Effect of SiO 2 and Y 2 O 3 additives on the anisotropic grain growth of dense mullite
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T-I. Mah and T.A. Parthasarathay Materials Directorate, Wright Laboratory, WL /MLLN, Wright-Patterson Air Force Base, Ohio 45433, and UES, Inc., 4401 Dayton-Xenia Road, Dayton, Ohio 45432 (Received 27 May 1999; accepted 13 December 1999)
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 m1/2 for the undoped mullite to values as high as 4.0–4.5 MPa m1/2 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.
I. INTRODUCTION
Anisotropic grain growth is a type of abnormal (or exaggerated) grain growth in which the grains develop elongated shapes with faceted sides. When used in a controlled manner, it can lead to the achievement of unique microstructures such as an interlocking network of randomly oriented, elongated grains and preferential grain alignment. With proper manipulation of the grain and grain boundary characteristics, it can lead to the development of materials with enhanced properties when compared to similar systems with equiaxial grains. The approach has been used to enhance the fracture toughness of silicon nitride1,2 and silicon carbide,3,4 to enhance anisotropic dielectric properties in ferroelectric ceramics5,6 and to improve the properties of superconducting ceramics.7,8 Anisotropic grain growth is frequently observed in materials with hexagonal, chainlike and layered crystal structures. The mechanism of growth is not clear but several factors have been suggested as being important for the process. These include anisotropic surface energies, twinning, segregation of impurities and dopants, and anisotropic wetting by liquid phases.9–11 Mullite is an important material for several applications in the traditional ceramics sector, such as refracto718
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J. Mater. Res., Vol. 15, No. 3, Mar 2000 Downloaded: 18 Mar 2015
ries and refractory lining for furnaces in the manufacture of glasses. It is also an important material for advanced structural applications at high temperatures because of its high melting point (1
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