The Effect of Oxygen on the Ion-Acoustic Signal Generation Process

Thermal wave imaging (TWI) describes a family of methods for materials characterization based on temperature changes induced by an external source. The source can be a laser [1,2,3] or an electron [4,5,6] beam modulated to produce time-varying surface and bulk temperatures in the specimen. Recently, ion sources have been used for excitation [7,8,9] and share some imaging features in common with laser and electron sources. All three types of sources have the ability to detect buried defects in opaque solids and to locate tightly closed cracks [10]. However, the fundamental physical mechanisms of signal generation and image contrast vary to some extent based on the source and, in particular, on the physics of ionacoustic signal generation process. Non-thermal acoustic wave generation by particle beam-specimen interactions may play a role in this process along with the thermal generation processes familiar from the case of laser excitation. In a continuing effort to study acoustic generation mechanisms employing an ion beam source, the question of sensitivity to surface chemistry arose. Since our apparatus was equipped to conduct experiemnts under O2 partial pressures, the effect of O2 on ion-acoustic generation was a natural choice. Oxygen is one of the most common impurities encountered in materials development, and its presence in materials such as turbine blade coatings raises questions about propagation through microstructures containing different phases