How to reduce internal pores in pressureless sintered silicon carbide sealing rings

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The formation of pores inside the pressureless sintered silicon carbide sealing ring is a multi-factor problem, mainly involving raw material characteristics, process parameters and other aspects.
1. Raw material powder characteristics
Powder particle size and distribution: If the raw material powder is too coarse or the particle size distribution is uneven, it will be difficult to fully fill the particles during sintering, forming gaps and pores.
Impurities in silicon carbide powder (such as free silicon, metal oxides, etc.) may react chemically or volatilize during high-temperature sintering, resulting in pore formation.
Micropowder is left for too long: SIO2 is generated on the surface and the charge changes.
Sintering aid selection: If the addition amount or distribution of sintering aids (such as B, C, Al₂O₃, etc.) is unreasonable, it may affect the liquid phase generation or diffusion dynamics, resulting in insufficient local densification.
2. Molding process
Brite density and uniformity: If the density of the billet is low or the density distribution is uneven during the molding process (such as dry pressing, isostatic pressing), pores are likely to remain in the low-density area during vacuum sintering.
Green body defects: air or organic volatilization residues that are not exhausted during molding may form closed pores during sintering.
3. Vacuum sintering process parameters
Heating rate: Too fast heating will lead to premature surface densification, and internal gases (such as additive decomposition gases and residual volatiles) cannot escape, forming closed pores.
Vacuum sintering temperature and holding time:
Insufficient temperature: It is difficult to activate sufficient material diffusion, densification is not complete, and residual pores.
Insufficient holding time: Insufficient time for pore migration and discharge, some pores are “frozen” in the material.
Atmosphere control: If there is oxygen or impurity gas in the sintering atmosphere (such as Ar, N₂ or vacuum), it may trigger side reactions (such as oxidation or gas escape), leading to pore formation.
4. Additive decomposition and volatilization
Residual organic additives: If the binder or lubricant used during molding is not completely removed, it will decompose at high temperature to produce gas (such as CO₂, H₂O), forming pores.
Volatilization of sintering aids: Some low-melting-point aids (such as Al) may volatilize at high temperatures, leaving holes.
5. Pore evolution dynamics
Pore migration is blocked: In the later stage of sintering, pores need to be discharged through grain boundary diffusion or lattice diffusion. If the grains grow too fast (such as abnormal grain growth), the pores may be “wrapped” by the grains and cannot be discharged.
6. Cooling process
Cooling rate is too fast: it may cause thermal stress concentration, induce microcracks or local pore expansion (but the main cause of pores is still concentrated in the sintering stage).

Optimization measures to reduce pores

1. Raw material optimization: select high-purity, fine-grained powder, and reasonably control the proportion of sintering aids.
2. Forming improvement: increase the density of the green billet (such as isostatic pressing), and optimize the degreasing process to remove organic matter.
3. Vacuum sintering process adjustment:
Use segmented heating (such as slow degreasing section + fast sintering section).
Extend the high-temperature insulation time to promote pore discharge.
Optimize sintering atmosphere (such as vacuum or inert gas protection).
By systematically optimizing raw materials, molding and sintering processes, the porosity of pressureless sintered silicon carbide sealing rings can be significantly reduced, and their density and mechanical properties can be improved.

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