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Why do worn alumina surfaces often look matte or rough?
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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.
Worn surfaces of alumina often appear matte or rough due to the material's susceptibility to grain pullout during use.1
As the ceramic undergoes wear, individual grains are loosened from the surface structure. This process weakens the surface, leading to more rapid erosion. Furthermore, these loosened grains possess sharp edges, which can then act as abrasive particles that contribute to further impingement wear elsewhere. This combination of grain loss and surface degradation results in the characteristic matte, roughened appearance of the worn material.1