Abstract:
The present invention provides a high-purity fused and crushed stabilized zirconia powder. The powder—with or without further processing, such as plasma spheroidization—is used in thermal spray applications of thermal barrier coatings (TBCs) and high-temperature abradables. The resulting coatings have a significantly improved high temperature sintering resistance, which will enhance the durability and thermal insulation effect of the coating.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. no. 11/520,043, filed Sep. 13, 2006, which application claims priority under 35 U.S.C.§119(e) from U.S. Provisional Patent Application No. 60/724,268, filed on Oct. 7, 2005, both of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       REFERENCE TO SEQUENCE LISTING 
       [0003]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This invention relates generally to powders for use in thermal barrier coatings and high-temperature abradable coatings, and more particularly to high-purity fused and crushed zirconia alloy powder for use with thermal spray deposition processes. 
         [0006]    2. Description of Related Art 
         [0007]    Superior high-temperature properties are required to improve the performance of heat resistant and corrosion resistant members. These members include, for example, gas turbine blades, combustor cans, ducting and nozzle guide vanes in combustion turbines and combined cycle power plants. Turbine blades, for example, are driven by hot gasses, and the efficiency of the gas turbine increases with the rise in operation temperature. The demand for continued improvement in efficiency has driven the system designers to specify increasingly higher turbine inlet temperatures. Thus, there is a continuing need for materials that can achieve higher operational temperatures. 
         [0008]    Ceramic coatings generally have been used to allow standard materials to operate in higher temperature environments. Ceramic thermal barrier coatings and high temperature abradable coatings may be applied using, for example, a thermal spray process. In one form of this process, ceramic powder feedstock is injected into a high velocity plasma stream where it is simultaneously melted and propelled toward a substrate. 
         [0009]    One factor that impacts coating lifetime is the sintering rate of the coating. When the coating is cycled above half of its absolute melting temperature, the coating begins to sinter causing volume shrinkage. As the coating shrinks, the stress difference between the coating and substrate increases. At a certain amount of shrinkage (which varies depending on the type of structure), the stress difference exceeds the adhesive or cohesive strength of the coating and it becomes detached. Decreasing the sintering rate increases the amount of time before the critical shrinkage is achieved, so it can become a major design consideration. 
         [0010]    As industry demands continue to drive higher operating temperatures, there remains a need in the art for coating materials that can meet performance requirements at increasingly higher temperatures. 
       SUMMARY OF THE INVENTION 
       [0011]    Accordingly, the invention is directed to a material for obtaining coatings for high temperature cyclic applications that have both improved sintering resistance to achieve a high service lifetime and low thermal conductivity to achieve high operating temperatures. The present invention meets the aforementioned needs by providing a high-purity fused and crushed stabilized zirconia (ZrO 2 ) and/or hafnia (HfO 2 ) powder that has about 60-95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ), and about 5-40 weight percent of a stabilizer (which may be yttria, ceria, ytterbia, or other rare earth oxides individually or in any combination). Any other oxide or chemical species in the inventive composition must be less than about 0.1 weight percent. 
         [0012]    Relative to currently-available fused and crushed stabilized zirconia powder, the high-purity fused and crushed stabilized zirconia powder of the present invention has a lower content of impurities such as soda, silica, alumina and titania. Thermal barrier coatings and high temperature abradable coatings made from the high-purity fused and crushed stabilized zirconia powder of the present invention exhibit significantly improved sintering resistance, which will lead to prolonged coating durability and enhanced thermal insulation effect. 
         [0013]    In one aspect, the invention provides a fused and crushed stabilized powder for thermal spray applications that includes between about 60 and 95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ), between about 5 and 40 weight percent of a stabilizer, and between about 0 and 0.1 weight percent other oxides or chemical species. The stabilizer may be one or more of yttria (Y 2 O 3 ), ceria (CeO 2 ), dysprosia (Dy 2 O 3 ), ytterbia (Yb 2 O 3 ), or other rare earth oxides. 
         [0014]    According to another aspect of the invention, a fused and crushed stabilized powder for thermal spray applications is provided that includes between about 60 and 95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ), typically between about 0 and 2.5 weight percent hafnia (HfO 2 ), between about 5 and 40 weight percent of a stabilizer, and between about 0 and 0.1 weight percent other oxides or chemical species. 
