Monolithic SiOC ceramics with tailored porosity

Mixing two catalytically cross-linking liquid polysiloxanes leads to porous but crack-free bodies after pyrolysis at 1100 degreesC in nitrogen, The low viscosity of the polysiloxanes allows the application of polymer-based moulding techniques like casting of complex-shaped SiOC components. Porosity and pore formation can be varied by changing the mixing ratio of the polysiloxanes. Pore formation affects strength, elastic modulus, fracture toughness, and creep behaviour. Four point bending strength varies between about 125 and 150 MPa (specimen size 3.5 x 3.0 x 50 mm(3)). Analysis of fracture surfaces shows crack deflection at the pores leading to fracture toughness values of up to 2.0 MPa m(1/2). investigation of the creep behaviour in compression revealed an improved creep resistance compared to silica glass depending on the thermal history of the amorphous material.