Patent Application
20150159507
This invention relates to coatings and components for high temperature applications, such as gas turbine assemblies.
The design of modern gas turbines is driven by the demand for higher turbine efficiency. It is widely recognized that turbine efficiency can be increased by operating the turbine at higher temperatures. In order to assure a satisfactory life span at these higher temperatures, thermal barrier coatings (hereinafter referred to as “TBCs”) are applied to airfoils and combustion components of the turbine, such as transition pieces and combustion liners, using various techniques.
A key concern for turbines used in both power generation and propulsion applications is with harmful effects of ingested dust, sand, volcanic ash, and other species entrained in turbine intake air. These species can adhere to TBCs and damage them through the formation of various comparatively low-melting point phases collectively referred to as “CMAS” due to their typical inclusion of such oxide components as calcia, magnesia, alumina, and silica. CMAS material generally melts around 1200° C. (about 2250° F.), which is below the surface temperature expected for TBC's in high-performance turbine components; once molten, the liquid CMAS infiltrates the cracks, pores, columnar grain boundaries, and open defects of TBCs and solidifies to form a glass when the TBCs cool to room temperature. As a result, the TBCs lose compliance and spall prematurely.
The industry standard 8YSZ material (zirconia stabilized with approximately 8 weight percent yttria) used for TBCs is particularly susceptible to degradation via CMAS. One technique to combat spallation resulting from CMAS ingestion involves TBC compositions with higher rare earth contents as compared to conventional TBCs. These high-rare-earth TBCs are designed to react with ingested CMAS and thereby limit its penetration. These high rare earth TBCs, however, have lower fracture toughness than conventional YSZ-based thermal barrier coatings, and thus, while attractive for some turbine applications, simply changing the chemistry of the coating may not be an ideal solution for all turbine designs.
As a result of the above, a need persists in the industry for thermal barrier coatings and related methods for fabricating coated components, where the coatings are resistant to CMAS ingestion (i.e., spallation resistant), include high strain tolerance, are scalable (i.e., compatible with large components), and are relatively inexpensive as compared with conventional thermal bather coatings.
Embodiments of the present invention are provided to meet this and other needs. One embodiment is an article. The article comprises a substrate; and a coating system disposed over the substrate. The coating system comprises a first layer comprising a first material and a second layer comprising a second material, with the first layer disposed between the second layer and the substrate. The second layer comprises proximal and distal surfaces relative to the substrate, and has a thickness defined between the proximal and distal surfaces; this second layer also has a distal surface region extending from the distal surface to a depth below the distal surface of 20% of the thickness of the second layer. The first material has a higher fracture toughness than the second material, and the second material is more resistant to infiltration by a nominal CMAS composition relative to 8 weight percent yttria-stabilized zirconia at a temperature of 1300 degrees Celsius. The second layer comprises a plurality of through-thickness cracks, wherein at least 90 percent of the cracks have a mean crack opening displacement, measured in the distal surface region, of up to about 5 micrometers.
Another embodiment is an article comprising a substrate comprising a superalloy; and a coating system disposed over the substrate. The coating system comprises a bondcoat comprising an aluminide or an MCrAlY material and disposed over the substrate; a first layer, comprising yttria-stabilized zirconia, disposed over the bondcoat; and a second layer, comprising yttria-stabilized zirconia having at least about 55 weight percent yttria, disposed over the first layer, the second layer comprising proximal and distal surfaces relative to the substrate, and the second layer having a distal surface region extending from the distal surface to a depth below the distal surface of 20% of the thickness of the second layer. The second layer comprises a plurality of through-thickness cracks, wherein at least 90 percent of the cracks have a mean crack opening displacement, measured in the distal surface region, of up to about 5 micrometers.
Another embodiment is an article comprising a substrate comprising a ceramic-matrix composite; and a coating system disposed over the substrate. The coating system comprises a bondcoat comprising a silicide or elemental silicon and disposed over the substrate; a first layer, comprising a silicate, disposed over the bondcoat; and a second layer, comprising a rare earth silicate disposed over the first layer. The second layer comprises proximal and distal surfaces relative to the substrate, and has a distal surface region extending from the distal surface to a depth below the distal surface of 20% of the thickness of the second layer. The second layer comprises a plurality of through-thickness cracks, wherein at least 80 percent of the cracks have a mean crack opening displacement, measured in the distal surface region, of up to about 5 micrometers.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Embodiments of the present invention include a coating having a unique microstructure that provides desirable resistance to CMAS infiltration while maintaining desirable levels of adhesion, strain tolerance, and other mechanical properties. The coating may include a wide range of materials, including stabilized zirconia systems, and may be deposited via scalable processes such as plasma spray techniques.
