Patent Publication Number: US-2005129868-A1

Title: Repair of zirconia-based thermal barrier coatings

Description:
FIELD OF THE INVENTION  
      This invention relates generally to the field of materials and more particularly to ceramic thermal barrier coatings.  
     BACKGROUND OF THE INVENTION  
      Thermal barrier coating materials are commonly used to protect underlying substrate materials from a high temperature environment. In modern gas turbine engines, hot gas path components formed of metal alloys such as nickel-based or cobalt-based superalloys are often coated with a layer of ceramic insulating material. Zirconia-based coatings, in particular 6-8% yttria stabilized zirconia (YSZ), is a material that is widely used for such applications. Zirconia may be deposited onto the substrate surface by a variety of processes, including for example plasma spray or physical vapor deposition (PVD). Plasma spray provides a coating formed of multiple overlapping splats of previously molten material. Physical vapor deposition provides a columnar-grained structure that may perform better than plasma sprayed coatings in certain applications due to an enhanced porosity control (lower thermal conductivity) and improved strain tolerance due to the inherent directionality of its structure (improved thermal shock performance).  
      Methods for repairing damaged ceramic thermal barrier coatings are known. U.S. Pat. No. 5,723,078 describes the use of a plasma spray process to repair a columnar-grained coating. The extremely high temperatures produced during a plasma spray process, as high as 15,000° C. for example, necessitate that such repairs be performed in a shop environment following disassembly of the machine containing the component to be repaired. U.S. Pat. No. 6,413,578 describes the use of a ceramic paste that can be applied to a damaged gas turbine component while the component remains installed. The paste includes a ceramic powder and a binder material that is thermally reacted to form the repair. Such chemically bonded repair materials generally do not perform as well as the original coating material, especially under conditions of cyclic thermal exposures.  
      U.S. Pat. No. 4,588,655 describes a ceramic coating consisting of alumina and zirconia particles, and U.S. Pat. No. 5,059,095 describes applying a dense coating of this material to a gas turbine rotor blade tip using a high velocity oxy-fuel (HVOF) process. The dense layer of alumina-zirconia material is useful for a gas turbine blade tip application due to its friction and abrasion qualities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic illustration of a portable low velocity oxy-fuel tool being used to deposit a ceramic coating on a surface of a component that is in its operating position in a machine.  
       FIG. 2  is a partial cross-sectional illustration of a ceramic coating obtained by depositing a relatively low melting point powder and a relatively high melting point powder using a low velocity oxy-fuel process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present inventors have found that prior art coatings of alumina and zirconia applied by thermal processes such as air plasma spray (APS) or high velocity oxy-fuel (HVOF) have a life expectancy that is less than that of zirconia coatings not containing alumina due to spalling caused by the differential thermal expansion between the zirconia and the alumina. Furthermore, the thermal conductivity of such coatings is higher than that of pure zirconia coatings. An improved coating is described herein as combining zirconia (unstabilized or stabilized) with a material having a coefficient of thermal expansion and/or a thermal conductivity that is closer to that of zirconia than the corresponding property of alumina. The coefficient of thermal expansion of the material that is mixed with the zirconia may be within 30% of that of the zirconia in one embodiment, or within 20% or 10% of that of the zirconia in other embodiments. The thermal conductivity of the material may be no more than that of the zirconia in one embodiment, or no more than 20% higher than that of the zirconia in another embodiment. The coefficient of thermal expansion may be an important variable in the selection of the mix material in applications where coating life is a primary concern. In applications where thermal protection is a primary concern, the thermal conductivity may become a more important consideration to be balanced against coating life. The mix material combined with zirconia advantageously has an incipient melting point sufficiently low so that particles of the material are at least partially melted during a low velocity oxygen fuel (LVOF) process so that the combined particle mix may be applied to a component of a machine such as a gas turbine by using LVOF equipment. The low velocity oxygen fuel process may be an oxy-acetylene flame spray (OFS), for example, or it may be a low velocity oxy-fuel process that utilizes hydrogen or other fuel.  
      The coefficient of thermal expansion (10 e 6 /° K.), thermal conductivity (W/mK) and melting point (° C.) of 8% yttria stabilized zirconia (8YSZ), alumina (Al 2 O 3 ), calcium titanate (CaTiO 3 ), strontium titanate (SrTiO 3 ) and sodium-zirconium-phosphate-silicate (NZPS) are as shown in Table 1. NZPS is a family of materials that can have several different stoichiometries. The values provided in Table 1 are for the specific combination of Na 3 Zr 2 Si 2 PO 12 , although other stoichiometries of NZPS are included within the scope of the present invention.  
                           TABLE 1                       MATERIAL   COE 10e −6 /° K.   conductivity W/mK   MP ° C.                                                8YSZ   12.0   2.0   2,700       Al 2 O 3     ˜8.0   ˜30   2,100       CaTiO 3     ˜14.0   4.4   1,975       SrTiO 3     ˜11.4   2.3   2,080       NZPS   ˜6   1.75   1,275                  
 
