This basic research investigates the microstructure evolution of a composite based on ZrB 2-MoSi 2 from the as-sintered features to the changes occurring upon oxidation at ultra-high temperatures, 1650 and 1800°C, in a bottom-up loading furnace for 15 minutes. Scanning and transmission electron microscopy evidenced the formation of a matrix typified by ZrB 2-cores surrounded by (Zr,Mo)B 2-rims with dispersed MoSi 2 particles and SiO 2 glass trapped at the triple junctions. The oxidation at 1650°C induced the migration of silica to the surface, which formed a continuous and protective scale. Below this scale, the matrix evolved into ZrO 2 grains encasing MoB nano-inclusions, as a result of the oxygen and boron oxide partial pressures established in the subscales. Underneath, a MoSi 2-depleted boride region, but substituted by SiO 2 and MoB was found. The same phases were observed upon oxidation at 1800°C, but a thicker and more turbulent oxidized layer formed as a consequence of the rapid evolution of MoO 3 , SiO and B 2 O 3 gases from the scales beneath the outermost silica-layer. According to the observed phases and the calculated phase stability diagrams, the partial pressures gradient within the oxide layer were defined and the effect of Mo-doping in boride matrices on the oxidation behavior was compared to that of other transition metals to establish a criterion design for the realization of ceramics with improved oxidation resistance.

This basic research deals with the microstructure evolution of a W-doped ZrB 2 ceramic, as-sintered and upon oxidation at 1650°C. Transmission electron microscopy enabled to disclose microstructural features occurred during oxidation never observed before. In the pristine material, (Zr,W)B 2 solid solutions surround the original ZrB 2 nuclei, whereas refractory W-compounds at triple junctions and clean grain boundaries are distinctive of this ceramic. After oxidation, the microstructure is typified by intragranular nanostructures, in which nanosized W inclusions remained trapped within ZrO 2 grains, or decorate their surfaces. The understanding of the oxidation reactions occurring in the system as a function of the oxygen partial pressure was fundamental to conclude that W-based compounds do not notably suppress or retard the oxidation of ZrB 2 ceramics. K E Y W O R D S ceramic, inclusions, oxidation, SEM, TEM

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.

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