Abstract:
A multilayer thermal barrier coating (TBC) having a low thermal conductivity that is maintained or even decreases as a result of a post-deposition high temperature exposure. The TBC comprises an inner layer and an insulating layer overlying the inner layer. The inner layer is preferably yttria-stabilized zirconia (YSZ), while the insulating layer contains barium strontium aluminosilicate (BSAS). After deposition, the TBC is heated to a temperature and for a duration sufficient to cause a decrease in the thermal conductivity of the BSAS-containing layer and, consequently, the entire TBC.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
         [0001]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to a multilayer thermal barrier coating (TBC) system characterized by a low coefficient of thermal conductivity.  
           [0003]    The use of thermal barrier coatings (TBC) on components such as combustors, high pressure turbine (HPT) blades and vanes is increasing in commercial as well as military gas turbine engines. The thermal insulation of a TBC enables such components to survive higher operating temperatures, increases component durability, and improves engine reliability. TBC is typically a ceramic material deposited on an environmentally-protective bond coat to form what is termed a TBC system. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.  
           [0004]    Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by air plasma spraying (APS), flame spraying and physical vapor deposition (PVD) techniques. TBC&#39;s formed by these methods have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries of the TBC microstructure. TBC&#39;s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can 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., cathodic arc, laser melting, etc.).  
           [0005]    In order for a TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC has and maintains a low thermal conductivity throughout the life of the component, including high temperature excursions. However, the thermal conductivities of TBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC&#39;s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency.  
           [0006]    In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC&#39;s are employed to thermally insulate components intended for more demanding engine designs. A TBC with lower thermal conductivity would allow for higher component surface temperatures or reduced coating thickness for the same surface temperature. Reduced TBC thickness, especially in applications like combustors which require relatively thick TBC&#39;s, would result in a significant cost reduction as well as weight benefit.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a thermal barrier coating (TBC) and method by which a low thermal conductivity of the TBC is maintained or even decreased as a result of a post-deposition high temperature exposure. The TBC is part of a TBC system that includes a bond coat by which the TBC is adhered to a component surface. The TBC of this invention preferably comprises an inner layer on the bond coat and an insulating layer overlying the inner layer. According to one aspect of the invention, the inner layer preferably contains yttria-stabilized zirconia (YSZ), while the insulating layer contains barium strontium aluminosilicate (BSAS; (Ba 1!x Sr x )O—Al 2 O 3 —SiO 2 ) The thermal conductivity (T c ) of BSAS is approximately equal to that of YSZ. However, the thermal conductivity of BSAS has been surprisingly observed to decrease with sufficiently high temperature exposures, with the result that, though having similar as-deposited thermal conductivities, BSAS can become a better thermal insulator than YSZ if it undergoes an appropriate thermal treatment.  
           [0008]    Because BSAS has a low coefficient of thermal expansion (CTE) (about half that of YSZ), and therefore a BSAS coating may not adequately adhere directly to a metal substrate. In addition, alumina (Al 2 O 3 ) scale that forms on aluminum-containing bond coats may react with the silica content of the BSAS coating to form silicate-type phases that would further diminish the adhesion of the coating. Therefore, the present invention provides the YSZ-containing inner layer, which has a sufficiently high CTE to mitigate the CTE mismatch between the BSAS-containing insulating layer and the underlying metal substrate (e.g., bond coat).  
           [0009]    In view of the above, the present invention provides a TBC with a low-T c  outer coating (BSAS) whose thermal conductivity is reduced from its as-deposited T c  through an intentional high temperature thermal treatment. While not wishing to be held to any particular theory, the thermal conductivity of BSAS is believed to decrease with temperature exposure as a result of grain shape changes driven by the surface energy reduction, which causes pores to form in the BSAS coating. The resulting porosity decreases the thermal conductivity of the BSAS coating, with the result that the BSAS coating has significantly lower thermal conductivity than a conventional YSZ coating of the same thickness. As a result, a TBC containing a BSAS insulating layer in accordance with this invention is particularly suitable for thermally insulating components intended for demanding applications, including advanced gas turbine engines in which higher component surface temperatures are required. Alternatively, the lower thermal conductivity of the TBC allows for reduced coating thicknesses for the same surface temperature, resulting in a significant cost reduction as well as weight benefit.  
