US3442720A - Method of forming ti-modified silicide coatings on cb-base substrates and resulting articles - Google Patents

Description

May 6, 51969 g s] F.-B-RA-0LEY I-:rAL 3,

METHOD QF FORMING Ti'fMODIFIED SILICIDE COATINGS O -BASE SUBSTRATES AND RESULTING ARTICLES Sheet Q of 3 Filed Oct. 23, 1965 F/GJ SILICIDE ZONE SUBSILICIDE ZONE Cb-ZOTol5W-5Mo ALLOY SUBSTRATE 500x PHOTOMICROGRAPH OF SILICIDE COATING OVER Cb-ZOTu-ISW- 5M6 ALLOY SUBSTRATE.

"DISTURBED" Cr-MODIFIED SILICIDE COATING cb-uMo-zct ALLOY SUBSTRATE PHOTOMICROGRAPH OF SILICIDE COATING MODIFIED BY ABOUT 1% BY WEIGHT Cr OVER Cb-HMO2Cr ALLOY SUBSTRATE.

INVENTORS ELIHU F- BRADLEY EDWIN S. BARTLETT HORACE R. OGDEN ROBERT I. JAFFEE Era/2 eyaxz @{He/Zc/ersan ATTORNEYS y 6, 1969 F. BRADLEY ETAL 3,442,?39

METHOD OF FORMING Pi-MODIFIED SILICIDE OEWMNGS 0N Cb-BASE SUBSTRATES AND RESULTING ARTIGLES 3 Sheet Filed Oct. 23, 1965 Iooox V-MODIFIED SILICIDE COATING, EQUIAXED STRUCTURE SUBSILICIDE ZONE Cb-IZV ALLOY SUBSTRATE PHOTOMICROGRAPH OF SILICIDE COATING OVER Cb-l2V ALLOY SUBSTRATE,SHOWING EQUIAXED STRUCTUREOF PROPORTIONATELY VANADIUM MODIFIED SILCIDE COATING.

FIG. 4

TITANIUM-MODIFIED SILICIDE COATING NON-OXIDATION RESISTANT SUBSILICIDES CONTAMINATED Cb- |2Ti ALLOY SUBSTRATE PHOTOMICROGRAPH OF SILICIDE COATING, MODIFIED BY 7% BY WElGHTTi OVER Cb -l2Ti SUBSTRATE,AFTER OXIDATION AT2200 F FOR 100 HOURS.

ATTORNEYS y 1969 E. F. BRADLEY ETAL; 3,442,726

METHOD OF FORMING III-MODIFIED SILICIDE COATINGS ON Cb-BASE SUBSTRATES AND RESULTING ARTICLES Filed Oct. 25, 1965 Sheet 3 of 3 FIG. 5

Ti -VMODIFIED SILICIDE COATING OXIDATION RESISTANT SUBSILICIDES PROTECTED Cb9Ti-3V ALLOY SUBSTRATE 2 x PHOTOMICROGRAPH OF 6Ti-2VMODIFIED SILICIDE COATING OVER Cb-9Ti-3V ALLOY SUBSTRATE AFTER OXIDATION AT 2200F FOR 100 HOURS.

Ti-Cr-MODIFIED SILICIDE COATING OXIDATION RESISTANT SUBSILICIDES PROTECTED Cb- 6Ti 3Cr ALLOY SU BST RATE PHOTOMICROGRAPH OF 4Ti-2Cr -MODIFIED SILICIDE COATING OVER Cb-6Ti-3C ALLOY SUBSTRATE AFTER OXIDATION AT 2200F FOR 100 HOURS.

INVENTORS ELIHU F- BRADLEY EDWIN S. BARTLETT HORACE R.OGDEN ROBERT LdAFFEE ATTORNEY 5 United States Patent METHOD OF FORMING Ti-MODIFIED SILICIDE COATINGS ON Cb-BASE SUBSTRATES AND RE- SULTING ARTICLES Elihu F. Bradley, West Hartford, Conn., and Edwin S.

Bartlett, Worthington, and Horace R. Ogden and Robert I. Jaifee, Columbus, Ohio, assignors, by direct and mesne assignments, to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Oct. 23, 1965, Ser. No. 506,144 Int. Cl. C231? 7/00; C22c 27/00 US. Cl. 148-6.3 26 Claims ABSTRACT OF THE DISCLOSURE Oxidation resistant articles are provided comprising a Cb-base alloy substrate having superimposed thereon a surface coating zone consisting essentially of columbium silicide modified by 7-35% by weight of the coating of a metal modifier selected from the group consisting of Ti, and Ti in combination with at least one of V and Cr. A pack cementation process is preferably employed for depositing the silicide coating.

This invention relates to novel coatings for columbiumbase alloys that will protect the base metal or substrate from oxidation in high-temperature environments and to a method for creating such coatings.

More particularly, this invention relates to modified columbium silicide coatings for columbium-base alloys in which the silicide portion of the coating is created by methods, such as vapor deposition (particularly pack-cementation), eleotrophoretic deposition, and the like, and the modifiers for the coatings are titanium or a combination of titanium and at least one of vanadium and chromium. The invention also particularly relates to a method for creating such titanium-modified silicide coatings on columbium-base substrates by vapor deposition to produce a protective surface layer or zone over such substrates that provides oxidation-resistance at high temperatures, such as, for example, up to at least 2200 F. in air, or even higher for short exposure times.

The principal limitation in gas turbine technology today is the maximum turbine inlet temperature. The maximum turbine inlet temperature is, in turn, controlled by the maximum temperature that turbine blades and vanes are able to withstand without danger of failure. Formerly, the best available high-temperature alloys were nickel-and cobalt-base superalloys, but critical structural components, such as turbine blades and vanes made from such alloys, are limited to maximum operating temperatures of between 1800 and 1900 F.

For many years it has been generally known that the high-temperature strength properties of metals are closely related to their melting points. In general, metals having high melting points are capable of forming alloys having high strength at high temperatures.

The need for structural materials that can be used at temperatures in excess of those obtainable with existing structural materials has stimulated interest in the metals having the highest melting points, i.e., the refractory metals, particularly, chromium, columbium, vanadium, hafnium, tantalum, molybdenum, and tungsten.

Molybdenum was once considered the chief candidate for use as a base metal in high-temperature alloys. At the elevated temperature service conditions needed, however, molybdenum not only oxidizes, but the oxide formed is also volatile. Once the oxidation reaction sets in, it tends to progress rapidly until molybdenum is consumed at a catastrophic rate.

3 ,442,720 Patented May 6, 1969 As an alloy base material for high-temperature service, columbium ultimately offers more promise, and considerable interest has been shown in its development as a structural alloy base for use in high-temperature environments. Among the technically most important physical qualities of columbium as an alloy base are its high melting temperature (about 4475 F.) and its low neutroncapture cross section. Columbium is, therefore, potentially useful in such structures as test aircraft, space flight vehicles, and nuclear reactors.

Moreover, columbium is inherently a soft, ductile, readily fabricable material, and although it becomes too weak for practical structural uses at temperatures above 1200 B, it can be readily strengthened for use at much higher temperatures by alloying with various other metals, and particularly by alloying with other refractory metals. Columbium is also a highly reactive metal in that it dissolves large quantities of oxygen and also nitrogen, upon exposure to air or to atmospheres containing even small amounts of these elements at relatively modest temperatures.

Although columbium oxidizes rapidly at high temperatures, in contrast to molybdenum, which oxidizes catastrophically, columbium oxide does not volatilize. It is thus potentially possible to prevent oxygen attack on columbium by coating the metal, and if premature localized coating failure should occur, to restrict such failure and oxygen, attack to the localized site. Further advantages offered by Cbover Mo-base alloys are that Cb-base alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum.

The history of columbium alloy technology has, however, demonstrated the incompatibility of achieving oxidation resistance and high-temperature strength through alloying alone. Since the major uses for Cb-base alloys are as structural components in high-temperature applications, it is apparent that useful classes of high-temperature columbium alloys will require protective coatings in their normal high-temperature oxidizing environments.

