Patent Publication Number: US-2012034471-A1

Title: Thermal barrier systems including yttrium gradient layers and methods for the formation thereof

Description:
TECHNICAL FIELD 
     The following disclosure relates generally to thermal barrier systems and, more particularly, to embodiments of a thermal barrier system including an yttrium gradient layer, as well as to methods for the formation of such a thermal barrier system. 
     BACKGROUND 
     Thermal barrier systems are commonly formed over heat-exposed surfaces of combustor cans, heat shields, turbine blades, nozzle guide vanes, duct members, and other such components included within modern gas turbine engines (commonly referred to as “hot section components”). During engine operation, a thermal barrier system thermally isolates the hot section component, and specifically the underlying superalloy from which the hot section component is typically fabricated, from high temperature combustive gas flow. In addition, thermal barrier systems reduce structural degradation of the hot section component due to hot gas corrosion, oxidation, erosion, and the like. Thermal barrier systems thus enable gas turbine engines to operate at a higher core temperatures, and therefore at greater efficiencies, for longer periods of time and over longer operational lifespans. 
     A thermal barrier system typically includes a bond coat, which is formed over the heat-exposed surface of the hot section component, and at least one thermal barrier coating (also commonly referred to as a “TBC” or “top coat”), which is formed over the bond coat. The thermal barrier coating, in turn, often includes at least one ceramic layer formed primarily from zirconia (ZrO 2 ). In many cases, one or more secondary oxides are added to the zirconia-based ceramic coating to improve coating stability. Although other stabilizing oxides have been utilized (e.g., hafnium), and other weight percentages have been employed, zirconia-based coating containing approximately 7% to 8% yttrium (Y 2 O 3 ), by weight (commonly referred to as “7-8YSZ coatings”), have emerged as the predominate thermal barrier coating composition utilized by the majority of manufacturers and suppliers within the aerospace industry. The widespread usage of 7-8YSZ coatings is due, at least in part, to the excellent machinabillity, fracture toughness, and adherence to metallic bond coats provided by such coatings. 
     While providing the above-noted advantages, 7-8YSZ coatings are generally limited to maximum operational temperatures near 1260° C. When a 7-8YSZ coating is exposed to temperatures exceeding this upper threshold, the coating undergoes a phase change (in particular, the coating&#39;s crystalline structure changes from a meta-stable tetragonal to cubic at elevated temperatures and then to monoclinic upon cooling) and the volume of the coating increases. Although the coating&#39;s volumetric increase may be relatively minor (e.g., approximately 3%), the ceramic coating is typically unable to accommodate even a modest volumetric increase due to its inherent rigidity. As a result, 7-8YSZ coating continually exposed to temperatures exceeding 1260° C. tend to erode, possibly buckle and separate along the barrier coating-bond coat interface (commonly referred to as “spallation”), and ultimately flake away leaving the hot section component unprotected. As a further limitation, conventionally-employed 7-8YSZ coatings are prone to sintering during elevated temperature exposure. That is, when exposed to elevated temperatures, the particles within a given 7-8YSZ coating tend to adhere one another, which reduces the coating&#39;s porosity and creates tensile strain in the coating caused by the associated shrinkage. As the coating&#39;s porosity is reduced, the coating&#39;s thermal conductivity is increased and the ability to accommodate strain is reduced. The overall effectiveness of the 7-8 YSZ coating as a thermal barrier is consequently diminished. 
     Considering the above, it is desirable to provide embodiments of a thermal barrier systems that provides the benefits of conventionally-employed 7-8YSZ thermal barrier coatings, while also providing improved phase stabilities at higher operating temperatures (e.g., operational temperatures exceeding approximately 1260° C.) and improved sintering resistances. It would also be desirable to provide methods for forming such a thermal barrier coating over a gas turbine engine component. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying Drawings and this Background. 
     BRIEF SUMMARY 
     Embodiments of a thermal barrier system are provided for formation over a gas turbine engine component. In one embodiment, the thermal barrier system includes a bond coat formed over a surface of a gas turbine engine component, and an yttrium-stabilized zirconia thermal barrier coating formed over the bond coat. The yttrium-stabilized zirconia thermal barrier coating includes an yttrium gradient layer having an yttrium content that increases gradually with increasing distance from the bond coat. 
