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 kp′ = 6 × 10−13 g2/cm4 s. Nb3Co3CrSi6 oxidises by formation of a duplex SiO2–Cr2O3 layer but is mainly a Cr2O3-forming compound with kp′ = 1.8 × 10−11 g2/cm4 s and the linear volatilisation constant kl′ = 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.