In one embodiment, as depicted schematically in
Coating system 104 comprises a first layer 106 comprising a first material, and a second layer 108 comprising a second material. First layer 106 is disposed between the second layer and the substrate. The first material has a higher fracture toughness than the second material to provide desirable levels of strain and damage tolerance to coating system 104. The second material is a CMAS-resistant material, meaning that the second material is more resistant to infiltration at 1300° C. by molten “nominal CMAS” than is 8YSZ. Some of these materials owe their effective resistance to CMAS infiltration due to their tendency to react very slowly with CMAS, while others react very quickly but rapidly form a reaction product that effectively seals off the material from further exposure to the CMAS. In either case, the material demonstrates overall degradation rates at 1300° C. that are less than what is known in the art to be the case for 8YSZ. For the purposes of this description, the term “nominal CMAS” refers to the following composition, with all percentages in mole percent: 41.7% silica (SiO2), 29.3% calcia (CaO), 12.5% alumina (AlO1.5), 9.1% magnesia (MgO), 6.0% iron oxide (FeO1.5), and 1.5% nickel oxide (NiO). It will be appreciated that the 1300° C. temperature and the nominal CMAS composition given in this definition represent a reference temperature and a reference composition to define a benchmark for the material's CMAS resistance in a way that can be compared to the CMAS resistance of 8YSZ; use of these reference values does not limit in any way the actual temperature at which article 100 may operate or the actual composition of ingested material that becomes deposited on the coating during operation, both of which, of course, will vary widely in service.
Second layer 108, as shown in cross section in
A plurality of cracks 116 runs through the thickness 114 of second layer 108. Typically these cracks run through at least 75% of thickness 114 and are generally oriented substantially vertically, meaning within about 45 degrees to either side of perpendicular relative to proximal surface 110. These cracks 116 are thus referred to herein as “through-thickness cracks,” and provide strain tolerance to second layer 108.
The presence of through-thickness cracks in second layer 108 is contrary to conventional wisdom in the art, which typically envisions a continuous sealing layer to be applied to the task of isolating the CMAS from the underlying TBC. See, for example, U.S. Pat. No. 7,875,370. Such structures generally lack the degree of strain tolerance attributable to vertically cracked coatings. However, the present inventors have made the surprising discovery that even cracked coatings may sufficiently mitigate CMAS infiltration to allow the use of more strain-tolerant architectures, so long as the population of through-thickness cracks are engineered appropriately, in accordance with the descriptions herein.
A distal surface region 118 is herein defined within second layer 108 as extending from the distal surface 112 to a depth of 20% of the thickness 114 below the distal surface 112. In embodiments of the present invention, at least 90 percent of the cracks have a mean crack opening displacement, measured in the distal surface region, of up to about 5 micrometers. As described in more detail below, coatings with cracks meeting this criterion remarkably resisted CMAS infiltration, while coatings having cracks outside of the stated criterion showed detrimental levels of CMAS infiltration and resultant degradation. In particular embodiments, at least 95% of the cracks have a mean crack opening displacement, measured in the distal surface region, of up to about 5 micrometers. In certain embodiments, in addition to the 90% or 95% criteria discussed above, at least 50 percent of the cracks 116 have a mean crack opening displacement, measured in the distal surface region 118, of up to about 1.5 micrometers. In some embodiments, 50% of the cracks measured in the distal surface region 118 have a mean crack opening displacement of up to about 1.25 micrometers.
“Mean crack opening displacement” of a through-thickness crack, as used herein, is defined to be the arithmetic mean of at least 14 measurements of a given crack's width taken per 10 micrometers of thickness within the defined distal surface region 118. A method for measuring crack opening displacement, based on an image analysis technique, may be applied to obtain the mean crack opening displacement, and a more detailed account of one such technique is provided in the Examples section of this description, below.
The results noted herein are consistent with a mechanism whereby the infiltration of CMAS into the cracks of a coating is modeled as a competition between capillary forces drawing liquid into a crack, on the one hand, and a repellant back-pressure generated by gas trapped within the crack on the other hand. Thus it can be theorized that, if this mechanism is indeed operating, sufficiently thin cracks may trap enough gas to balance the capillary force drawing the fluid into the crack, thereby limiting or eliminating the degree of CMAS penetration that is possible. This phenomenon allows the use of remarkably strain tolerant, vertically cracked coatings in second layer 108 that maintain desired levels of CMAS infiltration resistance.
