory inerts) within the broad outlines of the foregoing disclosure and teachings. As Will be understood, however, too large a proportion of ranges.

inert filler may interfere with any diffusion coating mere-- hereof, it will be understood that mere dilution of the diffusible ingredients of the pack, without other controls, may produce unsatisfactory results regardless of how effective it is for inhibiting the transfer or presentation of the diffusible metallic component to the surface of the article being coated.

Although satisfactory results in accordance herewith are achieved in the absence of any inert filler at all, some such filler component is usually to be preferred, and commercial results may be generally enhanced if the active (i.e., diffusibl-e metallic) components of the pack are kept above about by weight of the pack. Conversely, and recalling that it is the aluminum component which is to be diffused (and which, inherently, will both melt and diffuse at lower temperatures than the chromium component), some proportion of inert filler or diluent (such as alumina) is generally to be preferred in cases Where the aluminum proportion of the pack approaches or exceeds the chromium proportions, at least in certain temperature At a given temperature, the coating thickness achieved may generally increase as the metallic components of the pack increase, yet purely economic considerations may indicate the desirability for a substantial proportion of filler in any case.

At treating temperatures susceptible to aluminum diffusion, many, if not most, proportions of chromium (as the inhibiting ingredient) do not require an inert filler for operability, satisfactory results being achieved without any inert filler in situations where the chromium proportion predominates (e.g., in the range of a pack composed of 80% metallic chromium and 20% metallic aluminum). From the standpoint primarily of ready commercial applicability to high-nickel and high-cobalt alloys, however, chromium-aluminum proportionings in the pack ingredients above 90% chromium are not preferred (although they may be operative) in accordance herewith primarily because of the thermodynamically engendered .difficulty of driving the aluminum component of such compositions adequately or controllably into diffusion relation with the article being coated, although such commercial difficulties may primarily relate to sintering of the pack ingredients around or to the surface being coated or similar purely mechanical side effects which can, if desired, be controlled by other techniques. As will also be understood in accordance with the foregoing, excessively high aluminum proportions (predominating over chromium) in the pack ingredients may defeat the ratecontrolling effect of the chromium, while excessively high or preponderantly chromium proportions over the aluminum may defeat or inhibit the desired aluminum diffusion.

Thus, generally speaking, chromium-to-aluminum ratios (by weight) in the pack ingredients substantially less than 0.5 may include so little chromiurn that the ratecontrolling effect thereof is either negligible or less than to be desired as compared with a straight aluminum pack; whereas, chromium-to-aluminum ratios substantially above 4.6 may produce an ultimate product having less than optimum oxidation resistance, perhaps because of too great an inhibition of aluminum available for diffusion coating or because of some actual diffusion coating of chromium instead of aluminum. Various determinations throughout such proportion ranges indicate that, in accordance herewith, chromium-to-aluminum ratios Within the broad ranges of from about 0.1 to 8 may be considered operative, although such chromium-to-aluminum ratios in the pack ingredients initially beyond the range of about from 0.5 to 4.6 are generally not preferred for standard commercial operations with most types of superalloy and other articles to be aluminized in accordance herewith. For certain applications, of course,

and especially to protect the article being coated fromother than oxidation (as, for example, diffusion coatings to protect nickel articles from sulphur attack, etc), satisfactory results may be achieved outside these ranges, While yet utilizing the rate-controlling advantages hereof and obtaining the enhanced results of this invention in the diffusion coating of articles with one or another of the ingredients of the coating pack While benefitting from the rate-control or diffusion-inhibiting advantages obtainable in accordance herewith.

