Interface structure, chemistry and properties of NiAl composites fabricated from matrix-coated single-crystalline Al2O3 fibres (sapphire) with and without an hBN interlayer

NiAl composites with and without an hBN interlayer were produced from matrix-coated single-crystalline Al2O3 fibres (sapphire) by diffusion bonding. The evolution of interface structure and chemistry during the fabrication processes (fibre coating, diffusion bonding and embedded casting) was characterized by electron microscopy. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature. Interface structural analysis by transmission electron microscopy demonstrates that a high interface shear strength (about 230–250 MPa) in the composites without hBN interlayers is achieved by direct contact of NiAl with aluminium oxide (intrinsic coherence). In the composites with hBN interlayers the boron nitride was partially (as-diffusion-bonded composite) or completely (as-cast composite) transformed to AlN owing to a chemical reaction with NiAl at high temperatures. The low interface shear strength (about 70 MPa) of the as-diffusion-bonded composites was caused by sliding of textured hBN basal planes. The low interface shear strength (about 75 MPa) of the as-cast composites was attributed to segregation of aluminium boride to triple junctions and grain boundaries of AlN. The interfacial reactions in the composites with hBN interlayers can be rationalized from thermodynamic calculations.