Tough cordierite glass-ceramics

Abstract

An internally nucleated, toughened cordierite glass-ceramic is disclosed. The cordierite glass-ceramic has good oxidation resistance and fracture toughness and coefficient of thermal expansion rivaling that of silicon nitride. The glass-ceramic may be cast as a liquid. Annealing produces a material of high crystallinity combining high hardness, high Young's modulus, good thermal stability, high strength, low density and good dielectric properties. The glass-ceramic comprises interlocking crystalline phases dominated by cordierite and a second phase having an elongated or acicular structure. A third phase may comprise a crystalline ceramic that promotes acicularity of the second phase. The third phase is preferably capable of twinning.

1 . A glass-ceramic having high fracture toughness and low coefficient of thermal expansion, the glass-ceramic formed from a composition comprising, in weight percent: 
     
       
         
               
               
               
             
                   
                   
               
                   
                 SiO 2   
                 35-50% 
               
                   
                 Al 2 O 3   
                 10-35% 
               
                   
                 MgO 
                 10-25% 
               
                   
                 TIO 2   
                  7-20% 
               
                   
                 CaO 
                 up to 1% 
               
                   
                 SrO 
                  up to 10% 
               
                   
                 F 
                 up to 5% 
               
                   
                   
               
           
              
             
             
              
              
              
              
              
              
              
              
             
          
         
       
       provided that the sum of CaO and SrO comprises at least 0.5%, and wherein the glass-ceramic includes a microstructure comprising interlocking first and second crystal phases in which at least one phase is acicular. 
     
   
   
       2 . The glass-ceramic of  claim 1 , wherein the first phase comprises hexagonal cordierite. 
   
   
       3 . The glass-ceramic of  claim 1 , wherein the second phase is acicular and comprises a titanate. 
   
   
       4 . The glass-ceramic of  claim 3 , wherein the titanate is selected from a group consisting of magnesium titanate, aluminum titanate and combinations thereof. 
   
   
       5 . The glass-ceramic of  claim 2 , wherein the first phase comprises from 50-80 vol. % of the glass-ceramic. 
   
   
       6 . The glass-ceramic of  claim 1 , wherein the acicular phase comprises from 8-20 vol. % of the glass-ceramic. 
   
   
       7 . The glass-ceramic of  claim 1 , wherein the glass-ceramic comprises up 20% of a third crystal phase comprising a twinning ceramic compound. 
   
   
       8 . The glass-ceramic of  claim 7 , wherein the twinning ceramic compound is selected from a group consisting of anorthite, Sr-feldspar, Ba-feldspar, enstatite and aluminous enstatite. 
   
   
       9 . The glass-ceramic of  claim 1  wherein the glass-ceramic includes up to 20% of a third crystal selected from the group consisting of fluormica, fluoramphibole, norbergite, spinel, sapphirine, mullite, forsterite and xonotlite. 
   
   
       10 . The glass-ceramic of  claim 7 , wherein the glass-ceramic comprises up to 20 vol. % of the third phase. 
   
   
       11 . The glass-ceramic of  claim 1 , wherein the glass-ceramic has a CTE from 20-50×10 −7 /° C. in the temperature range 25-1000° C. and a Knoop hardness greater than 800. 
   
   
       12 . The glass-ceramic of  claim 1 , wherein the glass-ceramic has a Young's modulus greater than 10 GPa. 
   
   
       13 . The glass-ceramic of  claim 1 , wherein the glass-ceramic has a fracture toughness from 2.5 to 6.0 MPa·m 0.5 . 
   
   
       14 . A method of forming a toughened, cordierite glass-ceramic article, the method comprising:
 a) blending a composition comprising, in weight percent, 35-50% SiO 2 , 10-35% Al 2 O 3 , 10-25% MgO, 7-20% TiO 2 , up to 5% CaO, up to 10% SrO, where the amount of CaO and SrO comprises at least 0.5%, and F is up to 5%;   b) heating the composition to a fluid glass;   c) forming the composition to a desired shape;   d) cooling the shape to solid glass;   e) annealing the solid glass at 1100-1300° C. for a sufficient amount of time to achieve a crystallinity of at least 85 vol. %.   
   
   
       15 . The method of  claim 14 , wherein forming includes casting the fluid glass. 
   
   
       16 . The method of  claim 14 , wherein the method further includes heat-treating after cooling the shape and before annealing, where heat-treating includes heating the shape to at least 800° C. for more than one hour. 
   
   
       17 - 23 . (canceled)