Silicon carbide (SiC) ceramics occupy an important place among high‑temperature structural ceramics due to their low thermal expansion coefficient, high thermal conductivity, high hardness, good thermal stability, and excellent chemical stability. They are widely used in aerospace, nuclear energy, military, and semiconductor applications.
Because SiC has extremely strong covalent bonds and a very low diffusion coefficient, achieving full densification of SiC ceramics is extremely difficult. To address this, several sintering techniques have been developed, including reaction sintering, pressureless solid‑state sintering, pressureless liquid‑phase sintering, hot pressing, and recrystallization sintering.
01 Reaction Sintering
The preparation process for reaction‑bonded silicon carbide (RBSC) ceramics is as follows: first, SiC powder, carbon powder, and organic binders are mixed uniformly in a certain ratio and formed into a green body; then, silicon is infiltrated at high temperature. The silicon reacts with carbon to form secondary SiC, which bonds the original SiC particles together. Finally, the residual free silicon fills the pores, achieving high densification of the RBSC ceramic.
During reaction sintering, to ensure complete silicon infiltration, the green body (α‑SiC + C) must have sufficient porosity; otherwise, only the surface region will be silicified, while unreacted carbon and a small number of pores remain in the centre. Therefore, the green density must be strictly controlled. Appropriate green density can be obtained by adjusting the contents of α‑SiC and carbon in the initial mixture, the particle size distribution of α‑SiC, the shape and size of carbon, and the forming pressure.
Reaction sintering is a near‑net‑shape process with almost no shrinkage or dimensional change during sintering. It offers advantages such as low sintering temperature, high product density, and low production cost, making it suitable for manufacturing large‑sized, complex‑shaped SiC ceramic products. In recent years, with the increase in wafer size and heat‑treatment temperatures, reaction‑bonded SiC has gradually replaced quartz glass. High‑purity SiC components containing some residual silicon phase can be produced using high‑purity SiC powder and high‑purity silicon, and these are widely used as support fixtures for electron tubes and semiconductor wafer manufacturing equipment.
Pressureless Sintering Processes
Pressureless sintering is mainly divided into solid‑state sintering and liquid‑phase sintering.
Solid‑State Sintering
In 1974, S. Prochazka first used a pressureless sintering process by adding small amounts of boron and carbon to high‑purity β‑SiC fine powder, successfully obtaining SiC sintered bodies with a relative density greater than 98% at 2020 °C. Solid‑state sintered SiC ceramics require high temperatures, but their physicochemical properties are stable, especially with no change in high‑temperature strength, giving them special application value.
In solid‑state sintering systems, various sintering additives are introduced. The most common system is the B‑C system, where B and C powders or directly B₄C powder are added as sintering aids. This system typically densifies at 1900–2100 °C and yields SiC ceramics with good properties. The Al‑C system is also frequently used; Al metal is the second most effective additive after boron. In addition, the AlN‑C system is used for SiC sintering. Compared with B, the addition of AlN effectively suppresses SiC grain growth.
Solid‑state sintering of SiC ceramics relies mainly on solid‑state diffusion mechanisms for densification. The addition of solid‑state aids promotes SiC grain growth while avoiding liquid‑phase formation. However, this system has certain limitations: first, the sintering temperature must be maintained in the high range of 1900–2100 °C, which not only consumes significant energy but also imposes severe requirements on equipment; second, the high temperature can cause abnormal grain growth, damaging material properties; third, full densification often requires prolonged holding times; furthermore, this process has strict demands on the particle size, purity, and uniformity of the starting powder.
Liquid‑Phase Sintering
Liquid‑phase sintering involves adding a certain amount of multi‑component low‑eutectic oxides to the raw materials, which form a liquid phase at high temperature. This changes the mass‑transfer mechanism from diffusion to viscous flow, lowering the energy required for densification and the sintering temperature. At the same time, the introduction of a grain‑boundary liquid phase significantly improves the strength and toughness of the material.
In the Al₂O₃‑participated liquid‑phase sintering of SiC, Al₂O₃ reacts with SiO₂ on the SiC particle surfaces at high temperature to form a liquid phase, which acts as a grain‑boundary phase and provides diffusion paths, lubricating the SiC particles and promoting particle rearrangement and densification. The liquid‑phase sintering of SiC is controlled by a dissolution‑reprecipitation mechanism, and its sintering behaviour is significantly influenced by raw material characteristics, including starting particle size, size distribution, impurity content, and crystalline phase composition. Meanwhile, the sintering temperature is affected to some extent by the amount of sintering aids; an appropriate amount of liquid‑phase aid can significantly lower the sintering temperature, but may affect the properties to a certain degree.
The most common liquid‑phase sintering system for SiC is the Al₂O₃‑Y₂O₃ system, which further introduces Y₂O₃ on the basis of Al₂O₃ as an additive, forming the Al₂O₃‑Y₂O₃ system. This system effectively reduces the sintering temperature of SiC and improves the microstructure and properties of the material.
03 Hot Pressing
Hot pressing involves filling dried SiC powder into a high‑strength graphite die and applying an axial pressure while heating, controlling the process with suitable pressure‑temperature‑time parameters to achieve sintering and forming of SiC. Additives for hot‑pressed SiC include Al, Fe, B, B₄C, Al₂O₃, AlN, BeO, B+C, etc. The densification mechanisms of these additives can be roughly divided into two categories: one forms a liquid phase with impurities in SiC to promote sintering, and the other forms solid solutions with SiC, reducing grain‑boundary energy and promoting sintering.
Because heating and pressing occur simultaneously in hot pressing, the powder is in a thermoplastic state, which facilitates contact diffusion and viscous flow mass transfer. This allows SiC ceramic products with fine grains, high relative density, and good mechanical properties to be obtained at lower sintering temperatures and shorter times. The drawbacks of this process are its complex equipment and procedures, demanding die materials, limited ability to produce only simple shapes, low productivity, and high production cost. However, in semiconductor manufacturing, where the performance requirements for ceramic materials used in precision instruments and components are extremely high, the control of composition, purity, and densification is far more important than economic cost. Additionally, the high added value of the products makes hot pressing particularly important.
04 Recrystallization Sintering
Recrystallization sintering has attracted widespread attention because it does not require sintering aids. It is the most common method for producing ultra‑high‑purity, large‑sized SiC ceramic components. The preparation process for recrystallized SiC ceramics is as follows: coarse and fine SiC powders of different particle sizes are mixed in a certain ratio, and green bodies are formed by slip casting, die pressing, extrusion, or other methods; then, the green bodies are sintered at 2200–2450 °C under an inert atmosphere. Finally, the fine particles gradually evaporate into the vapour phase and condense at the contact points of coarse particles, forming a recrystallized ceramic.
Recrystallization sintering offers additional advantages: during sintering, the green body exhibits almost no shrinkage, making it less prone to cracking or distortion, so complex‑shaped, high‑precision ceramic components can be produced; when preparing porous recrystallized ceramics, the porosity and pore size distribution can be easily controlled within narrow ranges; the recrystallized ceramic has a three‑dimensional interconnected porous structure, mostly open pores, which makes it suitable for infiltration with a second phase to produce composites.