The oxidation of cobalt based alloys containing partially dissolved particles of Si3N4 at 1000°C

The isothermal oxidation behavior of Co-Cr and Co-Cr-Si alloys with and without 5, 10, and 15 vol.% dispersions of unstable Si₃N₄ particles was studied in 1 atm of oxygen at 1000°C. The dispersion of Si₃N₄ which dissolved partially in the matrix, greatly reduced the oxidation rate of Co-Cr alloys. Silicon nitride was found to promote the formation of continuous Cr₂O₃ layers at low chromium concentrations. Furthermore, the unstable Si₃N₄ was more effective in reducing the oxidation rate than an alloy containing an equivalent amount of silicon. Additions of 15vol.% Si₃N₄ tended to increase the oxidation rate by forming nonprotective SiO₂ particles which disrupted the protective Cr₂O₃ scale. The mechanism of oxidation was altered due to the Si₃N₄ additions. Marker experiments indicated oxygen diffusion inward via the CoO lattice, rather than a combination of both oxygen and metal ion motion which is encountered in Co-Cr alloys.