Processing, microstructure and elastic properties of mullite-based ceramic foams prepared by direct foaming with wheat flour

Mullite-based ceramic foams with bulk densities as low as 0.4 g/cm3, porosities between 49 and 88% and average foam cell sizes in the range 100–320 μm are fabricated by direct foaming from suspensions containing wheat flour and partial sintering at 1600 °C. Stereology-based image analysis is used to determine a complete set of global microstructural descriptors, i.e. the porosity from foam cells (8–88%), interface density, mean curvature integral density and the related pore size measures (mean chord length and Jeffries size). Elastic constants are determined via impulse excitation (Young’s moduli 2.3–21.4 GPa, shear moduli 0.9–8.8 GPa, bulk moduli 1.5–12.9 GPa, Poisson ratios 0.193–0.234). Elastic moduli obey the power-law prediction (Gibson–Ashby relation for open-cell foams) for foams with porosities higher than 70%, but for lower porosities they are significantly lower, due to concave pores in the partially sintered matrix.