Actually, in most instances, the character of the particular coating actually diffused into the surface of the article changes substantially little throughout a very wide range of chromium-to-aluminum ratios in the pack, although the depth of coating and other operating characteristics may be predictably and controllably varied for commercial production reasons Within suchranges. For example, even when the chromium component of the coating pack is as high as 92% by weight, it is primarily aluminum which is diffused intothe surface of the article (although such aluminum diffusion may be excessively inhibited by reaction or merely dilution at such high chromiurn levels). Furthermore, extensive and careful analyses (by micro-probe techniques) on coated superalloys (as well as on coated pure nickel and pure cobalt articles) indicate primarily aluminum diffusions in accordance herewith. Although some chromium aluminides (and/ or other chromium components) in the coated surface of the article can be analytically detected, they are apparently due to the presence of substantial portions of chromium in the base alloy itself (since they do not occur to the same extent in articles having no chromium in the base alloy), and rarely exceed fractions of one percent even when the invention was applied to pure nickel or pure cobalt articles and notwithstanding a preponderant proportion of chromium in the coating pack.

This is believed to be a further indication that the presence of materials such as chromium (and capable of combining with the aluminum in the pack preliminarily) are effective in accordance herewith primarily as a transfer-inhibiting or rate-controlling ingredient, rather than a diffusion component of the pack, at least at operating temperatures where the desired aluminum diffusion coating is to be obtained with such metal articles and within the proportion ranges noted above. With excessively high chromium components in the pack, however, some little chromium diffusion may occur (and is to be noted by the appearance of minor portions of green chromium oxide after severe oxidation testing of the coated article) although such effects are both minor and to be understood as completely ancillary to the principal rate-controllingfunction of an extra metallic ingredient (such as chromium) in the coating pack itself for the purposes of rate control of transfer of the coating metal from the pack to the article being coated in accordance herewith.

As will be understood, although the foregoing illustrative explanation is primarily stated in terms of a highnickel superalloy material as the article to be coated, substantially the same disclosure obtains with regard to high-cobalt superalloys (as mentioned above) and the formation in the diffusion coated surface layer thereof of an appropriate cobalt aluminide. For example, in addition to the Udimet 500 nickel alloy and the X40 cobalt alloy noted above, satisfactory results have also been achieved in accordance herewith in the provision of aluminum diffusion coatings (and utilizing chromium as the rate-controlling ingredient of the pack) with a variety of other base metal alloys and articles. For example, among these may be noted high-cobalt alloys such as that designated as WI-52 and containing approximately 63% cobalt, 20% chromium, 11% tungsten, 2% nickel, 1.5% colombium, 0.4% carbon, and the balance iron. Similarly, satisfactory results have also been achieved in accordance herewith on such base metal mai i terials as standard specification SAE 1045 and SAE 52100 steels, etc., as Well as on such materials as relatively pure iron (Armco iron) and commercially pure nickel (A nickel).

In such coating operations, the coating ack initially contained metallic chromium metal and metallic aluminum metal approximately in the ratios by Weight of chromium in the amount of 24 times the amount of aluminum and with the aluminum comprising about 3% to 20% by weight of the coating pack, with a substantial amount of the pack (e.g., 60%70%) being made up of powdered alumina and with about 1% of ammonium fluoride as the carrier component. One satisfactory composition is that noted above for all of these commercial treatments of the various metals noted-Le, 69% alumina, 22% metallic chromium, 8% aluminum metal, and 1% ammonium fluoride. In all instances, when the treating retort was opened and unpacked, the powdered pack materials readily fell away from the articles being coated, which had a desired smooth surface and distinct color with coating layer or casing depths of the order of 0019-0022 on the nickel and cobalt articles and higher on the ferrous articles. Severe oxidation testing at 2000 F. in an oxidizing atmosphere provided in a standard testing furnace indicated no failure of any of these coated-parts in over 85 hours of testing treatment.

As will also be understood in accordance with the foregoing, a variety of other materials or ingredients may be included in the coating pack composition to achieve the desired rate-controlling or inhibiting action therein for systems providing an aluminized diffusioncoating, as

well as for other coating materials. For example, satisfactory results have also been achieved in accordance herewith utilizing nickel as the rate-controlling ingredient in an aluminizing pack for the coating of, not only highnickel or high-cobalt superalloys, but also such refractory metals as molybdenum and with illustrative pack compositions such as 40% metallic nickel, 20% metallic aluminum, 40% inert filler (alumina) and a vaporizable halogen carrier component such as ammonium fluoride. Similarly, such materials as cobalt, vanadium, carbon, silicon, and even, in some cases, iron, produce satisfactory results in accordance herewith, as rate-controlling ingredients for diffusion coating packs where aluminum and/or various other metals are being diffused into the surface of a variety of materials.

