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°C and 1650°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.