     Embodiments of a method are further provided for forming a thermal barrier system over a gas turbine engine component. In one embodiment, the method comprises the step of continually depositing yttrium-stabilized zirconia over a surface of the gas turbine engine component while increasing the yttrium content thereof to form an yttrium-stabilized zirconia thermal barrier coating comprising an yttrium gradient layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and: 
         FIG. 1  is a cross-sectional view of a thermal barrier system formed over a generalized gas turbine engine component in accordance with an exemplary embodiment; 
         FIG. 2  is a flowchart illustrating an exemplary method suitable for forming the thermal barrier system shown in  FIG. 1 ; 
         FIG. 3  is a generalized diagram of a plasma spray apparatus that can be utilized to form the yttrium gradient layer included within the thermal barrier system shown in  FIG. 1  in accordance with one possible implementation of the exemplary method shown in  FIG. 2 ; and 
         FIG. 4  is a graph of spray powder composition (vertical axis) versus time (horizontal axis) illustrating one manner in which the feed ratio of two different powder feedstocks may be varied over time to yield the yttrium gradient layer included within the thermal barrier system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description. As appearing herein, the term “over” is utilized to denote the relative positioning of layers and other structural elements (e.g., gas turbine engine components) and does not necessarily denote direct contact between the specified layers or structural elements, unless otherwise stated. 
       FIG. 1  is a cross-sectional view of a thermal barrier system  10  and a generalized gas turbine engine (“GTE”) component  12  in accordance with an exemplary embodiment. Thermal barrier system  10  includes a metallic bond coat  14  and a ceramic thermal barrier coating  18  (“TBC  18 ”). TBC  18  is formed over bond coat  14 , which is, in turn, formed over at least one surface  16  of GTE component  12 . In the illustrated example, TBC  18  includes three layers: a first yttrium-stabilized zirconia (“YSZ”) layer  20 , an yttrium gradient layer  22 , and a second YSZ layer  24 . The layers of thermal barrier coating  18  are successively deposited over bond coat  14  such that second YSZ layer  20  overlies yttrium gradient layer  22 , yttrium gradient layer  22  overlies first YSZ layer  20 , and first YSZ layer  20  overlies bond coat  14 . In view of their relative proximity to GTE component  12 , first YSZ layer  20  and second YSZ layer  24  are referred to herein as “inner YSZ layer  20 ” and “outer YSZ layer  24 ,” respectively. It will be recognized, however, that thermal barrier system  10  may be formed over an inner surface of GTE component  12  and, further, that component  12  may be mounted within a gas turbine engine in any orientation in three dimensional space. Consequently, in certain embodiments, outer YSZ layer  24  may reside closer to the longitudinal axis of GTE component  12  and/or closer to the longitudinal axis of the host gas turbine engine than does inner YSZ layer  20 . 
     GTE component  12  may comprise any structural element or assemblage of structural elements included within a gas turbine engine and subjected to elevated temperatures during engine operation. In many cases, GTE component  12  will assume the form of a component included within the gas turbine engine&#39;s combustor section or turbine section and having at least one surface exposed to hot gas flow produced by combustion of a fuel-air mixture. To list but a few examples, GTE component  12  may assume the form of a combustor can, a heat shield, a turbine shroud, a turbine blade, a nozzle guide vane, or a duct member. GTE component  12  is conveniently, although not necessarily, formed from a nickel- or cobalt-based superalloy, such as Inconel®. 
     Bond coat  14  may comprise any metallic material, whether currently known or later developed, suitable for bonding TBC  18  to GTE component  12 . In one embodiment, bond coat  14  is an aluminide-based alloy. In a second, preferred embodiment, bond coat  14  is a MCrAlY-based alloy, wherein M represents nickel, cobalt, or a nickel-cobalt alloy; and wherein Cr, Al, and Y represent chromium, aluminum, and yttrium, respectively. A given bond coat alloy can, and typically will, include lesser amounts of one or more additional metallic or non-metallic constituents, which may be added in powder form to a master alloy during processing to optimize the metallurgical properties of the resulting alloy. For example, small amounts of nickel, cobalt, hafnium, tantalum, zirconium, and the like may be added to a MCrAlY-based bond coat to optimize, for example, the bond coat&#39;s oxidative resistance. Bond coat  14  may be formed utilizing any one of a number of conventionally-known deposition processes, such as the processes set-forth below in conjunction with  FIG. 2 . 
