Patent Publication Number: US-7910173-B2

Title: Thermal barrier coating and process therefor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a division patent application of U.S. patent application Ser. No. 11/160,164, filed Jun. 10, 2005, now U.S. Pat. No. 7,597,966. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a thermal barrier coating (TBC) deposited on a surface to have a columnar microstructure, wherein the TBC overlying at least certain portions of the surface has an interior region that is denser than an exterior region overlying the interior region to improve the impact resistance of the TBC. 
     Components within the hot gas path of gas turbine engines are often protected by TBC&#39;s that are typically formed of ceramic materials deposited by plasma spraying, flame spraying, and physical vapor deposition (PVD) techniques. TBC&#39;s employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can also be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.). 
     Various ceramic materials have been proposed as TBC&#39;s, the most widely used being zirconia (ZrO 2 ) partially or fully stabilized by yttria (Y 2 O 3 ), magnesia (MgO), or ceria (CeO 2 ) to yield a tetragonal microstructure that resists phase changes. Though various other stabilizers have been proposed for zirconia, yttria-stabilized zirconia (YSZ) is often preferred due at least in part to its high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying, and PVD techniques. Nonetheless, considerable effort has been made to formulate ceramic materials with reduced thermal conductivity, improved resistance to spallation and sintering, and other properties and characteristics that detrimentally affect the thermal insulating capability of a TBC. 
     In addition to low thermal conductivity and spallation resistance, TBC&#39;s on gas turbine engine components are required to withstand damage from erosion and impact by particles of varying sizes that are generated upstream in the engine or enter the high velocity gas stream through the air intake of a gas turbine engine. The damage can be in the form of erosive wear (generally from smaller particles, lower particle velocities, and/or lower impingement angles) and impact spallation (generally from larger particles, greater particle velocities, and/or greater impingement angles). Commonly-assigned U.S. Pat. No. 5,981,088 to Bruce et al. teaches that YSZ containing less than six weight percent yttria exhibits improved impact resistance. In addition, commonly-assigned U.S. Pat. No. 6,352,788 to Bruce and U.S. Pat. No. 7,060,365 to Bruce teach that small additions of oxides such as magnesia, hafnia, lanthana, neodymia, and/or tantala can improve the impact and erosion resistance of zirconia partially stabilized by about four weight percent yttria (4% YSZ). Aside from compositional approaches, improvements in erosion and impact resistance have been achieved by forming the outer region of a PVD TBC to be denser than an underlying interior region of the TBC, as taught in commonly-assigned U.S. Pat. No. 5,683,825 to Bruce et al. and commonly-assigned U.S. Pat. No. 6,982,126 to Darolia et al. 
     Notwithstanding the above-noted advancements, it would be desirable if TBC&#39;s were available that exhibited further improvements in resistance to particle damage, and particularly impact damage. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention generally provides a TBC and deposition process for a component intended for use in a hostile thermal environment, such as the turbine, combustor and augmentor components of a gas turbine engine. The TBC has a first coating portion on at least a first surface portion of the component. The first coating portion is formed of a ceramic material to have at least an inner region, at least an outer region overlying the inner region, and a columnar microstructure whereby the inner and outer regions comprise columns of the ceramic material. The columns of the inner region are more closely spaced than the columns of the outer region so that the inner region of the first coating portion is denser than the outer region of the first coating portion. According to a preferred aspect of the invention, the higher density of the inner region promotes the impact resistance of the first coating portion. 
     The TBC and process of this invention allow for the TBC to have a second coating portion on a second surface portion of the component. The second coating portion can be formed to have a columnar microstructure of the same ceramic material as the first coating portion, but with a denser outer region overlying a less dense inner region. For example, the inner region of the second coating portion can be deposited to be similar to the outer region of the first coating portion, while the outer region of the second coating portion can be deposited to be similar to the inner region of the first coating portion. With this embodiment, the inner region of the second coating portion can be simultaneously deposited with the outer region of the first coating portion so as to be a continuum thereof. With this approach, the first coating portion is capable of being more impact resistant than the second coating portion, while the second coating portion is more erosion resistant than the first coating portion. 
     The TBC and process of this invention also allow for the TBC to have a third coating portion on a third surface portion of the component, with the third coating portion being formed of the ceramic material but thinner than the first and second coating portions. For example, the entire third coating portion can be deposited during the simultaneous deposition of the outer region of the first coating portion and the inner region of the second coating portion. With this approach, the third coating portion can be deposited on less critical surface regions of the component and/or on those surfaces that are less prone to impact and erosion damage, thereby minimizing the weight of the TBC. 
