Coating with dense columns separated by gaps

A strain-tolerant coating for use in gas turbine engines can include a plurality of dense, generally vertically-oriented columnar structures formed during deposition by preferential material accumulation, and a plurality of inter-columnar gaps separating the columnar structures. The columnar structures can include a plurality of randomly-oriented particle splats and can have a porosity of less than four percent.

BACKGROUND

The present invention relates generally to coatings formed by plasma spray. More particularly, the present invention relates to a coating having dense columns separated by gaps.

Liquid carrier plasma spray techniques, such as suspension plasma spray and solution precursor plasma spray techniques, have been developed to produce coatings having generally vertically-oriented columnar structures, similar to those produced by electron beam-physical vapor deposition (EB-PVD), separated by inter-columnar gaps or cracks, which can increase tolerance to the effects of thermal cycling during operation. The columnar structures are formed during material deposition by preferential material accumulation on surface asperities on a substrate. The columnar structures are porous, having nano-size pores, and are separated by porosity bands or areas of low material accumulation, which can accommodate thermal expansion and contraction of the coating. While the columnar microstructures can improve strain tolerance, the intra-columnar porosity can compromise the durability and toughness of the coating. Prior art liquid carrier plasma spray techniques teach modifying spray parameters such as standoff distance (distance between the plasma spray torch and the substrate), to increase the coating density. However, increasing density in such manner can result in a more uniform deposition of particles on the substrate and the loss of strain-tolerant columnar structures formed by preferential material accumulation.

SUMMARY

A strain-tolerant coating for use in gas turbine engines can include a plurality of dense, generally vertically-oriented columnar structures formed during deposition by preferential material accumulation, and a plurality of inter-columnar gaps separating the columnar structures. The columnar structures can include a plurality of randomly-oriented particle splats and can have a porosity of less than four percent.

A method of forming a strain-tolerant coating can include providing first and second suspension feedstocks, simultaneously feeding the first and second suspension feedstocks to a plasma spray torch, and depositing the particles of the first and second suspension feedstocks on a substrate to form a first plurality of columnar structures of preferential material accumulation. The first suspension has particles of a first material suspended in a liquid medium. The second suspension has particles of a second material, different from the first material, suspended in a liquid medium. The columnar structures of the first plurality of columnar structures have a porosity of less than four percent.

While the above-identified figures set forth embodiments of the present invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale, and applications and embodiments of the present invention may include features, steps and/or components not specifically shown in the drawings.

DETAILED DESCRIPTION

A strain-tolerant coating having high density columnar structures can be formed using a liquid carrier plasma spray technique that combines a conventional coating material with a second material capable of filling nanopores, which would otherwise form by the accumulation of the conventional material particles. The incorporation of the second material can increase the density of the coating matrix above that which has been obtained with single and mixed material coatings in the prior art while still retaining the columnar structures formed by preferential material accumulation. Increasing material density while preserving the columnar structures can improve coating durability and toughness, while maintaining the improved strain tolerance provided by inter-columnar gaps. While the present invention is particularly directed to the design of thermal barrier coatings, it will be understood by one of ordinary skill in the art that the dense columnar microstructure and method of manufacture disclosed has application outside of thermal barrier coatings, including but not limited to environmental barrier coatings for ceramic matrix composites, tribological or wear coatings, and thin-film solid oxide fuel cells.

FIG. 1is a perspective view of one embodiment of coating system10as applied to substrate12. Coating system10can include outer coating14and optional inner coating16. Outer coating14can have dense generally vertically-oriented columnar structures18separated by inter-columnar gaps20. Inner coating16can also have generally vertically-oriented columnar structures22separated by inter-columnar gaps24. Inter-columnar gaps20and24can extend from an outermost layer26,28of outer coating14and inner coating16, respectively. In some embodiments, inter-columnar gaps20and24can extend a full thickness of outer coating14and inner coating16, respectively. As used herein, the term “generally vertically-oriented” refers to columns extending from an innermost coating layer30,32of inner and outer coatings16and14to the respective outermost coating layer28,26of inner and outer coatings16and14. Columnar structures18and22formed during deposition can be generally aligned with the spray angle, which can typically vary between 45° and 90° from the substrate. While the spray angle can be set to optimize mechanical properties of the coating, such as adhesion strength, complex component geometries can sometimes necessitate variations in spray angle across the substrate. Substrate12can be a component of a gas turbine engine. Substrate12can be a nickel- or cobalt-based superalloy, a refractory alloy, or a ceramic matrix composite (CMC), among other materials. In some embodiments, coating system10can also include a bond coat (not shown), which can be applied to substrate12prior to coating deposition. The bond coat can be a diffusion or overlay bond coat including, for example, an aluminide or MCrAlY (where M can be iron, cobalt, or nickel) applied to substrate12, as known in the art. In some embodiments, a surface of substrate12and/or the bond coat can be roughened, such as by grit blasting or peening, to improve adhesion of coating14or16.

