What are the process methods for silicon nitride ceramics

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Silicon nitride (Si3N4) ceramics are known as “versatile ceramics” due to their excellent comprehensive properties. It has the advantages of high strength, high hardness, excellent wear resistance, outstanding thermal shock resistance (able to withstand rapid temperature changes), good chemical stability, and low density. These characteristics make it an ideal material for solving technical problems under extreme working conditions, widely used in fields such as aerospace, automotive industry, mechanical manufacturing, electronic semiconductors, and healthcare.
However, obtaining silicon nitride ceramic products with these excellent properties requires a series of precise and complex preparation processes.

Core raw material: Preparation of silicon nitride powder
The starting point of all high-performance ceramics is high-quality powder. The synthesis method of silicon nitride powder directly determines its purity, particle size distribution, and sintering activity, which in turn affects the performance of the final product.

1. Carbon thermal reduction method
   Process: Mix silica (SiO ₂) powder with carbon powder (C) and react at high temperatures of 1400-1550 ℃ in a nitrogen (N ₂) atmosphere. The chemical equation is: 3SiO ₂+6C+2N ₂ → Si ∝ N ₄+6CO.
   Characteristics: With a wide range of raw material sources and low costs, it is currently the most mainstream method in industrial production. The specific surface area and particle morphology of the powder can be adjusted by controlling the process parameters.
2. Direct nitriding method
   Process: High purity silicon (Si) metal powder is directly reacted with nitrogen at high temperature (1200-1400 ℃) to generate silicon nitride: 3Si+2N ₂ → Si ∝ N ₄.
   Characteristics: The process is simple, but the reaction is a strong exothermic reaction that requires precise temperature control to prevent silicon powder from melting and clumping. The obtained powder usually needs to be crushed and ground before it can be used.
3. Silicon imine decomposition method
   Process: Silicon tetrachloride (SiCl ₄) is reacted with ammonia gas (NH3) in an inert solvent to form silicon imine. After washing, drying, and calcination, high-purity silicon nitride powder is finally decomposed.
   Features: It can obtain sub micron powders with ultra-high purity and high sintering activity, but the cost is high, mainly used in high-end fields with extremely high performance requirements.
4. Gas phase reaction method
   Process: Utilizing silane (such as SiCl ₄ or SiH ₄) to undergo gas-phase reaction with ammonia at high temperature, directly generating nanoscale silicon nitride powder.
   Characteristics: The powder purity is extremely high, the particle size is small and uniform, but the technical difficulty is high, the output is low, and the cost is the highest.

Forming process: giving the material a preliminary shape
Converting powder into a specific shaped blank is a crucial step in the manufacturing process. The choice of molding method depends on the shape, size, and precision requirements of the product.

1. Dry pressing molding
   Process: Mix silicon nitride powder with a small amount of organic binder, load it into a metal mold, and press it into a dense body through unidirectional or bidirectional mechanical pressure.
   Features: High efficiency, low cost, high degree of automation, suitable for producing parts with simple shapes and large batches, such as ceramic bearing balls, sealing rings, etc.
   Problem solving: Suitable for large-scale, standardized product manufacturing, reducing the cost of individual parts.
2. Isostatic pressure forming
   Process: Load the powder into a rubber or plastic mold with elasticity, seal it, and place it in a high-pressure container. Apply isotropic ultra-high pressure (up to 300MPa or more) through a liquid or gas medium to make the blank uniform and dense.
   Characteristics: It can produce large green bodies with complex shapes, uniform density, and isotropic properties, avoiding the density gradient problem that may occur during dry pressing.
   Problem solving: Used for manufacturing high-performance, high reliability, and complex shaped components such as large crucibles, Radomes, and insulators.
3. Injection molding
   Process: Mix silicon nitride powder with a large amount (usually up to 30-50%) of thermoplastic binder, pelletize and send it into an injection molding machine. Inject it into the metal mold cavity in a heated and melted state, cool and demold to obtain a “green body”.
   Features: It can form extremely complex three-dimensional shapes, has high dimensional accuracy, and does not require subsequent mechanical processing. But the subsequent process requires a long and complex degreasing (removal of adhesive).
   Problem solving: Perfectly solved the near net forming problem of complex irregular components, such as turbine rotors, engine swirl chambers, etc., greatly reducing material waste and processing costs.
4. Casting molding
   Process: Disperse the powder in a solvent, add binders, plasticizers, etc. to make a uniform and stable slurry. Use the scraper of the casting machine to scrape a thin layer of slurry on the base tape, and dry it to form a continuous thin sheet body.
   Features: Specially designed for producing ultra-thin and flat sheet-like ceramics.
   Problem solving: Mainly used in the field of electronics industry, such as preparing silicon nitride ceramic substrates, solving chip heat dissipation and insulation problems.

