Abstract:
A process for extending the cyclic service life of thermal barrier coatings made of yttrium-stabilized zirconium oxide (YSZ) or the like which have been applied to a substrate with an oxidizing bond coat in between includes increasing or long-term stabilizing the strain tolerance of the thermal barrier coating.

Description:
This application is a divisional application of U.S. Pat. No. 12/014,893, now U.S. Pat. No. 8,021,720, filed Jan. 16, 2008 which claims priority to Swiss Patent Application No. CH 00061/07, filed Jan. 16, 2007. The entire disclosure of both applications is incorporated by reference herein. 
    
    
     The present invention deals with the field of thermal machines. It relates to a process for extending the cyclic service life of thermal barrier coatings, in particular, thermal barrier coatings made of yttrium-stabilized zirconia or related chemical compositions. 
     BACKGROUND 
     In thermally stressed gas turbine components, such as for example guide vanes or rotor blades or liners in the combustion chamber, Ni-based superalloys, the surfaces of which are additionally protected by the application of a ceramic thermal barrier coating, are often used in order to reach high operating temperatures and thereby to achieve improved efficiencies. An example of a structure of a component of this type in the form of a turbine blade is illustrated in excerpt form in  FIG. 1 . A bond coat (BC)  12 , for example made of MCrAlY or PtAl, has been applied to the substrate  11  composed of a Ni-base superalloy. The actual thermal barrier coating (TBC)  14  has been applied to the bond coat  12 . At high temperatures, a thermally grown oxide layer (TGO)  13  composed predominantly of α-Al 2 O 3  is formed at the interface between thermal barrier coating  14  and bond coat  12 , which slows down the oxidation of the bond coat. 
     The service life of the thermal barrier coating  14  under cyclic oxidation is dependent not only on the thickness of the thermally grown oxide layer  13  (on account of increasing stresses at the interface as the oxidation increases) but also on the strain tolerance of the thermal barrier coating  14 . The service life is limited in particular by delamination from the substrate  11 . 
     The mechanical properties of the thermal barrier coating  14  are determined by various mechanisms: 
     The following are driving forces in the TBC delamination:
     A1: transient strains ε1 in the event of thermal shocks (temperature gradients for example when starting up and shutting down a gas turbine) and   A2: stationary mismatch strains ε2 (mismatch between the thermal expansions of adjacent layers for example during steady-state gas turbine operation)   A3: intrinsic strains ε3 caused by TGO growth at the TBC/BC interface (oxidation rate of the bond coat  12 )   A4: although further strain components (e.g. mechanical) are present, they can substantially be ignored for the use of coated parts in gas turbines.   

     The result of the local strains A1-A4 is as follows for the local stress σ TBC  in the TBC:
     A5: σ TBC =E TBC  Σεi, where i=1 . . . 4; E TBC : macroscopic modulus of elasticity of the TBC   

