What is silicon nitride ceramic material

2020-07-08


Silicon nitride is an important structural ceramic material. It is a superhard substance, with inherent lubricity and abrasion resistance, and is an atomic crystal; Antioxidant at high temperatures. Moreover, it can resist cold and hot shocks, and when heated to over 1000 ℃ in the air, it will not disintegrate after rapid cooling and heating. It is precisely because silicon nitride ceramics have such excellent properties that people often use them to manufacture mechanical components such as bearings, gas turbine blades, mechanical sealing rings, and mold parts. If silicon nitride ceramics, which are resistant to high temperatures and not easy to heat transfer, are used to manufacture the heating surfaces of engine components, it can not only improve the quality of diesel engines, save fuel, but also improve thermal efficiency. China, the United States, Japan and other countries have developed this type of diesel engine.

The process for preparing silicon nitride ceramic products generally consists of raw material treatment, powder synthesis, powder treatment, forming, green body treatment, sintering, and ceramic body treatment.

The types of preparation processes for silicon nitride ceramics are mainly distinguished by different methods and sequences of synthesis, molding, and sintering.

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

1、 Dry pressing molding

Dry compression molding, also known as molding, is one of the commonly used molding methods. Dry compression molding is the process of placing powder with good fluidity and suitable particle grading after granulation into a metal mold cavity, and applying pressure through the pressure head. The pressure head displaces in the mold cavity, transferring pressure, causing the powder particles in the mold cavity to rearrange and deform, and then being compacted to form a ceramic blank with a certain strength and shape.

2、 Process principle and influencing factors of dry compression molding

1. Process principle

The essence of dry compression molding is that under the action of external forces, particles approach each other in the mold, and firmly connect the particles with the aid of internal friction to maintain a certain shape. This internal friction acts on the thin layer of adhesive surrounding the particles that are close to each other.

As the pressure increases, the billets will change their shape, slide against each other, reduce the gap, gradually increase contact, and adhere to each other. Due to the further proximity of the particles, the force between the colloidal molecules and the particles is strengthened, and the green body has a certain mechanical strength.

2. Influencing factors

The main factors affecting dry compression molding are:

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

Selection of adhesives and lubricants;

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Mold design;

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

To sum up, if the particle grading of the billet is appropriate, the binder is used correctly, and the pressurization method is reasonable, the dry compaction method can also obtain a relatively ideal billet 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: reactive sintering, conventional sintering, and hot pressing sintering.

1. Reactive sintering

It is based on the reaction: 3Si (s)+2N2 (g)=Si3N4 (s) The nitridation reaction starts at 1100 and gradually heats up 1420. The entire process takes several CC days. As this reaction is an exothermic reaction, the heating rate should be carefully controlled. Generally, the product obtained by holding at a temperature below 1400 is α-、β- A mixture of Si3N4 having a porosity of 15C30%.

Silicon nitride ceramics

1. Reactive sintering

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

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