Creep fracture experiments with planar sapphirecopper interfaces stressed in tension

Abstract

Planar interfaces between sapphire and copper were prepared by diffusion bonding. Constant load experiments with these specimens exhibited a threshold strees for creep-fracture. The magnitude of the threshold stress was small, about 2–4 MPa, and it decreased with increasing temperature. Above the threshold the fracture surfaces were featureless and fracture appeared to have occurred by crack propagation. The estimates of a threshold stress intensity for crack propagation yielded values of only 0.01–0.02 MPa √m. In experiments carried out below the threshold stress, cavitation was observed at the interface. The cavities nucleated very quickly with a spacing of about 2–5 μm. However, the rate of cavity growth slowed considerably with time. Thus, while the early growth rate was faster than an equivalent measurement in copper-copper bicrystals, the cavity growth rate became slower than copper-copper as time progressed. We suggest that, initially, diffusion occurs quickly at the coppersapphire interface, but then a spinel or an intermetallic type reaction layer slows the diffusion rate.