Effectiveness of Diffusion Barrier Coatings for Mo-Re Embedded in C / SiC and C / C

Advanced high-temperature cooling applications such as heat-pipe-cooled leading edges may often require the elevated-temperature capability of carbon/silicon carbide (C/SiC) or carbon/carbon (C/C) composites in combination with the hermetic capability of metallic tubes. In some applications, it may be advantageous to co-process metallic tubes with the composite. Under those conditions, it is important to understand the effect of the harsh environment of co-processing and additional thermal exposure on the metallic tubes in contact with carbon and silicon carbide. In this paper, the effects of C/SiC (using pyrolytic carbon (PyC) and CVI SiC processing steps) and C/C on tubes fabricated from several different refractory metals were evaluated. Though Mo, Nb, and Re were evaluated in the present study, the primary effort was directed toward two alloys of Mo-Re, namely, arc cast Mo-41Re and powder metallurgy Mo-47.5Re. Samples of these refractory metals were subjected to either the PyC/SiC deposition or embedding in C/C. MoSiz(Ge), R512E, and TiB 2 coatings were included on several of the samples as potential diffusion barriers. Also, several of these samples were exposed to time at temperature in a vacuum as controls. The effects of the processing and thermal exposure on the samples were evaluated by conducting burst tests, microhardness surveys, and scanning electron microscopic examination (using either secondary electron or back scattered electron imaging and energy dispersive spectroscopy). The results showed that a layer of brittle Mo-carbide formed on the substrates of both the uncoated Mo-41Re and the uncoated Mo-47.5Re, subsequent to the C/C or the PyC/SiC processing. Both the R512E and the MoSiz(Ge ) coatings were effective in preventing not only the diffusion of C into the Mo-Re substrate, but also the formation of the Mo-carbides. However, Mo-silicide layers formed on the Mo-Re substrates as a result of the chemical interaction with both the R512E and the MoSiz(Ge ) coatings. This Mo-silicide layer grew with heat treatment time, albeit at a slower rate than the Mo-carbide layer in the uncoated condition, and thus reduced the effective thickness of the substrate. The development of lateral and transverse cracks in the R512E coating may have diminished the efficacy of that coating. For the Mo-47.5Re alloy, burst strength degraded moderately (relative to the uncoated condition) when the alloy tubes were coated with either MoSiz(Ge ) or TiB 2. The degradation in burst strength was more drastic subsequent to the coating followed by the PyC/SiC deposition, with the degradation from TiB 2 being more severe than from MoSiz(Ge ). MoSiz(Ge ) thus appeared to be more effective than TiB 2 as a diffusion barrier coating for Mo-47.5Re. However, none of the coatings were effective at preventing both C and Si diffusion without some degradation of the substrate.