Prevention of SiC-fiber decomposition via integration of a buffer layer in ZrB2-based ultra-high temperature ceramics

A ZrB2-based ceramic, containing short Hi-Nicalon SiC fibers, was fabricated with a Mo-impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB2-MoSi2 layer, in order to prevent inward Mo diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB2 doped with either Si3N4 or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning electron microscopy imaging and elemental mapping via energy-dispersive X-ray spectroscopy showed that this tailored sample geometry provides an effective diffusion barrier to prevent the SiC fibers from deterioration due to reactions with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without buffer layer is strongly degraded by MoSi2 diffusion into the fiber core. The comparison of the three buffer-layer systems showed a moderate alteration of the fiber structure in the case of Si3N4 addition, whereas in the PDC-doped samples hardly any structural change within the fibers was observed. A stepwise reaction mechanism is deduced, based on the continuous progression of a reaction zone that propagates toward the ZrB2-MoSi2 top layer. The progression of such a reaction zone as a consequence of the different eutectic melts forming in the different layers, that is, first in the SiC-fiber-containing bulk, then in the buffer layer itself, and finally in the top layer at high temperature, allows for an effective separation of the ZrB2-MoSi2 top layer from the SiC fibers. Subsequent oxidation at 1500 degrees C and 1650 degrees C for 15 min did not affect the efficiency of all three buffer layers, since no structural changes regarding buffer layer and fibers were observed, as compared to the non-oxidized samples.