BRIEF SUMMARY OF THE INVENTION
The present invention provides slurry coating processes for selectively enriching surface regions of metal-based substrates, for example, the under-platform regions of a turbine blade, with chromium.
The process of this invention preferably employs a slurry coating composition containing a metallic powder whose bulk composition contains metallic chromium, optionally metallic aluminum in a lesser amount by weight than chromium, and optionally other constituents. The composition further includes colloidal silica, and may also include one or more additional constituents, though in any event the composition is substantially free of hexavalent chromium and sources thereof.
The slurrying coating process generally entails preparing the slurry coating composition, applying the slurry coating composition to the surface region of the substrate to form a slurry coating on the surface region, and then heat treating the slurry coating to remove any volatile components of the slurry coating composition and thereafter cause diffusion of chromium from the slurry coating composition into the surface region of the substrate to form a chromium-rich diffusion coating.
Notable advantages associated with the slurry coating process of this invention include its effectiveness in chromizing a metal substrate, the ease with which the slurry can be economically prepared, and the ease with which the content of the coating species in the slurry can be readily adjusted to meet the requirements for a particular substrate. Moreover, the slurry coating composition employed by the process of this invention exhibits highly desirable stability characteristics while being free of chromate compounds, including hexavalent chromium, and free of phosphoric acid. Furthermore, the slurry coating composition can be applied by a number of different techniques, and its wetting ability promotes the formation of a relatively uniform coating.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing a representative example of a high pressure turbine blade.
FIGS. 2 and 3 are scanned images of cross-sections through substrates protected with a chromide diffusion coating deposited in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The slurry coating process of the present invention is adapted to selectively enrich surface regions of substrates with chromium and preferably also aluminum. A particular application is the under-platform regions on turbine blades of gas turbine engines, an example of which is a high pressure turbine blade 10 shown in FIG. 1. The blade 10 generally includes an airfoil 12 against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to severe attack by oxidation, corrosion and erosion. For this reason, the airfoil 12 is typically protected from the hostile environment of the turbine section by an environmentally-resistant coating, for example, a diffusion coating such as an aluminide or platinum aluminide coating often deposited by pack cementation or noncontact vapor deposition. The blade 10 is configured to be anchored to a turbine disk (not shown) with a dovetail 14 formed on a root section of the blade 10. A platform 16 separates the airfoil 12 and dovetail 14, such that the root section, its dovetail 14, and the underside of the platform 16 can be are referred to as under-platform regions 18 of the blade 10. Though not directly exposed to the hot gas path of a turbine engine, the under-platform regions 18 are nonetheless susceptible to oxidation and corrosion. Slurry coating compositions and processes of this invention are particularly adapted to selectively form a chromium-containing coating on the surfaces of the under-platform regions 18 of the blade 10 of FIG. 1, as well as surfaces of other components similarly subjected to oxidation and corrosion.
The slurry coating compositions of this invention contain a powder of metallic chromium (i.e., in a zero oxidation state), and preferably also metallic aluminum. The composition preferably contains colloidal silica as the liquid vehicle. The term “colloidal silica” is meant to embrace any dispersion of fine particles of silica in a medium of water or another solvent, with water being preferred such that the slurry composition is a water-based (aqueous) system. Dispersions of colloidal silica are available from various chemical manufacturers in either acidic or basic form. Moreover, various shapes of silica particles can be used, e.g., spherical, hollow, porous, rod, plate, flake, or fibrous, as well as amorphous silica powder. Spherical silica particles are generally preferred. The particles may have an average particle size in a range of about 10 nanometers to about 100 nanometers. Nonlimiting examples of references which describe colloidal silica include U.S. Pat. Nos. 4,027,073 and 5,318,850, which are incorporated herein by reference. Commercial examples of colloidal silica are available under the names Ludox® and Remasol® from REMET Corporation, of Utica, N.Y., USA.
The amount of colloidal silica present in the composition will depend on various factors, for example, the amount of metallic powder used and the presence (and amount) of any other constituents in the slurry, for example, an organic stabilizer as discussed below. Colloidal silica appears to function primarily as a very effective binder in the slurry composition. Processing conditions are also a consideration, for example, how the slurry is formed and applied to the under-platform regions 18. The colloidal silica may be present at a level in the range of about 1% to about 25% by weight, based on silica solids as a percentage of the entire composition. In especially preferred embodiments, the amount is in the range of about 10% to about 20% by weight.
The metallic powder may constitute, by weight, about 25% to about 80%, more preferably about 30% to about 50%, of the entire slurry composition. The powder particles may be in the form of spherical particles, though other forms are possible as well, such as wire, wire mesh, and those described above for the colloidal silica. The metallic powder can be used in a variety of standard sizes. Preferred sizes for the powder particles will depend on several factors, such as the alloy of the under-platform regions 18, the technique by which the slurry is to be applied to the under-platform regions 18, and the presence and amounts of other potential constituents in the slurry. An example of a suitable average particle size range is about 0.5 to about 200 micrometers. In some preferred embodiments, the powder particles have an average particle size in the range of about 1 to about 50 micrometers, with a particularly preferred range being about 1 to about 20 micrometers. The powder particles can be produced by various processes, including gas atomization processes, rotating electrode techniques, etc.
