The invention relates to thermal barrier coatings and, more particularly, relates to reduced conductivity thermal barrier coating systems having at least two layers, each layer exhibiting a different microstructure.
Thermal barrier coatings (hereinafter “TBCs”) are employed in turbine engines in an effort to shield and protect the structural metallic components from the high temperature conditions present in a combustion environment. These ceramic coatings effectively lower the substrate metal surface temperature and slow the kinetics of oxidation which degrades the metallic substrate. Reduced conductivity TBCs have provided an even greater benefit to turbine engines than conventional TBCs by allowing higher turbine engine operation temperatures or even further reduced metal substrate temperatures. The microstructure of a TBC is dictated by processing. The microstructure also contributes to the physical properties of the coated article, in particular, the thermal conductivity. When creating thermal barrier coatings, it is desirable to reduce the thermal conductivity of the TBCs as much as possible.
In one aspect of the present disclosure, a coated article broadly comprises an article having at least one surface; and a thermal barrier coating system disposed upon at least one surface and comprising at least two layers, each layer having a different microstructure, wherein the thermal barrier coating system exhibits a thermal conductivity of no more than 16 BTU in/hr ft2 F.
In another aspect of the present disclosure, a coated article broadly comprises a turbine engine component having at least one surface; and a thermal barrier coating system disposed upon the at least one surface. The thermal barrier coating system comprises at least two layers, with each layer having a different microstructure. In a preferred embodiment, the at least two layers broadly comprises: a first layer having a first microstructure; a second layer having a second microstructure; and an interlayer having a third microstructure and formed between the first and second layers, wherein the first and second microstructures comprise a microstructure selected from the group consisting of columnar, amorphous, randomized, and splat-like, wherein the third microstructure comprises a combination of the first and second microstructures, and wherein the thermal barrier coating system exhibits a thermal conductivity of no more than 16 BTU in/hr ft2 F.
In yet another aspect of the present disclosure, a process for coating an article broadly comprises applying a first layer of a thermal barrier coating system having a first microstructure on at least one surface of an article; applying upon the first layer a second layer of the thermal barrier coating system having a second microstructure that is different from the first microstructure; and forming between the first and second layers an interlayer having a third microstructure comprising a combination of the first and second microstructures.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
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If desired, an optional bond coat layer may be applied on at least one surface of the article at step 10 prior to the application of the thermal barrier coating system. The bond coat layer may be applied using any suitable technique known in the art. If desired, a thermally grown oxide layer (“TGO”) may be formed upon the bond coat layer at step 12 using any suitable technique known in the art. Alternatively, the thermal barrier coating system may be directly applied to, or deposited on, the at least one surface of the article.
A first layer of a thermal barrier coating may be applied upon the at least one surface of the article, or the bond coat layer if present or the thermally grown oxide layer if present, at step 14. A second layer of the thermal barrier coating may be deposited on the first layer at step 16. During the second layer deposition process, an interlayer typically forms between the first and second layers of the thermal barrier coating at step 18. One or more additional layers may be applied upon the second layer at step 20 such that additional interlayer(s) form between each layer subsequently applied at step 22. Each layer of the thermal barrier coating system preferably has a different microstructure. For example, the first layer has a first microstructure, the second layer has a second microstructure, and the interlayer has a microstructure exhibiting a combination of the first and second microstructures.
The application of the bond coat and the first, second and any subsequent layers of the thermal barrier coating system may be achieved using either a vapor deposition process (e.g., physical vapor deposition) or a thermal spray process (e.g., plasma spraying) as known to one of ordinary skill in the art. Whether using a vapor deposition process or a thermal spray process, each layer of the thermal barrier coating system, and the bond coat layer, is applied so that each layer exhibits a different microstructure. The microstructures contemplated herein include, but are not limited to, columnar, amorphous, randomized, and splat-like microstructures. Whether using a vapor deposition process or a thermal spray process, each layer of the thermal barrier coating may be applied using a vacuum-plasma spraying torch apparatus known as the O3CP, commercially available from Sulzer Metco Ltd., of Westbury, N.Y. The O3CP vacuum-plasma spraying apparatus allows a user to apply a first coating exhibiting a microstructure such as a columnar microstructure, and then adjust the operating parameters of the spraying apparatus to apply a subsequent coating exhibiting a different microstructure. Prior processes required one of ordinary skill in the art to utilize two entirely different spraying apparatus to apply coatings having different microstructures as disclosed herein. In using the O3CP vacuum-plasma spraying apparatus to perform the exemplary process described herein, one recognizes benefits such as reduced time and costs, increased efficiency, and minimized likelihood of contaminating the thermal barrier coating system being applied.
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Each layer of the thermal barrier coating system may include a ceramic base material and at least one dopant oxide of a metal present in an amount of about 1 wt % to about 99 wt %, and from about 5 wt % to about 99 wt %, and from about 30 wt % to about 70 wt %, of the total weight of the layer. Suitable ceramic base materials may include any one of the following: a zirconate, a hafnate or a titanate. Suitable dopant oxides of a metal may include oxides of any one of the following metals: lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutelium, indium, scandium, and yttrium. For example, a representative thermal barrier coating system may comprise yttria stabilized zirconia having from about 1.0 wt % to about 25 wt % yttria of the total weight of the layer and a balance of zirconia, or gadolinia stabilized zirconia having from about 5.0 wt % to about 99 wt % gadolinia, from about 30 wt % to about 70 wt % gadolinia, of the total weight of the layer and a balance of zirconia or both yttria stabilized zirconia and gadolinia stabilized zirconia.
One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.