Method to prevent low temperature degradation of zirconia

Abstract

The invention is directed to a method of producing the material that is unaffected by the low-temperature degradation, humidity-enhanced phase transformation typical of yttria-stabilized zirconia in general, as well as of yttria-stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP). Because of the high fracture toughness and high mechanical strength, this class of materials is widely used, including as implants, such as for the packaging material for small implantable neural-muscular sensors and stimulators. The destructive phase transformation is eliminated by converting the surface to stable cubic or T-prime zirconia by post-densification thermal treatment in a cation-rich milieu.

1 . A method of producing a stable tetragonal zirconia polycrystal ceramic having an outer surface, comprising the steps of: 
 selecting a densified tetragonal zirconia polycrystalline ceramic;    placing said ceramic in a powder bed of a cation-rich material; and    converting said outer surface to a stable ceramic phase at a controlled temperature, at a controlled pressure, and in a controlled atmosphere to achieve an average grain size of less than about 0.5 micron, thereby substantially eliminating low-temperature degradation of said polycrystal ceramic.    
     
     
         2 . The method according to  claim 1 , further comprising the step of stabilizing said tetragonal zirconia polycrystal ceramic with yttria.  
     
     
         3 . The method according to  claim 1 , further comprising the step of stabilizing said tetragonal zirconia polycrystal ceramic with three mole percent of yttria.  
     
     
         4 . The method according to  claim 1 , further comprising the step of stabilizing said tetragonal zirconia polycrystal ceramic with at least one oxide selected from calcia, magnesia, and ceria.  
     
     
         5 . The method according to  claim 1 , comprising the step of converting said outer surface at a controlled temperature of 1400° C. to 1800° C.  
     
     
         6 . The method according to  claim 1 , comprising the step of converting said outer surface at a controlled temperature of 1600° C. to 1700° C.  
     
     
         7 . The method according to  claim 6 , comprising the step of converting said outer surface at between about 1600° C. to 1700° C. for at least about one and less than about two hours.  
     
     
         8 . The method according to  claim 1 , comprising the step of converting said outer surface to cubic phase.  
     
     
         9 . The method according to  claim 1 , comprising the step of converting said outer surface to T-prime phase.  
     
     
         10 . The method according to  claim 1 , comprising the step of coating said outer surface with a hermetic coating.  
     
     
         11 . The method according to  claim 1 , comprising the step of thermally treating said stabilized tetragonal zirconia polycrystal ceramic in a bed of cation-rich material.  
     
     
         12 . The method according to  claim 11 , comprising the step of thermally treating said stabilized tetragonal zirconia polycrystal ceramic in a powder bed of cation-rich material.  
     
     
         13 . The method according to  claim 11 , comprising the step of thermally treating said stabilized tetragonal zirconia polycrystal ceramic in a bed of yttria-rich material.