Transition metal nitrides have practical importance in abrasives, cutting tools, and coatings exhibiting high elastic modulus, excellent wear resistance, high hardness, and low coefficient of friction. Due to its low electrical resistivity and chemical inertness, cubic TaN has been used as a diffusion barrier; however, the accumulation of crystallized TaN at the grain boundary results in device failure. Adding a small amount of oxygen to -TaN can avoid this failure. The photocatalytic activity of Ta2O5 is limited to the UV region. Nevertheless, nitrogen doping can expand this behavior to the visible range. Under heavy load in harsh environments, hafnium nitride undergoes brittle fracture, and it can be prevented by incorporating tantalum into it. Addition of Ta to cubic HfN improves its wear resistance, coefficient of
friction, electrical conductivity, and oxidation resistance. The material selection and process optimization would require knowledge of the phase behavior and thermodynamic properties of the ternary and subsystems of Ta-N-O and Hf-Ta-N systems. One of the most common methods to establish such information is to perform the thermodynamic modelling (Calphad approach) of the systems, considering all stable phases. No attempt so far has been reported in this direction. Hence, the objective of the present thesis is to perform the thermodynamic modelling of these systems.
It is now well established that Ta3N5 is a stable phase in the Ta-N system. This phase is a promising candidate for photocatalytic application. However, the currently accepted version of the Ta-N phase diagram does not include Ta3N5 as a stable phase. In the present work, the thermochemical data, such as decomposition temperature and heat capacity of Ta3N5, are experimentally determined. The Ta-N system is reassessed using the CALPHAD approach combined with experiments and ab initio calculations. The revised version of the phase diagram includes Ta3N5.
The Ta-N phase diagram indicates that the thermal decomposition of pure Ta3N5 leads to the formation of -TaN. However, Ta3N5 is usually associated with some amount of oxygen as an impurity mainly due to its synthesis route. We found that the θ-TaN phase, usually observed at high pressures, is forming during the thermal decomposition of oxygen containing Ta3N5. The presence of θ-TaN is verified using several experimental techniques. Elemental distribution analyzed through energy dispersion X-ray spectroscopy reveals about 7 at.% of oxygen in θ-TaN. First-principle calculations compare the thermodynamic stability of oxygen substituted θ-TaN and pure θ-TaN. The computational studies confirm that oxygen in θ-TaN enhances its thermodynamic stability.
The model parameters in the currently accepted version for the Hf-N system cannot reproduce the corresponding phase diagram. Hence, the Hf-N system is reassessed using the CALPHAD approach combined with ab initio calculations. The revised Gibbs energy functions are reproducing the phase diagram.
Keywords: CALPHAD, ab initio calculation, Ta3N5, High-pressure TaN, Oxygen doping, Experiments.