New applications of the flash method

Two methods for measuring the thermal diffusivity of semitransparent solid ceramics and ceramic coatings on metallic substrates are described. Tests conducted on various zirconias have shown these materials to be highly unstable and tend to show that such materials exhibit an optically dense behaviour at high temperature. The use of a phosphate glass Q-switched laser with very short pulse (25 ns) allows the characterisation of a thin coating of ceramic with thickness between 15 and 50 μm. A new method for measuring the thermal diffusivity in the three principal directions of a three-dimensional composite material is described. The principle is derived from the flash method. It consists of heterogeneously illuminating one of the faces of a square-base specimen (by two parallel bands) in order to analyse heat transfer in the parallel direction (x and y directions) and the perpendicular direction (z direction) of the faces of the specimen. An example of a result obtained on anisotropic graphite demonstrates the validity of the method. This method applied to a homogeneous material gives two thermal diffusivity values (α x and α y ), thereby improving the measurement accuracy and reliability.