Kinetics and mechanism of the low-pressure,high-temperature oxidation of tungsten silicides-III

Oxidation studies were performed on silicon-coated tungsten which had been treated to form various intermetallic compounds on the surface. Two classifications of behavior were noted (1) a passivated state in which a protective layer of SiO₂ formed at low temperatures and (2) a combustion state in which volatile oxides such as SiO, WO₂, and WO₃ formed. A simple kinetic model has been proposed to account for the abrupt transition from one state to the other. The temperature-pressure conditions under which the transition occurs have been delineated by a thermodynamic study. The combustion state is characterized by the linear growth of W₅Si₃ on WSi₂ surfaces and by the growth of a solid solution of silicon in tungsten on W₅Si₃ surfaces. The oxygen consumption on W₅Si₃ surfaces followed first-order kinetics with an activation energy of 19±1 kcal/mole. The activation energy for oxygen consumption on the solid solution was the same as for pure tungsten, 28±1 kcal/mole, but the kinetic order of the solid solution was 0.9 compared to 0.7 for pure tungsten.