Abstract text (incl. figure legends and references)

A ZrB₂–based ultra-high temperature ceramic (UHTC), containing short Hi-Nicalon SiC fibers, was separated from the outermost oxidation resistant ZrB₂-MoSi₂ layer by sandwiching it with a Mo-impermeable buffer layer, in order to prevent fiber decomposition.

This additional layer consisted of ZrB₂ doped with either Si₃N₄ or with the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning in addition to transmission electron microscopy imaging (SEM/TEM) and elemental mapping via energy-dispersive X-ray spectroscopy (EDS) showed that this tailored sample geometry provides an effective Mo-diffusion barrier, preventing the SiC fibers from deterioration due to their reaction with Mo or Mo-compounds. In contrast, the structure of the SiC fibers in a reference sample without such a 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 SiC fibers was observed.

A stepwise reaction mechanism is proposed, based on the continuous progression of a reaction zone that propagates towards the ZrB₂-MoSi₂ top layer. The progression of such a reaction zone, which is seen as a consequence of the different eutectic melts forming in the different layers, i.e., first in the SiC-fiber containing bulk, followed by the buffer layer itself, and finally in the top ZrB₂-MoSi₂ layer at high temperature, allows for an effective separation of the oxidation resistant top layer from the SiC fibers that are embedded in the bulk.

Subsequent oxidation of the UHTCs at 1500°C and 1650°C for 15 min did not affect the efficacy of all three buffer layers containing Si₃N₄, SiCN or SiHfBCN, since no structural changes within the corresponding buffer layer as well as of the fibers were observed, as compared to the non-oxidized samples.