Smooth outer coating for combustor components and coating method therefor

Abstract

A coating and method for overcoating a TBC on a component used in a high-temperature environment, such as the combustor section of an industrial gas turbine. The coating defines the outermost surface of the component and is formed of at least two layers having different compositions. An inner layer of the coating contains alumina in a first silica-containing matrix material that is free of zinc titanate. An outer layer of the coating contains alumina, a glass material, and zinc titanate in a second silica-containing matrix material. The outer layer of the coating has a surface roughness of not greater than three micrometers Ra and forms the outermost surface of the component. The coating reduces the component temperature by reducing the convective and radiant heat transfer thereto.

1 . A method of reducing convective and radiant heat transfer to a combustor component of a gas turbine, the method comprising the steps of: 
 depositing a thermal barrier coating on the component;    preparing first and second slurries, the first slurry being free of zinc titanate and containing first and second alumina particles in a first silica-forming binder material, the first alumina particles having a particle size range of about 3 to about 6 micrometers, the second alumina particles having a particle size range of about 0.05 to about 0.8 micrometers, the second slurry containing third alumina particles, a glass material, and zinc titanate in a second silica-forming binder material, the third alumina particles having a particle size range of less than the first and second alumina particles;    depositing and firing the first and second slurries to form first and second layers of a multilayer outer coating overlying the thermal barrier coating, the first layer being free of zinc titanate and comprising alumina in a first silica-containing matrix material, the second layer comprising alumina, the glass material, and the zinc titanate in a second silica-containing matrix material;    wherein the second layer defines an outermost surface of the component, the outermost surface having a surface roughness of not greater than 3 micrometers Ra.    
   
   
       2 . A method according to  claim 1 , wherein the first alumina particles constitute about 20 to about 40 weight percent of the first slurry.  
   
   
       3 . A method according to  claim 1 , wherein the second alumina particles constitute about 20 to about 55 weight percent of the first slurry.  
   
   
       4 . A method according to  claim 1 , wherein the third alumina particles constitute about 25 to about 65 weight percent of the second slurry.  
   
   
       5 . A method according to  claim 1 , wherein the first and second slurries are deposited to yield the outer coating having a thickness of less than the thermal barrier coating and the outermost surface has a surface roughness of not greater than 1 micrometer Ra.  
   
   
       6 . A method according to  claim 1 , wherein the first slurry consists of the first and second alumina particles, the first silica-forming binder material, and a carrier liquid that is eliminated during the firing of the first slurry.  
   
   
       7 . A method according to  claim 1 , wherein the second slurry consists of the third alumina particles, the glass material, the zinc titanate, the second silica-forming binder material, and a carrier liquid that is eliminated during the firing of the second slurry.  
   
   
       8 . A method according to  claim 1 , further comprising depositing a bond coat on the component prior to depositing the thermal barrier coating.  
   
   
       9 . A method according to  claim 8 , wherein the bond coat has a chemical composition consisting essentially of nickel, chromium, aluminum, yttrium and incidental impurities, and the bond coat has an average surface roughness Ra of at least about 8 micrometers.  
   
   
       10 . A method according to  claim 1 , wherein the thermal barrier coating is deposited by air plasma spraying.  
   
   
       11 . A method according to  claim 10 , wherein the thermal barrier coating has a chemical composition consisting essentially of zirconia, yttria and incidental impurities.