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The thermal diffusivity of brittle coatings cannot be measured by the flash method directly because of the difficulty of preparing free-standing samples. Adopting the flash method using a two-layer composite sample, it is possible to measure thermal diffusivity if the radiant pulse is well defined and good thermal contact on the interface of the composite sample can be ensured. Using an equilateral trapezoidal pulse of an Nd-glass laser measuring the dimensionless temperature history of the rear face of the sample, we determined the thermal diffusivity of thermal control coatings in the temperature range of 80 to 200 degrees C. The results for different thicknesses of substrate showed that the thermal contact resistance of the interface can be neglected.