Stability of nickel coated sapphire whiskers

The effect of high temperature annealing (1100 to 1300° C) on the stability of nickel coated purified sapphire whiskers has been investigated. It was found that the initially coherent coating spheroidized to form a series of partially adherent nickel particles, which increased in size with continued time at temperature. The time for complete adhesion of the particles, which was established from the product of the number and volume of particles per unit area of surface, decreased from 72 h at 1100° C to 3 h at 1300° C, giving an activation energy of 70 kcal/mol. In addition at 1300° C, after 8 h, there was evidence for whisker breakdown from a sapphire-nickel reaction.