April 1, 2024

Technical Overview of Silicon Nitride Materials

The process flow for preparing Silicon Nitride ceramic products generally consists 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 sequences of synthesis, shaping, and sintering.

Silicon Nitride Tapered Crucible 2
Silicon Nitride Tapered Crucible 2

01. Reaction sintering (RS)

Reactive sintered Silicon Nitride is formed by forming Si powder or a mixture of Si powder and Si3N4 powder, then passing nitrogen gas at about 1200 ℃ for pre-nitriding, then machining into the required parts, and finally performing final nitriding and sintering at about 1400 ℃. There is no need to add burning aids during this process, so the material strength will not be significantly reduced at high temperatures. At the same time, reaction-sintered Silicon Nitride has non-shrinkage characteristics and can prepare parts with complex shapes. However, because the density of the product is 70% to 90% low and there are a large number of pores, the mechanical properties are greatly affected.

02. Normal pressure sintering (PLS)

Atmospheric pressure sintered Silicon Nitride is prepared by mixing high-purity, ultra-fine, high α-phase content Silicon Nitride powder with a small amount of sintering aid, and through processes such as forming and sintering. During the sintering process, the α phase dissolves into the liquid phase and then precipitates on the β-Si3N4 crystal nucleus to become β-Si3N4, which is beneficial to the sintering process. Nitrogen must be introduced during sintering to inhibit the high-temperature decomposition of Si3N4. Ceramics with complex shapes and excellent properties can be obtained by normal pressure sintering. The disadvantage is that the sintering shrinkage is relatively large, generally 16% to 26%, which can easily cause cracking and deformation of the products.

03. Re-sintering (PS)

The reaction-sintered Si3N4 sintered billet is placed in Silicon Nitride powder in the presence of a sintering aid and re-sintered at high temperatures to obtain a dense Si3N4 product. The sintering aid can be introduced during silica powder ball milling, or it can be added by impregnation after reaction sintering. Since the reactive sintering process can be pre-processed and the shrinkage during the re-sintering process is only 6% to 10%, parts with complex shapes and excellent performance can be produced.

04. Hot pressing sintering (HP)

Place the Silicon Nitride powder and sintering aid in the graphite mold and sinter under one-way pressure at high temperatures. As the external pressure increases the sintering driving force, the α→β transformation and densification speed are accelerated. The hot pressing method can obtain high-strength Silicon Nitride ceramics with a density greater than 95%, with high material performance and a short manufacturing cycle. However, this method can only produce products with simple shapes, and the processing cost for parts with complex shapes is high. Moreover, due to one-way pressing, the structure has a preferred orientation, which makes the performance different in parallel and perpendicular directions to the hot-pressed surface.

05. Gas pressure sintering (GPS)

Air pressure sintering is to sinter the Si3N4 compact in 5 to 12MPa nitrogen at 1800 to 2100°C. Due to the high pressure of nitrogen, the decomposition temperature of Si3N4 is increased, which is conducive to the selection of a burning aid that can form an intergranular phase with high refractoriness to improve the high-temperature performance of the material.

06. Hot isostatic pressing (HIP)

The mixture powder of Silicon Nitride and sintering aid is encapsulated in a metal or glass envelope and then sintered at high temperature through high-pressure gas after vacuuming. The commonly used pressure is 200MPa and the temperature is 2000℃. Hot isostatic pressing of Silicon Nitride can reach the theoretical density, but the process is complicated and the cost is high. In recent years, some other sintering and densification processes have been developed, such as ultra-high pressure sintering, chemical vapor deposition, explosive forming, etc.

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