DEVELOPMENT OF A SI C DIFFUSION BARRIER LAYER BASED ON THE MO-SI-C-N SYSTEM/

A critical component of a high-temperature oxidation-resistant coating system being developed for molybdenum is a diffusion barrier layer wh ich would minimize interdiffusion between the MoSi2 + X SiC coating an d the molybdenum substrate. Previous results have shown that both sili con and carbon diffuse into the substrate, necessitating the developme nt of a barrier layer for both elements. This paper will examine the p ossibility of using reactive rf sputtering for synthesizing a 'nitride d' layer of MoSi2 or MoSi2 + X SiC. The deposition process consisted o f rf magnetron sputtering of either a pure MoSi2 or a MoSi2 + 1.96 SiC composite target in a nitrogen ambient. A MoSixNy layer, synthesized by reactive sputtering of a pure MoSi2 target, has been shown to be an effective silicon diffusion barrier layer up to 1100 degrees C, by re searchers at Los Alamos National Laboratory. In the present study, the feasibility of producing a diffusion barrier layer for both silicon a nd carbon has been examined in the quaternary system of Mo-Si-C-N. It has been shown that reactive, rf magnetron sputtering of a composite t arget (MoSi2 + 1.96 SiC) in a nitrogen ambient results in the formatio n of an amorphous, diffusion barrier layer for both silicon and carbon diffusion from the overlying coating to the substrate. The barrier la yer remains amorphous after a heat treatment at 1000 degrees C for 30 min. A ball-cratering technique was used, in conjunction with Auger li ne scans, to assess the efficacy of the barrier layer, following a dif fusion anneal treatment. Details of Auger electron spectroscopy, scann ing/transmission electron microscopy and X-ray diffraction analyses ar e presented. (C) 1997 Elsevier Science S.A.