Silicon nitride is an important structural ceramic material. It is a super-hard substance, which has lubricity and is wear-resistant. It is an atomic crystal; it resists oxidation at high temperatures. Moreover, it can resist the impact of cold and heat. It will not be broken if it is heated to over 1000℃ in the air, cooled rapidly and then heated rapidly. It is precisely because of the excellent characteristics of silicon nitride ceramic that people often use it to manufacture mechanical components such as bearings, gas turbine blades, mechanical seal rings, and sex molds. If silicon nitride ceramics, which are resistant to high temperatures and difficult to transfer heat, are used to make the heating surfaces of engine components, not only can the quality of the diesel engine be improved, fuel savings, and thermal efficiency can be improved. my country, the United States, Japan and other countries have developed such diesel engines.
The process of preparing silicon nitride ceramic products generally consists of raw material processing, powder synthesis, powder processing, forming, green processing, sintering and ceramic body processing.
The types of silicon nitride ceramic preparation processes are mainly distinguished by different methods and sequences of synthesis, molding and sintering.
The commonly used ceramic molding methods include dry pressing, casting, and injection. Dry pressing is a widely used molding process, and it is also one of the mainstream molding processes for ceramic backplanes of mobile phones. The ceramic back covers of the Mi MIX series are all dry pressed. shaped. Today, let’s take a closer look at the ceramic dry pressing process.
1. Dry pressing
Dry pressing, also known as compression molding, is one of the commonly used molding methods. Dry press molding is to fill the powder with good fluidity and suitable particle size after granulation into the metal mold cavity, apply pressure through the indenter, and the indenter will move in the cavity and transfer the pressure to make the powder in the cavity The body particles are rearranged and deformed and compacted to form a ceramic green body with a certain strength and shape.
2. The process principle and influencing factors of dry pressing
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 internal friction force is used to firmly connect the particles to maintain a certain shape. This internal friction acts on the thin layer of bonding agent on the periphery of the particles that are close to each other.
As the pressure increases, the blanks will change their shape, slide each other, reduce the gap, gradually increase the contact, and stick to each other. As the particles get closer, the force between the colloidal molecules and the particles is strengthened so that 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, moisture content, etc.;
Choice of binder and lubricant;
During the pressing process, pressing force, pressing method, pressing speed and holding time.
In summary, if the billet particles are properly graded, the binder is used correctly, and the pressure method is reasonable, the dry pressing method can also obtain a relatively ideal green body density.
Silicon Nitride Ceramics
The high-purity silicon nitride powder hardly shrinks in the hot pressing liquid at 1700C. There are three main sintering methods for silicon nitride: reaction sintering, conventional sintering and hot pressing sintering.
1. Reaction sintering
It is based on the reaction: 3Si(s)+2N2(g)=Si3N4(s) The starting temperature of the nitridation reaction is at 1100, and then the temperature is gradually increased by 1420. The whole process takes several CC days. Since this reaction is an exothermic reaction, the temperature is increased 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. Features: the strength of the material will not be significantly reduced at high temperatures; the product size and shape are unchanged, and products with complex shapes can be made; when two parts are to be welded, they only need to be connected 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, which is often referred to as over-temperature nitriding.