           [0010]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIGS. 1 through 3 represent cross-sectional views a thermal barrier coating systems in accordance with three embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    The present invention is generally applicable to components subjected to high temperatures, and particularly to components such as the high and low pressure turbine vanes (nozzles) and blades (buckets), shrouds, combustor liners and augmentor hardware of gas turbine engines. The invention provides a thermal barrier coating (TBC) system suitable for protecting those surfaces of a gas turbine engine component that are subjected to hot combustion gases. While the advantages of this invention will be described with reference to gas turbine engine components, the teachings of the invention are generally applicable to any component on which a TBC may be used to protect the component from a high temperature environment.  
         [0013]    TBC systems  10 ,  110  and  210  in accordance with three embodiments of this invention are represented in FIGS. 1 through 3. In each embodiment, the TBC system  10 ,  110  or  210  is shown as including a metallic bond coat  12  that overlies the surface of a substrate  14 , the latter of which is typically a superalloy and the base material of the component protected by the TBC systems  10 ,  110  and  210 . As is typical with TBC systems for gas turbine engine components, the bond coat  12  is preferably an aluminum-rich composition, such as an overlay coating of an MCrAlX alloy or a diffusion coating such as a diffusion aluminide or a diffusion platinum aluminide of a type known in the art. Aluminum-rich bond coats of this type develop an aluminum oxide (alumina) scale  16 , which is grown by oxidation of the bond coat  12 . The alumina scale  16  chemically bonds a multilayer TBC  18 ,  118  or  218  to the bond coat  12  and substrate  14 .  
         [0014]    The TBC&#39;s  18 ,  118  and  218  of FIGS. 1, 2 and  3  are only schematically represented. As known in the art, one or more of the individual layers of the TBC&#39;s  18 ,  118  and  218  may have a strain-tolerant microstructure of columnar grains as a result of being deposited by a physical vapor deposition technique, such as EBPVD. Alternatively, one or more of the layers may have a noncolumnar structure as a result of being deposited by such methods as plasma spraying, including air plasma spraying (APS). Layers of this type are in the form of molten “splats,” resulting in a microstructure characterized by irregular flattened grains and a degree of inhomogeneity and porosity. In each case, the process by which the layers of the TBC  18 ,  118  and  218  are deposited provides microstructural defects and pores that are believed to decrease the thermal conductivity of the TBC  18 ,  118  and  218 .  
         [0015]    The present invention provides compositions and structures for the TBC&#39;s  18 ,  118  and  218  that further reduce thermal conductivity as a result of including a layer that contains barium strontium aluminosilicate (BSAS; (Ba 1−x Sr x )O—Al 2 O 3 —SiO 2 ) Similar to YSZ, BSAS is not volatile in water vapor at high temperatures, and therefore would be expected to be capable of surviving the hostile environment of the hot gas path within a gas turbine engine. However, while preliminary data indicated that the thermal conductivity (T c ) of BSAS is slightly lower than YSZ, the CTE of BSAS is about half that of YSZ. The T c  and CTE data for YSZ and BSAS are summarized in Table 1 below (“RT” stands for “room temperature,” or about 25° C.).  
                                                         TABLE 1                                       CTE   Melting   Thermal               (RT to 1200° C.)   Temperature   Conductivity           Material   (×10 −6 /° C.)   (° C.)   at RT (W/mK)                                        YSZ   9.40   about 2600   &gt;2           BSAS   5.27   about 1700   1.72                      
 
         [0016]    Because BSAS has a significant CTE mismatch with metal surfaces, a BSAS coating would be expected to be prone to spallation from the bond coat  12  or metal substrate  14 . Another problem with the use of BSAS in a TBC system is that the alumina scale  16  that forms on the surface of the bond coat  12  would be expected to have a tendency to react with the silica content of a BSAS coating, forming silicate-type phases that could promote interface degradation and failure from thermal fatigue. In view of these concerns, and because BSAS would be expected to provide only a modest improvement in thermal insulation, BSAS has not been utilized as a thermal-insulating layer for high temperature (e.g., gas turbine engine) applications.  
