Abstract

The adhesion strength of surface coatings is a crucial parameter in view of the performance of such coatings. We present a noncontact technique that permits a quantitative characterization of the coating adhesion. It is based on laser-induced acoustic waves and interferometric detection of the generated transient surface displacement. Nd:YAG laser pulses are used to generate the required acoustic pulses at the back side of the sample. The solid coating at the front surface of the sample will undergo delamination if the superposition of the generated incoming compressive and of the reflected tensile wave exceeds the adhesion strength of the coating. The time-resolved monitoring of the coating displacement yields information on the adhesion properties. The transient surface displacements are measured with a specially designed, homodyne fiber-optic interferometer. It enables the detection of displacements of >1 nm at a large bandwidth of 300 MHz.¹ Furthermore, an analysis software has been developed that allows an interactive calculation of the time-dependent displacement from the interferometer signal. Here we present new results obtained for plasma-sprayed Cr₂O₃ and Mo/Mo₂O₃ layers on steel substrates. An example is shown in Fig. 1 for a 120-μm-thick molybdenum ceramic layer. Seven consecutive laser shots were fired onto the same spot on the rear side of the steel substrate. Figure 1(a), shows the interferometer signals and Fig. 1(b), the corresponding calculated displacement signals of the molybdenum layer opposite the laser impact for the first three shots. Obviously the first shot (numbered 1) produced a delamination of the layer with a gap of ≈ 1 μm remaining between substrate and layer. Because of this delamination gap, the longitudinal acoustic pulses generated by further laser impacts cannot reach the on-axis detection point directly but only by circumventing the gap by its still adhering rim. The resulting extension of the path explains the observed delay of the arrivals of signals 2 and 3. The delay of approx. 20 ns (Fig. 1(c)) corresponds to a lateral diameter of the delamination of 320 μm. Although this delamination can clearly be detected, the surface of the ceramic layer does not show any visible damage. This demonstrates that conventional, e.g., microscopic inspection of the surface is not an adequate means to detect the delamination of coatings while our technique is capable of producing and detecting minute delaminations. Further examples for nickel-coated steel samples will be presented, including detailed analyses of the interferometer signals in view of the adhesion strength of these coatings.

© 1994 IEEE