Strong Interface in CMCs, a Condition for Efficient Multilayered Interphases

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Abstract
A fiber treatment was used to change the bonding strength of the Nicalon NLM 202 SiC fiber from weak to strong, in a series of 2D-SiC/SiC composites with multilayered interphases. The materials with the pre-treated fibers were compared to the same materials but reinforced with as received fibers. The stress-strain behavior and the fracture toughness were examined as a function of crack patterns identified by TEM. All the materials could be grouped into two distinct families: (i) materials reinforced with untreated fibers have a weak fiber bonding and are characterized by a low strength and a low toughness and (2) materials with the pre-treated fibers have a strong fiber bonding and are characterized by a high strength and a high toughness. This latter behavior is identified by TEM. It corresponds to a new interfacial behavior with a cohesive mode of interfacial cracking, involving branching and deflection by the successive interfaces. In the former family, the adhesive interfacial failure mode corresponds to the classical debond/sliding mechanism.

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