A particularly important potential area of use for Cbbase alloys as dictated by economic and technological factors is in structural materials, such as turbine blades for jet engines, which are designed for exposure to oxidizing and corrosive combustion gas environments at temperatures up to about 2000 F. (a temperature that clearly establishes utility for these alloys) and higher. Concomitantly, such alloys must be able to resist mechanical stresses for appreciable periods of time at these high temperatures.

About 500 F. is the maximum operating temperature to which Cb-base alloys can be subjected for extended times in an unprotected or uncoated condition without serious resultant oxidation. At temperatures much above 500 F. the oxidation problem becomes acute.

The art has previously recognized certain oxidation-resistant intermetallic coatings as exhibiting particular potential for protecting refractory metals (e.g., columbium, molybdenum, tantalum, and tungsten) from oxidation at high temperatures. In general, the more effective of these ilntermetallic coatings are silicides, aluminides, and beryllides of the base metal.

In considering coatings for the refractory metals, both the coating and substrate material importantly affect the performance of the coated systems. For example, a silicide coating over columbium may perform quite differently from one over molybdenum with the dilference in performance attributable to the substrate rather than to the coating type. As an additional confirmation of the importance of the substrate, some species of coating that are reliably protective over certain of the refractory metals are ineffective over columbium and are susceptible to failure on columbium at high temperatures. Coating and substrate must thus be coordinated and treated as an integrated system. Success with a particular coating on a particular refractory metal base does not mean the coating will be successful when used on a different refractory metal base.

Several methods, such as, flame or plasma torch spraying, slurry application techniques, electrophoretic deposition, hot pressure bonding, or vapor deposition have been used for applying intermetallic coatings to Cb-base alloys. A vapor deposition process that can be used advantageously to achieve some types of coatings is the so called pack-cementation process, in which the object to be coated is surrounded by a particulate pack mixture containing, for example, (1) the metal to be reacted with (or deposited upon) the object to be coated (e.g., silicon, aluminum, beryllium), (2) an activator or energizer (usually a halide salt, such as, NaCl, NaF, KF, NH Cl, and the like), and (3) an inert filler material (e.g., A1 SiO BeO, MgO, and the like).

This mixture, held in a suitable container (such as, a steel box, a graphite boat, or a refractory oxide crucible), is then heated to the desired coating temperature in a prescribed atmosphere and held for a length of time suflicient to achieve the desired coating. When conducted properly, the pack-cementation process may be used to produce controlled-thickness coatings on columbium, the major portions of which will be compounds, such as, CbA13, CbSi and the like.

The more favorable coatings for columbium (columbium aluminides, silicides, and beryllides) possess certain intrinsic deficiencies such as rapid oxidation failure at low temperatures (in the vicinity of about 1300 F.) or at high temperatures (about 2000 F. and above). Perhaps, the most serious deficiency of existing coatings for columbium, however, is their propensity for failing at localized sites.

Silicide coatings on columbium and its structural alloys are more stable than aluminides and have a better thermal expansion match with the substrate than beryllides. In fact, beryllides have such a severe thermal expansion mismatch with columbium that their use is prohibited in many applications. With columbium, the silicides are thus coating materials of primary interest.

Silicide coatings on structural Cb alloy substrates, however, are prone to consumption by rapid oxidation at low (about 1300 F.) temperatures. This characteristic of silicide coatings is sometimes termed the silicide pest phenomenon. Modification of silicide coatings is thus highly desirable to impart sufficient longevity and reliability to give to them a utility they do not normally possess.

In view of the foregoing, it is a primary object of this invention to provide novel and improved Ti-modified columbium silicide coating compositions that will protect Cb-base substrates from the effects of oxidation at temperatures up to at least about 2500 -F. and that will achieve substantially uniform and homogeneous Timodified silicide coatings that exhibit equiaxed grain structures and high resistance to failure at localized sites.

Another object of this invention is to provide new and improved Ti-modified columbium silicide coatings for Cbbase substrates that overcome the silicide pest phenomenon characterized by rapid consumption of silicide coatings through oxidation at temperatures of about 1300 F., and that also overcome rapid consumption of columbium silicide coatings through oxidation at high temperatures of about 2000 F. or higher thereby providing coatings that give excellent oxidation resistance at temperatures up to at least 2500 F.

Further objects of this invention are to provide a novel and improved coating for Cb-base substrates that in addition to providing resistance to simple thermal oxidation will also be protective under other reasonably expected conditions of use, and to this end, the protective coatings of, this invention achieve good resistance to thermal cycl- 7 ing, thermal shock, formation of defects, and high velocity gas erosion, and exhibit and achieve good thermal chemical stability.

The term thermal chemical stability as used herein means the ability of a silicide coating to retain its coating integrity, adherence, and oxidation resistance at a relatively low temperature in the silicide pest phenomenon range (i.e., about 1300 F.) following a substantial exposure (e.g., hours) at relatively high temperatures, such as, for example, 2000 F. or higher.

Other objects of this invention are to provide for columbium and its alloys:

(1) A coating that in nominal thicknesses of 2 mils or more is capable of providing protection for exposures to high-temperature oxidizing environments for times in excess of 100 hours at temperatures of up to at least about 2700 F.;

(2) A coating that exhibits good resistance to thermalshock failure;

(3) A coating that displays excellent resistance to the formation of defects at both higher and lower temperatures of exposure;

(4) A coating that achieves significant resistance to high velocity gas erosion; and

(5) A coating that is relatively insensitive to substrate geometry effects, or the shape of the substrate on which it is applied.

A further object of this invention is to provide a new and improved Ti-modified silicide coating for columbium and its alloys that includes a sublayer or subzone formed from subsilicides of the substrate which subzone acts as an oxidation penetration barrier to give back-up protection to the substrate should a defect in the primary surface coating occur, and which also acts as a thermal expansion buffer and thereby prevents failure of the coating due to thermal expansion mismatch between the Timodified silicide compound of the coating surface zone and the substrate itself.

A still further object of this invention is to provide a new and improved method for coating Cb-base substrates with a columbium silicide coating by vapor deposition (pack-cementation) of a silicide coating over a Cb-base substrate that has been previously modified by alloying with titanium, or by alloying with titanium and at least one metal selected from the group consisting of vanadium and chromium, to create thereby a substantially homogeneous and uniformly modified columbium silicide coating. Such coatings have an equiaxed structure, maintain their uniformity and homogeneity on even intricately shaped parts and at the edges and corners of parts, and resist local failure.

Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention, the objects and advantages being realized and attained by means of the composition, methods, and processes particularly pointed out in the appended claims.

In its broadest embodiment this application provides an oxidation-resistant coating for Cb-base alloys, the coating consisting essentially of columbium silicide modfied with from about 7 to 35% by weight of the coating of a metal modifier selected from the group consisting of: (a) titanium (Ti), and (b) titanium (Ti) and at least one metal selected from the group consisting of vanadium (V) and chromium (Cr).

To achieve the foregoing objects, and in accordance with its purpose, this invention in one embodiment provides an article of manufacture having good resistance to oxidation that comprises a substrate consisting essentially of Cb and from 12 to 65% by weight of the substrate of Ti, the article having an oxidation resistant surface zone consisting essentially of columbium silicide modified by a Ti content of from 7 to 35% by weight of the surface zone.