     Inner YSZ layer  20 , yttrium gradient layer  22 , and outer YSZ layer  24  may be formed as an integral structure utilizing a single, continually-performed process. Alternatively, layers  20 ,  22 , and  24  may be formed discrete layers utilizing a series of sequentially performed fabrication steps. In this latter case, layers  20 ,  22 , and  24  may be formed utilizing the same type of process or, instead, utilizing two or more different types of processes. In either case, yttrium gradient layer  22  is formed as a single, unitary layer utilizing a continual, uninterrupted deposition process, such as an electron beam physical vapor deposition (“EB-PVD”) process or the plasma spray process described below in conjunction with  FIGS. 3 and 4 . 
     As a point of emphasis, the yttrium content of yttrium gradient layer  22  increases with increasing distance from bond coat  14 . In a preferred embodiment wherein yttrium gradient layer  22  includes an inner surface  26  and an outer surface  28  (again, defined in view of their proximity to GTE component  12 ), yttrium gradient layer  22  has a minimum yttrium content adjacent inner surface  26 , has a maximum yttrium content adjacent outer surface  28 , and transitions from the minimum yttrium content to the maximum yttrium content when moving from inner surface  26  to outer surface  28 . The yttrium content of yttrium gradient layer  22  preferably increases in a gradual or progressive manner through the full thickness of layer  22 . In general, it is desirable to impart yttrium gradient layer  22  with a gradient profile that is substantially linear or exponential, as taken through the thickness of layer  22 ; however, due to practical limitations that may be inherent in the process utilized to form yttrium gradient layer  22 , such as the plasma spray process described below, it may be impossible or impractical to impart layer  22  with an infinitely smooth gradient profile. Thus, in many cases, the yttrium profile of layer  22  may comprise a series of incremental steps or gains in yttrium content, which collectively approximate a line or curve. 
     In contrast to yttrium gradient layer  22 , inner YSZ layer  20  and outer YSZ layer  24  each have a substantially uniform yttrium content through their respective thicknesses, with outer YSZ layer  24  having an yttrium content exceeding that of inner YSZ layer  20 . In embodiments wherein no intervening layers are formed between inner YSZ layer  20  and yttrium gradient layer  22 , and therefore inner surface  26  of yttrium gradient layer  22  is bonded directly to YSZ layer  20 , it is preferred that the yttrium content of inner YSZ layer  20  is substantially equivalent to the minimum yttrium content of yttrium gradient layer  22 . In this manner, the yttrium content across the interface of inner YSZ layer  20  and yttrium gradient layer  22  may be held substantially constant to promote durable interlayer bonding. Similarly, in embodiments wherein no intervening layers are formed between outer YSZ layer  24  and yttrium gradient layer  22 , and therefore outer surface  28  of yttrium gradient layer  22  is bonded directly to YSZ layer  24 , it is preferred that the yttrium content of outer YSZ layer  20  is substantially equivalent to the maximum yttrium content of yttrium gradient layer  22  to again promote durable interlayer bonding and optimize the structural integrity of the resulting thermal barrier coating. 
     The minimum and maximum yttrium contents of yttrium gradient layer  22  will inevitably vary amongst different embodiments of thermal barrier system  10 . However, in a preferred embodiment, the minimum yttrium content of yttrium gradient layer  22  is approximately 7% to approximately 8%, by weight of yttrium (“7-8YSZ”). Yttrium gradient layer  22  is preferably formed to have a minimum yttrium content of approximately 7-8% for multiple reasons. First, as noted in the foregoing section entitled “Background,” 7-8YSZ material is readily available within the aerospace industry, is highly machinable, and demonstrates excellent fracture toughness when deployed within a gas turbine engine environment. In addition, 7-8YSZ adheres well to MCrAlY-based bond coats. Durable bonding between bond coat  14  and inner YSZ layer  20  can thus be promoted by forming yttrium gradient layer  22  to have a minimum yttrium content of approximately 7-8% in preferred embodiments wherein bond coat  14  is formed from a MCrAlY-based alloy and the yttrium content of inner YSZ layer  20  is substantially equivalent to the minimum yttrium content of yttrium gradient layer  22 . 
