Testing Stability of Surfaces and Coatings by Resonant Photoacoustic Phase Transition Detection

A technique has been developed for the determination of the phase transition temperature of very small samples or thin films and coatings. The technique relies on the determination of changes in the heat capacity of the surface by resonant piezoelectric detection of the photoacoustic signal. The sample is illuminated by means of a low power diode laser modulated at the resonant frequency of a piezoelectric detector. The detector senses the acoustic signal generated by the expansion of the surface that has been slightly heated (about 1 K) by the laser. The signal is monitored as the sample is heated at an established rate, and the phase transition temperature is determined by the change in the amplitude and phase of the signal. As the penetration of the light in metals is very small (well below 1 μm), the technique is particularly suitable for the study of surface coatings and thin films.