Both nickel and cobalt, as well as silicon and silicon carbide, produce rate-controlling results in the diffusion coating of, for example, chromium as the coating material and on a variety of the superalloys and other materials noted, but especially on high-nickel alloys such as Incaloy articles containing about 13% chromium and 44% nickel with the remainder being smaller proportions of molybdenum, titanium, aluminum, iron, boron, and carbon. Also, the metals such as nickel and chromium, among other materials, are also satisfactory for inhibiting the transfer rate of lower melting coating materials (e.g., copper or iron) in the transfer and diffusion thereof from a coating pack into the surface of an article being coated and especially where the composition of the article and/or the thermal characteristics of the coating material are such as to promote too rapid transfer or diffusion at treating temperatures as high as may be desired in standard commercial operations. Similarly, in such applications as the siliconizing of steel or other ferrous alloys at temperatures high enough to produce internal diffusion rates of silicon higher than desired, control of transfer of silicon from the packto the metal article is readily achieved in accordance herewith by causing combination of the silicon with an added ingredient in the pack, and so that a treating temperature may be selected to produce the formation of a particular si-licide'in the article notwithstanding a normally excessive diffusion rate at such temperaturej Thus, generally and merely as illustrative, metals such as chromium, nickel, iron, or

12 cobalt may be considered as satisfactory rate-controlling ingredients for diffusion coatings of aluminum or silicon, while such materials as silicon or iron or carbon satisfactorily control the transfer of chromium, while silicon also is useful with aluminum coatings and iron similarly for silicon or aluminum, etc.

It is also to be understood that the utilization of the foregoing materials as rate-controlling or inhibiting ingredients in the various coating packs mentioned or included in accordance herewith does not necessarily exclude some diffusion of the rate-controlling ingredient itself, although usually of a minor and/ or ancillary nature, and such operations are generally to be distinguished from situations where an additional ingredient is actually added to the pack purposefully to be diffused along with the primary coating metal as, for example, the addition of carbon to the pack for carburizing or controlling the decarburizing of a ferrous alloy (and within the article, not by reaction in the pack) during the heating treatment for diffusing some other primary coating metal into the article. Also, to be distinguished are such situations where one or another of the materials noted may be used primarily as a getter for one or another of the possible resultants of one or another of the concurrent reversible reactions (e.g., to getter iron halide in the diffusion coating if iron materials using a halide carrier component) for accelerating or otherwise controlling which one of the several possible reactions goes farthest to completion.

Similarly, the foregoing disclosure is to be understood as relating primarily to rate-controlling techniques involving inhibiting the transfer of the coating material from the pack to the surface of the article by chemical reaction thereof with other ingredients in the pack (and/ or, as noted, by controlling the decomposition of any resultants of such chemical reaction by a selected degree of aggressiveness of the carrier component), rather than to such mechanical expedients of rate control as simple dilution of the pack with inert ingredients and/ or such techniques as may involve actual preliminary treatment of the surface of the article being coated for inhibiting penetration thereto (or thereinto) of pack ingredients (such as the techniques disclosed in the co-pending application of Martin Epner relating to preliminary coating or masking of the article surface for mechanical or chemical interference with transfer or diffusion of coating'materials from the pack, or that of Walter Butler relating to preliminary or simultaneous coating of the article itself with one diffusion material for altering the acceptance charac teristics of the article surface for another or primary diffusion material).

