Foaming methods for the production of porous ceramics based on ZrB2 and SiC

Different methods of direct and indirect foaming were applied to produce porous ceramics based on ZrB2 and/or SiC. Clot forming was used to produce highly porous ZrB2 ceramics by exploiting the foaming property of ovalbumin and its ability to thicken after heating around 80°C. SiC foams were developed by in situ foam formation in alkaline environment using a low temperature process such as a chemical consolidation. An alkali aluminosilicates binder was used. The foaming agent was metallic silicon iimpurity in SiC powder. Replica method was used to produce ZrB2- SiC porous composite. Polyurethane templates generated empty ceramic struts that decrease mechanical performances. Foams from PU showed very low compressive strength: 0.7 MPa with porosity >=85 vol% and 0.3 MPa with porosity >=88 vol%. Thank to hydrophilic surfaces, cellulose or natural sea sponges were permeated by the suspension, yielding open cells with dense wall structure after sintering, thus increasing mechanical strength up to 4.8 MPa. Finally, the possibility to shape into graded porous structures by freeze casting was investigated on ZrB2 and ZrB2-SiC composites. The total porosity was about 63-64 vol % and hierarchical architectures with anisotropic porosity were realized, which showed porosity formed by mainly unidirectional channels separated by ceramic plates. The compressive strength was improved compared to sponge-templated foams.