The number of through-thickness cracks 116 present in second layer 108 may directly affect the strain tolerance of second layer 108, in that more cracks (that is, a higher linear frequency, also referred to in the art as “linear density,” of cracks intersecting a theoretical horizontal line drawn at the 75% thickness point relative to distal surface 112) typically results in higher strain tolerance for the layer. Thus, in one embodiment, the plurality of through-thickness cracks 116 in second layer 108 has a linear crack frequency of at least about 5 cracks per inch (about 2 cracks per centimeter). In some embodiments, this crack frequency is at least about 25 cracks per inch (about 10 cracks per cm), and in particular embodiments, the frequency is at least about 100 cracks per inch (about 40 cracks per centimeter). Those skilled in the art typically use a measurement length of at least about 25 mm to obtain a reasonably representative measurement of linear crack frequency.
In addition to vertical cracks, coating porosity is also known in the art to contribute some measure of strain tolerance to ceramic coatings of the type described for coating system 104. In one embodiment, second layer 108 has a porosity in the range from about 2% by volume to about 30% by volume. Thus it is not necessary for second layer to have a true “dense vertically cracked” microstructure as that term is used in the art, though such an embodiment is not precluded. Regarding porosity in first layer 106, experimental results suggest both conventional “porous” layers (porosity generally approximately 10%-30%) and “dense vertically micro-cracked” layers (porosity generally approximately 2%-10%) have shown desirable results in coating system 104. Thus in some embodiments, the porosity of first layer 106 is in the range from about 2% to about 30% by volume, and in two alternative embodiments, the porosity is in the range from about 2% by volume to about 10% by volume, or from about 10% to about 30%. In particular embodiments, first layer 106 has either a columnar structure with elongated coating growth domains, or a dense, equiaxed matrix with vertical micro-cracks, both of which coating types are associated with liquid-injection plasma spray processes in accordance with U.S. Pat. No. 8,586,172. These particular coating types are noted for their high adhesion and strain tolerance.
As noted above, the first material, which is present in first layer 106, has higher fracture toughness than the second material, which is present in second layer 108. Fracture toughness of the layers may be characterized relative to one another, for example, according to one of several standard techniques, such as indentation techniques, known and widely used in the art. The first material may be a ceramic material, and in some embodiments is a material used in thermal barrier coatings or other high temperature applications. Yttria-stabilized zirconia, including YSZ having a yttria content in the range from about 7 to about 9 weight percent, is a well-known example of such a material, as are hafnia and titania (including stabilized compositions that include these oxides). In some embodiments, the first layer is a material, such as a silicate, commonly used in recession-resistant environmental barrier coating applications and often associated with silicon-bearing substrates exposed to high temperatures; examples of such silicate coating materials include barium strontium aluminosilicate and rare-earth disilicates and monosilicates. As used herein, the term “rare-earth” will be understood to include not only the lanthanide series elements, but scandium and yttrium as well. Specific examples include yttrium disilicate and yttrium monosilicate.
The second material, used in second layer 108, is applied for CMAS resistance, as noted above. Many different materials have been described in the art as providing enhanced CMAS protection relative to yttria-stabilized zirconia and other standard TBC materials, and any of these materials may be considered for use in the coating system described herein.
In one embodiment, the second material includes an oxide. Oxides that include one or more transition metal elements, rare-earth elements, silicon, and/or indium have been described in the art as being resistant to CMAS. In one embodiment, the oxide includes zirconium, hafnium, titanium, or combinations thereof. Zirconia, hafnia, and/or titania materials stabilized with one or more rare-earth elements have been described in the art of CMAS-resistant coatings. Examples of such materials include coatings containing gadolinia and zirconia, such as gadolinia-stabilized zirconia; and coatings containing mixtures of gadolinia and hafnia. Examples of other potentially suitable oxide materials include pyrochlores, such as lanthanum zirconate; garnets, such as those described in U.S. Pat. No. 7,722,959; and oxyapatites, such as those described in U.S. Pat. No. 7,722,959. Sodium-containing oxides, such as sodium oxide, sodium silicate, and sodium titanate, are other examples of CMAS resistant oxide materials.
In one particular example, the second material includes yttria-stabilized zirconia having higher yttria content (relative to the overall YSZ content) than typical 8YSZ. Generally, the yttria content in this example is greater than 38 weight percent, and in specific embodiments the yttria content is at least about 55 weight percent. Coatings as described herein using YSZ with yttria content greater than 38 weight percent were superior to coating made with lower-yttria YSZ materials.