         [0017]    Notwithstanding the above concerns, the present invention provides several different approaches to incorporating a BSAS-containing layer into the TBC systems  10 ,  110  and  210  of this invention. Contrary to the thermal data of Table 1, it was unexpectedly determined that the thermal conductivity of BSAS actually decreases with prolonged exposures to elevated temperatures. In one investigation, the thermal conductivity of air plasma sprayed (APS) BSAS coatings was measured in the as-deposited condition, after aging for about five hours at about 1482° C., and after aging for about fifty hours at about 1482° C. The measurements were made at temperatures of about 820° C., 890° C. and 990° C. The averages of these measurements are summarized in Table 2 below. It should be noted that the conductivities of the as-deposited BSAS specimens in Table 2 are lower than the conductivity indicated in Table 1 because Table 1 is based on bulk BSAS at room temperature, while Table 2 is based on plasma sprayed BSAS at elevated temperatures.  
                                                 TABLE 2                                       Thermal Treatment   Thermal Conductivity (W/mK) at:                (Time/Temperature)   820° C.   890° C.   990° C.                       As-deposited   1.53   1.51   1.53            5 hrs./1482° C.   1.28   1.30   1.33           50 hrs./1482° C.   1.33   1.32   1.35                      
 
         [0018]    The above results indicated that a significant improvement in thermal insulation could be achieved by the incorporation of BSAS into a TBC if the BSAS coating was subjected to an appropriate thermal treatment. While not wishing to be held to any particular theory, the basis for the decreasing thermal conductivity of BSAS evident in Table 2 is believed to be related to increased porosity created as a result of changes in grain shape driven by surface energy reduction during high temperature excursions. Thermal treatments sufficient to significantly decrease the thermal conductivity of BSAS (i.e,. below about 1.5 w/mK) are generally believed to be at least about 1200° C. if held for at least two hours.  
         [0019]    On the basis of the above results, the present invention provides the several approaches represented in FIGS. 1 through 3 for incorporating a BSAS-containing layer into the TBC systems  10 ,  110  and  210 . With reference to FIG. 1, the TBC  18  is shown as comprising an inner layer  20  lying directly on the bond coat  12  and a single outer layer  22  lying directly on the inner layer  20 . A preferred composition for the inner layer  20  is based on binary yttria-stabilized zirconia (YSZ), a particular notable example of which contains about 6 to about 8 weight percent yttria, with the balance zirconia. Other zirconia-based ceramic materials could also be used with this invention, such as zirconia fully stabilized by yttria, nonstabilized zirconia, or zirconia partially or fully stabilized by ceria, magnesia, scandia and/or other oxides. According to one aspect of the invention, a particularly suitable material for the inner layer  20  is YSZ containing about 4 to about 8 weight percent yttria (4-8% YSZ). In the embodiment of FIG. 1, the outer layer  22  is entirely BSAS. According to a preferred aspect of the first embodiment of FIG. 1, the inner layer  20  is deposited to a thickness that is sufficient to provide a suitable stress distribution within the TBC system  10  to promote the mechanical integrity of the coating. A suitable thickness for this purpose is generally on the order of about 3 to about 30 mils (about 75 to about 750 micrometers), which is also believed to be sufficient to provide a physical barrier to a possible reaction between the alumina scale  16  and the silica content of the BSAS outer layer  22 . The BSAS outer layer  22  is sufficiently thick to provide the desired level of thermal insulation in combination with the YSZ inner layer  20 . While coating thickness depends on the particular application, a thickness ratio of YSZ/BSAS of about one is believed to be suitable, such that a suitable thickness for the BSAS outer layer  22  is also about 3 to about 30 mils (about 75 to about 750 micrometers).  
