NIOBIUM-SILICIDE ALLOYS HAVING A SURFACE REGION OF ENHANCED ENVIRONMENTAL-RESISTANCE, AND RELATED ARTICLES AND PROCESSES

Abstract

Niobium silicide articles are described. They include a surface region enriched with at least about 25 atom % germanium, which can enhance the properties of the article. Methods for preparing these articles are described as well. According to one method, an article is formed from a niobium silicide composite material which contains a selected amount of germanium. The article is then heat-treated under conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region. In another embodiment, a germanium-containing material is applied over a niobium-silicide article, and then diffused into the surface region of the article by way of a heat treatment.

Description

BACKGROUND OF THE INVENTION

The present invention generally relates to refractory metal intermetallic composites. Some specific embodiments of the invention are directed to the enhancement of various properties of niobium-silicide-based articles which are very useful as turbine engine components.

Turbines and other types of high-performance equipment are designed to operate in a very demanding environment which always includes high-temperature exposure, and often includes high stress and high pressure. Superalloys based on elements like nickel or cobalt have often provided the chemical and physical properties required for such operating conditions.

While the attributes of superalloys continue to ensure considerable interest in such materials, new compositions have been developed to meet an ever-increasing threshold for high-temperature exposure. Prominent among such materials are the refractory metal intermetallic composites (RMIC's). Examples include various niobium-silicide alloys. (The RMIC materials may also include a variety of other elements, such as titanium, hafnium, aluminum, and chromium). These materials generally have much greater temperature capabilities than the current class of superalloys. As an illustration, while many nickel-based superalloys have an operating temperature limit of about 1100° C., many RMIC alloys have an operating temperature in the range of about 1200° C.-1700° C. These temperature capabilities provide tremendous opportunities for future applications of the RMIC alloys. Moreover, the alloys are considerably lighter than many of the nickel-based superalloys.

While articles formed from the niobium-silicide alloys clearly possess very attractive properties, continued improvement in certain areas would be welcome in the art. As an example, great efforts are being made to improve environmental protection, e.g., resistance to oxidation and corrosion. Some of these efforts are necessary because niobium-silicide alloys can sometimes undergo rapid oxidation at temperatures above about 1000° C. Under very demanding operating conditions, oxidation in the surface region of the niobium-silicide article—even when the article itself is covered by protective coatings—could ultimately damage the article.

It should thus be clear that niobium-silicide articles having improved properties would be very welcome in the art. In particular, niobium-silicide-based turbine components having improved environmental resistance at elevated temperatures would be of considerable interest. The articles should also exhibit a general balance in other properties as well. For example, components such as turbine airfoils should also be characterized as having good low-temperature toughness and good high temperature strength. Moreover, it would also be desirable if the articles could be made in a timely, cost-efficient manner, using conventional manufacturing equipment.

BRIEF DESCRIPTION OF THE INVENTION

One embodiment of this invention is directed to a niobium silicide article which includes a surface region comprising at least about 25 atom % germanium, based on the composition of the surface region.

Another embodiment relates to a method for preparing a niobium silicide article which includes a surface region enriched in germanium. The method comprises the following steps:

(a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase, at least one metal silicide phase, and at least about 10 atom % germanium, based on total atom percent of the composite material; and then

(b) heat-treating the article formed in step (a), under heating conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region.

An additional embodiment is directed to another method for preparing such a niobium silicide article. The alternative method comprises the following steps:

(a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase and at least one metal silicide phase;

(b) applying a germanium-containing material (e.g., a coating) to a surface of the article formed in step (a); and then

(c) heat-treating the germanium-containing material and article, under conditions sufficient to cause at least a portion of the germanium to diffuse into a surface region of the article.

Other features and advantages of this invention will be better appreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an SEM (scanning electron microscope) image of a niobium-silicide test coupon treated according to some embodiments of the present invention.

FIG. 2 is an EDS (energy dispersive X-Ray spectroscopy) representation of the image of FIG. 1, showing germanium content in the test coupon.

FIG. 3 is an EDS representation of the image of FIG. 1, showing oxygen content in the test coupon.

FIG. 4 is a chart which demonstrates the effect of germanium levels on various sample alloys, after heat treatment steps.

FIG. 5 is another SEM image of a niobium-silicide test coupon treated according to some embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The articles described herein are formed from niobium-silicide alloys, which are generally known in the art. Many suitable examples are described in the following patents, which are all incorporated herein by reference: U.S. Pat. No. 5,833,773 (Bewlay et al); U.S. Pat. No. 5,932,033 (Jackson et al); U.S. Pat. No. 6,409,848 (Bewlay et al); U.S. Pat. No. 6,419,765 (Jackson et al); and U.S. Pat. No. 6,676,381 (Subramanian et al). The niobium-silicide alloys usually have a microstructure comprising a metallic Nb-base phase and an intermetallic metal silicide phase (e.g., Nb-silicide). However, they may include one or more other phases as well. (As used herein, “alloy” is meant to describe a solid or liquid mixture of two or more metals, or one or more metals with one or more non-metallic elements).