Other materials besides oxides have been described for use in resisting CMAS, and are also considered as potentially useful as second materials in coating system 104. Examples of such alternative materials include carbides (such as silicon carbide, tantalum carbide, titanium carbide, and others), nitrides (such as silicon nitride, zirconium nitride, tantalum nitride, boron nitride, and others), and silicides (such as chromium silicide, molybdenum silicide, tantalum silicide, titanium silicide, and others).
The materials described herein for both layers 106, 108 of coating system 104 may be applied to substrate 102 with the requisite microstructure using any of various deposition techniques commonly used in industry for the application of ceramic coatings for use in high-temperature components. Plasma spray techniques, including techniques applying liquid feedstock injection, are particularly attractive due to their scalability and relatively well understood relationships among processing parameters, coating structure, and resultant properties. For example, crack opening displacement and linear frequency are often controlled by parameters, such as (but not limited to) feedstock feed rate and particle size distribution, particle velocity, gun-to-substrate distance, substrate temperature, and plasma temperature, that affect particle temperature and deposition rate during deposition. Typically, hotter deposition temperatures tend to promote higher crack frequency, but may also promote more highly developed, that is, wider, cracks. Thus a plasma spray method for fabricating any particular coating system 104 will require some development of the proper combination of process parameters to develop a consistently acceptable microstructure as described herein.
A bondcoat 120 is disposed between first layer 106 and substrate 102 in some embodiments. Bondcoat 120 provides functionality—adhesion promotion and oxidation resistance, for example—in coating system 104 similar to what such coatings generally provide in conventional applications. In some embodiments, bondcoat 120 comprises an aluminide, such as nickel aluminide, or a MCrAlY-type coating well known in the art. These bondcoats may be especially useful when applied to a metallic substrate 102, such as a superalloy. In other embodiments, bondcoat comprises a silicide compound or elemental silicon, which are often associated with ceramic-based substrates, such as silicon carbide reinforced silicon carbide ceramic matrix composites (CMC's). These coatings may be applied using any of various coating techniques known in the art, such as plasma spray, thermal spray, chemical vapor deposition, or physical vapor deposition.
In a particular embodiment, associated with a metallic-based component—for example, a component of a gas turbine assembly—an article 100 includes a substrate 102 that includes a superalloy, and a multi-layered coating system 104 disposed over substrate 102. Coating system 104 includes a bondcoat 120 that comprises an aluminide or an MCrAlY-type material; a first layer 106, disposed over the bondcoat 120, that includes yttria-stabilized zirconia (such as YSZ with nominally 7-9 weight percent yttria content); and a second layer 108 disposed over first layer 106 and including yttria-stabilized zirconia having at least about 55 weight percent yttria content. Second layer 108 includes a plurality of through-thickness microcracks 116 having the characteristics set forth previously for such cracks 116. Such an embodiment may provide, among other things, durable thermal protection for metallic components used in high-temperature environments.
In another particular embodiment, associated with a ceramic-based component, which, like the above example, may include a component for a gas turbine assembly, an article 100 includes a substrate comprising a ceramic-matrix composite, such as a silicon carbide reinforced composite having a silicon carbide matrix; and a multi-layered coating system 104 disposed over substrate 102. Coating system 104 includes a bondcoat 120 comprising a silicide or elemental silicon; a first layer 106, comprising a silicate, disposed over bondcoat 120; and a second layer 108 comprising a rare earth monosilicate disposed over the first layer. Second layer 108 includes a plurality of through-thickness microcracks 116 having the characteristics set forth previously for such cracks 116. Such an embodiment may provide, among other things, durable environmental protection for silicon-bearing ceramic components, such as ceramic-matrix composites, used in moisture-containing, high-temperature environments.
The following examples are presented to further describe the fabrication of coatings of the present invention, but should not be read as limiting, because variations still within the scope of embodiments of the present invention will be apparent to those skilled in the art.
Thermal bather coatings were deposited onto 25 mm diameter, 3 mm thick Rene N5 alloy substrates that had approximately 150 microns of NiCrAlY bondcoat applied utilizing an air plasma spray process, which provided a surface roughness of about 400 microinch Ra. The first thermal barrier layer was deposited to a thickness of 430 microns onto the bondcoat surface. An 8 weight percent yttria-92 weight percent zirconia composition with a particle size median diameter (d50) of about 2.2 microns was suspended in ethanol at 20 wt % solids using polyethyleneimine as a dispersant (at 0.2 wt % of the solids). The suspension was injected into a Northwest Mettech Axial III torch through the center tube of a tube-in-tube atomizing injector with a nitrogen atomizing gas sent through the outer tube. A ⅜″ diameter nozzle was used at the end of the plasma torch. The suspension feed rate was 24 ml/min. The plasma torch was rastered across the substrate at 600 mm/sec, with stripes spaced at 4 mm intervals. Spray distance between the torch nozzle and the substrate samples was 75 mm. Plasma conditions used were 300 standard liter per minute (slpm) total gas flow with 10% nitrogen, 15% hydrogen, and 75% argon volumetric flow fractions. Plasma conditions used were 300 slpm total gas flow with 75% nitrogen, 15% hydrogen, and 10% argon volumetric flow ratios. A current of 200 A was used for each of the three electrodes, resulting in a total gun power of approximately 98 kW.