Sintering process: the key to achieving the final performance of materials
Sintering is the finishing touch of ceramic preparation, which densifies the powder body at high temperatures to obtain the final mechanical and physical properties. Due to its strong covalent bonding properties, silicon nitride is difficult to achieve density through solid-state sintering like oxide ceramics, and special sintering techniques must be used.

1. Atmospheric pressure sintering
   Process: Conducted in a sintering furnace under normal atmospheric pressure. In order to promote sintering, sintering aids (such as MgO, Y ₂ O3, Al ₂ O3, etc.) must be added to the powder, which will form a liquid phase with SiO ₂ on the surface of silicon nitride at high temperatures and achieve densification through the liquid-phase sintering mechanism.
   Features: Low cost, simple equipment, suitable for producing complex and large shaped products. It is currently the most economical and widely used sintering method.
   Problem solving: Provides the most cost-effective solution for most commercial applications, such as wear-resistant parts, corrosion-resistant nozzles, etc.
2. Hot pressing sintering
   Process: Load the powder into a graphite mold and apply unidirectional high temperature (usually>1700 ℃) and high pressure (20-40MPa) simultaneously during the sintering process.
   Characteristics: Pressure promotes particle rearrangement and diffusion, resulting in products with close to theoretical density, fine grain size, and extremely high mechanical properties (especially strength and toughness). But it can only produce block or plate shaped products with simple shapes, and the cost is high.
   Problem solving: Used for manufacturing components that require extremely high reliability, such as bearings for aerospace engines and gyro ball covers for missile guidance systems.
3. Pressure sintering
   Process: It is an upgraded version of atmospheric sintering. In the later stage of sintering, high-pressure nitrogen gas (1-10 MPa) is introduced into the furnace cavity to suppress the decomposition and vaporization of silicon nitride at high temperatures.
   Characteristics: High pressure atmosphere can effectively suppress material decomposition, allowing for the use of higher sintering temperatures, resulting in more complete grain development, superior performance (high toughness, high Weibull modulus), and almost fully dense products. It can produce complex shaped parts with performance similar to hot press sintering, but at a lower cost than the latter.
   Problem solving: This is the preferred method for manufacturing high-performance silicon nitride ceramic bearing balls, solving the technical bottleneck of high-speed, high load, and long-life bearings.
4. Reaction sintering
   Process: Using silicon powder as raw material, the blank is made through the above molding process and then subjected to nitriding treatment in a nitrogen atmosphere. Nitrogen infiltrates the interior of the billet and reacts with silicon to form silicon nitride, thereby bonding the billet together.
   Characteristics: During the nitriding process, the size change is minimal (<0.1%), and highly accurate near net shape products can be obtained. However, the product contains a small amount of unreacted free silicon, resulting in a low density (about 80%) and significantly lower mechanical properties (especially strength) compared to other sintering methods.
   Problem solving: Mainly used for manufacturing components with complex shapes, high dimensional accuracy requirements, but low stress, such as lifting tubes, fixtures, jigs, etc. in metal melt processing.

Through the combination and innovation of the above process methods, silicon nitride ceramics can be tailored to meet the demanding requirements of different fields. From cost reducing atmospheric sintering, to pursuing ultimate performance through hot pressing and pressure sintering, and to emphasizing dimensional accuracy through reaction sintering, each process contributes to solving specific industrial problems and continues to drive technological progress in high-end manufacturing.

Brudeze Ceramics supplies and sells a wide range of high-quality quartz glass, including alumina ceramics, zirconia ceramics, silicon nitride ceramics, aluminum nitride ceramics, silicon carbide ceramics, boron carbide ceramics, bioceramics, machinable ceramics, etc. We can meet the customization requirements of various ceramic products.