In a broader embodiment, this invention also provides a new and improved article of manufacture having good stress-rupture strength at high temperatures, high-temperature oxidation resistance, and resistance to cyclic fatigue failure, which article comprises a substrate selected from the group consisting of: (a) columbium and an alloying addition of from 12 to 65% by weight of the substrate of titanium (Cb-(12 to 6-5) Ti), and (b) columbium and an alloying addition of from 12 to 65% by weight of the substrate of a metal modifier consisting essentially of titanium and at least one metal selected from the group consisting of vanadium and chromium [Cb- (12 to 65)(Ti-V)], [Cb-(12 to 65)(Ti-Cr)], or [Cb- (12 to 65 )(Ti-V-Cr)]; the article having a thermal cyclic failure resistant, defect resistant, and broad range oxidation-resistant coating or surface zone consisting essentially of columbium silicide modified by a metal modifier in an amount of from 7 to 35% by weight of the coating, the metal modifier being selected from the group consisting of: (a) titanium, and (b) titanium and at least one metal selected from the group consisting of vanadium and chromium.

In a preferred embodiment, the invention also includes an article having good resistance to oxidation at elevated temperatures which comprises a substrate consisting essentially of Cb and from 12 to 65 by weight of the substrate of an alloying addition consisting essentially of Ti and V, the article having an oxidation-resistant coating consisting essentially of a surface zone of columbium silicide modified by a metal modifier in an amount of from 7 to 35% by weight of the coating, the metal modifier consisting essentially of Ti and V.

Further in accordance with its purpose, this invention provides a process for producing a coated metal article having resistance to oxidation at high temperatures, the metal article comprising a substrate consisting essentially of Cb and from 12 to 65% by weight of the substrate of an alloying addition of a metal modifier selected from the group consisting of: (a) Ti, and (b) Ti and at least one metal selected from the group consisting of Cr and V; which process comprises contacting the substrate with a powdered pack of a finely ground source of Si and a small amount of a volatilizable halide salt, as active ingredients, and in inert filler; heating the substrate and powdered pack for a time period sufiicient to cause volatilization of the halide salt, producting deposition of Si on the surface of the substrate, and thereby effecting the creation of an exterior surface zone on the substrate consisting essentially of columbium silicide modified by an amount of the metal modifier consisting essentially of 7 to 35% by weight of the exterior surface zone.

In accordance with the invention, most preferred results are obtained when the metal modifier includes V in addition to Ti. Preferred results, in general, are also obtained when the metal modifier is titanium alone, and the Ti is present in amounts from 13 to 28% by weight of the surface zone or coating. These coatings are produced by siliconizing Cb-alloy substrates containing 23 to 55% Ti (e.g., Cb-23Ti to Cb-55Ti). The most preferred Ti-modified coating of this invention contains 14% by weight Ti and is produced by siliconizing a Cb- 25Ti alloy substrate.

When the metal modifier consists essentially of both Ti and V, it is preferred that Ti content be from 4 to 26% by weight of the coating and V content be from 2 to 6% by weight of the coating, provided the aggregate of Ti and V in the coating is at least 7% by weight of the coating. Such coatings can be prepared by the proportionate modification of silicide coatings, by siliconizing, in accordance with the process of this invention Cb-base alloy substrates containing 6 to 50% by weight Ti and 3 to 12% by weight V. An aggregate Ti and V content of 12 to 16% in the alloy substrates siliconized in this manner is greatly preferred. These substrates 4 to 5% by weight of the coating of Ti and 4 to 5% by weight of the coating of V. These more preferred coatings are prepared by proportionate silicide modification of Cb-(69)Ti-(69)V alloy substrates. These coatings can therefore be represented by the formula [Cb- (6-9)Ti-(6-9)V]Si Coatings produced by siliconizing the following alloys in accordance with this invention produce the best results of all: Cb-6Ti-9V, and Cb-9Ti- 6V. Of these, alloy substrate (Cb-6Ti-9V), which contains about 6% by weight of Ti and about 9% by weight of V, is the optimum alloy of this invention.

In Ti-V (or Ti-Cr) modified coatings, in accordance with this invention, at least 2% V (or Cr) content (of the coating) is needed to give oxidation resistance to the subsilicide zone at manimum modification levels. On the other hand V contents (of the coating) above about 15% should not be used.

Thus, where the V content is inherent in the substrate, amounts above the critical 15% maximum limit in the coating should be avoided for two principal reasons:

(1) If the V content exceeds this maximum limit, excessive diffusional growth of the subsilicide zone results and produces a corresponding depletion of the protective disilicide coating or surface zone and a shorter life of the roating; and

(2) A V content in the substrate in excess of this maximum limit can produce V enrichment of the substrate zone immediately beneath the coating to an extent whereby any local coating failure results in formation of V 0 in large amounts, with resulting catastrophic oxidation of the coated system, thus negating one of the major advantages of Cb-base alloys over Mo-base alloys.

Where V containing coatings are applied to structural substrates it is also important that the V content not exceed this 15 maximum limit because if the applied coating has too high a V content, V will diffuse from the coating into the substrate and result in significant degradation of substrate strength. Typical Cb-base structural substrates to which these coatings can be applied include V-free or low-V containing Cb-base substrates, i.e., substrates containing V only in amounts up to about 5%. V may be desirable in some Cb-base structural substrates because, in low amounts, it has a modest strengthening effect on Cb-base substrates and also decreases the alloy density. In amounts above 5%, however, V tends to degrade the strength of the substrate.

In Ti-Cr modified silicide coatings, in accordance with this invention, Ti contents of 4 to 26% by weight of the coating and Cr contents of 2 to 15 by weight of the coating are preferred, provided that the aggregate of Ti and Cr is at least 7% and not in excess of 35% by weight of the coating. Such coatings can be prepared by siliconizing, in accordance with this invention, an alloy substrate of the formula Cb-(650) Ti-(329)Cr with the proviso that the Ti and Cr, in the aggregate, are between 12 and 65 by weight of the substrate. Even more preferred are Ti-Cr modified silicide coatings containing 4 to 6% Ti by weight of the coating and 2 to 5% Cr by weight of the coating, provided again that the aggregate of Ti and Cr is at least 7% by weight of the coating.

An optimum Ti-Cr modified coating is produced by siliconizing the following alloy substrate in accordance with this invention:

Coatings having Cr contents above 16% Cr have diffusion properties that could result in severe degradation of the ductility of the coated system when the coatings of this invention are used with structural Cb-base alloys.

.At minimum modification levels, with Ti-V or Ti-Cr coatings, it has been found that a Ti to V, or a Ti to Cr, ratio of 2:1 produces preferred results.

A combination metal modifier containing Ti, V, and Cr can also be used, although the combination of all three modifiers in one coating system does not produce the superior coating properties attainable with Ti-V modifiers. The aggregate amount of Ti, V, and Cr in these tri-modified coatings should be between 7 and 35% by weight of the coating. These coatings can be produced by siliconizing, in accordance with this invention, Cb-Ti-V-Cr alloy substrates containing, in the aggregate, from 12 to 65% by weight of the substrate of Ti, V, and Cr.

As pointed out above, the use of a Ti-V combination metal modifier is the most preferred embodiment of this invention. Although each of Ti-V, and Cr ias modifiers lends valuable oxidation resistance and other desirable properties to the coatings of this invention, each of these modifiers, in one manner or another, is capable of adversely affecting the structural properties of the substrate.

f the modifiers of this invention, Ti is the most detrimental to high-temperature structural strength of Cbbase alloys, while V is the least detrimental. Thus, while the presence of certain Ti levels is necessary to the achievement of the benefits provided by this invention, the use of V in combination with Ti permits use of lesser amounts of Ti, by substitution of less detrimental V. In addition to the fact that V is less detrimental to the structural properties of Cb-base alloys than is Ti (and thus substitution of V for Ti lessens the overall detrimental effect imposed by the total modifier), it has been found thatTi and V modifiers, when used in combination, produce a synergistic effect.

Much lower total amounts of combined Ti-V modifier can be used with benficial results equivalent to those obtained when much greater levels of straight Ti-modification are used. For example, the coatings produced by siliconizing Cb-6Ti-9V substrates (the most preferred substrate composition of this invention) achieve equivalent oxidation resistance, erosion resistance, thermal chemical stability, and thermal shock resistance to that obtainable with high Ti-modification (e.g., Cb-SOTi or Cb-25Ti), at much lower aggregate amounts of Ti-V modifier, and even lower amounts of the more deleterious Ti.

