Compatibility limits in sapphire whisker-nickel composites

The effects of anneals at 1200 to 1300 C on the compatibility of sapphire whiskers coated with nickel, with the coatings ranging from 20 to 320 nm in thickness, have been investigated. It was found that at 1200 C, all the coatings, which were initially continuous, spheroidized to form a series of adherent particles. The particles coarsened with continued time at temperature, but the whiskers remained coherent. In contrast, at 1300 C, most of the whiskers degraded after anneals longer than 2 hr, an event which was preceded by a transition in the wetting characteristics of the particles and hole formation in some whiskers. The whisker breakdown was correlated with the diffusion of iron impurity into the nickel particles.