Weak interface dominated high temperature fracture strength of carbon fiber reinforced mullite matrix composites

Weak fiber/matrix interface dominates the toughening properties of ceramic matrix composites. This paper reports a novel sol-gel fabricated carbon fiber reinforced mullite matrix composite, in which the fiber/matrix interface was inherently weak in shear properties (∼25 MPa), measured in-situ by fiber push-in tests. The interface microstructure was chemically sharp, characterized by transmission electron microscopy. The outcome of the weak interface was the full trigger of the toughening mechanisms like crack deflection, etc., leading to significant enhancement of the fracture toughness of the composite (∼12 MPa√m), measured by single edged notch beam method. Finally, due to the weak fiber/matrix interface and large thermal expansion mismatch of the fiber and matrix, the high temperature fracture strength was enhanced in the temperature range from 25 to 1200 °C, which is attributed to the enhancement of the interfacial property at elevated temperatures that favors better load transfers between composite constituents.