In addition to niobium and silicon, the alloys usually include at least one element selected from the group consisting of titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al). Ti and/or Hf are often preferred constituents. A typical range for Ti is about 2 atom % to about 30 atom % (based on total atom % for the alloy material), and preferably, about 12 atom % to about 25 atom %. A typical range for Hf is about 0.5 atom % to about 12 atom %, and preferably, about 2 atom % to about 8 atom %. A typical range for Cr is about 0.1 atom % to about 25 atom %, and preferably, about 2 atom % to about 20 atom %. A typical range for Al is about 0.1 atom % to about 15 atom %, and preferably, about 0.1 atom % to about 4 atom %.

The alloys frequently include other elements as well. Non-limiting examples are nitrogen, molybdenum, yttrium, tantalum, rhenium, ruthenium, zirconium, iron, tungsten, germanium, carbon, and tin. The particular inclusion and amount for any of these elements will of course depend on a variety of factors, such as the desired properties for the final alloy product. As one illustration, the presence of molybdenum in alloys with the enriched germanium surface region may, under some conditions (though not all), adversely affect the oxidation-resistance of the alloys.

The niobium silicide materials can be formed into useful articles by a variety of forming techniques. Casting is typically employed. Various details regarding the casting of these refractory materials are well-known in the art. Non-limiting examples of casting techniques are described by Subramanian et al, in U.S. Pat. No. 6,676,381 (incorporated herein by reference).

The niobium-silicide articles of this invention include a surface region enriched in germanium. As further described below, the presence of relatively high levels of germanium in this region results in significant improvements in some of the important characteristics of the article. The depth of the “surface region” will depend in part on the type of article in use. As an example, the surface region for an article with relatively thin walls, e.g., a turbine airfoil, may be more shallow than the surface region of an article which has a greater thickness.

In general, the “surface region” of the article (in terms of germanium enrichment) is defined as the region which extends to a depth of no greater than about 30% of the cross-sectional thickness of the article. In some specific embodiments, the depth is no greater than about 10% of the cross-sectional thickness. As used herein, “surface region” includes an affected region of the substrate, as well as any coating over the affected region which is formed during the surface treatment process. The “affected region” is the region of the substrate in which diffusion has occurred, according to some embodiments which are described below in detail.

As a non-limiting illustration in the case of a gas turbine blade, the “surface region” usually extends to a depth of about 50 microns into the surface, and preferably, no greater than about 25 microns into the surface. In the case of articles with greater cross-sectional thicknesses, the surface region could extend to a depth of about 250 microns. It should be understood that “surface region” refers to the surface of the bulk alloy itself, treated according the invention. In other words, the “surface” does not refer to oxidation layers which are formed on top of the bulk alloy, during one or more of the thermal treatments described below.

In terms of the level of germanium, “surface enrichment” is meant to define a concentration of at least about 25 atom % germanium, based on the composition of the entire surface region. In general, this level of germanium is much higher than that which would be present in a typical niobium-silicide alloy. In some specific embodiments, the level of germanium is at least about 40 atom %. In other embodiments, the level of germanium is at least about 50 atom %. The maximum amount of germanium in the surface region is usually about 67 atom %, e.g., as estimated according to stoichiometric NbGe₂, the preferred phase for some embodiments (as noted below). Those skilled in the art will be able to select the most appropriate amount of germanium for a given situation. As one illustration, higher amounts of germanium desirably increase the oxidation resistance of the article. However, an excessive amount of germanium (especially if in free form, i.e., not in a phase, as described below) may lower the melting point of the surface region to a level which is not suitable for some end uses.

The germanium in the surface region is usually present in the form of one or more phases. The types of phases present will depend on a variety of factors, such as the elements present in the alloy substrate; the respective proportions of those elements; and the heating and processing conditions which are used to incorporate the germanium into the region (discussed below). Non-limiting examples of germanium-containing phases which might be present are NbGe₂ (niobium digermanide); Nb₃Ge (e.g., the Beta phase); Nb₅Ge₃; TiGe₂, Ti₅Ge₃, Ti₆Ge₅, Ge—Hf, Ge—Si, Ge—Al, and Ge—Cr. Ternary and higher-order derivatives of these phases are possible as well. Moreover, one or more of the phases may be present as a solid solution, which might contain individual elements as well. Furthermore, those skilled in the art understand that the elements in these phases do not have to be present in stoichiometric proportions.

In some specific embodiments, at least a portion of the germanium is present in the form of the niobium digermanide phase (NbGe₂). The present inventors have discovered that the presence of this phase can considerably enhance oxygen diffusion barrier properties under some operational conditions. In some embodiments, at least about 40% of the germanium in the surface region is in the form of NbGe₂. In other embodiments, at least about 50% of the germanium is in the form of NbGe₂. In some especially preferred embodiments, at least about 70% of germanium in the surface region is present in the form of NbGe₂. (It should be understood that a niobium-germanide phase contains primarily the two elements, although other elements may be present in solid solution, in relatively minor amounts, e.g., a total of less than about 15 atom %).

