What is Silicon Nitride Ceramic Material?

Silicon nitride is an important structural ceramic material. It is a superhard material with inherent lubricity and wear resistance. It is an atomic crystal and antioxidant at high temperatures. It can also resist thermal shock. When heated to above 1000°C in air, it will not disintegrate even if it is rapidly cooled or heated. It is precisely because silicon nitride ceramics have such good properties that people often use it to manufacture mechanical parts such as bearings, gas turbine blades, mechanical seals, and mold parts. If high- temperature resistant and heat-resistant silicon nitride ceramics are used to manufacture the heating surface of engine parts, it can not only improve the quality of diesel engines, and save fuel, but also improve thermal efficiency. This type of diesel engine has been developed in China, the United States, Japan, and other countries.

The preparation process of silicon nitride ceramic products generally consists of raw material processing, powder synthesis, powder processing, molding, green body processing, sintering, and ceramic green body processing.

The preparation process types of silicon nitride ceramics are mainly distinguished by the methods and sequences of synthesis, molding, and sintering.

Common molding methods for ceramics include dry pressing, tape casting, injection molding, etc. Dry pressing is a widely used molding process and one of the mainstream molding processes for mobile phone ceramic back panels. The ceramic back covers of the Xiaomi MIX series are all dry-pressed. Today, let's take a closer look at the ceramic dry-pressing process.

1. Dry pressing

Dry pressing, also known as die pressing, is one of the commonly used molding methods. Dry pressing is the process of placing powder with good fluidity and suitable particle grading after granulation into the metal mold cavity and applying pressure through the pressure head. The pressure The head produces displacement in the mold cavity, transmits pressure, and rearranges and deforms the powder particles in the mold cavity, and then compacts to form a ceramic blank with strength and shape.

2. 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 particles are firmly connected with the help of internal friction to maintain the shape. This internal friction acts on the thin adhesive layer around the particles close to each other.

As the pressure increases, the blanks will change shape, slide against each other, the gap decreases, the contact gradually increases, and adhere to each other. As the particles get closer, the interaction between the colloid molecules and the particles is strengthened, and the green body has mechanical strength.

2. Influencing factors

The main factors affecting dry pressing are:

Powder properties: particle size, particle size distribution, fluidity, water content, etc.;

Selection of adhesives and lubricants;

Mold design;

Pressing force, pressing method, pressing speed, and holding time during the pressing process.

In summary, if the particle size of the blank is appropriate, the binder is used correctly, and the pressing method is reasonable, the dry pressing method can also obtain a relatively ideal blank density.

Silicon nitride ceramics

The hot-pressing liquid of high-purity silicon nitride powder does not shrink at 1700°C. 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 following reaction: 3Si(s) 2N2(g)=Si3N4(s) The nitriding reaction starts at 1100°C and gradually heats up to 1420°C. The whole process takes several CC days. Since the reaction is exothermic, the heating rate should be carefully controlled. Generally, the product obtained by keeping warm below 1400°C is a mixture of α- and β-Si3N4 with a porosity of 15C30%.

Silicon nitride ceramics

1. Reaction sintering

Advantages: No additives are allowed Features: The strength of the material will not be significantly reduced at high temperatures; The product size and shape remain unchanged, and complex-shaped products can be produced; To weld two parts, just connect them and nitride them . In reaction sintering, the key factor affecting product quality is the control of reaction temperature. After the three-step heating method, the furnace temperature is raised to above the melting point of silicon, which is often called over-temperature nitriding.