     The maximum yttrium content of yttrium gradient layer  22  is preferably greater than approximately 12%, by weight of yttrium; more preferably, between approximately 15% and approximately 65%, by weight of yttrium; and, still more preferably, approximately 20%, by weight of yttrium. In many cases, the maximum yttrium content of yttrium gradient layer  22  will be at least twice the minimum content of layer  22 . Forming yttrium gradient layer  22  as a compositional gradient structure that transitions from a first yttrium content (e.g., 7-8%, by weight of yttrium) to a second, significantly higher yttrium content (e.g., 20%, by weight of yttrium) provides at least two advantages. First, as the yttrium content of yttrium gradient layer  22  increases, so too does the overall phase stability of layer  22 . Thus, relative to a comparable YSZ layer having a lower yttrium content (e.g., a uniform YSZ layer having an yttrium content of 7-8%), yttrium gradient layer  22  can be subjected to higher temperatures without undergoing a phase change and a corresponding expansion in volume. Yttrium gradient layer  22 , and more generally TBC  18 , can consequently be subjected to higher combustive gas temperatures (e.g., temperatures exceeding 1260° C.) without experiencing a phase transformation otherwise causing the thermal barrier coating to degrade, as previously described. As a second advantage, by forming yttrium gradient layer  22  to gradually increase from a minimum to a maximum yttrium content, the overall sintering resistance of TBC  18  is significantly improved. Such an increase in sintering resistance allows yttrium gradient layer  22 , as well as outer YSZ layer  24 , to maintain a higher porosity through elevated temperature exposure. This, in turn, allows the thermal conductivity of TBC  18  to be maintained at low levels and thermal expansion and contraction strains to be accommodated through elevated temperature exposure thereby maintaining the overall effectiveness of TBC  18  as a thermal barrier. 
     Notably, the above-described advantages provided by yttrium gradient layer  22 , in combination with inner YSZ layer  20  and outer YSZ layer  24 , cannot be achieved by simply depositing an yttrium-stabilized zirconia layer having a higher yttrium content directly over bond coat  14 ; it has been found that YSZ layers having higher yttrium contents (e.g., yttrium contents approaching or exceeding 20%) tend to bond poorly with conventionally-employed bond coat materials. Furthermore, YSZ coatings having yttrium contents around 20%, by weight of yttrium, tend to form detrimental phases with MCrAlY bond coat alloys and are consequently incompatible therewith. Nor can the above-described advantages be achieved by simply depositing an yttrium-stabilized zirconia layer having a higher yttrium content (e.g., 20%, by weight of yttrium) over an yttrium-stabilized zirconia layer having a lower yttrium content (e.g., 7-8%, by weight of yttrium); such dual-layer combinations likewise tend to bond poorly and, therefore, are prone to separation over repeated thermal cycles within a gas turbine engine environment. In contrast, by forming a composition gradient structure that gradually transitions from a lower yttrium content to a higher yttrium content in the above-described manner, a thermal barrier coating can be produced that achieves the above-noted advantages, while also demonstrating superior structural durability within high temperature GTE environments. 
       FIG. 2  is a flowchart illustrating an exemplary method  30  that may be performed to form a thermal barrier system including an yttrium gradient layer of the type described above. For ease of explanation, exemplary method  30  is described below in conjunction with thermal barrier system  10  shown in  FIG. 1 ; it is noted, however, that exemplary method  30  can be utilized to form thermal barrier systems that differ materially from thermal barrier system  10 . Referring jointly to  FIGS. 1 and 2 , method  30  commences with the provision of a gas turbine engine component, such as GTE component  12  (STEP  32 ,  FIG. 2 ). As explained above, GTE component  12  can comprises any type of structural element included within a gas turbine engine and heated during operation thereof. As also noted above, GTE component  12  is conveniently fabricated (e.g., cast) from a nickel- or cobalt-based superalloy, such as Inconel®. If desired, one or more surfaces of GTE component  12  (e.g., surface  16  identified in  FIG. 1 ) may be cleaned (e.g., with a degreasing agent), planarized (e.g., via lapping, grinding, or chemical-mechanical planarization), and/or otherwise prepared for the subsequent formation of bond coat  14  during STEP  32  ( FIG. 2 ). 
     Next, during STEP  34  ( FIG. 2 ), bond coat  14  is formed over surface  16  of GTE component  12  ( FIG. 1 ). As noted above, in a preferred embodiment, bond coat  14  is formed via deposition of a MCrAly-based alloy. Any one of a number of conventionally-known deposition techniques can be utilized to form bond coat  14  over GTE component  12 . Processes suitable forming bond coat  14  include, but are not limited to, physical vapor deposition, cladding, high velocity oxygen fuel spraying, low pressure plasma spraying, vacuum plasma spraying, and air plasma spraying. As a non-limiting example, bond coat  14  may be deposited to a thickness of approximately 0.012 millimeter to approximately 0.254 millimeter. 
