Silicon nitride composite sintered body and process for producing same

A composite sintered body of silicon nitride-silicon carbide containing 1 to 30% by volume of silicon carbide as a dispersed phase in the silicon nitride, wherein an amorphous phase of silicon carbide is present in the grain boundaries of silicon nitride and microcrystals of silicon carbide having particle sizes of 5 to 500 nm. are dispersed in said amorphous phase of silicon carbide.

The composite sintered body of silicon nitride-silicon carbide according to claim (4), wherein the proportion of the amorphous phase of silicon carbide is 20 to 99% by volume based on the total amount dispersed of silicon carbide.

A process for producing a silicon nitride composite sintered body comprising;

• adding at least two sintering aids selected from the group consisting of alumina, yttria, magnesia, and aluminum nitride to an amorphous composite powder starting material comprising mixed powders of amorphous silicon nitride and carbon, to form a green compact, in which said sintering aid is present in a total amount of 1% to 15% by weight;
• firing, as a primary firing, the green compact in a nitrogen atmosphere by a hot pressing or an atmospheric pressure sintering process at 1350.degree. C. to 1650.degree. C. to form a solid solution of carbon and silicon nitride, crystallize said silicon nitride into .alpha.-silicon nitride, and simultaneously effect phase transition to a .beta.-crystal phase;
• and firing at 1800.degree. C. to 2200.degree. C. to precipitate .alpha.- or .beta.-silicon carbide crystals in particles of the silicon nitride, and to transfer a portion of the silicon carbide crystals precipitated in the particles of silicon nitride to grain boundaries thereof.

A process for producing a silicon nitride composite sintered body comprising;

• conducting a primary firing of an amorphous composite powder, comprising amorphous silicon nitride and carbon, in a nitrogen atmosphere by a hot pressing or an atmospheric-pressure sintering process at 1350.degree. to 1600.degree. C., to form a solid solution of carbon and silicon nitride, crystallize said silicon nitride into .alpha.-silicon nitride, and cause a phase transition to a .beta.-crystal phase to form a resulting material;
• adding to the resulting material at least two sintering aids selected from the group consisting of alumina, yttria, magnesia, and aluminum nitride, in a total amount of 1% to 15% by weight;
• firing the resulting mixture at a temperature of 1800.degree. C. to 2200.degree. C. to precipitate .alpha.- or .beta.-silicon carbide crystals in the particles of the silicon nitride, and transfer a portion of the silicon carbide crystals precipitated in particles of the silicon nitride to grain boundaries thereof.