Effect of SiO₂ and Y₂O₃ additives on the anisotropic grain growth of dense mullite

Mullite powder with a nearly stoichiometric composition was doped with 1.5–5 wt% SiO₂ or 0.5–1.0 wt% Y₂O₃ and hot pressed at 1525–1550 °C to produce almost fully dense materials. The effect of the additives on the grain growth of the dense systems was investigated during subsequent annealing at temperatures above that of the eutectic (∼1590 °C) for the SiO₂–Al₂O₃ system. The average length and width of the grains were measured by image analysis of polished and etched sections. At 1750 °C, anisotropic grain growth was relatively rapid, leading to the formation of rodlike grains. Compared to the undoped mullite, the addition of SiO₂ and Y₂O₃ produced a small reduction in the grain growth kinetics. Transmission electron microscopy revealed that the glassy second phase was concentrated at the three-grain junctions or distributed inhomogeneously at the grain boundaries. For the materials annealed at 1750 °C, the indentation fracture toughness at room temperature increased from 2.0 to 2.5 MPa m^1/2 for the undoped mullite to values as high as 4.0–4.5 MPa m^1/2 for the doped mullite. The implications of the data for enhancing the fracture toughness of mullite by the in situ development of a microstructure of elongated grains are considered.