Reactive Sintered Silicon Nitride

Reactive Sintered Silicon Nitride, Reactive Sintered Si3N4 Ceramics

Reactive Sintered silicon nitride refers to the process of obtaining a predetermined sintered body by chemically reacting the raw material body through a solid phase, liquid phase, and gas phase at a certain temperature, while simultaneously densifying and synthesizing the specified components.
Using the general molding method, high-purity silicon powder or a mixture of silicon powder and silicon nitride powder is first pressed into a green body of the desired shape and then pre-nitrided at a temperature of 1200°C in a nitriding furnace. The green body after pre-nitriding has a certain strength and can be subjected to various mechanical processing (such as turning, planing, milling, and drilling). Finally, sintering again at above 1400°C (the melting point of silicon) can obtain products with very small dimensional changes (i.e., the very small shrinkage rate of the green body after sintering), and the products can be used without grinding.

Reactive Sintered refers to the process of obtaining a predetermined sintered body by chemically reacting the raw material body at a certain temperature through solid phase, liquid phase, and gas phase, while simultaneously performing densification and synthesis of specified components.
Using the general molding method, high-purity silicon powder or a mixed powder of silicon powder and silicon nitride powder is first pressed into a green body of the desired shape and then pre-nitrided at 1200℃ in a nitriding furnace. The green body after pre-nitridation has a certain strength and can be subjected to various mechanical processing (such as turning, planing, milling, and drilling). Finally, it is sintered again at above 1400℃ (the melting point of silicon) to obtain a product with very small dimensional changes (that is, after the green body is sintered, the shrinkage rate is very small). At this time, the product can be used without grinding.
Reaction sintering is the earliest silicon nitride preparation process. The initially produced reaction sintered silicon nitride generally contains about 25% of pores, and the degree of densification is not high enough, so it is difficult to reflect the high strength and hardness of silicon nitride materials.

Silicon Nitride Nozzle

Processing process

Use a primary crystalline silicon block, wet grind it in a ball mill, use alcohol as a grinding medium and grind it to less than 0.07mm. Then purify the raw materials and make the blanks. The molding method can be casting, molding, hot pressing, or isostatic pressing. During molding, the density of the blank must meet certain requirements.
The blank is first nitrided in a nitriding furnace, and a molybdenum wire electric furnace or a molybdenum disilicide rod electric furnace can be used. The furnace must be tightly sealed to ensure vacuum and safety of use. Silicon and nitrogen begin to react at about 970~1000℃, and the reaction rate accelerates as the temperature rises. However, if the temperature rises quickly and exceeds the melting point of silicon, the blank will collapse due to the melting of silicon. Therefore, it must be pre-nitrided at a temperature far below the melting point. The nitriding furnace is a mixed atmosphere of 95% nitrogen and 5% hydrogen at 1180~1210℃ for 1-1.5 hours. The degree of nitriding is about 9%. The pad in the furnace is made of silicon nitride.
When machining the blank after bisque firing, avoid contact with water, and feed, and speed should not be too fast to process the blank to the required size of the finished product.
Finally, nitriding firing is carried out. The nitriding temperature can be nitrided in stages at 1420℃ below the melting point of silicon and above the melting point of silicon.

Advantages of Reactive Sintered Silicon Nitride

-Low shrinkage
-Suitable for preparing parts with complex shapes and precise dimensions
-Low cost

Disadvantages

-Long nitriding time
-Low strength
-High porosity

Application

Reaction-bonded silicon nitride still has sufficiently high-temperature resistance and corrosion resistance, so it is common in the field of refractory materials and is suitable for processing complex products with low strength requirements. It is mainly used to prepare thermocouple protection tubes, crucibles for melting metals, and rocket nozzles.