Development of a diffusion Barrier layer for silicon and carbon in molybdenum : a physical vapor deposition approach

During the last two decades, research on high-temperature, oxidation-resistant coating systems for refractory metals has focused on a variety of silicides (e.g., Mo and Ta silicides), due to their excellent resistance to oxidation. However, commercialization efforts have been thwarted in large measure due to the diffusion of silicon from the coating to the substrate, resulting not only in the depletion of silicon from the coating, but also the formation of less oxidation-resistant subsilicides. Consequently, the development of a high-temperature, diffusion barrier layer for silicon has assumed considerable importance. Furthermore, introduction of carbon in the system, e.g., during the synthesis of MoSi sub 2 -SiC composite thin films on molybdenum substrates, results in the diffusion of both silicon and carbon into the substrate, necessitating the development of a barrier layer for both elements. This article examines the possibility of using a novel approach, that of reactive radio frequency (RF) sputtering, for synthesizing a diffusion barrier (based on the Mo-Si-C-N quaternary system) for both silicon and carbon. It is shown that reactive if magnetron sputtering of a composite target (MoSi sub 2 +1.96 moles SiC) in an argon-nitrogen atmosphere results in the formation of an amorphous layer, of an as-yet undetermined stoichiometry, preventing the diffusion of both silicon and carbon into the molybdenum substrate. This layer is thermally and chemically stable up to at least 1260 deg C.