Sintered silicon nitride

Abstract

Sintered silicon nitride products comprising predominantly β-silicon nitride grains in combination with from about 0.1 to 30 mole % silicon carbide, and grain boundary secondary phases of scandium oxide and scandium disilicate. Such products have high fracture toughness, resistance to recession, and resistance to oxidation at temperatures of at least 1500° C. Methods for preparing sintered silicon nitride products are also disclosed.

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         5 . The sintered silicon nitride product of  claim 13 , wherein said sintered silicon nitride product has a fracture toughness, as measured by the indentation strength method at a 20 kg indentation load, of about 7.5 MPa.m0.5.  
     
     
         6 . The sintered silicon nitride product of  claim 11 , wherein said sintered silicon nitride product has an oxidation weight gain at a temperature of 1500° C. of 0.36 mg.cm −2 .  
     
     
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         9 . The sintered silicon nitride product of  claim 11 , wherein said sintered silicon nitride product has a sintered density of about 3.19 g.cm−3.  
     
     
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         11 . A sintered silicon nitride product comprising silicon nitride of about 84 weight %, silicon carbide of about 5 weight %, scandium oxide of about 7 weight %, and silicon dioxide of about 4 weight %.  
     
     
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         13 . The silicon nitride product of  claim 11 , wherein said product has a fracture toughness of about 7.5 Mpa/m0.5.  
     
     
         14 . The silicon nitride product of  claim 11 , wherein said product has a modulus of rupture of about 93.4 ksi.  
     
     
         15 . The silicon nitride product of  claim 11 , wherein said product has a Weibull distribution slope of about 13.82.  
     
     
         16 . The silicon nitride product of  claim 11 , wherein said product is a hot section component of a gas turbine engine.  
     
     
         17 . A method of preparing a sintered silicon nitride product, comprising: 
 a) providing a starting powder mix, said starting powder mix comprising silicon nitride of about 84 weight %, silicon carbide of about 5 weight %, and scandium oxide of about 7 weight %;    b) forming a green body from said starting powder mix; and    c) sintering said green body to form said sintered silicon nitride product, wherein said step c) comprises gas pressure sintering and includes a plurality of sintering stages, and wherein said sintered silicon nitride product comprises from about 60 to 95 mole % silicon nitride, from about 2.5 to 25 mole % silicon carbide, from about 1 to 10 mole % scandium disilicate, and from about 1 to 8 mole % scandium oxide, said sintered silicon nitride product having a fracture toughness in the range of from about 7.7 to 8.5 MPa.m 0.5 .