Oxidation modeling of a Si3N4-TiN composite : Comparison between experiment and kinetic models
Résumé
Based on microstructural observations, a phenomenological oxidation model and associated oxidation kinetics have been developed for a Si3N4-50 wt.%TiN ceramic composite. Above 1000 °C, the oxidation mechanism is complex and has been described in a three-step model. In a first step, the oxidation of TiN in TiO2 is controlled by titanium diffusion, whereas Si3N4 oxidation in SiO2 is controlled by O diffusion through SiO2. The outward diffusion of Ti creates pores in the sub-layer, which decrease the Ti migration rate. In a second step, the oxidation is controlled by oxygen diffusion, both through SiO2 and TiO2, and SiO2 progressively fills the porosity due to the outward diffusion of Ti. In the third step, SiO2 covers the sub-layer entirely, and the oxidation is controlled by O diffusion through SiO2. Isothermal and non-isothermal thermogravimetric analyses (TGA) have been carried out on the Si3N4-TiN composite under air up to 1200 °C, in order to compare experimental data with the three-step oxidation model. An analysis method for non-isothermal TGA is presented. The results of this analysis, and of isothermal TGA, have enabled the identification of the main kinetic parameters characterizing the three steps of the oxidation kinetic model. The kinetic models corresponding to the first two oxidation step are in proper accordance with the experiment, whereas a gap is observed between the parabolic kinetic model of the third step and the experiment.
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