Of course, the detrimental effect of the instant modifiers on the high-temperature structural properties of Cb-base substrates makes it desirable that minimum modification levels be used where possible, and in most instances the lowest amount of modifier that produces the desired oxidation resistance and other beneficial coating properties is desirable to preserve desired structural properties to the extent possible.

While in its preferred embodiments, this invention comprehends the production of the instant coating by siliconizing Cb-Ti, Cb-Ti-V, Cb-Ti-Cr, or Cb-Ti-V-Cr alloy substrates, resulting in the formation of proportionately modi fied columbium silicide coatings, it will be understood that, in its broader aspects, this invention comprehends application of the oxidation-resistant coatings of this invention to other Cb-base structural alloy substrates, such as, for example, Cb-ZOTa-ISW-SMO, and the like. Such application could be accomplished, for example, either by incorporating a metal modifier (Ti, Ti-V, etc.), in accordance with this invention in the substrate, or by physically adhering thin sheets or foils of the Cb-base alloy substrates of the coatings of this invention (e.g., Cb- 6Ti-9V, Cb-9Ti-6V, etc.) to structural alloy substrates before, or possibly even after, siliconizing. Such adherance also could be achieved, for example, by a combination of physical and chemical means.

Use of the instant coatings on structural alloy substrates is possible, because even after inclusion in the substrate of elements electronegative with respect to Cb, such as W or M0, the metal modifiers of this invention have sufficient beneficial effect on silicide oxidation resistance to overcome the expected detrimental effect on oxidation resistance of structural strengtheners which are electronegative with respect to Cb. For example, an alloy substrate consisting essentially of 8 Cb-20Ta-l5W-5Mo-21Ti when siliconized in accordance with this invention, achieves a modified silicide coating exhibiting the improved oxidation resistance and other benefits provided by this invention.

It will be appreciated that physical or a combination of physical and chemical adherence of coatings of this invention to substrates not containing the metal modifiers (Ti, Ti-V, etc.) may be desirable in some instances for structural reasons; hence, such means, rather than direct inclusion of the modifiers of this invention in the substrate can be used to achieve the beneficial results of this invention. Thus the coatings of this invention can be removed from the substrates of this invention by grinding, machining, etc., and applied to structural Cb-base alloy substrates by plasma or flame spraying, electrophoretic deposition, powder or slurry processes, or the like.

As previously set forth, conventional silicide coatings on structural Cb-base alloy substrates are prone to rapid consumption through oxidation at low (about 1300 F.) temperatures (this tendency is sometimes referred to as the silicide pest phenomenon) and at high (about 2000 F. or higher) temperatures. At the latter temperatures a rapid oxidation mechanism occurs which, though different from the pest phenomenon, is similar in its harmful end result.

Quite surprisingly, in accordance with the invention, it has been discovered that if silicide coatings for columbium and its structural alloys are modified through substrate modifications of:

(1) Titanium (Ti), or

(2) Titanium and vanadium (Ti-V), or

(3) Titanium and chromium (Ti-Cr), or

(4) Titanium and both vanadium and chromium (Ti-V-Cr) within the ranges taught herein for these respective e1ements, the deleterious effects of both the low temperature silicide pest phenomenon and the high temperature rapid oxidation mechanism are essentially overcome.

The Ti-modified silicide coatings of this invention are thus particularly outstanding in their ability to protect columbium and its alloys from oxidation under a wide variety of conditions of use and at temperatures ranging from room temperature up to about 2700 F. These coatings possess distinctly superior oxidation resistance and superior defect insensitivity up to at least about 2700" F. and overcome and counteract the tendency of unmodified or Ti-free silicide coatings on Cb-base substrates to fail at critical temperatures of about 1300 F. and about 2000 F. or above.

Coating failure at low temperatures (about 1300 F.) occurs by rapid disintegration of the coating to a fine, intermetallic powder that spalls from the surface of the substrate, leaving it unprotected against subsequent oxidation attack. Although the mechanism by which such low-temperature powdering occurs has not been established, two possible mechanisms that have been suggested by research observations are:

(1) Selective grain boundary (or other preferred directional) oxidation of the coating, and

(2) Localized detecting of the coating that allows substrate or subcoating oxidation with consequent voluminous oxide growth at the coating-substrate interface. This latter mechanism results in excessive pressure build-up at the interface and consequent spalling of the unoxidized coating.

In accordance with the invention, it has been discovered that both mechanisms can be altered and corrected by changes in coating chemistry and structure. For example, the normal disilicide structure, shown in FIG. 1 (photomicrograph of a disilicide coating over alloy showing structure enlarged 500 times), consists predominantly of columnar grains with axes oriented normal to the substrate surface. The grain boundaries disclose areas of atomistic imperfection that are highly susceptible to failure by either chemical or mechanical forces. With either of the two mechanisms just described, such a structure presents the least resistance to failure.

At higher temperatures (above about 2000 F.), coatings based on the CbSi phase oxidize by forming heterogeneous, non-protective oxide phases, and relatively rapid linear oxidation of the coating results. To correct this deficiency, in accordance with this invention, chemical modification of the columbium disilicide (CbSi coating has been achieved by adoption of principles analogous to those used in some areas of metallic alloy technology.

It has thus been discovered that during reaction of pack-cementation atmospheres with Cb-base alloy substrates, the chemical elements present in the substrate generally react on a proportionate basis with the coating at mosphere. This results in a distinct and controllable modification of coating chemistry.

For example, during pack siliconizing of Cb, the predominant resulting coating phase is CbSi When, however, a Cb-base alloy, such as, Cb-20Ta-15W-5Mo (additions expressed in percent by weight), is pack siliconized, a coating is formed that is structurally similar to CbSi but which has a chemical analysis corresponding to the compound OJB O.12 0.0 0.06 2, I

By chemically modifying the substrate upon which the pack-cementation-reacted coatings are formed, the resulting coating can thus also be modified in chemical composition.

In accordance with the invention, by this substrate modification technique it has been further discovered that modifications of columbium silicide coatings (particularly CbSi coatings) with titanium (Ti), or titanium and at least one metal from the group: vanadium and chromium (T i-V, Ti-Cr, or Ti-V-Cr) are particularly advantageous in improving both principal deficiencies of unmodified CbSi coatings, to wit:

(1) Rapid oxidation at low (about 1300 F.), and

(2) Rapid oxidation at high (above about 2000 F.) temperatures.

Thus, in accordance with this invention, specific elemental modifiers within specific ranges ofalloy content may be used to homogeneously and uniformly modify columbium silicide coatings to improve significantly coating performance over that attainable with unmodified columbium silicide coatings.

For a clearer understanding of the invention, specific examples are set forth in the description that follows. These examples are merely illustrative and are not to be understood as limiting the scope and underlying principles of the invention in any way.

In accordance with this invention, the Cb-base alloy substrates set forth below, most of which consist essentially of Cb and various amounts of alloying elemental additions of Ti, Ti-V, Ti-Cr, and Ti-V-Cr, were prepared as set forth in the description that follows this enumeration of the alloys.

For illustrative and comparison purposes a number of the alloys listed are not direct examples of this invention; however, Alloys through 12, 22 through 45, and 47 below are all true examples of the invention. The composition of each alloy, excluding incidental impurities, is given in percent by weight of each element present.