An additional embodiment of this invention relates to the formation of articles which include a surface region enriched in germanium. According to one such method, a niobium-silicide article is formed by one of the techniques mentioned previously, e.g., casting. The alloy used to form the article comprises a metallic niobium-base phase, and at least one metal silicide phase. (The alloy may include a variety of other elements, as discussed previously).

The niobium-silicide alloy used in this embodiment further includes germanium. When the article is heat-treated, the level of germanium in the surface region increases to at least about 25 atom %, based on the total composition of the surface region. The inventors do not wish to be bound by any particular theory regarding the mechanism by which the germanium level is increased. It is believed that, usually, at least a portion of the germanium in the bulk alloy migrates to the surface region of the article. (For simplicity, this embodiment will sometimes be referred to as the “migration” embodiment, although other mechanisms are suggested below).

The minimum amount of germanium present in the alloy is that which results in a surface region containing at least about 25 atom % germanium. (It should be understood that the enriched surface area would thus contain germanium which had migrated from the bulk alloy, i.e., the area below the surface region, along with germanium which was initially present in the surface region when the article was formed). When it is desirable that the surface region contain an amount of germanium greater than about 25 atom %, the level of germanium in the bulk alloy can be modified accordingly. For example, the bulk alloy can be formulated to comprise (i.e., prior to any heat treatment) at least about 25 atom % germanium, and more often, at least about 35 atom % germanium.

The heat treatment employed in this embodiment will depend on many factors. They include: the specific composition of the alloy; the microstructure of the alloy; the amount of germanium enrichment desired; the depth of the bulk alloy (which in some cases appears to serve as a reservoir of germanium); the heat-treatment environment (e.g., heating atmosphere; type of heating cycles); and the heating mechanism. Another factor influencing the selected heat treatment relates to the rate of oxide growth. As an example, if the heat treatment is carried out at too high a temperature and/or for too long a period of time, the overlying oxide layer may grow too fast, which may in turn prevent the formation of the enriched surface region. For a typical niobium-silicide alloy, the heat treatment will usually be carried out at a temperature in the range of about 600° C. to about 1400° C. In some specific embodiments, the heat treatment is carried out at a temperature in the range of about 1000° C. to about 1250° C.

The heat treatment can be carried by using various types of equipment, e.g., employing a suitable convection or conduction mechanism. As an example, a standard furnace could be used. An oxidizing atmosphere such as air is the most preferred heating environment for this embodiment.

Heating times will also depend on many of the factors set forth above, including the particular heating equipment employed. In the case of niobium-silicide alloys having approximate dimensions of 1 cm×1 cm×1 cm, the heating time will usually be in the range of about 30 minutes to about 200 hours. More often, the heating time will be in the range of about 1 hour to about 100 hours. Typically, longer heating time periods can compensate for lower heating temperatures (within the general ranges noted above), while higher temperatures can compensate for shorter time periods. (In some commercial applications, the heat treatment time is typically no greater than about 50 hours). The most appropriate heating regimen can readily be determined by a series of tests, to determine which parameters provide the desired amount of germanium enrichment to the surface region. As further described in the examples, the amount of germanium present in that region can accurately be determined by various techniques, e.g., X-ray diffraction, electron microprobe techniques; EDS (energy dispersive X-Ray spectroscopy); WDS (wavelength dispersive X-Ray spectroscopy); and wet chemical analysis. It should also be understood that a portion of the heat treatment can effectively occur when the article is put into service, e.g., a gas turbine component operating under normal conditions.

The heat treatment in this embodiment also results in the formation of one or more oxide layers over the enriched surface region. The oxide layer can be referred to as an oxide “scale”. It is formed primarily when the heat treatment is carried out in an oxidizing atmosphere. The oxide scale may contain different phases, depending in part on the content of the bulk alloy, along with the particular heat treatment conditions employed. Moreover, when the oxide scale is in the form of multiple layers, each layer may primarily contain one phase, e.g., a niobium-rich oxide phase or a silicon-rich oxide phase. The thickness of the layer will depend in part on the other factors described herein (especially heat treatment time and temperature; and bulk alloy composition). Usually, the layer will have a thickness of about 5 microns to about 250 microns.

In some embodiments, the oxide scale can remain on the article during a given end use. However, in other situations, it is desirable to remove the oxide scale. Removal can be undertaken by various techniques, e.g., abrasion with a suitable media such as glass beads; polishing, grinding, and the like.

As mentioned above, it appears that at least some of the germanium enrichment occurs by way of a migration mechanism. However, germanium enrichment may also be occurring due to other mechanisms. For example, the increase in germanium as a proportion of the constituents in the surface region can occur because other constituents in that region, such as niobium, silicon, and titanium, are leaving that region by way of transformation into the oxide scale discussed previously.

