Impact of fiber surface gas treatments on interfacial bonding in SiC/BN/SiC minicomposites

Abstract BN interphases with different microstructures and a structural gradient were prepared by chemical vapor deposition (CVD) within SiC/SiC minicomposites. The interfacial shear stress (ISS) measured by the fiber push‐out test and the debonding location in the fiber/matrix interface zone during mechanical loading depends on the BN interphase structure. The ISS is higher when the structural organization of the BN interphase is poor, in which case debonding occurs on the fiber side. The Hi‐Nicalon S fiber surfaces were gas‐treated to strengthen the bond with the interphase. BCl 3 ‐based reactive CVD treatments have produced a thin boron carbide layer on the fiber surface, resulting in a 30% increase in ISS. However, the minicomposites are brittle in tensile tests due to fiber bridging by parasitic boron clusters. Short‐time CVD of SiC increases the fiber surface roughness and doubles the ISS, but again the minicomposites are brittle, indicating the need to better modulate the resulting roughness and adjust the interphase. Finally, ammonia treatments have nitrided the fiber surface and increased the ISS by 12%. While the increase in interfacial bonding measured at the microscopic scale was small, this nitriding treatment did not embrittle the minicomposites at the macroscopic scale.