Ultrasonic characterization of metallic interfaces
A set of diffusion bonded copper samples was prepared at different temperatures and times allowing the bonding to proceed across the planar interfaces. To obtain a second set, interfaces were roughened to various degrees followed by diffusion bonding at a designated time/temperature condition. On all samples, ultrasonic reflection coefficient (R) maps of the bonded interfaces were obtained over a broad frequency range. In addition, the bond strengths, a, of the interfaces were determined, thus providing an empirical a-R correlation. Nearly all of the specimens tested failed along the interfaces, exposing fracture planes with distinctive features indicating originally bonded and unbonded areas. These features, examined metallographically, allowed for the successful testing of the "distributed spring model" by Baik and Thompson (J. NDE 4, 177, 1984). This model was used as an intermediate step in the development of a bond strength model to explain the observed σ-R correlation, the beginnings of which are discussed.