Effect of processing and fiber coating on fiber-matrix interaction in mullite fiber-mullite matrix composites

The fiber-matrix interaction in mullite fiber-mullite matrix composites was investigated with the composites fabricated using a commercial mullite powder and a mullite powder synthesized in our laboratory (via a diphasic gel route) as the matrix with uncoated and BN-coated fibers. Two different BN-coating thicknesses, 0.3 μm and 1 μm, were used. The composites made from the mullite powder synthesized in our laboratory showed less interaction between the fiber and matrix as well as no fiber deformation owing to a lower processing temperature, compared with the composite made from the commercial mullite powder. However, the fiber-matrix bonding in both the uncoated and even 0.3 μm BN-coated cases was still strong so that no fiber pullout was observed on the fracture surfaces. In the case of the 0.3 μm BN-coated fiber, the strong interfacial bonding resulted because the coating thickness was too thin to survive during processing. With 1 μm BN-coated fiber, however, extensive pullout could be obtained because the coating survived and prevented bonding between the fiber and matrix.