In some embodiments, inner coating16can be included to provide a layer of material chemically compatible with both substrate12and outer coating14, when outer coating14is chemically incompatible with substrate12. Inner coating16can improve adhesion between outer coating14and substrate12. In some embodiments, inner coating16can have a total thickness less than outer coating14. Both outer coating14and inner coating16can be designed to provide durability, toughness, and strain tolerance to coating system10.

Both outer and inner coatings14and16can comprise a ceramic material, such as an oxide, and including but not limited to rare-earth zirconates, hafnates, or titanates, silicates, aluminates, or titanates as known in the art for producing thermal barrier coatings for high temperature applications, environmental barrier coatings, tribological or wear coatings, and thin-film solid oxide fuel cells, among others. Both outer and inner coatings14and16can be deposited using suspension plasma spraying, in which the ceramic particles are suspended in a liquid medium. In some embodiments, as shown inFIG. 1, columnar structures22of inner coating16can be more porous than columnar structures18of outer coating14. Additionally, inter-columnar gaps24of inner coating16can be less defined than inter-columnar gaps20of outer coating14.

During deposition of inner coating16, partially or fully melted ceramic particles can impact a surface of substrate12or previously deposited particles. The particles can form both randomly oriented splats13and spheroids, which can be visible in a micrograph. Spheroids may be produced when the kinetic energy of the ceramic particle is less than the surface tension of the particle, which can allow the particle to generally maintain shape or be only minimally deformed upon impact. The majority of particles can solidify prior to or upon impact with substrate12. As such, the particles can accumulate to form a microstructure with nanopores formed between adjacent particles. Preferential accumulation on substrate asperities can produce porous columnar structures22separated by relatively higher porosity inter-columnar porosity bands or gaps24.

Outer coating14can be designed to improve the durability and toughness of coating system10. In addition to the deposition of conventional coating materials, which can solidify prior to or upon impact of substrate12or previously deposited particles, outer coating14can include a second material that can fill in nanopores, which would otherwise form between particles. In some embodiments, the second material may not immediately solidify and recrystallize upon impact. In some embodiments, the second material may retain substantial sintering energy and sinter outer ceramic coating14upon impact and with subsequent passes of the plasma torch as additional heat is applied. As shown inFIG. 1, adjacent dense and generally vertically-oriented columnar structures18can be separated by well-defined inter-columnar gaps20. Inter-columnar gaps20can define voids in material accumulation that substantially separate adjacent columnar structures18along a total length of columnar structures18. Columnar structures18, formed by the deposition of two materials, can be contrasted with columnar structures36shown inFIG. 2, which were formed upon deposition of a single material to form outer coating38of coating system40. Columnar structures18have a lower porosity than columnar structures36, which is visible in comparison of the micrographs ofFIGS. 1 and 2. Additionally, well-defined inter-columnar gaps20of coating system10(FIG. 1) can be contrasted with the inter-columnar gaps42of coating system40, which define regions of relatively high porosity between adjacent columnar structures36of relatively lower porosity. As viewed in the micrograph ofFIG. 1, inter-columnar gaps20can delineate distinct boundaries of adjacent columnar structures18; whereas boundaries of adjacent columnar structures36can be unclear. The size and orientation of both columnar structures18and inter-columnar gaps20can be varied to provide a desired tolerance to both thermal and mechanical stresses (e.g., abrasion or rub). In some embodiments, e.g., wear coatings, larger inter-columnar gaps may be beneficial. In some embodiments, a plurality of inter-columnar gaps20can have a length equal to at least 80 percent of the total length of adjacent columnar structures18and a median width greater than 5 μm. As shown inFIG. 1, the microstructure of inner coating16, which can be formed by deposition of a single material, can be similar to the microstructure of coating system40, having a higher porosity than outer coating14and less defined inter-columnar gaps24.