According to another embodiment, the germanium required for surface enrichment of the niobium-silicide article can be diffused from a material (e.g., a coating) over the surface of the article. Many different techniques can be used to carry out this technique. As an example, the germanium, or a germanium-containing compound or mixture, could be deposited on the substrate surface by using a slurry composition. According to this technique, the germanium could be used in the form of the metal itself; in the form of an alloy, or as a compound or metal mixture which melts at a temperature below the melting point of the substrate. The alloy, compound, or metal mixture may include other beneficial elements, such as chromium, niobium, aluminum, and the like. (Intermetallic compounds of germanium are usually not preferred, because their melting points may be too high. However, intermetallics may form in situ during the subsequent heat treatments, and this occurrence is desired). The amount of germanium selected for the slurry will depend in large part on the amount of germanium desired for the enriched surface region of the substrate.

Metal-containing slurry compositions are well-known in the art, as are their methods for preparation. Usually, the germanium or germanium alloy in the slurry will have an average particle size in the range of about 0.5 micron to about 50 microns, and more often, in the range of about 1 micron to about 10 microns. (Moreover, the alloy particles are preferably spherical or substantially spherical when the material is to be deposited by a spray technique). The slurry can be aqueous or organic, depending on various factors, such as its specific content, and the manner in which it will be applied to the article. Furthermore, the slurry can include various other ingredients, such as stabilizers (e.g., organic stabilizers), which chemically stabilize the slurry constituents. Stabilization of the slurry can be important, e.g., when very fine metal particles are incorporated therein. Additives which improve the wettability of the slurry to the substrate surface are also used when appropriate.

The slurry can contain various other ingredients as well. Many of these are known in the art to those involved in slurry preparations. Slurries are generally described in “Kirk-Othmer's Encyclopedia of Chemical Technology”, 3rd Edition, Vol. 15, p. 257 (1981), and in the 4th Edition, Vol. 5, pp. 615-617 (1993), as well as in U.S. Pat. Nos. 5,759,932 and 5,043,378. Each of these references is incorporated herein by reference. A good quality slurry is usually well-dispersed and free of air bubbles and foaming. It typically has a high specific gravity and good rheological properties adjusted in accordance with the requirements for the particular technique used to apply the slurry to the substrate. Moreover, the solid particle settling rate in the slurry should be as low as possible, or should be capable of being controlled, e.g., by stirring.

The slurry can be applied to the surface by many different techniques. For example, it can be slip-cast, brush-painted, dipped, sprayed, poured, rolled, or spun-coated onto the substrate surface. Spray-coating is often the easiest way to apply the slurry coating to substrates which have complex geometric shapes, such as turbine airfoils. The viscosity of the coating can be readily adjusted for spraying, by varying the amount of liquid carrier used. Spraying equipment is well-known in the art. Any spray gun should be suitable, including manual or automated spray gun models; air-spray and gravity-fed models, and the like. Adjustment in various spray gun settings (e.g., for pressure and slurry volume) can readily be made to satisfy the needs of a specific slurry-spraying operation. The slurry can be applied as one layer, or multiple layers.

After the slurry coating has been applied to the surface of the article, it is heat-treated. The heat treatment conditions are those which are sufficient to cause at least a portion of the germanium in the slurry to diffuse into the surface region of the article. As in the other embodiments, the heat treatment can be carried out by using various types of equipment, e.g., a standard furnace. In this embodiment, the heat treatment is carried out in either a vacuum or an inert atmosphere. A vacuum is preferred.

Heating times and temperatures for this embodiment will also depend on some of the factors set forth above, including the particular heating equipment employed. Usually, the heating temperature is based on the melting temperature (T_m) of the germanium-containing material (e.g., element/alloy/compound) in the slurry. Thus, the heating temperature is usually in the range of about (0.8)T_m to about (1.5)T_m of the material. In some preferred embodiments, the heating temperature is in the range of about (1.2)T_m to about (1.4)T_m. Thus, if elemental germanium (with a melting temperature of 937° C.) were used in the slurry, the broader range would be about 750° C. to about 1405° C. (It should be understood, however, that the temperature used should not exceed the melting point of the substrate).

Heating times will usually be in the range of about 10 minutes to about 10 hours. More often, the heating time will be in the range of about 30 minutes to about 90 minutes. As in the other embodiments, longer heating time periods can compensate for lower heating temperatures, while higher temperatures can compensate for shorter time periods. Moreover, the amount of germanium which is incorporated into the surface region can be ascertained by the various techniques discussed previously, so that the optimal diffusion conditions can be determined.

Other methods for applying a germanium-containing coating to the surface of an article are also possible. Non-limiting examples include plasma deposition (e.g., cathodic arc deposition; vacuum plasma spraying (VPS); high velocity oxy-fuel (HVOF) techniques; and air plasma spray (APS)); physical vapor deposition (PVD); chemical vapor deposition (CVD); pack deposition techniques; and sputtering. Those of ordinary skill in the art are familiar with details regarding each of these techniques. It is also understood that the germanium material would be used in a form which is compatible with the specific deposition technique. As an example, the thermal spray techniques (e.g., VPS, HVOF, and APS) would usually employ the germanium (or germanium alloy) in powdered form. The heating techniques to permit diffusion of the germanium can be adjusted to suit the particular deposition technique. (Those skilled in the art would understand that the heat treatment during any of the processes described herein may result in relatively minor changes in the thickness of the overall article, e.g., due to elemental interdiffusion and the like).

