Structure formation in porous materials produced by gravity-sensitive SHS

The paper presents some theoretical and experimental data on self-propagating high-temperature synthesis (SHS) of high-porosity materials. A classification of SHS systems by their sensitivity to deformation has been proposed. The classification takes into account the formation and lifetime of the volume fraction of melt in the combustion wave. To produce high-porosity cermet foams, a system should meet the following requirements: it has (i) to contain an optimal amount of liquid and solid phases in the product, (ii) to evolve impurity-born gases during formation of the liquid phase without breakdown;of steady combustion, (iii) to burn at a high velocity. Systems with the melt content about 80 vol.%, were found to form closed spherical macropores. High-porosity ceramics (TiC + TiB2) and cermet foams (NiAl-TiC0.6) were obtained by SHS. The porous ceramics contained largely open pores while the cermets closed pores. The effect of gravity on the formation of porous SHS was investigated. SHS was found to yield on inhomogeneous product in a vertically positioned sample and a homogeneous product in horizontally positioned sample.