RECENT DEVELOPMENTS IN THE LASER SPALLATION TECHNIQUE TO MEASURE THE INTERFACE STRENGTH AND ITS RELATIONSHIP TO INTERFACE TOUGHNESS WITH APPLICATIONS TO METAL-CERAMIC, CERAMIC-CERAMIC AND CERAMIC POLYMER INTERFACES

This paper summarizes recent developments in the laser spallation technique for measuring the tensile strength of planar thin film interfaces. In this technique, a laser-produced compressive stress pulse irk the substrate, reflecting from the coating's free surface, pulls the interface in tension and leads to its failure if the tensile amplitude is high enough. Earlier, the critical stress amplitude that accomplishes the removal of the coating was determined through a computer simulation of the process. Recently, the technique was modified so that the interface stress can be determined directly by recording the coating or substrate free-surface velocities using a Doppler interferometer. The recorded surface velocity is related to the interface stress via an elastic wave mechanics simulation. Interface strengths of several metal/ceramic, ceramic/ceramic and ceramic/polymer systems are summarized from our recent efforts. In addition, two developments, the first a novel interferometer to record velocities from rough surfaces, and the second a technique to produce subnanosecond rise-time stress pulses with no asymptotic post-peak decay, are discussed which further allows the technique to be applied to rough thermal spray coatings and also to films as thin as 0.1 mum. Finally, it is shown how the tensile strengths obtained from the laser spallation experiment can be related to the interface fracture energies through a Griffith-type relationship, which, in tum, is derived by using the concepts of universal bonding correlations. It is shown that the estimated values obtained through this relationship am in good agreement with experimentally obtained values if the interfaces are free of defects, suggesting that the laser spallation experiment measures a fundamental strength value that is intrinsic to the material system and makes the measured value a suitable parameter for characterizing the interface.