As noted above, at least with the high-nickel and highcobalt superalloy materials illustratively disclosed, satisfactory results are achieved with a coating pack having some 69% or 70% inert filler (such as alumina) and diffusible metallic components in the amount of, for example, 22% chromium and 8% aluminum to begin with, along with an effective amount of about 1% vaporizable halide carrier. In commercial operations, as will be understood, it is generally intended to reuse a given quantity of coating pack again and again. Since each use of a given quantity of pack exhausts some componentstat least the aluminum and carrier components) to some extent, effective reuse of the powdered pack ingredients requires some regeneration thereof (as, for example, adding an additional 20% of the metallic component thereto, along with additional carrier component), so that the precise composition of the pack itself constantly changes, as a result of reaction exhausting of reactant components thereof as well as the inevitable mechanical losses of handling. As has been found, in accordance herewith,

that, although a starting pack composition is preferred as above disclosed, continued use and reuse of such a pack, with the necessary replenishment thereof, results in a certain equilibrium condition in the pack, which condition has been noted to be approached even after utipeated uses of the pack ingredients and upon normal regeneration thereof with about 20% active ingredients. Similarly, although the temperature and time ranges noted above may also be preferred as a standard commercial operating condition, it is to be understood that, especially with chromium-aluminum packs in the aluminizing treatment of high-nickel and high-cobalt superalloys, satisfactory results are also obtained by holding the articles to be coated embedded in the pack at temperature (i.e., after heating the retort up to temperature) for a time range of approximately -%-40 hours within a temperature range of about l400-2200 F., depending, of course, on the particular material being coated and the depth or thickness of coating case desired.

The foregoing, of course, relates to a situation where the particular carrier material is selected specifically to attack and break down the rate-controlling intermetallic formed in the pack ingredients, as noted above, although the selection of a particular carrier material, with due regard to its aggressiveness of activeness with the coating metal (whether or not preliminarily combined with another ingredient in the pack), as well as the quantity thereof, are also to be considered as a rate-controlling technique within the disclosure above.

Accordingly, and as will understood from all of the foregoing, there are provided herewith techniques and compositions for controlling the diffusion coating of a variety of coating materials into a variety of coated metal articles in a manner so that the intermetallic compounds 4 or alloys formed in the surface of the article being coated may be adequately controlled and predicted notwithstanding the fact that the characteristics inherent in the coating pack and the constituents thereof, as well as the diffusion or transfer rates for any particular operating temperature, are different from or actually incompatible with the conditions desired for the formation in the coated article of the specifically and optimumly desired product; and the techniques and compositions in accordance herewith satisfactorily accommodate such inimical or incompatible situations for controlling the composition and activity of the coating pack in a manner, perhaps, different than the inherent characteristics thereof for the purpose of controlling the composition of the ultimately desired product by inhibiting or controlling the rate or quantity at which the coating material is presented or transferred from the powdered pack to the surface of the article being coated and, preferably, by the addition to the coating pack of a combining reactant for producing the controlled transfer effect desired of the coating material to the surface of the article being coated by or through the medium of a vaporizable carrier component.

While the methods and compositions herein disclosed form preferred embodiments of this invention, this invention is not limited to these precise methods and compositions and modifications thereof may be provided without departing from the scope of this invention which is defined in the appended claims.

What is claimed is:

1. In a method for the production of a diffusion coating of the character described on the surface of an alloy base metal having high temperature resistant characteristics and a substantial proportion of approximately half of a metal selected from the group consisting of nickel and cobalt, and a substantial proportion of chromium, thev steps which comprise embedding said alloy base metal in a diffusion coating pack including a source of chromium metal and sufficient aluminum metal for effecting diffusion coating thereof into the surface of said base alloy and a source of vaporizable halogen as a carrier for said base metal having high temperature resistant characteristics and having a substantial proportion of approximately half of a metal of the group consisting of nickel and cobalt and a substantial proportion of chromium, the steps which comprise embedding said alloy base metal in a diffusion coating pack including a source of chromium metal and sufficient aluminum metal for effecting diffusion coating thereof into the surface of said base alloy and a source of vaporizable halogen as a carrier for said aluminum in said diffusion coating thereof and powdered filler material, heating said base alloy in said pack to a temperature of at least about 1800 F. effecting diffusion coating of said aluminum into the surface of said article to produce said diffusion coating of aluminum thereon said aluminum being present in an amount at least 3% by weight of said pack.

