Interfacial thermal stability in BN-coated continuous Al2O3 fiber-reinforced NiAl composites

The microstructural changes at the interfaces in a NiAl composite reinforced with BN-coated continuous single crystal Al2O3 fibers were studied by analytical electron microscopy and high-resolution electron microscopy (HREM) after annealing at 1000 °C for 100 h and 500 h. Both NiAl/BN and BN/Al2O3 interfaces exhibited excellent stability during annealing, as did the AlN layer formed at NiAl/BN interface during processing. In the case of contamination which was introduced through the use of a glue during diffusion bonding of the composite, significant changes of the microstructure at both NiAl/BN and BN/Al2O3 interfaces were observed. A continuous reaction layer formed at the BN/Al2O3 interface during annealing. The reaction product was identified by electron diffraction as Al18B4O33. We propose to attribute this to the reaction: 9Al2O3+2B2O3→Al18B4O33. Many nanoparticles were observed in the BN matrix near the BN(AlN)/NiAl interface in the as-fabricated composite. Neither the AlN layer nor these nanoparticles grew during annealing. HREM images of the precipitates revealed a structure distinctly different from that of the BN matrix. B, C, N and Al were detected in these precipitates by EELS analysis.