Broadband fiber-optic interferometer as ultrasonic transducer suitable for coating adhesion measurements

Abstract

A novel scheme for the measurement and characterization of the adhesion of solid coatings on solid substrates is presented. It represents an appealing alternative to standard methods, which are not satisfactory in many respects. Our technique is based on laser-induced spallation of the coatings under test, combined with an interferometric monitoring of the spallation process which is manifested in the time dependence of the surface displacement of the coating. We induce the spallation of the coating by a shock pulse generated at the backside of the sample by the impact of a Nd:YAG laser pulse. At the coated front surface of the sample, compressive pulses are reflected as tensile pulses. The coating undergoes spallation if the effective tensile stress, which is the superposition of the stresses of the incoming and reflected waves, exceeds the adhesion strength of the coating. In order to measure adhesion strengths, one has to determine the critical tensile stress at the interface corresponding to the threshold of spallation. We present our experimental setup for the laser generation of high-amplitude acoustic waves, which induce the desired spallation of a coating under test. A fiber-optic interferometer has been developed, which features a low noise level of 1 nm and a large bandwidth of 300 MHz necessary for recording the fast transient surface displacements of the sample surface. The performance and versatility of this instrument are analyzed in theory and experiment. An interactive computer program has been developed which allows the correct reconstruction also of large surface displacements from interferometer signals that stretch over many interference fringes.