Kinetics of high temperature oxidation of (Nb,Co,Cr)₇Si₆ and (Nb,Co,Cr)₈Si₇ silicide compounds

Nb₃Co₃CrSi₆ (isostructural to Nb₃Fe₃CrSi₆) and Nb₄Co₄Si₇ (isostructural to Zr₄Co₄Ge₇) intermetallic compounds have been studied from the oxidation point of view. Isothermal oxidation results at 1300 °C are discussed. The volatilisation of the external chromia layer formed by oxidation on Nb₃Co₃CrSi₆ was quantified by analysing the thermogravimetric curves on the basis of the differential equation that describes the oxidation kinetics. Nb₄Co₄Si₇ is a typical SiO₂-forming compound with a parabolic oxidation constant k_p′ = 6 × 10^-13 g²/cm⁴ s. Nb₃Co₃CrSi₆ oxidises by formation of a duplex SiO₂-Cr₂O₃ layer but is mainly a Cr₂O₃-forming compound with k_p′ = 1.8 × 10^-11 g²/cm⁴ s and the linear volatilisation constant k_l′ = 1.85 × 10^-9 g/cm² s. Transformation of Nb₃Co₃CrSi₆ into Nb₄Co₄Si₇ slows down the diffusion of Cr towards the surface leading to an overall decreasing of the Cr₂O₃ formation rate.