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 synthesisand analysis, at various scales, of model composites representative of SiC/SiC composites prepared by siliconmelt infiltration. Comparing the SiC-Si,B and SiC-Si model composites clearly demonstrates the boron’s protectiveeffect. Yet, the SiC-Si,B sample shows a macroscopic reactivity gradient related to the progression of theSi,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 andat SiC/Si,B interfaces. Boron diffuses into bulk SiC, in agreement with thermodynamic calculations, but alsoconcentrates at structural defects in polycrystalline CVD SiC. EELS and APT both revealed a high boron overconcentrationover a few nanometers at SiC/Si,B interfaces, without intermediate layer visible by TEM. Ramanspectroscopy 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 toconclude that the addition of boron cumulates three beneficial effects limiting SiC reactivity: (1) a volume effectin SiC, stabilizing and accommodating structural defects (2) an interfacial effect, with B atoms concentrating andstabilizing SiC/Si,B interfaces, and (3) a volume effect in the Si,B alloy, forming B-C clusters inhibiting carbondiffusion. Further RMS, EELS and APT analyses at different locations in the model composites would be useful toexplain the macroscopic corrosion gradient.