In the illustrated example, the metallic powder serves as the source for the corrosion-resistant species, chromium and optionally aluminum, desired for the under-platform regions 18 of the blade 10. As such, the metallic powder contains particles of at least chromium, and optionally particles of both chromium and aluminum or additional and separate particles of aluminum, such that the bulk composition of the metallic powder contains less aluminum by weight than chromium. The powder may also contain other elements capable of imparting desired characteristics to the under-platform regions 18, e.g., enhanced oxidation resistance, phase stability, environmental resistance, and sulfidation resistance. For example, the powder may contain one or more platinum group metals (platinum, palladium, ruthenium, rhodium, osmium, and iridium), and/or one or more rare earth metals (lanthanides) such as lanthanum, cerium, and erbium. Elements which are chemically-similar to the lanthanides could also be included, such as scandium and yttrium. In some instances, it may also be desirable to include one or more of iron, cobalt, and silicon. Moreover, those skilled in the art understand that the powder may also contain various other elements and other materials at impurity levels, e.g., less than about 1% by weight. Techniques for preparing powders formed from any combination of the optional elements described above are also well known in the art and available from a number of commercial sources, and therefore will not be discussed in any detail here.
Suitable and preferred compositions for the chromium-based powder and its amount in the slurry composition will depend in large part on the amount of chromium desired for the under-platform regions 18. In general, suitable amounts of chromium and optionally aluminum in the slurry composition should exceed their respective amounts in the substrate to be protected. The chromium content of the slurry composition is also preferably sufficient to compensate for any projected loss of chromium from the under-platform regions 18 under expected operating conditions, such as temperatures, temperature/time schedules and cycles, and environmental conditions. Preferred coatings produced by this invention on nickel-base superalloy substrates contain at least 15 to less than 60 weight percent chromium, and preferably about 25 to about 30 weight percent chromium, and further contain aluminum in an amount below that at which a continuous beta intermetallic (NiAl) phase will form (for example, less than 18 weight percent aluminum, though this value will depend on the coating composition, including the amount of chromium), with the balance of the coating being nickel and other constituents present in the substrate. More generally, suitable powder materials contain more chromium than aluminum by weight, and
BRIEF SUMMARY OF THE INVENTION
The present invention provides slurry coating processes for selectively enriching surface regions of metal-based substrates, for example, the under-platform regions of a turbine blade, with chromium.
The process of this invention preferably employs a slurry coating composition containing a metallic powder whose bulk composition contains metallic chromium, optionally metallic aluminum in a lesser amount by weight than chromium, and optionally other constituents. The composition further includes colloidal silica, and may also include one or more additional constituents, though in any event the composition is substantially free of hexavalent chromium and sources thereof.
The slurrying coating process generally entails preparing the slurry coating composition, applying the slurry coating composition to the surface region of the substrate to form a slurry coating on the surface region, and then heat treating the slurry coating to remove any volatile components of the slurry coating composition and thereafter cause diffusion of chromium from the slurry coating composition into the surface region of the substrate to form a chromium-rich diffusion coating.
Notable advantages associated with the slurry coating process of this invention include its effectiveness in chromizing a metal substrate, the ease with which the slurry can be economically prepared, and the ease with which the content of the coating species in the slurry can be readily adjusted to meet the requirements for a particular substrate. Moreover, the slurry coating composition employed by the process of this invention exhibits highly desirable stability characteristics while being free of chromate compounds, including hexavalent chromium, and free of phosphoric acid. Furthermore, the slurry coating composition can be applied by a number of different techniques, and its wetting ability promotes the formation of a relatively uniform coating.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view showing a representative example of a high pressure turbine blade.
FIGS. 2 and 3 are scanned images of cross-sections through substrates protected with a chromide diffusion coating deposited in accordance with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The slurry coating process of the present invention is adapted to selectively enrich surface regions of substrates with chromium and preferably also aluminum. A particular application is the under-platform regions on turbine blades of gas turbine engines, an example of which is a high pressure turbine blade 10 shown in FIG. 1. The blade 10 generally includes an airfoil 12 against which hot combustion gases are directed during operation of the gas turbine engine, and whose surface is therefore subjected to severe attack by oxidation, corrosion and erosion. For this reason, the airfoil 12 is typically protected from the hostile environment of the turbine section by an environmentally-resistant coating, for example, a diffusion coating such as an aluminide or platinum aluminide coating often deposited by pack cementation or noncontact vapor deposition. The blade 10 is configured to be anchored to a turbine disk (not shown) with a dovetail 14 formed on a root section of the blade 10. A platform 16 separates the airfoil 12 and dovetail 14, such that the root section, its dovetail 14, and the underside of the platform 16 can be are referred to as under-platform regions 18 of the blade 10. Though not directly exposed to the hot gas path of a turbine engine, the under-platform regions 18 are nonetheless susceptible to oxidation and corrosion. Slurry coating compositions and processes of this invention are particularly adapted to selectively form a chromium-containing coating on the surfaces of the under-platform regions 18 of the blade 10 of FIG. 1, as well as surfaces of other components similarly subjected to oxidation and corrosion.
