The ZS and ZSC composites were selected for the study of strength degradation at high temperature. The degradation in the mechanical properties was studied as a function of exposure time to atmospheric air at 1400 °C. These results are summarized in Fig. 9, where the room temperature strength of the materials is plotted as a function of exposure time. For ZS, a reduction of 33% in strength occurred between 16 and 24 h, while for ZSC less than 16 h were needed. This can be explained considering that C burns in the oxidizing atmosphere, creating pores and channels through which air can permeate.
The microstructure and thickness of the oxide layers formed after exposure to atmospheric air at 1400 °C was studied for both ZS and ZSC samples, which were cut and polished for observation in the SEM. Fig. 10(left) shows a micrograph and compositional maps for a ZS sample annealed for 1 h. The outer, Si and O rich layer can be concluded to be SiO2, while the intermediate layer, which is O rich and B poor, is ZrO2. Similar conclusions can be drawn from Fig. 12(right), which represents compositional maps for a ZSC sample annealed for 24 h. Again, the outer layer is mainly composed of SiO2, and an intermediate layer of ZrO2 separates the former layer and the bulk interior of the sample.
These observations were confirmed by quantitative analysis, of which an example is presented. Fig. 11 is a cross section of a ZSC sample annealed for 24 h at 1400 °C in air, and contains several of the features previously observed. The microstructure of the oxide layer can be divided into four different zones or regions. The first, outer one is composed of ZrO2 grains embedded in a glassy SiO2 matrix, as can be deduced from the elemental composition. The zone labeled as number 2 is composed only of SiO2, while the zone labeled as 3 is composed almost exclusively of ZrO2 and contains no Si. The fourth zone corresponds to the composition of the bulk, as-fabricated material. For ease of comparison, raw spectra obtained from all four different zones are depicted.
These observations confirm the oxidation process already outlined in previous references, such as Refs. , , ,  and . Both ZrB2 and SiC are oxidized, producing B2O3 that evaporates at high temperatures. The SiO2 formed, which is liquid at the studied conditions, is expelled towards the surface of the sample by capillary forces, and acts as a protective layer. The intermediate layer is thus composed mostly of ZrO2 and pores that allow for oxygen permeation. It is thus expected that the ZSC samples, containing carbon that burns out at high temperature, will oxidize at a faster rate because of the porosity produced during carbon combustion.
To ascertain this effect, the thickness of both the SiO2 and ZrO2 oxide layers were measured as a function of annealing time, for both ZS and ZSC samples. These results are presented in Fig. 12. ZS material shows an increase of the oxide layer as , typical of diffusion controlled process. This is expected as oxygen diffusion trough the SiO2 layer, in either molecular or atomic form, is necessary to further increase the oxide layer thickness.
In the ZSC material the carbon readily burns out, creating channels and a rapid formation of a thick SiO2 layer. Once the initial SiO2 layer (with thickness of approximately 50 μm) is formed, the process is controlled by oxygen diffusion and the thickness increase of the reaction layer can be fit as , typical of diffusion process in which an initial condition exists. The good fit to the experimental data shown in Fig. 12 supports the hypothesis of a diffusion controlled oxidation process.