Experimental thermodynamic approach of solid solution boundaries of the ZrCxOy oxycarbide

International audienceThe establishment and modeling of the C-O-Zr ternary phase diagram rely on thermodynamic and diagrammatic data. The knowledge of the C-Zr and O-Zr binary phase diagrams, described independently, is required. A comprehensive thermodynamic description of these binary systems is needed in order to establish the associated ternary system. The ternary solid solutions are modeled using specific models and interaction parameters assessed through experiments and available data from the literature. Starting from already published thermodynamic descriptions of the different phases and models, the ternary system could not be satisfactorily described using conventional models because ZrCxOy is reported as a partial solid solution. In addition, the evolution of the boundaries of this solid solution is not well established as a function of the temperature and hence for the determination these boundaries, two complementary approaches were undertaken. Based on a previous work which allowed the understanding of the reactional mechanisms involved during the carbothermal reduction of zirconia, carbon black and ZrO2 powders were mixed in different proportions to synthesize oxycarbides. The mixtures were then heat treated under flowing Argon in a graphite furnace during a long dwell time to make sure that the reaction is complete. Several temperatures were chosen to check the evolution of the solid solution boundaries. The characterization methods were coupled and compared so as to determine the stoichiometry in carbon and oxygen of synthesized oxycarbides (elemental analysis, wavelength dispersive spectroscopy, Rietveld refinements and nuclear microprobe).Thus, the reaction between zirconium carbide and ZrO2 was studied at the same temperatures in order to understand the reactional mechanisms involved during this reaction and also to check the amount of oxygen which can be inserted in the oxycarbide lattice. These two approaches will be useful for the optimization of adjustable variables using the CALPHAD method