      Both calcium titanate and strontium titanate exhibit coefficients of thermal expansion that are closer to that of zirconia than that of alumina. The thermal conductivities of these materials are also close to that of the zirconia, especially when compared to the thermal conductivity of alumina, which is much higher than (an order of magnitude higher than) that of zirconia. The melting points of these materials are all lower than that of alumina and are sufficiently low so that particles of these materials that are delivered by a low velocity oxy-fuel process will be completely or at least partially melted to a degree sufficient to allow the materials to be effectively applied by this process.  
       FIG. 1  illustrates a low velocity oxy-fuel system  10  being used to spray a composite powder  12 . The composite powder  12  may include a first constituent  14  that is a relatively high melting point ceramic material that normally cannot be applied with a LVOF process, for example either stabilized or unstabilized zirconia. The composite powder  12  also includes a second constituent  16  that is a relatively low melting point ceramic material that can be at least partially melted or fully melted and successfully applied by a LVOF process, for example calcium titanate or strontium titanate. The two constituents are mixed together to form a homogeneous mixture prior to spraying, such as by ball milling or by wet chemical mixing. The portion of the composite powder  12  that is the low melting temperature material may range from less than or at least 20 vol. % to 40 vol. %, or more, of the composite powder  12 . While the proportions may vary for different materials and application temperature ranges, for the specific application of a gas turbine hot gas path component, the proportion of low-melting component will generally fall within the range of 20-40 vol. %. Particle sizes may be selected to ensure the proper operation of the LVOF system  10 , such as in the range from −120+325 mesh, from −140+325 mesh, or from −150+325 mesh for example.  
      Prior art low velocity oxy-fuel processes have not been used successfully to deposit zirconia due to the high melting point of zirconia. The prior art thermal spray processes used to apply zirconia coatings have included high velocity oxy-fuel (HVOF) and plasma spray. These processes are not useful for in-situ repairs of machines such as gas turbines due to the high temperature, high particle velocity, and/or high sound energy levels produced. The Figure illustrates a damaged region  18  of an existing coating  20  on a component  22  being repaired by the deposition of a repair coating  24  with the component  22  in place in its operating position within a machine of which it forms a part. Access is provided to the damaged region  18  without removing the component  22  from the machine. The damaged region may be cleaned with any known cleaning process, such as by grit blasting or chemical cleaning. The repair coating  24  may be applied onto the substrate  22 , onto a bond coat layer (not shown) covering the substrate  22 , or onto a portion of the existing coating  20 . Repair coating  24  may be applied to any desired thickness, such as in the range of 8-35 mils, for example.  
      The coefficient of thermal expansion of sodium-zirconium-phosphate-silicate is lower than that of alumina. However, NZPS does exhibit a thermal conductivity that is lower than that of both alumina and zirconia, and it also has the lowest melting temperature of the materials described above. NZPS may be selected as the low-melting temperature powder  16  for applications where thermal conductivity is especially important.  
      One may appreciate that it is possible to use a LVOF process to apply a variety of relatively high melting temperature ceramic powders  14  that are normally not successfully applied with LVOF by combining the high melting temperature powder  14  with a low melting temperature powder  16  in the LVOF process. A typical cross-section of the resulting coating  24  is illustrated in  FIG. 2 . The lower melting temperature constituent  16  has been at least partially melted by the spray process and has re-solidified to form splats  26 . The splats  26  surround and encase the unmelted or potentially partially melted particles of the high melting temperature material  14 . Complete melting of the low melting temperature particles  16  is not necessary. Surface melting of the particles  16  is sufficient. It may be difficult to quantify a specific amount of melting because a number of variables can affect the coating microstructure. Test data may be useful for identifying an acceptable microstructure for a particular application. The two constituent particles  14 ,  16  will sinter during a subsequent high temperature heat treatment and/or during the subsequent operation of the component. The resulting coating  24  is relatively porous when compared to a plasma sprayed coating (typically 10-15% void fraction), with a typical void percentage being in the range of 20-25%.  
      While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.