Toughening of Mullite/Cordierite Laminated Composites by Transformation Weakening of β‐Cristobalite Interphases

An interesting concept for achieving graceful failure in oxide composites is discussed. It is based on crack deflection in a weak interphase between a matrix and reinforcement (e.g. fiber), around a fibrous core in a fibrous monolith, or in an interphase in a laminated composite. The interphase can be phase transformation weakened by a crystallographic unit cell, volume contraction, and/or shape change. Mullite/cordierite laminates with a β→α‐cristobalite, transformation‐weakened interphase were investigated for interphase debonding behavior. The laminates were fabricated by stacking alternate, tape‐cast, green sheets of chemically doped β‐cristobalite, which was synthesized by an organic steric entrapment method, and a mullite/cordierite matrix mixture. The laminate showed fracture behavior depending on a critical particle size effect. The grain size of polycrystalline β‐cristobalite was controlled by annealing time at 1300°C. A hot‐pressed laminated composite, annealed for 10 h at 1300°C, had an average grain size of ∼4 μm and a 3‐point flexure strength of 131 MPa. Its work of fracture was 2.4 kJ/m ² but non‐catastrophic fracture behavior was demonstrated. The indentation response indicated crack deflection along the cristobalite debonding interphase. With increasing annealing time, the strength decreased due to the formation of internal macrocracks in the cristobalite layer, which occurred spontaneously during thermally induced transformation.