Anisotropic Grain Growth and Microstructural Evolution of Dense Mullite above 1550°C

Abstract

Mullite powder with a nearly stoichiometric composition was hot-pressed at 1550°C to produce an almost fully dense microstructure of fine, nearly uniaxial grains. The grain growth of the dense mullite was investigated during subsequent annealing at temperatures in the range of 1550 -1750°C. Grain growth was relatively slow at 1550°C and the microstructure remained nearly equiaxial. Annealing at temperatures above the eutectic temperature (ϳ1590°C) produced fairly rapid anisotropic grain growth. At 1750°C, the anisotropic grain growth can be divided into two stages. In the first stage, the initial microstructure with an anisometric shape factor of 1.7 evolved rapidly into a microstructure with a shape factor of 2.7, consisting of a significant fraction of highly elongated grains. In the second stage, the microstructure evolved slowly into a system consisting of somewhat "blocky" grains with a shape factor of 2.2. The Al 2 O 3 content of the mullite grains increased slightly and reached an equilibrium value during the first stage of anisotropic grain growth. For the samples annealed at 1750°C, the indentation fracture toughness (2.5 ؎ 0.2 MPa ⅐ m 1/2 ) was almost independent of the anisometric shape factor. The interaction between the indentation cracks and the microstructure showed a predominantly transgranular mode of crack propagation. The data indicate that while a network of highly elongated grains can be developed by the present approach, some further manipulation of the grain boundary chemistry is required for an improvement of the fracture toughness.

Low‐Temperature Mullite Formation in Ternary Oxide Coatings Deposited by ALD for High‐Temperature Applications

“…When processing mullite powders by conventional powder metallurgy, molding the powders and later sintering the compacts, this overlapping is seen as a problem, as the final structure presents then low densification. [24] Conversely, this feature is seen as an advantage for the inverse opals, hence it could aid to keep the original porous ordered structure, which is by definition composed mostly by macropores (originally occupied by the polystyrene (PS) spheres).…”

“…Mullite has been synthesized by conventional powder metallurgy method, [24] CVD, [25] and sol-gel synthesis. [26] Sintering of mullite powders usually requires high temperatures for densification because of the low bulk and grain-boundary diffusion coefficients and the processing of nanosized powders to full density is a remaining challenge.…”

“…[27] Manufacturing mullite by sintering of silica and alumina powders usually requires temperatures of 1450 °C or even higher for phase conversion, [28] with undesired resulting glassy phase from silica sources. [24] Although sol-gel approaches can lower the conversion temperature, their formed layer also undergoes large shrinkage during drying, except if the liquid is removed under very slow rates or by supercritical drying. This large amount of shrinkage may build substantial residual stresses, which in turn could cause cracks and film delamination.…”

In-situ synthesis and sintering of mullite glass composites by SPS

“…There are many ways to obtain mullite with higher aspect ratio, such as applying high temperature in sintering [15] or adding low melting-point components in liquid-solid reaction synthesis [16]. However, it is noted that the enhanced diffusion of Si 4+ ions is believed to be the underlying reason for oriented growth of mullite [15][16][17].…”

“…There are many ways to obtain mullite with higher aspect ratio, such as applying high temperature in sintering [15] or adding low melting-point components in liquid-solid reaction synthesis [16]. However, it is noted that the enhanced diffusion of Si 4+ ions is believed to be the underlying reason for oriented growth of mullite [15][16][17]. Several works have shown that electromagnetic field can enhance the movement of ions [8], and it is therefore possible that the field produced in the SPS machine may also have enhanced the diffusion of Si 4+ ions, and this is more so in the experiment with the higher SPS current.…”