Estimation of thermal diffusivity of thin film materials by a one-level, two-point 'diverging'-thermal-wave technique. Part I: Theory

Thin films and coatings widely used in different branches of modern technology are not appropriate for thermal nondestructive testing because of time limitations imposed on conventional front-face flash methods. Instead of taking temperatures directly at the pulsed heating region, as is traditionally done, it is suggested that the temperature be measured at any distance from the centre of the heating spot. Since this method does not require specific instrumentation, its realisation is very easy. The expression derived for the thermal diffusivity does not require knowledge of sample thickness, depth of absorption of the incident beam, or of radiative or convective losses. Application of the method requires only measurements of temperature excursion with time, on either side of the sample. This is an important improvement where applications to thin films are considered or where access to the sample is possible from one side only. The method is equally suited to opaque, transparent, and semitransparent materials. Applicability of the new method to thin metallic and dielectric films is discussed. Accuracy of the method is checked by comparison of the results obtained for different samples with finite-element calculations.