Ultrasonic measurement of the diffusion bond strength

This paper presents an ultrasonic method for measurement of diffusion bond strength between two identical materials. The method requires a single normal-incidence ultrasonic measurement. When a diffusion bond is not perfect some ultrasonic energy is reflected from the interface separating the two substrates. Amplitude of a reflected signal correlates qualitatively with the bond strength. However, a quantitative correlation is difficult to establish since reflected signal amplitude depends on parameters of the test equipment. We introduce a spectroscopic analysis of a reflected signal to measure the bond strength. We show experimentally that an imperfect diffusion bond can be characterized by a single parameter: interfacial spring stiffness. The stiffness can be determined from the reflected signal spectrum and does not depend on testing equipment. It is also demonstrated that for a particular diffusion bonding procedure as applied to tool steel the interfacial stiffness correlates quantitatively with bond strength.