AbstractsEngineering

Ceramic-metal composites offer unique combinations of hardness and toughness which make them attractive for a variety of applications. Among the ceramic-metal composites (cermets), metal-sulfur compositions may be produced using the Self-propagating High-temperature Synthesis (SHS) technique, a process where the material is created using a self-sustaining combustion reaction. The material studied in the present thesis consists of the product of the reaction between chromium and sulfur powders. The objectives of this study are i) to demonstrate the production of dense chromium-chromium sulfide cermets from the initial reactant powders using combustion synthesis, ii) to investigate the combustion process itself, including a microstructural analysis of the reactants and products, and iii) to evaluate the physical and mechanical properties of the cermets and to correlate them with chemical composition. We propose a new method for the preparation of the metal-sulfur precursor mixture based on the ability to melt-cast the precursor mixture. This ability, with the properties of the reaction products, enables the creation of near-net shape structures and offers the possibility of tuning the microstructure and material properties by changing the composition of the reactant. We began with thermochemical calculations and the phase diagram of the Cr-S reaction with varying equivalence ratios and the pressure at which the synthesis occurs to determine the adiabatic flame temperature and phases of the product at flame conditions using FactSage, a comprehensive multi-phase, multi-component equilibrium database. By modifying the synthesis process, it is found that with an improved manufacturing process, dense Cr-CrS cermet samples without any micro- and macro-cracks, and with porosity about 7% can be produced. To develop a better understanding of the Cr-S system, the combustion characteristics of the system were investigated including the quenching thickness, heating zone thickness, flame velocity, and burning rate. The quenching dimension (i.e., the minimum size of sample that can sustain flame propagation with heat losses) in various geometries was measured and linked to the reaction zone thickness of the flame front. The results obtained illustrate that flames in metal-sulfur compositions are dominated by the reaction zone thickness, in contrast to classical gaseous flames for which the flame thickness is dominated by the preheat zone. The chemical thickness of the front is estimated and it is found that the flame thickness in the present case is dominated by the thickness of the reaction zone. The flame propagation rate affects the heating and cooling rate, which influences the grain growth and other properties of the product. The observation of anomalous propagation modes in which the combustion front appears to lead at the edges of the charge reiterates the importance of differentiating the apparent propagation velocity from the fundamental (normal) burning velocity for SHS systems with liquid products. To investigate the…