     From the above, it can be appreciated that the present invention enables a TBC deposited on a component to be tailored to have different coating portions with different levels of erosion and impact resistance based on the location of a denser coating region within the different coating portions. As such, the TBC can be deposited so that certain surface portions more prone to impact damage are made more impact resistant due to the presence of a denser inner coating region, while other surface portions more prone to erosion damage are made more erosion resistant due to the presence of a denser outer coating region. The TBC can be deposited by PVD techniques to obtain the desired strain-resistant columnar grain structure noted above, with the closer column spacing of the outer surface region being achievable through compositional or processing modifications. 
     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 of a high pressure turbine blade. 
         FIG. 2  schematically represents a cross-sectional view of the blade of  FIG. 1  along line  2 - 2 , and shows a thermal barrier coating system on the blade with three coating portions in accordance with an embodiment of the invention. 
         FIGS. 3 and 4  are scanned images of prior art thermal barrier coatings that have suffered spallation from impact damage. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention is applicable to a variety of components subjected to high temperatures, such as the high and low pressure turbine nozzles and blades, shrouds, centerbodies, combustor liners, and deflectors of gas turbine engines, the invention will be discussed in reference to a high pressure turbine (HPT) 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 surfaces are therefore subjected to heat, oxidation, and corrosion from the combustion gases as well as impact and erosion damage from particles entrained in the combustion gases. The airfoil  12  is shown as being configured for anchoring to a turbine disk (not shown) with a dovetail  14 . For purposes of the following description, the leading edge  16  and the concave (pressure) surface  18  of the airfoil  12  are also identified in  FIG. 1 . 
     To protect the airfoil  12  from its hostile operating environment, at least the surfaces of the airfoil  12  are proved with a thermal barrier coating (TBC) system  20 , which is schematically depicted in  FIG. 2  in accordance with the present invention. The TBC system  20  is represented in  FIG. 2  as including a multilayer ceramic TBC  26  anchored with a metallic bond coat  24  to a surface region  22  of the airfoil  12 , which is usually a nickel, cobalt, or iron-based superalloy. As is typical with TBC systems for components of gas turbine engines, the bond coat  24  is preferably an aluminum-rich composition of a type known in the art, such as an overlay coating of a beta-phase NiAl intermetallic or an MCrAlX alloy, or a diffusion coating such as a diffusion aluminide or a diffusion platinum aluminide. As such, the bond coat  24  develops an aluminum oxide (alumina) scale  28  as a result of oxidation, such as during deposition of the TBC  26  on the bond coat  24 , as well as high temperature excursions of the blade  10  during engine operation. The alumina scale  28  chemically bonds the TBC  26  to the bond coat  24  and substrate  22 . The TBC  26  is represented in  FIG. 2  as having a strain-tolerant microstructure of columnar grains. As known in the art, such columnar microstructures can be achieved by depositing the TBC  26  using a physical vapor deposition technique, such as EBPVD or another atomic and molecular vapor process, as well as known melting and evaporation deposition processes. As with prior art TBC&#39;s, the TBC  26  is deposited to a thickness that is sufficient to provide the required thermal protection for the underlying surface region  22  of the airfoil  12 . 
     The TBC  26  is depicted in  FIG. 2  as having three coating portions  32 ,  34 , and  36  overlying different surface areas of the surface region  22 . A first coating portion  32  is shown made up two layers that form inner and outer regions  38  and  40  of the coating portion  32 , the latter of which also forms the outer surface of the TBC  26 . A second coating portion  34  is also shown as being made up two layers, in which the layer forming an inner region  42  of the coating portion  34  is a continuum of the layer forming the outer region  40  of the first coating portion  32 . Between the first and second coating portions  32  and  34  is a third coating portion  36  primarily formed by a single layer that is a continuum of the outer region  40  of the first coating portion  32  and the inner region  42  of the second coating portion  34 . It should be understood that  FIG. 2  is merely intended to help explain the invention, and that the proportions of the coating portions  32 ,  34 , and  36  and the transitions therebetween are not intended to limit or define the invention in any way. For example, in practice the coating portion  36  can cover a much larger surface area than the coating portions  32  and  34 , and the transitions are likely to be much more gradual and possibly irregular as compared to what is represented in  FIG. 2 . 
