Strength degradation mechanisms in h-BN/NiAl coated sapphire fibres with a reactive Hf or Y interlayer

NiAl strengthened with ceramic fibres is considered as a load-bearing component in the combustion zone turbine blades. Sapphire fibres coated with hexagonal-boron nitride (h-BN) and Y (or Hf) interlayers and NiAl were investigated to strengthen the fibre matrix interface by compound formation with the transition metals introduced. Our goal is to identify strength degradation relevant mechanisms active during composite formation and application. Therefore, the tensile strength of coated fibres before and after annealing was measured to simulate the effect of composite fabrication. Strength degradation mechanisms were identified by electron microscopy. Chemical reactions between Y or Hf and Al2O3, as well as surface diffusion of Al2O3 into irregularities in the adjacent coating, alter the surface morphology and may act as crack initiation sites. Based on these results, future strategies for avoiding or minimizing strength degradation during production of intermetallic matrix composites (IMCs) can be compiled.