As mentioned above, the germanium can also be diffused into the article surface by a “pack” process. Details regarding pack techniques (also referred to as “pack cementation” techniques) are known in the art and described, for example, in U.S. Pat. No. 6,110,262, which is incorporated herein by reference. As a general example, the article could be embedded in a powder pack containing germanium (in metal-, alloy-, or compound-form). The pack also contains an activator—typically an ammonium or alkali metal halide carrier—and an inert filler.

Once embedded, the article is usually enclosed in a sealed chamber, and then heated to a temperature similar to the diffusion temperatures mentioned above. Under these conditions, the halide activator dissociates, and reacts with the germanium from the metallic source. This reaction produces gaseous germanium halide species, which can migrate into the surface region of the article. The germanium-rich vapors are reduced by the metals at the alloy surface, to form intermetallic compounds which provide the enriched germanium content.

When the germanium-enriched surface region is formed by the diffusion techniques, the oxide scale described previously is not formed. (A very small, incidental amount may in some cases be unintentionally formed, e.g., when there is a variation in process steps). The substantial absence of an oxide layer represents a significant advantage for the formation of the germanium-enriched layer by the diffusion technique.

As mentioned above, the “surface region” includes an affected region of the substrate, as well as any overlying coating related to the diffusion process. Thus, in this embodiment, the affected region is the region in which diffusion has occurred, i.e., as contrasted with the underlying bulk alloy portion which is not affected at all by the diffusion treatment. The overlying coating in this embodiment is the non-diffused portion of the slurry or similar material that may sometimes remain on the surface after treatment is complete. (It is thought that this coating may remain on the surface more frequently when a solid state material is used as the treatment agent, as compared to a liquid-state material). In some instances, it may be desirable to remove the coating from the surface, e.g., using the techniques described above. However, in other cases the coating can remain, and enhance the overall properties of the article.

The germanium-enriched surface region can provide an improved degree of oxidation resistance to the niobium-silicide articles, over a range of aggressive environments. A variety of articles can benefit from this important characteristic. Many of them are components for turbines, e.g., land-based turbines, marine turbines, and aeronautical turbines. Specific, non-limiting examples of the turbine components are buckets, nozzles, blades, rotors, vanes, stators, shrouds, combustors, and blisks. Non-turbine applications are also possible. (In some preferred embodiments, the niobium-silicide articles, as ready for use, include a germanium level in the bulk region (below the surface region) of no greater than about 10 atom %).

Embodiments of this invention are useful for providing additional environmental resistance to articles which have internal surfaces, e.g., holes, cavities, depressions, passageways, and the like. As an example, a turbine blade formed from a niobium-silicide alloy may include a number of cooling holes and passageways, e.g., for channeling bypass air from the compressor of the turbine. In preferred embodiments, the diffusion process for internal surfaces can be carried out with a pack cementation technique. Alternatively, diffusion could be carried out by directing a slurry (e.g., by pumping) through the passageways. (Care should be taken to avoid blockage of the passageways). By using these techniques, the alloy surface of the hole or passageway can be enhanced with a germanium-based phase. In this manner, the interior surface—which is often difficult to efficiently coat and protect by other methods—can receive an added measure of environmental resistance, e.g., resistance to the harmful effects of oxidation.

In some embodiments, the surface region is compositionally graded, in regard to the level of germanium. For example, the concentration of germanium can vary through the depth of the region. (Some gradation may also be present in the bulk alloy). Preferably, however, the total level of germanium within the enriched region remains at the level described previously, i.e., at least about 25 atom %. The gradation may be substantially continuous, but this does not always have to be the case.

In the situation where enrichment is achieved by the migration mechanism, i.e., from within the bulk alloy, gradation may usually be evidenced by a gradual decline in germanium concentration in the upward direction, i.e., away from the substrate. In the situation where enrichment is achieved by diffusion from a layer deposited over the substrate, gradation may usually be evidenced by a gradual decline in germanium concentration in the downward direction, i.e., toward the substrate. There are advantages to gradation in some situations. For example, gradation of the germanium level can result in a gradation of “thermophysical properties”, i.e., the physical characteristics of a material at elevated temperatures. Examples of those properties are the coefficient of thermal expansion (CTE), thermal conductivity, and strength.

In addition to its function as an oxygen-barrier layer, the enriched germanium surface region can function as a bond layer for an overlying protective coating, e.g., a ceramic overcoat. An example of such an overlying coating is a thermal barrier coating (TBC). TBC's are often formed from materials like zirconia, stabilized zirconia (e.g., yttria-stabilized), zircon, mullite, and combinations thereof; as well as other refractory materials having similar properties. These coatings are well-known in the art and described, for example, in a patent issued to Zhao et al, U.S. Pat. No. 6,521,356, which is incorporated herein by reference.

TBC's and other types of overcoats can be applied by many conventional techniques. Some of the techniques were listed previously, such as PVD. The thickness of the TBC can vary greatly, depending on many factors. Usually, the coating has a thickness in the range of about 10 microns to about 600 microns. (In those cases where the enriched germanium region has been formed by heating the bulk alloy, the oxide scale formed on top of the enriched region should usually be removed, prior to deposition of the TBC or other top coating).

