Some Features and Limitations of Zirconia Toughened Nitrogen Ceramics
It has previously been assumed that good toughness can be achieved in silicon nitride based ceramics up to moderate temperatures through the addition of small amounts of zirconia particles to the ceramic matrix, providing the crystalline structure of the zirconia is partially stabilized. However, in all reactions involving the partially stabilized zirconia and silicon nitride in a nitrogen atmosphere, the non-transformable tetragonal zirconia (t’) phase forms by dissolution of small amounts of nitrogen and simultaneous introduction of oxygen vacancies in the crystal structure. The increasing numbers of vacant sites in the anion lattice produces additional stabilization to the zirconia and consequently t → m-ZrO2 transformation is prevented. The effect of nitrogen dissolution in zirconia is clearly evidenced in zirconia containing O’ and O’-β’ composites. As a result, no significant improvement in toughness is observed although the tetragonal zirconia phase is present in the samples.
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