On top of the first 8YSZ layer, a second layer with a composition of 55 weight percent yttria-45 weight percent zirconia and a particle size d50 of about 1.5 microns was deposited to a thickness of about 370 microns. This layer was deposited using the same plasma conditions as used for the previous 8YSZ layer.
As in Example 1, similar substrates and bondcoats were used. In this example, the first TBC 8YSZ layer deposited was approximately 300 microns thick and was applied using a conventional air plasma spray process with Sulzer Metco 204NS powder. In this example, the second layer of 55 weight percent yttria-45 weight percent zirconia (“55YSZ”), applied over the first 8YSZ layer, was deposited using the same process as described in Example 1, but was deposited to a thickness of about 200 microns.
A coating was produced using the same substrate and NiCrAlY bondcoat as described in Example 1. As in Example 1, the first TBC layer was also an 8YSZ composition, but with a d50 particle size of about 0.7 microns. It was deposited to a thickness of about 180 microns using similar conditions to those described in Example 1. A second layer of 55YSZ was applied over the 8YSZ layer to a thickness of about 660 microns. In this example the particle size d50 was about 0.5 microns and the coating deposited with the plasma torch angled 20 degrees off of perpendicular to the surface.
The samples generated in the previous examples were imaged with a Nikon Epiphot 200 inverted microscope using reflected light illumination. A 50× objective was used to maximize resolution of the crack morphology. A Lumenera digital camera with a resolution of 1600 by 1200 pixels was used to capture images and was interfaced to a computer using Clemex Vision image analysis software.
The 8-bit 256 grey level image was captured and stored into the image analysis system. The image was segmented into a binary image (black and white) by selecting the characteristic grey level range of the cracks. In this way crack features could be distinguished from the background or matrix microstructure. The field of view was typically about 100 μm below the surface.
There were several processing steps to eliminate small pores and extraneous cracks so that one is left with the extended cracks for analysis. Detected features less than 50 μm and with an aspect ratio less than 5 were eliminated. Once this size and shape filtering was implemented, the remaining feature images were dilated and then eroded in order to close off small internal gaps within the detected crack feature and to connect discontinuities. At this stage the remaining long cracks were processed using an algorithm to eliminate most branching (horizontal) crack structures from the longer main spine of the major detected cracks. This processing removed over 90% of the branches. Any remaining branches were manually removed to ensure that only the central spines of the major cracks were being measured.
The detected major cracks were then measured with test lines that were automatically generated using the built-in algorithm designated Thickness Grid Per Object. However, to ensure that the test lines were in the correct orientation relative to the crack, a processing step was added which smoothed the edges of the crack while artificially expanding the crack thickness.
The generated test lines spanned across the crack width, and were spaced at user-defined measurement intervals of approximately 0.5 microns. The test lines were combined with the processed binary image of the main cracks using a Boolean operation so that only the segment of the test line that coincided with the processed crack image would remain.
The image analysis software measured the length of the test lines which corresponded with the actual crack thickness dimensions in a sequential manner along with the X and Y centroid of each test line. The measurements were compiled with thickness variation versus distance from edge generated.
Specimens described above were tested in a high-temperature thermal gradient test facility where combustion flame conditions and bare metal backside cooling conditions were adjusted to produce an outer coating surface temperature of about 1400 degrees Celsius (about 2550 degrees Fahrenheit) and bare metal backside temperature of about 870 degrees Celsius (about 1600 degrees Fahrenheit). The outer coating surfaces were coated with 24 mg/cm2 of a CMAS composition and then exposed directly to the flame, whereupon the CMAS melted and infiltrated into the outer coating layer. Following infiltration, specimens were exposed for an additional 10 thermal gradient exposures consisting of 10 minutes flame exposures each. The amount of coating spalled from each of the specimen surfaces was measured. Specimens deemed to be good performers as in examples 1 and 2 exhibited approximately 4 times lower volume of coating spalled compared to example 3.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
