Quantitative Evaluation of the Effect of Black-Coating for Laser Flash Measurement

Abstract

Thermal diffusivity of solids is usually measured by the laser flash method above room temperature. This method has some advantages, for example, non-contact measurement and short time measurement. Generally, the surface of the specimen is a pulse-heated by near-infrared laser and the temperature change of the rear-surface is observed using an infrared radiometer. It is preferable that the front and rear surfaces of the specimen show high infrared absorptance and low infrared transmittance. In order to satisfy the optical conditions, black coatings using a dry graphite spray are commonly used. Although it is very convenient, it is pointed out that it shows poor repeatability and inhomogeneous coating. And it is said that the quantitative evaluation of the effect caused by this black-coating is difficult. In this study, we tried to evaluate the effect of the dry graphite spray coating. The variation of the thermal diffusivity caused by the dry graphite spray coating for measurement was investigated using the isotropic graphite and quartz substrates. The repeatability of the black-coating spraying was estimated as the standard deviation of measured thermal diffusivity values and thickness of black-coating. The quantitative effect by the black-coating was investigated as the weight and thickness dependence of thermal diffusivity. Thermal diffusivity of silicon nitride specimens are measured without and with coating. It is found that black-coating is effective to obtain reliable measurement-results using laser flash method.