In one embodiment, outer coating14can include equal parts of gadolinium zirconate (GZO), which has a melting point of 2850° C., and gadolinium aluminate (GdAlO3), which has a lower melting point of 2350° C. Both materials can be deposited simultaneously. The materials can be premixed or fed to the plasma spray torch from separate vessels at a predetermined rate. The GdAlO3can fill nanopores, which would otherwise form between adjacent GZO particles, to create columnar structures18having a porosity of less than four percent. In some embodiments, the porosity is less than three percent and in others, the porosity is less than 2.5 percent. As shown inFIG. 1, the highly dense columnar structures18remain separated by inter-columnar gaps20even as the theoretical density of columnar structures18approaches 100 percent (porosity nears zero percent). Inner coating16can be formed of yttrium-stabilized zirconia (YSZ).

FIGS. 3A and 3Bare x-ray maps of aluminum and zirconium, which show the formation of discrete layers44and46of GZO and GdAlO3, respectively, throughout the coating matrix, as represented by the elements (zirconium and aluminum) unique to each compound. Discrete layers44and46of GZO and GdAlO3can substantially correspond to a droplet size formed from plasma torch injectors. In some embodiments, discrete layers44and46can be approximately 1 μm. As shown inFIGS. 3A and 3B, discrete layers44and46of GZO and GdAlO3materials can be substantially evenly distributed throughout columnar structures18. GZO can provide CMAS resistance in thermal barrier coatings. GdAlO3, while increasing the density of outer coating14, may provide additional durability improvements.

FIG. 4provides a diagram of steps for producing coating system10. As shown inFIG. 4, first and second suspensions can be provided having particles of first and second materials, respectively, suspended in liquid mediums (steps100and102). The first and second suspensions can be simultaneously fed to a plasma spray torch (step104), and deposited on a substrate to form a first plurality of dense columnar structures separated by inter-columnar gaps (step106). In some embodiments, a suspension having particles of a third material can be suspended in a liquid medium (step108), fed to a plasma spray torch (110), and deposited on a substrate to form a second plurality of columnar structures (112). The second plurality of columnar structures can form an inner coating16(step114) to which outer coating14can be applied (step116) following steps100-106.

Example

FIGS. 1, 2A, and 2Bshow coatings deposited using a Progressive Surface 100HE™ plasma torch with a 9.5 mm diameter exit nozzle. Outer coating14was deposited with a torch power of 105 kW at a standoff distance of 63.5 mm. 25 wt % GZO particles, having a median particle diameter (d50) of 0.5 μm, were suspended in ethanol and fed to the plasma torch at a rate of 45 ml/min. 25 wt % GdAlO3particles, having a median particle diameter (d50) of 0.5 μm, were suspended in ethanol and fed to the plasma torch at a rate of 45 ml/min. GZO and GdAlO3suspensions were simultaneously injected from two different 230 μm orifices oriented approximately 30 degrees apart and impinging at the plasma torch exit center. Similar dense columnar structures were formed at torch powers of 95 kW and 85 kW and standoff distance of 76.2 and 88.9 mm, however, column diameter and inter-columnar porosity was increased with lower torch power and increased standoff distance. Resulting outer coating14is composed of discreet layers44,46of GZO and GdAlO3of about 1 μm in size, which corresponds to a droplet size formed from the injectors. Columnar structures18have an intra-columnar porosity of 0.6 percent as determined by contrast in scanning electron microscopy at a magnification of 2500×. Columnar structure porosity was determined for an image taken at a mid-coating thickness and included all pores.

The incorporation of a second material capable of filling nanopores, which would otherwise form by conventional material particles in a liquid carrier plasma spray technique can produce strain-tolerant coatings having high density columnar structures. The incorporation of the second material can increase the density of the coating matrix above that which has been obtained with single and mixed material coatings in the prior art while still retaining columnar structures formed by preferential material accumulation. Increasing material density while preserving the columnar structures can improve coating durability and toughness, while maintaining the improved strain tolerance provided by the inter-columnar gaps.

Summation

Any relative terms or terms of degree used herein, such as “substantially”, “essentially”, “generally”, “approximately” and the like, should be interpreted in accordance with and subject to any applicable definitions or limits expressly stated herein. In all instances, any relative terms or terms of degree used herein should be interpreted to broadly encompass any relevant disclosed embodiments as well as such ranges or variations as would be understood by a person of ordinary skill in the art in view of the entirety of the present disclosure, such as to encompass ordinary manufacturing tolerance variations, incidental alignment variations, transient alignment or shape variations induced by thermal, rotational or vibrational operational conditions and the like. Moreover, any relative terms or terms of degree used herein should be interpreted to encompass a range that expressly includes the designated quality, characteristic, parameter or value, without variation, as if no qualifying relative term or term of degree were utilized in the given disclosure or recitation.

