REVERSE REACTION SINTERING OF Si3N4/SiC COMPOSITES

Abstract

A method of making a composite sintered silicon nitride/silicon carbide body, including mixing a predetermined amount of silicon nitride powder with a predetermined amount of silicon carbide powder, heat-treating the resultant mixed powder at a temperature of between about 800 and 1500 degrees Celsius in a substantially nitrogen sintering atmosphere, and producing a thin film of silica around individual silicon nitride and silicon carbide grains. The thin film of silica is useful in retarding the diffusion of oxygen to the silicon nitride particles, slowing their oxidation. The pressure of the sintering atmosphere is not substantially greater than atmospheric pressure.

1 . A method of manufacturing a composite sintered body of silicon nitride and silicon carbide comprising:
 (a) mixing a predetermined amount of silicon nitride powder with a predetermined amount of silicon carbide powder to produce a resultant mixed powder;   (b) heat-treating the resultant mixed powder at a temperature of between about 800 and 1500 degrees Celsius in a substantially nitrogen sintering atmosphere;   (c) producing a thin film of silica around individual silicon nitride and silicon carbide grains;   (d) retarding the diffusion of oxygen through the silica film;   (e) retarding the oxidation of silicon nitride particles to produce silica and nitrogen gas;   (f) substantially eliminating porosity from the compact;   (g) sintering individual grains together; and   (h) cooling the sintered body to yield a sintered substantially silicon nitride/silicon carbide body;   wherein the substantially nitrogen sintering atmosphere has a finite and predetermined oxygen partial pressure;   wherein the pressure of the substantially nitrogen sintering atmosphere is not substantially greater than atmospheric pressure; and   wherein the combined heat-treating temperature and oxygen partial pressure of the sintering atmosphere minimizes the oxidation of silicon nitride while maximizing the sintering of the silicon nitride/silicon carbide body.   
   
   
       2 . The method of  claim 1  wherein the sintering occurs at a temperature in the range of between about 800 degrees Celsius and about 1200 degrees Celsius. 
   
   
       3 . The method of  claim 1  wherein the resultant mixed powder has a composition of about 85 weight percent silicon carbide and about 15 weight percent silicon nitride. 
   
   
       4 . The method of  claim 1  wherein the resultant mixed powder has a composition of about 80 weight percent silicon carbide, about 15 weight percent silicon nitride, and about 5 weight percent silicon metal. 
   
   
       5 . The method of  claim 1  wherein the resultant mixed powder has a composition of about 75 weight percent silicon carbide, about 15 weight percent silicon nitride, and about 10 weight percent silicon dioxide. 
   
   
       6 . The method of  claim 1  wherein the sintered body is substantially comprised of grains of silicon carbide distributed in a Si 2 N 2 O matrix. 
   
   
       7 . The method of  claim 1  wherein the sintered body is substantially comprised of grains of silicon carbide and grains of silicon nitride distributed in a Si 2 N 2 O matrix. 
   
   
       8 . The method of  claim 1  wherein the sintered body 6 wherein the silicon carbide grains have a thin outer layer of silicon dioxide. 
   
   
       9 . The method of  claim 1  wherein the substantially nitrogen sintering atmosphere is air. 
   
   
       10 . (canceled) 
   
   
       11 . (canceled) 
   
   
       12 . (canceled) 
   
   
       13 . (canceled) 
   
   
       14 . (canceled) 
   
   
       15 . (canceled) 
   
   
       16 . A method for making a SiC/Si 3 N 4  sintered composite body, comprising:
 a) forming a green body of a mixture of SiC powder, Si 3 N 4  powder, and a third silicon-containing powder;   b) heating the green body to a temperature of between about 800 and about 1450 degrees Celsius in a low oxygen partial pressure atmosphere;   c) forming a silicon-containing liquid throughout the body;   d) infiltrating substantially all open porosity with the silicon-containing liquid;   e) partially oxidizing Si 3 N 4  grains to form SiO 2  and nitrogenous byproducts;   f) sintering the green body to form a sintered body comprising SiC and Si 3 N 4  grains; and   g) forming a substantially silica layer around the SiC and Si 3 N 4  grains;   wherein the mixture of SiC powder, Si 3 N 4  powder, and a third silicon-containing powder includes SiC and Si 3 N 4  present in a weight ratio of between about 4:1 and about 7:1.   
   
   
       17 . The method of  claim 16  wherein the third silicon-containing powder is selected from the group consisting of silicon metal, silica, and mixtures thereof. 
   
   
       18 . The method of  claim 16  wherein the low oxygen partial pressure atmosphere is air. 
   
   
       19 . The method of  claim 16  wherein the low oxygen partial pressure atmosphere is substantially nitrogen. 
   
   
       20 . The method of  claim 16  wherein the low oxygen partial pressure atmosphere is characterized by a small but finite oxygen partial pressure. 
   
   
       21 . A method for making a substantially densified SiC/Si 3 N 4  body, comprising:
 a) mixing SiC grains, Si 3 N 4  grains, and a third silicon-containing powder to produce an admixture;   b) forming a green body from the admixture;   c) heating the green body to a temperature of between about 800 and about 1450 degrees Celsius in a low oxygen partial pressure atmosphere to produce a heated body;   d) coating substantially all of the SiC and Si 3 N 4  grains with respective substantially liquid silica films; and   e) cooling the heated body to yield a densified body comprising SiC and Si 3 N 4  grains; and   wherein the mixture of SiC grains, Si 3 N 4  grains, and a third silicon-containing powder includes SiC and Si 3 N 4  present in a weight ratio of between about 4:1 and about 7:1.   
   
   
       22 . The method of  claim 21  and further comprising f) infiltrating substantially all open porosity of the SiC grains with a silicon-containing liquid. 
   
   
       23 . The method of  claim 21  wherein the substantially liquid silica films function to slow the diffusion of oxygen to the respective grain surfaces to prevent complete oxidation of the grains while the body is heated. 
   
   
       24 . The method of  claim 21  and further comprising g) disposition of Si 2 N 2 O crystals on the Si 3 N 4  grains.