Patent Publication Number: US-2021163363-A1

Title: Ceramic material, method of production, layer and layer system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2018/083005 filed 29 Nov. 2018, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2017 223 879.8 filed 29 Dec. 2017. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to erbium oxide-stabilized zirconium oxide which can, in particular, be used as ceramic thermal barrier layer. 
     BACKGROUND OF INVENTION 
     Modern gas turbines having a high efficiency require thermal barrier layers which should not have any pronounced phase transitions in the entire operating range or temperature range. In addition, the crack resistance should not be adversely affected by thermal stress and sintering associated therewith. The thermal barrier layers used today, i.e. gadolinium zirconate or a zirconium oxide stabilized with 33.5 mol % of Y 2 O 3 , which crystallize in a fluorite or face centered cubic structure, have a crack resistance which is only about ⅓ of that of zirconium oxide partially stabilized with 4 mol % of Y 2 O 3  (YSZ). 
     In the case of the 4 mol % YSZ system known hitherto, which on the basis of most recent knowledge may possibly be capable of use up to about 1623 K, the crack resistance increases with increasing sintering temperature, with the crack resistance being a factor of 3 higher than that of face centered cubic systems such as 13 mol % YSZ or gadolinium zirconate because of the tetragonally distorted lattice. In the case of the systems, the operating states then have to be matched to the tolerable stress states of the ceramic, so that the energy liberation rate of the system is not sufficient to propagate the cracks in the ceramic. 
     SUMMARY OF INVENTION 
     It is therefore an object of the invention to solve the abovementioned problem. 
     The object is achieved by a ceramic material as claimed, a process as claimed, a ceramic layer as claimed and a ceramic layer system as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  and  FIG. 2  represent working examples of the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Some systems having a purely cubic structure do not have any phase transitions in wide temperature ranges. The crack resistance is reduced with increased sintering. The system composed of 13 mol % YSZ (fully stabilized zirconium oxide) and 33.5 mol % YSZ crystallizes out in the cubic phase. Additions of erbium zirconate (Er 2 O 3 ) stabilize the systems (ErSZ) in a phase having a low symmetry and an increased crack resistance. The crack resistance is significantly increased by reducing the symmetry of the system and is maintained even under presintering or is even increased in, for example, the system 4 mol % YSZ. The new system is simple to melt and is stabilized in a phase which has a low symmetry compared to the cubic phase. In final alloy compositions composed of the three elements ZrO 2 , Er 2 O 3 , Y 2 O 3 , the composition composed of ZrO 2  and Er 2 O 3  and also ZrO 2  and Y 2 O 3  should preferably firstly be partially alloyed, the melted alloys are milled again and then blended to give a homogeneous mixture and subsequently finally homogeneously melted in an electric furnace. 
     The following compositions are particularly suitable for this purpose: ErSZ: (2-6) mol % Er 2 O 3  in ZrO 2 , preferably 3.5 mol % erbium oxide (Er 2 O 3 ), 8-30 mol % ErSZ and 48YSZ (2ZrO 2 ×Y 2 O 3 ), preferably 15 mol % erbium oxide-stabilized zirconium oxide, 8-30 mol % ErSZ and yttrium-stabilized zirconium oxide containing 13-20 mol % of Y 2 O 3  as stabilizer for ZrO 2 , preferably 15 mol % yttrium oxide-stabilized zirconium oxide. 
     The ceramic preferably consists of Er 2 O 3 , Y 2 O 3  and ZrO 2 . 
     The zirconium oxide can be partially or fully stabilized; it is preferably fully stabilized. 
     The inventive step does not lie in the application or manufacture of the layer itself but in the selection of the concentration range to be set. The material having a cubic starting structure is stabilized by the additions in a low-symmetry structure (tetragonal). 
     The process comprises using at least 10% by volume and not more than 90% by volume of erbium oxide (Er 2 O 3 )-stabilized zirconium oxide (ZrO 2 ) for the partial melt and accordingly from 90% by volume to 10% by volume of Y 2 O 3 —ZrO 2 . 
     The following compositions are preferred for the partial melt:—fully stabilized zirconium oxide, in particular Y 2 O 3 -stabilized zirconium oxide, very particularly preferably zirconium oxide stabilized by 33.5 mol % of Y 2 O 3 , —Y 2 O 3 -stabilized zirconium oxide (ZrO 2 ), in particular zirconium oxide (ZrO 2 ) stabilized by 13-20 mol % of Y 2 O 3 , and also—from 2 mol %-6 mol %, in particular 3.5 mol %, erbium oxide (Er 2 O 3 )-stabilized zirconium oxide (ZrO 2 ),—8 mol %-30 mol %, in particular 15 mol %, erbium oxide (Er 2 O 3 )-stabilized zirconium oxide (ZrO 2 ). 
     This partial melt of Er 2 O 3  and ZrO 2  can be combined in any way. 
       FIG. 1  shows, in simplified form, the layer system  1  which comprises a metallic substrate  4  or a ceramic substrate, in particular composed of CMC. 
     An either metallic or ceramic bonding layer  7 , in particular an NiCoCrAlY layer in the case of a metallic substrate, is present on the substrate  4 , and on top of this bonding layer there is at least one ceramic layer  10  based on the ceramic material according to the invention. 
       FIG. 2  shows a further variant  1 ′ in which the ceramic layer  11  is made up of two layers and in addition to the bonding layer has a ceramic layer  8  located underneath in order to match the coefficient of thermal expansion. 
     This layer  8  can in this case be zirconium oxide which is stabilized with yttrium oxide and does not comprise any erbium oxide.