Effect of a BN interlayer on the tensile strength of NiAl coated sapphire fibers

The tensile strength of sapphire fibers coated with NiAl and BN before and after annealing was evaluated. Fibers without BN interlayer cracked during annealing. This may be due to thermal stress induced fiber fracture during cooling. Fibers coated with BN interlayer retained approximately 60% of their strength after annealing. Fiber surface diffusion and surface reactions with impurities may, in combination with thermal stress, be responsible for the strength degradation.