     The driving forces in the TBC delamination are in some cases independent of operating time (component A1, A2, A4) and in some cases dependent on operating time (component A3). 
     Resistance to TBC Delamination: 
     According to linear fracture mechanism theory, the thermal barrier coating fails as soon as the energy release rate G reaches a critical value G C , where: 
     B1: G&gt;=G C =π(σ TBC ) 2  a/E TBC =(K IC ) 2 /E TBC  with critical crack length a, fracture toughness of the TBC K IC  and σ TBC  according to A5. 
     (Source: Anderson, T. L.; Fracture Mechanics; 1994; ISBN 0-8493-4260-0; p. 16) 
     SUMMARY OF THE INVENTION 
     The thermal barrier coating therefore fails (schematically) as soon as the local stress σ TBC  (A5) exceeds a critical value defined by B1. A high fracture toughness and a low modulus of elasticity of the TBC accordingly have the effect of lengthening the service life. However, the modulus of elasticity of the thermal barrier coating increases during the operating time as a result of sintering processes. This loss of strain tolerance often constitutes the actual factor that limits the service life of the thermal barrier coating. 
     The invention is based, in part, on the consideration that the TBC strain tolerance is substantially influenced by what are known as microcontacts in the thermal barrier coating  14  (TBC). These punctiform contact locations in the TBC bridge microcrack flanks, where they transmit the stresses. If these microcontacts are weakened, the cohesion of the TBC is reduced and the TBC becomes more strain-tolerant, and the macroscopic modulus of elasticity drops. If the microcontacts are strengthened, as by sintering processes, the TBC becomes more rigid and for the same imposed strain (A1, . . . A4) the local stresses (A5) rise until the critical energy release rate B1 is reached and the TBC flakes off. This is important in particular if the progressing sintering processes are accelerated by oxidic impurities in the TBC (see in this respect for example the thesis by F. Szücs; Thermomechanische Analyse und Modellierung plasmagespritzter und EB-PVD aufgedampfter Wärmedämmschichtsysteme für Gasturbinen [thermomechanical analysis and modelling of plasma-sprayed and EB-PVD thermal barrier coating systems for gas turbines], VDI Fortschrittsberichte, volume 5, no. 518, VDI Verlag GmbH Diisseldorf, Deutschland, ISBN 3-18351805-8, pages 47, 96-102). 
     A macroscopic modulus of elasticity, which is reduced relative to thermal barrier coatings that have not undergone any aftertreatment, of plasma-sprayed thermal barrier coatings which have subsequently been infiltrated in aqueous hydrofluoric acid, is described in the abovementioned thesis by F. Szücs on page 104-106. It was in this way possible to demonstrably increase the TBC strain tolerance and cyclic service life of the coating system. 
     An object of the invention is to provide a process by which the cyclic service life of the thermal barrier coatings on, for example, gas turbine blades and vanes and other components in the hot gas path can be lengthened in a simple way. 
     The present invention provides a process for extending the cyclic service life of thermal barrier coatings made of yttrium-stabilized zirconia (YSZ) or related chemical compositions, which have been applied to a substrate with an oxidizing bond coat in between, wherein the strain tolerance of the thermal barrier coating is increased and/or long-term stabilized. 
     In one configuration of the process according to the invention, to increase the strain tolerance of the thermal barrier coating, punctiform contact locations or microcontacts, which bridge microcrack flanks in the thermal barrier coating, are weakened. 
     In particular, to weaken the microcontacts the SiO 2  that is present as an impurity at the inner surface of the open-pore thermal barrier coating, as well as further sinter-active oxidic impurities that are present, are decomposed. 
     In this case, the decomposition of the SiO 2  and of the further sinter-active oxidic impurities that are present is preferably effected chemically. 
     According to a first alternative, the chemical decomposition of the SiO 2  and of the further sinter-active oxidic impurities that are present takes place in the HF-containing gas phase of a fluoride ion cleaning installation. 
     According to a second alternative, the chemical decomposition of the SiO 2  and of the further sinter-active oxidic impurities that are present is effected by means of a hydrofluoric acid solution in vapor form, in particular in an autoclave. 
     According to a third alternative, the chemical decomposition of the SiO 2  and of the further sinter-active oxidic impurities that are present is effected by means of an aqueous hydrofluoric acid solution in a chemical bath. 
     In particular, the fluoride ion cleaning process is controlled, by means of the parameters partial pressure, HF gas content, temperature and time profile, in such a way that only the microcontacts are selectively weakened. 
     However, a shot-peening process can also be used to weaken the microcontacts. 
     Alternatively, a Trowal process is used to weaken the microcontacts. 
     Alternatively, an infiltration process using a medium, followed by a heat treatment, wherein the infiltrated medium undergoes a volumetric expansion during a liquid-solid phase change, is used to weaken the microcontacts. 
     A refinement of this variant is characterized in that the infiltration medium is water, which is introduced into the pores in the thermal barrier coating by immersion, spraying or via the gas phase and is briefly frozen by means of liquid nitrogen or dry ice, and in that the infiltration agent is then removed by standard drying or by means of vacuum drying. 
     In this case, preferably the penetration depth of the infiltration medium is restricted by suitable process parameters to the outer region of the thermal barrier coating, in order to minimize risks of the process having a negative effect on the adhesion of the thermal barrier coating. 
     The spatial propagation of the freezing operation can also be controlled by targeted dissipation of the heat into the component interior, so that the solidification front, starting from the interface of the thermal barrier coating, is guided in the direction of the surface of the thermal barrier coating, instead of prematurely closing off the porosity from the outside before the solidification has ended and the risks of the process having a negative effect on the adhesion of the thermal barrier coating are minimized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is to be explained in more detail below on the basis of exemplary embodiments in conjunction with the drawing, in which: 
         FIG. 1  diagrammatically depicts an excerpt from an example of a surface structure of a gas turbine blade with thermal barrier coating. 
     
    
    
     DETAILED DESCRIPTION 
     As has already been mentioned, in the context of the invention the thermal barrier coating  14  (the TBC) is to be made more strain-tolerant and sintering of the TBC in operation is to be minimized, without adversely affecting the adhesion of the layer. It is in this way possible to lengthen the cyclic service life of the thermal barrier coating. It is in this way possible to realize a higher number of cycles at the same operating temperature or a higher operating temperature for the same number of cycles. 
     In this context, the TBC strain tolerance is substantially influenced by what are known as microcontacts in the thermal barrier coating  14 . Since the thermal barrier coating has an open-pore structure, the microcontacts are accessible to gases and/or liquids. This fact is exploited in the context of the invention in order to attack and weaken the microcontacts using chemical, mechanical and physical methods. The microcontacts in the TBC can be attacked by means of the following methods:
         a. By means of HF gas phase in a fluoride ion cleaning (FIC) installation. An installation of this type and the processes which it can be used to carry out are disclosed for example in US-A1-2002/157277.   b. By means of hydrofluoric acid solution in vapor form, for example in an autoclave.   c. By means of aqueous hydrofluoric acid solution in a chemical bath.   d. By means of mild shot-peening of the TBC-coated surface, if appropriate in combination with smoothing of the TBC.   e. By means of the Trowal in combination with smoothing of the TBC.   f. By means of infiltration of a liquid which on solidification weakens the microcontacts on account of a volumetric expansion.       

     In this context, the process used should be selected in such a way that:
         the TGO  13  is not attacked and the adhesion to the bond coat  12  remains unaffected;   the SiO 2  that is present as an impurity at the inner surface of the TBC, as well as further oxidic impurities, which are significantly involved in the progressing sintering process, are selectively decomposed;   destabilization of the partially stabilized Y 2 O 3 —ZrO 2  crystal (bulk) is avoided   the TBC layer thickness remains substantially unchanged; and   damage to regions that are not TBC coated (blade or vane interior and regions with a metallic protective coating) is avoided.       

     The invention can in general terms be employed in regions in which the TBC flakes off prematurely and this has the effect of limiting service life or increasing the failure rate or repair/reconditioning costs as a result of the TBC flaking off prematurely. Experience has shown that flaking of this nature occurs in particular at rotor blades or guide vanes in the first or second row of a gas turbine and at entry segments or inlet segments of silo combustion chambers. 
     The invention is of particular interest for the development of retrofit solutions, in which an increase in service life can be achieved without changing the component casting and options for optimizing the coating have largely already been fully exploited.