Semiclosed-cell mullite foams: Preparation and macro- and micromechanical characterization

The preparation and characterization of the properties of mullite ceramic f oams suitable for diesel exhaust filtration systems are described. Two foam s of different cell sizes, 32 and 61 ppi (pores per inch), were prepared fr om slurries by the replication process with polyurethane sponges as the tem plates. The microstructure and the dependence of the porosity and the mecha nical properties on the preparation and resulting foam cell size and normal ized density are discussed. In addition the micro- and macromechanical prop erties have been characterized using instrumented indentation techniques; w ith sharp (Berkovich) and small spherical-tipped indenters the hardness and modulus of the dense strut materials have been measured. The macroproperti es have been measured with a large sapphire indenter, similar to 5 mm radiu s using a load partial-unloading technique. The latter enables the modulus and the contact pressure versus penetration to be measured. These values ar e compared with traditional bulk measurements for crushing strength and ela stic modulus, The results are discussed in terms of available theoretical t reatments of the indentation of brittle porous materials.