     The columnar grains of the layers forming the inner region  38  of the first coating portion  32  and the outer region  44  of the second coating portion  34  are represented as being more closely spaced than the grains of the layer forming the outer region  40  of the first coating portion  32 , the inner region  42  of the second coating portion  34 , and the third coating portion  36 , with the result that the TBC  26  is more porous within the outer region  40  of the first coating portion  32  and the inner region  42  of the second coating portion  34 . Consequently, the first coating portion  32  has a denser inner region  38  and the second coating portion  34  has a denser outer region  42 . While the denser outer region  42  of the second coating portion  34  promotes the erosion resistance of the second coating portion  34  in accordance with, for example, commonly-assigned U.S. Pat. Nos. 5,683,825 and 6,982,126, the denser inner region  38  of the first coating portion  32  is believed to promote the impact resistance of the first coating portion  32 . To promote the impact resistance of the denser inner region  38  and the erosion resistance of the denser outer region  44 , the columns within the separate layers forming these regions  38  and  44  should be sufficiently dense to yield a porosity of less than 20 percent by volume, preferably 15 percent or less by volume, while the columns within the layer forming the remaining regions  40 ,  42 , and  46  can have a porosity of greater than 20 percent by volume in order to minimize the thermal conductivity of the TBC  26  within these regions  40 ,  42 , and  46 . 
     As will be discussed below, it is believed that the denser inner region  38  is capable of promoting the impact resistance of the first coating portion  32  as a result of the failure mode by which spallation from impact damage occurs. Specifically, whereas damage from erosion occurs by the gradual removal of thin layers from the surface of a TBC, spallation from impact damage has been observed to initiate within the innermost portions of TBC&#39;s, generally in the vicinity of the interface between the TBC and its bond coat. Forming the inner region  38  of a denser columnar microstructure is believed to increase the fracture toughness of the inner region  38 , thereby raising the threshold required to initiate cracking and slowing the propagation of cracks that inevitably form. 
     Further improvements in fracture toughness is also believed to be obtainable by forming the inner region  38  of the first coating portion  32  to have the crystallographic texture [100], in which the columnar grains grow in a textured manner in the [100]. The advantage is believed to follow from the higher fracture toughness of tetragonal zirconia associated with such texture, as compared with the typically observed texture [111] or random orientation of TBC&#39;s. Improved resistance to crack propagation within the inner region  38  can also be achieved by grooving the surface of the bond coat  24  prior to depositing the TBC  26 , such as in accordance with commonly-assigned U.S. Pat. No. 5,419,971 to Skelly et al. In the context of the present invention, grooving is believed to slow the propagation of cracks that are caused by thermal fatigue and tend to propagate at or just above the TBC-bond coat interface. 
     By combining the different microstructures of the inner and outer regions  38 ,  40 ,  42 , and  44  into a TBC  26  as represented in  FIG. 2 , improved impact resistance can be achieved in selected surface areas of the blade  10  and improved erosion resistance can be simultaneously achieved on other surface areas of the blade  10 , while maintaining thermal protection of these and the remaining surface areas of the blade  10 . With particular reference to the blade  10 , such a TBC  26  can be deposited so that the more impact-resistant coating portion  32  is deposited on those areas most prone to damage from impact, such as the leading edge  16  of the airfoil  12 , and the more erosion-resistant coating portion  34  can be deposited on those areas most prone to erosion damage, such as the concave (pressure) surface  18  of the airfoil  12 . The remaining surfaces of the airfoil  12  requiring thermal protection can be coated with the coating portion  36 , which has minimum thickness as a result of lacking the denser coating regions  38  and  44 . Such an approach has the advantage of improving impact and erosion resistance of the blade  10  with minimal increase in blade weight attributable to the TBC  26 . Additionally, the dense, fracture-resistant microstructure of the inner region  38  on the leading edge  16  will result in improved performance and durability of the blade  10  and its TBC  26  beyond what could be achieved by simply increasing the thickness of a conventional TBC, while avoiding the additional weight that would be incurred with such an approach. 
     As evident from  FIG. 2 , the thickness of the TBC  26  within the first and second coating portions  32  and  34  is greater than within the third coating portion  36 , which is advantageous since the first and second coating portions  32  and  34  are intended to be applied where damage from particle impact and/or erosion is more likely. Generally, the maximum thickness of the TBC  26  can be in a range of about 50 to about 325 micrometers, with the thicknesses of the coating portions  32  and  34  being about two to about five mils (about 50 to about 125 micrometers) greater than the third coating portion  36 . It is believed that the dense inner region  38  of the first coating portion  32  and the dense outer region  44  of the second coating portion  34  should constitute up to about half the thickness of the TBC  26  within their respective coating regions  32  and  34 , for example, about 0.3 to about three mils (about 7.5 to about 75 micrometers) in thickness, more preferably about 0.5 to about 1 mil (about 12 to about 25 micrometers) in thickness. The inner and outer regions  38 ,  40 ,  42 , and  44  are illustrated in  FIG. 2  as being somewhat distinct, though it is within the scope of the invention that the transition between the porous to denser microstructures can be gradual or more distinct. 
