Theory of photothermal displacement method for determining physical properties of multilayered coatings

In this study theoretical principles underlying the photothermal method for determining thermal properties of opaque multilayered and functionally graded coatings are analyzed. The method is based on irradiation of the assembly by the repetitive pulse of focused laser radiation that is absorbed in the subsurface region and causes non-uniform heating and buckling of a coating. The irradiated surface of a coating is monitored by a low power beam of a second laser that is reflected from the specimen. The deflection angle of the monitoring beam, as a function of time, contains the relaxation and the "wave" components. It is shown that the phase of the "wave" component depends on the thermophysical properties (e.g., thermal diffusivity or thermal conductivity) of a coating. These properties can be determined by comparing experimentally measured values of the phase shift of the "wave" component with the theoretical values obtained from the analytical solution of the two-dimensional thermal elasticity problem for a multilayered coating-substrate assembly.