Thermal barrier coating systems and processes therefor

Abstract

Coating systems and processes by which the coating systems can be deposited to be resistant to contaminants, and particularly resistant to infiltration and damage caused by CMAS. The coating systems include inner and outer ceramic layers, each having a microstructure characterized by splats and horizontal porosity. The inner ceramic layer consists essentially of zirconia stabilized by about 6 to about 9 weight percent yttria. The outer ceramic layer overlies and contacts the inner ceramic layer to define the outermost surface of the coating system. The outer ceramic layer consists essentially of zirconia stabilized by about 25 to about 75 weight percent yttria, has a thickness that is less than the thickness of the inner ceramic layer, and has a porosity level that is lower than that of the inner ceramic layer.

1 . A process of forming a coating system on a component, the process comprising:
 depositing a bond coat on a surface of the component;   depositing an inner ceramic layer on the bond coat using a thermal spray process to yield a microstructure characterized by splats and horizontal porosity, the inner ceramic layer consisting essentially of zirconia stabilized by about  6  to about  9  weight percent yttria and being deposited to have a thickness and a porosity level;   depositing an outer ceramic layer on the inner ceramic layer using a thermal spray process to yield a microstructure characterized by splats and horizontal porosity, the outer ceramic layer defining an outermost surface of the coating system and consisting essentially of zirconia stabilized by about 25 to about 75 weight percent yttria, the outer ceramic layer being deposited to have a porosity level that is higher than the inner ceramic layer and to have a thickness that is less than the thickness of the inner ceramic layer such that a ratio of the thickness of the outer ceramic layer to that of the inner ceramic layer is less than 1.0; and then   heat treating the inner and outer ceramic layers in a vacuum to a temperature and for a duration sufficient to relieve stresses therein induced by the depositing steps.   
     
     
         2 . The process according to  claim 1 , wherein the heat treating step is performed at a temperature of about 1050 to about 1080° C. in a vacuum for a duration of about two to about four hours. 
     
     
         3 . The process according to  claim 1 , wherein the ratio of the thickness of the outer ceramic layer to that of the inner ceramic layer is not greater than 0.5. 
     
     
         4 . The process according to  claim 1 , wherein the porosity level of the outer ceramic layer throughout the thickness thereof is less than that of the inner ceramic layer. 
     
     
         5 . The process according to  claim 4 , wherein the outer and inner ceramic layers are deposited so that the porosity level of the inner ceramic layer is about 10 to about 25 volume percent 
     
     
         6 . The process according to  claim 4 , wherein the outer and inner ceramic layers are deposited so that the porosity level of the outer ceramic layer is about 3 to about 15 volume percent. 
     
     
         7 . The process according to  claim 1 , wherein the outer ceramic layer is deposited to consist essentially of zirconia stabilized by about 38 to less than 55 weight percent yttria and the outer and inner ceramic layers define a thickness ratio of not greater than 0.5. 
     
     
         8 . The process according to  claim 1 , wherein the outer ceramic layer is deposited to consist essentially of a cubic crystal phase and the inner ceramic layer is deposited to consist essentially of a tetragonal or modified tetragonal crystal phase. 
     
     
         9 . The process according to  claim 1 , wherein the component is a gas turbine engine component and the process further comprises reacting the outer ceramic layer with a deposit containing calcia, magnesia, alumina and silica to form calcium yttrium silicate. 
     
     
         10 . The process according to  claim 1 , wherein the inner ceramic layer is deposited to have a thickness of at least 50 to about 500 micrometers. 
     
     
         11 . The process according to  claim 1 , wherein the outer ceramic layer is deposited to have a thickness of up to 250 micrometers. 
     
     
         12 . The process according to  claim 1 , wherein the outer ceramic layer is deposited to have a thickness of at least about 25 micrometers. 
     
     
         13 . The process according to  claim 1 , wherein the outer ceramic layer reacts with a eutectic compound containing calcia, magnesia, alumina and silica to form calcium yttrium silicate at temperatures above 1200° C. 
     
     
         14 . The process according to  claim 1 , wherein the bond coat is a metallic bond coat chosen from the group consisting of MCrAlX overlay coatings and/or diffusion aluminide coatings. 
     
     
         15 . The process according to  claim 1 , wherein the component is a gas turbine engine component formed of a nickel-base or cobalt-base superalloy. 
     
     
         16 . The process according to  claim 1 , wherein the component is chosen from the group consisting of high and low pressure turbine vanes and blades, shrouds, combustor liners and augmentor hardware of a gas turbine engine.