         [0015]    In another aspect of the invention, a method of producing a stabilized powder for thermal spray applications is provided that includes the step of providing raw materials of about 60 to 95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ), about 5 to 40 weight percent stabilizer, and about 0 to 0.1 weight percent other oxides or chemical species. A further step of the method includes fusing the raw materials, followed by crushing the fused materials to achieve a particle size suitable for thermal spray applications. Additionally, the method may include the optional step of subjecting the crushed powder to a plasma spheroidization process. Typical particle sizes may be in the range of between about 5 to 200 microns, depending on, among other factors, the desired microstructure of coating. 
         [0016]    In one other aspect of the invention, a method of providing a thermal sprayed coating on a substrate is provided. The method includes the step of providing a powder with a particle composition of between about 60 and 95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ), between about 5 and 40 weight percent stabilizer, and between about 0 and 1.0 weight percent other oxides or chemical species. A next step includes applying the powder to the substrate using a thermal spraying technique. Suitable thermal spray techniques include plasma, detonation, high velocity oxy-fuel (HVOF) process, or powder combustion. 
         [0017]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0018]      FIG. 1  provides a chart showing the improved sintering resistance of coatings made from one embodiment of powder according to the present invention; 
           [0019]      FIG. 2  provides a flow chart of a method for producing high-purity stabilized zirconia powder in accordance with the present invention; and 
           [0020]      FIG. 3  provides a micrograph of a fused and crushed powder in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The present invention provides a high-purity fused and crushed zirconia alloy powder that has between about 60 to 95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ) and about 5 to 40 weight percent of a stabilizer. The stabilizer may be one of several stabilizers including yttria (Y 2 O 3 ), ceria (CeO 2 ), dysprosia (Dy 2 O 3 ), ytterbia (Yb 2 O 3 ), or other rare earth oxides individually or in any combination. To achieve the high sintering resistance and extended service lifetime, the inventive composition must have a high purity such that any other oxide or chemical species in the inventive composition must be less than 0.1 weight percent. In one embodiment, soda content will be less than 0.05%, silica content will be less than 0.05%, alumina content will be less than 0.01% and titania content will be less than 0.05%. 
         [0022]    When subject to high temperature heat treatment, coatings of zirconia alloys will shrink in size due to sintering. For coatings made from high purity zirconia alloys, the shrinkage due to sintering can be as much as 80% less than what is observed in coatings made from low purity zirconia alloys. As an example,  FIG. 1  shows that the coating made from high purity 8 wt. % Y2O3-ZrO2 powder has a greatly improved sintering resistance. The chart shows linear in-plane shrinkage of several coatings sintered at 1400° C. It shows that the coating made from the high purity powder has significantly less shrinkage due to its greatly improved sintering resistance. 
         [0023]      FIG. 2  provides a flow chart of a method for making powder of the above compositions. In step S 101 , raw materials of the appropriate composition are provided. The raw material composition is provided in the same weight percentages as desired in the final powder composition. Thus, raw materials will include between about 60 to 95 weight percent zirconia (ZrO 2 ) and/or hafnia (HfO 2 ) (typically including up to 2.5 weight percent hafnia), and between about 5 to 40 weight percent stabilizer(s) (such as one or more of yttria, ceria, dysprosia, ytterbia, and other rare earth oxides). In step S 102 , the raw materials are fused together into a solid mass. Impurities, such as silica, alumina, titania and other oxides or chemical species, in the fused powder must be no more than 0.1 weight percent. The fusing process is followed, in step S 103 , by crushing to achieve a suitable particle size. Suitable particle size will vary depending on the desired coating microstructure, the type of thermal spray technique being used, and other factors. Particle size may range between about 5 to 200 microns, with more typical ranges between about 15 to 125 microns. A micrograph of powder made in accordance with the above process is shown in  FIG. 3 . 
         [0024]    Subsequent to fusing and crushing, the high-purity powder may optionally be further processed by a plasma spheroidization process. This optional step is shown as step S 104  in  FIG. 2 . Plasma spheroidization processes are generally known to improve, among other properties, powder flow characteristics. As applied to the present invention, the process basically involves in-flight heating and melting of the crushed high-purity powder particles. Molten spherical droplets are formed, which are then cooled under free fall conditions. The flight time of the droplets is controlled so that they solidify before reaching the bottom of a primary collection chamber. The resulting powder particles have improved flow characteristics than the fused and crushed powder. 
         [0025]    The high-purity stabilized powder can be applied onto an object or substrate to provide a thermal barrier or abradable coating. Applying the powder to the substrate can be accomplished using one or more thermal spraying techniques such as plasma spray, detonation, HVOF, or powder combustion. 
         [0026]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general invention concept as defined by the appended claims and their equivalents.