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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This patent application claims priority to co-pending U.S. patent application Ser. No. 11 / 279,461, filed Apr. 12, 2006, which claimed priority to U.S. Provisional Patent Application Ser. No. 60,597,049, filed Nov. 7, 2005.BACKGROUND OF THE INVENTION[0002]Polycrystalline silicon nitride (Si3N4) bodies are becoming increasingly attractive as structural and mechanical materials due to their ability to provide high strength and durability under severe conditions, and especially under high temperature applications. Si3N4 is characterized by high heat resistance, mechanical strength, thermal shock resistance, wear resistance, chemical stability, and hardness. One reason that Si3N4 does not enjoy wider use is that Si3N4 green powder compacts or green bodies are inherently difficult to sinter.[0003]Known processes for producing a sintered silicon nitride body typically require the use of a sintering aid, such as Y2O3, Al2O3, MgO, or the like, add...

AI Technical Summary

Benefits of technology

This approach enables the production of high-strength Si3N4 / SiC composites with controlled microstructure and reduced porosity, maintaining high compact strength and uniform nitrogen ratio, while avoiding excessive oxidation and density gradients.

Problems solved by technology

One reason that Si3N4 does not enjoy wider use is that Si3N4 green powder compacts or green bodies are inherently difficult to sinter.

However, the relative amounts of O and N in the glass phase is an uncontrolled variable, and thus the composition of the glassy phase at the grain boundary is likewise uncontrolled and variable, resulting in density gradients in the sintered body and compositional gradients at the grain boundaries.

However, such a technique requires a bimodal PSD in the starting mixture of Si3N4 and SIC powders, and thus it is impossible to achieve a uniform mixture, resulting in a sintered body with an uneven grain structure.

Further, the bimodal PSD of the main constituent powders makes an even, uniform distribution of the yttria sintering aid unlikely, resulting in insufficient sinterability and poor mechanical strength in the resultant sintered body.

However, this technique suffers from the difficulty in uniformly nitriding the silicon metal from the surface to the core of the body, which typically results in at least some silicon metal unreacted inside the resulting sintered body.

However, as with the previously-descried technique, it is difficult to uniformly nitride the body from surface to interior.

However, the resulting sintered body typically suffers from the generation of pores and the deterioration of mechanical properties by decomposition of amorphous components.

Further, full density cannot be achieved via this technique absent elevated gas-pressure during sintering.

Also, since a sintering aid powder is mixed with the partially crystalline composite powder and then sintered, the dispersion of the sintering aid powder is typically uneven, resulting in segregation of the sintering aid and inconsistent density and other physical properties observed in the sintered body.

However, this technique suffers from the preferential formation of β-silicon nitride, making it difficult to increase the percentage of α-silicon nitride in the composite powder.

Since β-silicon nitride tends to grow in a needle shape, the resultant powder is suffers from the anisotropic particle shapes and is thus difficult to compact or pulverize.

There thus remains a need for a technique for evenly sintering a Si3N4 / SiC body to density that does not require a pressurized nitrogen atmosphere and / or excessively high firing temperatures, as both requirements greatly increase the expense of the process and, thus, the end product.

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