The slurry coating compositions of this invention contain a powder of metallic chromium (i.e., in a zero oxidation state), and preferably also metallic aluminum. The composition preferably contains colloidal silica as the liquid vehicle. The term “colloidal silica” is meant to embrace any dispersion of fine particles of silica in a medium of water or another solvent, with water being preferred such that the slurry composition is a water-based (aqueous) system. Dispersions of colloidal silica are available from various chemical manufacturers in either acidic or basic form. Moreover, various shapes of silica particles can be used, e.g., spherical, hollow, porous, rod, plate, flake, or fibrous, as well as amorphous silica powder. Spherical silica particles are generally preferred. The particles may have an average particle size in a range of about 10 nanometers to about 100 nanometers. Nonlimiting examples of references which describe colloidal silica include U.S. Pat. Nos. 4,027,073 and 5,318,850, which are incorporated herein by reference. Commercial examples of colloidal silica are available under the names Ludox® and Remasol® from REMET Corporation, of Utica, N.Y., USA.
The amount of colloidal silica present in the composition will depend on various factors, for example, the amount of metallic powder used and the presence (and amount) of any other constituents in the slurry, for example, an organic stabilizer as discussed below. Colloidal silica appears to function primarily as a very effective binder in the slurry composition. Processing conditions are also a consideration, for example, how the slurry is formed and applied to the under-platform regions 18. The colloidal silica may be present at a level in the range of about 1% to about 25% by weight, based on silica solids as a percentage of the entire composition. In especially preferred embodiments, the amount is in the range of about 10% to about 20% by weight.
The metallic powder may constitute, by weight, about 25% to about 80%, more preferably about 30% to about 50%, of the entire slurry composition. The powder particles may be in the form of spherical particles, though other forms are possible as well, such as wire, wire mesh, and those described above for the colloidal silica. The metallic powder can be used in a variety of standard sizes. Preferred sizes for the powder particles will depend on several factors, such as the alloy of the under-platform regions 18, the technique by which the slurry is to be applied to the under-platform regions 18, and the presence and amounts of other potential constituents in the slurry. An example of a suitable average particle size range is about 0.5 to about 200 micrometers. In some preferred embodiments, the powder particles have an average particle size in the range of about 1 to about 50 micrometers, with a particularly preferred range being about 1 to about 20 micrometers. The powder particles can be produced by various processes, including gas atomization processes, rotating electrode techniques, etc.
In the illustrated example, the metallic powder serves as the source for the corrosion-resistant species, chromium and optionally aluminum, desired for the under-platform regions 18 of the blade 10. As such, the metallic powder contains particles of at least chromium, and optionally particles of both chromium and aluminum or additional and separate particles of aluminum, such that the bulk composition of the metallic powder contains less aluminum by weight than chromium. The powder may also contain other elements capable of imparting desired characteristics to the under-platform regions 18, e.g., enhanced oxidation resistance, phase stability, environmental resistance, and sulfidation resistance. For example, the powder may contain one or more platinum group metals (platinum, palladium, ruthenium, rhodium, osmium, and iridium), and/or one or more rare earth metals (lanthanides) such as lanthanum, cerium, and erbium. Elements which are chemically-similar to the lanthanides could also be included, such as scandium and yttrium. In some instances, it may also be desirable to include one or more of iron, cobalt, and silicon. Moreover, those skilled in the art understand that the powder may also contain various other elements and other materials at impurity levels, e.g., less than about 1% by weight. Techniques for preparing powders formed from any combination of the optional elements described above are also well known in the art and available from a number of commercial sources, and therefore will not be discussed in any detail here.
Suitable and preferred compositions for the chromium-based powder and its amount in the slurry composition will depend in large part on the amount of chromium desired for the under-platform regions 18. In general, suitable amounts of chromium and optionally aluminum in the slurry composition should exceed their respective amounts in the substrate to be protected. The chromium content of the slurry composition is also preferably sufficient to compensate for any projected loss of chromium from the under-platform regions 18 under expected operating conditions, such as temperatures, temperature/time schedules and cycles, and environmental conditions. Preferred coatings produced by this invention on nickel-base superalloy substrates contain at least 15 to less than 60 weight percent chromium, and preferably about 25 to about 30 weight percent chromium, and further contain aluminum in an amount below that at which a continuous beta intermetallic (NiAl) phase will form (for example, less than 18 weight percent aluminum, though this value will depend on the coating composition, including the amount of chromium), with the balance of the coating being nickel and other constituents present in the substrate. More generally, suitable powder materials contain more chromium than aluminum by weight, and
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