     Exemplary method  30  continues with the formation of TBC  18  over bond coat  14 . As indicated in  FIG. 2  at  36 , at least three steps may be performed to produce the successive layers of TBC  18 . First, at STEP  38  ( FIG. 2 ), inner yttrium-stabilized zirconia layer  20  is deposited over bond coat  14  ( FIG. 1 ). Next, at STEP  40  ( FIG. 2 ). yttrium gradient layer  22  is deposited over inner yttrium-stabilized zirconia layer  20  ( FIG. 1 ). Finally, at STEP  42  ( FIG. 2 ), outer yttrium-stabilized zirconia layer  24  is deposited over yttrium gradient layer  22  ( FIG. 1 ). In certain embodiments of method  30 , STEPS  38 ,  40 , and  42  may be performed in a continual manner. That is, the process utilized to form inner YSZ layer  20  may be continued, without interruption, to form yttrium gradient layer  22 ; and/or the process utilized to form yttrium gradient layer  22  may be continued, without interruption, to form outer YSZ layer  24 . Such a continuous process yields an integrally-formed thermal barrier coating having exceptional structural integrity. This notwithstanding, STEPS  38 ,  40 , and  42  may be performed in a non-continuous or intermittent manner in further embodiments. In such embodiments, layers  20 ,  22 , and  24  will comprise discrete, separately-formed structures; however, durable bonding between neighboring layers  20 ,  22 , and  24  can still be achieved, especially in embodiments wherein the yttrium contents of YSZ layers  20  and  24  are substantially equivalent to the minimum and maximum yttrium contents of yttrium gradient layer  22 , respectively. In embodiments wherein STEPS  38 ,  40 , and  42  are discretely performed, different techniques may be utilized to form each of layers  20 ,  22 , and  24 ; e.g., layers  20  and  24  may be formed utilizing a conventional deposition technique, such as a conventional plasma spray technique or an EB-PVD technique, while yttrium gradient layer  22  may be formed utilizing a modified plasma spray technique of the type described below. By way of non-limiting example, inner YSZ layer  20  may be deposited to a thickness between approximately 0.025 millimeter and approximately 2.0 millimeters; yttrium gradient layer  22  may be deposited to a thickness of approximately 0.127 millimeter to approximately 2.0 millimeters; and outer YSZ layer  24  may be deposited to a thickness between approximately 0.025 millimeter and approximately 0.75 millimeter. The foregoing notwithstanding, it will be appreciated that the thicknesses of inner YSZ layer  20 , yttrium gradient  22 , and outer YSZ layer  24  will vary, at least in part, based upon the selected deposition process and desired microstructure; e.g., whether layers  20 ,  22 , and  24  are formed utilizing EB-PVD, plasma spray, or another suitable process and are formed to have a low density (porous) microstructure, a vertically cracked microstructure, or another microstructure. 
       FIG. 3  is a generalized schematic of a plasma spray apparatus  50  that may employed to produce yttrium gradient layer  22 , and potentially also YSZ layers  20  and  24 , in an exemplary implementation of method  30 . As can be seen in  FIG. 3 , plasma spray apparatus  50  includes a plasma spray gun  52  and a multi-feedstock powder supply system  54 , which supplies plasma spray gun  52  with a powder mixture in the manner described below. Plasma spray gun  52  includes a spray gun housing  56 ; liquid inlet and outlet ports  58 , which can be connected to a pump (not shown) to circulate a liquid coolant (e.g., water) through spray gun housing  56 ; a gas inlet port  60 , which can be connected to a plasma source (not shown) to supply plasma spray gun  52  with a combustible plasma; and a gas outlet port  62 , which can be used for a carrier and/or plasma shrouding gas during bond coat and/or top coat deposition. Plasma spray gun  52  also includes first and second electrodes  64  and  66 , which are energized during operation of plasma spray gun  52  to create an electrical arc to create the plasma and produce a flame  68 . Lastly, at least one powder feed port  70  couples plasma spray gun  52  to powder supply system  54 . During operation of plasma spray gun  52 , powder feed port  70  directs a powder mixture received from powder supply system  54  into flame  68 , which rapidly melts the powder mixture particles and propels the particles from the nozzle of gun  52  and against GTE component  12 . As they impinge upon the surface of GTE component  12  (or, more accurately, on bond coat  14 ), the particles gradually produce a dense, adhesive ceramic coating having the above-described properties. In embodiments wherein it is desired to impart TBC  18  with a porous, low density microstructure, process parameters can be controlled to cause the impinging particles to deform or flatten into numerous lamellae or plate-like formations commonly referred to as “splats” and provide a controlled porosity. Alternatively, in embodiments wherein it is desired to impart TBC  18  with a vertically cracked microstructure, a similar but modified process may be employed wherein GTE component  12  is heated in a controlled manner to induce vertically-propagated cracks during coating deposition. 