Discussion of Possible Embodiments

A strain-tolerant coating for use in gas turbine engines can include a plurality of dense, generally vertically-oriented columnar structures formed during deposition by preferential material accumulation, and a plurality of inter-columnar gaps separating the columnar structures. The columnar structures can include a plurality of randomly-oriented particle splats and can have a porosity of less than four percent.

The strain-tolerant coating of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:

The strain-tolerant coating of the preceding paragraphs, wherein the columnar structures can have a porosity of less than three percent.

The strain-tolerant coating of any of the preceding paragraphs, wherein the columnar structures can include a first material and a second material, which can be different than the first material.

The strain-tolerant coating of any of the preceding paragraphs, wherein the columnar structures can include a plurality of discrete layers of the first material and a plurality of discrete layers of the second material.

The strain-tolerant coating of any of the preceding paragraphs, wherein the first and second materials can be substantially evenly distributed throughout the columnar structures.

The strain-tolerant coating of any of the preceding paragraphs, wherein the first material can have a first melting point, and the second material can have a second melting point that is less than the first melting point.

The strain-tolerant coating of any of the preceding paragraphs, wherein the first material can be gadolinium zirconate.

The strain-tolerant coating of any of the preceding paragraphs, wherein the second material can be gadolinium aluminate.

The strain-tolerant coating of any of the preceding paragraphs, wherein the columnar structures can comprise 50 percent gadolinium zirconate and 50 percent gadolinium aluminate.

The strain-tolerant coating of any of the preceding paragraphs, wherein the plurality of columnar structures can constitute a first plurality of columnar structures and the plurality of inter-columnar gaps can constitute a first plurality of inter-columnar gaps. The first plurality of columnar structures and first plurality of inter-columnar gaps can be located in an outermost section of the coating. The coating can further include an innermost section, including a second plurality of generally vertically-oriented columnar structures formed during deposition by material accumulation and having randomly-oriented particle splats, and a second plurality of inter-columnar gaps separating the second plurality of columnar structures.

The strain-tolerant coating of any of the preceding paragraphs, wherein the coating can have a total thickness, which includes a total thickness of the outermost section and a total thickness of the innermost section, with the total thickness of the innermost section being less than the total thickness of the outermost section.

The strain-tolerant coating of any of the preceding paragraphs, wherein the inter-columnar gaps of the first plurality of inter-columnar gaps can define voids in material accumulation that substantially separate adjacent columnar structures along a total length of the columnar structures.

The strain-tolerant coating of any of the preceding paragraphs, wherein the inter-columnar gaps can extend a length equal to at least 80 percent of the total length of the columnar structures.

A method of forming a strain-tolerant coating can include providing first and second suspension feedstocks, simultaneously feeding the first and second suspension feedstocks to a plasma spray torch, and depositing the particles of the first and second suspension feedstocks on a substrate to form a first plurality of columnar structures of preferential material accumulation. The first suspension has particles of a first material suspended in a liquid medium. The second suspension has particles of a second material, different from the first material, suspended in a liquid medium. The columnar structures of the first plurality of columnar structures have a porosity of less than four percent.

The method of the preceding paragraphs can further include forming inter-columnar gaps, wherein the inter-columnar gaps define voids in material accumulation that substantially separate adjacent columnar structures along a total length of the columnar structures.

The method of any of the preceding paragraphs, wherein the first material can have a first melting point and the second material can have a second melting point that is less than the first melting point.

The method of any of the preceding paragraphs, wherein the first material can be gadolinium zirconate.

The method of any of the preceding paragraphs, wherein the second material can be gadolinium aluminate.

The method of any of the preceding paragraphs can further include providing a third suspension feedstock comprising particles of a third material suspended in a liquid medium, which can be fed to a plasma spray torch and deposited on the substrate to form a second plurality of columnar structures of preferential material accumulation. The second plurality of columnar structures can make up an inner coating and the first plurality of columnar structures can make up an outer coating.

The method of any of the preceding paragraphs, wherein the third material can be yttrium stabilized zirconia.