3. In a method for the production of a diffusion coating of the character described on the surface of an alloy base metal having high temperature resistant characteristics and having a substantial proportion of approximately half of a metal selected from the group consisting of nickel and fusion coating pack including a source of chromium metaland sufficient aluminum metal for effecting diffusion coating thereof into the surface of said base alloy and a source of vaporizable halogen as a carrier for said aluminum in the diffusion coating thereof and powdered filler material, heating said base alloy in said pack effecting diffusion coating of said aluminum into the surface of said article, said chromium being present in said pack in a ratio of about 2-4 times by Weight the amount of aluminum therein.

4. In a method for the production of a diffusion coating of the character described on the surface of an alloy base metal having high temperature resistant characteristics and having a substantial proportion of approximately half of a metal of the group consisting of nickel and cobalt and a substantial proportion of chromium, the steps which comprise embedding said alloy base metal in a diffusion coating pack including a source of chromium metal and sufficient aluminum metal for effecting diffusion coating thereof into the surface of said base alloy and a source of vaporizable halogen as a carrier for said aluminum in the diffusion coating thereof and powdered filler material, heating said base alloy in said pack effecting diffusion coating of said aluminum into the surface of said article, said aluminum comprising about 3%-20% of said pack by 'weight and said chromium being present in an amount about 2-4 times by weight the amount of said aluminum.

5. A metal article of the character described and susceptibleto long exposure to an oxidizing and corrosive atmosphere at high temperature and resistant to the thermal shock incident to repeated heating and cooling between ambient temperature to high temperature, and comprising a base alloy including a substantial proportion of aproximately half of at least one of the metals selected from the group consisting of nickel and cobalt and a substantial proportion of chromium, which article is enclosed within a diffused outer layer case including aluminum. and said outer layer case having been formed on said article in accordance with the method recited in claim 1.

6. A metal article as recited in claim 5 in which said outer layer case is at least about 0.00l"0.002" thick over said article.

7. In a method for the production of a diffusion coating of the character described on the surface of an alloy base metal having high temperature resistant characteristics and a substantial proportion of approximately half of a metal selected from the group consisting of nickel and cobalt, and a substantial proportion of chromium, the steps which comprise embedding said alloy base metal article in a diffusion coating pack including a source of chromium metal and sufiicient aluminum metal for eflfecting diffusion coating thereof into the surface ofsaid base alloy and a source of vaporizable halogen as a carrier for said aluminum in said diffusion coating thereof, the weight ratio of said chromium to said aluminum in said powdered pack being substantially within the range of about 0.5 to 4.6, and heating said basemetal alloy in said pack effecting diffusion coating of said aluminum into the surface of said article.

8. A method as recited in claim 7 in which said heating step is accomplished at temperatures within the range of about 1400-2200 F. and prolonged for a time at said temperature within the range of about 4-40 hours.

9. In a diffusion coating process where a metallic coating is diffused into the surface of a metal article by heating such metal article in a non-oxidizing atmosphere in a sealed powdered diffusion coating pack including the metallic coating material to be diffused into said article and a carrier component for effecting the transfer of said metallic coating material from said pack to the surface of said article, the steps which comprise incorporating in said diffusion coating pack an additional metallic ingredient for combining chemically with said coating material in said pack for inhibiting and controlling said transfer thereof to the surface of said article to be coated, effecting said chemical combination with said additional ingredient, and transferring said coating material from said chemical combination to the surface of said article for said diffusion coating thereinto of only said coating material and substantially in the absence of diffusion of said additional metallic ingredient into said article.

10. A process as recited in claim 9 in which said metallic coating material to be diffused into said article is selected from the group consisting of aluminum, chromium, iron, silicon, and mixtures thereof.

11. A process as recited in claim 9 in which said metal article to be coated comprises a substantial proportion of a metal selected from the group consisting of chromium, cobalt, copper, iron, nickel, the refractory metals, and alloys and mixtures thereof.

12. A process as recited in claim 9 in which said additional metallic ingredient added to said pack for combining chemically with said coating material comprises a material selected from the group consisting of aluminum, beryllium, boron, carbon, chromium, cobalt, iron, nickel, the refractory metals, silicon, and mixtures and alloys thereof.