In other embodiments, a separate protective coating can be applied over the article having the germanium-enriched surface region. This protective coating could serve as the sole overlying coating (i.e., the top layer of the article, providing further oxidation resistance), or it could function as a bond coat for a TBC or other protective topcoat. In those instances in which an oxide scale is present over the enriched surface region, it may sometimes be desirable to remove the scale before application of this protective coating.

Useful protective coatings of this type (i.e., serving as bond coatings or oxidation-resistant coatings) often comprise silicon, titanium, chromium, and niobium, as described in U.S. Pat. No. 6,521,356. Some compositions of this type contain about 43 to about 67 atom % silicon; between about 2 and about 25 atom % titanium; between about 1 and about 25 atom % chromium; and a balance of niobium. Many other constituents can be incorporated into the compositions. Non-limiting examples include boron, tin, iron, germanium, hafnium, tantalum, aluminum, tungsten, and molybdenum.

As another example, coatings based on chromium, ruthenium, and aluminum can also be used to effectively protect niobium silicide components. Examples of this type can be found in a patent issued to M. Jackson, U.S. Pat. No. 4,980,244, which is incorporated herein by reference. Many of these coatings comprise about 32 atom % to about 62 atom % chromium; about 19 atom % to about 34 atom % ruthenium; and about 19 atom % to about 34 atom % aluminum. They may also include one or more other elements, such as yttrium, iron, nickel, and cobalt.

Other coatings which promote oxidation resistance are based on silicon-iron-chromium alloys. Specific examples are described in U.S. Pat. No. 5,721,061 (Jackson et al), which is incorporated herein by reference. For example, some embodiments contemplate materials which comprise (in weight percent) about 26% to about 32% iron, and about 24% to about 30% chromium; with the balance being silicon. In some cases, these types of coatings are heat-treated after being applied over the substrate (e.g., at about 1250° C. to about 1400° C.). As described by Jackson et al, the heat treatment (which could sometimes be combined with or satisfied by other heat treatments on the article) results in a coating which comprises an outer layer and an interaction layer between the outer layer and the substrate material. The interaction layer includes one or more metallic-silicide phases which further enhance the protective capabilities of the overall coating.

The thickness of the protective coating can vary greatly, depending on many factors like those described above, and also depending on whether or not a TBC or other top coat is to also be used. In some specific embodiments, the coating has a thickness between about 10 microns and about 400 microns. Moreover, the coating can be applied by a variety of techniques, as also described above, such as APS, HVOF, slurry deposition, and the like.

EXAMPLES

The examples which follow are merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention.

Example 1

A niobium-silicide sample was prepared by dry-mixing a composition with the following nominal constituents: 38.7 atom % Nb, 2.0 atom % Hf. 18.4 atom % Ti, 0.9 atom % Al, 2.7 atom % Cr, 12.2 atom % Si, and 1.9 atom % Sn. The sample also contained 23.2 atom % Ge. The composition was arc-melted, to prepare an alloy sample in the shape of a disc. A test coupon was cut from the sample, and had approximate dimensions of 1 cm×1 cm×1 cm. A surface of the coupon was polished to remove any dirt and impurities. The coupon was then placed in a conventional box furnace, and heated in an air atmosphere for 100 hours, at a temperature of 1150° C.

FIG. 1 is a cross-sectional representation of the test coupon (obtained by SEM (scanning electron microscope) after removal from the box furnace. As depicted in the figure, article 10 includes a bulk alloy portion 12, i.e., the main portion of the alloy body. The bulk alloy portion, both before and after the heat treatment, was primarily made up of an (Nb,Ti)₅(Si,Ge)₃ phase.

Surface region 14, having an average depth of about 50 microns, is disposed over bulk alloy portion 12. The surface region was enriched in germanium, and primarily comprised the niobium digermanide (NbGe₂) phase. As described above, surface region 14 was formed during the heat treatment, presumably by the upward migration of germanium from the bulk alloy portion 12. A sectional analysis of the composition of the niobium digermanide phase itself, via microprobe, indicated a composition of approximately 28 atom % Nb, 64.5 atom % Ge, 2 atom % silicon, 5 atom % Ti, and 0.5 atom % Hf.

The heat treatment in the oxidizing atmosphere also resulted in the formation of an oxide layer over the surface region. The oxide layer, i.e., oxide scale 16, primarily contained niobium-rich oxide phases and silicon-rich oxide phases which appeared to be somewhat layered. The thickness of the overall oxide layer was about 250 microns.

FIG. 2 is an EDS representation of the image of FIG. 1, in which the spectroscopic scan is programmed to represent germanium levels, by color differentiation. The enriched surface region 14 is disposed over bulk alloy portion 12. (It should be noted that the EDS instrument provides a very clear display of enriched layer 14, in color).