     With continued reference to  FIG. 3 , powder supply system  54  includes a first powder feedstock  74 , a second powder feedstock  76 , and a bifurcated flow passage  78 , which couples powder feedstocks  74  and  76  to powder feed port  70  of plasma spray gun  52 . A first flow control valve  80  is positioned across a first leg of bifurcated flow passage  78  downstream of powder feedstock  74 ; and a second flow control valve  84  is positioned across a second, opposing leg of bifurcated flow passage  78  downstream of powder feedstock  76 . A controller  82  is operably coupled to flow control valves  80  and  84  and, during the plasma spray process, adjusts the position of valves  80  and  84  to control the rate of powder flowing from feedstocks  74  and  76  to plasma spray gun  52  and, therefore, the overall composition of the powder mixture supplied to plasma spray gun  52 . In one embodiment, controller  82  adjusts the position of valves  80  and  84  in accordance with a predetermined flow schedule, such as that described below in conjunction with  FIG. 4 . 
     The yttrium content of powder feedstock  74  is less than the yttrium content of powder feedstock  76 . In a preferred embodiment, the yttrium content of powder feedstock  74  is substantially equivalent to the desired minimum yttrium content of yttrium gradient layer  22 ; and the yttrium content of powder feedstock  76  is substantially equivalent to the desired maximum yttrium content of yttrium gradient layer  22 . In this manner, a relatively straightforward plasma spray process can be performed wherein the powder mixture supplied to plasma spray gun  52  gradually transitions from an initial composition comprising substantially 100% powder drawn from feedstock  74  and substantially 0% powder drawn from feedstock  76  to a final composition comprising substantially 0% powder drawn from feedstock  74  and substantially 100% powder drawn from feedstock  76 . Further emphasizing this point,  FIG. 4  is a graph of spray powder composition (vertical axis) versus time (horizontal axis) illustrating one manner in which controller  82  may adjust ratio of the powder mixture drawn from feedstock  74  and  76  during the thermal spray process. As indicated in  FIG. 4  at  86 , controller  82  may modulate flow control valves  80  and  82  (i.e., gradually open valve  80  while gradually closing valve  82 ) to progressively change the powder mixture composition from a low yttrium content powder mixture composed substantially entirely of powder drawn from feedstock  74  to a high yttrium content powder mixture composed substantially entirely of powder drawn from feedstock  76  over the time period during which yttrium gradient layer  22  is formed. Yttrium gradient layer  22  is thus formed to have a minimum yttrium content substantially equivalent to the yttrium content of feedstock  74  adjacent inner surface  26  ( FIG. 1 ), to have a maximum yttrium content substantially equivalent to the yttrium content of feedstock  76  adjacent outer surface  28  ( FIG. 1 ), and to gradually transition from the minimum yttrium content to the maximum yttrium content when moving from inner surface  26  to outer surface  28  ( FIG. 1 ). 
