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Why are sintering aids added during advanced ceramic processing?

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Steps in doctor blading, a tape-casting process employed in the production of ceramic films. Ceramic powder and solvent are mixed to form a slurry, which is treated with various additives and binders, homogenized, and then pumped directly to a tape-casting machine. There the slurry is continuously cast onto the surface of a moving carrier film. The edge of a smooth knife, generally called a doctor blade, spreads the slurry onto the carrier film at a specified thickness, thereby generating a flexible tape. Heat lamps gently evaporate the solvent, and the dry tape is peeled away from the carrier film and rolled onto a take-up reel for additional processing.
advanced ceramics Steps in doctor blading, a tape-casting process employed in the production of ceramic films. Ceramic powder and solvent are mixed to form a slurry, which is treated with various additives and binders, homogenized, and then pumped directly to a tape-casting machine. There the slurry is continuously cast onto the surface of a moving carrier film. The edge of a smooth knife, generally called a doctor blade, spreads the slurry onto the carrier film at a specified thickness, thereby generating a flexible tape. Heat lamps gently evaporate the solvent, and the dry tape is peeled away from the carrier film and rolled onto a take-up reel for additional processing.

In advanced ceramic processing, sintering aids are primarily used to enhance densification and eliminate porosity, which is critical for achieving desired material properties.1

While solid-state sintering is the standard method for bonding advanced ceramic particles, it often struggles to remove pores that become trapped inside growing grains. Sintering aids address this in two main ways:

  • Reactive or Transient-Liquid Sintering: Chemical additives create a temporary liquid phase that facilitates the initial stages of sintering. This liquid is later evaporated, resorbed by the solid particles, or crystallized, allowing for densification without leaving behind a permanent glassy phase that could degrade the material's performance.1
  • Grain Boundary Control: Certain additives can modify the migration of grain boundaries. For example, adding magnesia to alumina slows grain boundary migration, ensuring that pores remain on the boundaries where they can be effectively eliminated through diffusion. This process is essential for achieving high levels of translucency and extremely low porosity in specialized applications.1