Alloy 3 Titanium 3 Columbium 97 Alloy 4 Titanium 10 Columbium 90 Alloy 5 Titanium 12 Columbium 88 Alloy 6 Titanium 12.5

Columbium 87.5

Alloy 7 Titanium 15 Columbium 85 Alloy 8 Titanium 17.5

Columbium 82.5

Alloy 9 Titanium 20 Columbium 80 Alloy 10 Titanium 23 Columbium 77 Alloy '11 Titanium 25 Columbium I Alloy 12 Titanium x 50 Columbium 50 Alloy 13 Titanium 75 Columbium 25 Alloy 14 Titanium 3 Chromium 1 Columbium 96 Alloy 15 Titanium 3 Vanadium 3 Columbium 94 Alloy 16 Titanium 3 Chromium 3 Columbium 94 Alloy 17 Titanium 4 Vanadium 3 Columbium 93 Alloy 18 Titanium 6 Vanadium 3 Columbium 91 Alloy 19 Titanium 6 Chromium 3 Columbium 91 Alloy 20 Titanium 3 Chromium 6 Columbium 91 Alloy 21 Titanium 9 Chromium 1 Columbium 90 Alloy 22 Titanium 9 Vanadium 3 Columbium 88 Alloy 23 Titanium 6 Vanadium 6 Columbium 88 Alloy 24 Titanium 9 Chromium 3 Columbium 88 Alloy 25 Titanium 6 Chromium a 7 Columbium 87 Alloy 26 Titanium 6 Vanadium 9 Columbium 85 Alloy 27 Titanium 9 Vanadium 6 Columbium 85 Alloy 28 Titanium 9 Chromium 6 Columbium 85 Alloy 29 Titanium 12 Vanadium 3 Columbium 85 Alloy 30 Titanium 15 Chromium Columbium 80 Alloy 31 Titanium Chromium 13 Columbium 72 Alloy 32 Titanium 15 Chromium 29 Columbium 56 Alloy 33 Titanium 25 Vanadium 6 Columbium 69 Alloy 34 Titanium 25 Chromium 6 Columbium 69 Alloy 35 Titanium 25 Chromium 13 Columbium 62 12 Alloy 36 Titanium 25 Chromium 29 Columbium 46 Alloy 37 Titanium 50 Vanadium 6 Columbium 44 Alloy 38 Titanium 50 Vanadium 12 Columbium 38 I Alloy 39 Titanium 3 Vanadium 6 Chromium 4 Columbium 87 Alloy 40 Titanium 6 Chromium 4 Vanadium 3 Columbium 87 Alloy 41 Titanium 3 Chromium 8 Vanadium 3 Columbium 86 Alloy 42 Titanium 15 Chromium 13 Vanadium 6 Columbium 66 Alloy 43 Titanium 15 Chromium 29 Vanadium 6 Columbium 50 Alloy 44 Titanium 25 Chromium 30 Vanadium 6 Columbium 39 Alloy 45 Titanium 3 Chromium -i 8 Columbium 89 Alloy 46 Titanium 3 Chromium 3 Vanadium 3 Columbium 91 Alloy 47 Titanium 21 Tantalum 20 Tungsten 15 Molybdenum 5 Columbium 39 The foregoing alloys were consolidated by standard nonconsumable arc-melting in a chilled copper crucible, using a tungsten electrode, in a high-purity helium atmos phere. The as-cast buttons were machined to provide a number of nominal X A x 4 inch rectangular tabs. Sharp corners and edges were rounded off by filing. Before Percent by weight Si powder 17 NaF powder 3 A1 powder 80 These packs, contained in covered steel or graphite cans, were then subjected to a temperature of about 2200 F in an argon atmosphere for about 4 hours, Mter this treatment, the specimens were cooled and recovered from the first pack, repacked in a fresh pack mix of the same composition as the first pack mix, and recycled for times ranging /2 to 12 hours at about 2200 F. The resulting modified disilicide coatings were homogeneous, sound, and uniform. They ranged in thickness from 3 to 6 mils.

As described below, the compositions of the modified silicide coatings created by such treatment were determined or controlled by substrate composition. The improved behavior displayed by such modified coatings, however, primarily resulted from the chemical modification achieved and was dependent upon proportions of modifying ingredients in the coating. The proportions of modifiers in each coating were determined in turn largely by the proportions of elemental modifiers in the substrate.

Several modified silicide coatings produced in this manner were chemically analyzed by electron-beam microprobe analytical techniques. Analytical results for two of these coatings (Alloys 2 and are shown in Table l, and demonstrate modification of columbium silicide coat ings by proportionate reaction between the siliconizing atmosphere and the alloy substrates to yield substantially proportionately modified silicide coatings.

The Ti-modified columbium silicide coatings created by the process described above were evaluated by the following tests:

(1) Cyclic oxidation tests in air at: (a) 1300 F., and (b) 2200 F.

(2) Metallographic examination of the coating structures: (a) as-coated, (b) after 1300 F. oxidation, and (c) after 2200 F. oxidation.

(3) Electron-beam microprobe analysis to determine the chemical composition of the coatings.

(4) Thermal chemical stability tests.

Oxidation tests were conducted in ambient air without forced air flow. During testing, specimens, supported on refractory oxide boats, were inserted in an electrically heated mufile furnace preset at the desired temperature. Specimens were removed periodically from the furnace, and cooled to room temperature for visual examination and weighing, after which they were returned to the furnace for additional oxidation exposure.

TABLE l.--CHEMICAL ANALYSIS OF MODIFIED DISILICIDE COATINGS Anl t Nominal a y real Results Substrate Cb Ta W Mo Ti Si Composition, Area of Analyzed Alloy werght percent Analysis w/o a/o w/o a/o w/o a/o w/o a/o w/o a/o w/o e/o Composition 01.0 74.2 20.7 13.0 13.3 8.2 4.0 4.7 0 0 0 0 Cb-21Ta-l3W-4Mo. 2 38.2 14.6 4.4 9.6 3.0 2.0 1.1 0 0 35.6 09.2 (Cb23Ta-l5W-3Mo)Slg. 5 CMZTL 87.8 79.0 0 0 0 0 0 0 12.1 21.0 0 0 b-12'li.

,0 24,3 0 0 o 0 0 0 7.1 6.7 42.9 6 -1 n By this concept of proportionate reaction, the modified The time intervals for cyclic exposures were as set columbium silicide coatings of Alloys 1-47, produced by forth in Table 3 below: siliconizing these alloys in accordance with the procedures of this invention, were determined to have the com- 50 position set forth in Table 2 below. TABLE 3 TABLE 2.COMPOSITIONS OF MODIFIED SILICIDE COAT- Time for Cumulative INGS PRODUCED BY PROPORTIONATE MODIFICATION Cycle in Time in OF SILICIDE COATINGS BY ELEMENTAL ALLOYING Cycle Hours Hours ADDITIONS TO Cb-BASE SUBSTRATES 1.5 1.5 Substrate 1. 5 3. 0 Composition, Modified Silieide 1 5 4, 5 weight Coating Composition, 15. 5 20, 0 Alloy No percent weight percent 5 0 25, 0 25.0 50.0 1 100Gb 62Cb-38S1. 25.0 75,0 2 60Cb-20Ta-l5W-5Mo. 38Cb-15Ta-10W-2Mo-35Si. 25, 100, 0 3. 97Cb-3Ti 60Cb-2Ti-38Si. 4- 90Cb-10Ti 54Cb'6Ti-40Si. 5- 88Cb-12Ti. 53Cb-7Ti-40Si. 7" ili s i 2383*?? 1 i -9 i i.v 8 82.5CM7'5TL isobdOTHzsL Test1ng at each temperature was drscontrnued upon 800M013 460M2TM2SL failure of the speclmen, or after a total of 100-hours 10 77Cb-23Ti 44Cb-13Ti-43Si. n 750mm 43CM4TM3SL 5 oxidation wtrhout failure was achieved, except that cer 12 C .50Ti 2 5 .2 5'1 47 v tern of the alloys were sub ected to 2200 F. llfe tests to 13 25Cb-75Ti 12Cb-37Ti-51Si.