         [0020]    In FIG. 2, the TBC  118  differs from the TBC  18  of FIG. 1 by having a multilayer outer coating  122 . As before, an inner layer  120  lies directly on the bond coat  12 , and the outer coating  122  lies directly on the inner layer  120 . A preferred composition for the inner layer  120  is again based on YSZ, preferably 3-20% YSZ. In contrast to the embodiment of FIG. 1, the outer coating  122  is formed to include a graded region of alternating thin YSZ and BSAS layers  124  and  126 , respectively, followed by an outer layer  128  formed entirely of YSZ. The YSZ layers  124  and  128  may have the same composition as the inner layer  120  (3-20% YSZ), though it is foreseeable that their compositions could differ. For example, a higher yttria content may be desired in the outer YSZ layer  128  to improve high temperature phase stability, or a lower yttria content may be desired to improve erosion resistance.  
         [0021]    In the embodiment of FIG. 2, the YSZ inner layer  120  promotes stress distribution between the bond coat  12  and the TBC  118 , the BSAS layers  126  serve to reduce the overall thermal conductivity of the TBC  118 , the YSZ outer layer  128  promotes the erosion resistance of the TBC  118 , and the thin YSZ layers  124  provide a grading effect between the BSAS layers  126  and the YSZ inner and outer layers  120  and  128 . As such, the YSZ inner layer  120  can have a thickness similar to that of the YSZ inner layer  20  of FIG. 1. The individual thin layers  124  and  126  preferably have thicknesses of about 2 mils (about 50 micrometers) for a combined thickness of about 10 to about 30 mils (about 250 to about 750 micrometers), though thicknesses of as little as 5 (about 125 micrometers) and as much as 50 (about 1250 micrometers) are foreseeable. The combined thickness of the BSAS layers  126  preferably constitutes at least about one-third of the combined thickness of the YSZ layers  124  in order for the TBC  118  to contain sufficient BSAS to have a significant impact on heat transfer. Any number of YSZ and BSAS layers  124  and  126  can be combined to form the graded region of the outer coating  122 . However, the layers  124  and  126  are preferably arranged so that the layer contacting the YSZ inner layer  120  is YSZ to promote mechanical compliance. The YSZ outer layer  128  should be sufficiently thick to provide erosion protection to the graded layers  124  and  126 . A suitable thickness for this purpose is generally on the order of up to about 10 mils (about 250 micrometers).  
         [0022]    In FIG. 3, the TBC  218  is similar to that of FIG. 2 by the inclusion of a YSZ inner layer  220  and a multilayer outer coating  222  that includes a YSZ outer layer  228 . However, the TBC  218  differs in that the outer coating  222  comprises a BSAS/YSZ composite layer  224  between the inner and outer YSZ layers  220  and  228 . A preferred composition for the composite layer  224  is a uniform mixture of about 25 to about 75 weight percent BSAS, with the balance 4-8% YSZ. Equal parts of BSAS and YSZ in the composite layer  224  are believed to provide an adequate stress field. The stated range for the BSAS/YSZ ratio is believed to achieve stress distribution for varying relative thicknesses of the YSZ inner and outer layers  220  and  228 . A suitable thickness for the composite layer  224  is up to about 10 mils (about 250 micrometers), preferably about 4 to about 7 mils (about 100 to about 175 micrometers). The composition and thickness of the composite layer  224  provide a sufficient amount of BSAS to significantly lower the thermal conductivity of the TBC  218 . For the same reasons discussed above, suitable thicknesses for the YSZ inner and outer layers  220  and  228  are again up to about 10 mils (about 250 micrometers).  
         [0023]    In view of the above, it can be appreciated that each of the TBC systems  10 ,  110  and  210  of this invention employs a TBC  18 ,  118  and  218  whose thermal conductivity is reduced by the addition of a constituent having a lower thermal conductivity than YSZ and other conventional TBC materials. Because a larger CTE mismatch exists with a metal bond coat  12  and substrate  14  when BSAS is used as the low thermal conductivity material, each of the TBC&#39;s  18 ,  118  and  218  includes an intermediate YSZ layer  20 ,  120  or  220  that reduces the CTE mismatch. The TBC&#39;s  118  and  218  also employ an outer layer  128  and  228  that is entirely or predominantly YSZ, whose erosion resistance properties are better than BSAS and conventional TBC materials.  
         [0024]    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. Therefore, the scope of the invention is to be limited only by the following claims.