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Why do worn transformation-toughened zirconia surfaces become polished rather than roughened?
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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.
In transformation-toughened zirconia (TTZ), worn surfaces become polished rather than roughened due to the specific nature of the material's wear process. When the surface is subjected to stress during wear, the toughening particles are induced to transform, which places the surface into compression and strengthens it.1
If particles do pull out from the surface, they are typically in the submicrometre range. Because these particles are extremely small, they act to polish the surface rather than abrade it, preventing the roughened, matte appearance often seen in conventional ceramics like alumina, where larger, sharp-edged grains can cause further erosion.1