Method of forming silicon carbide and silicon nitride composite
View Patent ↗Densified composites of silicon nitride and silicon carbide that exhibit high creep resistance are obtained by mechanically activating a mixture of amorphous powders of silicon nitride and silicon carbide and sintering the mechanically activated mixture in the presence of an electric field under high pressure. The grain size in the resulting composite is less than 100 nanometers for all components of the composite, and the composite exhibits high creep resistance.
1. A method for forming a dense composite of silicon nitride and silicon carbide, said method comprising:
(a) mechanically activating a powder mixture of amorphous silicon nitride and silicon carbide in the presence of at most 1% by weight of metal oxide densification aids, said powder mixture consisting essentially of particles of about 1 nanometer to 100 nanometers in diameter; and
(b) consolidating said powder mixture so activated into a continuous mass by compressing said powder mixture while passing an electric current through said powder mixture, to achieve a fused mass of silicon nitride and silicon carbide crystals.
2. The method of claim 1 in which said mechanically activated powder mixture resulting from step (a) consists essentially of particles of about 1 micron to about 10 microns in diameter, and said fused mass produced in step (b) consists essentially of crystalline grains less than 100 nm in diameter.
3. The method of claim 1 in which said mechanically activated powder mixture resulting from step (a) consists essentially of particles of about 1 micron to about 5 microns in diameter, and said fused mass produced in step (b) consists essentially of crystalline grains less than 50 nm in diameter.
4. The method of claim 1 in which any metal oxide densification aid present in said powder mixture constitutes at most about 0.5% by weight of said powder mixture of step (a).
5. The method of claim 1 in which any metal oxide densification aid present in said powder mixture of step (a) constitutes at most about 0.1% by weight of said powder mixture of step (a).
6. The method of claim 1 in which said powder mixture of step (a) is devoid of metal oxide densification aids.
7. The method of claim 1 further comprising forming said powder mixture of step (a) by pyrolysis of a polyorganosilazane in an inert atmosphere.
8. The method of claim 7 in which said polyorganosilazane is a polyureasilazane.
9. The method of claim 1 in which step (b) comprises compressing said powder mixture of step (a) at a pressure of about 10 MPa to about 200 MPa and a temperature of from about 900° C. to about 3,000° C., and said electric current is a pulsed direct current of about 1,000 A/cm 2 to about 10,000 A/cm 2 .
10. The method of claim 9 in which said pressure is about 40 MPa to about 100 MPa.
11. The method of claim 9 in which said temperature is about 1,000° C. to about 2,000° C.
12. The method of claim 9 in which said pulsed direct current is about 1,500 A/cm 2 to about 5,000 A/cm 2 .
13. The method of claim 1 in which step (a) comprises milling said powder mixture by high-energy ball milling.