MEASUREMENTS OF COATING ADHESION STRENGTHS BY TIME-RESOLVED INTERFEROMETRIC MONITORING OF LASER-INDUCED SPALLATION

A novel scheme for the measurement and characterization of the adhesion strength of solid coatings on solid substrates is being presented. Although the method is presently restricted to purely elastic and isotropic materials, 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 that is manifest in the time-dependence of the surface displacement of the coating. The spallation of the coating is induced by a shock pulse generated at the back side of the sample by the impact of an 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 lo the threshold of spallation. In a theoretical section, the transmission and reflections of the laser-generated elastic waves at the interface between substrate and coaling are reviewed. Under the assumption that the cohesion strengths of the substrate and the coating exceed the adhesion strength of the coating the spallation of the coating is modelled, and the effect of spallation on the measured surface-displacement signal is discussed. The results of spallation studies performed on steel substrates coated with nickel or plasma-sprayed ceramics with thicknesses between 100 and 500 u,m are presented. Spallation has been achieved in all cases, yielding interesting results including adhesion strengths in the range of 0.2-2 GPa for the nickel coatings, which represent an ideal case for the demonstration of our scheme. We have even been able to analyse the dynamics of the formation of small delaminations for ceramic coatings. Although such delaminations with gap heights between 0.9 and 2um cannot be detected by visual inspection of the sample, it is possible with our method to measure their geometrical extension in all three dimensions.