14 gfiobyfidcr 59Cb 2Ti 1Cr 38si a maximum of 300 hours. The specimens used 1 n these 15 94Cb 3Ti.3v 57Ob.2Ti.2v.39si tests were examlned every 25 hours after the 1n1t1al 100- 94Cb-3Ti-3Cr 58Cb-2Ti-2Cr-38Si. 1

93CMTL3VU sficbsTmvagsi. hours 8 cycle exposure, with testing berng dlscontrnued on 9lCb-6Ti-3 Cb-4Ti-2V-39Si. fallure or on reaching 300 hours. 9lCb-6Ti'3Cr 55Cb-4Ti-2Cr-39Si. MCMTHSCL" EwMTHCHQSL Metallographic examlnatlon of the coatrngs prior to (mean-10 540b.5'1-i 1 r40s testing revealed very srgnlficant structural features, par- 88Cb-9Ti-3V 53Cb-5Ti-2V-40Si. 88Cb 6Ti 6v 52Gb 4T1 4V 405i. trcularly when modified with preferred amounts of T1 88Cb-9Ti-3C 53Cb-5Ti-2Cr40St. or T1 and at least one metal of the group: V and Cr, 87Cb-6T1-70r--- 51Cb-4Ti-5Cr-40Si.

all in accordance with the invention.

15 Whereas normal MSi (where M represents proportionate ingredients as they occur in the substrate) reaction coatings possess continuous columnar grains (see FIG. 1, photomicrograph of disilicide coating over 16 coating performance begins to fall off. And at very high modification levels, such as about 75% by weight of Ti in the substrate, the coating formed is primarily TiSi Because of its characteristic properties, TiSi does not achieve the desired improvement in performance at either cfj'zoTa'lswtsMo 5 low or high" temperatures.

alloy enlarged 500 chemlcal modlficatlon f thlS Oxidation tests at 1300 F. showed very significant yp f With Small amounts of improvement in the performance of Ti-modified columbicombmations of these) resulted in disturbed strucum ili id s, as ll s Ti-V-, Ti-Crand Ti-V-Cr-mQdityres that eXhibited a tendency to ak down the unde- 10 fied columbium silicides. Results of oxidation tests consirable columnar structure. Such a disturbed strucducted at 1300 F. on Alloys 1 through 44 and 47 are tum can f ff 110WS the structllre summarized in Table 4. From the information shown of columbium d1s1l1c1de (Cb1 modlfied by an addltlon in Table 4 and related data, the following conclusions of about 1% of chromium in the coating (photomlcroon improvement in 1300 F. oxidation performance of graph enlarged 500 times). 15 columbium disilicide coatings are apparent:

Greater amounts (on the order of about 7% or (1) Modification with from 7 to 35% by weight of greater) of the modifiers of tins inventlon 1n he pin S12 the coatings with Ti results in distinctly improved per- Coatlllg Were found to result In complete dlsassoclatlon formance. The columbium disilicide structure in the norof the continuous columnar grain structure and to create mally critical low temperature range is resistant to a cs ab eq i xe ype grain structure. Such an equi- 20 failure by powdering or local defecting within the 7 to axed structure is shown in FIG. 3, which shows the struc- 3 5 range f Tidifi ti A h 37% Tidifi. ture of columbium disilici de modified by the additiofl cation level, when the structure becomes basically TiSi of about 7% V as a modifier 1n the coating (photomlrather than the desired CbSi -base, performance was crograph enlarged 1000 times). definitely inferior.

In accordance with the invention, it has also been dis- 25 (2) In combination, Ti with either V, or Cr, or with covered that increasing the Ti content (and to a lesser both V and Cr exerts a beneficial influence on 1300 F. extent, Cr content) in columbium siliclde cOatmgS o oxidation performance at all total modifier levels between amounts above about 10% y Welght f Second 7 and 35 by weight of the coatings. When these comphase dispersion in the equiaxed columbium s1l1c1de coatbirr d odifi r were used within the 7 to 35% range, ings (CbSi or (Refractory Metal) Sig and TiSi are of 0 distinctly improved performance was obtained. Morediiferent crystallographic types). Broad, diffused grain over, within this range, the use of combined modifiers, boundaries are particularly evident in the Ti-, Ti-V, Ti-Cr and particularly V, was unusually beneficial and in some and Ti-V-Cr-modified coatings of this invention, which cases achieved results improved even beyond those atsuggests that important and beneficial chemical segregatainable with the primary single Ti-modification. In many tion effects take place at grain boundaries. 35 cases it was thus possible to use lesser amounts of Ti At modification levels above about 60% by weight of and V, or Ti and Cr, or Ti, V and Cr, than would have Ti in the substrate, the desired improved behavior in been necessary if Ti alone had been used as a modifier.

TABLE 4.-BEHAVIOR OF VARIOUS MODIFIED SILICIDE COATINGS DUR- ING CYCLIC OXIDATION IN AIR AT 1.300 F.

Time to Total Weight Substrate Composition Failure, Change During Modeoi Alloy N0. weight percent hours Test, mgJcm. Failure 1 Unmodified Cb 20 --1.4 Local defect. 2 60 Cb-2OTa-15W-5Mo 20 52 Powdering. 97Cb-3T 20 -2.0 Local detect. 4.. -75 2.7 Do. 5-. 100 0.1 Unfailed. 5-. 100 0.5 Do. 7-. 100 0.3 Do. 8.. 100 0.15 Do. 0.. 100 0.2 Do. 10--- 100 0.1 Do. 11 100 0.2 Do. 12..- 100 0. 3 D5. 13 100 4.3 Local defect. 14. 20 6.7 Do. 15- 291 Do. 16. -75 0.1 Do. 17- -75 3.0 Do. 18. -75 1.2 Do. 19. -2.7 Do. 20. 100 -0.7 Untailed. 21.-. 75 2.8 Local defect. 22- 100 1.8 nfaile 23- 100 0.7 Do. 24- 100 0.1 Do. 25..- 100 0.15 Do. 20.-- 100 1.3 Do. 27- 100 0.5 Do. 28. 100 0.25 Do. 29- 100 0.5 Do. 30- 100 0.1 Do. 31- 100 -0.s Do. 32. 100 0.1 Do. 33. 100 1.5 Do. 34- 100 -05 Do. 35- 100 --0.3 Do. 36. 100 -0.1 Do. 37- 100 -0.2 Do. as. 100 -0.3 Do. 30..- 100 0.65 Do. 40 100 0. Do. 41--- 0.3 Do. 42--- 66Cb-15Ti-13Cr-6V.-. 100 -0.3 Do. 43... 500b-15Ti-29Cr-6V... 100 0.2 Do. 44--- 39Cb-25Ti-30Cr-6V 100 0.1 Do. 47 390b-21Tl-20Ta-15W-5M 100 -0.5 Do.

At 2200 F., Ti-modification, Ti-V-modification, and Ti-Cr-modification of the disilicide structure achieved improvements of primary significance, with certain Ti-V- modifications producing coatings exceeding all others tested in oxidation resistance.

Further, it has been established that at 2200 F. other apparently possible modifying elements, such as, Ta, W, Mo, Hf, Zr, Fe, Ni, Al, Si, and Y did not cause any significant improvements in coating performance. Results of oxidation testing conducted at 2200 F. on Alloys 1-44 and 47, specimens of which were also tested for oxidation testing at 1300 F. (see Table 4), are summarized in Table 5 below.

oxidation, however, cracks appeared through a number of the modified silicide coatings. These cracks allowed oxidation to penetrate to the subsilicide region. In many instances these subsilicide zones then grew to appreciable thicknesses in 100 hours at 2200 F.

In regard to this growth of the subsilicide zone, it should be noted that the thermal expansion of the subsilicide (M Si is less than that of either CbSi or (Cb- 20Ta-15W-5Mo)Si and therefore the subsilicide zone is in compression at low temperatures, i.e., below the coating temperature or higher exposure temperatures. Exposure at higher temperatures permits growth of M Si as pointed out above, and as this zone becomes thicker it TABLE 5.BEHAVIOR OF VARIOUS MODIFIED SILICIDE COATINGS DUR- ING CYCLIC OXIDATION IN AIR AT 2,200 F Total Weight Time to Change, at Fail- Substrate Composition, Failure, ure or After 100 Mode of Failure General oxidatlon.

