Structure and Chemistry of Oxide Films Thermally Grown on Molybdenum Silicides

The oxides grown during thermal oxidation of molybdenum silicides were studied using electron microscopy, infrared absorption, and thermogravimetric techniques. Analysis of the oxidation mechanism based on thermochemical and diffusion rate data was correlated with experimental results. Because of the higher oxidation potential of silicon, it may be preferentially depleted from the silicide substrate to form a single‐phase silica layer. Whether this occurs depends also on the relative diffusion rates in the oxide and substrate layers. Alternately, a duplex oxide layer containing both silica and molybdenum oxide phases will form. The molybdenum oxide phase is a necessary condition for the silicide pest, a low‐temperature failure mechanism in which oxidation results in a nonadherent powder rather than in a protective film.