U.S. patent application number 11/612743 was filed with the patent office on 2008-06-19 for niobium-silicide alloys having a surface region of enhanced environmental-resistance, and related articles and processes.
This patent application is currently assigned to GENERAL ELECTRIC. Invention is credited to Bernard Patrick Bewlay, Ramgopal Darolia, Voramon Supatarawanich Dheeradhada, Richard DiDomizio, Joseph David Rigney, Pazhayannur Ramanathan Subramanian.
Application Number | 20080142122 11/612743 |
Document ID | / |
Family ID | 39525718 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080142122 |
Kind Code |
A1 |
Bewlay; Bernard Patrick ; et
al. |
June 19, 2008 |
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.
Inventors: |
Bewlay; Bernard Patrick;
(Niskayuna, NY) ; DiDomizio; Richard; (Scotia,
NY) ; Subramanian; Pazhayannur Ramanathan;
(Niskayuna, NY) ; Dheeradhada; Voramon
Supatarawanich; (Latham, NY) ; Rigney; Joseph
David; (Milford, OH) ; Darolia; Ramgopal;
(West Chester, OH) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
GENERAL ELECTRIC
Schenectady
NY
|
Family ID: |
39525718 |
Appl. No.: |
11/612743 |
Filed: |
December 19, 2006 |
Current U.S.
Class: |
148/243 ;
148/281; 148/422 |
Current CPC
Class: |
C22C 27/02 20130101;
C23C 8/12 20130101; C22F 1/18 20130101; C23C 12/02 20130101; C23C
10/20 20130101; C23C 16/30 20130101; C23C 14/06 20130101 |
Class at
Publication: |
148/243 ;
148/422; 148/281 |
International
Class: |
C23C 22/05 20060101
C23C022/05; C22C 27/02 20060101 C22C027/02 |
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 (NbGe.sub.2).
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.degree. C. to about
1400.degree. C.
31. The method of claim 26, wherein the heat treatment causes the
formation of the niobium digermanide (NbGe.sub.2) 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)T.sub.m to about
(1.5)T.sub.m of the germanium-containing material, where "T.sub.m"
represents the melting temperature.
Description
BACKGROUND OF THE INVENTION
[0001] 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.
[0002] 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.
[0003] 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.degree. C., many RMIC alloys have
an operating temperature in the range of about 1200.degree.
C.-1700.degree. 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.
[0004] 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.degree. 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.
[0005] 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
[0006] 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.
[0007] 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:
[0008] (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
[0009] (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.
[0010] An additional embodiment is directed to another method for
preparing such a niobium silicide article. The alternative method
comprises the following steps:
[0011] (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;
[0012] (b) applying a germanium-containing material (e.g., a
coating) to a surface of the article formed in step (a); and
then
[0013] (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.
[0014] Other features and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is an SEM (scanning electron microscope) image of a
niobium-silicide test coupon treated according to some embodiments
of the present invention.
[0016] FIG. 2 is an EDS (energy dispersive X-Ray spectroscopy)
representation of the image of FIG. 1, showing germanium content in
the test coupon.
[0017] FIG. 3 is an EDS representation of the image of FIG. 1,
showing oxygen content in the test coupon.
[0018] FIG. 4 is a chart which demonstrates the effect of germanium
levels on various sample alloys, after heat treatment steps.
[0019] 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
[0020] 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).
[0021] 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 %.
[0022] 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.
[0023] 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).
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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 NbGe.sub.2, 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.
[0028] 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
NbGe.sub.2 (niobium digermanide); Nb.sub.3Ge (e.g., the Beta
phase); Nb.sub.5Ge.sub.3; TiGe.sub.2, Ti.sub.5Ge.sub.3,
Ti.sub.6Ge.sub.5, 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.
[0029] In some specific embodiments, at least a portion of the
germanium is present in the form of the niobium digermanide phase
(NbGe.sub.2). 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 NbGe.sub.2. In other embodiments, at least
about 50% of the germanium is in the form of NbGe.sub.2. In some
especially preferred embodiments, at least about 70% of germanium
in the surface region is present in the form of NbGe.sub.2. (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 %).
[0030] 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).
[0031] 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).
[0032] 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.
[0033] 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.degree. C. to about
1400.degree. C. In some specific embodiments, the heat treatment is
carried out at a temperature in the range of about 1000.degree. C.
to about 1250.degree. C.
[0034] 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.
[0035] 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.times.1 cm.times.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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.sub.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.sub.m to about (1.5)T.sub.m of the material. In some
preferred embodiments, the heating temperature is in the range of
about (1.2)T.sub.m to about (1.4)T.sub.m. Thus, if elemental
germanium (with a melting temperature of 937.degree. C.) were used
in the slurry, the broader range would be about 750.degree. C. to
about 1405.degree. C. (It should be understood, however, that the
temperature used should not exceed the melting point of the
substrate).
[0045] 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.
[0046] 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).
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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 %).
[0052] 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.
[0053] 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.
[0054] 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.
[0055] 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.
[0056] 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).
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.degree.
C. to about 1400.degree. 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.
[0061] 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
[0062] 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
[0063] 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.times.1 cm.times.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.degree. C.
[0064] 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).sub.5(Si,Ge).sub.3
phase.
[0065] 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 (NbGe.sub.2) 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.
[0066] 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.
[0067] 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).
[0068] 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
[0069] 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.degree. C. and 1000.degree. 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.
[0070] 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.
[0071] After the 1150.degree./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 %).
[0072] 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.
[0073] After the 1000.degree. 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/cm.sup.2.
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/cm.sup.2
for sample 1). Sample 3 did show a weight loss (-17 mg/cm.sup.2),
indicating some degradation of the alloy article, although the
weight loss was not as great as in the 1150.degree. C. test.
Example 3
[0074] 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.RTM. 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:
[0075] 50.0 g Ge
[0076] 27.5 g Remet.RTM. Ethyl Silicate 40
[0077] 27.5 g Ethyl alcohol
[0078] 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.degree. C. for 60 minutes, followed by 120.degree. C.
for 30 minutes, followed by 220.degree. C. for 60 minutes.
[0079] The coated coupon was then diffusion heat-treated in a
vacuum oven, at a temperature of about 1000.degree. 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.
[0080] 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.
* * * * *