An Electron Microscopy Study on the Prevention of SiC-Fiber Decomposition via the Integration of a Buffer Layer into ZrB₂-based Ceramics

SiC-fiber (SiCf) reinforced ZrB₂-based ultra-high temperature ceramics (UHTCs) demonstrate improvements in several critical aspects of pure ZrB₂. These include the poor failure tolerance, challenging densification at high pressure and temperature, the low oxidation resistance, and the high overall density. Further, MoSi₂ is a notable sintering additive for ZrB₂, which not only enhances densification and mechanical properties at high temperatures but also provides elevated oxidation resistance up to a minimum of 1650°C. The combination of both, SiC fibers and MoSi₂ within one ZrB₂ ceramic shows promise. However, active oxidation of the SiC fibers limits its possible application to temperatures of 1500°C and is not compatible with the required sintering temperature of MoSi₂ of approximately 1750°C. Moreover, the fibers suffer from deterioration due to reactions with Mo or Mo-compounds, yet taking place during the sintering process and thus jeopardizing its beneficial effects to the ZrB₂-SiCf ceramic. Hence, combining SiC fibers and MoSi₂ in a pure ZrB₂ ceramic has not been feasible thus far. A novel approach, based on the concept of functionally graded materials (FGM), aims to obviate the adverse reactions between the fibers and Mo or Mo compounds during the sintering process. A Mo-diffusion-resistant buffer layer is introduced between the SiCf-containing ZrB₂-ZrSi₂ bulk and the outermost oxidation-protective ZrB₂-MoSi₂ layer. This buffer layer consists of ZrB₂ doped with either Si₃N₄ or the polymer-derived ceramics (PDCs) SiCN and SiHfBCN. Scanning Electron Microscopy (SEM) joint with Energy-Dispersive X-ray Spectroscopy (EDS) investigations confirmed that this sample geometry and selected compositions effectively prevent SiC-fiber degradation by interactions with MoSi₂. In contrast, the SiC fibers in a reference sample lacking a buffer layer experienced significant degradation due to MoSi₂ diffusion into the fiber core. A comparison of the three buffer layer systems studied here revealed that incorporating Si₃N₄ resulted in moderate changes in the fiber structure, whereas PDC-doped samples exhibited minor structural changes within the fibers. Microstructural investigations via (scanning) transmission electron microscopy ((S)TEM) endorse the buffer layer in its function as a diffusion barrier for MoSi₂, as only negligible amounts of Mo inside the buffer layers and the fiber/bulk are detectable. Within the buffer layer, the microstructure is dominated by the formation of SiC, BN, Zr(Hf)O₂, and Zr₂CN, which are high-temperature stable compounds and do not impair the overall UHTC-performance of the ZrB₂ ceramic. The changes in the SiC-fibers structure can be primarily ascribed to reactions with the ZrB₂-ZrSi₂ bulk. Derived from the microstructural investigations the overall diffusion mechanisms in the reference sample without a buffer layer and the samples containing a buffer layer were deduced. The layered structure of the samples is concomitant with different eutectics formed in each zone of the composites. Due to the buffer layer, the formation of an early eutectic at the top layer-bulk interface is avoided, being otherwise responsible for unhampered Mo diffusion towards the fibers. Moreover, the sequential progression of the eutectic melts, beginning in the SiC-fiber-containing ZrB₂-ZrSi₂ bulk and extending through the buffer layer up to the top layer, ultimately prohibits a progressive diffusion of a Mo-Si melt phase towards the fibers, preventing their structural disintegration by MoSi₂. Subsequent oxidation at temperatures of 1500°C and 1650°C for 15 min demonstrates the sustained efficacy of all buffer layers. Only very minor modifications were observed in the buffer layer or fibers compared to non-oxidized samples, affirming the persistent effectiveness of the buffer layers in impeding the UHTC-composite degradation during oxidation.