FIG. 3 is another EDS representation of the image of FIG. 1. In this instance, the spectroscopic scan is programmed to represent oxygen content, by color differentiation. Region 18 is the bulk alloy, on which the germanium-enriched surface layer has been formed (not fully visible in this image). Region 16 is the oxide layer formed over the enriched surface layer. It is clear from the figure that there has not been any oxygen penetration into the surface layer or the bulk alloy, indicating that the integrity of the alloy can be maintained under these temperature and oxidation conditions.

Example 2

Niobium-silicide test samples were prepared by arc-melting, according to the general procedure described in Example 1. The specific composition of each sample is indicated in FIG. 4. Each of the samples was subjected to oxidation testing at temperatures of 1150° C. and 1000° C., for 100 hours. Sample 1 was based on embodiments of the present invention. The sample contained 23.2 atom % germanium, which migrated to the surface to form the enriched layer during the oxidizing heat treatment. Samples 2 and 3 were comparative alloys. They contained relatively low levels of germanium, and were outside of the scope of this invention.

The chart in FIG. 4 depicts the weight-change in the sample, after the particular heat-treatment tests. As those familiar with these tests understand, weight loss is often an indication of oxide spallation. (Weight loss can also occur due to the evaporation of gaseous oxides). Oxide spallation is usually initiated by the penetration of oxygen and/or oxygen compounds into at least the surface region of the article, resulting in the loss of non-adherent (or loosely-adherent) material from the article. An excessive amount of spalling can lead to serious degradation of the article.

After the 1150°/100 hour thermal exposure test, comparative sample 2, which contained a relatively low germanium level, and did not have a germanium-enriched surface region, spalled badly, resulting in a large weight loss. Comparative sample 3, which also did not include a germanium-enriched surface region, spalled to some degree, with a consequent, significant weight loss. It is believed that the spallation occurred because of the penetration of oxygen, as discussed previously. Moreover, although only a theory, it appears that sample 2 degraded more than sample 3, because of the presence of significant amounts of molybdenum (5 atom %).

In contrast to samples 2 and 3, sample 1 showed no weight loss, and in fact showed a weight gain. This sample was within the scope of the present invention, and contained the enriched germanium surface region. The weight gain is an indication that the sample did absorb some oxygen, but the resulting microstructure in the general surface region remained strongly adherent to the underlying bulk alloy. Sample 1 represents an article which provides much greater resistance to oxidation and the other accompanying degrading effects, as compared to samples 2 and 3.

After the 1000° C./100 hour thermal exposure test, comparative sample 2 did not show a weight loss under these conditions, instead exhibiting a weight gain of 43 mg/cm². However, the weight gain in this instance was not as high as sample 1, which was based on the present invention (i.e., 49 mg/cm² for sample 1). Sample 3 did show a weight loss (−17 mg/cm²), indicating some degradation of the alloy article, although the weight loss was not as great as in the 1150° C. test.

Example 3

In this example, the germanium-enriched layer was formed by way of the diffusion technique described previously. A germanium (Ge) slurry was created by mixing Ge metal with an organic binder and carrier. The binder for this example was a Remet® product called Ethyl Silicate 40. The carrier was ethyl alcohol, which also functioned as an agent to adjust the viscosity of the slurry. Ge metal powder (purchased from Alfa Aesar), sieved to a −325 mesh, was combined with the binder and carrier in the following concentration:

50.0 g Ge

27.5 g Remet® Ethyl Silicate 40

27.5 g Ethyl alcohol

The mixture was sealed in a container and mixed via a paint shaker for 30 minutes, prior to being loaded into a gravity-fed spray gun. The mixture was air-sprayed on a niobium-silicide substrate surface, using a conventional DeVilbiss spray gun. The slurry was allowed to air-dry on the coupon, and then a second layer was sprayed over the first. The second layer of slurry was allowed to air-dry on the substrate. After being air-dried, the coated coupon was cured in an oven, according to this heating regimen: 80° C. for 60 minutes, followed by 120° C. for 30 minutes, followed by 220° C. for 60 minutes.

The coated coupon was then diffusion heat-treated in a vacuum oven, at a temperature of about 1000° C. for 60 minutes. FIG. 5 is a cross-sectional representation of the resulting sample, obtained by SEM. As depicted in the figure, article 20 includes a non-diffused alloy region 22. An enriched region 24 is present over region 22. Enriched region 24 had a thickness of about 10-25 microns, and had a concentration of germanium within the acceptable ranges set forth above. No evidence of coating spallation was seen. The top of the coating had a friable layer 26 that is easily removed via a light glass bead-blasting.

Various embodiments of this invention have been described in rather full detail. However, it should be understood that such detail need not be strictly adhered to, and that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the appended claims.

Claims

1. A niobium silicide article which includes a surface region comprising at least about 25 atom % germanium, based on the composition of the surface region.

2. The article of claim 1, wherein the surface region comprises at least about 40 atom % germanium.

3. The article of claim 1, wherein the amount of germanium in the surface region is in the range of about 25 atom % to about 67 atom %.

4. The article of claim 1, wherein at least a portion of the germanium in the surface region is in the form of a niobium germanide phase.

5. The article of claim 4, wherein the niobium germanide phase is niobium digermanide (NbGe2).

6. The article of claim 5, wherein at least about 40% of the germanium in the surface region is in the form of the niobium digermanide phase.