Local defect. General oxidation.

0. Local defect. Unfailed.

Do. Local defect.

n Inaccurate weight gain because of oxide spalling. b Data not obtained.

From the results shown in Table 5 and related data the following conclusions can be drawn:

(1) Ti-modification within the limits of from 7 to by weight of the coatings results in pronounced improvement in performance of columbium silicide coatings during oxidation at 2200 F. In fact, the propensity of columbium silicide to rapid oxidation at this temperature is essentially overcome and resistance to local defecting is greatly enhanced.

(2) Ti-modification in combination with V-, Cr-, or V-Cr-modification at total modifier levels from 7 to 35% by weight of the coating, in most instances, leads to excellent performance of columbium silicide coatings at 2200 F. and in some cases, particularly when T i-V com bination modifiers are used, produces results equivalent to those achieved using straight Ti-modified coatings, at greatly reduced total modifier levels. An unusually effective coating was produced by siliconizing a Cb-6Ti-9V substrate to produce a coating containing 4% by Weight of Ti and 5% by weight of V.

Metallographic examination of the specimens of the examples (Alloys 5-12, 20, and 22-44), oxidized at 1300 F., showed no structural change. After 2200 F.

is better able to resist a given load, or, conversely, the actual stress level imposed on M Si becomes lower.

Subsilicides are defined as the phase or phases of the coating having substantially less Si than CbSi which phase or phases can be determined by X-ray d'iifraction to be crystallographically distinct from the most protective CbSi phase.

With the Ti-modified silicide coatings at the minimal effective modifier lever of 7% by weight of Ti, the subsilicides were found to be not completely oxidationresistant after hours of exposure. The 7Timodification thus did not prevent some undesirable contamination of the alloy substrate after 100 hours at 2200 F.

In accordance with the preferred form of the invention, however, it was found that even at the minimal modification level, when co-modification of the colum bium silicide coating with V or Cr in addition to Ti was used, the subsilicide zone then effectively inhibited substrate attack. Accordingly, an important attribute of this invention is its capability of enhancing oxidation resistance of the subsilicide region through co-modification of low level Ti-modified silicide coatings by V or Cr. This effect is shown in the photomicrographs of FIGS. 4-6.

At modification levels of by weight of Ti and greater, however, the subsilicides associated with simple Ti-modification were all sufficiently oxidation resistant to form an effective barrier to substrate contamination regardless of whether cracks developed in the primary coating zone.

As pointed out above, certain of the coating compositions of this invention were subjected to cyclic oxidation at 2200 F. for up to 300 hours to test the coating life of these alloys at these conditions. The results of these tests are set forth in Table 6. These results, and related data, indicate:

(1) Protection for 300 hours was obtained with Tisubstrate modification as low as 12.5%. Based on weight gain data, however, straight Ti-substrate modifications in excess of would probably be required for reliable 300-hour service.

(2) Optimum protection for 300 hours was obtained by combination Ti-V substrate modification in the preferred amounts of this invention (i.e., 12 to 16% by weight in the aggregaate of Ti and V in the substrate or 8 to 10% in the aggregate of Ti and V in the coating).

(3) Combination Ti-Cr and Ti-V-Cr modifications, in accordance with this invention, do not produce 300- hour protection results equivalent to those obtained with equivalent amounts of Ti-V or straight Ti modification.

(4) Any of the modifications of this invention, i.e., Ti, Ti-V, Ti-Cr, or Ti-V-Cr produce greatly improved 300-hour protection over unmodified silicide coatings or coatings modified by Ti, V, or Cr in amounts outside the limits of this invention.

TABLE 6.COATING LIFE OF VARIOUS MODIFIED SILI- CgDEgOATINGS SUBJECTED TO CYCLIC OXIDATION AT 2, 00

The thermal chemical stability of the coatings of this invention was tested by subjecting certain of the abovelisted alloys to a standard 100 hour, 8-cycle exposure at 2200" F., then slow cooling the specimens by a standard slow cool cycle, and then subjecting the specimens to a standard 8-cycle, up to 100 hour exposure at 1300 F.

The results of these thermal chemical stability tests are indicated in Table 7 below. These tests were designed to simulate certain operating conditions to which parts coated with the alloys of this invention would be likely to be subjected. For example, a part in a gas turbine or jet aircraft engine may be subjected to exposure to high temperatures (above 2000 F.) for extended periods during operation, and then to cooling and resulting intermittent exposures at low temperatures (around 1300 F.) during slowdown or shut-down of the turbine or engine.

The slow-cool resistance was of great interest because some coatings have a tendency to exhibit powdering during slow cool cycles.

The results of these tests, as set forth in Table 7, and related data, indicate:

(1) Both the slow-cool resistance to powdering and the overall thermal chemical stability of the samples tested was greatly improved by modification of the coatings in accordance with this invention.

(2) Straight Ti-modifications resulting in the greatest improvement in thermal chemical stability were those in the relatively higher ranges of modification allowed by this invention (i.e., 25 to 50% by weight modification of the substrate, which produces 14 to 26.5% by weight modification of the coatings).

(3) Much lower aggregate amounts of Ti-V combination modifiers were required for equivalent improvements of the thermal chemical stability of the samples tested (i.e., 15% Ti-V by weight in the substrate, or 9% Ti-V by weight in the coatings produced results equivalent to those produced with 2550% by weight straight Ti substrate modification or 14 to 26.5% Ti coating modication).

(4) The slow-cool resistance of Ti-Cr and Ti-V-Cr modified coatings was satisfactory, but the overall thermal chemical stability of these samples tested did not measure up to the results achieved with Ti-V and straight Ti-modification.

TABLE 7.-THERMAL CHEMICAL STABILITY BEHAVIOR OF VARIOUS MODIFIED SILIOIDE COATINGS Life at 1,300 F.

A Not tested.

The most preferred coating of this invention, i.e., the coating produced by siliconizing a Cb-6Ti-9V alloy substrate, was also tested for thermal shock resistance and diffusional stability. In the former test the sample was subjected to air-quench cycles during 100 hours at 2200 F. The thermal shock resistance of this sample was excellent. The coating was not degraded and 100-hour weight gain was only 0.73 mg./cm.

Additional testing of this most preferred coating (Cb-6Ti-9V)Si included oxidation of a specimen containing an intentional defect in the coating for 25 hours at 2200" F., thereby exposing both the subsilicide zone and the substrate to direct environmental attack. In this test the subsilicide zone was not preferentially oxidized, and oxidation and contamination of the substrate was localized within the region of the defect and did not cause catastrophic destruction of the sample.

Oxidation tests of the most preferred coating (Cb-6Ti-9V)Si were also conducted at higher temperatures. These cyclic tests were conducted for 100 hours on duplicate specimens at 2500 F. and 2700 F., respectively, and produced no failures of the test specimens. Weight gains after 100 hours of testing, using the standard eight cycles between room and test temperatures, were 0.54 mg./cm. and 0.69 mg./c m. respectively, for the specimens oxidized at 2500 F., and 0.90 mg./cm. and 156 mg./cm. respectively, for the specimens oxidized at 2700 F.

Metallographic examination of these test specimens after oxidation showed that the (Cb-6Ti-9V)Si coatings have superior diffusional stability compared to that of (Cb-25Ti)Sicoatings, although the latter coatings also exhibited outstanding oxidation resistance at temperatures up to 2700 F withstanding failure at these temperatures for the entire 100-hour test period. For example, after exposure for 100-hours at 2500 F. the thickness of the subsilicide region under a (Cb-6Ti-9V)Si coating was 1.2 mils, compared to 3.6 mils under a (Cb-25Ti)Si coating. A corresponding thicker residual protective di- 21 silicide coating was retained when the preferred (Cb-6Ti-9V)Si coating was used.

