Thermal Stability of W-xRe/TiC/SiC Systems (x=0, 5 and 25 at % Re) at High Temperature

During this work, we examined the effect of an additional TiC interlayer between SiC and the W-xRe substrate (x=0.5 and 25 at% Re). The first aim was to limit the chemical reaction leading to the formation of the silicide W5Si3 and the carbide WC. Samples were prepared by hot-filament rapid chemical vapour deposition. The TiC layer thicknesses were between 2 and about 100 μm. After thermal treatment between 1573 and 1873 K, the samples were characterized and revealed the existence of a reaction whatever thickness was used, with predominant formation of the carbide W2C. This is the first effect induced by the addition of TiC; the reaction scheme is deeply modified. Another main fact is the role played by the TiC thickness. Depending on layer's size, we can have a variable reduction of the reaction zone with the existence of a maximum effect. Indeed, it appears that the total elimination of the reaction at the highest temperatures is hardly conceivable owing to the intrinsic diffusion of carbon atoms into the TiC. In many cases, a reaction occurs with the formation of the carbide W2C. It appears during our investigations that, at above 1673 K, molten silicon is obtained between the SiC and the TiC. In comparison with the data obtained at 1573 K, the presence of this molten phase induces a strong limitation for carbon diffusion, even for thin TiC layers. Nevertheless, it seems impossible to reach an efficiency of 100% above 1673 K. This aim is probably more accessible at lower temperatures according to extrapolations at 1573 K. The third and last parameter considered herein was the role played by the concentration of rhenium into the substrate. According to our results, the addition of this metal is quite beneficial; a more-important effect is clearly obtained, especially for the thinner TiC layer, the maximal effect appearing to be slightly more pronounced when the rhenium concentration increases. We can deduce from this study that TiC is an efficient diffusion barrier especially for silicon, but its effect is less notable for carbon. Nevertheless, we can propose some methods to perfect the protection of metallic substrates in the presence of SiC. The first is to use a compound without carbon; the most evident option is TiN. We are actually considering this possibility. Another interesting possibility is to use WC because it is stable in presence of SiC. Temperature is, however, a limiting factor, because this compound WC reacts with W at 1420 K to form W2C, which is not stable in the presence of SiC. As a consequence, this carbide can only be used up to 1420 K. The carbon mobility in these two compounds has to be established. (O.M.)