     While the TBC  26  is depicted in  FIG. 2  as containing not more than two layers over any given area of the surface region  22 , the TBC  26  can comprise any number of alternating dense and porous interior layers between the inner regions  38  and  42  and their respective outer regions  40  and  44 , with these dense and porous interior layers having microstructures similar to the dense regions  38  and  44  and porous regions  40  and  42 , respectively. The denser of such interior layers preferably have thicknesses of up to about 0.5 mil (about 12 micrometers), and are preferably spaced apart by the porous layers whose thicknesses are about 0.5 to 2 mils (about 12 to 50 micrometers). With a combination of denser and porous interior layers within the TBC  26 , improvements in both impact resistance and erosion resistance can be obtained in a single region of the TBC  26 . 
     A suitable ceramic material for the TBC  26  is YSZ, though it is foreseeable that various other ceramic materials proposed for TBC&#39;s could be used instead, as well as different ceramic materials for the layers forming the regions  38 ,  40 ,  42 ,  44 , and  46 . According to one embodiment, the entire TBC  26  is formed of YSZ, such as about 6-8% YSZ (zirconia stabilized with about six to about eight weight percent yttria). Alternatively, the denser regions  38  and  44  can be formed of the impact-resistant YSZ compositions taught in U.S. Pat. No. 5,981,088 to Bruce et al., U.S. Pat. No. 6,352,788 to Bruce, and U.S. patent application Ser. No. 10/063,962 to Bruce. By maintaining a substantially constant composition through the thickness of the TBC  26 , the formation of interfaces that could serve as paths for crack propagation through the TBC  26  is minimized or avoided. 
     Various process and composition-related approaches can be used to obtain the different microstructures within the regions  38 ,  40 ,  42 ,  44 , and  46  of the TBC  26 , as will be discussed below. As noted above, the compositions of the regions  38 ,  40 ,  42 ,  44 , and  46  may be identical (resulting in a constant composition throughout the TBC  26 ), have the same base composition but modified with certain additions, or have different base compositions. If the regions  38 ,  40 ,  42 ,  44 , and  46  have the same composition, processing modifications must be made to result in the denser microstructures desired for the regions  38  and  44 . If the regions  38 ,  40 ,  42 ,  44 , and  46  have the same base composition, minor chemistry modifications can be made to the denser regions  38  and  44  to enhance surface diffusion processes and promote flatness of the crystallization front, causing a majority of the inter-columnar gaps to decrease during deposition by PVD. Examples of such chemistry modifications include additions of nickel, titanium, chromium, and/or their oxides to enhance sintering processes in zirconia during deposition of the dense regions  38  and  44 . 
     A process suitable for achieving the TBC  26  of the type represented in  FIG. 2  with only modifications to an otherwise conventional EBPVD process can be achieved as follows. Deposition is initiated on the blade  10  with the blade  10  held stationary and its leading edge  16  facing the molten pool of ceramic material (e.g., YSZ) being evaporated with an electron gun. Deposition in this manner continues until the desired thickness for the inner region  38  has been deposited on the leading edge  16 . Alternatively, the blade  10  can undergo slow and/or limited oscillation as needed to control and increase the density of the deposited ceramic. Thereafter, a typical rotation pattern can be initiated while deposition continues to deposit the ceramic that forms the more porous regions  40 ,  42 , and  46  of the TBC  26  over the entire airfoil  12 . Once the desired thickness for these regions  40 ,  42 , and  46  has been obtained, rotation is stopped to position the blade  10  with its concave surface  18  facing the molten pool to deposit the dense outer region  44  on only the concave surface  18 . As before, the blade  10  may be held stationary or undergo a limited and/or slow oscillation to increase the density of the deposited ceramic. To deposit the interior regions of alternating dense and porous regions described above, rotation of the blade  10  can be periodically stopped during that part of the deposition process following deposition of the inner layer  38  on the leading edge  16  and before deposition of the outer layer  44  on the concave surface  18 . To create the [100] texture in the dense inner region  38  on the leading edge  16 , additional variation of process parameters may be required. It is believed that the [100] texture can be achieved with a combination of stationary deposition and increasing the deposition temperature, such as by generating additional heat with a second electron beam gun during deposition on the leading edge  16 . 
