Effects of the content of in-situ grown mullite on the microstructure and mechanical properties of 3Y-TZP ceramics fabricated by gel-casting

The effects of different content of mullite on the microstructure and mechanical properties of in-situ mullite toughened 3Y-TZP ceramics (Mullite/3Y-TZP composite ceramics) prepared by gel-casting combined with pressureless sintering were observed in this study. The results indicated when the content of mullite was low (2-4 mol%), especially in 4 mol%, mullite showed regular columnar structure and the size of columnar mullite was larger. When the content got higher (> 6 mol%), mullite showed a granular or even fluid shape and was connected to each other. In addition, the bending strength and fracture toughness of the samples showed a trend of rising first and then decreasing with the increase of the content of mullite, but the hardness had been declining. The sample containing 4 mol% mullite had the best mechanical properties (hardness reached 13.02 GPa, bending strength reached 907.31 MPa and fracture toughness reached 10.38 MPa m(1/2)). Furthermore, mullite, alumina and zirconia particles played an important role in toughening reinforcement due to pinning effect.