In summary, this invention is directed to protective coatings for Cb-base substrates based on modified columbium silicide structures. The primary structure of the coating comprises a vapor-deposited columbium silicide surface zone, but in accordance with the invention this surface zone is Ti-modified. As a result of such Ti-modification, the basic chemical and microstructural characteristics of the columbium silicide coating are changed, and these changes result in a pronounced improvement in coating behavior at both low and high temperatures. In other embodiments of this invention, the Timodification is combined with V or Cr, or with both V and Cr modification. The very best results are achieved with a combination of Tiand V-modification.

{In brief, modification of basic columbium silicide coatings with the following elements and combinations of elements at levels in the aggregate of from 7 to 35% by weight of the silicide coating are particularly etfective in promoting significantly improved coating behavior at both low and high temperatures: Ti, Ti-V, Ti-Cr, and Ti-V-Cr.

In addition to the benefits accruing from chemical modification with Ti and the combinations Ti-V, Ti- Cr, and Ti-V-Cr, important structural modifications are also achieved in which equiaxed structures are created that are better able to resist stress-induced failure than are the columnar structures that are characteristic of the usual columbium silicide coatings that result from ordinary siliconizing of columbium by vapor deposition processes.

We claim:

1. An article of manufacture comprising a Cb-base alloy substrate having superimposed thereon a thermalcyclic-failure resistant, defect resistant and broad range oxidation resistant surface coating zone consisting essentially of columbium silicide modified by 7 to 35% by Weight of the coating of a metal modifier selected from the group consisting of: (a) Ti, and (b) Ti and at least one of V and Cr.

2. The article of claim 1 wherein the metal modifier of the coating consists essentially of Ti and V.

3. The article of claim 2 wherein the surface coating zone consists essentially of columbium silicide modified by from 4 to 26% of Ti and from 2 to 6% of V.

4. The article of claim 3 wherein the surface coating zone consists essentially of columbium silicide modified by about 4 to 5% of Ti and about 4 to 5% of V.

5. The article of claim 1 wherein the metal modifier of said coating consists essentially of Ti.

7 6. The article of claim 5 wherein the surface coating zone consists essentially of columbium silicide modified by 13 to 28% of Ti.

7. The article of claim 1 wherein the metal modifier of the coating zone consists essentially of Ti and Cr.

8. The article of claim 7 wherein said surface coating zone consists essentially of columbium silicide modified by from 4 to 26% of Ti and 2 to 15% of Cr, the aggregate Ti and Cr in said coating not exceeding 35% by weight thereof.

9. An article of manufacture having good resistance to oxidation at elevated temperatures which comprises: a substrate consisting essentially of Cb and an alloying addition of from 12 to 65% by weight of the substrate of a metal modifier selected from the group consisting of (a) Ti, and (b) Ti and at least one of V and Cr; and a thermal-cyclic-failure resistant, defect resistant, and broad range oxidation resistant coating on the surface of said substrate, said coating consisting essentially of columbium silicide modified by from 7 to 35% by weight of the coating of the selected metal modifier.

10. The article of claim 8 wherein the metal modifier consists essentially of Ti and at least one of V and Cr.

11. The article of claim 10 wherein the metal modifier consists essentially of Ti and V.

12. The article of claim 11 wherein the substrate consists essentially of Cb modified by an alloying addition of from 6 to 50% by weight of the substrate of Ti and from 3 to 12% by weight of the substrate of V, and the coating essentially of columbium silicide modified by from 4 to 26% by weight of the coating of Ti and from 2 to 6% by weight of the coating of V.

13. The article of claim 12 wherein the substrate consists essentially of 6 to 9% by weight of Ti, 6 to 9% by weight of V, balance essentially Cb, and the coating consists essentially of columbium silicide modified by 4 to 5% by weight of the coating of Ti and 4 to 5% by weight of the coating of V.

14. The article of claim 13 wherein the substrate consists essentially of about 6% by weight of Ti, about 9% by weight of V, balance essentially Cb, and the coating consists essentially of columbium silicide modified by about 4% by weight of the coating of Ti, and about 5% by weight of the coating of V.

15. The article of claim 13 wherein the substrate consists essentially of about 9% by weight of Ti, about 6% by weight of V, balance essentially Cb, and the coating consists essentially of columbium silicide modified by about 5% by weight of the coating of Ti and about 4% by weight of the coating of V.

16. The article of claim 9 wherein the metal modifier consists essentially of Ti.

17. The article of claim 16 wherein the substrate consists essentially of from 23 to 55% by weight of Ti, balance essentially Cb, and the coating consists essentially of columbium silicide modified by 13 to 28% by weight of the coating of Ti.

18. The article of claim 17 wherein the substrate consists essentially of about 25% by weight of Ti, balance essentially Cb, and the coating consists essentially of columbium silicide modified by about 14% by weight of the coating of Ti.

19. The article of claim 9 wherein the substrate consists essentially of at least 35 by weight of Cb, from 6 to 50% by weight of Ti, and from 3 to 29% by weight of Cr, and the coating consists essentially of columbium silicide modified by from 4 to 26% by weight of the coating of Ti and 2 to 15% by weight of the coating of Cr, the aggregate Ti and Cr in said coating not exceeding 35% by weight thereof.

20. The article of claim 9 wherein the substrate consists essentially of at least 35% by weight of Cb, from 3 to 25% by weight of Ti, from 4 to 29% by weight of Cr, and from 3 to 12% by weight of V, the aggregate Ti, Cr and V being at least 12% by weight of the substrate, and the coatng consists essentially of columbium silicide modified by 2 to 13% by weight of the coating of Ti, 3 to 16% by weight of the coating of Cr, and 2 to 6% by weight of the coating of V.

21. The article of claim 9 wherein said article has a subsurface zone between the coating and exterior of the substrate, said subsurface zone predominantly comprising oxidation-resistant subsilicides of the substrate.

22. A method of producing a coated metal article having resistance to oxidation at elevated temperatures, the metal article having a substrate consisting essentially of Cb and an alloying addition of from 12 to 65% by weight, in aggregate, of a metal modifier selected from the group consisting of (a) Ti, and (b) Ti and at least one of V and Cr, which process comprises subjecting the substrate to an environment produced by heating a mixture of powders comprising a finely ground source of Si and a small amount of a volatilizable halide salt, as active ingredients, and an inert filler; heating the substrate and powder mixture for a time sufficient to cause volatilization of the halide salt and deposition of Si on the surface of the substrate, thereby effecting the creation of an exterior coating on the surface of the substrate consisting essentially of columbium silicide modified by the selected 23 metal modifier in an amount of from 7 to 35% by weight of the coating.

23. The process of claim 22 wherein the metal modifier consists essentially of from 12 to 65% by weight of the substrate of Ti.

24. The proces of claim 22 wherein the metal modifier consists essentially of from 6 to 50% by weight of the substrate of Ti and from 3 to 12% by weight of the substrate of V.

25. The process of claim 24 wherein the metal modifier consists essentially of from 12 to 16% by weight of the substrate, in the aggregate, of Ti and V.

26. The process of claim 22 wherein the metal modifier consists essentially of from 6 to 50% by weight of the substrate of Ti and from 3 to 29% by weight of the substrate of Cr, the aggregate Ti and Cr not exceeding 65 by weight of the substrate.

24 References Cited UNITED STATES PATENTS 3,015,579 1/1962 Commanday et al.

3,037,883 6/ 1962 Wachtell et al.

3,090,702 5/1963 Commanday et al.

3,219,474 11/1965 Priceman et al. 117131 X 3,307,964 3/1967 Jacobson 1l7l35.1 X 3,317,343 5/1967 Jefferys.

3,337,363 8/1967 Chao et al.

ALFRED L. LEAVITT, Primary Examiner.

J. R BATTEN, JR., Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,442,720 May 6, 1969 Elihu F. Bradley et al.

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

Column 21, line 72, the reference numeral "8" should read 9 Column 22, line 51, "coatng" should read coating Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, IR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer

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