     In embodiments wherein inner YSZ layer  20  is integrally formed with yttrium gradient layer  22 , the plasma spray process may commence prior to formation of layer  22 . In particular, as indicated in  FIG. 4  at  88 , inner YSZ layer  20  may first be formed by plasma spraying a powder mixture over bond coat  14  ( FIG. 1 ) while drawing substantially 100% powder from feedstock  74  (flow control valve  80  fully open) and substantially 0% powder from feedstock  76  (flow control valve  82  fully closed). In this manner, inner YSZ layer  20  is formed to have an yttrium content substantially equivalent to the minimum yttrium content of yttrium gradient layer  22 . After formation of inner YSZ layer  20 , the plasma spray process may be continued, without interruption, to form yttrium gradient layer  22  in the above-described manner. Similarly, in embodiments wherein outer YSZ layer  24  is integrally formed with yttrium gradient layer  22 , the plasma spray process may continue, without interruption, after formation of yttrium gradient layer  22  to further form outer YSZ layer  24 . More specifically, as indicated in  FIG. 4  at  90 , outer YSZ layer  24  may be formed by plasma spraying a powder mixture over yttrium gradient layer  22  while drawing substantially 0% powder from feedstock  74  and substantially 100% powder from feedstock  76 . In this manner, outer YSZ layer  24  may be formed to have an yttrium content substantially equivalent to the maximum yttrium content of yttrium gradient layer  22 . 
     During the above-described plasma spray process, various process parameters (e.g., the proximity of spray gun  52  to GTE component  12 , powder velocity, powder mixture composition, etc.) to impart inner YSZ layer  20  and outer YSZ layer  24  with desired porosities. In addition, GTE component  12  may be successively heated and cooled during plasma spraying to form vertically-propagated microcracks within inner YSZ layer  20 , gradient layer  22 , and outer YSZ layer  24 . In most cases, the above-described plasma spray process will be performed in an open atmosphere (commonly referred to as “air plasma spraying”); however, the possibility that other types of plasma spray processes (e.g., argon shroud plasma spraying) can be employed is by no means excluded. 
     Although, in the above-described exemplary embodiment, the yttrium contents of powder feedstocks  74  and  76  are substantially equivalent to the minimum and maximum yttrium contents of yttrium gradient layer  22 , respectively, this need not always be the case. If desired, yttrium gradient layer  22  can be formed utilizing powder feedstocks having yttrium contents different than the maximum and minimum yttrium contents of yttrium gradient layer  22 , providing that at least one powder feedstock has an yttrium content less than the minimum yttrium content of layer  22  and at least one powder feedstock has an yttrium content greater than the maximum yttrium content of layer  22 . If desired, additional powder feedstocks containing one or more additives may also be employed in further embodiments of the above-described plasma spray process. 
     There has thus been provided multiple exemplary embodiments of a thermal barrier system that achieves the benefits of conventionally-employed 7-8YSZ thermal barrier coatings, while also exhibiting improved phase stabilities at higher operating temperatures (e.g., operational temperatures exceeding approximately 1260° C.) and improved sintering resistances. The foregoing has also provided exemplary embodiments of a method suitable for forming such a thermal barrier coating over a gas turbine engine component. Although preferably formed utilizing a modified plasma spray technique of the type described above, it is emphasized that the yttrium gradient layer (or layers) can be formed utilizing any suitable process, whether currently known or later developed. For example, in certain embodiments, a modified EB-PVD technique can be employed to form yttrium gradient layer wherein the vaporization rate of at least two ingots, each having a different yttrium content, is varied with time to vary the yttrium content of the gasses within the vacuum chamber and, therefore, the composition of the yttrium gradient layer cumulatively deposited over the gas turbine engine component. In one exemplary case, during STEP  40  of exemplary method  30  ( FIG. 2 ), the vaporization rate of a first ingot having an yttrium content substantially equivalent to the gradient layer&#39;s desired minimum yttrium content may be gradually decreased from a high level to a low level (e.g., via a continual reduction in electron beam intensity), while the vaporization rate of a second ingot having an yttrium content substantially equivalent to the gradient layer&#39;s desired maximum yttrium content is gradually increased from a low level to a high level (e.g., via a continual increase in electron beam intensity), to progressively increase the overall yttrium content of the vacuum chamber atmosphere over the duration of the EB-PVD process to form an yttrium gradient layer of the type described above. In embodiments wherein layer  20  and/or layer  24  are integrally formed with yttrium gradient layer  22  ( FIG. 1 ), the vaporization rate of the first and second ingot may also be controlled, as appropriate, to form inner YSZ layer  20  during STEP  38  of exemplary method  30  (e.g., the vaporization rate of the first ingot may be maintained at a relatively high level, while the vaporization rate of the second ingot is held at or near zero) and/or to form outer YSZ layer  24  during STEP  42  of exemplary method  30  (e.g., the vaporization rate of the second ingot may be maintained at a relatively high level, while the vaporization rate of the first ingot is held at or near zero). 
     While at least one exemplary embodiment has been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended Claims.