Experiments and numerical modelling of gasless combustion processes (Lecture)

Experiments have been performed to produce TiC and TiB2-based cermets by Self-sustained High temperature Reactions or gasless combustion starting from the pure elements: titanium, carbon and boron and the admixing of inert metallic powders. Experimentally it has been observed, that porosity in the reactant mixture is needed for the reaction front to propagate. Furthermore, due to the exothermicity of the process and the increased density of the reacted material, the final product is characterized by a large remaining porosity (typically 50%). In order to produce dense ceramics or cermets, there is a need for a subsequent densification step which is often hard to achieve in ceramic composite materials due to their high deformation resistance. Therefore, the densification pressure must be applied within seconds after the Self-sustained High-temperature Reactions when the temperature of the reacted material is still above the ductile-to-brittle transition temperature and/or the melting temperature of the metallic phase, which acts as a binder.
The time window for densification is determined by the end of the combustion process on the one hand, and the solidification of the final product on the other hand. In order to be able to design the process, numerical simulations are carried out and will be compared to experiments.
In this paper, numerical modelling will focus on the physics of the heat generation and heat conduction process in Self-sustained High-temperature Reactions. Later the chemistry will be taken into account as well. Incorporation of the chemistry in gasless combustion processes is essential to have a predictive capability for example in the production of ceramics and cermets by the process described above and in the design of pyrotechnics delays as well. In small geometries a conflicting requirement between a reliably sustained reaction and a finite burning velocity is anticipated because of heat losses to the surroundings. Comparison of experiments with detailed numerical modelling is aimed to find the limits of gasless combustion processes in relation to the material properties.