TEM characterization of turbostratic and rhombohedral BN interphases synthesized by chemical vapour infiltration in SiC/SiC-Si composites

BN interphases were synthesized by chemical vapour infiltration (CVI) from BCl3 and NH3 at 900 degrees C and 1200 degrees C within preforms made of a single 2D woven SiC fibre ply. Then CVI of a SiC matrix followed by molten silicon infiltration resulted in laboratory-made single-ply SiC/SiC-Si composites representative of similar industrial ceramic matrix composites. The interphases and interfaces were characterized by transmission electron microscopy. It has been found that, in the two cases, the interphases are mostly turbostratic BN. Those synthesized at 900 degrees C are very poorly crystallized and exhibit a quasi-isotropic microstructure. An equiaxed interlayer is obtained prior to the columnar CVI growth of SiC matrix. Interphases synthesized at 1200 degrees C exhibit high degrees of crystallization and structural anisotropy. The SiC matrix grows directly in columns on BN. For interphase thicknesses of 300 nm and more, triangular-shaped crystallites of rhombohedral BN appear in turbostratic BN and the columnar growth of SiC is disoriented.

Automated summary

The BN interphases synthesized by CVI at 900 and 1200 degrees C exhibit varying degrees of crystallization and microstructure, affecting the growth of the SiC matrix in different ways.