7. The article of claim 6, wherein at least about 50% of the germanium in the surface region is in the form of the niobium digermanide phase.

8. The article of claim 1, wherein the surface region extends to a depth of about 30% of the cross-sectional thickness of the article.

9. The article of claim 1, wherein the surface region extends to a depth of about 50 microns.

10. The article of claim 1, comprising a bulk alloy region below the surface region, wherein the bulk alloy comprises a metallic niobium-base phase and at least one metal silicide phase.

11. The article of claim 10, wherein the bulk alloy region further comprises titanium and at least one element selected from the group consisting of rhenium and ruthenium.

12. The article of claim 10, wherein the bulk alloy further comprises titanium and at least one element selected from the group consisting of hafnium, chromium, and aluminum.

13. The article of claim 11, wherein the bulk alloy further comprises at least one element selected from the group consisting of silicon, zirconium, tin, tungsten, and carbon.

14. The article of claim 1, wherein at least one oxide layer is disposed over the surface region.

15. The article of claim 14, wherein the oxide layer is formed by a heat treatment which is carried out to form the germanium-containing surface region.

16. The article of 1, wherein the germanium in the surface region is compositionally graded.

17. The article of claim 1, wherein at least one protective coating is disposed over the surface region.

18. The article of claim 17, wherein the protective coating is an oxidation-resistant coating.

19. The article of claim 18, further comprising a thermal barrier coating disposed over the oxidation-resistant coating.

20. The article of claim 1, in the form of a turbine engine component.

21. The article of claim 20, wherein the turbine engine component is selected from the group consisting of turbine buckets, nozzles, blades, rotors, vanes, stators, shrouds, combustors, and blisks.

22. A turbine component, formed at least in part from a niobium-silicide alloy which comprises niobium (Nb), silicon (Si); and at least one element selected from the group consisting of titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al);

wherein a surface region of the niobium-silicide alloy comprises at least about 25 atom % germanium, and at least a portion of the germanium in the surface region is in the form of the niobium digermanide phase.

23. The turbine component of claim 22, further comprising at least one protective coating over the surface region of the alloy.

24. The turbine component of claim 22, further comprising an oxidation-resistant coating over the surface region of the alloy; and a yttria-stabilized zirconia thermal barrier coating disposed over the oxidation-resistant coating.

25. The turbine component of claim 22, comprising at least one hole or passageway in a niobium-silicide portion of the component, wherein the interior alloy surfaces of the hole or passageway also comprise at least about 25 atom % germanium.

26. A method for preparing a niobium silicide article which includes a surface region enriched in germanium, comprising the following steps:

(a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase, at least one metal silicide phase, and at least about 10 atom % germanium, based on total atom percent of the composite material; and then

(b) heat-treating the article formed in step (a), under heating conditions sufficient to increase the level of germanium in the surface region to at least about 25 atom %, based on the total composition of the surface region.

27. The method of claim 26, wherein at least a portion of the increase in the level of germanium in the surface region is caused by the migration of germanium from the article formed in step (a), up to the surface region.

28. The method of claim 26, wherein the surface region extends to a depth of about 50 microns.

29. The method of claim 26, wherein the heat treatment is carried out in an oxidizing atmosphere.

30. The method of claim 29, wherein the heat treatment is carried out at a temperature in the range of about 600° C. to about 1400° C.

31. The method of claim 26, wherein the heat treatment causes the formation of the niobium digermanide (NbGe2) phase in the surface region.

32. The method of claim 31, wherein at least about 40% of the germanium in the surface region is in the form of the niobium digermanide phase, after the heat treatment.

33. A method for preparing a niobium silicide article which includes a surface region enriched in germanium, comprising the following steps:

(a) forming an article from a refractory metal intermetallic composite material which comprises a metallic niobium-base phase and at least one metal silicide phase;

(b) applying a germanium-containing material to a surface of the article formed in step (a); and then

(c) heat-treating the germanium-containing material and article, under conditions sufficient to cause at least a portion of the germanium to diffuse into the surface region of the article.

34. The method of claim 33, wherein the surface region extends to a depth of about 30% of the cross-sectional thickness of the article.

35. The method of claim 33, wherein the germanium-containing material comprises elemental germanium, or a germanium-containing compound or mixture.

36. The method of claim 33, wherein the germanium-containing material is applied in the form of a slurry.

37. The method of claim 36, wherein the slurry is applied by a technique selected from the group consisting of slip-casting, brushing, dipping, spraying, pouring, roll-coating, and spin-coating.

38. The method of claim 33, wherein the germanium-containing material is applied to the surface of the article by a technique selected from the group consisting of plasma deposition; physical vapor deposition (PVD); chemical vapor deposition (CVD); sputtering; and pack processes.

39. The method of claim 33, wherein the heat treatment of step (c) is carried out in a vacuum or an inert atmosphere.

40. The method of claim 39, wherein the heat treatment is carried out at a temperature in the range of about (0.8)Tm to about (1.5)Tm of the germanium-containing material, where “Tm” represents the melting temperature.