     In investigations leading to this invention, YSZ TBC&#39;s having a nominal yttria content of about seven weight percent were deposited by EBPVD to have thicknesses of about 125 micrometers. Each of the TBC&#39;s were deposited on pin specimens formed of René N5 (nominal composition of, by weight, about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2% Al, 5.0% W, 3.0% Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B, 0.01% Y, the balance nickel and incidental impurities), on which a platinum aluminide (PtAl) bond coat had been previously deposited. The microstructures of the TBC&#39;s differed from each other as a result of modifications to the EBPVD process. Specifically, a baseline group of pins were coated using a deposition pressure of about 12 microbars, while two additional sets of pins were coated at a lower rate as a result of being coated at a deposition pressure of 5 microbars in an oxygen-containing atmosphere or an argon atmosphere. Following deposition, the porosities of the TBC&#39;s were determined to be about 24 to about 30 percent by volume for the baseline pins, about 17 percent by volume for the pins coated in the oxygen atmosphere at 5 microbars, and about 19 percent by volume for the pins coated in the argon atmosphere at 5 microbars. 
     The impact performance of these specimens was assessed by cycling the coated pins in and out of a jet stream into which alumina particulate was injected. Coating loss was then correlated to the mass of the particulate required to wear through (spall) the TBC. The results were normalized to the coating thickness and recorded in grams of particulate per one mil (25 micrometers) of coating thickness (g/mil) to permit comparison between coatings of different thicknesses. The results were as follows: about 70 to about 110 g/mil for TBC&#39;s with porosities of about 24 to 30%, about 170 to about 190 g/mil for TBC&#39;s with porosities of about 17%, and about 160 to about 180 g/mil for TBC&#39;s with porosities of about 19%. These results demonstrated that improved impact resistance can be achieved with 7% YSZ by increasing the density of the columnar microstructure. While increased density was achieved by varying the deposition pressure, similar increases in density and impact resistance should be attainable by depositing TBC on a substrate held stationary or slowly rotated or oscillated as described previously. 
     Further analysis conducted to investigate the present invention also demonstrated that the erosion and impact behavior of TBC is determined at least in part by overall porosity levels and the stability of the zirconia lattice. The analysis was performed with more than fifty experimental data points for erosion and impact performance obtained from coatings having various different compositions deposited by EBPVD, including 7% YSZ, 4% YSZ, YSZ modified to contain limited additions of carbon, and zirconia containing limited additions of lanthana or ytterbia oxide. Observations made with cross-sections through the TBC&#39;s eroded at high temperatures suggested that multiple mechanisms of material removal were occurring and influenced by particle size, velocity, temperature, and material. The mechanisms were distinguished by the time scales for stress wave transit relative to those for plastic deformation, and were able to be described in terms of different domains that also represent different observed failure modes. The typical impact failure mode was with particle impingement at about ninety degrees to the surfaces of TBC&#39;s on pin specimens, and on the leading edges of HPT blades. Impact resistance can be estimated with the following equation:
 
 I≡Γ   TBC   E   YSZ   α+1 /(σ tbc   y ) 2+α−β 
 
From this formula, it can be seen that impact resistance (I) is increased with higher fracture toughness (Γ), higher elastic modulus (E), and lower yield strength (σ). Lower yield strengths allow plastic deformation to occur so that part of the impact energy can be absorbed by deformation before causing initiation of cracks.
 
     The above investigation and analysis illustrated that an important aspect of the impact failure mode is that material removal does not occur in a gradual fashion, as is the case with erosion. Instead, cracks propagate to the interface between the bond coat and TBC, where spallation occurs as seen in  FIGS. 3 and 4 . Final delamination was observed to typically occur about twelve micrometers from the bond coat-TBC interface. It was also observed that periodically located horizontal cracks were present in the TBC&#39;s at distances of about 80, 40, 24, and 12 micrometers from the bond coat-TBC interface. From  FIG. 3 , it can be seen that some TBC&#39;s detached in tiers at these subsurface locations. From these observations, it was concluded that improved impact resistance could be achieved with the dense inner region  38  located immediately adjacent the bond coat  24 , and that impact resistance can be further improved with additional dense interior regions periodically located between the inner region  38  and the surface of the TBC  26 , as described above. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.