BN/SiC Coating on SiC Tows Prepared by Chemical Vapor Infiltration

Boron nitride (BN) and silicon carbide (SiC) coatings were prepared in sequence on desized SiC tows by chemical vapor infiltration (CVI) at the temperature of 950°C. BN was synthesized by the reaction between boron trichloride (BCl₃) and ammonia (NH₃) while SiC was from the thermal decomposition of methyl trichlorosilane (MTS). The thickness of the coatings was uniform on a certain cross section of a monofilament, and the thickest/thinnest ratio is less than ∼2 across the whole SiC tows for both SiC and BN coatings, as observed by scanning electron microscopy (SEM). Distinct β-SiC powder X-ray diffraction (XRD) pattern was obtained for as-prepared CVI SiC while as-prepared BN coating only showed a bump and a tiny peak corresponding to hexagonal BN (h-BN). This result was verified by transmission electron microscopy (TEM) images, in which CVI BN only had curved diffraction fringes within small pockets, known as turbostractic structures(t-BN). However, t-BN possesses similar short-range structure to h-BN as demonstrated by ¹¹B solid-state NMR spectrum. SiC_f/SiC minicomposites using the interphase coating mentioned above is currently under investigation.