Silicon nitride is an important structural ceramic material. It is a super-hard substance, which has lubricity and wear resistance, and is an atomic crystal; it is resistant to oxidation at high temperatures. And it can resist the impact of cold and heat, heating in the air to more than 1000 ℃, rapid cooling and then rapid heating, will not crack. Because silicon nitride ceramics have such excellent characteristics, people often use it to manufacture mechanical components such as bearings, gas turbine blades, mechanical seal rings, and molds. If silicon nitride ceramics that are resistant to high temperatures and difficult to transfer heat are used to manufacture the heating surfaces of engine components, not only can the quality of the diesel engine be improved, fuel can be saved, but also the thermal efficiency can be improved. my country and the United States, Japan and other countries have developed this kind of diesel engine.
The technological process for preparing silicon nitride ceramic products is generally composed of raw material processing, powder synthesis, powder processing, forming, green body processing, sintering and ceramic body processing.
The types of silicon nitride ceramic preparation processes are mainly distinguished by different methods and orders of synthesis, molding and sintering.
Commonly used molding methods for ceramics include dry pressing, casting and injection. Dry pressing is a widely used molding process and one of the mainstream molding processes for mobile phone ceramic backplanes. The back cover of Xiaomi MIX series ceramics is dry pressed shaped. Today, let's take a closer look at the ceramic dry press forming process.
1. Dry press molding
Dry press molding, also known as compression molding, is one of the commonly used molding methods. Dry press molding is to put the powder with good fluidity after granulation into the metal mold cavity, apply pressure through the indenter, the indenter is displaced in the mold cavity, and the pressure is transmitted to make the powder in the mold cavity. The bulk particles are rearranged and deformed to be compacted to form a ceramic green body with a certain strength and shape.
2. Dry press molding process principles and influencing factors
1. Process principle
The essence of dry pressing is that under the action of external force, the particles are close to each other in the mold, and the particles are firmly connected by the internal friction force to maintain a certain shape. This internal frictional force acts on a thin layer of binder around the peripheral particles.
As the pressure increases, the blanks will change shape, slide against each other, the gap decreases, and gradually increase the contact and close to each other. As the particles get closer, the force between the colloid molecules and the particles is strengthened, so the green body has a certain mechanical strength.
2. Influencing factors
The main factors affecting dry pressing are:
Powder properties: particle size, particle size distribution, fluidity, water content, etc.;
Binder and lubricant selection;
Pressing force, pressing method, pressing speed and holding time during pressing.
To sum up, if the gradation of the blank particles is appropriate, the binder is used correctly, the pressing method is reasonable, and the dry pressing method can also obtain a more ideal green body density.
Silicon nitride ceramics
High-purity silicon nitride powder hardly shrinks when hot-pressed at 1700C. There are three main sintering methods for silicon nitride: reaction sintering, conventional sintering, and hot-press sintering.
1. Reaction sintering
It is based on the reaction: 3Si(s)+2N2(g)=Si3N4(s) The starting temperature of the nitridation reaction is 1100, and then it gradually heats up 1420. The whole process takes a few CC days. Because this reaction is exothermic, it heats up. Speed should be carefully controlled. Generally, the product obtained by holding at a temperature lower than 1400 is a mixture of α- and β-Si3N4 with a porosity of 15C30%.
Silicon nitride ceramics
1. Reaction sintering
Advantages: No additional additives are allowed. The strength of the material will not be significantly reduced at high temperatures; the product size and shape will not change, and products with complex shapes can be made; when welding two parts, just connect them together. Nitriding. In reaction sintering, the key factor affecting product quality is to control the reaction temperature. After the three-step heating method, the furnace temperature is raised above the melting point of silicon, often referred to as overtemperature nitridation.