Significantly improved mechanical properties of mullite ceramics by adding AlOOH with different sizes and morphologies

Mullite ceramics with high purity and toughness were prepared by hot-press sintering of pyrophyllite at 1300°C using AlOOH nanomaterials with different sizes and morphologies (nanoparticles, nanorods, nanoflakes, and micro-sized sea urchin–like) as additives. Among the four types of AOOH additives, the incorporation of nanoflakes and sea urchins resulted in the formation of a relatively uniformly distributed and tightly packed microstructure within the ceramics, which significantly improved the density and mechanical properties of the ceramic materials. Compared to nano-sized AlOOH, the addition of micron-sized sea urchin–like AlOOH could produce mullite ceramics with best purity and flexural strength. The flexural strength and fracture toughness of ceramics prepared from micro-sized sea urchin–like AlOOH and pyrophyllite reach 427.34 ± 1.99 MPa and 4.68 ± .31 MPa m^1/2, respectively. During the ball milling process, the originally micron-sized sea urchin–like AlOOH particles were broken down into micro- and nano-sized AlOOH particles. The resulted micron and nanoscale AlOOH particles exhibited synergistic and multi-scale effects with pyrophyllite, which contributed to the formation of uniformly sized and densely arranged mullite crystals within the ceramics. Additionally, the bridging between the mullite crystals further improved the mechanical properties of the mullite ceramic material.