Strength degradation of NiAl coated sapphire fibres with a V2AlC interlayer

The tensile strength of sapphire fibres coated with NiAl containing a V2AlC interlayer was investigated. The effect of the deposition processes as well as the influence of temperature experienced during diffusion bonding onto the tensile strength was analysed. Strength degradation mechanisms were identified by chemical and morphological analysis. The investigated material systems show significant strength degradation at room temperature. This may be due to thermal stress induced twin formation as well as formation of fracture mirrors. Thermal stresses arise upon cooling after the deposition and the consolidation process, respectively. With the introduction of a V2AlC interlayer chemical reactions between fibre and matrix could be prevented and the population of morphological features on the fibre surface due to Al2O3 surface diffusion acting as stress concentration sites could be reduced. The extensive reduction in fibre strength can be understood based on thermal stress induced twinning and fracture mirror formation.