13. A process as recited in claim 9 in which said metal article to be coated comprises a substantial proportion of a metal selected from the group consisting of chromium, cobalt, copper, iron, nickel, the refractory metals, and alloys and mixtures thereof; and in which said metallic coating material to be diffused into said article is selected from the group consisting of aluminum, chromium, iron, silicon, and alloys and mixtures thereof; and in which said additional metallic ingredient added to said pack for combining chemically with said coating material is selected from the group consisting of aluminum, beryllium, boron, carbon, chromium, cobalt, iron, nickel, the refractory metals, silicon, and alloys and mixtures thereof.

14. In a method for the production of a diffusion coating of the character described on the surface of an alloy base metal having high temperature resistant characteristics and a substantial proportion of approximately half of a metal selected from the group consisting of nickel and cobalt, and a substantial proportion of chromium, the steps which comprise embedding said alloy base metal article in a diffusion coating pack including a source of sufficient aluminum for effecting ditfusion coating thereof into the surface of said base alloy and a source of vaporizable halogen as a carrier for said aluminum in said diffusion coating thereof and an additional separate metallic ingredient for combining chemically with said aluminum in said pack for inhibiting and controlling said transfer thereof to the surface of said alloy base metal, and heating said alloy base metal in said pack effecting diffusion coating of said aluminum only into the surface of said article substantially in the absence of diffusion of said separate metallic ingredient thereinto.

References Cited by the Examiner UNITED STATES PATENTS 1,899,569 2/1933 Howe. 2,536,774 1/ 1951 Samuel. 2,811,466 10/1957 Samuel 117-22 2,837,442 6/1958 Seelig et al. 2,874,070 2/1959 Galrniche 117-1072 2,875,090 2/1959 Galmiche 117-1072 X 3,061,462 10/ 1962 Samuel. 3,073,015 1/1963 Wachtell et al. 3,079,276 2/1963 Puyear et al. 117-1072 3,096,160 7/1963 Puyear et al. 3,096,205 7/1963 De Guisto 117-1072 3,108,013 10/1963 Pao Jen Chao et al. 117-1072 3,117,846 1/1964 Pao Jen Chao 117-1072 X FOREIGN PATENTS 586,241 3/1947 Great Britain.

685,683 1/1953 Great Britain.

722,797 2/ 1955 Great Britain.

749,056 5/1956 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. RICHARD D. NEVIUS, Examiner. R. S. KENDALL, Assistant Examiner.

Claims (1)

1. 9. IN A DIFFUSION COATING PROCESS WHERE A METALLIC COATING IS DIFFUSED INTO THE SURFACE OF A METAL ARTICLE BY HEATING SUCH METAL ARTICLE IN A NON-OXIDIZING ATMOSPHERE IN A SEALED POWDERED DIFFUSION COATING PACK INCLUDING THE METALLIC COATING MATERIAL TO BE DIFFUSED INTO SAID ARTICLE AND A CARRIER COMPONENT FOR EFFECTING THE TRANSFER OF SAID METALLIC COATING MATERIAL FROM SAID PACK TO THE SURFACE OF SAILD ARTICLE, THE STEPS WHICH COMPRISE INCORPORATING IN SAID DIFFUSITON COATING PACK AN ADDITIONAL METALLIC INGREDIENT FOR COMBINING CHEMICALLY WITH SAID COATING MATERIAL IN SAID PACK FOR INHIBITING AND CONTROLLING SAID TRANSFER THEREOF TO THE SURFACE OF SAID ARTICLE TO BE COATED, EFFECTING SAID CHEMICAL COMBINATION WITH SAID ADDITIONAL INGREDIENT, AND TRANSFERRING SAID COATING MATERIAL FROM SAID CHEMICAL COMBINATION TO THE SURFACE OF SAID ARTICLE FOR SAID DIFFUSION COATING THEREINTO OF ONLY SAID COATING MATERIAL AND SUBSTANTIALLY IN THE ABSENCE OF DIFFUSION OF SAID ADDITIONAL METALLIC INGREDIENT INTO SAID ARTICLE.