The role of boron in interactions between liquid Si,B alloy and SiC at high temperature: Contribution of Raman spectroscopy, electron energy loss spectroscopy (EELS) and atom probe tomography (APT)

This study examines the protective role of boron towards chemical vapor deposited (CVD) SiC via the synthesis and analysis, at various scales, of model composites representative of SiC/SiC composites prepared by silicon melt infiltration. Comparing the SiC-Si,B and SiC-Si model composites clearly demonstrates the boron's protective effect. Yet, the SiC-Si,B sample shows a macroscopic reactivity gradient related to the progression of the Si,B alloy through the porous SiC network. Electron energy loss spectroscopy (EELS) and atom probe tomography (APT) were used to assess elemental composition at the quasi-atomic and trace scales, in the different phases and at SiC/Si,B interfaces. Boron diffuses into bulk SiC, in agreement with thermodynamic calculations, but also concentrates at structural defects in polycrystalline CVD SiC. EELS and APT both revealed a high boron overconcentration over a few nanometers at SiC/Si,B interfaces, without intermediate layer visible by TEM. Raman spectroscopy also evidenced a microscale gradient of substituted boron in the Si,B alloy, near SiC/Si,B interfaces, which was related to a high carbon concentration and an unexpected carbon environment. These results led to conclude that the addition of boron cumulates three beneficial effects limiting SiC reactivity: (1) a volume effect in SiC, stabilizing and accommodating structural defects (2) an interfacial effect, with B atoms concentrating and stabilizing SiC/Si,B interfaces, and (3) a volume effect in the Si,B alloy, forming B-C clusters inhibiting carbon diffusion. Further RMS, EELS and APT analyses at different locations in the model composites would be useful to explain the macroscopic corrosion gradient.