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Why are sintering aids used when making dense advanced ceramics?
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advanced structural ceramics
Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.
Sintering aids are used in the production of advanced ceramics primarily to overcome the difficulty of eliminating pores that become trapped inside growing grains during the sintering process. While grain boundary diffusion can effectively remove pores located at grain boundaries, pores trapped within grains are extremely challenging to eliminate through standard solid-state sintering alone.1
Sintering aids address this in two main ways:
- Facilitating densification: In reactive-liquid or transient-liquid sintering, chemical additives create a temporary liquid phase that promotes the initial stages of densification before being evaporated, resorbed, or crystallized.1
- Controlling grain boundary migration: Certain additives can slow the movement of grain boundaries. For example, magnesia is used as a sintering aid for alumina to prevent grain boundaries from migrating away from pores. By keeping pores on the grain boundaries, the material can achieve extremely low porosity through grain boundary diffusion, which is essential for achieving properties like translucency.1
By enabling the removal of these internal pores, sintering aids allow for the creation of denser, higher-performance materials that would otherwise remain porous.1